Particle transport by continental water flows in relation to erosion, deposition, soils, and human activities

Sedimentary Geology, 20 (1978) 81--139 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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PARTICLE TRANSPORT BY CONTINENTAL WATER FLOWS IN RELATION TO EROSION, DEPOSITION, SOILS, AND HUMAN ACTIVITIES

A.J. MOSS and P.H. WALKER

Sediment Transport Group, Division of Soils, CSIRO, Canberra, A.C.T. (Australia) (Revised version accepted March 12, 1976)

ABSTRACT Moss, A.J. and Walker, P.H., 1978. Particle transport by continental water flows in relation to erosion, deposition, soils, and human activities. Sediment. Geol., 20: 81--139. Field and laboratory studies, and the results of an experiment, combine to show that erosional, transportational and depositional processes in shallow, overland flows are basically the same as those of streams. Thus essentially fluviatile processes can operate, ephemerally, over entire landscapes. Separation into behaviourally distinct suspended and bed loads occurs even in flows a millimetre deep. Normally, suspended-load transport rate is limited by detachability or availability, whereas bed-load transport rate is governed by highly slope-sensitive capacity limits. Because most slopes must ultimately decline in gradient downhill and because bedload capacity falls rapidly with decreasing slope, 'hydraulic mantles', deposits built of excess bed load, are almost universal features around the bases of hills. Most natural slopes are consequently divisible into an upper zone of net erosion and a lower zone of net deposition. In these depositional zones, solid--fluid interactions exert major control over their own physical environments by adjusting such parameters as slope, depth, and velocity. Consequently, given a supply of heterogeneous detritus, an extremely consistent succession of sediment types differentiates downhill, with distally decreasing surface slopes, as transporting power and turbulence decline. This succession is represented in the well-known catenary sequence of Milne (1936a, b) and also occurs, with little modification, in rivers. Overland flow transportation is strongly inhibited by dense plant cover. Thus, whereas landscapes with sparse vegetative cover can maintain hydraulic adjustment to overland flows, heavily vegetated landscapes are less able to make such adjustments and tend to remain protected from erosion, often for long periods. During such periods tectonic movements may modify slopes, pedogenesis may alter textures, or deposition of fine material may result from the dissipation of fluid energy by plant cover. A backlog of needed hydraulic adjustments is thus built up and sudden baring of such long-protected surfaces can lead to drastic readjustment by surface flows. Much of the more severe human-activated water erosion evidently results from the baring of surfaces in such naturally disequilibrated states. Many hillslope soil materials must have developed on bed-load sediments. Some examples have been shown to have this origin. Fine organic and inorganic materials, with which soil fertility is closely associated, are, if entrained by flowing water, maintained largely in suspension in steeply sloping hillside

82 environments. However, where turbulence has waned sufficienctly, probably from a stage in which the bed surface grains are first immersed in the viscous sublayer, redeposition of these particles takes place, often in high concentration. This evidently happens on footslopes and m a n y floodplains and is part of a natural system of fertility transference and renewal. Diversion by man of natural flows and the building of dams interfere with this natural process and can therefore have a long-term detrimental effect on natural soil fertility.

INTRODUCTION

General considerations and purpose of investigation Gilbert (1882) ascribed the entire topography of the Lake Bonneville area to the action of surface water. He considered such action worldwide, writing, 'Our eyes are so accustomed to these forms that we unconsciously anticipate them and readily detect the exceptional nature of all other forms of sculpture'. He asserted that 'an alluvial footslope is the inevitable concomitant of an angular summit'. Davis (1909) attributed the movement of solids down hillslopes to many causes but stressed the overall similarities between such transportation and that of rivers. He noted that both the 'waste sheets' of valley sides and rivers moved fastest at the surface, made similar grading adjustments and matured in similar ways. The often enormous transporting power of comparatively shallow water, flowing down hillslopes, has been unwittingly demonstrated in many areas as a result of human activities. Mere reduction of plant cover can allow major disturbance of solids, the coarser grades of which are often rapidly redeposited as slope declines downhill. So widespread is this redistribution process that there can be little d o u b t that sediments thus formed in the past have provided the materials on which many soils have developed. Milne (1936a, b), working in eastern Africa, gave the first detailed description of the relationships between the disposition of soils on hillslopes and past erosional and depositional events of unexceptional magnitude. He defined the soil catena in terms of downslope translocation and selective redeposition of both solids and solutes. Holmes (1937) observed comparable 'slope successions' in New South Wales, Australia, associating them with past climatic changes. Evidence of slope erosion and associated differential redeposition, in ancient landscapes and paleosols, has been widely reported (Eargle, 1940; Ruhe, 1954, 1956; Parizek and Woodruff, 1957; Berry and Ruxton, 1959; Butler, 1959; Gile and Hawley, 1966). Ruhe (1959) considered stone lines, in both Africa and the U.S.A., to result from water erosion, the pebbles evidently being left behind as finer particles were washed away. He demonstrated the relationship of soil properties to the processes of erosion and deposition that gave rise to differing parent materials. Many old hillslope landscapes have highly complex

83 sediment and soil distributions evidently resulting from repeated episodes of erosion and deposition (Ruhe, 1956; Butler, 1959, 1967; Gile and Hawley, 1966; Ojanuga et al., 1976). In more modern (Holocene) landscapes, systematic downslope variations in soil properties (particularly particle size) are widespread (Walker, 1966; Ruhe and Walker, 1968; Walker and Ruhe, 1968; Kleiss, 1970; Malo et al., 1974). In closed drainage basins on late glacial drift (<13,000 y B.P.), Walker (1966) showed that gross sedimentological differentiation had produced discrete bodies of coarse and fine sediments whose disposition was radial in relation to the clay-floored depressions. This differentiation evidently took place in overland flows. Similar conclusions have been drawn for open drainage systems (Daniels and Jordan, 1966; Ruhe and Walker, 1968). However, in open systems, the finer fractions must often be carried away by streams. Kleiss {1970) studied soils on relatively young surfaces (<3,000 y B.P.), sculptured from pre-Wisconsin till. He concluded, 'Profile characteristics indicate little pedogenic soil formation, rather, many of the soil properties are inherited from the sedimentary nature of the soil material'. He listed particle size, organic carbon, bulk density and cation exchange as soil properties largely influenced by the nature of hillslope sedimentation. In fluvial flow and transportation studies, the depth--slope product has long been recognized as an important parameter. Overland flows typically have lesser depths but higher slopes than do rivers so this product could often be of the same order in both environments. However, overland flows are more often supercritical and are frequently subject to direct effects from raindrop impact. Are erosion, transportation, and deposition in the two environments homologous, analogous or quite different in physical nature? Published evidence suggests analogy, if not homology. Ellison (1947, 1950) described what was essentially the differentiation of solids into suspended and bed loads at erosion sites, using the term 'fertility erosion' to describe how fine organic and inorganic materials are transported rapidly away, often leaving behind deposits of coarse, infertile sands. Dory and Carter (1965) and Swandon and Dedrick (1967) made similar observations. The suspension of fine solids by shallow flowing water and their subsequent rapid entry into river systems is one of the major characteristics of mancaused erosion. Saltation and rolling, the major modes of bed-load transportation in rivers, have been observed in shallow flows (Ellison and Ellison, 1947; Meyer and Monke, 1965; Foster and Meyer, 1972). Moreover, very shallow water can transport significant quantities of solids without the aid of rainfall impact (Emmett, 1970; Foster and Meyer, 1972). McGee (1897) reported very strong transportation of solids by a sheet flood when no rain was falling. Deposits of overland flows and rivers have many features in common. Both are often laid as series of laminae; ripples, widespread in rivers, occur in fine sands laid by overland flows; sorting often produces very clean sands

84 in both environments. Bull (1963) noted that some alluvial fan sands were as well sorted as beach sands. Bed-load equations, developed for application to rivers, have been found to adapt quite well to thin flow transportation (Young and Mutchler, 1969; Foster and Meyer, 1972). Many students of geomorphology have tended to examine the current product of sculpturing -- the gross landscape features and to draw inferences concerning the processes that produced these current configurations. But it is also possible, following the evolutionary direction of the landscape itself, to study the relevant processes directly and thence attempt to explain, in terms of them, the geometry of landforms. The present investigation, following the latter course, represents a detailed study of transportation b y overland flows which, in combination with knowledge of fluviatile processes, is later applied to landscape sculpturing, soils, natural fertility, and man-caused erosion. The physical solid--fluid interactions of overland flows must be basically the same whether happening naturally or initiated by human activities. Studies of these interactions are thus equally applicable to natural landscape evolution and man-caused erosion. Because of the great importance of past history in landscape and soil studies, we have tried to relate properties of sediments closely to their formative environments. This approach not only facilitates understanding of the processes themselves but often allows environmental identification of materials formed in the past. The study was largely based on field work, much of which was carried out under heavy rain, and many of the modern field samples studied were from deposits actually seen to form. The results of a single, relevant experiment are also reported. Because of close similarities between processes in overland flows and rivers, the investigation was conducted mainly as a comparison between the two environments and their sediments. Before reporting our results, therefore, a brief review of fluviatile processes, as relevant to sediment formation, will be given.

Deposition and textural properties o f river sediments Detailed studies of river sediments, in relation to their formative physical environments, were made by Moss (1962, 1963, 1972). A brief synopsis of this work is given here. Greater detail is available in the original papers and their bibliographies. The total load of a stream is the sum of several loads transported by distinct mechanisms. These are here regarded as floating load (buoyed at the surface), dissolved load (in ionic solution), suspended load (ranging from colloids to coarser particles in turbulent suspension) and bed load (relatively coarse particles moving in continuous to intermittent contact with the bed). Bed load has two components, saltation load and contact load (usually rolling, sometimes sliding along the bed). Bed-load particles, resting on others their

85 own size or larger, tend to move by saltation; if resting on others smaller than themselves, they tend to roll. Both saltation and rolling respond to increasing particle concentration, and resulting interparticle collisons, by changing in nature. Saltatory particles form a ballistic dispersion whose tendency to dilate is balanced by gravity. They thus form a layer which flows rather like a viscous fluid and was called the 'rheologic layer' by Moss (1972). Whilst saltation is essentially threedimensional, rolling tends to be two-dimensional. At high concentrations, rolling particles tend to jam up as one halts another, and so on, until a juxtaposed halted layer forms which is often immovable to the current. River sediments are composites, comprising two or more distinct populations of particles, intimately admixed but emplaced by different physical mechanisms. In sands, the quantitatively most important is the framework population (A), derived from either individually saltating bed load or bed load moving as a rheologic layer. By repeated grain emplacement and subsequent removal of those in less stable positions, a well-packed sediment framework is built of particles of almost equal size. Finer particles, landing from suspension or from saltatory leaps, may pass contemporaneously into the interstices of the framework and become trapped, unless or until the protecting, larger particles are moved. This second m o d e of emplacement gives rise to the interstitial population (B) which is highly sensitive environmentally. Contact load particles that halt on accruing beds often become interred by the growing framework population, and give rise to the contact population (C). If contract load is coarse and abundant, and its particles (usually pebbles) jam together on the bed, the contact load-flow interactions dominate the bed and the framework population merely builds up its level between the pebbles. The result is typical river gravel. Moss (1972) described the depositional sequence that consistently occurs, in rivers fed with heterogeneous detritus, as transporting power, turbulence, and grain size decline downstream. Successive differing solid--fluid interactions dominate the bed--flow interface, leaving behind different types of sediments. These sediment types tend to meet each other along sharp junctions, a reflection of their physically distinct formative mechanisms. Such junctions were reproduced in flume experiments. The sequence not only occurs downstream, along the thalweg, but tends to repeat itself laterally, from thalweg to bank and, vertically, in the flood plain deposits of meandering streams. It is closely linked to the well-known sequence of fluviatile sedimentary structures. The bed stage sequence of sediments (or solid--fluid interactions) with declining transporting power, was given for rivers by Moss (1972) as follows:

Rheologic bed stage with clogged contact population. Typified by the traction clog gravels of streams; framework population, built from the rheologic layer is stacked between juxtaposed coarse, halted contact population particles; interstitial population copiously present and usually including

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clay, infiltrates the voids of the two coarser populations; deposits react to erosion by forming armoured monolayers of coarse contact load, the normal bed lining of mountain streams.

Rheologic bed stage without clogged contact population. Sands laid from the rheologic layer; often clayey; mean framework grain size is variable; interstitial population is copiously present and often includes some clay. These sands are frequently interstratified with gravels. Dune bed stage. Characterized by dunes in medium to coarse sand; mean framework grain size about 0.25 to at least 2.2 mm. During deposition, turbulence probably penetrates to below the base of the surface layer of particles; interstitial population is sparse and relatively coarse with clay typically absent; contact population is normally sparse to absent and relatively fine. Coarse ripple bed stage. Characterized by ripples in clean, medium sand; mean framework grain size range is 0.25--1.00 mm but rare above 0.70 mm. Summits of surface layer of framework grains in turbulent water, but the viscous sublayer probably extends above their bases. Interstitial population is very sparse and relatively coarse with clay normally absent; contact population is sparse to absent and relatively fine. Fine ripple bed stage. Characterized by ripples in rather clayey fine sand or silt; mean framework grain size ranges from as little as 0.01 mm to about 0.25 mm; surface framework grains are probably entirely within the viscous sublayer; interstitial population copiously present and usually rich in clay; contact population is relatively fine and sparse to absent. Suspended load bed stage. Fine material deposited directly from suspension; less studied, in the present context, than the other bed stages. In the first two stages, the rheologic layer acts as a buffer zone between bed and flow. The remaining four, in which the flow acts directly on particles on the bed, are termed, for convenience, the 'bare bed stages'. This differentiation sequence apparently also exists on sea floors with little modification (Stubblefield et al., 1975). Even cursory inspection of hillside mantle deposits suggests that a similar sequence has differentiated widely on landscapes, producing a variety of soil parent materials. For example, gravels, bearing strong resemblances to river gravels, occur widely in mantles and are often interstratified with clayey sands. FIELD OBSERVATIONS

Field observation was facilitated because processes at the bed--flow interface are more readily visible in shallow, overland flows than in streams. For

87 this reason, much field work was conducted while heavy rain was falling. Flows 1--50 mm deep were examined on slopes of 1--15 °. Many sites were visited, b u t three areas, showing geological variation and slightly disturbed b y human activities, were studied in detail. These were: (1) Gibraltar Creek. The valley of this creek, which drains rugged country a b o u t 30 km southwest of Canberra, is cut mainly into deeply weathered Shannons Flat Adamellite (Joyce, 1973). Dominantly concave depositional lower slopes pass down to the main channel. Heterogeneous granitic detritus, ranging downwards from huge boulders, groups of which cap the hilltops, is yielded. Soils range from red podzolics (Stace et al., 1968) or Hapludalfs (Soil Survey Staff, 1967) on upper slopes to yellow podzolic soils or Umbraqualfs on lower slopes. (2) Black Mountain. Black Mountain, within Canberra City, is largely fault-bounded and is dominantly composed of marine Silurian fine greywacke sandstones (Crook et al., 1973). It rises 200--300 m above the local plain and is surrounded by extensive mantles (Costin and Polach, 1973). Detritus yielded comprises sandstone pebbles and boulders, fine sand, silt and clay. Medium and coarse sand are rare. Soils range from lithosols (Ustorthents) on mid-slopes to yellow podzolic soils (Albaqualfs) on the lower slopes. (3) Mount Ainslie. Mount Ainslie, also within Canberra City, is similar to Black Mountain in dimensions, soils, and general nature b u t is composed of Devonian acid and dacitic flows and pyroclastic rocks (Opik, 1958), cut by many closely spaced incipient fractures. Detritus comprises mainly rock fragments, ranging from boulders to particles of fine sand and silt size, together with some clay. Quartz is a minor constituent. Shallow-water flows; their transportation mechanisms and sediments General nature. Significantly transporting sheet flows were not encountered. All flows that were capable of transporting the materials over which they flowed, cut channels and concentrated transportation along them. Save at very low water surface slopes, all flows were supercritical and turbulent. In the three main study areas, visible moving particles were already geological in nature comprising sand grains and pebbles. Soil aggregates were rare and evidently easily destroyed. However, in some other areas, more persistent aggregates occurred. Sediment transported by the local rivers contains little or no material specific to soils except for rare argillan flakes. Particle motion and sediments on hillslopes. In water as little as a millimetre deep, new detritus, even as entrained, became immediately separated behaviourally into suspended and bed loads. The former moved downstream at the speed of the current, whereas the latter, moving in saltation or rolling, joined the other slowly and intermittently moving bed-load particles. Because no suspended-load deposits formed on the hillslopes, the bulk of this fine material must have been carried, swiftly and directly, to the local streams.

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Subsequent textural analysis of sediments revealed only very small proportions in the interstices of coarser hillslope sediments. This initial separation caused eroding soils to lose their previous identities as they were disaggregated and set in motion. On steeper slopes, down to about four degrees for the more strongly flowing natural channels seen, the saltation load moved as a rheologic layer which sometimes occupied most of the flow depth b u t was never actually observed to fill the flow completely. Thus, rapidly moving bed-load-free water, above the layer, was always able to carry suspended load away, as a spatially separate process. Pebbles were transported in such flows, often rolling at a b o u t half the speed of the current and even continuing in motion while protruding from the flow ~ as has also been noted in ephemeral streams (Leopold and Miller, 1956). Coarse contact load was sometimes seen to jam on the bed and initiate the formation of gravel deposits which closely replicated those of rivers. Associated with the gravels, and typically marginal to them, were discrete deposits of often clayey sands, usually pebble-free. These, too, were laid from the rheologic layer. The two types of sediment, often closely associated, were clearly homologues of the rheologic bed stage deposits of rivers. Individual saltation of medium to coarse sand was the dominant m o d e of bed-load transportation in somewhat gentler flows, typically on slopes of 1 - 4 ° . This type of transportation, as in streams, also took place towards margins of flows whose centres were occupied by the rheologic layer. Rolling particles, sometimes up to 10 mm in diameter, were often moved along with the individually saltating sand and tended to concentrate towards channel thalwegs. Often, these rolling particles clogged, seeding fine gravel deposits. This led, as in the rheologic bed stages, to the side-by-side formation of two types of deposit. Fine gravels were typically concentrated along the thalweg, and sands, built almost entirely of saltatory particles, tended to occupy more marginal sites and to build bars. Both types of deposit were remarkably clean, not only lacking clay and silt, but often having fine sand strongly suppressed. In formative mechanism and general nature these medium and coarse sands (and fine gravels) were clearly closely akin to the dune and coarse ripple bed stage deposits of rivers. However, dunes were not seen and ripples were recorded only in pools where flow had become subcritical. Also, contact load transportation appeared more important than in rivers. Thus, the evident d i c h o t o m y of these sediments was not in terms of structures, as in rivers, b u t paralleled the theologic bed stages, manifesting the relative importance of contact load. Together, we call them the 'coarse bare bed stages'. Individual saltation of fine sand and silt, over beds of the same material, t o o k place only where coarser material was absent from the bed load, either from the start or due to previous differentiation. Such fine material remained highly mobile over beds of medium or coarse sand but, once allowed to

89 dominate bed--flow interactions, formed the most distinctive sediments encountered in the study. Typically, these deposits occurred on slopes of less than 2 ° . The interstices of these deposits were rich in both clay and fine organic matter, in extreme contrast to their normal upslope precursors. In addition, coarser organic matter often tended to concentrate by becoming stranded and clogging on the growing surface of these sediments, later to be interred. Ripples often formed in this bed stage. They were usually of very low amplitude with flat tops. Sometimes they would be 100 mm or more long with foreset heights of only 1--2 mm. The lees of these features were often collecting areas for organic matter. In general nature these deposits were virtually homologous with those of the fine ripple bed stage of rivers. Significant deposits of fine suspended load did not occur on the hillsides examined. Thin veneers of clay were seen in local depressions which had been flooded and left filled with static water which subsequently soaked away. It was evident, however, that much fine sand and silt travelled down the steeper slopes, suspended in turbulent water. On lower slopes this material returned to bed-load motion before being built into sediments of the fine ripple bed stage in relatively quiescent Water. Throughout the study, it was noticed that the bed stages occurred in sequence, downslope, as gradient declined. This ordering was particularly marked where overall deposition was taking place. The deposits of neighbouring bed stages met along remarkably discrete junctions and the scale of such differentiation was often quite small (Figs. 1 and 2). Floating load, essentially plant debris, is often disregarded in fluviatile studies. However, in some streams, large floating elements, such as entire trees, can jam on the bed, halting rolling boulders and combining with them to form temporary dams (Dobbie, 1954). In shallow overland flows, even leaves and small twigs can drag on the bed or channel sides and jam together in much the same way as do pebbles of the contact load. Thus, because of scale effects, floating load is relatively more important in shallow overland flows. Floating load dams and combined floating and bed-load dams were major features of many of the channels investigated. They frequently followed a regularly repeated pattern. A dam would block a channel and a pool would form on its upstream side, reducing mean water velocity and inducing bed-load sedimentation. If flow in these pools became subcritical, deltas would often form. As a result, water pouring over, or filtering through, such a dam would be devoid of bed load and would immediately scour severely, regaining bed load and entraining more floating load. This would stimulate the formation of a further dam downstream and so on. Repetition of this process led to some channels having staircase-like longitudinal sections, with alternating pools and falls. Fig. 3 shows mixed plant debris and pebble dams in a vehicle rut. When small trenches were cut in the bed-load deposits of overland flows, lamination, very like that of river deposits, was frequently revealed. This was noted by Ireland et al. (1939) and Williams (1970).

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Fig. 1. Bed-load differentiation sequence beside a road. Soil material, dumped outside the photographed area to the right, has been eroded during a heavy rainstorm. The area in the right background is dominantly erosional with some local deposition in the rheologic bed stages. Running from top centre to right foreground is a depositional belt occupied by well-developed fans. These are built of clean sands of the coarse bare bed stages. They meet the much finer deposits of the fine ripple bed stage (on which the matchbox rests) along an abrupt angular junction. The through flow, along the side of the road, was also transporting in the fine ripple bed stage and shows ripple trains. Canberra ACT.

Channels, their processes and deposits. The term 'rill' is often applied to channels small enough to be eliminated by tillage, whereas 'gully' is applied to larger channels of overland flows. Our observations, however, support the view of Fenneman (1908) who thought that there were two physically distinct types of overland flow channel. In areas characterized by net erosion, channels typically have steep, high banks, cut in the material being eroded and are only partially lined with their own sediments. Such channels tend to flow straight and to unite downhill into larger ones (Fig. 4). In areas characterized by net deposition, channels are shallow, with low banks and are cut into, and entirely lined by, their own deposits. Their courses meander, they braid, and they always tend to split, downhill into smaller channels. We apply the terms 'gully' to the first type and 'rill' to the second, irrespective of size. The existence of two types of channel is clearly related to bed-load capacity. Slight downhill steepening, by increasing bed-load capacity and allowing net erosion, can change a rill to a gully and a slight fall in slope often has the reverse effect.

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Fig. 2. Differentiated sediments laid by a thin flow discharging into a flooded hole left by a fallen tree during a storm. Clean, coarse sands of the coarse bare bed stages have built deltaic features which overstep deposits of the fine ripple bed stage, the latter rich in clay and organic matter. Fine sands of the rheologic bed stage, probably laid late in the event, mark paths of distributaries atop the coarse sand bodies.

H o w e v e r , the m o r e n o r m a l overall relationships e v i d e n t l y result b e c a u s e a l m o s t all hillslope gradients m u s t e v e n t u a l l y decline downhill, and so also m u s t the b e d - l o a d c a p a c i t y o f flowing water. T h u s e v e r y slope t e n d s to have an u p p e r loading z o n e , or z o n e of n e t erosion, w h i c h is c h a r a c t e r i z e d b y gullies. B e l o w this z o n e is a z o n e of n e t d e p o s i t i o n , fed b y rills, a n d in w h i c h a m a n t l e (or fan) o f b e d - l o a d s e d i m e n t s is built. T h e s e relationships e v i d e n t l y h a v e universality, being little d e p e n d e n t on a b s o l u t e size scale. T h e y can be o b s e r v e d in r o a d cuttings, quarries (Fig. 5) and even o n w a t e r - e r o d e d piles o f e a r t h less t h a n half a m e t r e high. T h e t r a n s i t i o n f r o m erosion a n d gullies to d e p o s i t i o n a n d rills is o f t e n r e m a r k a b l y a b r u p t . As p r e v i o u s l y n o t e d , n e w l y e r o d i n g m a t e r i a l can be a d d e d to gully flows o n l y in areas w h e r e b e d - l o a d linings d o n o t occur. This leads to c o n c e n t r a t e d local a t t a c k on s o m e f l o o r areas a n d banks. If particles large e n o u g h to s h o w b e d - l o a d b e h a v i o u r are c o m m o n in t h e m a t e r i a l being e r o d e d , particles t h a t can be s u s p e n d e d m u s t a w a i t t h e e n t r a i n m e n t o f p r o t e c t i n g b e d - l o a d particles b e f o r e t h e y t h e m s e l v e s can b e e n t r a i n e d . M o r e o v e r , new s u s p e n d e d load c a n n o t be e n t r a i n e d f r o m a n y area c o v e r e d b y a v e n e e r o f b e d - l o a d deposits. This screening e f f e c t was r e p e a t e d l y n o t i c e d in t h e field. A t t h e initiation o f

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Fig. 3. Depositional amelioration of a vehicle rut. The rut, originally about 200 m m deep, runs up a slope of a b o u t 6 °. The soil is rich in sandstone pebbles and a b u n d a n t plant debris is available. In some settings such a rut would probably develop as a gully. However, in this case, overland flow from a single storm has largely eliminated the rut by building a series of traction clog deposits, initiated by jams of pebbles and plant d e b r i s along its length. Slope is upwards from observer. Black Mountain, Canberra, ACT.

an eroding event, much suspended load, representing particles initially exposed to the flow, would be entrained. After a short time the water in the channels would partially clear as suspended load transport rate fell. Clearly, after an initial surge, suspended load transport rate must become broadly proportional to bed-load transport rate. These observations, the near absence of suspended load deposits on hillslopes and the fact that water is able to carry large concentrations of fine material in suspension, led us to conclude that, normally, the dominant control of suspended load transport rate in overland flows is availability which, in turn, is largely controlled by bedload transport rate. If bed-load material is not available, of course, it can exert no such constraint. In strong contrast, saltation load is clearly controlled by capacity limits which increase rapidly with slope. We observed that, if suitable loose particles were available, a flow would load to capacity over a distance of a few centimetres. Deposition of excess load was almost as rapid. Equally rapid adjustments were noted in flumes by Moss (1972). The swift reactions to slight slope changes or the formation of plant debris dams have already been

93

Fig. 4. Gullies. Zone of net erosion on a recently artificially graded surface on weathered granite terrain. Slope is about 12 ° . The small gullies, which tend to unite as they pass downhill, have only partial depositional linings comprising traction clog gravels. Blocking by these deposits is evidently a major cause of the small zig-zag features of the channel courses. Gully sides are steep and cut in the material being eroded. Gibraltar Creek, ACT.

discussed. O f t e n , b e d - l o a d d e p o s i t s are laid o n l y a m e t r e or t w o d o w n s t r e a m o f gully heads. In such cases t h e flow m u s t b e c o m e c o m p l e t e l y l o a d e d w i t h saltation load virtually at t h e gully h e a d itself. T h e discrete z o n e s of erosion and d e p o s i t i o n on hillsides a p p e a r to be s c u l p t u r e d in t e r m s o f saltation load c a p a c i t y , their j u n c t i o n r e p r e s e n t i n g the line along w h i c h c a p a c i t y n o r m a l l y falls b e l o w actual load. C o n t a c t load characteristics c o n t r a s t s t r o n g l y w i t h t h o s e of b o t h s u s p e n d e d a n d saltation loads. E v i d e n t l y b e h a v i o u r d e p e n d s largely on local c o n c e n t r a t i o n o f rolling particles on the bed. If c o n c e n t r a t i o n is low, c o n t a c t load a p p e a r s u n i m p o r t a n t , b u t if high, clogging t a k e s place and gravel f o r m a t i o n is initiated. C o n t a c t l o a d b e h a v i o u r a p p e a r e d less closely related to the e x i s t e n c e o f z o n e s o f net erosion and d e p o s i t i o n t h a n t h a t of saltation load. I t was also n o t e d t h a t , t h e coarser the c o n t a c t load, t h e m o r e p e r s i s t e n t w e r e t h e a r m o u r e d m o n o l a y e r s f o r m e d f r o m it. Steep-sided, n a r r o w gullies caused c r o w d i n g o f c o n t a c t load a n d m u c h o f t h e i r b e d s was o f t e n c o v e r e d b y such coarse, a r m o u r e d m o n o l a y e r s . T h e p r e s e n c e o f these e r o s i o n - r e s i s t a n t pavem e n t s e v i d e n t l y m o d i f i e d gully d e v e l o p m e n t p r o f o u n d l y , causing t h e flow to a t t a c k the gully walls, a n d to u n d e r c u t t h e m . T h e result was t h a t gullies b e c a m e

94

Fig. 5. Zones of net erosion and deposition on a small scale. A quarry in deeply weathered granite shows, in the background, gullies (steep-sided, uniting downhill and cut in the material being eroded). In the zone of net deposition (foreground) a depositional fan is surfaced by a b a n d o n e d rill channels (less steep-sided, bifurcating downhill and cut in their own deposits). Small areas apparently belonging to the fine ripple bed stage (lacking coarse particles) occur near the b o t t o m right-hand corner. Otherwise the fan is composed of deposits of the rheologic and coarse bare bed stages with characteristic local c o n c e n t r a t i o n s of coarse particles (clogged c o n t a c t load). Geological h a m m e r gives the scale. Paddy's River, ACT.

broad, flat-floored and relatively shallow rather than steep-sided, deep, and ravine-like. Jacobson (1965) noted such a contrast between gullies in loess country and those in mixed loess and boulder clay country. We noted that, if the monolayer was broken, or if bare bed areas occurred along the gully floors, local floor erosion would cause a nick point to develop at the downstream ends of armoured areas. Pebbles and boulders became removable at such points and the feature moved upstream, lowering the bed level. However, gravels with armoured monolayers almost invariably formed again, downstream, from the detritus thus set in motion. We observed that all the bed stages that were dominated by bed load-flow interactions occurred in both net erosional and net depositional situations. Normally, however, the gullying part of a system was cut into relatively steep slopes and rheologic bed stage relationships prevailed. It is well-known that near-ground plant cover, dead or alive (e.g. leaf litter or grass), strongly inhibits transportation by water, dissipating the energy

95 of both overland flows and falling rain. During the present study it was repeatedly noticed that this effect did not necessarily prevent erosion but often merely caused transference of attack from one point to another. Such effects arose because of the bed-load capacity phenomenon and the ability of flows to load up to capacity very rapidly, given suitable and entrainable materials. Overland flows, forming over densely ptant-c0vered ground, were normally unable to channel and did not concentrate. They moved relatively slowly because of the large resistance to flow, typically acquiring little suspended load and virtually no bed load. On suddenly encountering the low flow resistance of bare ground such flows would immediately become shallower, faster-flowing, and erosive just where bed-load material became readily available. This often led to the development of an uphill-migrating gully head, fed by virtually unloaded water pouring over its lip.

Transportation with and without rainfall impact. In rivers and deep gully flows the direct effects of individual raindrops on the bed must be negligible. In very shallow flows they can supply energy at the bed--flow interface and hence may affect transportation. Some relevant field observations and approximate measurements, admittedly far from comprehensive, were made, mainly on the Gibraltar Creek area. With rainfall fine and sparse, a flow 1.5 mm deep on a bare slope of 2.5 ° had a surface velocity of 200 mm/sec over medium and coarse sand. The flow carried suspended load, medium sand was in rapid individual saltation and coarse sand rolled. With no rain falling, a 5-mm deep flow on a 2.5 ° bare slope had a surface velocity of 250 mm/sec over medium and coarse sand, both of which were in rapid individual bed-load motion. Quartz grains up to 4 mm in diameter were being rolled along the bed. A 5--6 mm deep flow on a 3 ° bare slope, with rainfall insignificant, had a surface velocity of 300 mm/sec over sand and fine gravel. A 6 mm rock particle rolled at 100 mm/sec; 4--6 mm particles rolled at 60--160 mm/sec. Water 20 mm deep, passing over sand and fine gravel on a bare slope of 1 ° 10' with no rain falling, had a suspended load of 270 ppm in i~s upper t~alf. The largest quartz grain in the sample had a dimater of 0.18 mm. Three equivalent samples, taken just after the onset of a sharp burst of rain, had 1370, 1540 and 1600 ppm suspended load with quartz grains up to 0.40 mm in diameter. The increase, both in quantity and maximum size of suspended load, was evidently directly caused by rainfall impact. These results show that very shallow flows, on low slopes, have significant transporting power without the aid of rainfall impact which can, however, affect transportation. In general, the transportation processes appeared to remain the same in nature whether rain was falling or not. Bed-load transportation, erosion and deposition The field observations, taken alone, demonstrate that significant transportation of solids can take place in overland flows, even in very shallow

96 water and without the aid of rainfall impact. The several operative transporting processes are essentially the same as those of rivers including the splitting off, and rapid transportation, of suspended load, and the differentiation of solids, during deposition, into a series of discrete sediment types as transporting power and turbulence wane downstream. The evidence suggests that, in general, and provided that erosion produces some material that cannot be suspended, bed-load transportation rate is controlled by capacity whereas the transport rate of suspended load is controlled by availability. Bed-load deposits line large proportions of channel systems, barring flows from access to potential suspended load. Suspended load transportation thus becomes largely controlled by bed-load transportation. The lagging nature of bed-load sedimentary processes has a major retarding effect on erosion. We call this effect 'bed-load amelioration'. Because bed-load capacity often tends to control erosion rate, surfaces that resist particle entrainment, of which armoured layers and heavily plantcovered areas are but two cited examples, do not necessarily cause overall reductions in erosion. Rather, they often transfer underloaded (and hence loadable) flowing water to downstream, less resistant areas where severe erosion may ensue. However, if water loses capacity while flowing over such resistant areas (e.g. by reaching a lesser slope) some overall reduction in erosion can be achieved. The gradient of nearly all hillslopes must eventually decline, gradually or suddenly, away from the hill. Because erosion of most hillslopes provides detritus coarse enough to travel as capacity-limited bed load, mantles of bedload sediments around the bases of hills are ubiquitous. LABORATORY STUDIES OF NATURAL MATERIALS To facilitate comparison, and because field work had demonstrated strong resemblances between overland flow and river deposits, sampling and analytical techniques almost identical with those used by Moss (1972) for river and flume sediments, were employed. This involved subjecting very small samples, often of less than a gram in the case of sands, to both size-shape and size analysis. Sieving was used for the coarser fractions and a Coulter counter (Walker et al., 1974) for silt-sized particles. To act as a sedimentological link between river and overland flow environments, six fluviatile bed-load sediments, seen to form in very shallow water (17 -100 mm) were studied initially. This work, not reported in detail, revealed no discernible contrasts with sediments formed in deeper river water.

Modern hillslope surface samples Most overland flow deposits sampled were actually accruing at the time of collection. The results confirmed, in detail, the conclusions drawn from field studies, i.e. that processes, sediments and the ordered downstream differentia-

97 tion sequence of sediments are essentially the same in overland flows and rivers. Table I briefly describes the samples. Figs. 6--10 give the results. Fig. 6 shows results for three rheologic bed stage gravels. Sample 2013 (Gibraltar Creek) was not related to a specific event but samples 2029 (Black Mountain) and 2043 (Mount Ainslie) formed during thunderstorms on slopes of 8 ° and 4 ° respectively. Sample 2043 was seen to form from coarse contact load and the rheologic layer, in a small gully. Associated with two of these gravels were rheologic bed stage sands, samples 2041 and 2042 (Fig. 7) from Mount Ainslie and samples 2026 (Fig. 7) and 2027 (Fig. 8) from Black Mountain. Both gravels and sands gave results closely resembling those of their fluviatile equivalents (Moss, 1972). The only difference noted between rheologic bed stage sediments of overland flows and rivers involved the 'suspensive diminution' feature, shown by interstitial populations of this type of sediment (Moss, 1972). This feature is manifested by a sharp minimum in the elongation function curve and a corresponding inflection in the size analysis curve. It reflects a load stratification in the flow in that there is a lower zone, comprising the rheologic layer plus suspended load and a upper zone comprising suspended load only. As water becomes shallower the lower zone occupies progressively more of the flow depth and the upper zone becomes thin and tends to disappear. Thus the suspensive diminution feature must also tend to disappear. This effect was evidently shown by some of the samples. Sample 2043 was collected under 40 mm of water with most of the flow free of bed load. Its curves (Fig. 6) show the feature well. Sample 2044, representing the suspended load being carried above sample 2043 while the latter was forming, has an elongation function curve which rises to high values (as suspended load diminishes in quantity with increasing grain size) over the suspensive diminution minimum of the gravel curve. However, the curves (Fig. 7) of the rheologic bed stage sands, formed in water only 2--3 mm deep in the same flow, have the suspensive diminution feature greatly suppressed. Clean medium and coarse sands, built from particles in individual saltation, together with some fine gravels, again gave results very like those of their fluviatile equivalents in the coarse ripple and dune bed stages. Their most striking feature was that, in spite of quite strong transportation of clay, fine organic matter, silt and fine sand along the same channels, they usually contained little or none of these fine materials. Laboratory investigations confirmed the field observation that these sediments split into two types according to the importance of contact population. Samples 2022, 2024 and 2036 (Fig. 9) are examples with clogged contact population which can evidently constitute over half of such sediments by weight. The other type of sediment is represented by samples 2020, 2021 and 2031 in Fig. 10. In these, contact load was minor or lacking. The fine ripple bed stage was represented by samples 2025 and 2037 (Fig. 8), the latter t o o small for a satisfactory size analysis. Like their fluviatile equivalents, sediments of this stage proved rich in fine, interstitial population.

6

10

10

9

9

8

2020

2021

2022

2024

2025

Figure showing results

2013

Sample number

Black Mountain, Canberra

Gibraltar Creek, A C T

Gibraltar Creek, A C T

Gibraltar Creek, A C T

Gibraltar Creek, A C T

Gibraltar Creek, A C T

Location

Description of modern natural samples

TABLE I

S h a l l o w rill; sandstone terrain.

S h a l l o w rill; granite terrain.

S h a l l o w rill; granite terrain.

S h a l l o w rill; granite terrain.

S h a l l o w rill; granite terrain.

Gully on granite hillside.

Environment

3020

1100

D e p t h 2 m m ; s l o p e 1 °. P a r e n t m a t e r i a l was i m p o r t e d t o p s o i l ( f r o m river f l o o d p l a i n ) c o n s t i t u t i n g n e w l y s o w n l a w n . Individual s a l t a t i o n o f fine s a n d a n d silt o v e r l o w - a m p l i t u d e ripples. S a m p l e laid, a f t e r r a i n h a d ceased, as r i p p l e foreset.

1960

D e p t h 5 m m ; slope 3 ° ; s u r f a c e v e l o c i t y 33 c m / s e c . M e d i u m t o c o a r s e s a n d in strong individual saltation; 5 mm particles rolling.

D e p t h 5 m m ; slope 2.5 ° ; s u r f a c e v e l o c i t y 25 c m / s e c ; n o rain. S t r o n g i n d i v i d u a l s a l t a t i o n o f m e d i u m s a n d ; 3--4 m m particles rolling a n d clogging o n p l a n e bed. F r o m t h a l w e g o f rill y i e l d i n g 2 0 2 0 a n d 2021.

1840

1680

D e p t h 1.5 m m ; slope 2.5 ° ; rain light ; surface v o l o c i t y 19 c m / s e c . M e d i u m s a n d in s t r o n g s a l t a t i o n ; c o a r s e s a n d rolling. F r o m p l a n e - b e d d e d rill floor. Associated with 2020 but depth 1--1.5 m m . F r o m l o n g i t u d i n a l bar.

2800

Number of particles measured

P a t c h o f s e d i m e n t w i t h surface i n c l i n e d a t 2 ° o n largely b a r e gully f l o o r w i t h average i n c l i n a t i o n o f 20 ° . N o f l o w w h e n collected.

Nature of occurrence

¢,D

Mount Ainslie, Canberra.

Mount Ainslie, Canberra.

2043

2044

Gibraltar Creek, A C T

2039

Mount Ainslie, Canberra.

Gibraltar Creek, A C T

2037

2042

Gibraltar Creek, A C T

2036

Mount Ainslie, Canberra.

Black Mountain, Canberra.

2029

2041

Black Mountain, Canberra.

2027

10

Black Mountain, Canberra.

2026

G u l l y over old volvanic rocks.

G u l l y over old volcanic rocks.

G u l l y over old v o l c a n i c rocks.

G u l l y over o l d volcanic rocks.

S h a l l o w rill ; granitic terrain.

S h a l l o w rill; granitic terrain.

S h a l l o w rill ; granitic terrain.

S h a l l o w rill ; sandstone terrain.

S h a l l o w rill; sandstone terrain.

S h a l l o w rill; sandstone terrain.

S u s p e n d e d l o a d s a m p l e f r o m t o p 20 m m o f w a t e r over site o f 2 0 4 3 .

D e p t h 40 m m ; slope 4 ° ; s u r f a c e v e l o c i t y 35 c m / s e c ; c o l l e c t e d d u r i n g h e a v y thunderstorm from thalweg of channel. R h e o l o g i c m o t i o n w i t h p e b b l e s rolling.

D e p t h 3 m m ; general slope 4 - - 5 ° ; sedim e n t surface slope 1 ° ; c o l l e c t e d d u r i n g h e a v y t h u n d e r s t o r m . Marginal b a r t o 50 m m deep c h a n n e l . P a r t i c l e m o v e m e n t rheologic.

D e p t h 2 m m ; general slope 4--5 ° ; sedim e n t surface slope 2 ° ; c o l l e c t e d d u r i n g h e a v y t h u n d e r s t o r m . Marginal b a r t o 50 m m d e e p c h a n n e l .

D e p t h 20 m m ; slope 1 ° 1 0 ' ; r a i n i n g fairly heavily. F r o m t h a l w e g o f c h a n n e l yielding 2036 and 2037.

D e p t h 3 m m ; slope 1 ° 1 0 ' ; n o rain falling. Marginal bar, u n a r m o u r e d , t o c h a n n e l y i e l d i n g 2 0 3 6 . A c t i v e s a l t a t i o n o f med i u m sand.

D e p t h 20 m m ; slope 1 ° 1 0 ' ; l i t t l e rain. A r m o u r e d p l a n e - b e d d e d rill f l o o r b e l o w thalweg. Sample includes both armoured layer a n d u n d e r l y i n g m a t e r i a l .

C o l l e c t e d d r y a f t e r t h u n d e r s t o r m . Slope o f d e p o s i t surface 8 ° ; general slope o f rill f l o o r 9 - - 1 0 ° .

As 2 0 2 6

Collected dry after thunderstorm. Slope o p o f d e p o s i t surface 1 20 ; general slope o f rill f l o o r 5o20 ' .

320

3780

2000

2200

2000

1600

3180

3620

1900

2400

¢D

100 2.5 F

i ,2044 2"0

/z

~a_

/

~

2013 2043

1-0

20 I / ' I A

2.0 mm

2013 _ _

2043

~

2029

15 •

2

/~

6 P

8

100180 . . . . .

lOmm

//'-,.,......-'~//7~ 2043~/z z ,,-;," 2029,'" /

*~ 60-'

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....J

001

01

.

t

2013

10

10mm

Grain size Fig. 6. Results for t h r e e t r a c t i o n clog gravels derived f r o m differing source rocks. T o p - e l o n g a t i o n f u n c t i o n curves up t o p = 2.0 m m . Middle - - e l o n g a t i o n f u n c t i o n curves cont i n u e d i n t o coarser size ranges. B o t t o m - - size cumulative curves dissected in t e r m s o f p o p u l a t i o n c o m p o s i t i o n (A - - solid line; B - - d o t t e d line; C - - dashed line). N o t e the general similarity o f results in spite o f the fact t h a t the t h r e e s e d i m e n t s are derived f r o m granite (Sample 2013), s a n d s t o n e (Sample 2029) and volcanic rocks (Sample 2043). The m a i n s o u r c e - r o c k variability is m a n i f e s t e d as size variations in t h e f r a m e w o r k p o p u l a t i o n s . Sample 2044 ( t o p ) is t h e e l o n g a t i o n f u n c t i o n curve for the coarser s u s p e n d e d material c o l l e c t e d f r o m above, and at the time o f f o r m a t i o n of, S a m p l e 2043. Samples are described in Table I.

In general, the textural properties of overland flow sediments closely replicated those of rivers, the main differences being the relative abundance of different populations in the coarse bare bed stages. Firstly, contact load was more important in the overland flow sediments and secondly, analysis of proportions of interstitial population revealed that it, too, was relatively more important. It averaged 17.1% in samples of the coarse bare bed stages

101 25!

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20~ 2041

. 2042

••2026"

1.5

1'0

2~0mm

100 r

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:: . : ~ : ~ / i l i : : i : i .............. ,

0.05

0.1

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Grain size

Fig. 7. A b o v e • - e l o n g a t i o n f u n c t i o n curves o f rheologic bed stage sands, rich in b o t h interstitial and contact p o p u l a t i o n s , f o r m e d in shallow overland flows at slopes o f a b o u t five degrees. Below - - size cumulative curves for the samples, dissected in terms o f population c o m p o s i t i o n ( A - - solid line: B - - d o t t e d line; C - - dashed line). N o t e t h e similarity o f results between Sample 2026, derived f r o m quartz sandstone, and Sample 2041, derived f r o m q u a r t z - p o o r volcanic rocks and largely c o m p o s e d o f their fragments. Samples are d e s c r i b e d in Table I.

as against 7.1% (dune) and only 2.9% (coarse ripple) in the river sediments studied by Moss (1972). Most of this increase was ascribable to a greater abundance of particles with diameters over a tenth of those of associ~/ted framework population particles. Finer material (silt and clay) was still rare to absent. The characteristic structures (dunes and ripples) of the fluviatile coarse bare bed stages are greatly suppressed in overland flows and, in them, these stages evidently split, more obviously, in terms of the importance of the contact population. It therefore seems practical to express the differentiation sequence as a descending energetic hierarchy for thin flows on hillslopes, as follows:

Rheologic bed stage with clogged contact population Rheologic bed stage without clogged contact population

102 2.5

2025

2.0

_

~ ' -

~

-

'

-

"

"

1.5

~

2037

-2027

oi

012

0:4

013

0.Smm

100 80-

60"

y

-~ 40. 2 20" L

0.001

.,,

..... , = : . : : ............ :':"................. ' " "

001

01

1~0 mm

Gruin size

Fig. 8. A b o v e -- elongation function curves of three bed-load sediments formed in very shallow water. S a m p l e 2 0 2 7 , f o r m e d o n a slope of a b o u t five degrees, shows feature 2 at p = 0.13 m m . This feature is characteristic of the rheologic bed stages and is not s h o w n b y the curves of Samples 2 0 2 5 and 2037 which f o r m e d o n slopes of a b o u t a degree. B e l o w - size cumulative curves of Samples 2025 and 2027 dissected in terms of population c o m p o s i t i o n s ( A - - solid line: B - - dotted line; C -- dashed line). S a m p l e 2 0 2 7 was t o o small for satisfactory separation of its finer fractions; Sample 2037 was too small t o yield a satisfactory size analysis at all. Samples are described in Table I.

Coarse bare bed stage with clogged contact population Coarse bare bed stage without clogged contact population Fine ripple bed stage Suspended load bed stage

Hillslope soil samples Near-surface modifications of hillslope mantles produce soils which must therefore, in the light of our present findings, frequently have bed-load sediments as their parent materials. With the techniques we have used, therefore, many soil materials, if not too pedologically modified, should be still identifiable as bed-load deposits. To this end, associated pairs of soil samples, from each of the three main field areas, were studied in the same way as had been the modern hillslope sediments. The samples were taken from

103 2.5 [

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~

2022

2036

~

2024

15~

i

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................•..-.:.:..~"";;;~!::::ii

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Grain size

Fig. 9. E l o n g a t i o n f u n c t i o n curves ( a b o v e ) a n d size c u m u l a t i v e curves ( b e l o w ) o f thinflow s e d i m e n t s b e l o n g i n g t o t h e coarse bare b e d stages a n d rich in c o n t a c t p o p u l a t i o n . S a m p l e 2 0 3 6 i n c l u d e d t h e a r m o u r e d m o n o l a y e r w h i c h is r e s p o n s i b l e for t h e size m o d e at a b o u t 10 m m . T h e size c u m u l a t i v e curves are dissected in t e r m s o f p o p u l a t i o n c o m p o s i tion. Samples are described in T a b l e I.

beneath fairly steeply inclined surfaces, high on hillslopes, to avoid risk of collecting originally fluviatile materials. Hence, all the samples could be anticipated to belong to the rheologic bed stages. Green (1974) has already applied size--shape analysis to soil materials. G i b r a l t a r C r e e k area. Samples 2051 and 2052 (Fig. 11), from granitic terrain, were derived from the lower B horizon of a well-differentiated, sedimentologically stratified, red podzolic soil at depths of 0.60 and 0.65 m respectively. The modern surface slope was 6 ° but, only some 20 m uphill, the gradient steepened to 9 °. The curves for the stratigraphically higher sample (2051) show it to be a rheologic bed-stage coarse sand with only 17% contact population, n o t enough to have been juxtaposed in a clogged and packed state. The elongation function curve shows a minimum at p = 0.4 mm; a corresponding inflection occurs in the size distribution curve at 0.25 mm (p expresses length, whereas sieve size approximates breadth). This feature almost certainly represents the suspensive diminution. The result strongly suggests that the sediment was laid from the rheologic layer, that there was a

104 25

~

20

2020

~

2021

/

tO, 1.

2Omm

100

6O 40

20 0'1

012

0)5

1"0mm

•

Grain size Fig. 10. A b o v e - - e l o n g a t i o n [ u n c t i o n curves f o r s e d i m e n t s f o r m e d in v e r y s h a l l o w f l o w s o n slopes. These samples c o n t a i n e d l i t t l e or no c o n t a c t p o p u l a t i o n . B e l o w size c u m u l a t i v e c u r v e s o f t h e s a m e s a m p l e s , d i s s e c t e d in t e r m s o f t h e i r p o p u l a t i o n c o m p o s i t i o n s ( A - s o l i d l i n e ; B -- d o t t e d l i n e ; C - - d a s h e d line). S a m p l e s are d e s c r i b e d in T a b l e I.

significant depth of bed-load-free water above this layer and that particles up to a diameter of 0.25 mm were carried in turbulent suspension by the flow. The flow was therefore rather powerful and relatively deep although essentially depositional. The lower sample (2052) had a framework population of about the same coarseness but was otherwise quite different from the higher (younger) one. It contained 70% contact load, dominantly coarse, and was therefore a rheologic bed stage gravel. Secondly, the suspensive diminution feature occurred at p = 0.2 mm with a size distribution equivalent at about 0.1 mm. This suggests, again, the presence of bed-load-free water but a much less powerful current with less ability to suspend particles. Because the slope must have been about the same for both samples, it seems probable that they reflect different discharges. These results suggest that overland flows, sometimes powerful, laid the bed-load sediments which were the parent materials of this soil. They were loaded to capacity high on the hillside, where there was probably a paucity of plant cover. Sediments of this type can accumulate very rapidly and it is

105 2.0!

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2051

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1'0 mm

Grain size

Fig. 11. S e d i m e n t o l o g i c a l t e x t u r a l plots for t w o soil samples f r o m t h e l o w e r B h o r i z o n of a red p o d z o l i c soil (Hapludalf), G i b r a l t a r Creek, ACT. A b o v e -- e l o n g a t i o n f u n c t i o n curves, based o n m e a s u r e m e n t of 2 8 0 0 particles f r o m S a m p l e 2051 a n d o n 4 8 4 0 particles f r o m S a m p l e 2052. T h e full curve for S a m p l e 2 0 5 2 is n o t s h o w n ; it c o n t i n u e s a l m o s t h o r i z o n t a l l y t o 20 ram. B e l o w - size c u m u l a t i v e curves of t h e samples split, a p p r o x i m a t e l y , a c c o r d i n g to a p p a r e n t d e p o s i t i o n a l p o p u l a t i o n c o m p o s i t i o n s (A - - solid line; B - d o t t e d line; C - d a s h e d line).

not unlikely that the whole profile developed on the deposits of a single exceptional flow-producing event. Black Mountain. The sampled site at Black Mountain was towards the upper edge of its extensive depositional mantle (Costin and Polach, 1973}. The present surface slope is 8 ° and rock outcrops start to occur a few metres uphill from the site. Sample 2053, a sand from a depth of 0.33 m and sample 2054, a gravel from 0.45 m, came from the lower part of the A horizon of a very strongly differentiated red podzolic soil. Both samples contained charcoal which was evidently detrital. The sand (Fig. 12) gave an elongation function curve with the characteristics of the rheologic bed stage. A suspensive diminution feature suggested a sieve size of about 0.07 mm for the largest suspendable mineral particles. Framework particle size was rather fine (mean about 0.2 mm), a feature probably inherited from the parent sandstone. A relatively gentle depositional evironment is indicated, for such a steep slope, with bed-load-free water above the rheologic layer. The results for the gravel (Fig. 13} which still contained 5% clay in spite of being in the

106 2-5

\O"

15 I ~

2053

f

2O55

02

0-6

0'4

0"8

1'0 mm

P 2,0,

2053

1-5i 2055 1"0 ,

t0

{0

3'~0

100 -

4:0

5'~9

j~-

~0 mm

~-7

g E c~ 40~ 20 i

..,.. ..................... -"

.......

0'01

,,, ...

...... .y:

/"

/

-

!

: 0:1

1"0 mm

Gro~n size

Fig. 12. Sedimentological textural plots of two sandy soil samples. Sample 2053 was from the l o w e r A h o r i z o n o f a s a n d s t o n e - d e r i v e d , v e r y s t r o n g l y d i f f e r e n t i a t e d r e d p o d z o l i c soil ( H a p l u d a l f ) f r o m B l a c k M o u n t a i n , ACT. S a m p l e 2 0 5 5 w a s f r o m t h e l o w e r B h o r i z o n o f a s t r o n g l y d i f f e r e n t i a t e d y e l l o w p o d z o l i c soil ( A l b a q u a J f ) f o r m e d o n m a t e r i a l d e r i v e d f r o m the o l d v o l c a n i c r o c k s o f M o u n t Ainslie, ACT. A b o v e - - e l o n g a t i o n f u n c t i o n c u r v e s u p t o p = 1.0 m m ( 2 6 0 0 p a r t i c l e s m e a s u r e d f r o m S a m p l e 2 0 5 3 ; 7 7 2 0 f r o m S a m p l e 2055). C e n t r e - - e l o n g a t i o n f u n c t i o n c u r v e s for t h e c o a r s e r p a r t i c l e s . B e l o w - - size c u m u l a t i v e c u r v e s split, a p p r o x i m a t e l y , in t e r m s o f their apparent d e p o s i t i o n a l p o p u l a t i o n c o m p o s i tions (A s o l i d line; B - d o t t e d line; C - - d a s h e d line).

eluvial part of the profile, suggest even lower powers of suspension up to a sieve size of about 0.05 mm. The Black Mountain samples do not suggest an exceptional flow event as was implied by those from Gibraltar Creek, but appear to be the products of what, for the gradient, were relatively gentle flows. The charcoal fragments, perhaps, provide a clue, suggesting that sediment transportation by unexceptional events may have been facilitated by the temporary stripping of plant cover by fire.

107

Mount Ainslie. T h e M o u n t A i n s l i e s o i l s a m p l e s w e r e , again, a s a n d ( S a m p l e 2 0 5 5 ; Fig. 1 2 ) o v e r l y i n g a gravel ( S a m p l e 2 0 5 6 ; Fig. 1 3 ) . T h e y w e r e l o a m y a n d o c c u r r e d in t h e l o w e r B h o r i z o n o f a w e l l - d i f f e r e n t i a t e d y e l l o w p o d z o l i c soil, t h e o r i g i n a l s u r f a c e o f w h i c h h a d b e e n m o d i f i e d b y q u a r r y i n g o p e r a t i o n s .

25 r I I

i! 2.0~ o_

2056 205/. 1-5 -

2056

205L

0:2

o:~.

0'.6

0'.8

1.~

(o

p mm 2-5 F

2056 2-0 ~

2054 c~. 1"5-

2054 10

2'0 p

100

i

T

--

ml'Tk

T

T

~--"

80 ~

// /' //

60-

40 ~

2054

= o 20 ~

0 O01

/ .

0'01

2056

0"1

i:0 Grain

size

11

r

q

//"/' / /

/ //

/

~ ~"

I

1'0

100

ram.

Fig. 13. Sedimentological textural plots of two gravelly soil samples. Sample 2054 was from the lower A horizon of a sandstone-derived, very strongly differentiated red podzolic soil (Hapludalf) from Black Mountain, ACT. Sample 2056 was from the lower B horizon of a strongly differentiated yellow podzolic soil (Albaqualf) formed on material derived from the old volcanic rocks of Mount Ainslie, ACT. Above -- elongation function curves up t o p = 1.2 mm (3080 particles measured from Sample 2054; 8080 from Sample 2056). Centre -- elongation function curves of the coarser particles in the samples. Below -- size cumulative curves split, approximately, in terms of their apparent depositional population compositions (A - solid line; B -- dotted line; C - - dashed line).

108

Sedimentary stratification paralleled an underlying rock surface which was inclined at 9 ° . The site was on a spur of the mountain. The sand was very rich in interstitial population and contained 10% clay, some of which could have been pedologically translocated or be a product of weathering. No suspensive diminution effect was evident in either the sand or the gravel. The results suggest that these deposits were laid by a flow with little or no bedload-free water above the rheologic layer so that efficient removal of fine suspended load was not possible. The results confirm that the original sedimentary nature of water-laid soil materials can often be established by the use of textural analysis and that some features of depositing currents can even be deduced. The present inferences have, deliberately, been somewhat bold in order to exemplify the type of information that must be available in many soil materials. E X P E R I M E N T A L D I F F E R E N T I A T I O N OF SEDIMENTS BY SHALLOW WATER FLOWS ON SLOPES

The ubiquity of the thin flow differentiation sequence, its occurrence around even small piles of soil and other granular materials, and the rapid environmental reactions of bed-load transportation all suggested that such a sequence could be formed in the laboratory. This would allow its detailed study and, more particularly, comparisons of differentiates with each other and their parent materials. A simple experiment was therefore conducted.

The experiment 50 kg of granitic gravelly loam, derived from the A2 and B1 horizons of a yellow podzolic soil from the Gibraltar Creek area were well mixed and slowly fed into a varnished, rectangular wooden chute down which flowed a small water discharge of 0.154 1/sec at 10°C. The chute, 2.5 m long, 70 mm deep and 110 mm wide, was inclined at 13 ° to the horizontal with one end resting on a concrete floor. The soil, rapidly wettable, slid down the chute in viscous, m u d d y masses, forming a roughly conical mass movement deposit, about 90 mm high at the downstream end. Further moving masses, some like mud flows, others resembling debris flows, moved down the slopes of this deposit, reducing its side angles to 10 -16 ° . A few large particles, about 20 mm in diameter, remained in the chute. Mass movement brought about little differentiation but, as it declined, unladen water from the chute began to attack the cone summit. Suspended load was detached, bed load set in motion, and gullies up to 10 mm deep were cut near the summit of the cone. The gullies declined in depth downhill and, at the base of the slope, gave way to a multiply bifurcating system of rills on the surface of a growing fan, formed from material eroded near the summit. This fan, representing the zone of net deposition, grew and extended

109 u p the c o n e until, w h e n the e x p e r i m e n t was arbitrarily t e r m i n a t e d a f t e r 71 m i n u t e s , the z o n e o f n e t erosion e x t e n d e d o n l y 0.3 m f r o m the s u m m i t whereas the m a n t l e e x t e n d e d f o r a f u r t h e r t w o metres. G u l l y floors, b y this stage, had slopes o f a b o u t 5 ° whereas the fan surface slopes were less and declined distally to v e r y low values. It was obvious f r o m near the start o f t h e e x p e r i m e n t t h a t the d i f f e r e n t i a t i o n sequence, seen m a n y times in the field, was developing. T h e e x p e r i m e n t e v i d e n t l y replicated, in m i n i a t u r e , the processes described for large desert fans b y Bull (1963). Fig. 14 is a section, showing the main features d e v e l o p e d along a line c o n t i n u i n g the d i r e c t i o n o f t h e c h u t e . Figs. 15, 16 and 17 show details of t h e s e d i m e n t a r y surfaces after the e x p e r i m e n t had ceased. Saltation was rheologic in the z o n e o f net erosion, gravel deposits f o r m i n g on t h e gully floors t o g e t h e r with m o r e t r a n s i e n t sandy sediments. Rheologic t r a n s p o r t a t i o n c o n t i n u e d o n t o the z o n e o f n e t d e p o s i t i o n b u t was s u c c e e d e d b y a z o n e o f individual saltation o f m e d i u m and coarse sand 0 . 5 9 - - 0 . 8 4 m f r o m the s u m m i t along the section line at the t i m e the e x p e r i m e n t ceased. Here, rills 1---2 m m deep laid the two characteristic t y p e s of d e p o s i t o f the coarse bare bed stages, i.e. with and w i t h o u t clogged c o n t a c t p o p u l a t i o n . T h e r e was little angular break b e t w e e n surfaces laid in these stages and those o f rheologic bed stage sands. H o w e v e r , a v e r y m a r k e d and s u d d e n decline in gradient o c c u r r e d b e t w e e n the clean sands and the m o r e distal fine ripple b e d stage deposits. This was a c c o m p a n i e d b y a very m a r k e d t e x t u r a l break. A similar s u d d e n change was n o t e d b y H o o k e (1968). T h e f r a m e w o r k grain

10~

1

i

4

e 5~

5 6

8

7910 12 ~ _ e l

o L

•

~

..............

~

~

j

so

"~11

.

.

.

.

13

14

~ loo

.

15

.

.

.

16

17

l ~so

.

.

.

.

~--~ 200

....

,. . . .

~. . . .

~-.

1 250

crn

Fig. 14. Profile of surface of fan produced in thin flow erosion experiment, measured along a line continuing direction of chute. Numbers refer to features and samples taken: 1, TF14 (in gully); 2, TF13 (in gully); 3, TF19 (in gully); 4, TF12 (approximate junction between zones of net erosion and deposition); 5, point above which rills and gullies have depths up to 5 mm (profile is of main surface); 6, probable junction between rheologic bed stage deposition and coarse bare bed deposition; 7, TF17; 8, TF 18, 9, T F l l ; 10, TF10; 11, junction between coarse bare bed stage and fine ripple bed stage deposition; 12, TF16; 13, TF9; 14, TF8; 15, TF7; 16, TF15; 17, TF5.

110

Fig. 15. Detail f r o m the fan p r o d u c e d in t h e t h i n flow erosion e x p e r i m e n t . T o p right -mass m o v e m e n t deposits t h a t have b e e n s o m e w h a t e r o d e d , leaving p r o t r u d i n g p e b b l e s a n d small gullies. E x t r e m e right c e n t r e a n d t o p left - - rheologic b e d stage deposits. C e n t r e a n d left c e n t r e -- coarse bare b e d stage deposits s h o w i n g c h a r a c t e r i s t i c d i c h o t o m y , s e d i m e n t s w i t h clogged c o n t a c t p o p u l a t i o n ( a r o u n d m a t c h ) a n d s e d i m e n t s d o m i n a t e d b y f r a m e w o r k p o p u l a t i o n (finer a n d c o n c e n t r a t e d in rills). B o t t o m -- fine ripple b e d stage w i t h jams of organic debris. M a t c h is 44 m m long.

111

Fig. 16. Detail from the fan p r o d u c e d in the thin flow erosion experiment. A marked textural contrast exists b e t w e e n the clean sands of the coarse bare bed stages (top) and the fine sands of the fine ripple bed stage. The varied surface t o p o g r a p h y of the deposits of the latter stage is due partly to wakes formed around jams of organic debris and partly to a t e n d e n c y to form ripples. Note that the stage junction is discrete, with an overstep relationship, rather than transitional. Match is 44 m m long.

112

Fig. 17. Distal region of the fan p r o d u c e d in the thin flow erosion experiment, a b o u t 2 m from the chute. Clayey silts of the fine ripple bed stage, with associated jams of organic debris, are inclined at angles of less than half a degree. Animal trail is post-depostional. Scale is in inches.

size and surface slope of the fine ripple bed stage deposits declined distally (Fig. 18) until, by 2 m from the summit, they were silts. They were laid, from individually saltating particles, in rills often less than a millimetre deep. Patches of low ripples developed on their surfaces together with jams of organic debris, much of which became stranded in the very shallow water. Handling the material revealed that much clay and fine organic matter had collected interstitially to the framework populations. Beyond 2 m from the summit, a slight increase in the slope of the concrete floor appeared to inhibit further sedimentation. The flow, now evidently loaded only with fine silt, clay and some fine organic matter, flowed away towards a drain. Flow was supercritical and visibly turbulent in the zone of net erosion and over the fan where the rheologic and coarse bare bed stages prevailed. In the very shallow rills depositing in the fine ripple bed stage, dye trains revealed little or no sign of turbulence, suggesting that the flow was transitional to laminar. We therefore had little d o u b t that the viscous sublayer at least enclosed the summits of the surface framework particles. Noteworthily, the onset of concentrated redeposition of fine organic material was associated with the c o m m e n c e m e n t of this type of flow.

113 "E

0'04

X

¢ ®

\\ 003 -

~

\

\

E

tn

c

0'02 ~

~D

001 I00

180 cm

140

Distance from fan

summit

Fig. 18. D o w n s t r e a m decline in s e d i m e n t grain size, w i t h i n t h e fine ripple bed stage, in t h e t h i n flow erosion e x p e r i m e n t .

As with many observed natural erosion events the amount of suspended load in water leaving the experimental area declined visibly during the 71 minutes of the experiment. Superimposed on this general cleaning effect were short bursts of turbidity that occurred when the gullies cut into previously undisturbed mass movement deposits. Two to four minutes from the start, suspended load was 57,000 ppm and very clayey. At 17--19.5 minutes the value was 4,100 ppm and, at 50--51 minutes, only 82 ppm. This very large decline (seven hundred-fold) was clearly due to bed-load deposits lining the channels, i.e. the bed-load amelioration effect. Thus, much suspended-load transporting power went unused.

Analysis of samples A series of small samples, taken (save for TF6) from along the section line of Fig. 14, were texturally analysed to trace, in detail, the downslope differentiation processes. T F 1 4 (Fig. 19), a mass movement deposit, differed little from the parent material but did show some concentration of 0.5--5 mm particles. TF13 (Figs. 19,21), a gully floor gravel, showed strong enrichment of pebbles (due to clogging), another slight concentration at 0.25 mm (framework population) and marked paucity of fine material (due to suspension). TF12 (Fig. 19) was similar. Formation of these gravel deposits removed virtually all pebbles from the moving detrital load. Rheologic bed stage sands were represented b y TF19 (Figs. 19, 20, 21) which, although collected only 0.23 m from the summit, contained virtually no particles coarser than 2 mm. Its suspensive diminution feature, corresponding to a sieve size of 0.07 mm, makes an interesting comparison with the equivalent figures for the soil samples studied. The value for the sand on Black Mountain (Sample

114

I

100

o~ i ~ ~ ~

80 6oi 40

~~ ~/

o.0o~

O-Ol

o.~

1'o

1~

1'10

1C)

80 "6

~

~

.

~

.

"

~ 20 L

0'001

0"01

0'1

#

60i

,,,// " ,o j

4

20c

i

[

o.ool

o:oi

~

±

t

o.1

1.o

lO

J

Grainsize mm, Fig. 19. Size cumulative curves o f s e d i m e n t a r y d i f f e r e n t i a t e s , p r o d u c e d in t h e thin flow erosion e x p e r i m e n t , c o m p a r e d with the curve for their p a r e n t material. The relative p o s i t i o n s o f m o s t o f t h e s e samples are s h o w n in Fig. 14. T F 1 4 - - mass m o v e m e n t d e p o s i t ; T F 1 2 and 13 .... rheologic bed stage gravels; T F 1 1 and 18 - - coarse bare bed stage w i t h clogged c o n t a c t p o p u l a t i o n ; T F 1 0 and 17 - - coarse bare b e d stage w i t h o u t clogged c o n t a c t p o p u l a t i o n ; TF5, 6, 7, 8, 9 and 16 - - fine ripple b e d stage; T F 1 , 2 and 3 -- material still in s u s p e n s i o n , after passing over t h e visible part o f the fan, after 2--4 m i n u t e s , 17--19.5 m i n u t e s and 50--51 m i n u t e s respectively.

2 0 5 3 ) was the same, whereas one of the Gibraltar Creek samples (Sample 2 0 5 1 ) gave a value of 0.25 mm. The experimental flow was, of course, rather small and this result supports the view that an unexceptional flow laid the Black Mountain materials, whereas a much stronger one laid the samples from the Gibraltar Creek area.

115

25[ J

2.0~ TF19 (23cm)

D,.

1.5~ L 3

1

4

5mm

P

25[

2"0~

TF17(72cm)

13,.

TF18(75cm) 15-

i

~

3

~

J

Sam

3-0 F

1

5

(137cm)

2.5 2.0

i !

I

1'5 0.1

0.2ram

P

Fig. 20. E l o n g a t i o n f u n c t i o n curves of s e d i m e n t s f o r m e d in t h e t h i n flow e r o s i o n experim e n t . S a m p l e d i s t a n c e f r o m t h e c o n e s u m m i t is s h o w n on each curve. T o p -- T F 1 9 ( 3 7 8 0 particles m e a s u r e d ) s h o w s t h e c h a r a c t e r i s t i c s of a rheologie b e d stage sand. T h e suspensive d i m i n u t i o n e f f e c t e v i d e n t l y o c c u r s a r o u n d p = 0.15 m m . N o t e r e s e m b l a n c e t o curves in Fig. 7. C e n t r e -- coarse bare b e d stage s e d i m e n t s . T F 1 7 ( 1 9 0 0 particles m e a s u r e d ) lacks clogged c o n t a c t p o p u l a t i o n a n d c o m p a r e s w i t h curves in Fig. 10. T F 1 8 ( 2 6 2 0 particles m e a s u r e d ) has clogged c o n t a c t p o p u l a t i o n a n d c o m p a r e s w i t h curves in Fig. 19. Below k s e d i m e n t s o f t h e fine ripple b e d stage. T F 1 6 ( 1 4 6 0 particles m e a s u r e d ) has s o m e c o n t a c t p o p u l a t i o n a n d c o m p a r e s w i t h S a m p l e 2 0 2 5 (Fig. 8). T F 1 5 ( 1 0 6 0 particles m e a s u r e d ) lacks c o n t a c t p o p u l a t i o n b u t ranges t o very high values in its f r a m e w o r k p o p u l a t i o n b e c a u s e o f a high c o n c e n t r a t i o n o f e l o n g a t e p h y t o l i t h s .

116

10

/

T F19 Parent soil 1

material

~

TF13

~ 0'1

0-01 F

0"001

0"01

01

1'0

10

Grain size mm

Fig. 21. Enrichment factors, plotted against size, for two rheologic bed stage sediments, a gravel (TF13) and a sand (TF19), formed in the thin flow erosion experiment. In the gravel, pebbles have been concentrated by up to a factor of five, due to the traction clog process. A m o d e in the fine to medium sand range represents concentration in the framework population. The sand shows marked concentration in the fine to medium sand range (framework population) with a small coarse 'shoulder' probably representing slight concentration of contact population. Fine material is much reduced in the gravel and almost absent in the sand.

T F l l (Fig. 19) and TF 18 (Figs. 19, 20, 21) represent the coarse bare bed stages with clogged contact population. The elongation function curve of T F 1 8 closely replicates that of the natural material (Sample 2036, Fig. 9). Strong enrichment of the coarsest particles remaining in motion (up to 5 ram) is shown. A m o d e at 0.2 mm represents the framework population. Silt and clay were virtually absent. Formation of these sediments left no particles coarser than 2 mm sill in motion. TF19 (Figs. 19, 22) and T F 1 7 (Figs. 19, 20, 22) represent the coarse bare bed stage without clogged contact population. TF17 was very clean, lacking silt and clay but TF10 broke away from the normal pattern by containing significant clay. The reason for this was not observed. Possibly infiltration was responsible. So complete was the selection of medium and coarse sand for sediment building in the coarse bare bed stages that the flow, on leaving the area they dominated, transported almost nothing coarser than fine sand. Only a few

117

Ii 10 ~ TF17

10 ~iTFIO,

TF18

Parent soil

E _u ~

0"1

001 0.001

001

Oi

1'0

10

Orain size mm

Fig. 22. E n r i c h m e n t factors for samples of coarse bare b e d stage s e d i m e n t s f o r m e d in t h e t h i n flow e r o s i o n e x p e r i m e n t . T F 1 8 h a d clogged c o n t a c t p o p u l a t i o n , c o n c e n t r a t i o n r e a c h i n g a f a c t o r of a b o u t six for 3--4 m m particles. A fine t o m e d i u m sand m o d e r e p r e s e n t s its f r a m e w o r k p o p u l a t i o n ; clay a n d silt are virtually absent. T F 1 7 , w i t h sparse c o n t a c t p o p u l a t i o n s h o w s a s t r o n g f r a m e w o r k p o p u l a t i o n c o n c e n t r a t i o n in the m e d i u m sand range b u t c o n t a i n s virtually n o m a t e r i a l finer t h a n 0.08 m m in d i a m e t e r . T F 1 0 is unusual, s h o w i n g a slight e n r i c h m e n t of clay a l t h o u g h silt is s t r o n g l y reduced.

medium sand grains were rolled across the marked junction onto the fine ripple bed stage surface. Textural results for seven samples (TF5, 6, 7, 8, 9, 15 and !6), all belonging to the fine ripple bed stage, are shown in Figs. 19, 20 and 23. They ranged from fine sands to silts, showed marked concentration in their framework populations and were rich in interstitial population. Some contained higher percentages of clay than did the parent material. The finest framework population encountered had a fifty percentile of only 0.012 mm. Size analyses were made of the suspended load samples already mentioned (Figs. 19, 24). TF1 (2--4 minutes) had a fifty percentile of 3 pm and contained 30% clay, representing a tenfold enrichment over the parent material. TF2 (17--19.5 minutes) had a coarser fifty percentile (6 pm) and contained only 20% clay. TF3 (50--51 minutes) had a fifty percentile of 7 pm with only 5% clay. These results suggest that, in addition to waning with time, suspended load changed its composition, clay being, so to speak, cleared out more rapidly than silt.

t

118

10.,. ;~

TF5

TF16

Parent soil material

2 L~ 0'1

0'01 b t

otool

o.ol

t

o:1

1.o

~o

Grain size mm

Fig. 23. Enrichment factors of two fine ripple bed stage sediments formed in the thin flow erosion experiment. Both show strong framework population concentrations and some enrichment of clay. However, they are slightly deficient in fine silt, compared with their parent material.

Microscopy revealed other manifestations of the powerful differentiation of solids that t o o k place during the experiment. The parent material contained small amounts of dumortierite (sp.gr. 3.3) occurring as fine, equant grains. In differentiates the mineral was often noticed in samples of the rheologic and coarse bare bed stages but never in samples of the fine ripple bed stage. Several types of phytolith (inequant with relatively low specific gravities) also occurred in the parent material. None was seen in deposits of the rheologic and coarse bare bed stages but, in those of the fine ripple bed stage, phytoliths were often common, even reaching several per cent of the sample in the case of the very fine TF15. No attempt was made to fit a curve to the fan profile in Fig. 14. Firstly, the experiment was arbitrarily stopped while the fan was still evolving. Secondly, at least five distinct physical environments were known to have occurred along the line. Thirdly, the evidence suggests that the profile was related to the composition of the parent material and would vary between parent materials. For example, decreasing the proportion of pebbles and increasing that of medium sand would probably suppress the high energy stages and enhance development of the coarse bare bed stages. To some extent, all these considerations must apply to natural slopes. Indeed, some

119 I

10-TF,2

i

TF3 I L o

Parent

soil material

1.0

?, U-

TF3

._u UJ 0,1: TF2

0.01

o:ool

1

i

0'01

0'1 Grain size

~TF1 t10

io

mm

Fig. 24.

E n r i c h m e n t factors o f materials left in suspension after passing over the visible e x t e n t o f the d e p o s i t i o n a l area in the thin f l o w e x p e r i m e n t . Sampling times, from comm e n c e m e n t o f e x p e r i m e n t , were: TF1, 2--4 rain; TF2, 17--19.5 rain; TF3, 50--51 rain. N o t e the decline in clay e n r i c h m e n t w i t h time and the strong c o n c e n t r a t i o n o f fine silt.

of our observations suggest that discernible relationships may exist between mantle surface profiles and size analysis curves of materials being fed to the mantles. In general, the angles at which the different bed stages were laid were about equal to, or slightly more than, the angles of their natural equivalents. In the zone of net deposition rheologic bed stage sands were inclined, on average, at 4040 ' . Average slope for the coarse bare bed stages (too intimately mixed to be separated) was 406 ' . However, slight concavity was suggested because their proximal half averaged 4037 ' and their distal half 3 ° 34'. The visibly concave surface of the fine ripple bed stage had an average slope, between 0.84 and 0.94 m from the summit, of 1°57 '. However, between 0.94 and 2 . 1 5 m from the summit, slope averaged only 0°19 '. Surface slope in this stage was clearly related to framework particle coarseness. The experiment confirmed the suspicion that some particles travel downslope in suspension later to move as bed load in gentler flows before final deposition. Particles up to 0.08 mm in diameter were evidently in suspension in the theologic bed stages but much finer particles were in bed-load motion over the distal parts of the fan.

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In summary, the experiment closely reproduced many natural phenomena involving solid--fluid interactions. During deposition, each successive physical environment brought about its own downhill demise usually by exhausting the supply of particles suitable for its continuance. It was then abruptly replaced by the next environment in the hierarchical order. This highly consistent series of interactions is probably so widespread because, in the zone of net deposition, solid--fluid interactions build the surface on which they take place and thus come largely to control slope, depth, velocity and other parameters in their own environments. So marked was this control that, once initiated, the experiment, in effect, conducted itself. DISCUSSION

The complex interactions between overland flows and solids, evident in laboratory experiments, are further complicated in nature by external factors. The purpose here, therefore, is to explore more fully the nature of sediment transport in overland flows and to consider the specific effects of external factors such as tectonics, climate, and vegetation. Consideration is then given to landscape stability, pedogenesis, and soil renewal.

Sediment transport and differentiation in overland flows The differentiation of transported sediment into suspended and bedload modes is fundamental to overland flows. The maintenance of suspensions in shallow water flows is better understood if the twofold nature of the phenomenon is appreciated. Firstly, a repulsion of fine particles from the bed, a boundary effect, is required (Moss, 1972). Secondly, a dispersive mechanism, attributable mainly to turbulence, is needed to spread the particles t h r o u g h o u t the flow. Suspension has generally been described in terms of this second factor (Allen, 1970). However, dispersion not only moves particles upwards from the bed b u t also must return them towards the bed and could cause many to enter the interstices of coarser stationary particles. We know that, in the coarse bare bed stages and their fluviatile equivalents, fine particles do not thus enter the bed. It seems reasonable to propose therefore that repulsive forces, acting at the bed--flow interface, are responsible for this bed-cleaning effect. When flow mechanisms causing suspension lose their effectiveness, a dramatic increase in fine sediment deposition occurs. The fine ripple bed stage described in earlier sections is an example of this latter effect. Because of the shallow depths of overland flows generally, the spatial separation of suspended and bed loads is inhibited compared with that of stream flows. It is probable that, as flows become shallower, suspension owes progressively less to turbulence and more to the bed repulsion mechanism. Bed-load transportation, involving essentially boundary phenomena, is more readily understood in thin flows. Forces causing saltation arise because

121 particles are positioned along the bed--flow interface. In this context, a thin flow can be regarded as a moving boundary layer. The ready transport of contact load in thin flows may be related to the steep velocity gradients acting across these larger particles, which often span most or all of the flow depth, and the consequent high moments tending to roll them. Because most detritus entering erosional systems is of widely varying grain size, the movement of suspended and bed-load components together in overland flows must be the norm. Because they have low transportation rates and largely line drainage systems with their deposits, bed-load processes will control bed roughness and geometry and will greatly restrict suspended load transportation rate by limiting availability. Many particles, maintained in suspension in the higher energy bed-load environments, later become involved in bed-load transport themselves in areas with lower bed roughness and gentler flows. The redistribution of granitic detritus in thin water flows, shown in the experiment described in the previous section, is an almost perfect example of these downslope changes in depositional environment. If, on the other hand, the maximum size of available particles is less than the maximum suspendable size, bed-load amelioration cannot take place and enormously increased exposure and entrainment of suspended particles takes place. The loess landscapes of Iowa, U.S.A. are a case in point. In western Iowa, where the loess is thick (Oschwald et al., 1965), erosion gullies tend to enlarge excessively in the general absence of particles greater than silt size (Daniels, 1960; Daniels and Jordan, 1966). Detachment of highly suspendable particles, rather than limited flow capacity, becomes the dominant factor in soil erosion under these conditions. The great efficiency of channel flow then to transport sediment is evidenced by reported concentrations of 50% or more, by weight, of sediment carried in turbulent suspension (Pitty, 1971). Efficient separation of suspended and bed loads depends on their ability to move freely relative to each other. In the bare bed stages, sparse bed-load motion allows virtually unimpeded suspended load transport. Under more energetic transport conditions, the rheologic layer can be bypassed by rapidly moving suspended load, provided that bed-load-free water occurs above it. However, in the relatively shallow flows which are of particular interest here, the rheologic layer may occupy the whole flow depth, and the bypassing mechanism may become inoperative. The rheologic layer is then no longer sheared from above and must be driven directly by gravity. Addition to the load cannot be balanced by further dispersion, and interparticle distances must therefore decrease. Under such high energy conditions, suspended and bed loads largely lose their identities and transport approaches mass movement in nature. Rheologic bed-load transport probably grades into either mudflows or debris flows, according to bulk composition. Bull {1963) reported every gradation from mudflows to washed stream deposits on some alluvial fans in California. Also in California, Hooke (1967) thought debris flows were important in building some of the steeper slopes of alluvial fans. Sediment transport processes and the downhill differentiation sequence

122 described here obviously relate to a wide variety of landscapes subject to modification by flowing water. Because features such as alluvial fans and mantles can be built by other mechanisms, we refer to those built specifically by flowing water as 'hydraulic mantles'. Whereas, in landscape zones of net erosion, gullies behave in a similar way to mountain streams, water flowing across hydraulic mantles in the zone of net deposition does not appear to have close fluviatile analogues. Rill patterns on hydraulic mantles resemble channel patterns of braided rivers. However, the tendency in the former is to continue to bifurcate downslope (see illustration by Bull, 1968). The physical differentiation of chemically varying solids in hydraulic mantles is, in effect, chemical differentiation. Such differentiation is usually most evident in the zone of net deposition where clay and fine organic matter are strongly partitioned. Some of this fine material can re-enter the bed from the rheologic layer, but almost non re-enters in the bare bed stages. Fine material remaining entrained in overland flows can, however, commence redeposition in some concentration where fine ripple stage bed conditions are developed in relatively low energy depositional environments. Another important process associated with the proposed sedimentary differentiation sequence is the transmission of water and solutes downslope through mantle deposits. The coarse nature of these deposits on upper slopes encourages infiltration of significant amounts of water which flows from zones of net erosion. McGee (1897) witnessed the complete disappearance of a major overland flow into a large alluvial fan. H o o k e {1967) described the sieving action of some coarse fan deposits in California. Significant deposition from a sediment-laden flow may be induced by loss of water to highly permeable fan substrates. H o o k e referred to lobate masses thus formed as 'sieve deposits'. The re-emergence, downslope, of runoff water intercepted on upper slopes can result in the increase of local load capacity of flows and be accompanied by accelerated erosion {Ireland et al., 1939). Seepage waters of this kind contain solutes derived from higher slopes of the mantle (Van Dijk, 1958). The downslope transferrance of colloidal materials in subsurface waters can also be an important consideration in studies of weathering profiles where significant thicknesses of mantle deposit occur (Ruxton, 1958; Van Dijk, 1969).

Hydraulic adjustment of mantle slopes As a basis for discussion we propose here a simplified model (Fig. 25a) consisting of two planar surfaces, one horizontal, the other rising above it at a steep angle. Assume that the sloping surface is such as to permit the free flow of water and to allow flows to load to capacity in terms of bed load. By analogy with the experiment described here in a previous section, as overland flows act, zones of net erosion and net deposition will form. The latter will have outwardly decreasing gradients, with zones representing the sequence

123

\

\ \

\

\

\

(2.

jJJ

~7

Fig. 25. Diagrammatic representation of the development of a hydraulic mantle. In Fig. 25a, an initial state is envisaged with two plane surfaces intersecting at an abrupt angle. In Fig. 25b, as a result of hydraulic adjustment, zones of net erosion and deposition have developed. Within the latter are depositional environments of decreasing energy downslope.

of energetically decreasing physical environments and their sediments, culminating in clay deposits on the horizontal surface (Fig. 25b). This configuration of surfaces and deposits in the model is closely analogous to alluvial fan landscapes in arid regions (Bull, 1968). The mere presence of a steeply sloping segment in the model manifests disequilibrium. However, if the amount of granular solids moved in each flow event is small compared with the volume of the hill that the model represents, the system can be envisaged as approaching a steady state as it adjusts to each successive event. Provided that extreme conditions, relating to external factors, are not imposed on the system, it can be considered as

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remaining in a 'normal state of hydraulic adjustment'. This concept will be used and elaborated further in later parts of this discussion. We now consider the model, initially in a state of normal hydraulic adjustment (Fig. 25b), to be affected b y an external impulse (tectonic) which causes the steeply sloping surface to increase in gradient. The response to such a change in the zone of net erosion would be an increase in energy of overland flows and an intensification of gullying. A new pattern of energetically decreasing depositional environments, similar in kind to that initially developed (Fig. 25b), would then be superimposed on the hydraulic mantle. The natural equivalents of this kind of hydraulic readjustment are found in alluvial fans where the mountain front is uplifted rapidly (Bull, 1968; Hooke, 1972). The initially adjusted model (Fig. 25b) is now considered in relation to climatic changes and intense rainfall events. The model response here would relate to larger volumes of overland flow being discharged from the zone of net erosion. Such stimuli for hydraulic readjustment would lead to displacement of the array of depositional environments and their bed stages on lower slopes. Bull (1968) has described shifts of the loci of deposition, in alluvial fans of arid regions, as a result of climatic changes and intense rainfall events. Because of previous episodes of size differentiation of mantle sediments, however, all depositional zones may not immediately react by moving to the next highest bed:stage. An area normally in the fine ripple bed stage, and lacking medium and coarse sand, cannot change to the coarse bare bed stages unless suitable particles are brought in from upslope. The immediate transition would probably be to rheologic motion of the fine sands. Gravels laid on unusually low slopes during exceptional depositional events would, helped by their armouring capability, remain immovable to lesser flows for long periods. Gravel lenses in otherwise sandy mantle deposits thus often record past exceptional climatic events (Gile and Hawley, 1966). Many intense storms have horizontal dimensions that are smaller than some of the slopes they affect. Overland flows may even originate in areas that are normally in the zone of net deposition and parts of hydraulic mantles may become temporary eroding source areas. McGee (1897) recorded the reverse occurrence -heavy rain falling on upper slopes b u t not on the mantle.

Stored hydraulic adjustment potential The concept of a state of normal hydraulic adjustment, discussed in the previous section, is now expanded to include states of abnormal hydraulic adjustment. If a landscape element in a state of normal hydraulic adjustment is modified (e.g. by a tectonically caused overall increase in slope), the need for hydraulic adjustment is increased. If, however, associated conditions are such that the needed adjustments do not immediately occur, the landscape element can be considered to have an increased potential for hydraulic adjustment. The greater the increase in the potential for hydraulic adjust-

125

ment, the more catastrophic the hydraulic adjustments will be when they eventually occur. Conversely, a landscape element can acquire a decreased potential for hydraulic adjustment, for example, if its overall slope is decreased by tectonic movements. Landscape elements in this state will be relatively stable, nullifying the effects of translocation processes. In arid landscapes, greatly reduced vegetative cover allows the almost free interaction between flowing water and sediment. Under such conditions, the tendency is for natural systems to approach most closely normal hydraulic adjustment. For landscapes in many other climatic regions, however, the presence of a continuous and thick vegetative cover greatly reduces the free interaction between flowing water and sediment. Transportation by overland flows is strongly dependent on the existence of a sharp boundary between bed and flow. Here, the forces that move particles are generated and, in general, the sharper the boundary the more effective are these forces. Plants grow through this ephemeral boundary region and provide debris that falls into it. Because they are held by roots, plants are also immune to removal by all but the most powerful flows. The combined effect of plants in increasing the resistance to flow, dissipating the erosive energy of raindrops, and reducing the sharpness of the solid--fluid boundary, is profound. Surface flows are made deeper, slower and less turbulent than would be the case on bare ground. Only if flow depth exceeds that of the plant cover can high velocities develop. Extreme flows can evidently rip up turf (Horton, 1945). In the more general cases the damping of fluid motion by plant cover not only inhibits erosion but also stimulates deposition, notably of suspended loads, from both air and water (Brown, 1943; Beadle, 1948; Wilson, 1967). In many landscapes, plant cover migrates upward through sediment increments, a rising soil surface being the result. The presence of forest cover at the onset of aeolian deposition of the late Pleistocene loess of Iowa is implicit in the work of Ruhe (1956). An important consequence of erosional--depositional surfaces being vegetated is that overland flows passing across them entrain very little sediment. Such surfaces may remain unresponsive to changes such as uplift for long periods, so that a backlog of needed hydraulic adjustments accumulates. We refer to this condition as 'stored hydraulic adjustment potential'. In the case of a landscape where stored hydraulic adjustment potential has increased, the advent of fire, climatic change, or human activities represents a time of reckoning in geological terms. Because both hydraulic adjustment and plant cover are dependent on rainfall and runoff, stored hydraulic adjustment will tend to be the norm on vegetated slopes and erosion will tend to be ephemeral. Plant cover and tectonic movements, therefore, are powerful factors which may combine to produce a greatly increased potential for landscapes to erode. During periods of hydraulic inertia, weathering and pedogenesis proceed and can significantly alter materials in situ. For example, fine material may be produced, pebbles may decay, or states of aggregation or cementation

126 may change. Thus grain size distributions may be modified. Because grain size is critical in overland flow processes, potential reaction to future overland flows may be changed. Such changes can also alter permeability significantly. The disruption of established geomorphic surfaces and surficial deposits following tectonic movements has been reported in a wide range of environments (Bowler and Harford, 1966; Ruhe, 1967; Bull, 1968; Pillans, 1974). In the non-glaciated landscapes of humid to sub-humid southeastern Australia, widespread depletion of vegetative cover resulting from climatic change is considered to have triggered the alternating episodes of erosion and stability observed in Quaternary landscapes (Butler, 1967; Costin, 1971; Costin and Polach, 1973). Enhanced erosion resulting from the combined effect of tectonics, climatic change, weathering and pedogenesis, and reduced vegetative cover does not appear to have been documented in relation to the contemporary scene. Interactions of these factors may be more important than hitherto realised in many areas.

Interaction of transporting agencies Besides overland flow, many other agencies transport solids in landscapes. These include types of mass movement (landslips, mudflows, debris flows, creep), transport in air (suspension and saltation), transport by glacial ice, and stream flow. Many of them could be considered analogously to the manner in which we have treated overland flow processes. All have physically distinct actions and leave behind deposits whose textural properties and geometries are peculiar to their different modes of transport. Thus, if deposits of one transporting agency are attacked by a different transporting agency, there will almost certainly be an adjustment of the deposits towards a new dynamic equilibrium. This adjustment may vary from negligible to extreme. We are here concerned only with the manner in which overland flows and their deposits are involved in such interactions. Reference is made particularly to properties of transporting agencies such as slope-sensitivity, grain-size capacity, load capacity, and hydraulic adjustment. Most mass-movement mechanisms have large grain-size capacities but need relatively steep slopes for their initiation (Sharpe, 1938). If their deposits have high surface slopes and small maximum grain size, they will be vulnerable to attack by overland flows. Some mass movements, notably debris flows, can carry coarse materials onto low-gradient slopes, and their deposits tend to resist the action of overland flows (Walker, 1963). Transport of sediment by aeolian suspension is not slope-sensitive but has a low grain-size capacity (<0.1 mm) (Bagnold, 1941). Deposits of aeolian silt and clay thus tend to be draped as sheets across geomorphic surfaces developed by other agencies (Smith, 1942; Butler, 1956; Ruhe, 1956). Where these earlier geomorphic surfaces have been subjected to overland flows of relatively large grain-size capacities, high rates of erosion of the

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incoming aeolian sediment can be expected (Daniels et al., 1963). Holeman (1968) compiled values for the annual discharges of solids (mainly suspended loads) of the world's major rivers. His results suggest strongly that the bulk of the grand total is provided by rivers draining areas of Pleistocene loess or areas of strong tectonic uplift. In areas where neither influence is strong (e.g. Australia) loads are comparatively small. The greatest loads of all occur where both influences coincide, as happens in eastern Asia. Aeolian saltation is also not a slope-sensitive process and normally has a grain-size capacity of approximately 1 mm (Bagnold, 1941). Saltating sands may form relatively steep depositional features, such as dunes. High initial permeability of dune sands (Edelman, 1950) tends to reduce susceptibility to water erosion, despite the steep slopes. Glacial ice has large grain-size and load capacities and the ability to move detritus over very low slopes. Its deposits have a wide range of grain size, some being quite fine (see discussion of glacial till in Ruhe, 1975). Melting ice provides water for the redistribution of glacial deposits b y overland flows and streams as outwash deposits. The relatively high rates of initial erosion of glacial deposits by overland flows has been documented by Walker (1966). Morris (1942) reported severe contemporary water erosion of a cultivated drumlin. As shown in the earlier part of this paper, stream deposits closely resemble those of overland flows but are laid by deeper, more powerful flows on lesser slopes. Contemporary channel, point-bar, and overbank deposits, being located on valley floors of low gradient, are generally protected from erosion by overland flows. The finer, overbank stream deposits become exposed to erosion when new valley floors and floodplains are developed at lower levels. Erosion by overland flows of relict floodplain deposits has been reported in eastern Australia b y Warner (1972). Coarser, channel and point-bar deposits, however, may resist erosional processes so strongly as to create conditions favouring landscape inversion. Relict fluviatile gravels are found capping hills in erosional landscapes in many areas, even where there has been considerable uplift (Coventry, 1967; Williamson, 1969). Bed-load deposits of overland flows have well-packed framework grains as a structural skeleton. To this is often added a coarser skeleton of clogged contact population. Such deposits thus have a built-in resistance to deformation which is also aided b y their occurrence on relatively gentle slopes. These factors combine to make this type of deposit resistant to mass movement and enable them to occur as ubiquitous hillslope mantles. If left bare, however, fine differentiates of hillslope mantles are susceptible to wind erosion (Coventry, 1973). The action of streams in downcutting or meandering can upset the hydraulic adjustment of slopes by eroding their lower portions. Davis (1909) noted that resulting hydraulic adjustments moved upslope. Significant variations in sea level also affect the hydraulic adjustment of rivers in their coastal reaches. These in turn may affect the hydraulic adjustment of

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adjacent hillslopes. 'Trough valleys', occurring in the near-coastal reaches of the Hawkesbury River in eastern Australia (Hall, 1926), are an example of dramatic hydraulic changes being brought about by postglacial rise of sea level, their effect being transferred from the main river to small, steep tributaries. Landscape stability and pedogenesis

For the purposes of the present discussion we define an effectively stable landscape as a geomorphic surface which withstands erosional or depositional episodes, including extreme events, without significant hydraulic adjustment. Apparently landscapes may have acquired considerable increases in stored hydraulic adjustment potential, or be quasi-stable, but our definition requires only that there is negligible surface erosion and deposition. A variety of factors, including climate, relief, and vegetation, have been shown to contribute to the stability of landscapes. However, it is apparent, from an earlier section of this discussion, that these factors, acting in combination rather than singly, provide the most significant effects. As already mentioned, an important consequence of prolonged landscape stability is the development of weathering profiles and soils beneath geomorphic surfaces. Indeed, the presence of soils in Quaternary stratigraphies has been widely used as a criterion of landscape stability (Butler, 1959; Gile and Hawley, 1966; Birkeland, 1974). Similar inferences have also been drawn from soil and weathering depth functions in much older sedimentary rocks (Summerson, 1959. Jensen, 1975). By its nature, pedogenesis brings about changes of the materials in which soil profiles develop and thus changes in landscape stability occur. Pedogenesis involves production of fine-grained materials and, as it becomes more advanced, their downward translocation leaves the A horizon relatively coarse-grained and permeable, but rich in biological materials (Bullock et al., 1974). Both translocation and weathering enrich soil texture B horizons with fine-grained materials (Brewer, 1968) which, in consequence, tend to lose permeability. Further stages of soil profile development involve local to more continuous cementation of the subsoil by iron and manganese oxides, silica, and carbonates. The erosional resistance of such materials is well known (Ireland et al., 1939) and becomes extreme in laterites (Mulcahy, 1960). It is significant also that relict river gravels which cap hillslopes in erosional landscapes often have an associated lateritic weathering profile (Walker and Hawkins, 1957; Coventry, 1967). In these, prolonged pedogenesis has reinforced the stabilising effect of the river gravels. Given that landscape stability and pedogenesis are covariants in erosional landscapes and hydraulic mantles, it is of interest to re-state here the functional relationship of soil to the various soil-forming factors, as proposed by Jenny (1941): S - - f (cl, o, r,p, t . . . . )

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where S --- soil, c l = climate, o = organisms (plants and animals), r = relief, p = parent material, and t = time. Of the five listed soil-forming factors, only time is a truly independent variable (Crocker, 1952). The remaining four variables not only state an important functional relationship for soils, but they are an equally valid statement of the factors of landscape stability in terms of the hydraulic adjustment model proposed here. Soil scientists have long recognised the relationships between essentially lateral sedimentological processes and essentially vertical pedological processes. This relationship is embodied in the catena concept of Milne (1936a, b). There can be little d o u b t that the repeatedly occurring hillslope soil sequences he described represent, for the lower slopes, relatively uncomplicated examples of the depositional differentiation sequence of this study. Residual granite hills, south of Lake Victoria, were capped by shallow, skeletal, dark grey loams; red earths occurred downslope of these. The remainder of this sequence, considered by Milne to be water-laid and to result from 'ordinary slow denudation', is tabulated below as the soils occurred, in downslope order with decreasing surface slope for each successive type. The apparent sedimentological equivalents, in our terminology, are also listed. Milne

Present study

Coarse granite grit Washed sand Silty sand Clayey sand Clay

Rheologic bed stage deposits Deposits of coarse bare bed stages Proximal fine ripple bed stage deposits Distal fine ripple bed stage deposits Suspended load deposits

The correspondence is extremely close, even to the cleanliness of the 'washed sand'. The consistent repetition of the sequence, over wide areas, was probably aided by the availability of granite as a c o m m o n source rock, and the control that solid-fluid interactions exert over their own physical environments in the zone of net deposition. Analogous sequences have been recognised in other parts of the world (Holmes, 1937; Nye, 1954; Gunn, 1974; Valentine and Dalrymple, 1975). In many landscapes, however, the effects of tectonics, climate, and vegetation have been such during the Quaternary that repeated phases of stability and instability have occurred. Rather than one sequence such as that described b y Milne (1936a, b), several may occur in stratigraphic succession (Valentine and Dalrymple, 1976). The stratigraphies of many hillslope sites indicate reaction to severe, infrequent events. Field evidences include the truncation of soils of upper slopes with development of lag gravels or stone lines (Ruhe, 1956, 1959). Some stone layers are lenticular (Parizek and Woodruff, 1957), suggesting the formation of rheologic bed stage gravels on channel floors. If loaded above bed-load capacity, overland flows deposit sediment, even gravels, on

130 potentially highly erodible surfaces. Occurrences of this kind were reported by Eargle (1940) w h o described gravels overlying paleosols. In New Mexico, tectonics have played a major role in creating conditions conducive to episodic hydraulic adjustment. A succession of geomorphic surfaces and soils occurs (Ruhe, 1967) in fans, pediments and basins. In part of the arid zone of central Australia, Quaternary climatic fluctuations are considered to have resulted in episodic alluviation and a corresponding sequence of geomorphic surfaces and soils (Litchfield, 1969). In Iowa, a multiplicity of glacial deposits reflects major climatic and vegetational changes during the Quaternary (Ruhe, 1969). Erosion of these landscapes and the development of hydraulic mantles occurred during the interglacials. That these erosional-depositional landscapes also attained stability for significant periods of time is indicated b y the association of paleosols with their surfaces (Ruhe, 1956). In southeastern Australia, the direct effect of Pleistocene glaciations have been minimal and much of the Quaternary stratigraphy relates to the action of overland flows and fluviatile transport. Butler (1959, 1967) proposed widespread climatically induced instabilities for much of this region. The episodic nature of these is indicated by the presence of buried soils in a variety of landscapes, especially in hydraulic mantles. The more general occurrence of episodic landscape stabilities and instabilities of this kind was indicated for the Murray Basin of Victoria and southern New South Wales b y Lawrence (1976). It is important to emphasise that the erosional and depositional instabilities referred to here are part of the natural weathering and transportation systems which modify all continents. These processes play a vital role in stripping and redistributing the products of weathering, and are important mechanisms by which soil renewal takes place. The massive removals and redistributions associated with glaciations have only limited counterparts in continents such as Australia. Here, much removal and redistribution are carried out by overland flows and fluviatile agencies. That these t)rocesses do not inevitably result in landscape instability is shown by the widespread occurrence of very old lateritised ut)lands in Australia, especiallv those in Western Australia (Mulcahy and Bettenav, 1971). In terms of fertility maintenance, erosion and sedimentation are not necessarily opposites. The importance of erosional renewal is indicated by the work of Cole (1963) who investigated savanna areas in central Africa. Fertility increases were noted at some sites of erosion and were attributed to removal of infertile surface materials and exposure of relatively nutrient-rich horizons of the subsoil. Morrison et al. (1948), who also worked in central Africa, noted that hillwash preferentially deposited fine organic matter on some areas of low slope and that these areas were favoured for cultivation by local farmers. This observation of preferred deposition of organic matter appears to have confirmation in the experiment described here in an earlier section. The fine ripple bed stage of the experiment occurred on very gentle

131 slopes and was shown to favour the deposition of suspended load, including organic matter, being transported in overland flows. Strong association of both nitrogen and phosphorus with fine solids, during transportation, has been demonstrated by Schuman et al. (1976). To the 'fertility erosion' of Ellison (1947, 1950) we can therefore add 'fertility deposition'. Some of the fine material entrained during erosion can re-enter the bed from the rheologic layer but almost none can do so in the coarse bare bed stages. Fine material, remaining in suspension and being transported onto gentler slopes, can be readily deposited with the onset of the fine ripple bed stage. Depositional fertility renewal in the equivalent fluviatile environments is familiar. Of major rivers, the Nile provides a well-known example of how important this process is in maintaining nutritional balances in soils (Jenny, 1962). Areas where neither erosion nor deposition have been significant for long periods should, in these terms, be typically infertile. Such infertility is exemplified by many relict spreads of river gravel (Storrier and Walker, 1955). The same concepts can be applied on a larger, worldwide scale. In regions of active major tectonic uplift, fresh rock debris, together with contained nutrient elements, is made rapidly available to fast, highly turbulent, water flows. By the time these turbulent flows reach adjacent, lower, flatter terrain they typically carry not only rock-derived nutrient elements but much organic matter derived from upland plants. Proportions, often large, of nutrient elements are specifically associated with fine solids, both inorganic and organic. Where near-bed turbulence and bed-flow interface repulsive forces are sufficiently reduced, deposition of these fine particles and fertility renewal takes place on active river floodplains, lowly sloping areas traversed by overland flows and in areas where flows are damped by plant cover. Thus, all else being favourable, low-gradient continental areas that flank tectonically active mountain ranges, tend to be persistently fertile and to support dense populations. Modern civilizations were, of course, seeded in such areas because they allowed permanent settlement. Via such processes, agriculture and m a n y other human activities may be related to plate tectonics. However, detailed consideration of such topics is outside the scope of this study. The above reasoning also suggests that areas of minimal tectonic activity will, by contrast, have old, infertile soils. This appears generally true. Volcanoes, of course, also function as 'nutrient fountains', the fertility of their ejected materials, if geologically recent, being legendary. Glacial ice, too, has an analogous effect in that, together with associated transporting agents, it spreads material, relatively new to the Earth's surface, over large areas. Human activities, erosion, and natural solid translocation systems

Human interactions with natural particle translocation systems mainly arise through partial or complete removal of plant cover, various modifications to landscape geometry, and alterations to the routes taken by natural, flowing water and to solid fluid interactions. The concept of hydraulic

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adjustment appears basic to consideration of man-induced water erosion. Susceptibility of suddenly bared land to thin flow erosion is low on active river floodplains which are almost level and naturally maintained in a state of hydraulic adjustment. Analogously, were the hydraulically adjusted surface of a desert fan ploughed, probably no significant erosion would result. The other extreme, already mentioned, is exemplified by loess laid on sloping surfaces. Fine and heavily dependent on plant cover, these materials often react violently to baring. Piest and Spomer (1968) who studied erosion in a loess area, found that, although thin flows redeposited some material at the bases of slopes, once in a gulley, the fine particles composing these soils had little chance of local redeposition. Between these extremes are many possibilities, depending, largely, on state of hydraulic adjustment. If, as is usual, bed-load material is set in motion, erosion is accompanied by local redeposition which represents a reapproach to a state of hydraulic adjustment. In such cases, man-caused erosion will gradually slow up and local translocation of solids will soon become sufficiently restrained to allow plant recolonization. Gilbert (1917) described the restabilization processes which t o o k place, over several years, in the Sierra Nevada, after hydraulic mining had left behind huge quantities of exposed debris. Bed-load amelioration is probably responsible, in part, for the temporal erosion pattern often found where logging or forest clearance takes place in hilly areas. An initial burst of erosion wanes in a year or two as bed-load deposits, notably armouring gravels, gradually bring restabilization. Tillage is potentially damaging because it not only bares and loosens soil but it upsets the process of bed-load amelioration. The normal pattern of erosion, during an individual event, is that exposed fine material is rapidly entrained and removed at first but, as happened in our experiment, bedload amelioration soon causes its transport rate to fall and protects all fine material underlying bed-load deposits. Normally, the bed-load linings of one event are left to protect fine material from erosion by the next, and so on. Tillage destroys these bed-load layers and replaces the highly erodible fine material at the surface. Thus repeated tillage, even if water erosion is gentle, must gradually cause soil impoverishment as clay and fine organic matter are preferentially removed. Sharp boundaries between areas of different land treatment often invite concentrated water erosion. Such effects occur where, for example, flowing water, previously denied bed-load acquisition by dense plant cover, suddenly enters an erodible area from which plant cover has been removed. Rapid loading to bed-load capacity can then take place, virtually at the same spot, during an entire event. C o m m o n l y this type of situation is created in valley sides where cultivation often extends uphill on the relatively deep soils of the hydraulic mantle but ceases where the thin, stony soils of the zone of net erosion are reached. This upper zone is often left uncleared. A line of gullies is a characteristic manifestation of these circumstances.

133 As already implied, gullies usually result from sudden local availability of detritus to overland flows. To some extent gullies are self-causatory because initial excavation produces a depression. Water pouring down the upstream side of the depression accelerates, acquiring more detaching power and capacity as it does so. The result is the characteristic, often dumbell-shaped, uphill-migrating gully head. A few metres downstream of the head, where the gully floor begins to slope less steeply, bed-load linings usually st-,~t to occur. Areas artificially made effectively impermeable and inerodible, e.g. by concreting, behave as do their natural counterparts, often translocating water attack rather than preventing it. Modifications to surface geometry, such as 'landscaping' or the making of road cuttings often create situations of severe hydraulic maladjustment (Diseker and Richardson, 1962). In general, man-caused erosion represents natural erosion, triggered by human activities. Much or all of it would eventually have taken place naturally but has been brought forward in time. Prevention of erosion seems more useful than 'rectification' of its effects. Understanding the nature of a landscape is clearly the key to deciding what risks can be taken with any part of it. For example, the mere existence of a well-differentiated soil is evidence that significant hydraulic adjustments have not been made in the recent past and therefore, in itself, sounds a warning. Several indications may be present to suggest whether the drift from hydraulic adjustment (or lack of it from the start)has caused relative stability or instability. For example, fine subsoils may have unusually high surface angles, gravels may have level surfaces, pebbles may have decomposed, or an almost impermeable B horizon may have developed. A large proportion of natural overland flows discharges into streams before reaching sufficient states of tranquillity to deposit their fertilityassociated finer differentiates. Consequently rivers become carriers of natural fertility and often lay rich deposits on their flood plains. Flooding and meandering tend to add to, or bring about renewal of, such deposits. Human settlement in these areas often brings river control and flood prevention schemes. These entail such procedures as prevention of meandering, or confinement of the flow between levees. In general, water is passed through the area concerned, often towards the oceans, with little chance of bringing a b o u t natural fertility deposition. Thus short-term relief from flooding must often be at the long-term sacrifice of natural fertility. Similarly, in order to prevent hillslope erosion, surface water that would have moved as overland flows, is frequently discharged along artificial channels and given no chance to spread over lower slopes and to deposit its finer solid fractions. Dams are built across many rivers, usually to provide water supplies, electricity, or both. In the resulting masses of impounded water, both mean velocity and turbulence are enormously reduced, resulting in the premature deposition of both fine inorganic and organic materials. Thus, what would have been naturally deposited as a thin veneer over extensive, well oxygenated

134

areas, is d u m p e d in a relatively small area in a poorly oxygenated environment. Again, there must be a long-term detrimental effect on natural fertility. Whereas man-caused water erosion is essentially a natural process, stimulated by man, human interference with natural drainage systems and, more particularly, with the natural translocation of solids by water, represents a marked deviation from natural processes and may therefore be, in the long term, more damaging. CONCLUSIONS

(1) Overland flows and rivers transport solids by the same mechanisms. Even in flowing water a millimetre deep, behaviourally distinct suspended and bed loads can exist. Thus essentially fluviatile processes act over entire landscapes, often, albeit with extreme ephemerality. (2) In general, suspended-load transportation rate is controlled by availability whereas bed-load transportation rate is limited by load capacity. By lining large proportions of channel systems or erosion surfaces, bed-load deposits limit availability of new suspended load. We call the resulting curbing effect on erosion 'bed-load amelioration'. (3) Bed-load capacity is highly slope-sensitive and most hills eventually decline in gradient downslope. An almost inevitable consequence is the formation of hydraulic mantles, built of bed-load deposits, around the bases of hills. Most slopes can be divided into an upper zone of net erosion and a lower zone of net deposition in which is built the hydraulic mantle. (4) Hydraulic mantle deposition is extremely consistent in nature because, as these landforms develop, solid--fluid interactions largely control such parameters as depth, slope and velocity in their own physical environments. Given a supply of heterogeneous detritus from upslope, overland flows will build much the same textural types of differentiation products, virtually throughout the world. • (5) Depositional differentiation on hydraulic mantle slopes is characterised b y a series of discrete physical environments, each with characteristic sediments, as transporting power and turbulence wane downhill. The sequence is: Rheologic bed stage with clogged contact population. Rheologic bed stage without clogged contact population. Coarse bare bed stage with clogged contact population. Coarse bare bed stage without clogged contact population. Fine ripple bed stage. Suspended load bed stage. This represents the overland flow expression of what is probably a universal differentiation sequence, also occurring in rivers and, probably, in marine environments.

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(6) Properties of soils on hillslope sequences (catenas) are controlled to a considerable degree by the pattern of bed-load differentiation in shallow flows. (7) A slope able to respond to each successive overland flow event, by transference of material from upslope to its mantle, approaches a steady state relationship with its physical environment and is said to be in 'normal hydraulic adjustment'. Dense plant cover can strongly suppress overland flow transportation for long periods while other processes, such as tectonic movements, pedogenesis, and deposition of fine materials in tranquil, interplant environments, build up a backlog of needed hydraulic adjustment or 'stored hydraulic adjustment potential'. Hence, land cleared of plant cover may often react drastically to the action of overland flows. (8) Fertility-associated fine inorganic and organic particles are largely kept in suspension while the high-energy bed stages are being deposited, b u t enter sediments, often in some concentration, in the low-energy fine ripple and suspended load bed stages. Hence, fertility is, in effect, transferred from areas of erosion to areas of gentle deposition around the bases of hillslopes and on the floodplains of rivers. In the long term, landscape instability is necessary to the maintenance of this fertility renewal process. (9) Much man-caused erosion represents the artificial and premature instigation of events which would eventually have taken place naturally. The severity of such effects is closely related to state of hydraulic adjustment. Effects caused by the diversion of natural water flows and the building of dams show less natural analogy and may have long-term detrimental effects by upsetting the natural processes of fertility transference. ACKNOWLEDGMENTS

We thank Miss M.P. Green and Messrs J. Hutka and P.I.A. Kinnell for their assistance, particularly in conducting the experiment. We are also grateful to Dr R. Brewer, Messrs B.E. Butler and W.H. Litchfield, and Miss M.P. Green for criticism of early drafts and to Dr A.N. Gillison for helpful discussions.

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