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Sedimentary structures formed by flash floods in southern Israel
Sedimentary Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
SEDIMENTARY STRUCTURES F O R M E D BY FLASH FLOODS IN SOUTHERN ISRAEL IAAKOV KARCZ 1
Geological Survey of Israel, Jerusalem (Israel) (Received December 15, 1970,
ABSTRACT Karcz, I., 1972. Sedimentary structures formed by flash floods in southern Israel. Sediment. Geol., 7 : 161 182. During the rainy season the ephemeral streamcourses of southern Israel are swept by flash floods of low volume and short duration, which produce meander bars, megaripples and ripples, harrow marks, current lineations, obstacle marks, washouts and remnant bars, rhomboid marks, rill marks, rare convolute lamination, imbrication, mud pebbles and polygonal desiccation patterns. The rapid abatement of floods results in a variety of coexisting, interfering and superimposed associations of bed forms, which shed light on some aspects of provenance and origin of alluvial bed forms.
INTRODUCTION AND GENERAL SETTING
This paper summarizes a study of sedimentary structures produced by individual flash floods along the main ephemeral streamcourses (wadis) in the Negev, and the southern Coastal Plain of Israel (Fig. 1). Under a flash-flood regime, conditions of varied flow prevail and the resulting assemblages of sedimentary structures differ somewhat from those encountered in a perennial fluvial environment. In the southern Coastal Plain of Israel, the drainage pattern is dominated by several main streamcourses running down from the Judea-Hebron foothills. Commonly, dendritic patterns develop and the individual wadi catchments are dissected by close-spaced, rapidly headward-advancing trapezoidal gullies, which provide a major source of the sediment load. Streambeds consist mainly of moderately sorted, fine to medium-grained sand, but silty muds prevail in areas rich in loess and alluvial soils. The main part of the Negev drainage system is controlled by a series of gentle, northeast-southwest aligned folds; whereas on the east the network is determined mainly by the Jordan-Dead Sea rift valley, and the tilted blocks and Present address : Department of Geology, State University of New York, Binghamton, N.Y. 13901, U.S.A.
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SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
163
fractures along its edges. The run-offis carried mainly along the wide (6-7 km) and flat-bottomed, synclinal alluvial valleys, separated by anticlinal ridges. The floods, low in volume, occupy but a minor part of the valleys, and their tortuous or braided paths change not only from year to year, but also from one flood to another. Streambeds are coarse, alluvium commonly having 40-65~o material coarser than 5 mm. Many gravelly stretches are lined with a 5 4 0 cm thick veneer of medium to coarse sand with protruding calcareous or chert pebbles and boulders. The sand is generally covered by a 0.5-2 mm crust of clay which hardens and protects the various elements of bed sculpture from wind erosion.
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The climate in southern Israel is arid to semi-arid. The average annual rainfall increases gradually northwards, from 25 mm at the GulfofElat to 350-400 mm in the southern Coastal Plain (Fig.2). Precipitation occurs between October and May, in a highly varying number of low-intensity frontal rainstorms. Rain intensity is also fairly low; 50-60~o of rain days in southern Israel have falls of less than 5 mm/day, and only 10~o of the hourly amounts recorded exceed 5 mm (Israel Meteorological Service, 1967; F.A.O., 1967). This suggests that catastrophic floods are improbable. On the other hand, surface runoff in this region is facilitated by favourable topographic conditions along the drainage system, by scarcity of vegetation and by hardpan crusts on loess-soils which reduce their surface permeability• The annual runoff is highly variable. Unfortunately, the number of hydrometric stations in southern Israel has been very small, and detailed information about surface runoff is scarce. Long-range records are lacking and reconstruction of flood series, and evaluation of reliable models for runoff prediction can not be attempted. Such data as cited below are based mainly on the records of Israel Hydrological Service, and some information gathered by the Tahal (Water Planning for Israel, Ltd.) and F.A.O. teams (F.A.O., 1967 ; Dalinski, 1969; Tahal, 1970a, b). In conformity with the precipitation pattern, the annual frequency and volume of individual floods are highly variable. In the southern Coastal Plain (Nahal
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SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
Shiqma) there are 3-10 floods per year with volumes ranging from about 0.03 to 8 million m 3 per flood. In the northern Negev (for example, Nahal Besot and Nahal Lavan) there are 0-7 floods per year and the flood volume ranges from 0.05 to 2.8 million m 3, whereas further south in Nahal Tsin there are only 0-4 floods per year with volumes from 0.05 to 1.8 million m 3. Flood duration is short and decreases southwards (Fig.3). At Nahal Tsin, for example, 70~ of the floods last for less than 12 h. Floods build up very rapidly and usually the peak flow lags only slightly behind peak rainfall, especially where the antecedent moisture is high. Conditions of peak flow usually develop within an hour or two from the start of the flood, and are followed by a rapid deceleration and abatement (Fig.4). The latter are striking especially along the wide valleys of the Negev, where the rapidly spreading, low-volume flood results in a short-lived sheet flow. Occasionally, abating floods are strengthened by subsequent rainstorms, which may be expressed on the limnigrams as small surges or by new prominent peaks, and serve to extend the duration of the receding stage (Fig.4). Discharge at peak flow may reach 500-1,000 m3/sec range, but usually remains well below 100 m3/sec. Peak velocity occasionally attains 5-6 m/sec and the flow depth 2.5-3 m. Such conditions last briefly, and during most of the receding stage the depth is below 0.5 m and velocity is within 10-80 cm/sec range. b
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Information about bed load is even scarcer. Sporadic tests by the Hydrological Service indicate contents ranging from 0 to 86 g/1. F.A.O. (1967) suggests an average content of suspended material of about 10 15 g/l, which agrees fairly well with computed values based on the observed silting of floodwater reservoirs in the Coastal Plain and northernmost Negev (F.A.O., 1967; Tahal, 1970a, b). Analysis of suspended material from Nahal Besor and Nahal Shiqma (Tahal, 1970a) indicates that 60-70~ are finer than 2/~ and consist of montmorillonite and some kaolinite and illite. No further information is available about the traction load and actual sediment movement and distribution during individual floods. SEDIMENTARY STRUCTURES
The rapid alternations of floods and drought during the rainy season provide a natural laboratory for the study of streambed relief. The present examination was restricted to elements produced by single-flash floods, including: (1) meander bars; (2) megaripples and ripples; (3) sand ribbons (harrow marks) and current lineation; (4) obstacle marks; (5) washouts and remnant bars; (6) rhomboid marks; (7) occasional rill marks, convolute lamination and imbrica-
Fig.5. A sigmoidal pattern o f meander bars along Nahal Shiqma, sketched from a photograph. The channel width is about 10 m. S C = sigmoidal crests of meander bars; A S ~ alternate scours; R = parasitic ripples.
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
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tion; (8) mud cracks, mud pebbles and polygonal desiccation patterns. Observations were made after the flow vanished or during the last stages of the abatement, so that most comments on formation of these structures are necessarily interpretative.
Meander bars Sigmoidal and transverse systems of large-scale bed undulations are locally encountered along the main wadi channels. Length/height ratio ranges from 25 to about 120. The individual undulations are strongly asymmetrical. The lee sides are 0.15-0.45 m long and dip at 2 0 - 30° (somewhat steeper where covered by a veneer of mud), whereas the stoss faces are up to 10 m long and dip at less than 2°. Some crests are normal to the channel banks, but most are aligned in a roughly sigmoidal pattern (Fig.5). In sigmoidal patterns, crests are skewed at 25-60 ° to the banks and extend across the channel. Triangular or elliptical scour hollows ranging in depth from several centimeters to half a meter, commonly appear at alternate positions along the banks just below the point where the crest approaches the bank. Individual crests are straight, or convex downstream, with the apex alternately closer to one of the banks. In such forms the configuration resembles two rows of asymmetrically lobate bars aligned at offset along the channel (Fig.6A). The bars usually have a considerable lateral slope away from the lobe.
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Along curved channel stretches, the sigmoidal patterns are often distorted and asymmetrical. Roughly "s" and "z"-shaped alignments with a limb ratio of 1.5-3.5 commonly appear to the downstream of left and right hand bends respectively (Plate IA).
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SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
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Rapid abatement of flash floods appears to favour preservation of the rather delicate sigmoidal patterns. Occasionally, however, parts of the pattern are obliterated by superimposed smaller structures, mainly the harrow marks. Where flood duration is longer and abatement more gradual, deformation of the streambed advances beyond the sigmoidal configuration, and features of inner stream sinuosity develop, resembling those reported from many straight erodible river channels (Shen and Komura, 1968 ; Hickin, 1969). The streambed is then dominated by a series of fairly regular, alternating side shoals with the thalweg meandering between (Plate IB). When the flash floods are followed by short-lived rainstorms, which prolong the abatement stage and produce small surges, breaching and erosion of the side shoal occur, yielding a pattern of irregular shoal remnants and "remnant" middle bars (Plate I, C, D). Meander bars and megaripples are nearly always accompanied by penecontemporaneous "parasitic" small-scale ripples and local current lineation. The distribution of the small-scale structures along the meandering pattern sheds some light on the details of the bed molding process. Plates IE, F show a stoss side of a bar, deformed and scoured by flow retreating laterally towards the alternate scours. The penecontemporaneous ripples are dissected and locally replaced by rill marks, current lineation and occasionally also by newly formed ripples, whereas the lee faces of the bars are modified by small fan-like slip faces built up of the scoured-out material (Plate IG). Plate IH shows a new generation of ripples formed along the developing meandering thalweg. The side shoal carries somewhat obscured penecontemporaneous ripples, whereas along its edges, ripples of a later generation are disposed. It appears that during abatement and shrinkage of flow, water drains towards the side scours laying bare the tops of the sigmoidal undulations. The emerged tops are inert from then on, and form the shoals, whereas the surrounding flow continues to scour and remold the submerged parts. The sediment is carried along the path meandering between the shoals, reshuffling the bed material and smoothening the pre-existing bed relief.
PLATE I Meander bars along the main channels in the southern Coastal Plain of Israel. (Flow towards reader.) A. Asymmetrical sigmoidal pattern of meander bars, asymmetry presumably related to the bend upstream. The crests were slightly eroded during the abatement stage. B. A pattern of side shoals with the thalweg meandering between. C. Side shoals and r e m n a n t bars after a prolonged flash flood, Width of channel 8 m. D. A breached and dissected side shoal. Width of shoal 4.5 m. E. Internal structure of a m e a n d e r bar which carries superimposed rill marks. F. A stoss side of a m e a n d e r bar, scoured and rilled by flow retreating towards the alternate scours. G. Small fan of scoured-out material, advancing towards the center of a side scour. H. Ripples of a later generation disposed around a side shoal with earlier ripples on its surface. Width of side shoal 2.8 m.
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Megaripples and ripples Trains of regular megaripples appear mainly along the deeper parts of the confined wadi channels in the Coastal Plain. Locally they are found also within channel bends (close to the concave banks) and along parts of the meandering thalweg (Plate II, A-C). The height ranges from 0.1 to 0.3 m and spacing from 0.75 to 1.8 m. In sections the megaripples are cross-stratified (Plate IIE); the individual layers are slightly concave and asymptotic to the base. Some clayey layers thicken towards the troughs. Plate IIA shows a fairly typical transverse megaripple pattern. The crests are normal to the banks of the channel and extend across its entire width. Though the overall geometry may be regarded as two-dimensional, small-amplitude, spanwise perturbations are evident. Flow-aligned spurs (Allen, 1968a) extend along the stoss faces and frequently taper upstream (Plate IID). In a cross-section the spurs are sharply cuspate. These and similar subsidiary elements are not superimposed on the primary two-dimensional pattern due to some change in flow. They appear to represent an essential feature of the instability mechanisms operating in generation of the seminal ripple and megaripple patterns, though the explanations offered differ in details (Allen, 1968a; 1969 ; Karcz, 1970). Persistent patterns of regular three-dimensional megaripples in the sense of Allen (1968a) are rare. On several occasions however, forms related to the somewhat controversial "elliptical scours" described in the literature (see Harms and Fahnestock, 1965; Allen, 1968a) were encountered. They include flow-aligned, partly or completely infilled hollows, which range in depth from 0.4 to 0.8 m, in length from 1 to 3 m and in width from 0.4 to 1 m. These oval, elliptical or ogiveshaped scours are thatched or appear as isolated elements scattered along the streambed (Plate IIF, G). The origin of these structures, which in the geological literature is usually related to the origin of the trough-type cross-stratification, was attributed to a migration of ripples containing enclosed hollows within their geometrical pattern (see Allen, 1968a). Another hypothesis assumed a "scour and fill" mechanism, in which "kolk"-like eddy activity (Matthes, 1947) induces scour and the resulting hollows are filled by the advancing bed material. The morphology
PLATE II Megaripples in the main wadi channels. A. Transverse, two-dimensional megaripples with parasitic ripples. Flow from upper right. Width of channel 4 m. B. Oblique megaripples in a channel bend. Flow towards the reader. Heightofmegaripple20cm. C. Megaripples along the meandering thalweg. M megaripples; S S - - side shoals; R B - - remnant bar. Flow from upper left. Width of channel 6 m. D. Flow-aligned ridges (spurs) across the stoss faces of transverse megaripples. Flow from upper right. Height o f megaripple 25 cm. E. Internal cross-stratification of a transverse megaripple, covered by mud. Height of megaripple 30 cm. F. Elliptical scour, sediment infilled. Flow from right to left. G. Nested elliptical scours formed by flow from upper left.
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
PLATE II
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of the elliptical scours, and the accounts of their associated turbulence patterns (Coleman, 1969) suggest that they are the product of a three-dimensional instability configuration, presumably dominated by hairpin-shaped vorticity systems extending across the flow, with secondary currents towards and away from the boundary (Karcz, 1970). Megaripples are nearly always covered by penecontemporaneous smallscale current ripples (Plate IIA, D) and local backflow ripples appear in their lees (Plate IIIA). Small-scale ripples are almost ubiquitous and appear either in association with other structures or as the only element of bed sculpture. Along many of the main wadi channels, patterns of small-scale ripples extend for many kilometers without significant change in shape or dimensions. Along the wadi plains in the Negev, the geometry of ripples produced by the unconfined flow is more variable due to rapid changes in bed relief, bed material and conditions of flow. Ripple-spacing ranges from 5 to 25 cm, the height from 0.5 to 3.5 cm and the index usually lies between 10 and 20. The crests are aligned transverse to the flow with varying degrees of sinuosity, which transforms the two-dimensional pattern into a sinuous, catenary, linguoid and lunate configuration (Plate III, B-D). Some spurs are very strongly developed and produce a marked longitudinal alignment in addition to the transverse one (Plate IIIC). Often internal cross-lamination is evident, and sections and cuts in the channel bed and its banks reveal several types of climbing ripple bedding (Jopling and Walker, 1968) which occur with no apparent regularity (Plate IIIE, F). This lack of ordered sequences of ripple types is due to the variability between individual floods, as well as due to the uneven surges and pulses within single floods. Such surges and the reshuffling of the bed material during the abatement often result in strongly interfering ripple patterns, and cause the appearance side by side of two or three generations of current ripples (Plate IIIG, H). Harrow marks and current lineations
Harrow marks (sand ribbons) are patterns of flow-aligned alternating ridges and troughs in silt, sand and gravel (Karcz, 1967). They are very c o m m o n along the unconfined wadi reaches in the Negev in which short-lived conditions of sheetflow prevail and where they represent a predominant feature of the bed sculpture (Plate IV, A, B). PLATE III Ripple patterns along ephemeralchannels. A. Backflowripples in the lee of a megaripple. B, C, D. Three-dimensionalpatterns of ripples. Flow towards the reader. E, F. Climbing ripples in cuts in the channel bed. G. Interfering ripples after the abatement of flow. The primary flowwas towards the reader. H. Three generations of ripples along the streambed, marking the stages of abatement.
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Along the wadi channels in the Coastal Plain harrow marks are rare and always appear in association with other structures. N o connection exists between the appearance of the harrow marks and the channel sinuosity, but the patterns usually adapt themselves to any local changes in direction of flow and often show deflections and bends of up to 135 ° . On the whole the patterns are regular and continuous, and individual elements may sometimes extend for distances of well over 100 m. The ridges, remarkably uniform within any individual pattern, range in height from 1 to 10 cm and in spacing from 3 to 50 cm. Superposition of harrow marks on a pre-existing bed relief is very common. Plate IVC shows harrow marks on a rippled bed, and Plate IVD a superposition on a stoss side of a meander bar. Locally the superimposed ridges carry still later ripples (Karcz, 1967), and also minute obstacle marks, which formed during the last stages of the flow. Current lineations are relatively rare, and appear only sporadically along the sandy streambeds of the wadi channels in the Coastal Plain, along the stoss sides of ripples, megaripples and meander bars (Plate IVE). The origin of these flow-aligned patterns was attributed to a transverse instability of flow (Allen, 1964, 1968a; Karcz, 1967). The parent flow configuration envisaged for harrow marks consists of longitudinal vortex rolls with alternating sense of rotation, whereas the current lineation is related to the analogous "streak patterns" observed by Kline et al. (1967) in the wall region of turbulent flow (Allen, 1969b; Karcz, 1970).
Obstacle marks Obstacle marks (Richardson, 1968; Karcz, 1968), result from the deformation of flow around obstacles within the streampath. Such structures are very c o m m o n along the pebble and boulder-lined streambeds of the Negev. They include current crescents, sand shadows and composite shadows, which in many places dominate completely the bed sculpture (Plate IVF). Dimensions of the obstacle marks are proportional to the height and width of the parent obstacles PLATE IV Flow-aligned sedimentary structures along the ephemeral streambeds. A. Harrow marks in sand and gravel. Flow towards the reader. B. Harrowmarks in sand and gravel, showingthe segregation of finer and coarser material. Note the difference from the obstacle marks. Spacing 15 cm. C. Harrow marks superimposed on a transverse ripple pattern in silt and sand. D. Fine harrow marks superimposed on the stoss side of a meander bar. Flow divergesaway from the reader. E. Current lineation and minute obstacle marks superimposed on transverse current ripples. Flow towards the reader. F. Small-scale obstacle marks along a pebble-lined streambed. Flow from lower left. G. Rhomboid marks on a stoss side of a meander bar. Flow towards the reader. H. Rhomboid pattern in the making, obliterates transverse ripples in sand and fine gravel (the dark lines are wheel tracks of a jeep). Flow towards the reader.
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and range from 0.005 m to about 14 m. Along the streambeds of the Coastal Plain, the obstacle marks are rare and appear either around some shrubs and boulders, or on a minute scale (0.3-1.5 cm) around tiny irregularities of the streambed. The latter appear in the very last stages of the abating flow, and often disclose the structure of the vanishing flow pattern (Fig. 7).
Fig.7. Minute obstacle marks, which reveal the orientation of the flow pattern in the last stage of the vanishing flood.
Rhomboid marks Rhomboid-mark patterns were widely described from recent beach deposits (Hoyt and Henry, 1963; Otvos, 1965), however, many authors indicated that they might appear also in terrestrial channels. For that matter, rhomboid marks appear quite commonly along the roadside gutters after rainstorms heavily laden with dust, and are related to the criss-cross patterns of shear waves on the gutter flow surface. Plate IVG shows a diamond-shaped pattern superimposed on the stoss side of a meander bar. The individual rhombs are somewhat irregular and are elongated parallel to the flow. Thei~ outlines, especially downcurrent, are marked by a slight drop in height and by a somewhat coarser material. Similar rhomboid patterns were encountered along some sandy stretches in the Negev, either along a smooth bed or even replacing ripples. Plate IVH shows a rhomboid pattern in the making, presumably aided by small pebbles along the bed and thus related to obstacle mark patterns. The origin of rhomboidal marks was attributed to the oblique hydraulic jumps formed around resistant spots along the streambed or to an interference of crossing wave fronts in the swift sheet flow on a beach slope. The recent accounts of origin and behaviour of diagonal shear waves presented by Kennedy and Roubillard (1967) and Kennedy and Iwasa (1968), as well as the analysis presented by Chang and Simons (1970) suggest that at Froude numbers somewhat in excess of 1 the rhomboid pattern may represent the dominant bed form.
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Washouts and remnant bars These two types of structures are related to obstacle marks and reflect the flow abatement. In a diminishing flow, megaripples and meander bars emerge gradually and obstruct the stream. The flow deviates, climbs or surrounds the crest and enters the adjoining trough from the side. When the remaining flow is weak the megaripple is unaffected and the change in flowpath is reflected (if at all) only by the minute obstacle marks close to the crest and in the lee of the megaripple (Plate VA). When the flow is stronger, water jets into the trough from the side, scouring the slip face and eroding at least a part of the ridge. The resulting hollow is oval or circular and often bears evidence to the flow pattern within (Plate VB, C). Along a smooth but somewhat uneven streambed, rapid abatement exposes some higher parts (Plate V). The deflected flow surrounds and scours the emerged remnants, which rapidly acquire a quasi-equilibrium rhomboidal or elliptical shape similar to that of many middle bars described in literature. Geometry of the flow deformation around such remnant bars is occasionally reflected in the pattern of surrounding ripples (Plate VE). Along some of the remnants patchy current lineation appears, which is formed during the short-lived sheet flow over the remnant, immediately prior to its emergence.
Mud pebbles and mud cracks Mud pebbles are a common occurrence in fluviatile deposits (Ball, 1940). In the present study these pebbles were found mostly along the streambeds of the main wadi channels in the Coastal Plain and within their banks. They originate mostly in undercutting and in mudcrack-controlled collapse of the clayey banks and terraces (Karcz, 1969). During drought periods following flash floods, scree slopes frequently form in the lee of the channel banks (Plate VF). The scree consists of poorly rounded mud pebbles of a high sphericity, which diminish in size away from the banks. The pebbles are buried in situ, occasionally leading to the appearance of mud cones (Karcz, 1969, fig.8) or are swept downstream (Plate VG). The shape, size and consistency of the mud pebbles are a function of duration of exposure, composition, and of the number of successive cycles of wetting and drying by the intermittent rain showers, rather than a function of transport. Mud cracks develop also within the muddy pavements which occasionally form in the last stages of a flood. This relatively thin crust (0.5-3.5 cm) of silt and claY desiccates and crumbles away. Where the underlying sand forms a plane bed and the crust is thin, much of the cracked pattern is obliterated after a week or so. However, when the underlying bed is rippled and the mud thickens towards the troughs, desiccation and crumbling controlled by the sharp edges of the crests and spurs lead to striking polygonal mud patterns (Plate VH) which remain preserved along the bed for extensive periods (Karcz and Goldberg, 1967).
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Other structures A m o n g the sporadically occurring structures, rill marks, formed along and across the streambed in the last stages o f shrinking flow, are most common. They are prominent especially where they cross meander bars and sections of abandoned thalweg. Convolute lamination was encountered in several places, either in association with ripple trains in which the ripples are upwarped, or amongst horizontally laminated layers. On the whole, the convolutions appear to be much rarer than in flood deposits described by McKee et al. (1967). Imbrication of pebbles and boulders is fairly c o m m o n along the streambeds in the Negev, but was not studied in detail. DISCUSSION The paper is essentially descriptive and discussion is restricted to a few brief comments. Allen (1968b) has recently emphasized the hierarchical structure of flow systems, which often determines the provenance and orientation of coexisting bed forms and results in the appearance of bed form hierarchies. In a flash-floods environment the hierarchical ordering is not pronounced, and the assemblages of coexisting structures reflect mainly the changes in structure and energy of the abating flow. These continuous changes often are rapid and lead to heterogeneous and interfering associations o f structures, which may be complicated further by fresh upsurges o f flow (due to subsequent rainstorms) or by a pronounced bed relief. It is prefererable therefore, to think, in this case, in terms o f bed form associations rather than in terms o f hierarchies. The associations encountered in the present study are summarized below.
Uniform associations Bed relief is dominated by bed forms o f one type only, though variations in PLATE V Washouts, remnant bars and mud pebbles. A. Deformation of a part of a megaripple by the diverted shrinking flow. The change in direction of flow is demonstrated by the minute obstacle marks. B, C. Washouts in the lee of megaripples, obliterating a greater part of the ridge. D. Remnant bar in the stream center, showingprogressivescour by the retreating flow.Width of channel 6 m. E. Deformation of flow in front of a remnant bar and the resulting ripple pattern. Flow towards the reader. F. Scree of mud pebbles in the lee of a channel bank two weeks after the flash flood. Height of bank 2m. G. Mud pebbles embedded along the cross-strata in a trough cross-set. H. Polygonalpattern formed after desiccation of a mud-coveredrippled sand. The thin crust along the stoss sides of the ripples crumbled away, whereas the thicker mud along the troughs and in the lee of spurs remained. Photograph taken four weeks after the flood.
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1. KARCZ
dimensions and geometry are common. The encountered patterns are : (a) rippleconfiguration, which is very common both along the wadi channels of the Coastal Plain and along the stretches of unconfined flow along the main wadis of the Negev; it includes also patterns of interfering ripples; (b) harrow marks; and (c) obstacle mark patterns, both of which are common in the Negev but rare along the channels in the Coastal Plain.
Heterogeneous associations In such configurations two or more types of sedimentary structures appear together along the streambed. The encountered assemblages include: (a) Associations in which small-scale bed forms are carried by larger ones of a similar orientation. Such structures may form contemporaneously according with the hierarchy principle of Allen (1968b) (see also the "dunes and ripples" stage of sediment transport of Simons et al., 1965), or may be superimposed by a gradually waning flow of unchanged orientation. Here belong ripple-carrying megaripples and bars, wich are common only in the Coastal Plain, and the much rarer bars, megaripples, and ripples carrying current lineations. (b) Associations, which form as a result of a rapid change in flow. Usually the evidence of superposition is clear, even when the change in orientation is small. Most common of such configurations are bars, megaripples and ripples partly obliterated and replaced by harrow marks, rhomboidal marks or obstacle marks. Less common are ripples with superimposed harrow marks, which in turn carry superimposed ripples and obstacle marks. (c) More complex associations arise when the change in conditions and in volume of flow are rapid, and the primary bed relief determines the orientation of the shrinking flow. Here belong bars (usually with parasitic ripples) with accompanying alternate scours, scoured and deformed by one or more of the following: harrow marks; current lineations; ripple marks; rill marks; rhomboid marks and obstacle marks. Such associations were encountered only along the main wadi channels. The superposition, interference and replacement of one bed form by another, which are characteristic of the flash-floods regime, lead to yet another question. Allen (1968a, b) classified the alluvial bed forms according to the geometry of their parent flow-instability patterns. The coexistence and superposition of bed forms related to different instability patterns suggest, that such flow configurations are genetically related and appear in an ordered sequence. The recent advances in study of hydrodynamic stability indicate that this indeed is the case (Stuart, 1965; Tani, 1969), and that instability is not a catastrophic event, but a gradual process which consists of a series of stages, each with a characteristic structure of flow. The orderly succession of the flow configurations, which proceeds from two-dimensional to three-dimensional wave pattern, and then on to flow-aligned vortex rolls and to hairpin-shaped migrating systems of turbulent motion, should
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
181
be kept in mind if classification of bed forms according to their instability patterns is to be fully meaningful. ACKNOWLEDGEMENTS
I wish to thank G. M. Friedman, J. P. B. Lovell, I. Perath and T. Scoffin for their comments and criticism. The access to the records and files of the Israel Hydrological Service is gratefully acknowledged. REFERENCES Allen, J. R. L., 1964. Primary current lineation in the Lower Old Red Sandstone (Devonian) AngloWelsh basin. Sedimentology, 3:89-108. Allen, J. R. L., 1968a. Current Ripples. North-Holland, Amsterdam, 433 pp. Allen, J. R. L., 1968b. The nature and origin of bed form hierarchies. Sedimentology, 10:161-182. Allen, J. R. L., 1969a. On the geometry of current ripples in relation to stability of fluid flow. Geogr. Ann., 51A: 61-96. Allen, J. R. L., 1969b. Erosional current marks of weakly cohesive mud beds. J. Sediment. Petrol., 39:607 623. Ball, M. S., 1940. Armored mud balls, their origin and role in sedimentation. J. Geol., 48 : 1-31. Chang, H. Y. and Simons, D. B., 1970. The bed configuration of straight sand bed channels when flow is nearly critical. J. Fluid Mech., 42:491495. Coleman, J. M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3 : 129-239. Dalinsky, Y., 1969. Silting in the Shiqma Reservoir. Tahal Water Planning for Israel Ltd., Int. Rept. PM 723, 7 pp. (in Hebrew). Food and Agriculture Organization of the United Nations, 1967. Pilot Project in Watershed Management on the Nahal Shiqma, Israel, 2. F.A.O., Rome, 266 pp. Harms, J. C. and Fahnestock, R. K., 1965. Stratification, bed forms and flow phenomena (with an example from the Rio Grande). In: G. V. Middleton (Editor), Primary Sedimentary Structures and their Hydrodynamics Interpretation-Soc. Econ. Paleontol. Mineral., Spec. Publ., 12: 84-115. Hickin, E. J., 1969. A newly identified process of point bar formation in natural streams. Am. J. Sci., 267:999-1010. Hoyt, J. H. and Henry, V. J., 1963. Rhomboid ripple mark, indicator of current direction and environment. J. Sediment. Petrol., 33:604-608. Israel Meteorological Service, 1967. Climatological Standard Normals of Rainfall, 1931-1960. Israel Meteorological Service, Beit Dagan, pp. I/1-II/17. Jopling, A. V. and Walker, R. G., 1968. Morphology and origin of ripple-drift cross-lamination with examples from Massachusetts. J. Sediment. Petrol., 38:971-984. Karcz, I., 1967. Harrow marks, current aligned sedimentary structures, J. Geol. 75:113-121. Karcz, I., 1968. Fluviatile obstacle marks from the wadis of the Negev (southern Israel). J. Sediment. Petrol. 38 : 1000-1012. Karcz, I., 1969. Mud pebbles in a flash floods environment. J. Sediment. Petrol., 39: 333-337. Karcz, I., 1970. Possible significance of transition flow patterns in interpretation of some natural bed forms. J. Geophys. Res. 75:2869-2873. Karcz, I. and Goldberg, M. 1967. Ripple-controlled desiccation patterns from Wadi Shiqma, southern Israel. ). Sediment. Petrol., 37:1244-1245. Kennedy, J. F. and lwasa, Y., 1968. Free surface shear flow over a wavy bed. J. Hydraul. Div. A.S.C.E., 94: 431 4 5 4 . Kennedy, J. F. and Roubillard, L., 1967. Some experimental observations on free surface shear flow over a wavy boundary. Proc. 12th Congr. Int. Assoc. Hydraul. Res., 1:4148.
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Kline, S. J., Reynolds, W. C., Schraub, F. A. and Runstadtler, P. W., 1967. The structure of turbulent boundary layers. J. Fluid Mech., 30:741-775. Matthes, G. H., 1947. Maeroturbulence in natural stream flow. Trans. Am. Geophys. Union, 28:255262. McKee, E. D., Crosby, E. J. and Berryhill, Jr., H. L., 1967. Flood deposits, Bijou Creek, Colorado, June 1965. J. Sediment. Petrol., 37:829-851. Otvos, E. G., 1965. Types of rhomboid beach surface patterns. Am. J. Sci. 263:271-276. Richardson, P. D., 1968. The generation of scour marks near obstacles. J. Sediment. Petrol., 38:965970. Shen, H. W. and Komura, S., 1968. Meandering tendencies in straight alluvial channels. J. Hydraul. Div., A.S.C.E., 94:997-1016. Simons, D. B., Richardson, E. V. and Nordin, C. F., 1965. Sedimentary structures generated by flow in alluvial channels. In: G. V. Middleton (Editor), Primary Sedimentary Structures and their Hydrodynamic lnterpretation--Soc. Econ, Paleontol. Mineral., Spec. Publ., 12:34-52. Stuart, J. T., 1965. Hydrodynamic stability. Appl. Mech. Rev., 18:523-531. Tahal Consulting Engineers Ltd., 1970a. Ramat Hanegev-Floodwaters Utilization Study. Int. Rept. Tahal, A-0973, Tel Aviv, 10 pp. (in Hebrew). Tahal Water Planning for Israel Ltd., 1970b. Study of Clogging in Floodwater-Spreading Fields. Int. Rept. Tahal, HG-70/32, Tel Aviv, 33 pp. (in Hebrew). Tani, I., 1969. Boundary layer transition. In: W. R. Sears and M. van Dyke (Editors), Annual Reviews of Fluid Mechanics, 1. Annual Reviews, Palo Alto, Calif., pp. 169-196.
SEDIMENTARY STRUCTURES F O R M E D BY FLASH FLOODS IN SOUTHERN ISRAEL IAAKOV KARCZ 1
Geological Survey of Israel, Jerusalem (Israel) (Received December 15, 1970,
ABSTRACT Karcz, I., 1972. Sedimentary structures formed by flash floods in southern Israel. Sediment. Geol., 7 : 161 182. During the rainy season the ephemeral streamcourses of southern Israel are swept by flash floods of low volume and short duration, which produce meander bars, megaripples and ripples, harrow marks, current lineations, obstacle marks, washouts and remnant bars, rhomboid marks, rill marks, rare convolute lamination, imbrication, mud pebbles and polygonal desiccation patterns. The rapid abatement of floods results in a variety of coexisting, interfering and superimposed associations of bed forms, which shed light on some aspects of provenance and origin of alluvial bed forms.
INTRODUCTION AND GENERAL SETTING
This paper summarizes a study of sedimentary structures produced by individual flash floods along the main ephemeral streamcourses (wadis) in the Negev, and the southern Coastal Plain of Israel (Fig. 1). Under a flash-flood regime, conditions of varied flow prevail and the resulting assemblages of sedimentary structures differ somewhat from those encountered in a perennial fluvial environment. In the southern Coastal Plain of Israel, the drainage pattern is dominated by several main streamcourses running down from the Judea-Hebron foothills. Commonly, dendritic patterns develop and the individual wadi catchments are dissected by close-spaced, rapidly headward-advancing trapezoidal gullies, which provide a major source of the sediment load. Streambeds consist mainly of moderately sorted, fine to medium-grained sand, but silty muds prevail in areas rich in loess and alluvial soils. The main part of the Negev drainage system is controlled by a series of gentle, northeast-southwest aligned folds; whereas on the east the network is determined mainly by the Jordan-Dead Sea rift valley, and the tilted blocks and Present address : Department of Geology, State University of New York, Binghamton, N.Y. 13901, U.S.A.
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SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
163
fractures along its edges. The run-offis carried mainly along the wide (6-7 km) and flat-bottomed, synclinal alluvial valleys, separated by anticlinal ridges. The floods, low in volume, occupy but a minor part of the valleys, and their tortuous or braided paths change not only from year to year, but also from one flood to another. Streambeds are coarse, alluvium commonly having 40-65~o material coarser than 5 mm. Many gravelly stretches are lined with a 5 4 0 cm thick veneer of medium to coarse sand with protruding calcareous or chert pebbles and boulders. The sand is generally covered by a 0.5-2 mm crust of clay which hardens and protects the various elements of bed sculpture from wind erosion.
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The climate in southern Israel is arid to semi-arid. The average annual rainfall increases gradually northwards, from 25 mm at the GulfofElat to 350-400 mm in the southern Coastal Plain (Fig.2). Precipitation occurs between October and May, in a highly varying number of low-intensity frontal rainstorms. Rain intensity is also fairly low; 50-60~o of rain days in southern Israel have falls of less than 5 mm/day, and only 10~o of the hourly amounts recorded exceed 5 mm (Israel Meteorological Service, 1967; F.A.O., 1967). This suggests that catastrophic floods are improbable. On the other hand, surface runoff in this region is facilitated by favourable topographic conditions along the drainage system, by scarcity of vegetation and by hardpan crusts on loess-soils which reduce their surface permeability• The annual runoff is highly variable. Unfortunately, the number of hydrometric stations in southern Israel has been very small, and detailed information about surface runoff is scarce. Long-range records are lacking and reconstruction of flood series, and evaluation of reliable models for runoff prediction can not be attempted. Such data as cited below are based mainly on the records of Israel Hydrological Service, and some information gathered by the Tahal (Water Planning for Israel, Ltd.) and F.A.O. teams (F.A.O., 1967 ; Dalinski, 1969; Tahal, 1970a, b). In conformity with the precipitation pattern, the annual frequency and volume of individual floods are highly variable. In the southern Coastal Plain (Nahal
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165
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
Shiqma) there are 3-10 floods per year with volumes ranging from about 0.03 to 8 million m 3 per flood. In the northern Negev (for example, Nahal Besot and Nahal Lavan) there are 0-7 floods per year and the flood volume ranges from 0.05 to 2.8 million m 3, whereas further south in Nahal Tsin there are only 0-4 floods per year with volumes from 0.05 to 1.8 million m 3. Flood duration is short and decreases southwards (Fig.3). At Nahal Tsin, for example, 70~ of the floods last for less than 12 h. Floods build up very rapidly and usually the peak flow lags only slightly behind peak rainfall, especially where the antecedent moisture is high. Conditions of peak flow usually develop within an hour or two from the start of the flood, and are followed by a rapid deceleration and abatement (Fig.4). The latter are striking especially along the wide valleys of the Negev, where the rapidly spreading, low-volume flood results in a short-lived sheet flow. Occasionally, abating floods are strengthened by subsequent rainstorms, which may be expressed on the limnigrams as small surges or by new prominent peaks, and serve to extend the duration of the receding stage (Fig.4). Discharge at peak flow may reach 500-1,000 m3/sec range, but usually remains well below 100 m3/sec. Peak velocity occasionally attains 5-6 m/sec and the flow depth 2.5-3 m. Such conditions last briefly, and during most of the receding stage the depth is below 0.5 m and velocity is within 10-80 cm/sec range. b
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166
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Information about bed load is even scarcer. Sporadic tests by the Hydrological Service indicate contents ranging from 0 to 86 g/1. F.A.O. (1967) suggests an average content of suspended material of about 10 15 g/l, which agrees fairly well with computed values based on the observed silting of floodwater reservoirs in the Coastal Plain and northernmost Negev (F.A.O., 1967; Tahal, 1970a, b). Analysis of suspended material from Nahal Besor and Nahal Shiqma (Tahal, 1970a) indicates that 60-70~ are finer than 2/~ and consist of montmorillonite and some kaolinite and illite. No further information is available about the traction load and actual sediment movement and distribution during individual floods. SEDIMENTARY STRUCTURES
The rapid alternations of floods and drought during the rainy season provide a natural laboratory for the study of streambed relief. The present examination was restricted to elements produced by single-flash floods, including: (1) meander bars; (2) megaripples and ripples; (3) sand ribbons (harrow marks) and current lineation; (4) obstacle marks; (5) washouts and remnant bars; (6) rhomboid marks; (7) occasional rill marks, convolute lamination and imbrica-
Fig.5. A sigmoidal pattern o f meander bars along Nahal Shiqma, sketched from a photograph. The channel width is about 10 m. S C = sigmoidal crests of meander bars; A S ~ alternate scours; R = parasitic ripples.
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
167
tion; (8) mud cracks, mud pebbles and polygonal desiccation patterns. Observations were made after the flow vanished or during the last stages of the abatement, so that most comments on formation of these structures are necessarily interpretative.
Meander bars Sigmoidal and transverse systems of large-scale bed undulations are locally encountered along the main wadi channels. Length/height ratio ranges from 25 to about 120. The individual undulations are strongly asymmetrical. The lee sides are 0.15-0.45 m long and dip at 2 0 - 30° (somewhat steeper where covered by a veneer of mud), whereas the stoss faces are up to 10 m long and dip at less than 2°. Some crests are normal to the channel banks, but most are aligned in a roughly sigmoidal pattern (Fig.5). In sigmoidal patterns, crests are skewed at 25-60 ° to the banks and extend across the channel. Triangular or elliptical scour hollows ranging in depth from several centimeters to half a meter, commonly appear at alternate positions along the banks just below the point where the crest approaches the bank. Individual crests are straight, or convex downstream, with the apex alternately closer to one of the banks. In such forms the configuration resembles two rows of asymmetrically lobate bars aligned at offset along the channel (Fig.6A). The bars usually have a considerable lateral slope away from the lobe.
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Along curved channel stretches, the sigmoidal patterns are often distorted and asymmetrical. Roughly "s" and "z"-shaped alignments with a limb ratio of 1.5-3.5 commonly appear to the downstream of left and right hand bends respectively (Plate IA).
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SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
169
Rapid abatement of flash floods appears to favour preservation of the rather delicate sigmoidal patterns. Occasionally, however, parts of the pattern are obliterated by superimposed smaller structures, mainly the harrow marks. Where flood duration is longer and abatement more gradual, deformation of the streambed advances beyond the sigmoidal configuration, and features of inner stream sinuosity develop, resembling those reported from many straight erodible river channels (Shen and Komura, 1968 ; Hickin, 1969). The streambed is then dominated by a series of fairly regular, alternating side shoals with the thalweg meandering between (Plate IB). When the flash floods are followed by short-lived rainstorms, which prolong the abatement stage and produce small surges, breaching and erosion of the side shoal occur, yielding a pattern of irregular shoal remnants and "remnant" middle bars (Plate I, C, D). Meander bars and megaripples are nearly always accompanied by penecontemporaneous "parasitic" small-scale ripples and local current lineation. The distribution of the small-scale structures along the meandering pattern sheds some light on the details of the bed molding process. Plates IE, F show a stoss side of a bar, deformed and scoured by flow retreating laterally towards the alternate scours. The penecontemporaneous ripples are dissected and locally replaced by rill marks, current lineation and occasionally also by newly formed ripples, whereas the lee faces of the bars are modified by small fan-like slip faces built up of the scoured-out material (Plate IG). Plate IH shows a new generation of ripples formed along the developing meandering thalweg. The side shoal carries somewhat obscured penecontemporaneous ripples, whereas along its edges, ripples of a later generation are disposed. It appears that during abatement and shrinkage of flow, water drains towards the side scours laying bare the tops of the sigmoidal undulations. The emerged tops are inert from then on, and form the shoals, whereas the surrounding flow continues to scour and remold the submerged parts. The sediment is carried along the path meandering between the shoals, reshuffling the bed material and smoothening the pre-existing bed relief.
PLATE I Meander bars along the main channels in the southern Coastal Plain of Israel. (Flow towards reader.) A. Asymmetrical sigmoidal pattern of meander bars, asymmetry presumably related to the bend upstream. The crests were slightly eroded during the abatement stage. B. A pattern of side shoals with the thalweg meandering between. C. Side shoals and r e m n a n t bars after a prolonged flash flood, Width of channel 8 m. D. A breached and dissected side shoal. Width of shoal 4.5 m. E. Internal structure of a m e a n d e r bar which carries superimposed rill marks. F. A stoss side of a m e a n d e r bar, scoured and rilled by flow retreating towards the alternate scours. G. Small fan of scoured-out material, advancing towards the center of a side scour. H. Ripples of a later generation disposed around a side shoal with earlier ripples on its surface. Width of side shoal 2.8 m.
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I. KARCZ
Megaripples and ripples Trains of regular megaripples appear mainly along the deeper parts of the confined wadi channels in the Coastal Plain. Locally they are found also within channel bends (close to the concave banks) and along parts of the meandering thalweg (Plate II, A-C). The height ranges from 0.1 to 0.3 m and spacing from 0.75 to 1.8 m. In sections the megaripples are cross-stratified (Plate IIE); the individual layers are slightly concave and asymptotic to the base. Some clayey layers thicken towards the troughs. Plate IIA shows a fairly typical transverse megaripple pattern. The crests are normal to the banks of the channel and extend across its entire width. Though the overall geometry may be regarded as two-dimensional, small-amplitude, spanwise perturbations are evident. Flow-aligned spurs (Allen, 1968a) extend along the stoss faces and frequently taper upstream (Plate IID). In a cross-section the spurs are sharply cuspate. These and similar subsidiary elements are not superimposed on the primary two-dimensional pattern due to some change in flow. They appear to represent an essential feature of the instability mechanisms operating in generation of the seminal ripple and megaripple patterns, though the explanations offered differ in details (Allen, 1968a; 1969 ; Karcz, 1970). Persistent patterns of regular three-dimensional megaripples in the sense of Allen (1968a) are rare. On several occasions however, forms related to the somewhat controversial "elliptical scours" described in the literature (see Harms and Fahnestock, 1965; Allen, 1968a) were encountered. They include flow-aligned, partly or completely infilled hollows, which range in depth from 0.4 to 0.8 m, in length from 1 to 3 m and in width from 0.4 to 1 m. These oval, elliptical or ogiveshaped scours are thatched or appear as isolated elements scattered along the streambed (Plate IIF, G). The origin of these structures, which in the geological literature is usually related to the origin of the trough-type cross-stratification, was attributed to a migration of ripples containing enclosed hollows within their geometrical pattern (see Allen, 1968a). Another hypothesis assumed a "scour and fill" mechanism, in which "kolk"-like eddy activity (Matthes, 1947) induces scour and the resulting hollows are filled by the advancing bed material. The morphology
PLATE II Megaripples in the main wadi channels. A. Transverse, two-dimensional megaripples with parasitic ripples. Flow from upper right. Width of channel 4 m. B. Oblique megaripples in a channel bend. Flow towards the reader. Heightofmegaripple20cm. C. Megaripples along the meandering thalweg. M megaripples; S S - - side shoals; R B - - remnant bar. Flow from upper left. Width of channel 6 m. D. Flow-aligned ridges (spurs) across the stoss faces of transverse megaripples. Flow from upper right. Height o f megaripple 25 cm. E. Internal cross-stratification of a transverse megaripple, covered by mud. Height of megaripple 30 cm. F. Elliptical scour, sediment infilled. Flow from right to left. G. Nested elliptical scours formed by flow from upper left.
SEDIMENTARY STRUCTURES FORMED BY FLASH FLOODS
PLATE II
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of the elliptical scours, and the accounts of their associated turbulence patterns (Coleman, 1969) suggest that they are the product of a three-dimensional instability configuration, presumably dominated by hairpin-shaped vorticity systems extending across the flow, with secondary currents towards and away from the boundary (Karcz, 1970). Megaripples are nearly always covered by penecontemporaneous smallscale current ripples (Plate IIA, D) and local backflow ripples appear in their lees (Plate IIIA). Small-scale ripples are almost ubiquitous and appear either in association with other structures or as the only element of bed sculpture. Along many of the main wadi channels, patterns of small-scale ripples extend for many kilometers without significant change in shape or dimensions. Along the wadi plains in the Negev, the geometry of ripples produced by the unconfined flow is more variable due to rapid changes in bed relief, bed material and conditions of flow. Ripple-spacing ranges from 5 to 25 cm, the height from 0.5 to 3.5 cm and the index usually lies between 10 and 20. The crests are aligned transverse to the flow with varying degrees of sinuosity, which transforms the two-dimensional pattern into a sinuous, catenary, linguoid and lunate configuration (Plate III, B-D). Some spurs are very strongly developed and produce a marked longitudinal alignment in addition to the transverse one (Plate IIIC). Often internal cross-lamination is evident, and sections and cuts in the channel bed and its banks reveal several types of climbing ripple bedding (Jopling and Walker, 1968) which occur with no apparent regularity (Plate IIIE, F). This lack of ordered sequences of ripple types is due to the variability between individual floods, as well as due to the uneven surges and pulses within single floods. Such surges and the reshuffling of the bed material during the abatement often result in strongly interfering ripple patterns, and cause the appearance side by side of two or three generations of current ripples (Plate IIIG, H). Harrow marks and current lineations
Harrow marks (sand ribbons) are patterns of flow-aligned alternating ridges and troughs in silt, sand and gravel (Karcz, 1967). They are very c o m m o n along the unconfined wadi reaches in the Negev in which short-lived conditions of sheetflow prevail and where they represent a predominant feature of the bed sculpture (Plate IV, A, B). PLATE III Ripple patterns along ephemeralchannels. A. Backflowripples in the lee of a megaripple. B, C, D. Three-dimensionalpatterns of ripples. Flow towards the reader. E, F. Climbing ripples in cuts in the channel bed. G. Interfering ripples after the abatement of flow. The primary flowwas towards the reader. H. Three generations of ripples along the streambed, marking the stages of abatement.
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Along the wadi channels in the Coastal Plain harrow marks are rare and always appear in association with other structures. N o connection exists between the appearance of the harrow marks and the channel sinuosity, but the patterns usually adapt themselves to any local changes in direction of flow and often show deflections and bends of up to 135 ° . On the whole the patterns are regular and continuous, and individual elements may sometimes extend for distances of well over 100 m. The ridges, remarkably uniform within any individual pattern, range in height from 1 to 10 cm and in spacing from 3 to 50 cm. Superposition of harrow marks on a pre-existing bed relief is very common. Plate IVC shows harrow marks on a rippled bed, and Plate IVD a superposition on a stoss side of a meander bar. Locally the superimposed ridges carry still later ripples (Karcz, 1967), and also minute obstacle marks, which formed during the last stages of the flow. Current lineations are relatively rare, and appear only sporadically along the sandy streambeds of the wadi channels in the Coastal Plain, along the stoss sides of ripples, megaripples and meander bars (Plate IVE). The origin of these flow-aligned patterns was attributed to a transverse instability of flow (Allen, 1964, 1968a; Karcz, 1967). The parent flow configuration envisaged for harrow marks consists of longitudinal vortex rolls with alternating sense of rotation, whereas the current lineation is related to the analogous "streak patterns" observed by Kline et al. (1967) in the wall region of turbulent flow (Allen, 1969b; Karcz, 1970).
Obstacle marks Obstacle marks (Richardson, 1968; Karcz, 1968), result from the deformation of flow around obstacles within the streampath. Such structures are very c o m m o n along the pebble and boulder-lined streambeds of the Negev. They include current crescents, sand shadows and composite shadows, which in many places dominate completely the bed sculpture (Plate IVF). Dimensions of the obstacle marks are proportional to the height and width of the parent obstacles PLATE IV Flow-aligned sedimentary structures along the ephemeral streambeds. A. Harrow marks in sand and gravel. Flow towards the reader. B. Harrowmarks in sand and gravel, showingthe segregation of finer and coarser material. Note the difference from the obstacle marks. Spacing 15 cm. C. Harrow marks superimposed on a transverse ripple pattern in silt and sand. D. Fine harrow marks superimposed on the stoss side of a meander bar. Flow divergesaway from the reader. E. Current lineation and minute obstacle marks superimposed on transverse current ripples. Flow towards the reader. F. Small-scale obstacle marks along a pebble-lined streambed. Flow from lower left. G. Rhomboid marks on a stoss side of a meander bar. Flow towards the reader. H. Rhomboid pattern in the making, obliterates transverse ripples in sand and fine gravel (the dark lines are wheel tracks of a jeep). Flow towards the reader.
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and range from 0.005 m to about 14 m. Along the streambeds of the Coastal Plain, the obstacle marks are rare and appear either around some shrubs and boulders, or on a minute scale (0.3-1.5 cm) around tiny irregularities of the streambed. The latter appear in the very last stages of the abating flow, and often disclose the structure of the vanishing flow pattern (Fig. 7).
Fig.7. Minute obstacle marks, which reveal the orientation of the flow pattern in the last stage of the vanishing flood.
Rhomboid marks Rhomboid-mark patterns were widely described from recent beach deposits (Hoyt and Henry, 1963; Otvos, 1965), however, many authors indicated that they might appear also in terrestrial channels. For that matter, rhomboid marks appear quite commonly along the roadside gutters after rainstorms heavily laden with dust, and are related to the criss-cross patterns of shear waves on the gutter flow surface. Plate IVG shows a diamond-shaped pattern superimposed on the stoss side of a meander bar. The individual rhombs are somewhat irregular and are elongated parallel to the flow. Thei~ outlines, especially downcurrent, are marked by a slight drop in height and by a somewhat coarser material. Similar rhomboid patterns were encountered along some sandy stretches in the Negev, either along a smooth bed or even replacing ripples. Plate IVH shows a rhomboid pattern in the making, presumably aided by small pebbles along the bed and thus related to obstacle mark patterns. The origin of rhomboidal marks was attributed to the oblique hydraulic jumps formed around resistant spots along the streambed or to an interference of crossing wave fronts in the swift sheet flow on a beach slope. The recent accounts of origin and behaviour of diagonal shear waves presented by Kennedy and Roubillard (1967) and Kennedy and Iwasa (1968), as well as the analysis presented by Chang and Simons (1970) suggest that at Froude numbers somewhat in excess of 1 the rhomboid pattern may represent the dominant bed form.
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Washouts and remnant bars These two types of structures are related to obstacle marks and reflect the flow abatement. In a diminishing flow, megaripples and meander bars emerge gradually and obstruct the stream. The flow deviates, climbs or surrounds the crest and enters the adjoining trough from the side. When the remaining flow is weak the megaripple is unaffected and the change in flowpath is reflected (if at all) only by the minute obstacle marks close to the crest and in the lee of the megaripple (Plate VA). When the flow is stronger, water jets into the trough from the side, scouring the slip face and eroding at least a part of the ridge. The resulting hollow is oval or circular and often bears evidence to the flow pattern within (Plate VB, C). Along a smooth but somewhat uneven streambed, rapid abatement exposes some higher parts (Plate V). The deflected flow surrounds and scours the emerged remnants, which rapidly acquire a quasi-equilibrium rhomboidal or elliptical shape similar to that of many middle bars described in literature. Geometry of the flow deformation around such remnant bars is occasionally reflected in the pattern of surrounding ripples (Plate VE). Along some of the remnants patchy current lineation appears, which is formed during the short-lived sheet flow over the remnant, immediately prior to its emergence.
Mud pebbles and mud cracks Mud pebbles are a common occurrence in fluviatile deposits (Ball, 1940). In the present study these pebbles were found mostly along the streambeds of the main wadi channels in the Coastal Plain and within their banks. They originate mostly in undercutting and in mudcrack-controlled collapse of the clayey banks and terraces (Karcz, 1969). During drought periods following flash floods, scree slopes frequently form in the lee of the channel banks (Plate VF). The scree consists of poorly rounded mud pebbles of a high sphericity, which diminish in size away from the banks. The pebbles are buried in situ, occasionally leading to the appearance of mud cones (Karcz, 1969, fig.8) or are swept downstream (Plate VG). The shape, size and consistency of the mud pebbles are a function of duration of exposure, composition, and of the number of successive cycles of wetting and drying by the intermittent rain showers, rather than a function of transport. Mud cracks develop also within the muddy pavements which occasionally form in the last stages of a flood. This relatively thin crust (0.5-3.5 cm) of silt and claY desiccates and crumbles away. Where the underlying sand forms a plane bed and the crust is thin, much of the cracked pattern is obliterated after a week or so. However, when the underlying bed is rippled and the mud thickens towards the troughs, desiccation and crumbling controlled by the sharp edges of the crests and spurs lead to striking polygonal mud patterns (Plate VH) which remain preserved along the bed for extensive periods (Karcz and Goldberg, 1967).
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Other structures A m o n g the sporadically occurring structures, rill marks, formed along and across the streambed in the last stages o f shrinking flow, are most common. They are prominent especially where they cross meander bars and sections of abandoned thalweg. Convolute lamination was encountered in several places, either in association with ripple trains in which the ripples are upwarped, or amongst horizontally laminated layers. On the whole, the convolutions appear to be much rarer than in flood deposits described by McKee et al. (1967). Imbrication of pebbles and boulders is fairly c o m m o n along the streambeds in the Negev, but was not studied in detail. DISCUSSION The paper is essentially descriptive and discussion is restricted to a few brief comments. Allen (1968b) has recently emphasized the hierarchical structure of flow systems, which often determines the provenance and orientation of coexisting bed forms and results in the appearance of bed form hierarchies. In a flash-floods environment the hierarchical ordering is not pronounced, and the assemblages of coexisting structures reflect mainly the changes in structure and energy of the abating flow. These continuous changes often are rapid and lead to heterogeneous and interfering associations o f structures, which may be complicated further by fresh upsurges o f flow (due to subsequent rainstorms) or by a pronounced bed relief. It is prefererable therefore, to think, in this case, in terms o f bed form associations rather than in terms o f hierarchies. The associations encountered in the present study are summarized below.
Uniform associations Bed relief is dominated by bed forms o f one type only, though variations in PLATE V Washouts, remnant bars and mud pebbles. A. Deformation of a part of a megaripple by the diverted shrinking flow. The change in direction of flow is demonstrated by the minute obstacle marks. B, C. Washouts in the lee of megaripples, obliterating a greater part of the ridge. D. Remnant bar in the stream center, showingprogressivescour by the retreating flow.Width of channel 6 m. E. Deformation of flow in front of a remnant bar and the resulting ripple pattern. Flow towards the reader. F. Scree of mud pebbles in the lee of a channel bank two weeks after the flash flood. Height of bank 2m. G. Mud pebbles embedded along the cross-strata in a trough cross-set. H. Polygonalpattern formed after desiccation of a mud-coveredrippled sand. The thin crust along the stoss sides of the ripples crumbled away, whereas the thicker mud along the troughs and in the lee of spurs remained. Photograph taken four weeks after the flood.
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dimensions and geometry are common. The encountered patterns are : (a) rippleconfiguration, which is very common both along the wadi channels of the Coastal Plain and along the stretches of unconfined flow along the main wadis of the Negev; it includes also patterns of interfering ripples; (b) harrow marks; and (c) obstacle mark patterns, both of which are common in the Negev but rare along the channels in the Coastal Plain.
Heterogeneous associations In such configurations two or more types of sedimentary structures appear together along the streambed. The encountered assemblages include: (a) Associations in which small-scale bed forms are carried by larger ones of a similar orientation. Such structures may form contemporaneously according with the hierarchy principle of Allen (1968b) (see also the "dunes and ripples" stage of sediment transport of Simons et al., 1965), or may be superimposed by a gradually waning flow of unchanged orientation. Here belong ripple-carrying megaripples and bars, wich are common only in the Coastal Plain, and the much rarer bars, megaripples, and ripples carrying current lineations. (b) Associations, which form as a result of a rapid change in flow. Usually the evidence of superposition is clear, even when the change in orientation is small. Most common of such configurations are bars, megaripples and ripples partly obliterated and replaced by harrow marks, rhomboidal marks or obstacle marks. Less common are ripples with superimposed harrow marks, which in turn carry superimposed ripples and obstacle marks. (c) More complex associations arise when the change in conditions and in volume of flow are rapid, and the primary bed relief determines the orientation of the shrinking flow. Here belong bars (usually with parasitic ripples) with accompanying alternate scours, scoured and deformed by one or more of the following: harrow marks; current lineations; ripple marks; rill marks; rhomboid marks and obstacle marks. Such associations were encountered only along the main wadi channels. The superposition, interference and replacement of one bed form by another, which are characteristic of the flash-floods regime, lead to yet another question. Allen (1968a, b) classified the alluvial bed forms according to the geometry of their parent flow-instability patterns. The coexistence and superposition of bed forms related to different instability patterns suggest, that such flow configurations are genetically related and appear in an ordered sequence. The recent advances in study of hydrodynamic stability indicate that this indeed is the case (Stuart, 1965; Tani, 1969), and that instability is not a catastrophic event, but a gradual process which consists of a series of stages, each with a characteristic structure of flow. The orderly succession of the flow configurations, which proceeds from two-dimensional to three-dimensional wave pattern, and then on to flow-aligned vortex rolls and to hairpin-shaped migrating systems of turbulent motion, should
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be kept in mind if classification of bed forms according to their instability patterns is to be fully meaningful. ACKNOWLEDGEMENTS
I wish to thank G. M. Friedman, J. P. B. Lovell, I. Perath and T. Scoffin for their comments and criticism. The access to the records and files of the Israel Hydrological Service is gratefully acknowledged. REFERENCES Allen, J. R. L., 1964. Primary current lineation in the Lower Old Red Sandstone (Devonian) AngloWelsh basin. Sedimentology, 3:89-108. Allen, J. R. L., 1968a. Current Ripples. North-Holland, Amsterdam, 433 pp. Allen, J. R. L., 1968b. The nature and origin of bed form hierarchies. Sedimentology, 10:161-182. Allen, J. R. L., 1969a. On the geometry of current ripples in relation to stability of fluid flow. Geogr. Ann., 51A: 61-96. Allen, J. R. L., 1969b. Erosional current marks of weakly cohesive mud beds. J. Sediment. Petrol., 39:607 623. Ball, M. S., 1940. Armored mud balls, their origin and role in sedimentation. J. Geol., 48 : 1-31. Chang, H. Y. and Simons, D. B., 1970. The bed configuration of straight sand bed channels when flow is nearly critical. J. Fluid Mech., 42:491495. Coleman, J. M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3 : 129-239. Dalinsky, Y., 1969. Silting in the Shiqma Reservoir. Tahal Water Planning for Israel Ltd., Int. Rept. PM 723, 7 pp. (in Hebrew). Food and Agriculture Organization of the United Nations, 1967. Pilot Project in Watershed Management on the Nahal Shiqma, Israel, 2. F.A.O., Rome, 266 pp. Harms, J. C. and Fahnestock, R. K., 1965. Stratification, bed forms and flow phenomena (with an example from the Rio Grande). In: G. V. Middleton (Editor), Primary Sedimentary Structures and their Hydrodynamics Interpretation-Soc. Econ. Paleontol. Mineral., Spec. Publ., 12: 84-115. Hickin, E. J., 1969. A newly identified process of point bar formation in natural streams. Am. J. Sci., 267:999-1010. Hoyt, J. H. and Henry, V. J., 1963. Rhomboid ripple mark, indicator of current direction and environment. J. Sediment. Petrol., 33:604-608. Israel Meteorological Service, 1967. Climatological Standard Normals of Rainfall, 1931-1960. Israel Meteorological Service, Beit Dagan, pp. I/1-II/17. Jopling, A. V. and Walker, R. G., 1968. Morphology and origin of ripple-drift cross-lamination with examples from Massachusetts. J. Sediment. Petrol., 38:971-984. Karcz, I., 1967. Harrow marks, current aligned sedimentary structures, J. Geol. 75:113-121. Karcz, I., 1968. Fluviatile obstacle marks from the wadis of the Negev (southern Israel). J. Sediment. Petrol. 38 : 1000-1012. Karcz, I., 1969. Mud pebbles in a flash floods environment. J. Sediment. Petrol., 39: 333-337. Karcz, I., 1970. Possible significance of transition flow patterns in interpretation of some natural bed forms. J. Geophys. Res. 75:2869-2873. Karcz, I. and Goldberg, M. 1967. Ripple-controlled desiccation patterns from Wadi Shiqma, southern Israel. ). Sediment. Petrol., 37:1244-1245. Kennedy, J. F. and lwasa, Y., 1968. Free surface shear flow over a wavy bed. J. Hydraul. Div. A.S.C.E., 94: 431 4 5 4 . Kennedy, J. F. and Roubillard, L., 1967. Some experimental observations on free surface shear flow over a wavy boundary. Proc. 12th Congr. Int. Assoc. Hydraul. Res., 1:4148.
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Kline, S. J., Reynolds, W. C., Schraub, F. A. and Runstadtler, P. W., 1967. The structure of turbulent boundary layers. J. Fluid Mech., 30:741-775. Matthes, G. H., 1947. Maeroturbulence in natural stream flow. Trans. Am. Geophys. Union, 28:255262. McKee, E. D., Crosby, E. J. and Berryhill, Jr., H. L., 1967. Flood deposits, Bijou Creek, Colorado, June 1965. J. Sediment. Petrol., 37:829-851. Otvos, E. G., 1965. Types of rhomboid beach surface patterns. Am. J. Sci. 263:271-276. Richardson, P. D., 1968. The generation of scour marks near obstacles. J. Sediment. Petrol., 38:965970. Shen, H. W. and Komura, S., 1968. Meandering tendencies in straight alluvial channels. J. Hydraul. Div., A.S.C.E., 94:997-1016. Simons, D. B., Richardson, E. V. and Nordin, C. F., 1965. Sedimentary structures generated by flow in alluvial channels. In: G. V. Middleton (Editor), Primary Sedimentary Structures and their Hydrodynamic lnterpretation--Soc. Econ, Paleontol. Mineral., Spec. Publ., 12:34-52. Stuart, J. T., 1965. Hydrodynamic stability. Appl. Mech. Rev., 18:523-531. Tahal Consulting Engineers Ltd., 1970a. Ramat Hanegev-Floodwaters Utilization Study. Int. Rept. Tahal, A-0973, Tel Aviv, 10 pp. (in Hebrew). Tahal Water Planning for Israel Ltd., 1970b. Study of Clogging in Floodwater-Spreading Fields. Int. Rept. Tahal, HG-70/32, Tel Aviv, 33 pp. (in Hebrew). Tani, I., 1969. Boundary layer transition. In: W. R. Sears and M. van Dyke (Editors), Annual Reviews of Fluid Mechanics, 1. Annual Reviews, Palo Alto, Calif., pp. 169-196.