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Pipes and pipe flow process in an upland catchment, Wales
CATENA
Vol. 11, 145-158
Braunschweig 1984
PIPES AND PIPE FLOW PROCESS IN AN UPLAND CATCHMENT, WALES C.M. Wilson, Cambridge, and P.L. Smart, Bristol ABSTRACT The results of observations on the spatiallocalisation, origin and function of soil pipes in an upland catchment are discussed. The pipes occur in distinct zones in the brown earth soils of the lower slopes, and form a hydrological link conducting water between an upslope zone of highly permeable skeletal soils and the stream channel. It is proposed that in this catchment, pipes develop from an initial network of mole burrows, modified by hydraulic activity to produce an efficient downslope transmission network. A conceptual model for the pipe slope segment is proposed which recognises the importance of this transmission role. Slope discharge controls switching between saturated throughflow, pipe flow and overland flow, each of which has a specific threshold value for operation. Attempts to investigate the significance of pipe flow at the catchment scale met with limited success, but it appears to be important in increasing both contributing area and duration of storm flow.
1. INTRODUCTION Soil piping (JONES 1981) is well known from upland areas in Britain, where its origin and role in generation of storm flow have been the subject of previous research (GILMAN & NEWSON 1980, JONES 1978, 1982, McCAIG 1983). The alms of this study were threefold, firstly to investigate reasons for localisation of zones of piping within the catchment studied; secondly to establish a conceptual model describing the function of the pipe system, and finally to assess the significance of pipe flow as a process contributing to storm flow.
1.1. THE STUDY AREA The Cwm Llwch (GR SO005230) (Figure 1) drains northward from the scarp face of the Brecon Beacon Hills, 7 km south west of Brecon in South Wales, U.K. The catchment is underlain by impermeable rocks ranging in grain size from mudstones to grits (the Brownstones), but glacial deposition has produced a drift cover varying from 0 to over 20 metres deep. This drift cover rapidly thins towards the eastern and southern margins of the catchment. It is derived from the Brownstones and is compact, consisting of stones and boulders with a silty clay matrix. The stream network has incised into the superficial deposits to a depth varying from two meters in the upper reaches to twenty metres at the catchment outfall where bedrock is exposed. The backwall of the catchment is formed by the peak of Pen y Fan (886 m AOD) and is very steep with a conde lake and stand moraines at its base (MELLARD-READE 1895). Further down valley is an area of convexo - concave to planar slopes which stretch from the sides of the catchment to the incised channel. Resistant bands in the Brownstones produce bench features and many small waterfalls. ISSN 0341 - 8162 (~ Copyrighi lg84 by CATENA VERLAG, D - 3302 Cremlingen- Destedt, W. Germany
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Figure 1A shows the main soil types found in the catchment. Area A is characterised by thin stony or skeletal soils developed over scree, vestigial talus or morainic deposits. The average slope is 20° and these soils are free draining, stony, acid brown earths with a limited cover of wavy hair gra~s (Deschampsia flexuosa). Rocks are often exposed at the surface. Zone B is gently sloping (about 3°) and is characterised by waterlogging and bog peat in the core of the area with a peaty gley soil. In area C the ground slope is about 12°, the soil is well drained and heather and grass associations occur. The soil is relatively shallow, usually less than 0.6 m deep and consists of a stony brown earth with mor humus. Bilberry (Vaccinium myrtillus) is characteristic of this area. Zone D consists of an acid brown earth up to two metres deep with a mor humus of about 0.15 m depth. The slope ranges from 10 to 15°. Nardus stricta and Juncus squarrosus are characteristic of less free draining areas and Molinia caerulea gives a characteristic tussocky grass cover over most of this area. The long term regional rainfall for the adjacent Usk Valley (GR SN977262) is about 1600 m m per annum but rain gauges in the Cwrn Llwch catchment indicate that because of its altitude and exposure it may receive up to 33% greater rainfall in individual storms than this lower altitude station. Estimates of evaporation using Penman's method, and data from a meteorological station 5 km from the catchment (GR SN977262) indicated that monthly soil moisture deficits might occur in only two or three months a year on average. Daily
PIPE FLOW HYDROLOGY
147
evaporation estimates obtained from an evaporation tank at the same station indicated that even in the summer months evaporation losses are small and that typically, during storm events no significant evaporation occurs for several days after the cessation of the storm rainfall. The Cwm Llwch catchment with its higher altitude and increased rainfall and cloud cover must emphasise these characteristics. We have therefore an upland area whose hydrological regime is characterised by high inputs of water, low evaporation losses and limited groundwater storage. Consequently, there is frequent saturation in the soils of the catchment, giving rise to a flashy stream response dominated by quick-flow processes. It was during the study of these quick-flow processes (WILSON 1977) that the significance of pipe flow for water movement in the catchment became apparent.
1.2. CHARACTER OF THE SOIL-PIPES The pipes studied have been previously described in SMART & WILSON (1984). Briefly they comprise smooth walled approximately circular conduits about 6 cm in diameter, forming sinuous anastomosing bands in the upper horizons of the well drained brown earth soils of area D. These zones are frequently associated with seepage lines or slight hollows, and run downslope to the slope base. Using air photographic mapping validated by field inspection, it was found that some 3.5% of sub catchment W(Fig. 1A) was underlain by zones of pipes. These d!~lnot however extend into the steeper headwall slopes, or the upper portions of the valley side slopes. There does not appear to be any particular control by slope aspect. These pipes are distinguished from perennial flows in roofed channels, which are common in the peats of Area B. An area on the east flank of the basin, which has two well developed zones of pipes towards the base of the slope, was selected for further study (hereafter the Pipe Slope). The pipe network was defined using dyes and injections of water (SMART & WILSON, 1984). The higtiest part of the pipe network comprises a complex anastomosing net, which interconnects with the next pipe zone downstream. On the main slope, this complex is concentrated into two main pipes which led to a distributary network at the break of slope above the stream. At this point there is a further cross slope connection into the adjacent pipe zone. Some pipes can be followed to openings in the stream bank, but others do not penetrate the flat area adjacent to the stream, and discharge via overflows. Three discreet seepage zones which have no established connections with the pipe network are found on this fiat area, and are thought to be fed from saturated throughflow. At the head of the pipe network, bilberry becomes significant in the vegetation, indicating a shallower, more stony soil. Large pipes up to 8 cm in diameter emerge from between large sandstone clasts in the very stony upper horizons of the soil. No pipes can however be found at depth or further upslope despite excavation. An ephemeral seep (the Source Seep) emerges at the base of a small slip in this area. 2. SURFICIAL GEOLOGY OF THE PIPE-SLOPE AND THE DEVELOPMENT OF PIPES
2.1.
RELATION BETWEEN SURFICIAL GEOLOGYAND THE PIPE SYSTEM In view of the failure to locate pipes upslope of the Source Seep area, a detailed investi-
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gation of the geology of the surficial deposits of the slope was needed to explain the localisation of the pipes. Attempts to auger proved unsuccessful because of the high stone content of the deposits, which prevented penetration, made sample recovery difficult and gave false bedrock depths. A series of hammer seismic profiles were therefore made, initially using a Terrascout RI50 and later a Bison signal enhancement seismograph. Traverses were run along a slope contour and then reversed to allow a symmetrical velocity/distance plot to be drawn (MOONEY 1977). Traverse locations are shown on Fig. 1A and 2 and the detailed interpretation of the results in Fig. 2. To aid interpretation traverses were run on known
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PIPE FLOW HYDROLOGY
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materials, such as the moraines at the base of the valley headwall (Traverse 9), and bedrock near the summit of Pen y Fan (Traverse 8). However, some difficultywas still found due to lateral variations in slope deposits, which caused discontinuities in the velocity plots. Bedrock is exposed in the stream-bed and also in the Pen y Fan headwall and was identified in all traverses except 2 (Fig. 2). At the slope base there is a steep-walled bedrock concavity which is infilled by a dense stony compacted material comparable with moraine. The remaining slope deposits comprise a suite of related sediments. At the head of the slope the free face is replaced downslope by a mantle of stony unconsolidated material derived from the free face and hence termed here a mass-wasting deposit. Due to the interbedded sandstone/shale bedrock this contains considerable fines from weathering of the shales. However, the upper half metre has a much lower velocity due to the removal of many of these fines. We term this deposit skeletal mass-waste. Below traverse 7 the unaltered mass-waste deposits wedge out as bedrock approaches the surface in a bench feature, and pass laterally below traverse 6 into more consolidated less stony material. This is interpreted as wash and debris flow deposits derived from the mass-wasting material upslope and deposited in the basal concavity. The development of pipes on the slope, therefore, appears to be associated either with the deepening of the unconsolidated deposits due to the morainic infill or the lateral change in the top metre of slope deposits. Whilst the thick morainic infill is probably important in developing considerable saturated storage at the slope base, permitting prolonged discharge of the stream-side seeps, it is thought unlikely that it directly affects the occurrence of the pipe network. However, the change from skeletal mass-wasting material at the surface at traverse 5 and 6 to the wash and debris flows at traverse 4 may well be a causal factor. The former material is excessively freely draining, water poured onto the surface even in winter infiltrates rapidly and flows laterally between the clasts. By comparison the wash and debris flow sediments have a much lower permeability, infiltration rates are slower, and the capacity for lateral movement by saturated throughflow is more limited. Maximum winter infiltration rates on these materials are between 6 and 10 mm/min compared to between 6.5 and 42.0 mm/min for the skeletal soils. Thus the pipes may well function as a type of return flow at localities where permeability decreases downslope.
2.2.
ANIMAL BURROWING ACTIVITY AND THE DEVELOPMENT OF PIPES
The distribution of active pipe systems may therefore be controlled by hydrological factors. However, there is considerable evidence that the actual initiation of the network is due to animal burrowing activity, most probably by the mole Talpa europea. Moles are territorial animals which obtain food by collecting worms and coleoptera which fall into the burrow system. The extent of the individual burrows are therefore controlled by the availability of food. In rich grasslands up to 45 moles per hectare have been observed but, in poor sandy and acid upland soils, very extensive burrow systems are observed with densities of only 1 mole per hectare (MELLANBY 1971). Studies in Merioneth (HOPE-JONES 1969) and Snowdonia (MILNER & BALL 1970) indicate that the two major factors controlling mole distributions are the suitability of the soil for both the excavation of tunnel systems and production of a suitably rich invertebrate diet. The former appears to be controlled primarily by stoniness and it is therefore not surprising that mole signs at Cwm Llwch are completely absent from the skeletal soils of the upper slopes. The number of soil invertebrates is controlled by the complex interaction of soil drainage, pH and nutrient cycling. There is a clear association of
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WILSON & SMARI
moles with areas receiving large inputs of sheep dung and therefore with areas of better grazing, such as the flush areas which HOPE-JONES (1969) has identified as particularly favourable to moles in poor upland soils. Organic and gley soils and those ofpH less than 4.5 are generally not occupied by moles, the latter because of the limiting effect on earthworm distributions. The distribution of pipe networks at Cwm Llwch follows a remarkably similar localisation and signs of simultaneous pipe/burrow function were observed during the tracing work (SMART & WILSON, 1984). Whilst the diameter and location of the pipes at the A/B horizon interface are similar to mole burrows, the mole tunnel networks do not generally show the apparent organisation found in the pipes. Single unbranched tunnels 20 to 30 m long between isolated sections of range are however known, and bear a striking resemblance to an isolated pipe, which traverses an area of poor coarse grasses joining two separate piping zones. This lack of topological similarity may well be due to the effects of water flow in the burrows, which serves to maintain the system when moles are absent. The most efficient links, providing direct downslope flow, are utilised and swept clear, while dead-end passages and inefficient loops are choked with vegetation and sediment and so closed off. Thus it appears that it is the combined occurrence of the hydrological and ecological factors which give rise to the Cwm Llwch pipes. If it were not for the presence of the mole burrows, return flow at the junction of the skeletal soils and the wash and debris flow deposits would manifest itself as overland flow on the lower slopes. Where pipe development cannot occur, or is limited, for instance, by excessively stony soils, their hydrological role may be paralleled by stone filled gulley lines, such as found on the northern headwall of Pen y Fan (Fig. 1B).
3. OBSERVATIONS ON PIPE FLOW AND CONSTRUCTION OF A CONCEPTUAL MODEl= 3.1.
THE GENERATION OF PIPE FLOW
The onset of pipe flow is not dependent upon one set of rainfall or soil moisture conditions but can occur under various circumstances. It is also spatially variable in its occurrence over the hillslope, however some general conditions instrumental in its initiation can be described. Pipe flow can occur in response to a gradual build up of soil moisture over a period of hours, caused by rainfall with rates as low as 3 mm/hr. The delay before the initiation of flow depends upon the total volume of rain that has fallen, and to a lesser degree on antecedent conditions which control the capacity of slope moisture stores. During periods of high soil moisture or between storms, pipes may already contain a significant flow, and response to rainfall is very rapid, being within minutes of the start of heavy downpours. Finally, even after prolonged drainage, high intensity storms still produce pipe flow, even though the upper soil horizons are not saturated. These points are discussed further below with reference to catchment response. High intensity storms or prolonged storms of lower intensity which produce over 18 m m of rainfall cause the development of extensive overland flow on the Pipe Slope. This occurs due to the overflow of pipes which either debouch at the surface or have been damaged. It may result in either local or general sheet flow, which develops upslope as far as the Source Seep, but has never been identified further upslope in the skeletal mass-waste deposits. At the slope base local pipe overflow may well be related to the reduction in pipe flow capacity caused by the decrease of slope in the basal concavity adjacent to the stream. In
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general, it appears that constrictions and the presence of tributary pipes without change in pipe diameter are the major causes of overflow. These point discharges cause the development of infiltration excess overland flow. In general, however, once overland flow has developed, surface saturation soon occurs and the whole Pipe Slope area contributes to storm runoff. After the cessation of storm rainfall gaps quickly occur in the sheets of overland flow, but in many cases pipe flow continues under considerable hydraulic heads for a number of hours after the rainfall has ended, and is responsible at least in part for maintaining high discharges in the stream. The persistence of pipe discharge appears to be related primarily to the total storm rainfall, but it is noticeable that it falls off more rapidly under dry antecedent conditions. Recession from pipe-full conditions can take several days, during which flows can be observed from the head of the Pipe Slope right through to the stream. However, if no further rainfall occurs, flow has often been observed to decrease downstream in the pipe network. This loss of water must maintain moisture levels in the adjacent and underlying unconsolidated deposits (cf. SKOCZAN et al. 1976 and JONES 1982). These have a considerable volume at the Pipe Slope, and their significance in maintaining seep flows had already been mentioned. What is not apparent from our observations is whether significant discharge from this source into the pipe system occurs. The chemistry of pipe flow waters is generally not diagnostically useful, apart from a higher potassium concentration compared to the surface streams and rainfall (0.7 to 0.9 mg/l compared with 0.2 to 0.3 mg/l) (Fig. 3). Samples of overland flow from the non-peat areas of the basin also have high potassium concentrations and this implies a common source, probably from litter decomposition. However, as the pipes are generally in contact with the mineral soil, and have high potassium concentrations at the head of the Pipe Slope, this confirms earlier suggestions that flow through the litter rich skeletal soils above the Pipe Slope is a major source of water. Potassium concentrations generally decrease downstream through the pipes, often with a corresponding increase in calcium. This may well suggest discharge of water from the unconsolidated deposits at the slope base because rainfall contributions would not cause increased calcium concentrations, whereas longer residence saturated flow might well do so. Stream atW
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3.2.
WILSON & SMART
A CONCEPTUAL MODEL OF PIPE FLOW HYDROLOGY
Based on our observations, we propose the model of pipe flow hydrology in Fig. 4A. The essence of this model is that the slope unit containing pipes acts as a zone of transmission between an upslope water source and a downstream sink. In the Cwm Llwch case described, these are the skeletal mass-waste slope unit (which occupies a very substantial area upslope) and the stream, in the system described by GILMAN & NEWSON (1980) from Plynlimon, these are represented by a plateau peat bog and a valley mire without rapid connection to the stream. The model deliberately excludes the influence of direct precipitation on the pipe slope unit itself for reasons of clarity, although this can be readily incorporated. We consider this water source minor for the Cwm Llwch system, but it is more significant elsewhere, for example the Maesnant pipes (JONES 1978), and in Slithero Clough (McCAIG 1983). An upslope store feeds water downslope; if the discharge is low or the regolith thick, either unsaturated, or more probably saturated Darcian flow is capable of transmitting water to the slope base. Storage can occur by expansion of the saturated zone at the slope base, or at intermediate points on the slope, and discharge occurs by seepage into the downstream sink. If the total discharge from the upslope store exceeds the capacity of this route, water is diverted into the pipe system which has a considerably higher transmission capacity, but very much smaller storage. Finally, the pipe flow capacity can be exceeded and overland flow develops with some depression storage being satisfied before downslope transmission begins. We believe this threshold response system is basic to the operation of pipes, a contention supported by our qualitative observations above, the quantitative descriptions of catchment response discussed below, and the more comprehensive observation and modelling of McCA1G (1983). In addition to the downslope transmission, there is an inter-system transfer which occurs either in proportion to the size of the store, such as for depression and pipe storage, or when the particular store is full. The latter can occur where saturation in the regolith builds up initially to the level of the pipes, and finally to the surface, generating return flow and saturation excess overland flow. Approximate figures for the model components of storage, maximum discharge and total flow after a hypothetical major isolated storm of about 15 m m followed by five weeks of uninterrupted recession, are given for the Cwm Llwch Pipe Slope in Fig. 4B. The upslope store associated with the extensive area of mass-waste deposits is very large and only a small proportion of this will be discharged before the end of the recession. The second largest store is associated with the valley infill, which is about one-fifth of the size of the upslope store. Total depression and pipe storage are insignificantly small compared to these, as is confirmed by the very rapid initiation of pipe flow and sheet runoff. Peak pipe flow is estimated between 4.5 and 9.6 l/sec compared to 1.3-2.0 l/sec/m of slope for overland flow. The latter, however, only flows over about half the slope and for four to six hours after rainfall, while pipe flow is often maintained at a diminishing discharge for five days. Thus pipe flow is about three times more important than overland flow in draining the Pipe Slope. The volumes of saturated Darcian flow draining the waste mantle upslope and the infill downslope are more difficult to estimate as no hydraulic conductivity figures are available for the catchment. However, order of magnitude estimates suggest that discharge from the slope base is comparable in magnitude over the full five week recession with the six hours of overland flow, but tbur times less than five days of pipe flow. The observed discharge from the slope foot seeps is, however, too small to accommodate this output, and considerable diffuse seepage through the stream banks must occur.
PIPE FLOW HYDROLOGY
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154
3.3.
WILSON & SMART
HYDROLOGICAL ROLE OF PIPES IN CATCHMENT RESPONSE
We have suggested above that pipe flow in Cwm Llwch transmits water to the stream from source areas remote from the channel, at a rate which is similar to overland or channel flow. In fact, the stream network in the W catchment is markedly asymmetrical; on the eastern bank which is drained by pipe networks, there are no surface tributaries (Fig. 1), whilst there are many on the left bank, which drains a saturated peaty area in the west of the basin. Because the response and flow velocity of overland flow and pipe flow are similar, no great difference would be expected in the response of the rising limbs of hydrographs for catchments dominated by either of these processes. However, because of the observed tendency for pipe flow to persist after the cessation of overland flow, a significantly enhanced recession discharge might be expected. Furthermore, because of the spread of pipe-networks into areas otherwise distant from the channel, there may well be an enhanced contributing area in catchments dominated by pipe flow compared to overland flow. However, contributing area data obtained using both straight-line baseflow separation and chemical mass balance proved to be somewhat imprecise. Furthermore, it was difficult to obtain comparable matched catchments, catchment 4 (Fig. 1) for instance having a higher contributing area than W despite extensive pipe flow in the latter, due to the occurrence of significant overland flow on the impermeable peaty area (average contributing area for 13 paired storms catchment 4 18%, catchment W24%). For nested catchments 5 (with significant pipe flow) and 3 (little significant pipe flow) there is again no significant difference in the contributing areas (Table 1), even when direct runoff generated between the gauges is employed (5-3 in Table 1). However, the bowl shaped channel head area of catchment 3 would be expected to contribute significantly more to storm runoffthan the steeper straight side slopes of catchment 5 (WEYMAN 1974, BEVAN 1978). As this is clearly not the case enhanced runoff from the pipes may be inferred. Tab. 1: CONTRIBUTING AREAS OF STORM RUNOFF FOR NESTED SUB-CATCHMENTS 5 (SIGNIFICANT PIPE FLOW) AND 3(LITTLE PIPE FLOW), AND FOR THE AREA BETWEEN THESE GAUGES 5-3 (SIGNIFICANT PIPE FLOW). Storm Period hr 30 20 15 35 9 12 17 26 23 24
Storm Rainfall mm 140 60 20 56 18 13 32 75 44 23
Contributing Area (%) Site 3 Site 5 Site 5-3 27 19 13 18 20 22 12 20 26 20 18 16 18 19 20 22 28 32 20 16 13 21 20 20 22 16 11 20 16 13
This inference was investigated by examining individual hydrographs for the two catchments over a range of conditions. An important implication of the discharge threshold model is that antecedent conditions and both storm intensity and cumulative rainfall will all control the instantaneous slope discharge which, acts as the main switching function. Attempts to
PIPE FLOW HYDROLOGY
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define statistically significant differences in the response of the two basins using these three variables were unsuccessful. However, selected storm events do illustrate specific points. The three storm events presented in Figures 5A and 5B illustrate the importance of antecedent conditions and rainfall intensity for operation of the pipe system. All have similarly high 10-day cumulative antecedent precipitation. The intense rainfall of storm 1 gives a rapid response in both pipes and the headwater catchment; however, with reduction of intensity in storm 2, the 'primed' pipe system responds faster than the headwaters, where quick-flow develops less rapidly. Finally, storm 3 (Fig. 5B) with a very low intensity after a very heavy summer rain-storm, causes both more rapid initial response and faster time to peak in the pipe dominated catchment. Figure 5C illustrates that with a low antecedent precipitation, the thresholds in the pipe system are not exceeded and several storms are required before the pipe catchment starts t° respond, as can be seen by similarity in hydrograph areas for storms 4 and 5, compared to the additional runoffshown by the difference in areas between the two
WILSON & SMART
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catchments for storms 6 and 7. It should also be noted that the difference in response time is reduced as antecedent precipitation increases. Once a high soil moisture has been established by prior precipitation, there appears to be a simple linear relation between storm rainfall and peak runoff, as can be seen by reference to storms 1 and 2 of Fig. 5A. This is also illustrated for catchment 5 in Fig. 6, where peak discharge and storm rainfall are plotted for storms greater than and less than 50 mm 10-day cumulative antecedent precipitation index. The catchment appears to operate in both a 'dry' (no pipe flow) and 'wet' (pipe flow) mode, although there are clearly intermediate responses where during the storm itself, the mode changes as pipe flow thresholds are exceeded. This is certainly the case with storm A (Fig. 6), which is a heavy summer storm occurring after a prolonged dry period. Within this storm, the arbitrary 10-day cumulative antecedent precipitation index used in the compilation of Fig. 6 is exceeded. The aberrant storm B (wet antecedent response under dry conditions) is also an artefact of the 10-day limit used in calculation of this index, a very large storm occurring just prior to the start of the cumulation period. During our work, it became apparent that in order to develop a clear understanding of the significance of pipe flow in catchment hydrology, direct instrumentation and modelling of pipe flow would be necessary. This option was not unfortunately available to us. There are a number of reasons for this conclusion, some of which, such as the difficulties ofhydrograph separation and selection of paired basins (which differ only in the proportion of pipe flow), have been mentioned above. A more intractable problem is the complex response of catchments with several runoff producing mechanisms. It is not immediately apparent in such situations if the relationship observed between direct runoffand, for instance, storm runoffis dominated by the pipe flow mechanism or by another, such as surface runoff. Even if small pipe flow dominated catchments can be segregated, the complexity of response illustrated above suggests that gross parameters, such as antecedent precipitation indices, will only be partially successful in explaining the very dynamic response observed in these quick-flow processes. We therefore believe that the development of 'white-box' models, such as that
PIPE FLOW HYDROLOGY
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illustrated in Fig. 4 which deals explicity with the pipe flow slope hydrology system, will provide the most satisfactory route for explanation and prediction. Eventually, it may be possible to incorporate such sub-systems into a model of overall catchment response, as attempted by GILMAN & NEWSON (1980), but prior to this, direct instrumentation of pipe flow is needed, as in the more recent work of JONES (1982).
4.
CONCLUSIONS
The soil pipe network of the Cwm Llwch catchment provides an important route for discharge of return flow. This is generated by the sharp downslope decrease in the permeability of the upper soil horizons between the stony skeletal mass wasting deposits which mantle the long upper slopes in the basin, and the deeper but finer textured brown earths found adjacent to the streams. The pipes head in the stony soils and drain downslope in broad anastomosing bands, some passing right to the stream banks, and others discharging on the ground surface to generate surface runoff. This transmission function is we believe fundamental in understanding the pipe flow process. Whilst the active hydrological function is dominant in maintaining the network through time, it is suggested that burrows of the mole Talpa europea provided the original openings in the soil. Without these burrows pipe flow would be much less prevalent; and the hydrological role of pipes would be replaced by throughflow in gravelly gulley lines with very high permeabilities, or overland flow. In other areas however the initial openings exploited by the soil pipes may well be explained by a different process (See GILMAN & NEWSON 1980 and JONES 1978, 1981). The progressive development of pipe flow during storms is a relatively complex process requiring dense and sophisticated instrumentation. Our qualitative observations however, suggest a conceptual model which emphasises discharge thresholds in controlling slope flow process. Pipe flow has a transmission capacity intermediate between saturated throughflow and surface runoff. The instantaneous discharge at the top of the pipe slope controls switching of flow between these three routes. Thus under dry antecedent conditions or for small low intensity storms, saturated throughflow is capable of transmitting all the water moving to the stream from the upper slopes. Under wetter conditions or for higher more intense rainfall, the capacity of this route is exceeded and, initially pipe flow, then finally surface runoffoccurs. Similarly after cessation of rainfall, overland flow ceases first, while pipe flow may continue for several days. Estimates suggest that pipe flow may discharge four to five times the total output of either surface runoffor saturated throughflow on the slope studied. However attempts to estimate the role of pipe flow at the catchment scale were relatively unsuccessful because of the complexity of basin response. More work is dearly needed to elucidate the significance of pipe flow in controlling both the rate of response and the contributing area of storm flows.
ACKNOWLEDGEMENTS We would liketo thank the NationalTrust for permissionto work in the CwmLlwchcatchment,the UniversityofBristolfor financialand technicalsupport, and the Natural Environment ResearchCouncil for a Research Studentship to CMW during part of this research work. Many thanks also go to those friends who assistedin the fieldwork and Simon Godden who drafted the diagrams.
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REFERENCES
BEVAN, K. (1978): The hydrological response of headwater and sideslope areas. Hydrol. Sci. Bull. 23, 419-437. GILMAN, M.A. & NEWSON, M.D. (1980): Soil pipes and pipeflow - a hydrological study in upland Wales. Brit. Geomorph. Res. Grp. Mon. 1, 101. HOPE-JONES, P. (1969): Distribution of moles in part of Merioneth. Nature in Wales 11,164-168. JONES, J.A.A. (1978): Soil pipe networks: distribution and discharge. Cambria 5, 1-21. JONES, J.A.A. (1981): The nature of soil piping: a review of research. Brit. Geomorph. Res. Grp. Mon. 3,301. JONES, J.A.A. & CRANE, F.G. (1982): New evidence of rapid interflow contributions to the streamflow hydrograph. Beit. Hydrol. 3, 219-232. McCAIG, M. (1983): Contributions to storm quickflow in a small headwater catchment - the role of natural pipes and soil macropores. Earth Surf. Proc. Landforms 8, 239-252. MELLANBY, K. (1971): The mole. Collins, London, 159 pp. MELLARD-READE, T. (1895): The moraine of Llyn Cwm Llwch on the Beacons ofBrecon. Proc. Liverpool Geol. Soc. VII, 270-273. MILNER, C. & BALL, D.F. (1970): Factors affecting the distribution of the mole (Talpa Europea) in Snowdonia (North Wales). Journ. Zool. Lond. 162, 61-69. MOONEY, H.M. (1977): Handbook of engineering geophysics. Bison Instruments Limited., Minneapolis, U.S.A. SKOCZEN, S., NAGAWIECKA, H., BARON, K. & GALKA, A. (1976): The influence of mole tunnels on soil moisture in pastures. Acta. Theriol. 21, 38, 543-548. SMART, P.L. & WILSON, C.M. (1984): Methods for the tracing ofpipeflow on hillslopes. CATENA 11, 159-168. WEYMAN, D.1L (1974): Runoff processes, contributing area and stream flow in a small upland catchment. Inst. Brit. Geog. Spec. Pub. 6. Fluvial processes in instrumented watersheds. Eds. gregory, ICJ. and Walling, D.E., 33-43. WILSON, C.M. (1974): The generation of storm runoffin an upland catchment. Unpub. Ph.D. thesis, Department of Geography, University of Bristol.
Addresses of authors: C.M. Wilson, Land and Water Resource Consultants, Quy, Cambridge CB5 9AJ, England P.L. Smart, Department of Geography, University of Bristol, Bristol BS8 1SS, England
Vol. 11, 145-158
Braunschweig 1984
PIPES AND PIPE FLOW PROCESS IN AN UPLAND CATCHMENT, WALES C.M. Wilson, Cambridge, and P.L. Smart, Bristol ABSTRACT The results of observations on the spatiallocalisation, origin and function of soil pipes in an upland catchment are discussed. The pipes occur in distinct zones in the brown earth soils of the lower slopes, and form a hydrological link conducting water between an upslope zone of highly permeable skeletal soils and the stream channel. It is proposed that in this catchment, pipes develop from an initial network of mole burrows, modified by hydraulic activity to produce an efficient downslope transmission network. A conceptual model for the pipe slope segment is proposed which recognises the importance of this transmission role. Slope discharge controls switching between saturated throughflow, pipe flow and overland flow, each of which has a specific threshold value for operation. Attempts to investigate the significance of pipe flow at the catchment scale met with limited success, but it appears to be important in increasing both contributing area and duration of storm flow.
1. INTRODUCTION Soil piping (JONES 1981) is well known from upland areas in Britain, where its origin and role in generation of storm flow have been the subject of previous research (GILMAN & NEWSON 1980, JONES 1978, 1982, McCAIG 1983). The alms of this study were threefold, firstly to investigate reasons for localisation of zones of piping within the catchment studied; secondly to establish a conceptual model describing the function of the pipe system, and finally to assess the significance of pipe flow as a process contributing to storm flow.
1.1. THE STUDY AREA The Cwm Llwch (GR SO005230) (Figure 1) drains northward from the scarp face of the Brecon Beacon Hills, 7 km south west of Brecon in South Wales, U.K. The catchment is underlain by impermeable rocks ranging in grain size from mudstones to grits (the Brownstones), but glacial deposition has produced a drift cover varying from 0 to over 20 metres deep. This drift cover rapidly thins towards the eastern and southern margins of the catchment. It is derived from the Brownstones and is compact, consisting of stones and boulders with a silty clay matrix. The stream network has incised into the superficial deposits to a depth varying from two meters in the upper reaches to twenty metres at the catchment outfall where bedrock is exposed. The backwall of the catchment is formed by the peak of Pen y Fan (886 m AOD) and is very steep with a conde lake and stand moraines at its base (MELLARD-READE 1895). Further down valley is an area of convexo - concave to planar slopes which stretch from the sides of the catchment to the incised channel. Resistant bands in the Brownstones produce bench features and many small waterfalls. ISSN 0341 - 8162 (~ Copyrighi lg84 by CATENA VERLAG, D - 3302 Cremlingen- Destedt, W. Germany
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Figure 1A shows the main soil types found in the catchment. Area A is characterised by thin stony or skeletal soils developed over scree, vestigial talus or morainic deposits. The average slope is 20° and these soils are free draining, stony, acid brown earths with a limited cover of wavy hair gra~s (Deschampsia flexuosa). Rocks are often exposed at the surface. Zone B is gently sloping (about 3°) and is characterised by waterlogging and bog peat in the core of the area with a peaty gley soil. In area C the ground slope is about 12°, the soil is well drained and heather and grass associations occur. The soil is relatively shallow, usually less than 0.6 m deep and consists of a stony brown earth with mor humus. Bilberry (Vaccinium myrtillus) is characteristic of this area. Zone D consists of an acid brown earth up to two metres deep with a mor humus of about 0.15 m depth. The slope ranges from 10 to 15°. Nardus stricta and Juncus squarrosus are characteristic of less free draining areas and Molinia caerulea gives a characteristic tussocky grass cover over most of this area. The long term regional rainfall for the adjacent Usk Valley (GR SN977262) is about 1600 m m per annum but rain gauges in the Cwrn Llwch catchment indicate that because of its altitude and exposure it may receive up to 33% greater rainfall in individual storms than this lower altitude station. Estimates of evaporation using Penman's method, and data from a meteorological station 5 km from the catchment (GR SN977262) indicated that monthly soil moisture deficits might occur in only two or three months a year on average. Daily
PIPE FLOW HYDROLOGY
147
evaporation estimates obtained from an evaporation tank at the same station indicated that even in the summer months evaporation losses are small and that typically, during storm events no significant evaporation occurs for several days after the cessation of the storm rainfall. The Cwm Llwch catchment with its higher altitude and increased rainfall and cloud cover must emphasise these characteristics. We have therefore an upland area whose hydrological regime is characterised by high inputs of water, low evaporation losses and limited groundwater storage. Consequently, there is frequent saturation in the soils of the catchment, giving rise to a flashy stream response dominated by quick-flow processes. It was during the study of these quick-flow processes (WILSON 1977) that the significance of pipe flow for water movement in the catchment became apparent.
1.2. CHARACTER OF THE SOIL-PIPES The pipes studied have been previously described in SMART & WILSON (1984). Briefly they comprise smooth walled approximately circular conduits about 6 cm in diameter, forming sinuous anastomosing bands in the upper horizons of the well drained brown earth soils of area D. These zones are frequently associated with seepage lines or slight hollows, and run downslope to the slope base. Using air photographic mapping validated by field inspection, it was found that some 3.5% of sub catchment W(Fig. 1A) was underlain by zones of pipes. These d!~lnot however extend into the steeper headwall slopes, or the upper portions of the valley side slopes. There does not appear to be any particular control by slope aspect. These pipes are distinguished from perennial flows in roofed channels, which are common in the peats of Area B. An area on the east flank of the basin, which has two well developed zones of pipes towards the base of the slope, was selected for further study (hereafter the Pipe Slope). The pipe network was defined using dyes and injections of water (SMART & WILSON, 1984). The higtiest part of the pipe network comprises a complex anastomosing net, which interconnects with the next pipe zone downstream. On the main slope, this complex is concentrated into two main pipes which led to a distributary network at the break of slope above the stream. At this point there is a further cross slope connection into the adjacent pipe zone. Some pipes can be followed to openings in the stream bank, but others do not penetrate the flat area adjacent to the stream, and discharge via overflows. Three discreet seepage zones which have no established connections with the pipe network are found on this fiat area, and are thought to be fed from saturated throughflow. At the head of the pipe network, bilberry becomes significant in the vegetation, indicating a shallower, more stony soil. Large pipes up to 8 cm in diameter emerge from between large sandstone clasts in the very stony upper horizons of the soil. No pipes can however be found at depth or further upslope despite excavation. An ephemeral seep (the Source Seep) emerges at the base of a small slip in this area. 2. SURFICIAL GEOLOGY OF THE PIPE-SLOPE AND THE DEVELOPMENT OF PIPES
2.1.
RELATION BETWEEN SURFICIAL GEOLOGYAND THE PIPE SYSTEM In view of the failure to locate pipes upslope of the Source Seep area, a detailed investi-
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WILSON & SMART
gation of the geology of the surficial deposits of the slope was needed to explain the localisation of the pipes. Attempts to auger proved unsuccessful because of the high stone content of the deposits, which prevented penetration, made sample recovery difficult and gave false bedrock depths. A series of hammer seismic profiles were therefore made, initially using a Terrascout RI50 and later a Bison signal enhancement seismograph. Traverses were run along a slope contour and then reversed to allow a symmetrical velocity/distance plot to be drawn (MOONEY 1977). Traverse locations are shown on Fig. 1A and 2 and the detailed interpretation of the results in Fig. 2. To aid interpretation traverses were run on known
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PIPE FLOW HYDROLOGY
149
materials, such as the moraines at the base of the valley headwall (Traverse 9), and bedrock near the summit of Pen y Fan (Traverse 8). However, some difficultywas still found due to lateral variations in slope deposits, which caused discontinuities in the velocity plots. Bedrock is exposed in the stream-bed and also in the Pen y Fan headwall and was identified in all traverses except 2 (Fig. 2). At the slope base there is a steep-walled bedrock concavity which is infilled by a dense stony compacted material comparable with moraine. The remaining slope deposits comprise a suite of related sediments. At the head of the slope the free face is replaced downslope by a mantle of stony unconsolidated material derived from the free face and hence termed here a mass-wasting deposit. Due to the interbedded sandstone/shale bedrock this contains considerable fines from weathering of the shales. However, the upper half metre has a much lower velocity due to the removal of many of these fines. We term this deposit skeletal mass-waste. Below traverse 7 the unaltered mass-waste deposits wedge out as bedrock approaches the surface in a bench feature, and pass laterally below traverse 6 into more consolidated less stony material. This is interpreted as wash and debris flow deposits derived from the mass-wasting material upslope and deposited in the basal concavity. The development of pipes on the slope, therefore, appears to be associated either with the deepening of the unconsolidated deposits due to the morainic infill or the lateral change in the top metre of slope deposits. Whilst the thick morainic infill is probably important in developing considerable saturated storage at the slope base, permitting prolonged discharge of the stream-side seeps, it is thought unlikely that it directly affects the occurrence of the pipe network. However, the change from skeletal mass-wasting material at the surface at traverse 5 and 6 to the wash and debris flows at traverse 4 may well be a causal factor. The former material is excessively freely draining, water poured onto the surface even in winter infiltrates rapidly and flows laterally between the clasts. By comparison the wash and debris flow sediments have a much lower permeability, infiltration rates are slower, and the capacity for lateral movement by saturated throughflow is more limited. Maximum winter infiltration rates on these materials are between 6 and 10 mm/min compared to between 6.5 and 42.0 mm/min for the skeletal soils. Thus the pipes may well function as a type of return flow at localities where permeability decreases downslope.
2.2.
ANIMAL BURROWING ACTIVITY AND THE DEVELOPMENT OF PIPES
The distribution of active pipe systems may therefore be controlled by hydrological factors. However, there is considerable evidence that the actual initiation of the network is due to animal burrowing activity, most probably by the mole Talpa europea. Moles are territorial animals which obtain food by collecting worms and coleoptera which fall into the burrow system. The extent of the individual burrows are therefore controlled by the availability of food. In rich grasslands up to 45 moles per hectare have been observed but, in poor sandy and acid upland soils, very extensive burrow systems are observed with densities of only 1 mole per hectare (MELLANBY 1971). Studies in Merioneth (HOPE-JONES 1969) and Snowdonia (MILNER & BALL 1970) indicate that the two major factors controlling mole distributions are the suitability of the soil for both the excavation of tunnel systems and production of a suitably rich invertebrate diet. The former appears to be controlled primarily by stoniness and it is therefore not surprising that mole signs at Cwm Llwch are completely absent from the skeletal soils of the upper slopes. The number of soil invertebrates is controlled by the complex interaction of soil drainage, pH and nutrient cycling. There is a clear association of
150
WILSON & SMARI
moles with areas receiving large inputs of sheep dung and therefore with areas of better grazing, such as the flush areas which HOPE-JONES (1969) has identified as particularly favourable to moles in poor upland soils. Organic and gley soils and those ofpH less than 4.5 are generally not occupied by moles, the latter because of the limiting effect on earthworm distributions. The distribution of pipe networks at Cwm Llwch follows a remarkably similar localisation and signs of simultaneous pipe/burrow function were observed during the tracing work (SMART & WILSON, 1984). Whilst the diameter and location of the pipes at the A/B horizon interface are similar to mole burrows, the mole tunnel networks do not generally show the apparent organisation found in the pipes. Single unbranched tunnels 20 to 30 m long between isolated sections of range are however known, and bear a striking resemblance to an isolated pipe, which traverses an area of poor coarse grasses joining two separate piping zones. This lack of topological similarity may well be due to the effects of water flow in the burrows, which serves to maintain the system when moles are absent. The most efficient links, providing direct downslope flow, are utilised and swept clear, while dead-end passages and inefficient loops are choked with vegetation and sediment and so closed off. Thus it appears that it is the combined occurrence of the hydrological and ecological factors which give rise to the Cwm Llwch pipes. If it were not for the presence of the mole burrows, return flow at the junction of the skeletal soils and the wash and debris flow deposits would manifest itself as overland flow on the lower slopes. Where pipe development cannot occur, or is limited, for instance, by excessively stony soils, their hydrological role may be paralleled by stone filled gulley lines, such as found on the northern headwall of Pen y Fan (Fig. 1B).
3. OBSERVATIONS ON PIPE FLOW AND CONSTRUCTION OF A CONCEPTUAL MODEl= 3.1.
THE GENERATION OF PIPE FLOW
The onset of pipe flow is not dependent upon one set of rainfall or soil moisture conditions but can occur under various circumstances. It is also spatially variable in its occurrence over the hillslope, however some general conditions instrumental in its initiation can be described. Pipe flow can occur in response to a gradual build up of soil moisture over a period of hours, caused by rainfall with rates as low as 3 mm/hr. The delay before the initiation of flow depends upon the total volume of rain that has fallen, and to a lesser degree on antecedent conditions which control the capacity of slope moisture stores. During periods of high soil moisture or between storms, pipes may already contain a significant flow, and response to rainfall is very rapid, being within minutes of the start of heavy downpours. Finally, even after prolonged drainage, high intensity storms still produce pipe flow, even though the upper soil horizons are not saturated. These points are discussed further below with reference to catchment response. High intensity storms or prolonged storms of lower intensity which produce over 18 m m of rainfall cause the development of extensive overland flow on the Pipe Slope. This occurs due to the overflow of pipes which either debouch at the surface or have been damaged. It may result in either local or general sheet flow, which develops upslope as far as the Source Seep, but has never been identified further upslope in the skeletal mass-waste deposits. At the slope base local pipe overflow may well be related to the reduction in pipe flow capacity caused by the decrease of slope in the basal concavity adjacent to the stream. In
PIPE FLOWHYDROLOGY
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general, it appears that constrictions and the presence of tributary pipes without change in pipe diameter are the major causes of overflow. These point discharges cause the development of infiltration excess overland flow. In general, however, once overland flow has developed, surface saturation soon occurs and the whole Pipe Slope area contributes to storm runoff. After the cessation of storm rainfall gaps quickly occur in the sheets of overland flow, but in many cases pipe flow continues under considerable hydraulic heads for a number of hours after the rainfall has ended, and is responsible at least in part for maintaining high discharges in the stream. The persistence of pipe discharge appears to be related primarily to the total storm rainfall, but it is noticeable that it falls off more rapidly under dry antecedent conditions. Recession from pipe-full conditions can take several days, during which flows can be observed from the head of the Pipe Slope right through to the stream. However, if no further rainfall occurs, flow has often been observed to decrease downstream in the pipe network. This loss of water must maintain moisture levels in the adjacent and underlying unconsolidated deposits (cf. SKOCZAN et al. 1976 and JONES 1982). These have a considerable volume at the Pipe Slope, and their significance in maintaining seep flows had already been mentioned. What is not apparent from our observations is whether significant discharge from this source into the pipe system occurs. The chemistry of pipe flow waters is generally not diagnostically useful, apart from a higher potassium concentration compared to the surface streams and rainfall (0.7 to 0.9 mg/l compared with 0.2 to 0.3 mg/l) (Fig. 3). Samples of overland flow from the non-peat areas of the basin also have high potassium concentrations and this implies a common source, probably from litter decomposition. However, as the pipes are generally in contact with the mineral soil, and have high potassium concentrations at the head of the Pipe Slope, this confirms earlier suggestions that flow through the litter rich skeletal soils above the Pipe Slope is a major source of water. Potassium concentrations generally decrease downstream through the pipes, often with a corresponding increase in calcium. This may well suggest discharge of water from the unconsolidated deposits at the slope base because rainfall contributions would not cause increased calcium concentrations, whereas longer residence saturated flow might well do so. Stream atW
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152
3.2.
WILSON & SMART
A CONCEPTUAL MODEL OF PIPE FLOW HYDROLOGY
Based on our observations, we propose the model of pipe flow hydrology in Fig. 4A. The essence of this model is that the slope unit containing pipes acts as a zone of transmission between an upslope water source and a downstream sink. In the Cwm Llwch case described, these are the skeletal mass-waste slope unit (which occupies a very substantial area upslope) and the stream, in the system described by GILMAN & NEWSON (1980) from Plynlimon, these are represented by a plateau peat bog and a valley mire without rapid connection to the stream. The model deliberately excludes the influence of direct precipitation on the pipe slope unit itself for reasons of clarity, although this can be readily incorporated. We consider this water source minor for the Cwm Llwch system, but it is more significant elsewhere, for example the Maesnant pipes (JONES 1978), and in Slithero Clough (McCAIG 1983). An upslope store feeds water downslope; if the discharge is low or the regolith thick, either unsaturated, or more probably saturated Darcian flow is capable of transmitting water to the slope base. Storage can occur by expansion of the saturated zone at the slope base, or at intermediate points on the slope, and discharge occurs by seepage into the downstream sink. If the total discharge from the upslope store exceeds the capacity of this route, water is diverted into the pipe system which has a considerably higher transmission capacity, but very much smaller storage. Finally, the pipe flow capacity can be exceeded and overland flow develops with some depression storage being satisfied before downslope transmission begins. We believe this threshold response system is basic to the operation of pipes, a contention supported by our qualitative observations above, the quantitative descriptions of catchment response discussed below, and the more comprehensive observation and modelling of McCA1G (1983). In addition to the downslope transmission, there is an inter-system transfer which occurs either in proportion to the size of the store, such as for depression and pipe storage, or when the particular store is full. The latter can occur where saturation in the regolith builds up initially to the level of the pipes, and finally to the surface, generating return flow and saturation excess overland flow. Approximate figures for the model components of storage, maximum discharge and total flow after a hypothetical major isolated storm of about 15 m m followed by five weeks of uninterrupted recession, are given for the Cwm Llwch Pipe Slope in Fig. 4B. The upslope store associated with the extensive area of mass-waste deposits is very large and only a small proportion of this will be discharged before the end of the recession. The second largest store is associated with the valley infill, which is about one-fifth of the size of the upslope store. Total depression and pipe storage are insignificantly small compared to these, as is confirmed by the very rapid initiation of pipe flow and sheet runoff. Peak pipe flow is estimated between 4.5 and 9.6 l/sec compared to 1.3-2.0 l/sec/m of slope for overland flow. The latter, however, only flows over about half the slope and for four to six hours after rainfall, while pipe flow is often maintained at a diminishing discharge for five days. Thus pipe flow is about three times more important than overland flow in draining the Pipe Slope. The volumes of saturated Darcian flow draining the waste mantle upslope and the infill downslope are more difficult to estimate as no hydraulic conductivity figures are available for the catchment. However, order of magnitude estimates suggest that discharge from the slope base is comparable in magnitude over the full five week recession with the six hours of overland flow, but tbur times less than five days of pipe flow. The observed discharge from the slope foot seeps is, however, too small to accommodate this output, and considerable diffuse seepage through the stream banks must occur.
PIPE FLOW HYDROLOGY
153
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Fig. 4B: Maximum capacity, total discharged and maximum store size for a hypothetical majorisolated storm on the Pipe Slope at Cwm Llwch.
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3.3.
WILSON & SMART
HYDROLOGICAL ROLE OF PIPES IN CATCHMENT RESPONSE
We have suggested above that pipe flow in Cwm Llwch transmits water to the stream from source areas remote from the channel, at a rate which is similar to overland or channel flow. In fact, the stream network in the W catchment is markedly asymmetrical; on the eastern bank which is drained by pipe networks, there are no surface tributaries (Fig. 1), whilst there are many on the left bank, which drains a saturated peaty area in the west of the basin. Because the response and flow velocity of overland flow and pipe flow are similar, no great difference would be expected in the response of the rising limbs of hydrographs for catchments dominated by either of these processes. However, because of the observed tendency for pipe flow to persist after the cessation of overland flow, a significantly enhanced recession discharge might be expected. Furthermore, because of the spread of pipe-networks into areas otherwise distant from the channel, there may well be an enhanced contributing area in catchments dominated by pipe flow compared to overland flow. However, contributing area data obtained using both straight-line baseflow separation and chemical mass balance proved to be somewhat imprecise. Furthermore, it was difficult to obtain comparable matched catchments, catchment 4 (Fig. 1) for instance having a higher contributing area than W despite extensive pipe flow in the latter, due to the occurrence of significant overland flow on the impermeable peaty area (average contributing area for 13 paired storms catchment 4 18%, catchment W24%). For nested catchments 5 (with significant pipe flow) and 3 (little significant pipe flow) there is again no significant difference in the contributing areas (Table 1), even when direct runoff generated between the gauges is employed (5-3 in Table 1). However, the bowl shaped channel head area of catchment 3 would be expected to contribute significantly more to storm runoffthan the steeper straight side slopes of catchment 5 (WEYMAN 1974, BEVAN 1978). As this is clearly not the case enhanced runoff from the pipes may be inferred. Tab. 1: CONTRIBUTING AREAS OF STORM RUNOFF FOR NESTED SUB-CATCHMENTS 5 (SIGNIFICANT PIPE FLOW) AND 3(LITTLE PIPE FLOW), AND FOR THE AREA BETWEEN THESE GAUGES 5-3 (SIGNIFICANT PIPE FLOW). Storm Period hr 30 20 15 35 9 12 17 26 23 24
Storm Rainfall mm 140 60 20 56 18 13 32 75 44 23
Contributing Area (%) Site 3 Site 5 Site 5-3 27 19 13 18 20 22 12 20 26 20 18 16 18 19 20 22 28 32 20 16 13 21 20 20 22 16 11 20 16 13
This inference was investigated by examining individual hydrographs for the two catchments over a range of conditions. An important implication of the discharge threshold model is that antecedent conditions and both storm intensity and cumulative rainfall will all control the instantaneous slope discharge which, acts as the main switching function. Attempts to
PIPE FLOW HYDROLOGY
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define statistically significant differences in the response of the two basins using these three variables were unsuccessful. However, selected storm events do illustrate specific points. The three storm events presented in Figures 5A and 5B illustrate the importance of antecedent conditions and rainfall intensity for operation of the pipe system. All have similarly high 10-day cumulative antecedent precipitation. The intense rainfall of storm 1 gives a rapid response in both pipes and the headwater catchment; however, with reduction of intensity in storm 2, the 'primed' pipe system responds faster than the headwaters, where quick-flow develops less rapidly. Finally, storm 3 (Fig. 5B) with a very low intensity after a very heavy summer rain-storm, causes both more rapid initial response and faster time to peak in the pipe dominated catchment. Figure 5C illustrates that with a low antecedent precipitation, the thresholds in the pipe system are not exceeded and several storms are required before the pipe catchment starts t° respond, as can be seen by similarity in hydrograph areas for storms 4 and 5, compared to the additional runoffshown by the difference in areas between the two
WILSON & SMART
156
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catchments for storms 6 and 7. It should also be noted that the difference in response time is reduced as antecedent precipitation increases. Once a high soil moisture has been established by prior precipitation, there appears to be a simple linear relation between storm rainfall and peak runoff, as can be seen by reference to storms 1 and 2 of Fig. 5A. This is also illustrated for catchment 5 in Fig. 6, where peak discharge and storm rainfall are plotted for storms greater than and less than 50 mm 10-day cumulative antecedent precipitation index. The catchment appears to operate in both a 'dry' (no pipe flow) and 'wet' (pipe flow) mode, although there are clearly intermediate responses where during the storm itself, the mode changes as pipe flow thresholds are exceeded. This is certainly the case with storm A (Fig. 6), which is a heavy summer storm occurring after a prolonged dry period. Within this storm, the arbitrary 10-day cumulative antecedent precipitation index used in the compilation of Fig. 6 is exceeded. The aberrant storm B (wet antecedent response under dry conditions) is also an artefact of the 10-day limit used in calculation of this index, a very large storm occurring just prior to the start of the cumulation period. During our work, it became apparent that in order to develop a clear understanding of the significance of pipe flow in catchment hydrology, direct instrumentation and modelling of pipe flow would be necessary. This option was not unfortunately available to us. There are a number of reasons for this conclusion, some of which, such as the difficulties ofhydrograph separation and selection of paired basins (which differ only in the proportion of pipe flow), have been mentioned above. A more intractable problem is the complex response of catchments with several runoff producing mechanisms. It is not immediately apparent in such situations if the relationship observed between direct runoffand, for instance, storm runoffis dominated by the pipe flow mechanism or by another, such as surface runoff. Even if small pipe flow dominated catchments can be segregated, the complexity of response illustrated above suggests that gross parameters, such as antecedent precipitation indices, will only be partially successful in explaining the very dynamic response observed in these quick-flow processes. We therefore believe that the development of 'white-box' models, such as that
PIPE FLOW HYDROLOGY
157
illustrated in Fig. 4 which deals explicity with the pipe flow slope hydrology system, will provide the most satisfactory route for explanation and prediction. Eventually, it may be possible to incorporate such sub-systems into a model of overall catchment response, as attempted by GILMAN & NEWSON (1980), but prior to this, direct instrumentation of pipe flow is needed, as in the more recent work of JONES (1982).
4.
CONCLUSIONS
The soil pipe network of the Cwm Llwch catchment provides an important route for discharge of return flow. This is generated by the sharp downslope decrease in the permeability of the upper soil horizons between the stony skeletal mass wasting deposits which mantle the long upper slopes in the basin, and the deeper but finer textured brown earths found adjacent to the streams. The pipes head in the stony soils and drain downslope in broad anastomosing bands, some passing right to the stream banks, and others discharging on the ground surface to generate surface runoff. This transmission function is we believe fundamental in understanding the pipe flow process. Whilst the active hydrological function is dominant in maintaining the network through time, it is suggested that burrows of the mole Talpa europea provided the original openings in the soil. Without these burrows pipe flow would be much less prevalent; and the hydrological role of pipes would be replaced by throughflow in gravelly gulley lines with very high permeabilities, or overland flow. In other areas however the initial openings exploited by the soil pipes may well be explained by a different process (See GILMAN & NEWSON 1980 and JONES 1978, 1981). The progressive development of pipe flow during storms is a relatively complex process requiring dense and sophisticated instrumentation. Our qualitative observations however, suggest a conceptual model which emphasises discharge thresholds in controlling slope flow process. Pipe flow has a transmission capacity intermediate between saturated throughflow and surface runoff. The instantaneous discharge at the top of the pipe slope controls switching of flow between these three routes. Thus under dry antecedent conditions or for small low intensity storms, saturated throughflow is capable of transmitting all the water moving to the stream from the upper slopes. Under wetter conditions or for higher more intense rainfall, the capacity of this route is exceeded and, initially pipe flow, then finally surface runoffoccurs. Similarly after cessation of rainfall, overland flow ceases first, while pipe flow may continue for several days. Estimates suggest that pipe flow may discharge four to five times the total output of either surface runoffor saturated throughflow on the slope studied. However attempts to estimate the role of pipe flow at the catchment scale were relatively unsuccessful because of the complexity of basin response. More work is dearly needed to elucidate the significance of pipe flow in controlling both the rate of response and the contributing area of storm flows.
ACKNOWLEDGEMENTS We would liketo thank the NationalTrust for permissionto work in the CwmLlwchcatchment,the UniversityofBristolfor financialand technicalsupport, and the Natural Environment ResearchCouncil for a Research Studentship to CMW during part of this research work. Many thanks also go to those friends who assistedin the fieldwork and Simon Godden who drafted the diagrams.
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REFERENCES
BEVAN, K. (1978): The hydrological response of headwater and sideslope areas. Hydrol. Sci. Bull. 23, 419-437. GILMAN, M.A. & NEWSON, M.D. (1980): Soil pipes and pipeflow - a hydrological study in upland Wales. Brit. Geomorph. Res. Grp. Mon. 1, 101. HOPE-JONES, P. (1969): Distribution of moles in part of Merioneth. Nature in Wales 11,164-168. JONES, J.A.A. (1978): Soil pipe networks: distribution and discharge. Cambria 5, 1-21. JONES, J.A.A. (1981): The nature of soil piping: a review of research. Brit. Geomorph. Res. Grp. Mon. 3,301. JONES, J.A.A. & CRANE, F.G. (1982): New evidence of rapid interflow contributions to the streamflow hydrograph. Beit. Hydrol. 3, 219-232. McCAIG, M. (1983): Contributions to storm quickflow in a small headwater catchment - the role of natural pipes and soil macropores. Earth Surf. Proc. Landforms 8, 239-252. MELLANBY, K. (1971): The mole. Collins, London, 159 pp. MELLARD-READE, T. (1895): The moraine of Llyn Cwm Llwch on the Beacons ofBrecon. Proc. Liverpool Geol. Soc. VII, 270-273. MILNER, C. & BALL, D.F. (1970): Factors affecting the distribution of the mole (Talpa Europea) in Snowdonia (North Wales). Journ. Zool. Lond. 162, 61-69. MOONEY, H.M. (1977): Handbook of engineering geophysics. Bison Instruments Limited., Minneapolis, U.S.A. SKOCZEN, S., NAGAWIECKA, H., BARON, K. & GALKA, A. (1976): The influence of mole tunnels on soil moisture in pastures. Acta. Theriol. 21, 38, 543-548. SMART, P.L. & WILSON, C.M. (1984): Methods for the tracing ofpipeflow on hillslopes. CATENA 11, 159-168. WEYMAN, D.1L (1974): Runoff processes, contributing area and stream flow in a small upland catchment. Inst. Brit. Geog. Spec. Pub. 6. Fluvial processes in instrumented watersheds. Eds. gregory, ICJ. and Walling, D.E., 33-43. WILSON, C.M. (1974): The generation of storm runoffin an upland catchment. Unpub. Ph.D. thesis, Department of Geography, University of Bristol.
Addresses of authors: C.M. Wilson, Land and Water Resource Consultants, Quy, Cambridge CB5 9AJ, England P.L. Smart, Department of Geography, University of Bristol, Bristol BS8 1SS, England