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Hydrologic processes and geomorphic constraints on urbanization of alluvial fan slopes
Geomorphology 31 Ž1999. 325–335
Hydrologic processes and geomorphic constraints on urbanization of alluvial fan slopes A.P. Schick a,) , T. Grodek a , M.G. Wolman b b
a Department of Geography, Hebrew UniÕersity, 91905 Jerusalem, Israel Department of Geography and EnÕironmental Engineering, Johns Hopkins UniÕersity, Baltimore, MD 21218, USA
Received 15 April 1997; received in revised form 18 June 1997; accepted 20 July 1997
Abstract The natural array of processes conveying water and sediment from arid mountain catchments, through alluvial fans, into the base level below is affected by human intervention to an extent unknown until a few years ago. Previously permeable fan terrain has now been replaced by paved impermeable surfaces whose drainage becomes problematic. This problem is intensified with the growth of the town and its building density, particularly vis a vis the smaller catchments. The high sediment yields, with typically predominant bed material components that are supplied by steep catchments, create situations difficult to manage in terms of effective economical and environmentally sensitive criteria. Within the context of the general inadequacy of structural attempts to fully control the floods emanating onto an alluvial fan, a case can be made for exercising the option of local, low-key engineering intervention in protecting fan settlements. Several aspects of the considerations involved are illustrated by examples from the town of Eilat, Southern Negev Desert, Israel, a dynamically developing international tourist resort. q 1999 Elsevier Science B.V. All rights reserved. Keywords: arid environments; urban geomorphology; flooding; planning
1. Introduction Mountain margins in deserts are invariably rimmed by an assemblage of large and small alluvial fans with intervening alluvial slopes ŽBull, 1977; Rachocki, 1981.. This transition from montane terrains to intermontane depressions has been a preferred locus of human settlement, however sparse in the world’s deserts, since the dawn of history. Today
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Corresponding author. Fax: q972-2-5881442. E-mail address: [email protected] ŽA.P. Schick..
many of the larger desert cities blossom on or astride alluvial fans. Roads and railways connecting these cities tend to run for long distances along mountain margins and have to cross large and small ephemeral channels ŽSchick, 1974.. Runways of airports serving these cities are sited on the flatter terrain as close as possible downslope. Thus, the natural array of processes conveying water and sediment from the arid mountain catchment into the baselevel below is affected by human intervention to an extent unknown until a few years ago ŽFrench, 1987.. Alluvial fans in deserts are composed of predominantly clastic materials deposited by infrequent but
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occasionally very powerful floods originating in the mountain catchment ŽGraf, 1988.. They are shaped by complex processes and affected by climatic, tectonic, and other factors ŽBull, 1991; Blair and McPherson, 1994; National Research Council, USA, 1996.. In most fans, the widening of the channel, often associated with bifurcation through avulsion, conveys the water–sediment mixture to all parts of the fan surface, though the return time to a given point varies, but may exceed many decades. Due to the disproportionately high effect of extreme magnitude–low frequency flood events which characterize deserts, the recovery time from a big event may be very long. Prior to development, a substantial part of the floodwater volume reaching the fan infiltrates between the apex and the fan toe. Direct rain falling on the fan proper was of marginal consequence, but urban development has changed all this. The previously permeable fan terrain has now been replaced, to a varying but often considerable proportion, by impermeable surfaces such as roofs, roads, and parking lots. The drainage of these urban areas, though operating during only short and infrequent events, becomes a problem. The problem is insignificant in small localities with dispersed housing and on fans whose area is small relative to the catchment; it grows in importance along with the growth of the town and its building density, leading to a substantial increase of its paved areas. An alluvial fan town extending over a few square kilometres, even if fully developed, may have only a marginal effect on a flood emanating from a large catchment Ž) 500 km2 .. In such a case, the protection of the town from the channel flood will be by far the main concern. The smaller the catchment in relation to the urban area, the more important urban drainage will be. The sediment problem is of a cardinal importance in the context of urbanized alluvial fans and fan slopes. Steep arid mountain catchments yield sediment concentrations for the total flow volume of 5–15 = 10 4 mgrl, most of it in bed material sizes. The town itself and its associated transportation network have to contend with these large volumes and sizes. Furthermore, the introduction of protective devices such as check dams and drainage diversions result in a change in the sediment–water proportion,
which, in turn, has an impact on the surfaces affected by the flow further downstream. Urban runoff tends to be sediment-free, but the arid, vegetation-free environment provides ample opportunity for it to reclaim its proper load, not at source but rather further downstream. Large scale stormflow conveyors, wider and deeper than the original natural alluvial fan distributary braid have several impacts. They provide near-complete protection but enhance inchannel sedimentation; require continuous maintenance and strict zoning enforcement; exact a high cost of unproductive land appropriation; and evoke controversy over aesthetics and the environment. This paper confronts the question of how the natural processes on and around an alluvial fan are altered by urbanization. Further, we try to point out how some geomorphic principles applied to the urban alluvial fan situation might help provide better engineering solutions to prospective and resulting problems.
2. Main study area The venue of most of the observations on which this paper is based is the hyperarid Žmean annual rainfall 30 mm. western margin of the Southern ’Arava Valley, Israel. The ’Arava Valley is a link of the Syrian–African Rift extending from the Dead Sea in the north to the Gulf of ’Aqaba in the south. In its southernmost part, the 10-km wide ’Arava Valley ŽFig. 1., is bordered by steep mountain fronts which rise to elevations of 800 m.a.s.l. to the west and 2000 m.a.s.l. to the east. The towns of ’Aqaba and Eilat, located astride the Israel–Jordan border on the shores of the Gulf, provide an opportunity to investigate the interplay between physiographic constraints, engineering intervention, and geomorphic processes in this extremely arid environment. While the town of ’Aqaba is dominated by the large alluvial fan of Wadi Yutm ŽFig. 1., the town of Eilat is located on an alluvial fan slope created by coalescing fans draining areas of up to a few square km each, with a few somewhat larger ephemeral streams affecting areas to the north and south where the town is currently spreading. We will focus here on Eilat, and most of the type cases cited below will be taken from Eilat and its environs.
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Fig. 1. The Southern ’Arava Valley and the north shore of the Gulf of ’Aqaba: topography and urban development.
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3. Nature of the problem Assume a desert town occupies a fraction Že.g., one-third. of the area of an alluvial fan fed by a high relief catchment 10 times the size of the fan. Clearly, the locality has been experiencing the effects of floods since its establishment — a small one every few years, more rarely a large one, and perhaps a truly ‘catastrophic’ flood once in a few decades. Initial adaptation either by accepting the loss or by better siting away from the preferred floodpaths as perceived, usually gives way, as the town expands, to a more activist attitude leading to attempts to ‘control’ the flooding. Generally, the tools of ‘control’ are the dam, the ditch, and the embankment. These tools may be used in an attempt to isolate the town completely from flows from the outside, either by some form of watershed management, by damming upstream of the fan apex, or by isolating the built-up area from the rest of the fan by a series of embankments and ditches. A geomorphic evaluation of these two alternatives follows. Encircling the upslope urban boundary of an alluvial fan town by a series of embankments and ditches is designed to divert the floodwaters laterally around the town. It requires a dependable estimate of the maximum probable flood, both in terms of peak discharge and in terms of total flow, total sediment yield, and sediment composition. The former is needed to ascertain the floodway geometry Žcrosssection and slope., while the latter is needed to estimate the amount of deposition to be expected at the junction of the incoming natural channels into the protecting floodway. The angle at this junction is often such that much of the bed material transported by the floodwaters is deposited, raises the bed elevation, and tends to diminish the floodway cross-section to the point of complete obstruction. In its upslope parts, an encircling floodway tends to parallel the natural contours of the fan; hence, its longitudinal slope is much smaller than that of the original, natural channel. Therefore, considerable cut and fill work is required in order to provide the necessary slope, incurring considerable cost in land andror paving Žto decrease roughness.. Further downstream, the encircling floodway flow direction becomes more orthogonal to the fan contours, but there is additional water and sediment input from
neighbouring channels, thus increasing the probability of obstruction and subsequent ‘jumpout’ of the floodwaters in the direction of the protected town. Roads crossing the encircling floodway via bridges, as well as by dipping into the bottom of the floodway, tend to be a weak link in the protective system. In the first case, the cross-section under the bridge, even if sufficient at first, tends to accumulate refuse and become smaller in time. In the second case, ‘improving’ the descent into the crossing actually provides a better opportunity for the floodwaters to escape from the floodway. In general, maintenance of floodways in desert towns proves to be quite difficult to accomplish, especially over years without appreciable flows, so that as a rule, the big flood should be reckoned to tackle the protective system at a state much below its design capacity. Catchments upstream of desert towns, especially rocky and high relief ones, are not easily given to control by areal measures such as terracing and vegetation stripes because of the arid climate. The only viable option seems to be to dam the stream channel at its outlet from the mountains andror its tributaries at preferred sites further upstream. The first problem encountered when considering this solution is the very steep channel slope of the smaller catchments Žgenerally 0.05–0.10 for catchments 0.5–5 km2 in area., which dictates high structures if the capacity is designed to take care of the maximum probable flood volume. Sediment accumulated in such structures not only decreases the capacity over the years, but also causes the reservoir bottom, especially near the dam, to be less and less permeable, thus increasing the risk of overflow. For all of these reasons, structures capable of absorbing the exceptional flash flood are required to be huge, the consequent environmental drawbacks result in justification for the protection of highly valuable and selective sites only. They do not provide an acceptable solution in most urban situations. The required capacity of the protective dammed reservoirs is a cardinal point. In addition to the constraints of decreasing infiltration and accumulating sediment, man-made changes in the catchment must also be considered. Housing development or a major road built through the dammed catchment may substantially alter the hydrogeomorphic parameters used to estimate the size of the required structure.
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Overflow and concomitant breach of a protective dam is likely to cause damage of higher magnitude than the same flood event in a natural, pre-dam state. A case in point in the Rift Valley is the Nahal Ashalim dam designed to protect the main plant of the Dead Sea Works, located 2 km downstream. In 1991, a moderate flood in this hyperarid area breached the dam, flooded the plant, and caused major destruction and an extended stoppage of production. Without the dam, damage would have been minimal. Letting flows pass through alluvial fan developments is an attitude psychologically unacceptable to urban inhabitants. While at some past stage of growth, developers had some choice in siting, soon thereafter spatial constraints became such that the object tackled was not the layout of the houses and roads, but the stream channel, despite the difficulty of rigorously defining it in an alluvial fan environment. Using streets for urban drainage is a fairly common practice in arid towns, though our knowledge as to how a desert flash flood behaves on such a surface constrained by pavement berms is still relatively unknown in a field situation. The high and coarse sediment load coupled with the effects of street intersections is conducive to great temporal and spatial variations in the functioning of the street drainage system. Nevertheless, it is worthwhile to consider the option of local and low-key engineering intervention in protecting alluvial fan settlements and structures from floods. The probability of doing more harm than good by constructing major protecting works is enhanced by the fact that such structures broadcast a sense of total security, which, in time, gives way to complacency. The example of earthquake alerts and preparatory mechanisms in seismic areas has not yet been imitated in areas subject to infrequent but powerful desert floods.
4. Eilat scene — some examples The town of Eilat has been expanding over the last two decades in an upslope direction on a composite fan, and its urban fringe has now reached, at
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most places, the granitic mountain front ŽSchick, 1995.. Large scale expansion is currently taking place laterally to neighboring alluvial surfaces both northwards, onto a pediment-like surface bisected by the larger ephemeral stream channel of Nahal Roded, and southwards, onto a steep and deep alluvial veneer over granitic mountains overlooking the Gulf of ’Aqaba ŽFig. 2.. Various aspects related to geomorphic processes at these three boundary venues will be discussed shortly. The lower boundary of the fan town of Eilat, at its contact with the southernmost coastal segment of the ’Arava Valley, presents conflict between land uses and economic interests. The fluvial processes operating in this area present a set of different boundary conditions, with important applications to the design of drainage and flood protection. 4.1. Up-fan urban boundary The small Žup to 2 km2 in area. catchments draining towards the urban fringe resemble Nahal Yael, a research catchment 4 km NW of Eilat ŽFig. 2., from which detailed records of rainfall, infiltration, runoff, sediment transport, and associated geomorphic information have been assembled for the last 30 years ŽSchick and Lekach, 1993.. These records, augmented by indirect geomorphic evidence of extreme flood events in the hyperarid Southern Negev and the neighbouring territories of Sinai and Southern Jordan, as well as paleoflood studies more to the north ŽGreenbaum, 1996; Greenbaum et al., 1999., enable the evaluation of the high magnitude– low frequency hydrographs likely to affect the town fringe. Further, it is possible to estimate, with the aid of a simplistic procedure, the amount and composition of the sediment yielded by these flood events. Water volumes beyond the capacity of small check dams constructed between the granite footslopes and the uppermost residences may pose an occasional problem, especially if the area or hydrologic character of the contributing area is changed by subsequent construction. However, it is the sediment content of the floodwaters which presents a potentially more damaging scenario ŽSchick et al., 1997.. Peak discharges from these small catchments amount, for extreme events, to ca. 10 m3 sy1 , with total flow
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Fig. 2. The town of Eilat: present and future urban areas; hydrographic features; flood related structures Žexisting and proposed..
volumes of a few thousand m3. Numerous data point to a mean sediment concentration around 10%, data corroborated in a recent dambreak-induced flash flood simulation performed in the same area ŽSchick et al., 1996.. Hence, every one of these small catch-
ments would supply, during such short and powerful events, several hundred tons of sediment, most of it coarse bed material. The evaluation of the sediment varies according to several factors, but almost always the quantities are beyond the capacity that structures
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such as the circumferential floodways are designed to handle. Large inlets, large scale earthmoving works to maintain slope, unrealistically wide floodways, and paved stretches of the banks and bed are some of the techniques at the disposal of engineers to surmount these problems. All of these are expensive, though, and all require careful maintenance over the long years between big floods. A preferable method is based upon a record of hydrologic measurements and associated geomorphic research done over the years in the region, primarily in the Nahal Yael catchment ŽSchick, 1988.. In contrast to most methods employed in the engineering profession, which are based on sophisticated models predicting peak discharges or flood volumes fed by few and poor data, we employ a simplistic model fully supported by a wide array of relevant data. We rely on extensive infiltration tests on a variety of terrains, which enable an evaluation of several factors: a finely distributed runoff production rate; rainfall-correlated runoff hydrographs for catchments 0.05–0.5 km2 in area; channel bed infiltration rates both from ponded water and during an artificially controlled flood; a substantial body of suspended sediment samples from desert floodwaters; direct and indirect evaluations of bedload transport; and detailed analyses of the reservoir sedimentation accumulated in behind the Nahal Yael dam over the last 18 years. The simplistic procedure estimates the volumes of bedload transported by various flood events, which we consider the key factor in the adaptation to and protection from desert floods in this environment. The volume of sediment — both bed material and suspended sediment — will necessarily deposit somewhere en route, i.e., from the up-fan town boundary to the erosional baselevel below it. While nearly all the bed material will be deposited in some check dam or floodway conveyor, obstructing the latter if the volume is sufficiently great relative to the cross-section, the suspended fraction will tend to continue with the floodwaters to intermediate or terminal baselevels. The ponds of the Nature Beta Technologies plant on the lower margins of the Eilat alluvial fan slope Žbelow. is an illustration of the former. A massive blanketing of the coral population along the Eilat Nature Reserve coast — perhaps with irreversible ecological damage — to be expected as
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a result of any high magnitude–low frequency event in Nahal Garof or Nahal Shahmon, is an illustration of the latter. 4.2. Lateral urban fringes The southern additions to the urban area currently under construction are built on land created by the complete restructuring of the terrain surface of a 4-km2 area of steep, dissected alluvial fan slopes. The drainage is designed to close off the incoming flow from the mountain front by a series of check dams, small reservoirs, and flood conveyors to one of the two larger lateral stream channels — Nahal Shahmon and Nahal Garof — which, according to the plan, will be ‘regulated’. The internal drainage will be sewered and piped towards these regulated collectors. Due to the high density projected for the development, the runoff will be high but practically sediment-free, unless a big flood event occurs before construction is finished. However, any drainage mishap at any point within the protective devices upslope may result in the injection of sediment-laden floodwaters into the drainage system and its subsequent clogging and disruption, which will potentially lead to the breakdown of the entire system. The two main stream channels drain steeply towards the Eilat coast, with the port of Eilat and other major installations at their mouths. Although partly protected from flood damage by various local devices, these installations are at risk in case the fragile system contingent upon the new development breaks down. Furthermore, this southern coast of the town is the venue of the Coral Nature Reserve — the northernmost coral colony in the world. The detrimental effect of a major sediment-laden flash flood, fueled by the large construction and development project upstream, may be long lasting. Previously, due to natural infiltration in the terrain and channel beds, only a few flows reached the Gulf, probably with only a small fraction of the sediment load originally carried by these flows when exiting the mountains. 4.3. Fan slope toe The bottom parts of alluvial fans and slopes in the desert environment are the terminus of all flows that
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make it through the intermediate sinks of infiltration and, possibly, check dams and retarding basins. Prior to construction, the inherent structure of the alluvial fan dictates a degree of indeterminacy as to where exactly the next flood will exit the fan, and in how many loci. Construction, even with many flood control devices, tends to increase this indeterminacy. Eilat may be a typical example to illustrate this. The airport runway — the lifeline of this international resort — runs for 3 km along the toe of the fan slope, parallel to its contours, at an elevation just a few meters above the nearby sea level. The main downslope directed streets of Eilat, originally designed also as floodways, are actually aimed at the runway. A protecting ditch functions only for small flows, which also use some drainage tunnels under the runway. The latter are almost certain to be filled by sediment during a big event. The flows are perforce directed to the narrow land isthmus between the southern tip of the runway and the northwestern head of the Gulf of ’Aqaba, a strategic 150 m stretch of land which accommodates all traffic between the central business area of the town and its main hotel and recreation area. Fine sediment deposited on the road traversing this stretch is a common occurrence, even from small floods that leave whatever bed material they may have transported from further upstream. On a much larger scale, deposition of fines caused the main damage resulting from the 2 November 1994 flood in Nahal Roded. This relatively small flood was generated by an isolated rain cell which covered only a small, upstream part of the catchment. The flow that exited the mountains was further attenuated by filling extensive gravel quarrying holes covering much of the wide braided channel bed. These holes trapped all the bed material and much of the fines. Still, though the peak discharge on the fan was only 40 m3 sy1 , the flow had enough energy to carry towards the southern sector of the fan, a distance of 2 km, enough suspended material to fill the algae ponds of Nature Beta Technologies with ca. 40,000 m3 of sediment ŽFig. 2.. The diversion of the flow into the plant was the result of unrelated construction activities upstream of the flow path: the breach of a diversion built in conjunction with some roadwork and the construction of an elevated landing ground for migrating birds. By the same token, it
may be regarded as a normal reaction of an alluvial fan which, over time, distributes its flow patterns more or less equally over all of its sectors. This geomorphic indeterminacy tends to persist, irrespective of engineering efforts to subjugate it. Another facet of the fan toe flooding problem is exemplified by the relation between the Eilat fan slope and Nahal Ha-Arava. Nahal Ha-Arava drains towards the Gulf of ’Aqaba several catchments of the Southern Negev Mountains, extending ca. 50 km to the north of Eilat. However, its effective drainage area is, to some extent, indeterminate, as some of the flows are abstracted by two of the playa depressions within the rift valley. The effective integration of the entire hydrographic network ascribed to Nahal HaArava is a moot point, except perhaps for the very largest events. Still, Nahal Ha-Arava has been known to transmit large discharges, and over the years, its channel has been ‘regulated’ by widening, streamlining, and creating embankments to protect the Eilat hotel area located on its western side. With increasing pressure on the land spurred by the boom in tourism, plans are under consideration to divert the Nahal Ha-Arava channel-cum-floodway several hundred meters to the west, to coincide with the Israel– Jordan border. Thereby, the contiguity of the hotel area will be achieved, but the design presents many options, some strongly constrained by geomorphic features and processes. The design must consider the input, conveyance, and sediment deposition of floods from three different sources: local urban drainage, including whatever comes from the urban alluvial fan slope; the flow of Nahal Ha-Arava; and the impact of floods in the large catchment of Wadi Yutm, coming from the mountains of southern Jordan in the east ŽWolman, 1996.. Flood events from each of these may be isolated, any two may be active at one time, or all three may be active simultaneously. The time lag between rainfall of these three sources must be considered: for the first it may be an hour less; for the second, several hours; and for a Wadi Yutm flood, a day or two. A scenario is possible wherein a flood in Nahal Ha-Arava will disrupt its channel and protective devices to such an extent that a large flood coming from the same synoptic system from Wadi Yutm will cover the entire open space between ’Aqaba and Eilat.
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Flood attenuation in Nahal Ha-Arava might be achieved by having a retaining embankment only on one side of the channel and letting the high discharges temporarily spread over the other side. Diverting the channel to the international boundary probably precludes this option, as the envisaged economic development of the region will affect ’Aqaba as well, and using extensive areas of its western margins for floodwater spreading hardly seems acceptable. Another consideration related to baselevel concerns the substantial tidal differences, augmented by southerlies often associated with the Red Sea Trough — a synoptic configuration known to be associated with most of the high magnitude storms in the region ŽSharon and Kutiel, 1986.. It may well be that the floodwaters in the Nahal Ha-AravarGulf of ’Aqaba inflow area will be flowing towards a baselevel 1 to 2 m higher than the mean sea level, with serious consequences for the entire low-lying flat area, including a dramatic increase of the level of groundwater. Plans underway to extend the Eilat coastline by constructing one or more additional sealevel ‘‘lagoons’’ would bring a more easily accessible baselevel closer to the pathways of some of the anticipated flows, a possibility which requires either the acceptance of sedimentation of these recreational lagoons or the construction of massive protective devices to prevent the flows from accessing them.
5. ’Aqaba — some observations The town of ’Aqaba, symmetrically located opposite Eilat at the NE corner of the Gulf ŽFig. 1. and somewhat larger in population, presents interesting parallels as well as contrasts to the flooding problems of Eilat as outlined above. The northern part of the town is directly affected by the large Wadi Yutm, described below. The central part of the town, including the port and the main hotel area, occupies alluvial fan terrain at the outlet of three catchments 6 to 10 km2 in size each. The divides of these catchments have elevations around 1000 m.a.s.l. on peaks ca. 6 km east of the coastline ŽFig. 1.. The high relief of the bare granite provides the background for extremely destructive flooding in the downstream
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urbanized strip squeezed between the mountain front and the coast. Similar to Eilat, but with a larger damage potential due to larger catchments and higher relief, possible engineering solutions are hampered by existing urban and transportation structures. Besides the urban center, which in places leaves little room for floodway dimensions adequate to convey high magnitude–low frequency flash floods, the effects of a railway line built on an elevated embankment skirting the town along its upslope margins play a role. Although drainage openings through this embankment are provided, the possible flooding scenarios of large sediment-laden floods cum debris flows rushing down the steep Žslope 0.06–010. mountain channels are difficult to predict. The large fan of Wadi Yutm, with its apex located at an elevation of 200 m.a.s.l. 8 km north of the town center, is without parallel on the western side of the Southern ’Arava. The southernmost sector of this fan directs its distributaries towards the town. These endanger directly a quarter of unplanned, temporary dwellings for which new and safer sites are projected. The 1966 storm, known as the Ma’an flood, was centered north of the Yutm catchment, and yielded in Wadi Yutm several peaks, one of which reached a discharge of 500 m3 sy1 , with reported float velocities of 6 m sy1 and surges up to 12 my1 ŽSchick, 1971.. The steep overall slope of the fan Ž0.075. provides opportunities for floodwaters to reach any point along the 8 km circumference of the fan sector, and together with the smaller fan adjoining to the north, creates a flooding hazard to the ’Aqaba airport with its 3-km long runway ŽFig. 1.. Much of the fan area between the airport and the northern suburbs of ’Aqaba is occupied by a free customs zone, whose 2 km long ŽN–S. compound must likewise be protected from floods. This leaves only two ‘‘openings’’ several hundred meters wide each for a natural and relatively safe outlet for future floods. Long dikes and embankments are needed to direct floodwaters emanating from the Yutm away from new development quarters constructed beyond the fan toe in the flat ’Arava terrain, which are already close to the coastal playa located astride the international border. Both ’Aqaba and Eilat regard these coastal saline tracts as valuable for future develop-
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ment, particularly for tourist facilities. Hence, a long-range and mutually coordinated view in their flood control planning must be exercised. In at least three recorded events — two in the 1940s and one in the 1960s — extensive flooding of the playa lasting a number of days was reported ŽSchick, 1971.. In ’Aqaba as well as in Eilat, underground stormwater sewers have been recently built or are being planned in some developments. While in ’Aqaba this has been done on a moderately sloping terrain, in Eilat such a drainage system is about to be built on a steep alluvial fan slope development. Though the non-paved contributing area to both system is not large Žif protective devices do not fail., the natural affinity of floodwaters to sediment in arid environments is likely to cause clogging of inlets relatively frequently. Assuming that ’Aqaba, similarly to Eilat, will spread to the north in the near future, the flooding hazard from the bajada extending along the southeastern margins of the ’Arava must be considered before such a development takes place. This bajada comprises many adjoining steep alluvial fans of varying sizes and forms, with Wadi Muhtadi, 40 km north of ’Aqaba, large enough to locally raise the level of the rift valley to form the playa of Yotvata. The surface of the fans is covered in varying proportions with a light gray tone, indicating currently active, mostly spreading out channels. Between those there are tan colored areas, suggesting no flooding in the recent Ž; 100 years. past, and darkly colored areas composed of patinated pavement which seem to have stayed free of flooding for at least several hundred years. Occasionally, grey streaks on the dark surfaces testify to ‘overbank’ flows which diverged from one of the channels during high flow, but did not result in full avulsion. Despite similarities of environmental conditions, each fan shows a high degree of individuality in shape and structure. The fans extending to the north of ’Aqaba, like their counterparts in the west, are partly covered by Acacia trees. These are concentrated in marked preference in certain segments and generally avoid the dark, desert varnish areas. The dependence of the acacia in this environment on surface waters has been recently established ŽBenDavid-Novak and Schick, 1997.. This is a valuable tool in evaluating flooding probabilities within each fan.
6. Conclusions The projected development of the arid Southern ’Arava requires a detailed, science-based evaluation of the flooding hazard prior to major construction, whose optimal siting is of cardinal importance. The built-up areas on the fans, which are the only viable terrain types available, are bound to appreciably affect the flooding hazard further downstream. The evaluation of the hazard is not just a matter of rainfallrrunoff frequencies and catchment areas. A much more sophisticated approach, involving the application of diverse hydrologic, sedimentologic, geomorphic, and historical procedures to each and every fan separately, can greatly improve the efficiency of the necessary engineering solutions to the flooding problem. It is important to note that in both ’Aqaba and Eilat, the pristine waters of the Gulf of ’Aqaba represent an enormously valuable resource. Mere hastening of sediment-laden floodwaters may not be an appropriate resolution of the potential flood problem in this setting. Acknowledgements This study was done within the framework of research grant No. 12-090 of the United States–Israel BiNational Science Foundation. The assistance of authorities dealing with the flooding problem both in Eilat and in ’Aqaba and of Yehouda Enzel, Qinghong Huang, Jens Lange and Judith Lekach is gratefully acknowledged. References BenDavid-Novak, H., Schick, A.P., 1997. The Response of Acacia Tree Populations on Small Alluvial Fans to Changes in the Hydrological Regime: Southern Negev Desert, Israel. Catena 29, 341–351. Blair, T.C., McPherson, J.G., 1994. Alluvial fan processes and forms. In: Abrahams, A.D., Parsons, A.J. ŽEds.., Geomorphology of Desert Environments. Chapman & Hall, London, pp. 354–402. Bull, W.B., 1977. The alluvial fan environment. Prog. Phys. Geogr. 1, 222–270. Bull, W.B., 1991. Geomorphic Responses to Climate Change. Oxford Univ. Press, Oxford. French, R.H., 1987. Hydraulic processes on alluvial fans. Developments in Water Science 31.
A.P. Schick et al.r Geomorphology 31 (1999) 325–335 Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. Springer, Berlin. Greenbaum, N., 1996. Paleofloods in the Large Ephemeral Stream Systems of the Negev. PhD Thesis, Hebrew Univ., Jerusalem. Greenbaum, N., Margalit, A., Schick, A.P., Sharon, D., Baker, V.R., 1999. Reconstruction of a high magnitude rainstormflood in Nahal Zin — a large hyperarid catchment in the Negev Desert, Israel. Hydrological Processes 12, 1–23. National Research Council, USA, 1996. Alluvial Fan Flooding. National Academy Press, Washington, DC. Rachocki, A., 1981. Alluvial Fans: An Attempt at an Empirical Approach. Wiley, New York. Schick, A.P., 1971. A desert flood: physical characteristics, effects on man, geomorphic significance, human adaptation. Jerusalem Studies in Geography 2, 91–155. Schick, A.P., 1974. Alluvial fans and desert roads — a problem in applied geomorphology. Abh. Akad. Wiss. Goettingen, Math.–Phys. Kl., III Folge 29, 418–425. Schick, A.P., 1988. Hydrologic aspects of floods in hyperarid environments. In: Baker, V.R. ŽEd.., Flood Geomorphology. Wiley, New York, pp. 189–203. Schick, A.P. 1995. Fluvial processes on an urbanizing alluvial fan:
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Eilat, Israel. In: Costa, J. ŽEd.., Natural and Anthropogenic Influences in Fluvial Geomorphology. Geophys. Monogr. 89, pp. 209–218. Schick, A.P., Lekach, J. 1993. An evaluation of two ten-year sediment budgets, Nahal Yael, Israel. Phys. Geog. 14, 225– 238. Schick, A.P., Lekach, J., Grodek, T., Lange, J., Leibundgut, C., 1996. An artificial flash flood in a small arid stream channel, Eilat Mountains, Israel. Suppl. to EOS, F259, Nov. 12, 1996. Schick, A.P., Grodek, T., Lekach, J., 1997. Sediment management and flood protection of desert towns: effects of small catchments. In: Walling, D.E., Probst, J.-L. Žeds.., Human Impact on Erosion and Sedimentation. Int. Assoc. Hydr. Sci. Publ. 245, pp. 183–189. Sharon, D., Kutiel, H., 1986. The distribution of rainfall intensity in Israel, its regional and seasonal variations, and its climatological evaluation. J. Climatol. 6, 277–291. Wolman, M.G., 1996, Some considerations in selection of the design of floodwater drainage for the expanding development of the Eilat foreshore. Technical Report submitted to the Eilat Foreshore Development Company, Israel, May 1996.
Hydrologic processes and geomorphic constraints on urbanization of alluvial fan slopes A.P. Schick a,) , T. Grodek a , M.G. Wolman b b
a Department of Geography, Hebrew UniÕersity, 91905 Jerusalem, Israel Department of Geography and EnÕironmental Engineering, Johns Hopkins UniÕersity, Baltimore, MD 21218, USA
Received 15 April 1997; received in revised form 18 June 1997; accepted 20 July 1997
Abstract The natural array of processes conveying water and sediment from arid mountain catchments, through alluvial fans, into the base level below is affected by human intervention to an extent unknown until a few years ago. Previously permeable fan terrain has now been replaced by paved impermeable surfaces whose drainage becomes problematic. This problem is intensified with the growth of the town and its building density, particularly vis a vis the smaller catchments. The high sediment yields, with typically predominant bed material components that are supplied by steep catchments, create situations difficult to manage in terms of effective economical and environmentally sensitive criteria. Within the context of the general inadequacy of structural attempts to fully control the floods emanating onto an alluvial fan, a case can be made for exercising the option of local, low-key engineering intervention in protecting fan settlements. Several aspects of the considerations involved are illustrated by examples from the town of Eilat, Southern Negev Desert, Israel, a dynamically developing international tourist resort. q 1999 Elsevier Science B.V. All rights reserved. Keywords: arid environments; urban geomorphology; flooding; planning
1. Introduction Mountain margins in deserts are invariably rimmed by an assemblage of large and small alluvial fans with intervening alluvial slopes ŽBull, 1977; Rachocki, 1981.. This transition from montane terrains to intermontane depressions has been a preferred locus of human settlement, however sparse in the world’s deserts, since the dawn of history. Today
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Corresponding author. Fax: q972-2-5881442. E-mail address: [email protected] ŽA.P. Schick..
many of the larger desert cities blossom on or astride alluvial fans. Roads and railways connecting these cities tend to run for long distances along mountain margins and have to cross large and small ephemeral channels ŽSchick, 1974.. Runways of airports serving these cities are sited on the flatter terrain as close as possible downslope. Thus, the natural array of processes conveying water and sediment from the arid mountain catchment into the baselevel below is affected by human intervention to an extent unknown until a few years ago ŽFrench, 1987.. Alluvial fans in deserts are composed of predominantly clastic materials deposited by infrequent but
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occasionally very powerful floods originating in the mountain catchment ŽGraf, 1988.. They are shaped by complex processes and affected by climatic, tectonic, and other factors ŽBull, 1991; Blair and McPherson, 1994; National Research Council, USA, 1996.. In most fans, the widening of the channel, often associated with bifurcation through avulsion, conveys the water–sediment mixture to all parts of the fan surface, though the return time to a given point varies, but may exceed many decades. Due to the disproportionately high effect of extreme magnitude–low frequency flood events which characterize deserts, the recovery time from a big event may be very long. Prior to development, a substantial part of the floodwater volume reaching the fan infiltrates between the apex and the fan toe. Direct rain falling on the fan proper was of marginal consequence, but urban development has changed all this. The previously permeable fan terrain has now been replaced, to a varying but often considerable proportion, by impermeable surfaces such as roofs, roads, and parking lots. The drainage of these urban areas, though operating during only short and infrequent events, becomes a problem. The problem is insignificant in small localities with dispersed housing and on fans whose area is small relative to the catchment; it grows in importance along with the growth of the town and its building density, leading to a substantial increase of its paved areas. An alluvial fan town extending over a few square kilometres, even if fully developed, may have only a marginal effect on a flood emanating from a large catchment Ž) 500 km2 .. In such a case, the protection of the town from the channel flood will be by far the main concern. The smaller the catchment in relation to the urban area, the more important urban drainage will be. The sediment problem is of a cardinal importance in the context of urbanized alluvial fans and fan slopes. Steep arid mountain catchments yield sediment concentrations for the total flow volume of 5–15 = 10 4 mgrl, most of it in bed material sizes. The town itself and its associated transportation network have to contend with these large volumes and sizes. Furthermore, the introduction of protective devices such as check dams and drainage diversions result in a change in the sediment–water proportion,
which, in turn, has an impact on the surfaces affected by the flow further downstream. Urban runoff tends to be sediment-free, but the arid, vegetation-free environment provides ample opportunity for it to reclaim its proper load, not at source but rather further downstream. Large scale stormflow conveyors, wider and deeper than the original natural alluvial fan distributary braid have several impacts. They provide near-complete protection but enhance inchannel sedimentation; require continuous maintenance and strict zoning enforcement; exact a high cost of unproductive land appropriation; and evoke controversy over aesthetics and the environment. This paper confronts the question of how the natural processes on and around an alluvial fan are altered by urbanization. Further, we try to point out how some geomorphic principles applied to the urban alluvial fan situation might help provide better engineering solutions to prospective and resulting problems.
2. Main study area The venue of most of the observations on which this paper is based is the hyperarid Žmean annual rainfall 30 mm. western margin of the Southern ’Arava Valley, Israel. The ’Arava Valley is a link of the Syrian–African Rift extending from the Dead Sea in the north to the Gulf of ’Aqaba in the south. In its southernmost part, the 10-km wide ’Arava Valley ŽFig. 1., is bordered by steep mountain fronts which rise to elevations of 800 m.a.s.l. to the west and 2000 m.a.s.l. to the east. The towns of ’Aqaba and Eilat, located astride the Israel–Jordan border on the shores of the Gulf, provide an opportunity to investigate the interplay between physiographic constraints, engineering intervention, and geomorphic processes in this extremely arid environment. While the town of ’Aqaba is dominated by the large alluvial fan of Wadi Yutm ŽFig. 1., the town of Eilat is located on an alluvial fan slope created by coalescing fans draining areas of up to a few square km each, with a few somewhat larger ephemeral streams affecting areas to the north and south where the town is currently spreading. We will focus here on Eilat, and most of the type cases cited below will be taken from Eilat and its environs.
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Fig. 1. The Southern ’Arava Valley and the north shore of the Gulf of ’Aqaba: topography and urban development.
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3. Nature of the problem Assume a desert town occupies a fraction Že.g., one-third. of the area of an alluvial fan fed by a high relief catchment 10 times the size of the fan. Clearly, the locality has been experiencing the effects of floods since its establishment — a small one every few years, more rarely a large one, and perhaps a truly ‘catastrophic’ flood once in a few decades. Initial adaptation either by accepting the loss or by better siting away from the preferred floodpaths as perceived, usually gives way, as the town expands, to a more activist attitude leading to attempts to ‘control’ the flooding. Generally, the tools of ‘control’ are the dam, the ditch, and the embankment. These tools may be used in an attempt to isolate the town completely from flows from the outside, either by some form of watershed management, by damming upstream of the fan apex, or by isolating the built-up area from the rest of the fan by a series of embankments and ditches. A geomorphic evaluation of these two alternatives follows. Encircling the upslope urban boundary of an alluvial fan town by a series of embankments and ditches is designed to divert the floodwaters laterally around the town. It requires a dependable estimate of the maximum probable flood, both in terms of peak discharge and in terms of total flow, total sediment yield, and sediment composition. The former is needed to ascertain the floodway geometry Žcrosssection and slope., while the latter is needed to estimate the amount of deposition to be expected at the junction of the incoming natural channels into the protecting floodway. The angle at this junction is often such that much of the bed material transported by the floodwaters is deposited, raises the bed elevation, and tends to diminish the floodway cross-section to the point of complete obstruction. In its upslope parts, an encircling floodway tends to parallel the natural contours of the fan; hence, its longitudinal slope is much smaller than that of the original, natural channel. Therefore, considerable cut and fill work is required in order to provide the necessary slope, incurring considerable cost in land andror paving Žto decrease roughness.. Further downstream, the encircling floodway flow direction becomes more orthogonal to the fan contours, but there is additional water and sediment input from
neighbouring channels, thus increasing the probability of obstruction and subsequent ‘jumpout’ of the floodwaters in the direction of the protected town. Roads crossing the encircling floodway via bridges, as well as by dipping into the bottom of the floodway, tend to be a weak link in the protective system. In the first case, the cross-section under the bridge, even if sufficient at first, tends to accumulate refuse and become smaller in time. In the second case, ‘improving’ the descent into the crossing actually provides a better opportunity for the floodwaters to escape from the floodway. In general, maintenance of floodways in desert towns proves to be quite difficult to accomplish, especially over years without appreciable flows, so that as a rule, the big flood should be reckoned to tackle the protective system at a state much below its design capacity. Catchments upstream of desert towns, especially rocky and high relief ones, are not easily given to control by areal measures such as terracing and vegetation stripes because of the arid climate. The only viable option seems to be to dam the stream channel at its outlet from the mountains andror its tributaries at preferred sites further upstream. The first problem encountered when considering this solution is the very steep channel slope of the smaller catchments Žgenerally 0.05–0.10 for catchments 0.5–5 km2 in area., which dictates high structures if the capacity is designed to take care of the maximum probable flood volume. Sediment accumulated in such structures not only decreases the capacity over the years, but also causes the reservoir bottom, especially near the dam, to be less and less permeable, thus increasing the risk of overflow. For all of these reasons, structures capable of absorbing the exceptional flash flood are required to be huge, the consequent environmental drawbacks result in justification for the protection of highly valuable and selective sites only. They do not provide an acceptable solution in most urban situations. The required capacity of the protective dammed reservoirs is a cardinal point. In addition to the constraints of decreasing infiltration and accumulating sediment, man-made changes in the catchment must also be considered. Housing development or a major road built through the dammed catchment may substantially alter the hydrogeomorphic parameters used to estimate the size of the required structure.
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Overflow and concomitant breach of a protective dam is likely to cause damage of higher magnitude than the same flood event in a natural, pre-dam state. A case in point in the Rift Valley is the Nahal Ashalim dam designed to protect the main plant of the Dead Sea Works, located 2 km downstream. In 1991, a moderate flood in this hyperarid area breached the dam, flooded the plant, and caused major destruction and an extended stoppage of production. Without the dam, damage would have been minimal. Letting flows pass through alluvial fan developments is an attitude psychologically unacceptable to urban inhabitants. While at some past stage of growth, developers had some choice in siting, soon thereafter spatial constraints became such that the object tackled was not the layout of the houses and roads, but the stream channel, despite the difficulty of rigorously defining it in an alluvial fan environment. Using streets for urban drainage is a fairly common practice in arid towns, though our knowledge as to how a desert flash flood behaves on such a surface constrained by pavement berms is still relatively unknown in a field situation. The high and coarse sediment load coupled with the effects of street intersections is conducive to great temporal and spatial variations in the functioning of the street drainage system. Nevertheless, it is worthwhile to consider the option of local and low-key engineering intervention in protecting alluvial fan settlements and structures from floods. The probability of doing more harm than good by constructing major protecting works is enhanced by the fact that such structures broadcast a sense of total security, which, in time, gives way to complacency. The example of earthquake alerts and preparatory mechanisms in seismic areas has not yet been imitated in areas subject to infrequent but powerful desert floods.
4. Eilat scene — some examples The town of Eilat has been expanding over the last two decades in an upslope direction on a composite fan, and its urban fringe has now reached, at
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most places, the granitic mountain front ŽSchick, 1995.. Large scale expansion is currently taking place laterally to neighboring alluvial surfaces both northwards, onto a pediment-like surface bisected by the larger ephemeral stream channel of Nahal Roded, and southwards, onto a steep and deep alluvial veneer over granitic mountains overlooking the Gulf of ’Aqaba ŽFig. 2.. Various aspects related to geomorphic processes at these three boundary venues will be discussed shortly. The lower boundary of the fan town of Eilat, at its contact with the southernmost coastal segment of the ’Arava Valley, presents conflict between land uses and economic interests. The fluvial processes operating in this area present a set of different boundary conditions, with important applications to the design of drainage and flood protection. 4.1. Up-fan urban boundary The small Žup to 2 km2 in area. catchments draining towards the urban fringe resemble Nahal Yael, a research catchment 4 km NW of Eilat ŽFig. 2., from which detailed records of rainfall, infiltration, runoff, sediment transport, and associated geomorphic information have been assembled for the last 30 years ŽSchick and Lekach, 1993.. These records, augmented by indirect geomorphic evidence of extreme flood events in the hyperarid Southern Negev and the neighbouring territories of Sinai and Southern Jordan, as well as paleoflood studies more to the north ŽGreenbaum, 1996; Greenbaum et al., 1999., enable the evaluation of the high magnitude– low frequency hydrographs likely to affect the town fringe. Further, it is possible to estimate, with the aid of a simplistic procedure, the amount and composition of the sediment yielded by these flood events. Water volumes beyond the capacity of small check dams constructed between the granite footslopes and the uppermost residences may pose an occasional problem, especially if the area or hydrologic character of the contributing area is changed by subsequent construction. However, it is the sediment content of the floodwaters which presents a potentially more damaging scenario ŽSchick et al., 1997.. Peak discharges from these small catchments amount, for extreme events, to ca. 10 m3 sy1 , with total flow
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Fig. 2. The town of Eilat: present and future urban areas; hydrographic features; flood related structures Žexisting and proposed..
volumes of a few thousand m3. Numerous data point to a mean sediment concentration around 10%, data corroborated in a recent dambreak-induced flash flood simulation performed in the same area ŽSchick et al., 1996.. Hence, every one of these small catch-
ments would supply, during such short and powerful events, several hundred tons of sediment, most of it coarse bed material. The evaluation of the sediment varies according to several factors, but almost always the quantities are beyond the capacity that structures
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such as the circumferential floodways are designed to handle. Large inlets, large scale earthmoving works to maintain slope, unrealistically wide floodways, and paved stretches of the banks and bed are some of the techniques at the disposal of engineers to surmount these problems. All of these are expensive, though, and all require careful maintenance over the long years between big floods. A preferable method is based upon a record of hydrologic measurements and associated geomorphic research done over the years in the region, primarily in the Nahal Yael catchment ŽSchick, 1988.. In contrast to most methods employed in the engineering profession, which are based on sophisticated models predicting peak discharges or flood volumes fed by few and poor data, we employ a simplistic model fully supported by a wide array of relevant data. We rely on extensive infiltration tests on a variety of terrains, which enable an evaluation of several factors: a finely distributed runoff production rate; rainfall-correlated runoff hydrographs for catchments 0.05–0.5 km2 in area; channel bed infiltration rates both from ponded water and during an artificially controlled flood; a substantial body of suspended sediment samples from desert floodwaters; direct and indirect evaluations of bedload transport; and detailed analyses of the reservoir sedimentation accumulated in behind the Nahal Yael dam over the last 18 years. The simplistic procedure estimates the volumes of bedload transported by various flood events, which we consider the key factor in the adaptation to and protection from desert floods in this environment. The volume of sediment — both bed material and suspended sediment — will necessarily deposit somewhere en route, i.e., from the up-fan town boundary to the erosional baselevel below it. While nearly all the bed material will be deposited in some check dam or floodway conveyor, obstructing the latter if the volume is sufficiently great relative to the cross-section, the suspended fraction will tend to continue with the floodwaters to intermediate or terminal baselevels. The ponds of the Nature Beta Technologies plant on the lower margins of the Eilat alluvial fan slope Žbelow. is an illustration of the former. A massive blanketing of the coral population along the Eilat Nature Reserve coast — perhaps with irreversible ecological damage — to be expected as
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a result of any high magnitude–low frequency event in Nahal Garof or Nahal Shahmon, is an illustration of the latter. 4.2. Lateral urban fringes The southern additions to the urban area currently under construction are built on land created by the complete restructuring of the terrain surface of a 4-km2 area of steep, dissected alluvial fan slopes. The drainage is designed to close off the incoming flow from the mountain front by a series of check dams, small reservoirs, and flood conveyors to one of the two larger lateral stream channels — Nahal Shahmon and Nahal Garof — which, according to the plan, will be ‘regulated’. The internal drainage will be sewered and piped towards these regulated collectors. Due to the high density projected for the development, the runoff will be high but practically sediment-free, unless a big flood event occurs before construction is finished. However, any drainage mishap at any point within the protective devices upslope may result in the injection of sediment-laden floodwaters into the drainage system and its subsequent clogging and disruption, which will potentially lead to the breakdown of the entire system. The two main stream channels drain steeply towards the Eilat coast, with the port of Eilat and other major installations at their mouths. Although partly protected from flood damage by various local devices, these installations are at risk in case the fragile system contingent upon the new development breaks down. Furthermore, this southern coast of the town is the venue of the Coral Nature Reserve — the northernmost coral colony in the world. The detrimental effect of a major sediment-laden flash flood, fueled by the large construction and development project upstream, may be long lasting. Previously, due to natural infiltration in the terrain and channel beds, only a few flows reached the Gulf, probably with only a small fraction of the sediment load originally carried by these flows when exiting the mountains. 4.3. Fan slope toe The bottom parts of alluvial fans and slopes in the desert environment are the terminus of all flows that
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make it through the intermediate sinks of infiltration and, possibly, check dams and retarding basins. Prior to construction, the inherent structure of the alluvial fan dictates a degree of indeterminacy as to where exactly the next flood will exit the fan, and in how many loci. Construction, even with many flood control devices, tends to increase this indeterminacy. Eilat may be a typical example to illustrate this. The airport runway — the lifeline of this international resort — runs for 3 km along the toe of the fan slope, parallel to its contours, at an elevation just a few meters above the nearby sea level. The main downslope directed streets of Eilat, originally designed also as floodways, are actually aimed at the runway. A protecting ditch functions only for small flows, which also use some drainage tunnels under the runway. The latter are almost certain to be filled by sediment during a big event. The flows are perforce directed to the narrow land isthmus between the southern tip of the runway and the northwestern head of the Gulf of ’Aqaba, a strategic 150 m stretch of land which accommodates all traffic between the central business area of the town and its main hotel and recreation area. Fine sediment deposited on the road traversing this stretch is a common occurrence, even from small floods that leave whatever bed material they may have transported from further upstream. On a much larger scale, deposition of fines caused the main damage resulting from the 2 November 1994 flood in Nahal Roded. This relatively small flood was generated by an isolated rain cell which covered only a small, upstream part of the catchment. The flow that exited the mountains was further attenuated by filling extensive gravel quarrying holes covering much of the wide braided channel bed. These holes trapped all the bed material and much of the fines. Still, though the peak discharge on the fan was only 40 m3 sy1 , the flow had enough energy to carry towards the southern sector of the fan, a distance of 2 km, enough suspended material to fill the algae ponds of Nature Beta Technologies with ca. 40,000 m3 of sediment ŽFig. 2.. The diversion of the flow into the plant was the result of unrelated construction activities upstream of the flow path: the breach of a diversion built in conjunction with some roadwork and the construction of an elevated landing ground for migrating birds. By the same token, it
may be regarded as a normal reaction of an alluvial fan which, over time, distributes its flow patterns more or less equally over all of its sectors. This geomorphic indeterminacy tends to persist, irrespective of engineering efforts to subjugate it. Another facet of the fan toe flooding problem is exemplified by the relation between the Eilat fan slope and Nahal Ha-Arava. Nahal Ha-Arava drains towards the Gulf of ’Aqaba several catchments of the Southern Negev Mountains, extending ca. 50 km to the north of Eilat. However, its effective drainage area is, to some extent, indeterminate, as some of the flows are abstracted by two of the playa depressions within the rift valley. The effective integration of the entire hydrographic network ascribed to Nahal HaArava is a moot point, except perhaps for the very largest events. Still, Nahal Ha-Arava has been known to transmit large discharges, and over the years, its channel has been ‘regulated’ by widening, streamlining, and creating embankments to protect the Eilat hotel area located on its western side. With increasing pressure on the land spurred by the boom in tourism, plans are under consideration to divert the Nahal Ha-Arava channel-cum-floodway several hundred meters to the west, to coincide with the Israel– Jordan border. Thereby, the contiguity of the hotel area will be achieved, but the design presents many options, some strongly constrained by geomorphic features and processes. The design must consider the input, conveyance, and sediment deposition of floods from three different sources: local urban drainage, including whatever comes from the urban alluvial fan slope; the flow of Nahal Ha-Arava; and the impact of floods in the large catchment of Wadi Yutm, coming from the mountains of southern Jordan in the east ŽWolman, 1996.. Flood events from each of these may be isolated, any two may be active at one time, or all three may be active simultaneously. The time lag between rainfall of these three sources must be considered: for the first it may be an hour less; for the second, several hours; and for a Wadi Yutm flood, a day or two. A scenario is possible wherein a flood in Nahal Ha-Arava will disrupt its channel and protective devices to such an extent that a large flood coming from the same synoptic system from Wadi Yutm will cover the entire open space between ’Aqaba and Eilat.
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Flood attenuation in Nahal Ha-Arava might be achieved by having a retaining embankment only on one side of the channel and letting the high discharges temporarily spread over the other side. Diverting the channel to the international boundary probably precludes this option, as the envisaged economic development of the region will affect ’Aqaba as well, and using extensive areas of its western margins for floodwater spreading hardly seems acceptable. Another consideration related to baselevel concerns the substantial tidal differences, augmented by southerlies often associated with the Red Sea Trough — a synoptic configuration known to be associated with most of the high magnitude storms in the region ŽSharon and Kutiel, 1986.. It may well be that the floodwaters in the Nahal Ha-AravarGulf of ’Aqaba inflow area will be flowing towards a baselevel 1 to 2 m higher than the mean sea level, with serious consequences for the entire low-lying flat area, including a dramatic increase of the level of groundwater. Plans underway to extend the Eilat coastline by constructing one or more additional sealevel ‘‘lagoons’’ would bring a more easily accessible baselevel closer to the pathways of some of the anticipated flows, a possibility which requires either the acceptance of sedimentation of these recreational lagoons or the construction of massive protective devices to prevent the flows from accessing them.
5. ’Aqaba — some observations The town of ’Aqaba, symmetrically located opposite Eilat at the NE corner of the Gulf ŽFig. 1. and somewhat larger in population, presents interesting parallels as well as contrasts to the flooding problems of Eilat as outlined above. The northern part of the town is directly affected by the large Wadi Yutm, described below. The central part of the town, including the port and the main hotel area, occupies alluvial fan terrain at the outlet of three catchments 6 to 10 km2 in size each. The divides of these catchments have elevations around 1000 m.a.s.l. on peaks ca. 6 km east of the coastline ŽFig. 1.. The high relief of the bare granite provides the background for extremely destructive flooding in the downstream
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urbanized strip squeezed between the mountain front and the coast. Similar to Eilat, but with a larger damage potential due to larger catchments and higher relief, possible engineering solutions are hampered by existing urban and transportation structures. Besides the urban center, which in places leaves little room for floodway dimensions adequate to convey high magnitude–low frequency flash floods, the effects of a railway line built on an elevated embankment skirting the town along its upslope margins play a role. Although drainage openings through this embankment are provided, the possible flooding scenarios of large sediment-laden floods cum debris flows rushing down the steep Žslope 0.06–010. mountain channels are difficult to predict. The large fan of Wadi Yutm, with its apex located at an elevation of 200 m.a.s.l. 8 km north of the town center, is without parallel on the western side of the Southern ’Arava. The southernmost sector of this fan directs its distributaries towards the town. These endanger directly a quarter of unplanned, temporary dwellings for which new and safer sites are projected. The 1966 storm, known as the Ma’an flood, was centered north of the Yutm catchment, and yielded in Wadi Yutm several peaks, one of which reached a discharge of 500 m3 sy1 , with reported float velocities of 6 m sy1 and surges up to 12 my1 ŽSchick, 1971.. The steep overall slope of the fan Ž0.075. provides opportunities for floodwaters to reach any point along the 8 km circumference of the fan sector, and together with the smaller fan adjoining to the north, creates a flooding hazard to the ’Aqaba airport with its 3-km long runway ŽFig. 1.. Much of the fan area between the airport and the northern suburbs of ’Aqaba is occupied by a free customs zone, whose 2 km long ŽN–S. compound must likewise be protected from floods. This leaves only two ‘‘openings’’ several hundred meters wide each for a natural and relatively safe outlet for future floods. Long dikes and embankments are needed to direct floodwaters emanating from the Yutm away from new development quarters constructed beyond the fan toe in the flat ’Arava terrain, which are already close to the coastal playa located astride the international border. Both ’Aqaba and Eilat regard these coastal saline tracts as valuable for future develop-
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ment, particularly for tourist facilities. Hence, a long-range and mutually coordinated view in their flood control planning must be exercised. In at least three recorded events — two in the 1940s and one in the 1960s — extensive flooding of the playa lasting a number of days was reported ŽSchick, 1971.. In ’Aqaba as well as in Eilat, underground stormwater sewers have been recently built or are being planned in some developments. While in ’Aqaba this has been done on a moderately sloping terrain, in Eilat such a drainage system is about to be built on a steep alluvial fan slope development. Though the non-paved contributing area to both system is not large Žif protective devices do not fail., the natural affinity of floodwaters to sediment in arid environments is likely to cause clogging of inlets relatively frequently. Assuming that ’Aqaba, similarly to Eilat, will spread to the north in the near future, the flooding hazard from the bajada extending along the southeastern margins of the ’Arava must be considered before such a development takes place. This bajada comprises many adjoining steep alluvial fans of varying sizes and forms, with Wadi Muhtadi, 40 km north of ’Aqaba, large enough to locally raise the level of the rift valley to form the playa of Yotvata. The surface of the fans is covered in varying proportions with a light gray tone, indicating currently active, mostly spreading out channels. Between those there are tan colored areas, suggesting no flooding in the recent Ž; 100 years. past, and darkly colored areas composed of patinated pavement which seem to have stayed free of flooding for at least several hundred years. Occasionally, grey streaks on the dark surfaces testify to ‘overbank’ flows which diverged from one of the channels during high flow, but did not result in full avulsion. Despite similarities of environmental conditions, each fan shows a high degree of individuality in shape and structure. The fans extending to the north of ’Aqaba, like their counterparts in the west, are partly covered by Acacia trees. These are concentrated in marked preference in certain segments and generally avoid the dark, desert varnish areas. The dependence of the acacia in this environment on surface waters has been recently established ŽBenDavid-Novak and Schick, 1997.. This is a valuable tool in evaluating flooding probabilities within each fan.
6. Conclusions The projected development of the arid Southern ’Arava requires a detailed, science-based evaluation of the flooding hazard prior to major construction, whose optimal siting is of cardinal importance. The built-up areas on the fans, which are the only viable terrain types available, are bound to appreciably affect the flooding hazard further downstream. The evaluation of the hazard is not just a matter of rainfallrrunoff frequencies and catchment areas. A much more sophisticated approach, involving the application of diverse hydrologic, sedimentologic, geomorphic, and historical procedures to each and every fan separately, can greatly improve the efficiency of the necessary engineering solutions to the flooding problem. It is important to note that in both ’Aqaba and Eilat, the pristine waters of the Gulf of ’Aqaba represent an enormously valuable resource. Mere hastening of sediment-laden floodwaters may not be an appropriate resolution of the potential flood problem in this setting. Acknowledgements This study was done within the framework of research grant No. 12-090 of the United States–Israel BiNational Science Foundation. The assistance of authorities dealing with the flooding problem both in Eilat and in ’Aqaba and of Yehouda Enzel, Qinghong Huang, Jens Lange and Judith Lekach is gratefully acknowledged. References BenDavid-Novak, H., Schick, A.P., 1997. The Response of Acacia Tree Populations on Small Alluvial Fans to Changes in the Hydrological Regime: Southern Negev Desert, Israel. Catena 29, 341–351. Blair, T.C., McPherson, J.G., 1994. Alluvial fan processes and forms. In: Abrahams, A.D., Parsons, A.J. ŽEds.., Geomorphology of Desert Environments. Chapman & Hall, London, pp. 354–402. Bull, W.B., 1977. The alluvial fan environment. Prog. Phys. Geogr. 1, 222–270. Bull, W.B., 1991. Geomorphic Responses to Climate Change. Oxford Univ. Press, Oxford. French, R.H., 1987. Hydraulic processes on alluvial fans. Developments in Water Science 31.
A.P. Schick et al.r Geomorphology 31 (1999) 325–335 Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. Springer, Berlin. Greenbaum, N., 1996. Paleofloods in the Large Ephemeral Stream Systems of the Negev. PhD Thesis, Hebrew Univ., Jerusalem. Greenbaum, N., Margalit, A., Schick, A.P., Sharon, D., Baker, V.R., 1999. Reconstruction of a high magnitude rainstormflood in Nahal Zin — a large hyperarid catchment in the Negev Desert, Israel. Hydrological Processes 12, 1–23. National Research Council, USA, 1996. Alluvial Fan Flooding. National Academy Press, Washington, DC. Rachocki, A., 1981. Alluvial Fans: An Attempt at an Empirical Approach. Wiley, New York. Schick, A.P., 1971. A desert flood: physical characteristics, effects on man, geomorphic significance, human adaptation. Jerusalem Studies in Geography 2, 91–155. Schick, A.P., 1974. Alluvial fans and desert roads — a problem in applied geomorphology. Abh. Akad. Wiss. Goettingen, Math.–Phys. Kl., III Folge 29, 418–425. Schick, A.P., 1988. Hydrologic aspects of floods in hyperarid environments. In: Baker, V.R. ŽEd.., Flood Geomorphology. Wiley, New York, pp. 189–203. Schick, A.P. 1995. Fluvial processes on an urbanizing alluvial fan:
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Eilat, Israel. In: Costa, J. ŽEd.., Natural and Anthropogenic Influences in Fluvial Geomorphology. Geophys. Monogr. 89, pp. 209–218. Schick, A.P., Lekach, J. 1993. An evaluation of two ten-year sediment budgets, Nahal Yael, Israel. Phys. Geog. 14, 225– 238. Schick, A.P., Lekach, J., Grodek, T., Lange, J., Leibundgut, C., 1996. An artificial flash flood in a small arid stream channel, Eilat Mountains, Israel. Suppl. to EOS, F259, Nov. 12, 1996. Schick, A.P., Grodek, T., Lekach, J., 1997. Sediment management and flood protection of desert towns: effects of small catchments. In: Walling, D.E., Probst, J.-L. Žeds.., Human Impact on Erosion and Sedimentation. Int. Assoc. Hydr. Sci. Publ. 245, pp. 183–189. Sharon, D., Kutiel, H., 1986. The distribution of rainfall intensity in Israel, its regional and seasonal variations, and its climatological evaluation. J. Climatol. 6, 277–291. Wolman, M.G., 1996, Some considerations in selection of the design of floodwater drainage for the expanding development of the Eilat foreshore. Technical Report submitted to the Eilat Foreshore Development Company, Israel, May 1996.