Welsh floodplain studies: The nature of floodplain geometry

Journal of Hydrology, 25 (1975) 37--50 © North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands

WELSH FLOODPLAIN STUDIES: THE NATURE OF FLOODPLAIN GEOMETRY

JOHN LEWIN and M.M.M. MANTON

University College of Wales, Aberystwyth (Great Britain) The City University, London (Great Britain) (Received August 29, 1974; accepted September 4, 1974)

ABSTRACT Lewin, J. and Manton, M.M.M., 1975. Welsh floodplain studies: The nature of floodplain geometry. J. Hydrol., 25: 37--50. Field studies of floodplain geometry are necessary if the extent and patterns of inundation are to be related to discharge magnitudes. This is considered in relation to the floodplain geometry of parts of the three Welsh rivers -- the Ystwyth, the Rheidol and the Tywi. A photogrammetric approach has been used and tested, together with the development of computer procedures for automated plotting of floodplain profiles on any required scale. The complex and locally variable relief (1.7 to 3.3 m) of the floodplains studied is shown to be related to former braided and meandering river activity, some of it within the past 130 years for which reasonable historical evidence is forthcoming. Relief results dominantly from the presence of abandoned channel loops, from point bars and from former channel bedforms now incorporated into the floodplain surface. Local aggradation and incision are also evident. Variation along the rivers is such that a single discharge-related flood-stage is not identifiable: instead there are sequences of flooding and emptying beginning at relatively low discharges and related to floodplain relief. Some consequences for flood studies and floodplain management are suggested.

INTRODUCTION F o r s t u d y purposes, f l o o d s have b e e n d e f i n e d as " i n u n d a t i o n s o f n o r m a l l y dry l a n d " or as " e x c e p t i o n a l l y high rates o f discharge in w a t e r c o u r s e s " (Wolf, 1 9 6 6 ) . H o w e v e r , t h e relationship b e t w e e n discharge m a g n i t u d e s and the degree o f i n u n d a t i o n is n o t a simple one. Despite s o m e t e n t a t i v e claims ( D u t y et al., 1 9 6 3 ) , it is n o t even a c c e p t a b l e o n a general basis t o assign bankfull discharges t o s o m e specific r e c u r r e n c e interval based o n f l o o d f r e q u e n c y analysis. A v a r y i n g r e l a t i o n s h i p b e t w e e n t h e d i m e n s i o n s o f river channels a n d discharge r e c u r r e n c e interval is t h o u g h t t o arise b o t h because o f variation in t h e d u r a t i o n o f f l o o d s o f given f r e q u e n c y (reflecting h y d r o l o g i c regime), a n d because o f t h e diversity o f b e d a n d b a n k materials r e q u i r i n g v a r y i n g s t r e a m p o w e r (and t h e r e f o r e p e r h a p s flows o f differing m a g n i t u d e ) to f o r m channels in t h e m (Hack, 1 9 6 5 ; Harvey, 1 9 6 9 ; Tinkler, 1 9 7 1 ) .

38 Even when rivers do achieve discharges which reach or exceed bankfull stage, the spatial pattern of inundation is further critically affected by the geometry of the floodplain itself which is often far from simple. In Britain, concern over flood hydrology has recently led to concentrated studies, largely working on stream discharge and rainfall records,-at the Institute of Hydrology and the Meteorological Office (Anonymous, 1971). If actual patterns of flood inundation are to be understood, studies of floodplain geometry are also of considerable importance. Accordingly, we here report on analyses of floodplain geometry in Wales, together with some discussion of the method being employed. Both methods and examples may be found relevant to studies of flood extent and flood routing; this work can be extended further into flood probability mapping (Wolman, 1971 ), as has been undertaken in the United States (Hopkins, 1968), although there are few if any published British precedents for work in this field. In terms of natural processes, floodplain geometry essentially derives from patterns of fluvial erosion and sedimentation, and floodplain geometry may vary according to the type of sedimentary activity. A useful general review of such processes has been given by Allen (1965). One critical factor is the amount of overbank deposition; another is the prevailing channel pattern and the form of its migration. For the most part in the areas studied the amount of overbank deposition is slight, both as a result of insufficient fine materials being presentin overbank flows and because rapid migration of channels allows insufficient time for such materials to accumulate in depth. Thus natural levees (wedge-shaped ridges of sediment found adjacent to the channels) are not present, and flood deposits of vertically-accreted fine sediments are not found consistently in all the areas studied. Instead the major features of the floodplain topography are as follows: (a) Point bars consist of curving linear ridges (scroll bars) and depressions within developing meander loops: the ridges are deposited on the inside of channel bends during periodic high flows against the sloping face of the preceding bar. Depressions (swales) may be left between successive accretions. (b) Braid bars form lozenge-shaped alluvial islands between the branches of braided channel networks. These may be incorporated into the floodplain once the channels are abandoned when they may fill with sediment. (c) Cut-offs may be formed either as neck cut-offs late in the development of meander loops as the narrow gap between reaches is eventually breached, or as chute cut-offs when flood channels develop along point bar depressions. The latter are more common in the area studied. They may become plugged by hedload sediments, or excavated as a new main channel. In addition to these features, channels may also be characterized by general or local ag~adation or incision, such that the active channels are raised above or cut deep into floodplain deposits. The former is likely to present serious flood problems. In the a r e a s studied, aggradation and incision appears to be developed on a local scale, with closely adjacent reaches showing signs of both.

39 These physical processes of floodplain formation are as yet neither widely d o c u m e n t e d nor sufficiently understood. The number of available field surveys of floodplain geometry showing sufficient detail is also small, so that it would be premature to a t t e m p t a general typology of actual floodplain geometry. But the examples shortly to be presented may usefully be compared with others that have been made in greater or lesser detail, and it should eventually be possible to anticipate patterns of inundation in a given fluvial environment. Beyond the geomorphological features so far mentioned, two further floodplain characteristics need to be considered, although they will n o t be examined extensively in this paper. On river floodplains, artificial structures can have specific river training or flood protection functions, but there are also those which arise incidentally from road, rail, canal and field embankments. These are of considerable importance in restricting and compartmentalizing overbank flows and storage. Again, the extension of built-up areas on to floodplains, in Britain particularly since the nineteenth century (Nixon, 1966), has added further complication in the movement of inundating water. The geometry of floodplains continues to be modified b y the extension of urban and industrial development, and b y waste disposal, such that the passage of floodwaters is irrevocably modified. Secondly, the effects of vegetation can be significant. The woodland, scrub, and agricultural crops met with in a British c o n t e x t are all permeable to overbank water movement, but they may lower velocities, affect patterns of overbank sedimentation, hinder bank erosion, and restrict stock m o v e m e n t at flood times. Thus their extent and pattern are worth assessing. In the areas studied here, some floodplains are cultivated to the edge of the active river channel, but others can have a scrub zone of fluctuating extent, present particularly in areas of channel braiding and where rapid channel migration has left deposits initially unsuitable for cultivation. This palimpsest of geomorphological form, artificial structure and vegetation is combined in such a way as to produce complex patterns of effective flood~ plain geometry. Following discussion of methods employed, the nature of this geometry will be assessed with reference to four floodplain sites, summary details for which are given in Table [. SURVEY AND PLOTTING METHODS For various reasons, it was decided that photogrammetry based on specially commissioned air photography would provide the most useful and flexible basis for characterizing floodplain geometry. Published line maps, with limited height information, a concentration on the depiction of property lines, and a certain degree o f generalization for natural features and vegetation, can often be only of limited value in floodplain studies, particularly where channels have actively migrated since the maps were compiled. Conventional ground survey can also prove somewhat inflexible. Such survey is of course necessary

40

TABLE I Summary data for floodplain characteristics Ystwyth

Rheidol

Tywi (Llandeilo)

Floodplain width (m)

250---500

420--850

750--950

Valley floor gradient

0.004

0.00076

0.00125

Maximum migration plain relief (m)

1.7

2.5

2.3

Catchment area (kin ~)

155

179

633

Tywi (Llandove~-'~ 900--1100 ca.0.0032 3.3 331 ~

*Figure includes Bran and Gwydderig catchments.

to provide planimetric and height control for photogrammetric plotting and derivation of numerical data. But for the study of floodplain geometry, it was decided that photogmmmetry should play the major role because once suitable photography is available and the ground control obtained, then height and plan information for any point or points may subsequently be recovered photogrammet_ricatly, within specified limits of accuracy. In commissioning air photography, three major areas of decision arise. The first concerns the levels of accuracy required in the determination of height and plan information, because this in turn is related to the scale of the photography, as well as to conditions during photography, ground texture, and the plotting instrument and its operator. The plotting instruments available were a Kern P.G. 2, with " L " pantc~gaph, and a Santoni Stereo-Simplex IIID. Plotting was undertaken using 1: 5,000 photography of the rivers Ystwyth and Rheidol, and 1: 7,500 of the river Tywi. The accuracy of height determinations. as indicated by the standard error of observations of a single spot height, is better than + 0.10 m for the 1:5,000 photography and + 0.15 m for the 1: 7,500. Planimetric accuracy is +-0.30 m for both scales, when the data is read off numerically. Planning of flight lines next requires attention, in order to achieve the most economical coverage, the minimum of ground survey control, and minimum time to set up successive stereo models. This has to be considered for rivers individually. Finally, film and filter type require consideration. In the light of previous experience (Kidson and Manton, 1973) and comparisons between colour and false colour, plotting was undertaken usingcolour photography. No particular advantage was found in false cotour by comparison with colour obtained during "tow flow" conditions after a period of soil moisture deficit. Such condiJ;lons allow details of the channel to be observed, together with maximum differentiation of floodplain vegetation patterns. This shows up former channels and variation in floodplain sediments that cannot normally be seen.

41 Having obtained photography and ground control, it is possible to recover such vast amounts of numerical information concerning floodplain geometry that it is necessary to arrive at some m e t h o d of selection for display and interpretation, as much for the benefit of the photogrammetric plotter operator as the user. Whilst the numerical information is itself of direct value, we are concerned in this first instance with demonstrating the variability of floodplain geometry, and have been selective in taking serial floodplain profiles. Even then a single profile across the lower Rheidol floodplain may involve 120- 150 points for a profile 1100--1400 m in length. The recording of each point requires 18 characters for complete spatial definition, so that even to draw the profile involves considerable labour. Initial work on the Ystwyth involved hand plotting, but subsequently we have drawn on computer graphical plotting facilities, computer programmes in any case being necessary for transformation of the raw photogrammetric data into the most convenient plotting system. To do this we have used "PICASO"; a general-purpose pictorial o u t p u t system, the main area of which has been written by Rudling (n.d.) making use of the graphical o u t p u t peripherals on the Elliot 4100 computer at U.C.W., Aberystwyth. The structure of PICASO consists of a "main picture-defining, generating and control area" together with a number of sub-systems, some of which are cartographic in nature and these have been developed by us as useful for floodplain work. This allows automatic plotting of floodplain profiles at any scale consistent with the size of the plotting paper. Examples are shown in Figs.3, 4 and 6. THE STUDY AREAS

The Ystwyth at Llanilar (SN 6275) At Llanilar (Fig.l), the floodplain of the River Ystwyth locally contracts in width from approximately 500 to 250 m down valley within a distance of some 600 m. The floodplain is confined on the north by a solid-rock valley wall with some colluvial fans, and on the south by a terrace of Pleistocene silts, sands, and gravels rising some 6 m above the floodplain. Valley gradient is 0.0037. For 4 km above the area discussed the active channel has been realigned to run alongside the e m b a n k m e n t of a former railway, but the courses of rail and river then diverge for about 1 km so that the river channel is still locally free to migrate across a large part of the valley floor. The form of the floodplain is n o t simple, partly as a result of restraightening of the channel in 1969 following the century of channel migration subsequent to the initial railway construction and channel straightening of 1864. Generally speaking, the floodplain is higher on the channel side of the e m b a n k m e n t than on the other, locally by as much as 1 m. Additionally, the floodplain is convex in relation to the pre-1969 channel. Both of these features suggest recent, if local, aggradation. On the cross-profiles (Fig. 1), the floodplain is lowest

42

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Fig. 1. The Ystwyth floodplain at Llanilar (SN 6275).

43

adjacent to the valley sides, with at least 40% of the floodplain below the mean height of the present bank-tops on the surveyed profiles. Fig. 2 shows a plot of height against percentage of profile below it for selected profiles on the Ystwyth and other rivers. This provides a morphological indicator of liability to inundation but it is somewhat crude: surface:water profiles of inundating waters are c o m p l e x even in simple laboratory conditions, whilst the three-dimensional patterns of connection between areas of relatively low elevation must be important in determining actual inundation. Nonetheless, the illustrated curves do suggest a aistinctive flood-prone geometry. Floodplain relief in detail is furtlmr created by abandoned river channels which can be sharply defined and i m deep. These channels converge and diverge in this area, and although even some fairly recent abandoned channels (< 50 years) have been effectively eliminated by cultivation, interpretation of the commissioned photography is supported by examination of previous map and photographic evidence in suggesting some former braided channel develop2

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Fig. 2. Height plotted against percentage of profile length below height datum, in relation to mean banktop height, for selected profiles on the Rheidol, Ystwyth and Tywi.

ment. There is no evidence of substantial overbank deposition: we have here a floodplain c o m p o s e d of point and braid bars with infilled abandoned channels. Observations of an artifically straightened b u t unlined channel section show that channel migration and bar formation occurred at relatively high mean Froude number (> 0.61) during flows with a recurrence interval of approximately 0.8 year. The process of floodplain renewal, in the absence of constraints, is rapid, and the sinuous channel shown in Fig. 1 developed from a straightened channel during one winter. Migration of the sinuous channel takes place at a less rapid rate.

The Rheidol at Lovesgrove (SN 6280) The floodplain here varies from 400 to 850 m in width, though river

44 migration has been restricted by road and rail construction to as little as 300 m. Valley floor gradient is 0.00076. Cross valley profiles are illustrated in Fig. 3. These forms result from the migration of meander loops across the floor of the valley, leavingabaudoned channels and bars in a pattern of some complexity. Point bars are to be found on the inside bends of present and past meander loops, but some former mid-channel braid bars are also now incorporated into the floodplain surface. Meander loops have been abandoned when the channel still has comparatively low sinuosity following the formation of flood chutes across the floodplain. The development of this irregular floodplain surface may, however, be comprehended because much of the floodplain has been reformed within the period for which historical surveys are available (Lewin, 1972; Davies and Lewin, 1974). 65% of the floodplain between road and railway has been swept across since 1845, and historic maps and air photo~ graphs on eight occasions since this date allow reconstruction of stages in floodplain development. It is the arcuate cut-offs and convex profiles of the point-bar areas that tend to dominate floodplain relief today. A large number of cut-offs still contain perennial standing water, and at flood times these become reactived, initially by waters flowing up the channels from the open lower ends. In fact inundation can occur with flows of relatively low magnitude both following groundwater rise and flow up cut-offs to the extent that the concept of floods occurring above some "bank-full" stage is inapplicable. This floodplain thus resembles that of the fiver Oka in the early stages of inundation (Popov and Gavin, 1970); floods of greater magnitude in which floodwaters transgress right across the floodplain, obliterating from view the details of floodplain geometry, rarely occur. Within the area studied, there are some considerable differences in the form of surveyed profile (Fig. 3). In profile A floodplain relief is 2.5 m, two sequences of meander-loop development can be seen north of the present channel and these can be dated as having been active within the last century. In profile B, the channel adjacent to the railway was artifically cut off to prevent further erosion of the embankment: braiding of the steepened and straightened main channel resulted: Profiles C, D and E show very little floodplain relief, though interestingly it is the area north of the turnpike road (and thus remote and protected from channel migration for a century and a half) which is consistently the lowest part of the natural floodplain. Recent local a ~ t i o n of the floodplain is sttggested, as already described on the Ystwyth, but it must be em~A$ised that this e ~ m d a t i o n is not consistently developed on the R h ~ l o l . It is apparent on profile G but not on F. Profile G also shows two abandoned loops adjacent to the turnpike road (both active since 1845), with the present channel in a central valley position. Before river regulation under a hydroelectric scheme, this area was wellknown as exceptionally liable to inundation; as nearly 40% of the floodplain is below banktop height this does not seem surprisin.g, although it was not remarked upon until this detailed survey. This emphasises the value of surveys of this kind.

45

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Fig. 3. The Rheido] floodplain at Lovesgrove (SN 6280).

The illustrated profiles (Figs. 2 and 3) thus show a notable variety of'fioodplain geometry and relief within a short stretch of valley: such variability emphasises the need for extreme caution in assigning a degree of inundation to a magnitude of flow on an individual river. In addition, current river activity, involving both channel migration and locally both aggradation and incision, can be linked to changing patterns of flood liability. The Tywi at Llandeilo (SN 6423 and SN 6524) Fig. 4 shows two nearly adjacent reaches of the River Tywi east of Llandeilo; natural floodplain width (neglecting the effects of artifical structures) is 750--950 m, and valley gradient 0.00125. In the western reach (Fig. 4,1), river migration has been active in the past 130 years to the extent mapped in Fig. 5, and remains of abandoned channels may also be distinguished on the air photography beyond the confines of this activity. Floodplain relief approaching 2.3 m is present, with topographic "lows" related to abandoned

46

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Fig.4. The Tywi at Llandeilo; 1. SN 6423; 2. SN 6524.

channels, e.g. adjacent to the railway in profile B. The Tywi compares with the Rheidol in that the known history of river migration and channel abandonment can be used to account for some of the complexities of morphology, as in profile B, but the patterns of floodplain reliefis complex in detail (profile E). Recent active river .migrationis by no means invariablythe rule either on the Tywi or on other loealrivers: thus in the area mapped in Fig.4,2, the channel has maintained its present location for at least 130 years, without

considerable bank protection, a.r~lyet in an a~a with clear signs of abandoned meander loops on the floodplain itself(Fig.4, profilesG and H). Unusual degrees of stabilityhave been reported elsewhere (Alexander and NunaUy,

47

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Fig. 5. Dated locations for f o r m e r courses of the River T y w i in the area m a p p e d in Fig. 4,1.

1972) b u t the reasons for phases of activity and quiescence on a single river reach are not immediately clear. From a flood point of view, however, the floodplain geometry differs little; as on the Rheidol, a high proportion of the floodplain is below b a n k t o p height (Fig. 3, profiles G and D, and Fig. 2).

The site of Llandovery (SN 7634) The town of Llandovery is situated between the river Tywi and one of its major tributories, the Bran (Fig. 6). Whilst the earliest Roman settlement was clear of the floodplain, most of the present town lies at a low elevation and is potentially liable to flooding were the incised channels overtopped. The form of the valley floor is complex. The main arcuate lower course of the Tywi valley down to Carmarthen is continued headward by the Bran rather than the main river, which enters the major valley trough in a relatively confined valley from the north. Profiles A and B {Fig. 6) show the Tywi at a higher elevation than the Bran on the other side of the valley trough, with a noticeable slope to the valley floor from north to south. Apart from artifical structures, the banks of the Tywi are the highest parts of the valley floor. Approximately the same form can be seen in profile C, except that the actual channel of the Tywi is more greatly incised below the A40 roadbridge. Profiles D and E, which do n o t include the Tywi itself since it lies outside the photographic coverage to the north, again show that the land on which the town was built rises at most 2.5 m above the natural bank of the Bran, and for much of the area is considerably less than this. Both the Bran and its tributary the Gwydderig are actively migrating rivers, incisedslightly into the

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49 main valley floor on profile E. Here abandoned traces of former river courses may be seen for approximately 170 m to either side of the present channels, with the valley floors incised 1--2 m to this width (see also profiles F and G). Profiles in the built-up area were drawn using points located on the ground only: they thus take no account of buildings or artifical structures which in practice would considerably impede and channel flooding waters. Roads within the urban area in particular transmit rather than prevent flood flows, and spot heighting of points along roads (a selection of heights being shown in Fig. 6) suggests that this is the case in Llandovery. CONCLUSIONS

(1) Survey of floodplain sites on three rivers in Wales shows migration plains which are well-defined at the margins, and with relief varying up to 2.5 m in relatively simple situations. A high proportion of the floodplains is below mean b a n k t o p height, and the present river banks may be the highest parts of the "plain". Overbank deposition of fine flood sediments is slight and natural levees are n o t present at the sites studied. The major relief forms are produced by incision or aggradation during channel migration; with abandoned channels, undulating point bar relief and former channel bedforms (generally in this order of importance} forming relief elements of between 1 and 2 m, although local valley-floor forms (as at Llandovery) may be more complex. (2) The geometry of floodplain which results from geomorphological processes may be comprehended in part because historical documents are available covering a period when channel change has been active. The timescale over which such changes occur leads to alterations in flood-liability patterns within a matter of decades. But b y no means all the river reaches examined have been active in the last 130 years, although floodplain morphology and the presence of former channel traces are consistently developed throughout the areas studied. It seems likely that these forms resemble those of numerous "gravel" rivers in a British upland environment. Such forms contrast with those found in rivers that have well-developed levees and backswamps or large quantities of over-bank deposition, and their patterns of inundation are likely to be equally distinct. (3) No single relationship is apparent between the degree of inundation and the magnitude of flow along the valley floors. Parts of the floodplain may be inundated in the early stages of a flooding sequence (or in floods of small magnitude) b y means of the numerous abandoned channel loops. A sequence of filling, transmission and drying-out of the floodplain m a y follow, with the local pattern of inundation closely related to the local geometry of the floodplain. This sequence is varied spatially along each floodplain and is rendered more complex by the effects of communication lines and field embankments in rural areas, and of buildings in urban ones. (4) Photogrammetric survey methods provide a very useful means of obtaining information concerning floodplain geometry. It is possible using appropriate air photography to identify areas specially liable to inundation as

50 a result of recent fiver or constructional activity. The surveys undertaken show the high proportion o f f l o r l n s below bauktop height; the unsuspected existence of areas now cut-off from flooding and aggradation by the construction of road, rail, canal and other embankments but at risk from exceptional floods;and areas in which the potenti~| cost of flood damage requires regulation of floodplain land use. Further field studies of the rates and nature of channel activity, and of actual inundation patterns in relation to discharge magnitudes, are in hand in mid-Wales, and it would seem advisable for practical purposes to extend such work to other fluvial environments.

ACKNOWLEDGEMENTS

W e are grateful to the Natural Environmental Research Council for the provision of a research grant, and to R.P.A. Brown for undertaking some of the survey and photogrammetric work.

REFERENCES Alexander, C.S. and Nunaily, N.IL, 1972. Channel stabilityon the lower Ohio River. Ann. Assoc. Am. Geogr., 62:411--417 Allen, J.R.L., 1965. A review of the origin and characteristicsof recent alluvialsediments.

Sedimentotogy, 5; 87--191 Anonymous, 1971. Flood studies at the Institute of Hydrology, Wallingford. J. Inst. Water Eng., 25; 160--162 Davies, B.E. and Lewin, J., 1974. Chronosequences in alluvial soils with special reference to historical lead pollution in Cardiganshire, Wales. Environ. Pollut., 6:49--57 Dury, G.H., Hails, J.R. andRobbi@, M.B., 1963. Bankfuli discharge and the magnitudefrequency series. Aust. J. ScL, 26. 123- 124 Hack, J.T., 1965. Post-glacial d l ~ e v o t u t i o n and stream geometry in the Ontonagon area, Michigan. U.S. Geol. Surv., ProfessionalPaper No. 504-B Harvey, A.M., 1969. Channel capacity and the adjustment of streams to hydrologic regime. J. Hydrol., 8 : 8 2 - - 9 8 Hopkins, B.T., 1968. Map requirements for floodplain studies. Cir. Eng., 38:66--67 Kidson, C. and Manton, M.M.M., 1973. A u e u m e n t of coastal c ~ with the aid of photogrammetric and computer-aided taehniques. Estuarine Coastal Mar. Sci., 1 : 271--283 Lewin, J., 1972. Late-stage meander growth. Nat. Phys. Sci., 240:p.116 Nixon, M., 1966. Flood r~gul~tion and river training. In: R.B. Thorn (Editor), River Engineering and Water Com~Jrvation Works, Butterworth, London, pp. 239--271 Popov, I.V. and Gavin, Y.S., 1970. U l e of aerial p h o t ~ in evaluating flooding and emptying of river floodplains and the developmant of floodplain currents. Soy. Hydrol., 5:413--425 Rudling, B.T. A user's guide to PICASO. Department of Computer Science, U.C.W., Aberystw~yth (unpublimhed) Tinkler, K.J., 1971. Active valleymundem in south-central Texas and their wider implicationa Bull. Geol. Soc. Am., 82:1783--1800 Wolf, P., 1966. Compexieon~ of methods of flood estimation. In: Institution of Civil Engineem, River Flood H~drole~, The Irmtitution, London, pp.1- 24 Wolman, M.G., 1971. Evallb~ting alternative techniques of floodplain mapping. Water Resour. Res., 7: 1383--1391