Hydrology for the Water Management of Large River Basins (Proceedings of the Vienna Symposium, August 1991). IAHS Fubl. no. 201,1991.

BANK EROSION AND MEANDER MIGRATION OF THE RED AND MISSISSIPPI RIVERS, USA

COLIN R. THORNE Department of Geography, University of Nottingham, Nottingham NG7 2RD, UK

ABSTRACT On large rivers serious bank retreat usually occurs by a combination of fluvial attack of intact bank material plus mass failure under gravity, followed by basal clean-out of failed material. Both components of retreat are affected by bank material properties. The susceptibility of the bank to erosion by various erosive proces­ses depends on the engineering and geomorphic properties of the bank material and the distribution of materials with different pro­perties through the bank. The stability of the bank and its characte­ristic mode of failure depend on the geotechnical and geological properties of the bank materials. However, the rate of retreat of the bank that can be sustained over a considerable period is determined by the capacity of the flow at the bank toe to entrain and remove bed, bank and failed material. The balance between erosion and accretion of sediments at the toe can be described by the state of basal endpoint control. The importance of bank properties for bank retreat, channel geometry and meander evolution may be illustrated by reference to migration of meander bends of the Red and Lower Mississippi Rivers in the USA. Historical evidence shows how the nature of materials encountered in the outer bank affects both the rate and distribution of bank erosion in a bend, influencing the speed and direction of bend migration and so altering overall the pattern of channel evolution.

BANK EROSION PROCESSES

Bank erosion occurs when grains or assemblages of grains are removed from the bank face by the flow. Erosion consists of two distinct events: detachment; and entrainment. Sufficiently strong forces of lift and drag exerted on the bank by the flow may detach and entrain grains directly from the intact soil, but more commonly grains are loosened and even detached prior to entrainment by weakening and weathering under sub-aqueous or sub-aerial conditions. The nature of the processes responsible, and the form of grain or grain assemblage entrai­ned, depend on the engineering properties of the bank material. Of particular importance is the presence or absence of cohesion.

301

Colin R. Thome 302

Non-cohesive banks

Non-cohesive bank material is usually detached and entrained grain by grain. Stability depends on the balance of forces acting on surficial grains. Motivating forces are the downslope component of submerged weight and the applied fluid forces of lift and drag. Resisting forces are the slope normal component of submerged weight and inter-granular forces due to friction and inter-locking. Interlocking can be a major source of erosion resistance in imbricated alluvial deposits. When analyzing the stability of a non-cohesive bank with respect to flow erosion, the lift and drag forces can be represented by the boundary shear stress, because this scales on the same parameters of flow intensity (Wiberg & Smith, 1987). This approach yields reliable estimates of bank stability for non-cohesive materials. However, for natural rivers, the practical applications of analyses for non-cohesive banks are limited by the fact that most alluvial bank materials exhibit some cohesion. This is may be real cohesion due to the presence of silt and clay fractions, or apparent cohesion due to either capillary suction in the unsaturated zone, or the binding effect of vegetation roots and rhizomes.

Cohesive banks

Cohesive bank material is usually eroded by the detachment and entrainment of aggregates or crumbs of soil. The motivating forces are the same as those for noncohesive banks, but the resisting forces are primarily the result of cohesive bonds between particles and aggregates. The bonding strength, and hence the bank's erosion resistance, depends on the physio-chemical properties of the soil and the chemistry of the pore and eroding fluids (Arulanandan et al., 1980). Field and laboratory experiments show that intact, undisturbed cohesive banks are much less susceptible to flow erosion than non-cohesive banks (Thorne, 1982).

Usually, serious erosion of cohesive banks takes place through the loosening or detachment of aggregates by sub-aerial or sub-aqueous processes, followed some time later by entrainment of the disturbed material by the flow (Wolman, 1959). The processes responsible for loosening aggregates are driven by the dynamics and physical state of soil moisture close to the bank face. If the bank is poorly drained, positive pore water pressures act to reduce the effective cohesion and weaken the soil. In extreme cases, loss of strength may be com­plete, leading to bank failure by liquefaction. The most favourable conditions for high pore pressures occur in saturated banks following heavy and prolonged precipitation, snowmelt, and/or rapid drawdown in the channel. Where the soil surface is exposed, surface processes of raindrop impact and overland flow may also be important.

Most alluvial soils are expansive, that is they swell and shrink significantly during cycles of wetting and drying. This generates both a ped fabric, with desiccation cracks between peds, and a crumb structure to the soil. Cohesion between peds and crumbs is much weaker than within them, so that a heavily desiccated soil may have little erosion resistance.

Freezing of soil moisture can seriously reduce erosion resistance. This has been demonstrated for the case of needle ice formation by Lawler (1986). At

3 03 Bank erosion and meander migration

larger scales, ice lenses and wedges heave apart soil peds and blocks destroying inter-ped cohesion, and ice cantilevers left on the banks during the spring thaw can do serious structural damage (Church & Miles, 1982).

Stratified Banks

Alluvial banks often consist of layers of non-cohesive and cohesive materials. For example, non-cohesive point bar sands may be interspersed with fine-grained overbank deposits rich in silt and clay. Erosion processes on stratified banks proceed by a combination of those found on single material banks. Generally, noncohesive layers are eroded more quickly than cohesive ones. This leads to the generation of berms and terraces where cohesive material underlies non-cohesive material and under-cutting where cohesive material overlies non-cohesive. The presence of a weak non-cohesive layer low in a stratified bank can greatly reduce its resistance to bank retreat through fluvial erosion (Thorne, 1978; Pizzuto, 1984).

BANK FAILURE MECHANICS

Flow erosion generates bank retreat directly, but it also promotes mass instability leading to more rapid and more spectacular retreat. Failure occurs when flow erosion of the bank and/or the bed adjacent to the bank reduces the factor of safety with respect to the most critical mode of failure to unity. The type of failure depends on the geometry of the bank, the geotechnical properties of the bank material and the bank stratigraphy. Again, the presence or absence of cohesion is particularly important.

Non-cohesive banks

Mass failure of non-cohesive banks occurs by shearing along shallow, planar or slightly curved surfaces. The motivating force is shear stress on the potential failure plane due to the downslope component of weight. Resisting force is the shear strength of the potential failure plane, due to the slope normal component of weight, friction and granular inter-locking. Deep seated failures are rare because in a noncohesive material the shear strength increases more quickly with depth than does the shear stress (Terzaghi & Peck, 1948). In well drained banks, failure occurs when the bank slope angle exceeds the friction angle. This can result from bank weakening due to the loss of imbrication or from over-steepen­ing of the bank angle by basal scour. In poorly drained banks, failure can occur owing to a reduction in the effective friction angle due to positive pore pressures. Consequently, mass failure may be triggered by rapid drawdown or heavy precipitation. Generally though, the coarse texture of non-cohesive banks makes them well drained. The stability of non-cohesive banks with respect to mass failure may analyzed quite reliably using Taylor's infinite slope approach, which

Colin R. Thome 304

may be found in standard geotechnical engineering texts (Terzaghi & Peck, 1948).

Cohesive banks

For cohesive banks the motivating force is the downslope component of weight of the potential failure block. Shear strength depends not only on frictional forces, but also on cohesion. Deep seated failures are common on unstable cohesive riverbanks. This is the case because in a cohesive bank the shear strength increases less quickly with depth than does the shear stress (Terzaghi & Peck, 1948). The shape of the critical failure surface depends mostly on the geometry of the bank. Low, steep banks fail by sliding downwards and outwards along an almost planar surface. Usually, the upper half of the potential failure block is separated from the intact bank by a near vertical tension crack. This crack is the result of tensile stress that exists in the upper part of the bank adjacent to a steep slope (Terzaghi & Peck, 1948). During failure, a slab or block of soil topples forward into the channel and so this is called a slab-type or toppling failure. High, less steep banks fail by rotational slip along a curved surface passing close to, or just above, the toe of the bank. The failure block is back-tilted away from the channel. Generally, slab failures abound on unstable banks steeper than about 60 degrees and rotational slips on banks with slopes less than 60 degrees (Lohnes & Handy, 1968; Thome, 1988).

Most mass failures of cohesive banks occur following rather than during high flows in the channel. This is because the switch from submerged to satu­rated conditions that accompanies drawdown in the channel approximately doubles the bulk unit weight of the bank material, increasing the motivating force on the potential failure surface in about the same proportion. This is the case even in the absence of significant excess pore water pressures. In undrained banks the probability of failure being triggered by rapid drawdown is even greater.

The stability of cohesive banks with respect to rotational slip may be analyzed using well established techniques developed from Bishop's simplification of the method of slices (Bishop, 1955). These are found to give results which are very close to those of more sophisticated methods, and are much simpler to use. Slab-type failures have received relatively less attention and their analysis is less well established, but a method developed by Osman and Thorne (1988) seems to give reliable estimates of bank stability with respect to this failure mode (Thorne et ai, 1988).

Stratified Banks

Bank failures in composite banks can be complex. Flow erosion is not evenly distributed over the bank, but tends to etch out the less erosion resistant layers to produce a complicated bank profile. Erosion of weak layers low in the bank is particularly effective in producing mass failure of the overlying layers. The failure surface may lie entirely within one layer, or may cut across several

305 Bank erosion and meander migration

layers. Each case must be analyzed individually, depending on the sequence, thickness and geotechnical properties of each layer in the stratified bank. The pattern of drainage inside the bank is particularly important in stratified banks. For example, banks formed by thin layers of sand interspersed between thick layers of silt and clay are particularly prone to failure by piping due to strong seepage in the sand layers (Hagerty, 1986).

BASAL ENDPOINT CONTROL

The concept of basal endpoint control helps to explain the linkage between sedimentary processes operating exclusively on the banks and those operating in the channel as a whole. The concept characterises the balance in sediment coming into and going out of the basal area. Bank erosion and mass failures input sediment to the basal area, in a more or less disturbed state. The removal of this material depends entirely on its entrainment by the flow. Consequently, both the amount and the residence time of sediment stored in the basal area depends on the balance between the rates of supply from the bank and removal by the flow. There are three possible states for this balance: input > output (impeded removal); input = output (unimpeded removal); and input < output (excess basal capacity). The rate of bank retreat adjusts to the state of basal endpoint control as follows:

Impeded removal - Bank processes, plus any sediment inputs from upstream and laterally across the channel, supply material to the basal area at a higher rate than it is removed by the flow downstream. Basal accumulation results, decreasing the bank angle and height, and buttressing the bank. Bank stability increases and the rates of sediment input and bank retreat decrease, tending towards the second state; Unimpeded removal - Processes delivering and removing sediment are in balance. No change in basal elevation or storage takes place over time. The bank retreats by parallel retreat at a rate determined by the degree of fluvial activity at the base. If the sediment load at the base is zero, then the state of basal endpoint control is static and the bank retreat rate is zero; Excess basal capacity - Basal scour has a greater capacity to transport sediment than that supplied by bank processes, failures and fluvial inputs. Basal lowering and under-cutting result, increasing bank height and angle and decreasing bank stability. The rates of sediment input and bank retreat increase, tending towards the second state.

The concept demonstrates that the longterm rate of bank retreat at a section is fluvially controlled, regardless of the nature of the bank and the processes and mechanisms actually involved in bank retreat. That does not mean that these factors are irrelevant, however, because they control the bank geometry and stability. In particular, the bank properties and characteristics set limiting values for the stable bank height and angle.

The implications for channel evolution towards a stable, or regime, geometry are discussed at length in a recent paper (Thorne & Osman, 1988).

Colin R. Thome 306

Here, consideration is given to the impact of these geomorphic controls of bank erosion on bend migration in rivers which meander through flood plain deposits that include a mixture of easily erodible point bar alluvium and much stiffer clay-plug and backswamp materials.

THEORY OF BANK EROSION IN MEANDER BENDS

At a bend of a large river, it is very important to consider the stability of the outer bank when analyzing the cross-sectional geometry and the migration rate. In most bends the scour pool is located close to the outer, eroding bank. The depth of outer bank scour increases as a function of flow intensity and the acuteness of the bend, but many river banks fail before the scour depth reaches its theoretical maximum with respect to the bend flow hydraulics. This occurs when the limiting bank height with respect to mass failure is considerably less than the bank height associated with full hydraulic scouring of the bed adjacent to the bank (Thorne & Osman, 1988).

Following mass failure, the more or less disturbed failure block comes to rest at the bank toe, but due to the high velocities and high levels of turbulence and boundary shear stress in this area, its residence time there is likely to be short. Even at lower stages, although bed scouring is insignificant, at the outer bank processes of lateral erosion continue to attack slump debris, steepening the lower portion of the bank, and leading to mass failures higher up the bank. Consequently, a considerable amount of the sediment transport capacity of the river is satisfied from erosion of the outer bank material rather than scouring of the bed.

This geomorphic link between bank and bed erosion and sediment trans­port can be used to explain the commonly observed phenomena that the depth of scour in a migrating bend increases markedly where the bend encounters a resis­tant bank material, while the local rate of bank retreat decreases sharply.

In a meander migrating freely through alluvium, the scour pool depth is limited by the critical height of the outer bank and the supply of sediment into the pool by bank erosion and mass failures. Any attempt by the flow to increase the scour pool depth by eroding the bed simply has the effect of instead further destabilizing the bank and hastening its retreat. The increased input of bank sediment to the channel then satisfies the transport capacity of the flow at the bank toe, preventing further toe scour and continuing to limit scour pool depth.

But when a migrating bend encounters stiff materials in the flood plain such as those in either a clay plug or a backswamp deposits, this situation immediately changes in three important respects. Firstly, the increased erosion resistance of the cohesive material slows the rate of erosion by direct entrainment of intact material and hence reduces the rates of retreat and oversteepening. Secondly, the increased bank stability associated with the greater bank material strength allows an immediate increase in the scour depui adjacent to the bank, as the scour pool develops from a depth limited by the stability of the alluvial bank material towards its hydraulically determined maximum depth. Thirdly, bed scour is also promoted by the reduction in the supply of sediment from bank erosion

307 Bank erosion and meander migration

and mass failures, which means that the state of basal endpoint control switches at least temporarily, from unimpeded removal to excess basal capacity.

Where clay-plug or backswamp deposits outcrop throughout the bend at the outer bank, the bend becomes "constrained" to a geometry controlled by the distribution of the resistant materials in the flood plain. As these materials are contained in abandoned meandering channels and swales which are of actuate shape when viewed from the air, the planform of a constrained meander may resemble that of adjacent free meanders. The significance of the geomorphic control exerted by the nature of the outer bank materials and processes then only becomes apparent when historical maps or aerial photographs are used to plot bend migration through time. Constrained bends are then found to have rates of bank erosion and bend movement which are small compared to those of free alluvial bends. As a result of their slow movement in comparison to other, free meanders, constrained bends are liable to be overtaken by down-valley migration of the next bend upstream. This has two effects on the constrained bend. Firstly, the curvature of the constrained bend becomes more and more acute because its up-valley limb (driven by the free meander upstream) migrates faster than its down-valley limb (constrained by the resistant material). Secondly, as the bend becomes very acute separation may occur at the outer bank, further slowing the migration rate of the constrained bend. Both effects promote eventual abandon­ment of the constrained bend by the river through a neck cut-off.

More commonly though, the resistant materials are of limited extent at the outer bank. In this case the net result of geomorphic control of bank retreat is the production of a natural hard-point in the bank where the clay-plug or backswamp deposits outcrop. The hard point produces local scour at the toe of the hard-point and immediately downstream, and if this is sufficient to bring the bank height up to the limiting value for mass failure, then retreat by slumping and basal clean-out will proceed. Usually, however, clay-plug and backswamp materials fail by rotational slip rather than by the slab failures ubiquitous to eroding banks in point bar alluvium. But if the bank is still stable even when the scour hole is fully developed then it will retreat only by direct entrainment of intact material, and this is a comparatively slow process for such cohesive materials. As a result, the hard point retreats less rapidly than the alluvial bank around it and a local convexity develops in the otherwise concave planform of the outer bank. Eventu­ally, the hard point is either eroded by the increased levels of velocity, turbulence and shear stress that are experienced as flow is diverted around it, or it is flanked as the bank retreat each side of it reveals new, and possibly less resistant, areas for the river to attack. In some cases, a hard point at the outer bank diverts the flow so strongly that outer bank deposition and berm building are initiated. Under these circumstances the bend works around the obstruction to its downstream migration by eroding its inner bank, accumulating a bar at its outer bank, and reversing its curvature. Several clear examples of this effect of geomorphic controls of bank erosion have been documented on the Connecticut River by Reid (1984).

Colin R. Thorne 308

BEND EVOLUTION ON THE RED AND LOWER MISSISSIPPI RIVERS

The limited length of this paper precludes a thorough discourse on the impacts of bank erosion and failure processes on bend migration, but examples taken from an on-going studies of fluvial and sedimentary processes on the Red and Lower Mississippi Rivers being undertaken by the author in conjunction with the US Army Corps of Engineers serve to illustrate that they are of primary importance to understanding and explaining channel evolution.

The Red River. Louisiana

The study reach on the Red River extends from Shreveport, Louisiana to Index, Arkansas (Fig. 1). The systematic behaviour of meanders was investigated in parallel studies of bed scour and meander migration in relation to the nature of the materials found in their outer banks (Thome, 1988, Biedenharn et al., 1989). In the study of bend migration data were collected from historical maps, hydro-graphic surveys and aerial photographs for the years 1930,1938, 1959, 1969, 1980 and 1984. Annual bankline migration rates for 160 bends were determined by comparing bankline location on consecutive images to calculate the area eroded and dividing this by the bend length and time the interval in years to produce an average migration rate in feet per year. Bend radius was measured from the relevant planform image and width was defined by the average crossing width. The results were plotted on a graph of erosion rate versus R/w (Fig. 1).

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5 10 RADIUS OF CURVATURE/WIDTH

Fig. 1. Average annual erosion rate versus R/w for meander bends of the Red River. Open symbols represent free, alluvial bends and closed symbols, constrained bends (Developed from diagrams by Biedenharn et al., 1989).

309 Bank erosion and meander migration

In gentle bends, with R/w values greater than about 5, erosion rates are low and vary little with bend curvature. However, in tighter bends with R/w values between 5 and 2, rates are highly variable and maximum rates increase sharply as the bend becomes more acute. Maximum erosion rates peak for bends with R/w between about 2 and 3. Bends with R/w values in this range were observed to translate down-valley while maintaining their shape and planform geometry.

Some of the great variability in erosion rates can be explained by the nature of the outer bank materials. Geological maps were used to identify the distribution of clay plugs, backswamp materials and lithified Pleistocene deposits in the flood plain and terraces of the valley. The bends were then separated into "constrained" and "free" bends (Fig. 1). Constrained bends were those with erosion resistant materials in their outer banks. Free meanders had banks formed entirely from point bar alluvium.

For constrained bends erosion rates are much lower than for free mean­ders. Also, only where down-valley migration was impeded by a resistant material did the bend tighten sufficiently for its R/w to decrease to a value less than 2 to 3. Very few free meanders attained such tight curvatures. Considerable scatter remains in the data for free meanders. This is at least partially due to the presence of undetected resistant materials in some of the bends, although it is a characteristic of meander migration in general (Nanson & Hickin, 1983).

In the study of bed scour data were collected from the 1980/81 hydro-graphic survey. Information was assembled for 120 bends on meander radius of curvature, average channel width, average depth at the inflection points upstream and downstream and maximum scour depth adjacent to the outer bank in the meander. The results are summarized in Fig. 2.

•

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1 H • m

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D

'

™u D

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0 2 4 6 8 10 12 14 Radius of Curvature/width

Maximum depth versus R/w for meander bends of the Red River. Open symbols represent free, alluvial bends and closed symbols, constrained bends.

Fig. 2.

In gentle bends, with R/w values greater than about 4, maximum depth varies little with bend curvature. However, in tighter bends with R/w values between 4

Colin R. Thome 310

and 2, depths are variable and the deepest values observed increase sharply as the bend becomes more acute. Maximum depths observed in the Red River peak for bends with R/w between about 2 and 3. These bends also exhibit the most rapid migration rates through down-valley translation while maintaining their shape and planform geometry (Fig. 1).

Some of the variability in scour depths can be explained by the nature of the outer bank materials. Separation of the bends into "constrained" and "free" bends (Fig. 2) shows that in general constrained bends have much deeper scour pools at their outer banks than free meanders with the same R/w, reflecting the importance of bank erodibility and stability to the cross-sectional geometry and depth of the channel.

The Lower Mississippi River. Mississippi

The first example from the Lower Mississippi comes from the Rabbit Island reach at around river mile 690 (Fig. 3). The 1882-83 map shows how clay-plugs at the riverward ends of Council Lake (an abandoned meander bend) were producing hard points in the outer bank of the river, evident as convexities in the bank line. The two clay-plugs in the outer bank were located at and just down­stream of the bend apex. These are the points where erosion is normally concen­trated and retreat is fastest, producing the increases in amplitude and downstream progression characteristic of free alluvial meanders. At Rabbit Island the clay-plugs successfully stabilized the outer bank with respect to mass failure, and fluvial erosion of the intact, cohesive bank material was minimal, so that by 1947 the curve of the bend was flattened at the apex and the entrance and exit curves had become quite acute. After that bank attack switched to the inner bank in the fashion observed by Reid (1984), and by 1954 this bank had retreated significant­ly, while me outer bank had not moved appreciably.

The river, unable to meander through the clay-plugs, had worked around the obstruction by reversing its pattern of bend erosion.

Fig. 3. Channel evolution at Rabbit Island, river mile 690 on the Lower Mississippi.

311 Bank erosion and meander migration

1882-83 1947 1954

Fig. 4. Channel evolution at Island No. 69, river mile 608, Lower Missis­sippi.

The second example comes from Island number 69, at river mile 608 (Fig. 4). In 1882-83, this was a conventional meander bend with a smooth, concave outer bank, a broad point bar and a chute channel at the inner bank. But between 1882-83 and 1947 the bend encountered clay-plugs in the outer bank that imposed geomorphic controls on the rate and distribution of retreat. By 1947 the outer bank line had become highly deformed, with convexities around the bend apex, and stabilization of the bend exit. Geomorphic control at these locations coupled with continued erosion of the alluvial banks in between led to the single bend of 1882-83 becoming a compound, or double headed bend by 1947. The first loop was developing rapidly, upstream of the hard-points around the bend apex, as the flow entering the bend attacked the alluvium there. The second loop was the constrained meander bend from 1882-83, which had stalled against the hard-point at the bend exit. Up to 1947 the upstream loop continued to grow by extension along an axis in the up-valley direction, while the hard-points at the apex kept the outer bank there static. As this loop grew, its radius of curvature to width ratio became smaller and smaller and this and its up-valley orientation caused a chute cut-off sometime before 1954. Flow in the chute cut-off was, once again, directed against the hard points around the bend apex. Meanwhile, the locus of bank erosion at the bend apex switched from the outer, stable bank to the inner alluvial bank. In the 1954 survey the channel in the second half of the bend has begun to move away from the clayplug and to reverse its curvature.

Working with such large rivers as the Red and the Mississippi poses problems of scale both in terms of the physical size of the river and its basin and the length of time over which it is necessary to monitor the river in order to gain reliable insights into its channel evolution. These problems put the detailed, quantitative study of large rivers beyond the practical and financial capabilities of most university based researches. These problems were overcome in tfiis study through cooperative work wim the US Army Corps of Engineers. This organiz­ation has the necessary resources to conduct frequent hydrographie surveys and to monitor channel processes at both local and regional scales. Its goals are however not concerned primarily with basic research, but more with applied research for river management. The partnership of applied researchers from government agencies with basic researchers from universities can produce benefits for both,

Colin R. Thome 312

as a better understanding of river process and form constitutes the basis for the identification of improved strategies for river engineering and management.

CONCLUSIONS

The point of these examples of channel planform change through time on the Red and Mississippi Rivers is that they illustrate how geomorphic controls triggered by erosion resistant materials in the flood plain sediments encountered at the outer bank of a meander can alter the rate, direction and pattern of planform development of whole reaches of the river. The resulting channel development is totally unpredictable on the basis of its previous history, and at odds with the present theories and models for the evolution of free meanders in uniform alluvium. It is tempting to dismiss such examples as freak results from untypical bends which are unrepresentative of meanders in general. On this basis it might be concluded that the study of meander evolution under the influence of geomorp­hic controls on bank erosion is an interesting, but largely irrelevant special case. In fact the opposite is true. Work on the Red and Lower Mississippi Rivers shows the great majority of bends to be influenced by geomorphic controls associated with bank material properties. It is the ideal, classic free meanders that are hard to find. As these rivers have mature flood plains littered with sediment filled ox-bows, backswamps and abandoned channels, this is, perhaps, not surprising. But it does show that geomorphic control by bank properties acting through bank processes and basal endpoint control is normal in bend evolution in real rivers, and that channel development in uniform alluvium is a phenomenon mostly confined to the laboratory flume and the computer model.

These findings have important implications for the prediction of river response to changes in regime. The morphology of the channel has been shown to be function of the boundary materials present in the bank materials as well as of the gross basin parameters which determine the topography and the hydrological and sedimentary inputs to the river. This local influence can be seen to be particularly important in controlling the geometry of the channel and the pattern of channel evolution through time. To be successful, attempts to predict either the future natural development of die channel, or the channel's response to intervention by human activities at a reach scale must include a sound understand­ing of the role of boundary materials. This requires a thorough knowledge of bed and bank material characteristics and their geomorphological influence on scour, bank erosion and channel migration.

ACKNOWLEDGEMENTS

The data collection and analysis described in this paper was undertaken whilst the author was a lecturer at Queen Mary College, University of London and funded by a research grant from the US Army Research Group (London), who's support is gratefully acknowledged.

313 Bank erosion and meander migration

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