Hydrology 2000. IAHS Publ. no. 171, 1987.

Erosion and sedimentation

BRUCE W. WEBB

IITRODUCTIO!

Studies of river erosion and sedimentation date back to the pioneering investigations of major rivers, such as the Rhine and the Nile, during the nineteenth century. It is only in the last 20 to 30 years, however, that such studies have become a strong growth area within hydrology. Recent interest in river erosion and sedimentation is part of a wider recognition that water has not only a "quantity" but also a "quality" dimension. Further expansion and development of erosion and sedimentation studies can be anticipated in the period leading to the year 2000 and beyond because of the opportunities which this field offers the hydrologist both for academic and applied work. Scope also exists for advances in our understanding of rivers as erosion-transportation-deposition systems and for increased application of research findings to major environmental problems in developed and especially less developed countries of the world. Included within the latter category are several problems, such as accelerated soil erosion and the acidification and eutrophication of water bodies induced by man's activities, which not only have a deleterious impact in areas immediately affected but also lead to additional degradation in "off-site" locations.

River erosion and sedimentation is of interest to many scientific disciplines. In turn, the hydrologist is brought into contact not only with parallel investigations of drainage basins by geomorphologists, but also with studies by other scientists which have spatial and temporal perspectives very different from those typical of hydrological research. The latter range from the small plot experiments of agronomists and agricultural engineers to calculations of global budgets and cycles by oceanographers and geochemists. Within hydrology, studies of erosion and sedimentation have traditionally focused on the mobilization and transport of solid matter. More recently, "sediment" studies have also considered material carried by rivers in the dissolved and particulate-associated phases. The contribution of chemical denudation to continental erosion is also receiving increased attention from hydrologists.

Any speculation concerning the future of erosion and sedimentation research is fraught with uncertainty. However, one relatively sensitive barometer of past, present and future changes is represented by the activities of the International Commission on Continental Erosion of IAHS which has organized many symposia to discuss developments in erosion and sedimentation studies and has convened a Working Group which in 1985 completed its report on the

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52 Bruce W. Webb

IHP-II Project A.1-3-1 sponsored by UNESCO and enti t led "Recent developments in erosion and sediment yield s tudies"•

EVOLUTIOI AID PHESEIT POSITIOl

A vast amount of information has been collected over the present century on the solid and dissolved material carried by world r ive r s . I t i s now known that mean annual suspended sediment yields can range from <2 to >25 000 t km year and that the equivalent range of t o t a l dissolved solids yield i s from <1 to >500 t km" year" . The most recent estimates of annual t o t a l transport of material from the land surface of the globe to the oceans also demonstrate the dominance of suspended par t icu la te (13-5 x 10 t) over dissolved (3-7 x 10y t ) load.

Information concerning geographical differences in material transported by large and smaller r ivers of the world has s teadi ly grown as a resu l t of routine monitoring and surveillance a c t i v i t i e s . The present s i tua t ion, however, i s not without major gaps. Most information re la tes to suspended sediment and dissolved material , and information describing the chemical composition of r iver water i s less abundant. Very l i t t l e data are available on bed load transport because of the problems which attend f ield measurement of th i s phenomenon. Similarly, information on substances transported in part iculate-associated form also are generally unavailable because th i s f ie ld has a t t rac ted research in te res t only re la t ive ly recently and because the analyt ical techniques involved are often more demanding than i s the case with more routine determinations. The global d i s t r ibu t ion of stat ions monitoring r iver transport i s not even, and coverage of most continents in the southern hemisphere i s par t icular ly incomplete. Information describing the long-term variat ions of material transported in r ivers i s quite insuff ic ient . Worldwide, only a few continuous records of suspended sediment transport have been collected for a period of 40 years or more. Circumstantial evidence suggests that changes in climate and in land-use practices have had very major effects upon the suspended sediment loads of some large r ive r s . However, direct measurements of f luvial t ransport , which would prove these conclusions, are almost always lacking.

In recent years, there has been considerable in te res t in the r e l i a b i l i t y of many exist ing records of f luvia l t ransport . Estimates derived from infrequent sampling in routine monitoring programmes together with re la t ive ly simple interpolat ion and extrapolat ion procedures for calculating sediment and solute yields typical ly wi l l be subject to considerable inaccuracy and imprecision. This problem i s especially important in the case of pa r t i cu la t e and part iculate-associated flux and for r iver systems which are characterized by extreme flood events and by complex var ia t ions of sediment and solute concentrations with flow.

Techniques for measuring r ive r loads have been continuously improved. In recent years, there have been some remarkable innovations such as the introduction of nuclear gauges for measuring suspended sediment and the development of the vortex flume and

Erosion and sedimentation 53

electromagnetic devices for determining bed load transport r a t e s . There has been a general trend in r iver monitoring towards the increased use of automatic samplers and continuous recording equipment in order to reduce the problems associated with manual sampling.

Substantial progress has been made in defining, analysing and explaining the behaviour of sediment and solutes in r iver systems. Early effor ts centred on establishing the relat ionship between sediment or solute concentration and r iver flow, often in the form of a ra t ing curve. Many studies have shown that sediment and solute concentrations also vary according to season of the year and stage conditions, and i t i s now widely recognised that sediment and solute behaviour in r iver systems i s a function of mult ivariate control by many factors in addit ion to flow. The complex and often hys te re t ic response of sediment and solute concentratins to floods has been studied recently in some d e t a i l . These investigations suggest tha t storm-period sediment and solute responses will ref lect such factors as flushing and exhaustion in small drainage basins. Aggregation and transmission ef fec ts , from contrasting t r ibutary sub-basins however, wi l l dominate in larger drainage areas . The role of antecedent conditions and the length of time between storm events in which the depleted supply of readily available sediment and solute can recover have emerged as an important control on storm-period behaviour. Furthermore, i t i s recognised that biological , as well as physical, processes have an important role to play in making material available for transport during storm events. Increasingly, storm-period responses of sediment and solutes have been coupled to the origins of runoff and the routes by which i t reaches the r iver channel. Considerable a t t en t ion , for example, has been given to the influence of p rec ip i ta t ion , surface runoff, so i l water and groundwater on water chemistry. As concepts of runoff production in drainage basins have become more sophist icated, i t has been recognised tha t storm-period mobilization of material wi l l depend on a complex in te rac t ion between h i l l s lope hydrology and sediment and so lu te s tores in a catchment. In par t i cu la r , the dynamic and variable nature of areas contributing to storm runoff has been viewed as an important control on sediment and solute responses.

Hil lslope hydrology strongly influences the location of sediment sources and the specif ic erosion processes act ive on h i l l s l opes . Soil erosion i s recognised as a two-component process involving the detachment and the transport of soi l p a r t i c l e s . Furthermore, a d i s t i nc t i on must be drawn between erosion in r i l l and i n t e r i l l areas of the slope because i t has been discovered that r a in fa l l and runoff affect soi l detachment and transport qui te different ly in these zones. In the i n t e r i l l area, raindrop impact detaches so i l p a r t i c l e s , and raindrop splash and, to a lesser extent, shallow flow cause transport of soil pa r t i c l e s to the r i l l system. Raindrop impact has been ident i f ied as the major erosional process in the i n t e r i l l area. The processes involved, however, are complex and varied. Many studies have sought to determine the factors which control soi l detachment in i n t e r i l l areas and much a t t en t ion has been devoted to assessing the importance of raindrop size and in t ens i ty , momentum and k ine t ic energy of r a i n f a l l . Field and

54 Bruce W. Webb

laboratory experiments have been successful in developing empirical re la t ionships which r e l a t e the ra te of detachment to measurable r a i n f a l l cha rac te r i s t i c s . Additional factors, such as the occurrence of overland flow and the steepness of slope, have an influence on soi l detachment in i n t e r i l l areas. R i l l erosion occurs when overland flow i s concentrated in small, closely spaced channels and soil detachment takes place along the sides and floor of these fea tures . I n i t i a t i o n of r i l l s requires concentration of flow beyond a c r i t i c a l discharge which, in turn, depends on soi l cohesion and slope character. In sp i t e of considerable effor ts , a comprehensive descript ion of the complex physical processes which detach and transport soil par t i c les down a h i l l s lope has not been achieved. Furthermore, conflict ing relat ionships between soi l loss and slope parameters have been produced by different s tudies. The physical , chemical and biological properties of soi ls determine the i r suscep t ib i l i ty to a var ie ty of detachment and transport processes. I t has been established that soi l e rod ib i l i ty i s a dynamic property which varies during a rainstorm depending upon par t ic le s ize d i s t r ibu t ion , aggregate s t a b i l i t y and dispersible clay. Over a longer period of four to five years, land-use practice influences the so i l e rodib i l i ty by a l t e r ing organic matter content and the properties of the exposed subsoi l . The signif icant role of subsurface processes in so i l erosion has become widely recognised as the mechanisms of subsurface flow in so i l s are be t ter understood. Rapid concentrated pipe flow i s one of the most s ignif icant processes for removing par t i cu la te mater ia l . Related processes are beginning to be studied in d e t a i l , and these include the collapse of pipes and the subsequent development of gu l l i e s . Subsurface slumping due to l iqui fac t ion of clay-rich material in the s o i l , rupture of organic materials under high hydrostatic pressure during storm events and chemical dispersion in sodium rich s o i l s , which commonly occurs in semiarid and arid areas, are examples of subsurface erosionsal processes which deserve greater study.

Rapid mass movements, such as earthflows, debris s l ides and flows, and rock avalanches, are major contributors of sediment to r ive r systems especially in high r e l i e f areas affected by intense rainstorms. In sp i t e of the i r importance, rapid mass movements are less well understood compared to the h i l l s lope processes described above. Investigations of these phenomena require the development of sophisticated f ie ld measuring systems.

l o r many years, s tudies of upstream erosion and invest igat ions of downstream sediment yield were essent ia l ly conducted independently within hydrology. More recently, establishing the link between on-s i t e rates of soil erosion and sediment yields a t the drainage basin outlet has emerged as an important endeavour. In many drainage basins, only a small proportion of material eroded in upstream areas i s delivered to the outlet of the basin. Temporary or permanent storage may occur along the base of h i l l s lopes , on the val ley floor and floodplain and in the r iver channel. Although i t i s well known that sediment storage tends to increase with basin area due to greater t ravel distances, lower slope angles and a higher proportion of valley bottomland, a wide range of other factors influence sediment delivery and integrated physically-based

Erosion and sedimentation 55

models of this phenomenon have not been proposed. The inves t igat ion of sediment del ivery i s further complicated in that many published values of the sediment delivery ra t io were calculated by comparing measured sediment y ie ld a t the basin out le t with an estimate of gross erosion within the catchment. Recent attempts to apply a sediment budget approach to catchments, in which the various sources and sinks for sediment are identif ied and the conveyance processes linking these are establ ished, offer the prospect of a more sophisticated understanding of sediment delivery mechanisms.

Attention to the "quali ty" as well as the "quantity" of sediment transported by r ivers adds a new dimension to erosion and sedimentation s tud ies . Examination of r iver sediments in terms of the i r physical and chemical propert ies , such as par t ic le size d i s t r ibu t ion , organic matter content, clay mineralogy, carbon and nitrogen concentration and content of metals, PCB's and other pol lu tan ts , have provided a useful means of "fingerprinting" sediment sources in drainage basins. Furthermore, these studies have contributed to understanding the sediment delivery mechanisms, assis ted in the quant i f ica t ion of catchment denudation and revealed an important pathway for the transport of nutr ients and contaminants. Current investigations of sediment properties and par t icula te-associa ted fluxes are seeking to describe the re la t ionships between flow and the seasonal and storm-period variat ions in pa r t i cu la t e concentrations using much the same approach as adopted i n early studies of sediment y i e lds . Studies of sediment qua l i t i y , however, face addit ional complexities. For example, the pa r t i c l e size d i s t r ibu t ion obtained using t rad i t iona l laboratory techniques refers to an "absolute" chemically dispersed mineral f ract ion which differs subs tant ia l ly from the "effective" par t ic le size d i s t r i bu t ion of the r iver where s igni f icant aggregation of individual clay par t ic les occurs. In addit ion, se lec t ive entrainment of fine material and select ive deposition of coarser par t ic les often leads to an enrichment of the clay fract ion and i t s associated nu t r ien ts and contaminants in comparison with the source so i l mate r ia l s .

In many ways, the current s t a t e of knowledge in erosion and sedimentation s tudies i s summarized by the models which have been developed to explain and to predict upland erosion, sediment delivery and transport and r ive r yields of sediment and solutes . The diversi ty of th is f i e ld of invest igat ion i s mirrored by the number and sophis t ica t ion of models that have been proposed. At the simplest level , empirical equations, often based on regression analysis , have been developed to predict spa t ia l and temporal vaiations in sediment and solute concentrations and yields from information on hydrometeorological conditions and drainage basin cha rac t e r i s t i c s . This category includes so i l loss predict ive equations, such as the Universal Soil Loss Equation (USLE), and the sediment del ivery ra t io method for predicting sediment y ie ld . The USLE has been modified in several ways, by predicting sediment yields for individual storm events rather than on an annual bas is , and by using a d i s t r ibu ted , ra ther than a lumped approach, to erosion modelling. Although there has been considerable progress in physically-based predict ion ( e . g . the CREAMS model), development of

56 Bruce W. Webb

completely determinist ic models which employ physical pr inciples to describe downslope sediment movement are s t i l l a d is tant goal. Chemical weathering models based on thermodynamic and k ine t ic principles have also been developed to calculate solute mobilization i n r iver systems as well as the formation of solution landforms. The goal of recent years has been dynamic simulation models capable of predicting temporal variat ions of sediment and solute concentrations in streams during storm events or over longer periods. The approach to th is type of simulation frequently involves an in tegra t ion of specific subroutines, which r e l a t e to sediment and solute behaviour, with models that predict the rainfal l / runoff response and general hydrology of a drainage basin. Such models, however, vary in the extent to which they t r e a t processes in a dis t r ibuted or lumped fashion. Various s tochast ic and time series modelling procedures have also been successfully applied to the predict ion of sediment and solute behaviour in r ive r systems.

FOTUHE DEVELOP1EWE3

Extrapolation from the present posit ion of erosion and sedimentation studies suggests that a number of trends and developments may characterize the period leading to the year 2000.

Despite the effor ts of the l a s t three to four decades, there i s s t i l l a need for more basic data on erosion and sedimentation. There are , for example, few measurements of soi l e rod ib i l i t y for sub-Saharan Africa, and an understanding of the relat ionships between slope length, runoff and erosion i s r e s t r i c t ed by a l imited database. Concerning sediment and solute y ie lds , there i s insuff icient information pertaining to the world's major r i v e r s . Detailed long-term measurements of sediment and solute concentrations in any r iver are re la t ive ly ra re , and current effor ts to es tabl ish "baseline" or "background" conditions in catchments re la t ive ly undisturbed by man's influence would seem worth pursuing.

Increasing awareness of the importance of chemical substances transported in the solid phase should ensure continued i n t e r e s t in sediment t ransport , in sediment properties and in part iculate-associated fluxes. Major environmental contaminants, such as radionuclides, are transported in association with r iver sediment. Further studies are required to elucidate how contaminants become attached to and move with sediment, and measurement of physical and chemical sediment charac ter i s t ics should be included in many more routine and research programmes of monitoring and survei l lance.

Improved techniques for data acquis i t ion can be expected to continue during the next decade and a half . Advances in microprocessor technology already are being used in automatic monitoring equipment. Development of " in te l l igent" sampling devices, which are capable of "deciding" under what stage or other conditions to collect or record information, i s l ike ly to accelera te i n the near future. The usefulness of remote sensing techniques for col lect ing erosion and sediment data has not been fully rea l i sed .

Erosion and sedimentation 57

Development and evaluation of different types of imagery can be expected to expand.

In current l i t e r a t u r e , there i s a frequently expressed view tha t erosion research techniques should be standardized, because much of the existing information on upland erosion has been derived using incompatible methodologies and cannot be compared eas i ly . The value of a standard, and where possible simple and d i rec t , approach to measurement should be considered when select ing an established technique or developing a new one. As a complement to new effor ts in data col lect ion, i t i s l ike ly that our basic knowledge of erosion and sediment yields can be substant ia l ly improved by the development of eff icient systems for the compilation, storage and r e t r i e v a l of exist ing data. Establishment of a centralized computer database on sediment yields of r i ve r s , although a monumental task, would be of great value to future research.

The r e l i a b i l i t y of erosion and sedimentation information wi l l continue to be a prominent area of invest igat ion. In the case of r ive r y ie lds , addit ional research i s needed to quantify the effect of sampling frequency, co l lec t ion schedule and calculat ion procedure on the accuracy and precision of dissolved and par t icu la te load estimates. Frequently, researchers must make the most of routine sampling of r iver flows ra ther than employing a carefully designed data col lect ion programme. Considerable opportunity therefore ex is t s for developing approaches which give improved estimates of f luvia l transport from infe r io r databases. Greater use of sophist icated s t a t i s t i c a l techniques, such as stochastic processes and time ser ies techniques, can be anticipated in future attempts to estimate r ive r y ie lds .

The period leading to the year 2000 i s also l ikely to be characterized by increasing a t t en t ion to the fundamental physical, chemical and biological processes involved i n erosion and sedimentation. Although our understanding of overland flow hydraulics has improved in recent years, there i s scope for future inves t iga t ion of the effects of raindrop impact, surface roughness and dynamic bed configuration on shallow flow hydraulics. Fundamental physical processes such as diffusion and dispersion have been applied increasingly to the study of solute transport in s o i l s . Elaboration of basic concepts to accommodate the influence of macrostructure and v a r i a b i l i t y of soi l parameters in f ie ld s i tua t ions represents an ongoing challenge. Although thermodynamic and kinet ic principles have been applied to chemical weathering react ions , quant i f icat ion of reactions for specific rock types in particular•environments wi l l continue, and much work remains to be done on the fundamental processes which govern the pa r t i t i on of chemical.species between dissolved and par t icula te phases. Hecent invest igat ions of the processes involved i n acid ra in and ac id i f i ca t ion of water bodies are l ike ly to be expanded i n the next few years .

Increasing a t ten t ion has been and wil l be given to the role of biological processes in erosion and sedimentation. Some work has already been carried out on the biological sources of sediment and solutes in r ivers but these are more d i f f i cu l t to quantify than inputs from the rocks or the atmosphere. Future studies of

58 Bruce W. Webb

landscape denudation cannot assume that there i s a steady s t a t e between vegetational uptake, cycling and release of elements in drainage basins. The pool of nutr ients stored in the so i l and vegetation of a drainage basin i s commonly much in excess of the annual loss from the ecosystem in streamflow. Future catchment studies could profi tably develop the work already completed on nutr ient recycling mechanisms, and further invest igate the role of soil microbial populations and t e r r e s t r i a l inputs in regulating chemical transformations and governing solute losses respect ively. A var ie ty of biological processes occurs within streams and affects both dissolved and par t i cu la te transport of nu t r ien ts . The f ie ld of instream chemical transformations and solute dynamics i s r ipe for future expansion. Further investigations of erosion and sedimentation processes wil l undoubtedly address gaps in our knowledge of mass movements and gulleying. In the case of mass movement phenomena, some advance in understanding can be expected from a greater synthesis of currently available s i t e - spec i f i c information. Additional basic data, however, are required to fully invest igate the transformation from r i l l to gully erosion, future work wil l emphasise the dynamic and complex nature of erosion and sedimentation processes in drainage basins. The dynamic nature of soi l e rod ib i l i ty , for example, d ic ta tes that more information should be obtained for major world soi l groups concerning their r a t e of change with time under different land-use systems. Furthermore, because the suscep t ib i l i t y of a so i l to erosion depends in a complex way on such controls as par t ic le size d i s t r ibu t ion , heat of wetting, s t ruc tura l s t a b i l i t y and clay mineralogy, i t wi l l not be acceptable in the future to use indirect indices to express e rod ib i l i ty .

Eesearch into the linkages between the production of runoff and mobilization of sediment and solutes in drainage basins wi l l also be a continuing theme in the period leading to the year 2000, especially as knowledge of the complexities of runoff generation in different climatic environments increases. I t has been recently shown, for example, tha t spur-hollow systems in humid temperate environments have an important influence on h i l l s lope hydrology by conditioning the expansion and contraction of saturated areas during storm events, and, in turn, can strongly affect sediment and solute responses as well as the spa t i a l intensi ty of h i l l s lope denudation. Increased awareness of throughflow and pipeflow i n runoff production wi l l inevitably lead to a concomitant emphasis on subsurface processes of erosion in coming years. The concept of runoff originating from pa r t i a l and variable source areas has been employed in preliminary work to explain sediment behaviour in catchments. This approach i s one tha t could be profitably developed in the future.

Considerable opportunities exist for addit ional research on the controls as well as the processes of erosion and sedimentation. The influence of climate, geology, re l ie f and land use on sediment and solute yields of r ivers has been extensively studied. However, as more data become avai lable , t r ad i t iona l concepts concerning the relat ionships between r iver yields and environmental factors wil l f a l l under more c r i t i c a l scrut iny. For example, the long-standing "Langbein-Schumm rule" , which describes a relat ionship between

Erosion and sedimentation 59

specif ic sediment yield and effective prec ip i ta t ion i s not substantiated by global information which i s currently ava i lab le . Furthermore, research i s emphasising the mult ivariate nature of controls on sediment and solute yield, future research wil l undoubtedly abandon a simple "bivariate" and "blackbox" view of the re la t ionship between r iver yield and environmental fac tors . The important influence which man has on sediment and solute yields through changing the land use of drainage basins is a common theme. There i s , however, a need for future investigations to progress from studies of change in small experimental basins to monitoring of major impacts in large catchments, such as the effects of widespread forest clearance in the Amazon Basin.

"On-site" erosion studies should attempt to further quantify those factors which govern the "splashabi l i ty" , " r i l l a b i l i t y " and " g u l l i b i l i t y " of soi ls as well as the i r suscept ib i l i ty to mass movement. The effects of canopy cover on so i l splash, and especial ly the in te rac t ion between canopy charac te r i s t ics and r a i n f a l l in tens i ty which r e su l t s in concentrated droplets , awaits comprehensive study. The r e l a t i ve influence of ra in and hai l in determining the eros ivi ty of prec ip i ta t ion a t high a l t i tudes also has not been evaluated.

lew i n i t i a t i v e s in the modelling of erosion and sedimentation processes wil l be made in the period leading to the year 2000. The combination of deterministic and stochast ic models seem to offer good prospects for simulating varying sediment ava i l ab i l i t y in dynamic source areas which contribute both runoff and sediment to the r ive r channel. Two-dimensional mathematical models of water and sediment movement which are linked to the development of drainage networks provide an a l t e rna t ive means of spa t ia l ly "dis tr ibuted" simulation. Several numerical models describing the development of solut ion landforms have been proposed recently with the objective of combining hydrology and chemistry without undue complication. These invest igat ions will continue to develop modelling s t ra teg ies tha t a re f lexible in terms of time/space sca les . Furthermore, i t appears t ha t much progress can be made in solut ion modelling by employing simplified geochemical pr inciples in conjunction with exist ing knowledge of h i l l s lope hydrology.

One of the most exciting challenges in the f ie ld of erosion and sedimentation research l i e s in the in tegra t ion of erosional, t ranspor ta t ional and deposit ional processes within the f luvia l system. Central to th i s objective i s a proper understanding of sediment delivery dynamics which provides the essent ia l link between upstream erosion and downstream sediment yie ld . Although invest igat ions of sediment delivery ra t ios and mechanisms have been undertaken, the detailed processes involved remain a major unknown i n many sediment studies, and the link between delivery ra t ios and catchment charac ter i s t ics requires more invest igat ion. Future research must address both s p a t i a l and temporal var ia t ion i n sediment delivery, and should avoid a "blackbox" view of delivery mechanisms. I t i s already known that sediment delivery ra t ios vary between individual storm events as well as over longer time sca les . In the l a t t e r context, sediment storage and remobilization within a given landscape are affected by climatic or land-use changes. A

60 Bruce W. Webb

proper understanding of sediment delivery in drainage basins requires a thorough knowledge of the various sources and sinks for sediment in a catchment. In i t s conveyance, sediment may enter temporary or more permanent storage a t many s i t e s within the drainage basin, and current efforts to quantify the magnitude of storage and remobilization processes are l ike ly to be considerably expanded in coming years. Development of existing methods and innovation of new techniques are needed to help in the iden t i f i ca t ion of erosion and deposition within catchments. The study of the d i s t r ibu t ion of Cs i n s o i l s , for example, offers exciting prospects for mapping the detailed pat tern of erosion and deposition on h i l l s i de s and floodplains over the l a s t 30 years . Similarly, the or igin of eroded material may be ident i f ied from a comparison of the mineral magnetic properties of sediment transported in r iver channels with those of potent ia l so i l sources in the upstream catchment. New techniques are required to study storage and remobilization of sediment within the r ive r channel.

I t i s anticipated that a major objective in the period leading to the year 2000 wi l l be the quantif icat ion of sediment budgets for catchments of different scale in a range of climatic and physiographic s e t t i ngs . One finding of existing sediment budget studies is the recognition that temporal discont inui t ies may affect the delivery of material eroded upstream and the downstream sediment yield. Consequently, i t can be expected that there wi l l be a continuing trend to place erosion and sedimentation s tudies in a longer time perspective. Palaeohydrology and associated palaeo-sediment regime and dynamics of f luvial systems are growing specialisms. Studies evaluating the effectiveness of catastrophic events _vs_. lower magnitude/higher frequency storms in long-term sediment transport are part of the same trend.

In view of the dearth of long-term sediment yield records, more a t t en t ion i s being focused on lake sedimentation studies in an attempt to reconstruct basin sediment response over the l a s t few centuries rather than years. Eecent advances in methods for correlat ing and dating lake sediment cores, based upon magnetic properties and the analysis of Cs and Pb content, provide a detai led analysis of the timing and magnitude of sediment deposit ion. Future studies in th is f i e ld , however, need to overcome a number of problems such as identifying more accurately the sources of accumulated sediment.

Integrat ion of erosion and sedimentation studies with work carried out in other discipl ines is l ike ly to be of growing importance. Oceanographers, for example, share the in t e re s t of sediment spec i a l i s t s in global geochemical cycling and in the quantity of sediment and solutes delivered to the oceans. There i s also a natural associat ion between geomorphic research into r iver and h i l l s lope landforms and studies of sediment and solutes in the drainage basin. More jo in t work with b io logis ts would also be prof i table .

Future erosion and sedimentation research wil l continue to be directed to prac t ica l as well as academic i ssues . Control of soi l erosion represents an area of par t icular concern. New processes of erosion on t i l l e d f ie lds have been recently recognised and include

Erosion and sedimentation 61

"ephemeral gully erosion" which involves gully development in a zone of flow concentration in upland f ield areas. Although of s igni f icant s ize , these features are usually obli terated by normal farming pract ices in the area following thei r formation. However, the gul l ies often reform in the same location, and can account for appreciable soi l loss over the year. Research i s currently being undertaken to invest igate the processes responsible for th i s form of erosion and to recommend a l lev ia t ion measures.

At present there i s l i t t l e information available on the tolerance of different so i l s in different environments to erosion and to o f f - s i t e as well as on-s i te damage. Further research i s necessary to es tab l i sh the relat ionships between tolerance levels and rates of soi l renewal through weathering and soi l forming processes. In the pas t , there has often been a f a i lu re to analyse so i l erosion as an "ecological" problem and in the future studies could benefit from a more "hol i s t ic" approach. In the context of erosion control measures, i t i s important in future work to express.the economic consequences of soi l erosion i n monetary terms which r e l a t e to such factors as environmental damage, productivity loss and damage to in f ras t ruc ture and c iv i l property. Assessment of the effects of erosion on productivity should become more rigorous and more sophis t ica ted . Aspects such as changes in so i l nut r ient s ta tus and the potent ia l of future land-use scenarios require assessment. Approaches such as the Erosion Productivity Impact Calculator (EPIC), devised for the United Sta tes , should be developed for other a reas.

There i s also a considerable need for the development of farming technology to successfully meet the needs and aspirat ions of communities in less developed countries, and for the iden t i f ica t ion and gradual implementation of land-use and soi l management systems which prevent erosion problems from developing. However, research should also be carried out to develop techniques of land res tora t ion i n order to ameliorate the s i tua t ion in those areas which have already suffered degradation. The effectiveness of upstream conservation measures on downstream sediment yields needs to be more fu l ly tested in future work, and basin-wide s t ra teg ies for managing erosion need to be devised.

Although so i l erosion represents a major environmental problem, other applied issues re la t ing to erosion and sedimentation processes, including reservoir sedimentation, channel degradation, eutrophication, ac id i f ica t ion and pol lut ion by metals and other contaminants, wi l l also undoubtedly form foci for research in the period leading to the year 2000.

C0HCLUSI0IS

Erosion and sedimentation studies represent an area of hydrological research which i s l ike ly to grow further before the end of the present century. The sc i en t i f i c fundamentals of the processes involved wi l l become more firmly established and the temporal perspective of studies in th i s area wi l l further widen. Considerable benefits wi l l be gained from the linkage of research

62 Bruce W. Webb

into hydrological and erosion/sedimentation processes. In tegra t ion with research in discipl ines outside of hydrology can be expected and should be encouraged.