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Construction of Sediment Budgets for Drainage Basins

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Construction of Sediment Budgets for Drainage Basins Papers Construction of Sediment Budgets for Drainage Basins William E. Dietrich, Thomas Dunne, Neil F. Humphrey, and Leslie M. Reid ABSTRACT A sediment budget for a drainage basin is a quantitative statement of the rates of production, transport, and discharge of detritus. To construct a sediment budget for a drainage basin, one must integrate the temporal and spatial variations of transport and storage processes. This requires: recognition and quantification of transport processes, recognition and quantification of storage elements, and identifi- cation of linkages among transport processes and storage elements. To accomplish this task, it is neces- sary to know the detailed dynamics of transport processes and storage sites, including such problems as defining the recurrence interval of each transport process at a place. William E. Dietrich, Assistant Professor, Department of Geology and Geophysics, University of California, Berkeley, California 94720; Thomas Dunne, Professor; Neil F. Humphrey, Graduate Student, Leslie M. Reid, Graduate Student, Department of Geological Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195. 5 INTRODUCTION To construct a sediment budget for a drainage basin, the temporal and spatial variations of A sediment budget for a drainage basin is a transport and storage processes must be inte conceptual simplification of the interaction of grated, and to do so, these requirements should be processes that convey debris from bedrock down fulfilled: (1) recognition and quantification of hillslopes and channels and out of the basin. transport processes, (2) recognition and quantifi- Monitoring. the creation and movement of all cation of storage elements, and (3) identification particles in a drainage basin is impossible, and of linkages among transport processes and storage the recurrence and progress of many processes elements. To accomplish this task, it is necessary exceed the lifespan of the researcher. These to know the detailed dynamics of transport proc obstacles force us to develop conceptual models of esses and storage sites, including such problems sediment generation and transfer that can guide as defining the recurrence interval of each field measurements. Such a framework is valuable transport process at a place. in decisions about where to take representative measurements, about how to extrapolate data to RECOGNITION AND QUANTIFICATION OF TRANSPORT unmonitored parts of a basin, and about how to PROCESSES measure debris transport. A careful, long-term monitoring program designed Definition of sediment budgets has been inhibited to sample the spatial and temporal variations in by a lack of models for the mechanics of processes, elements of a sediment budget is an expensive by failure to define processes clearly, and by the venture. Such programs have usually been great difficulty in measuring rates of sediment undertaken to answer questions about rates of transfer. We cannot propose a single general landform evolution (Rapp 1960), effects of method for construction of a budget. Instead we climatic change (Leopold et al. 1966), and effects suggest an essential list of elements that all of land use (Fredriksen 1970, Janda et al. 1975, sediment budgets must have to be meaningful. We Swanson, et al. (1982). Yet, as illustrated by begin by proposing a definition of a sediment Leopold and his associates, a careful examination budget that leads to the requirements in construc- of the processes is needed both to interpret the tion of a sediment budget. The rest of the paper results of measurements and to evaluate the is devoted to an examination of these requirements, effects of changes in the major controls of using as illustrations problems we have encountered processes. Construction of predictive sediment in our fieldwork in the coastal mountains of Oregon budgets is hindered by the lack of understanding and Washington. A central theme of this paper is of the mechanics of processes. At present, no that, with a small, but essential amount of field- transport process is sufficiently well understood work, an approximate sediment budget can be that field data on major controls can be used to constructed that satisfies our suggested require predict transport rates. Until studies of such ments and can be used as a guide for a monitoring processes as soil creep, landslides, sheetwash, program. and sediment transport in river channels produce realistic mathematical models, we must rely on DEFINITION OF A SEDIMENT BUDGET extensive monitoring to provide data on transport rates. A sediment budget for a drainage basin is a quantitative statement of the rates of production, Before a monitoring program is begun, it is transport, and discharge of detritus. In most essential to recognize and quantify approximately studies, measurement of sediment production is the major processes that generate and transport assumed equivalent to quantification of sources of sediment. An initial field study should be made sediment discharged into channels. Sediment is to identify the distribution of dominant transport also produced by the chemical degradation and processes and interrelationships among them. From physical mixing of weathered bedrock to form a such a survey and from currently available theory soil, however, and material is transferred between for the physics of transport processes, a program size fractions as a result of attrition during of monitoring can be established to sample the transport. Also, the rate and nature of sediment range in values of major controls and the spatial production from the bedrock by soil-forming variation of processes as a function of these processes may affect significantly the rate of controls. The only way to generalize from a few hillslope erosion and mode of fluvial debris measurement sites to a landscape is to develop transport. In a sediment budget, the soil mantle predictive models of the relation of each should be treated as a separate element of transport process to its controls. sediment storage having a definable rate of inflow and outflow of detritus. Quantification of debris An important step in the simplication of a sedi- transport requires not only defining rates of ment budget to a form that can be examined in the sediment transfer between major storage elements field is the recognition of the dominant hillslope such as between soil and stream channels--but also erosion processes. Striking differences in computing rates of movement through these sediment dominant processes occur in forested areas of reservoirs. The latter is particularly difficult different climate, geology, and land use. The to do with present understanding of processes, but dominance of a process can also depend on stand it is from such a quantification that predictions history. A dramatic example is the period of can be made about the change in size of storage intense rain splash, sheetwash, and dry ravel that elements and the rapidity and magnitude of the occurs after a major burn in the chaparral of response of sediment discharge from a drainage Southern California (Rice, this volume). A central basin after some disturbance. Discharge of issue in this case becomes the assessment of the detritus is simply the rate of transport of transport of sediment (on hillslopes, into and sediment past a monitoring station. 6 along channels) after specific burns, relative to Much of the soil transported by these debris the total transport evaluated over a period suffi- slides may have been moved to the footslopes from ciently long to accommodate the spatial and further upslope by soil creep and earthflows. The temporal variation in magnitude and intensity of volume land frequency of this transfer across the burns. In this paper, we refer to the recognition hillslope-channel interface must be measured if and measurement of only a few processes that have this component of the sediment budget must be been shown to be important in wet, forested isolated. In some budgets where only the total environments. sediment flux is required, however, it may suffice to assess the sediment transport by other proc Slumps and Earthflows esses on the hillside and to ignore this last step in the transfer to channels (Dietrich and Dunne In some landscapes underlain by mechanically weak 1978). Debris slides and avalanches also occur rocks, massive earthflows and slumps move large along hillslope profiles and convey all or a quantities of debris to channel perimeters where portion of displaced sediment to the channel. shallow debris slides discharge sediment into the Again, it is necessary to obtain volume, channel (as described by Swanson and Swanston proportionn reaching the stream, the centroid 1977). These deep-seated movements involve soil, displacement of the remainder, and the frequency saprolite, weathered bedrock, or fresh bedrock and of transport. This may be done through a can extend to tens of meters below the soil sur combination of fieldwork and aerial photographic face. Engineering geologists who have measured interpretation (e.g., Simonette 1967). The relatively slow rates of movement in these deep- relatively short aerial photograph record can seated features have referred to the movement as sometimes be extended by using dendrochronology to creep (e.g., Wilson 1970, Swanston and Swanson date older landslides. Finally, debris slides and 1976). This has clouded the distinction between debris flows often originate in topographic earthflow and creep. For clarity, here we will
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