The production and transportation of sediment in a watershed are influenced by a complex set of geomorphic processes that vary in time and space. The erosion processes are:
- Soil detachment caused by rain and overland flow;
- Rill erosion and transport;
- Gully erosion;
- Channel degradation and bank erosion;
- Surficial gravity erosion; and
- Wind erosion
Erosion also occurs on a larger scale episodically due to channel bank and hill slope failures, landsliding, forest fires, and debris flows. Land use practices such as logging and clearing, grazing, road construction, agriculture, and urbanization activities also affect sediment production and delivery from a watershed. Sediment production may vary significantly with long-term cycles in drainage system development and rejuvenation, and zones of sediment production and/or deposition may shift in location with time (e.g., headward movement of nick points and/or channel migration and avulsions).
On very small scales of time and space, erosion rate is roughly equivalent to sediment yield. However, for larger basins, as the area and time scales to be considered increase, transport and deposition processes influence both the sediment yield and the timing of sediment discharge from a basin. Deposition occurs at the base of steep slopes; in lakes, reservoirs, and wetlands; in river flood plains; and on point bars in stream channels (lateral accretion and bed aggradation).
Episodic and spatially varying processes dominate sediment and water flow, but theories and quantification of these processes are not well developed. Biota play essential roles in the production, transport and storage of sediment and water, but knowledge of biological functions is poorly integrated into quantitative procedures for estimating sediment and water budgets.
On a geologic time scale, the surface of the earth is transformed by sediment production (erosion) in the upper part of the watershed, transportation of sediments in a fluvial system, and deposition in low-lying lakes, alluvial fans, deltas, and in the oceans. It is instructive to divide a watershed into major zones or provinces where different sets of physical processes interact to characterize sedimentation processes. There are three physiographic zones suggested in literature: production zone or sediment source area; transfer zone and, deposition zone or sediment sink area). The Corps further classifies zones according to dominant local sediment transport processes, and characterizes relative amounts of different sediment sizes within the regions. Development of basin sediment budgets and yield estimates requires an understanding of how these zones or provinces connect and the approximate quantities of materials entering and leaving each zone for the expected range of flow conditions.
For the purpose of estimating sediment production, important processes can be roughly divided into three categories of sheet and rill erosion, gully and channel erosion, and mass wasting processes. The relative magnitudes of each category depend on the soils, geomorphic, hydrologic and land use characteristics of the basin. The Natural Resource Conservation Service and U.S. Geological Survey are excellent sources for information.
Sediment Yield and Transport
Sediment yield, is the amount of sediment passing a specified channel location and is influenced by a number of geomorphic processes. It may be substantially less than the amount actually eroded in the basin. Sediment yield is typically expressed as the total sediment volume delivered to a specified location in the basin, divided by the effective drainage area above that location for a specified period of time. Yield typically has the units of cubic meters/square kilometer/year or acre-ft/square mile/year or metric tons or English tons per year. However, it is also necessary to estimate yield from a watershed from individual storm events of specified frequency (e.g., 5-, 25-, 50-, or 100-year events. Individual event yields are reported as metric tons or cubic meters per event. In some watersheds, single event sediment yields often exceed average annual values by several orders of magnitude.
Spatial and temporal variations in physical and biological features of the watershed make estimation of sediment yield an extremely difficult and imprecise task. Important variables include soils and geology, relief, climate, vegetation, soil moisture, precipitation, drainage density channel morphology, and human influences. Dominant processes within a watershed may be entirely different between physiographic or ecological provinces, and may change with time. The problem becomes even more complex when grain size distributions and sediment yield for particular events must be estimated for input to sedimentation transport simulation models. There is no widely accepted procedure for computing basin sediment yield and grain size distribution directly from watershed characteristics without measured information.
Sediment transport is influenced primarily by the action of wind and water, and deposition occurs in a number of locations where energy for transport becomes insufficient to carry eroded sediments. Colluvial deposits, floodplain, and valley deposits, channel aggradation, lateral channel accretion, and lake and reservoir deposits are examples of typical geomorphic deposition processes. The stability and longevity of sediment deposits vary. Lake and reservoir deposits tend to be long-term, whereas some channel and floodplain deposits may be remobilized by the next large scale flood event, only to be deposited downstream. The spatial and temporal variability of sediment production, transport and deposition greatly complicates the task of estimating sediment yield from a watershed.