3.2.2 Reservoir sedimentation
Reservoir sedimentation reflects sediment yield from source areas and can be used to estimate average annual unit-area sediment yield. The volume of sediment accumulated in a reservoir since dam construction can be determined using two methods:
- topographic differencing of reservoir floor elevations through time (e.g., difference between topography prior to sediment filling and current topography) and
- comparing two area-capacity curves (one based on the reservoir immediately following closure and the other based on current conditions).
Both methods require topographic data for the reservoir basin prior to sediment filling (typically derived from as-built construction drawings) and for current bathymetry. Current bathymetric data are typically useful for a wide range of other resource studies as well as ongoing hydropower operations and maintenance activities. The topographic differencing method has the benefit of describing the spatial distribution of sediment thickness (Figure 4), potentially useful information when evaluating relative sediment yield from geologically and/or geomorphically distinct reservoir source areas (e.g., two rivers flowing in from different geomorphic terrains).
Once the volume of sediment accumulated in a reservoir is determined, the annual unit-area sediment yield to a reservoir can be calculated using the following procedure:
- Accumulated volume is converted to accumulated mass by multiplying by an estimated or measured sediment density.
- Accumulated inorganic mass is estimated based on organic matter content determined from sediment sampled from the project reservoir or nearby reservoirs located in similar terrain.
- Total inorganic mass yield is calculated from accumulated inorganic mass using trap efficiency estimates derived from commonly used empirical equations.
- Average annual, unit-area sediment yield is calculated by dividing the total inorganic mass sediment yield by the reservoir's bedload (regulated) source area and the duration of accumulation.
- Average annual coarse sediment yield (> 2 mm) may be estimated by multiplying the accumulated inorganic mass by a coarse-to-total sediment ratio estimated from suspended load and bedload measurements or from core samples taken from reservoir deposits, and then dividing the total coarse inorganic mass by the reservoir's bedload (regulated) source area and the duration of accumulation.
Coring reservoir deposits is one useful approach for determining parameters that otherwise must be estimated from literature values, including sediment density and grain size distribution, organic matter content, age based on isotope geochronometers (i.e., 137Cs, 7Be, and 210Pb), and correlation of event stratigraphy to floods in the hydrologic record. A coring campaign designed to characterize the age and volume-averaged grain size distribution of reservoir sediment requires an array of sample sites extending the length of a reservoir's longitudinal axis. If the volume averaged grain size distribution of reservoir sediment is determined through coring and laboratory analysis, annual yields can be partitioned by grain size.
126.96.36.199 Advantages and disadvantages of approach
Measuring reservoir sedimentation to examine sediment supply has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
- Reservoir sedimentation is a direct measure of the effect of a dam on reducing sediment supply to downstream reaches.
- Large reservoirs efficiently trap sediment supplied by upstream source areas. Estimates of reservoir sedimentation therefore provide a robust measure of sediment yield.
- Reservoir delta deposits often contain event stratigraphy that can be correlated to flood events in the hydrologic record.
- Reservoir sedimentation provides a reliable measure from which to compare estimates of basin sediment yield and/or flux rate derived from other approaches.
- Estimates of sediment yield from reservoir sedimentation integrate variability in geomorphic terrains, climate, and changing land use within the reservoir source area.
- Historical topographic data are often coarse (>10 ft contour intervals) and may limit the accuracy of sediment yield estimates.
- Reservoir trap efficiency is commonly estimated from empirical relations with large uncertainty.
- Coring may be cost-prohibitive in large, deep reservoirs.
- Without information about the absolute or relative age of strata within the depositional profile, only the long-term average annual sedimentation rate can be determined based on the total accumulated sediment volume since dam closure. This approach does not account for temporal differences in sediment yield between flood and drought years.
188.8.131.52 Selected references
Ambers, R. 2001. Using the sediment record in a western Oregon flood-control reservoir to assess the influence of storm history and logging on sediment yield. Journal of Hydrology 244: 181-200.
Morris, G. L., and J. Fan. 1998. Reservoir sedimentation handbook: design and management of dams, reservoirs, and watersheds for sustainable use. McGraw-Hill, New York.
Snyder, N. P., D. M. Rubin, C. N. Alpers, J. R. Childs, J. A. Curtis, L. E. Flint, and S. A. Wright. 2004. Estimating accumulation rates and physical properties of sediment behind a dam: Englebright Lake, Yuba River, northern California. Water Resource Research 40.
Stillwater Sciences. 2006. Sediment budget for the Carmen-Smith Hydroelectric Project area, upper McKenzie River basin, Oregon. Final report. Prepared by Stillwater Sciences, Arcata, California for Eugene Water & Electric Board, Eugene, Oregon.