3.2.3 Extrapolation based on geomorphic processes and rates
Long-term monitoring of sediment flux and field inventory of erosion features required to completely and accurately describe sediment supply is often limited by lack of existing information, the scope of hydroelectric project effects, and the short time-frame encompassed by relicensing studies. A rapid approach to characterizing sediment supply involves extrapolation of geomorphic processes and rates based on terrain modeling, published process rates, analysis of historical aerial photographs, and direct field observations. This approach is commonly employed in source analyses for sediment Total Maximum Daily Load (TMDL) studies. The rapid approach typically involves the following procedure:
- Characteristics that influence erosion processes (e.g., geology, hillslope gradient, channel gradient, and vegetation cover) are identified and combined in a geographic information system to classify unique terrain types.
- A suite of dominant erosion processes are identified for each terrain type (e.g., shallow landsliding, deep-seated landsliding, soil creep, surface erosion, rockfall, and streambank erosion).
- Unique process rates reported in the literature for regions physiographically similar to the study area are compiled.
- Process rates are standardized to a common unit (e.g., t km-2 y-1 or mm y-1) and averaged to derive a typical average annual rate for each process.
- Sediment production under reference and current conditions is calculated by multiplying each specific terrain area by respective average annual process rates.
- Total production in each subbasin under reference and current conditions is calculated by summing production estimates within each terrain type.
Extrapolation of geomorphic processes and rates to terrains is well-suited where total production and storage estimates in reservoir source areas can be calibrated to reservoir sedimentation rates.
126.96.36.199 Advantages and disadvantages of approach
An extrapolation of sediment supply based on geomorphic processes and rates has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
- Geomorphic processes and rates in a drainage basin often cannot be completely inventoried within the limited time frame of a relicensing study. Furthermore, there is often little or no field evidence of historical processes and rates. In these cases, estimates of sediment production and delivery require extrapolation of published process rates and/or sample data. Extrapolation of rates by terrains is a process-based approach that allows flexibility to define terrain types and geomorphic processes according to the unique setting and objectives of an individual study.
- Published process rates estimates are often scarce, and application of rates may be somewhat subjective.
- Terrains need to be simplified to a small number of types where geomorphic processes and rates can be sampled in the field and/or from aerial photographs.
- The approach works best in landscapes where well defined terrain characteristics control geomorphic processes.
188.8.131.52 Selected references
Reid, L. M., and T. Dunne. 1996. Rapid evaluation of sediment budgets. Catena Verlag, GMBH, Reiskirchen, Germany.
EPA (U. S. Environmental Protection Agency). 2006. Examples of approved sediment TMDLs. https://www.epa.gov/owow/tmdl/examples/sediment.html.
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.