3.3 Sediment Transport and Channel Condition
A coarse-level analysis of channel morphology is commonly conducted for hydroelectric project relicensings to examine historical changes and identify locations for intensive study. Coarse-level analyses utilize historical aerial photographs, digital elevation models (DEM), digital orthophotography, and previous studies on channel geomorphology. A coarse-level analysis typically involves assessment of channel confinement, channel slope, channel sinuosity, sediment source area, presence of alluvial sediment (gravel bars), potential hillslope sediment source areas, and the distribution of side channels.
Based on the coarse-level analysis, intensive sites are selected for detailed geomorphic field studies designed to investigate channel morphology, grain size distributions of channel bed surface and subsurface material, and bed mobility. The general locations of intensive study sites are designed to characterize reaches potentially affected by project operations and to identify key geomorphic areas responsible for significant sediment supply and transport.
Within the general reach locations, intensive study sites are typically located in response reaches most likely to exhibit effects from changes in hydrology, sediment supply, or large woody debris (LWD) loading. Additional criteria that may be used to evaluate the suitability of response reaches includes applicability for sediment transport modeling, minimal direct input of sediment sources to the reach (e.g., severe bank erosion or shallow landsliding) that may locally control channel morphology or bed texture, and minimal localized land use impacts (e.g., riparian timber harvest, channel constrictions due to road construction or crossings, and artificial channel hardening from rip-rapping bank material).