Inundation Frequency Sample Clauses

Inundation Frequency. We estimated the magnitude of flows that inundated the alluvial deposits by surveying the topography of the Holocene alluvial valley along 78 cross sections in four locations (Table 3-1, Figure 3-2) during July and August 2005. The topography in the main channel of the Snake River was estimated with a digital depth sounder. Depth measurements were recorded at regular intervals across the channel and overlapped with at least two land-based survey measurements. We used the cross section data to develop one-dimensional flow models in HEC- RAS, a modeling program developed by the U.S. Army Corps of Engineers. We estimated the overbank roughness coefficient for all cross sections to be 0.08, a typical value for floodplains covered by grasses, bushes, and fallen and standing trees (Chow, 1959), such as is found along much of the Snake River alluvial valley. The stages of the modeled flows often did not precisely match those measured in the field during the specified flow; thus, we calibrated the flows at each cross section by changing the roughness coefficient of the channel until the stages of the modeled flows best matched those of the measured flows. We calibrated the flow models using water surface elevation data surveyed at each cross section during the 2005 dam-released flood, as this flood was closer in magnitude to the floods necessary to inundate the alluvial deposits than the late summer flows: the magnitude of the 2005 flows we surveyed ranged from 59 m3/s to 171 m3/s in Segment 3 (Table 3-2); the magnitudes of flows we modeled ranged from 213 m3/s to 889 m3/s in Segment 3. With the models calibrated to the 2005 peak flows, roughness coefficient values ranged from 0.015 to 0.05 in Segment 1, 0.02 to 0.048 in Segment 2, 0.022 to 0.05 in Segment 3, and 0.017 to 0.068 in Segment 4 (Table 3-3). Despite these adjustments to the roughness coefficient values, slight differences between modeled and measured water surface elevations for the 2005 peak flows remained. The greatest difference between measured and modeled water surface elevation was 0.11 m though the mean difference was less than 0.05 m for each reach (Table 3-4). All differences in stage were less than 7% of the maximum water depth along the cross section. After calibrating the models, we estimated the water surface elevation for up to 16 discharges at the upstream and downstream cross sections based on survey data and stage-discharge rating relationships, developed within HEC-RAS or from USGS ga...