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I I I I I I I I I I I I I I I I I I I 3. HYDRAULIC AND INCIPIENT MOTION ANALYSES Previous analyses of the hydraulic and incipient motion conditions at Mathers Hole and Cleopatras Couch demonstrated that the dynamics of these sites follow the PRM, Downstream backwater causes. deposition of material on the primary gravel bars during high flows and locally steep energy ( gradients at lower flows re-entrain portions of that material, creating localized areas of clean cobble/gravel substrate (Harvey et aI., 1993; Mussetter and Harvey, 1994). The HEC-2 hydraulic model used in the initial analysis for Cleopatras Couch was developed using six cross sections that were initially surveyed by the USFWS in 1983 and re-surveyed in 1991 (XS1 through XS6, Figure 2.5). The Mathers Hole hydraulic model was developed using 14 cross sections and detailed topography from surveys conducted in 1993. As previously discussed, comparison of the cross sections showed that they were nearly identical to the previous survey. The 1995 survey at Cleopatras Couch showed changes in the previously surveyed cross sections that are sufficient to affect the hydraulic conditions. The 1995 survey also provided topographic and water-surface profile data that were not previously available. 3.1. Hydraulic Analysis at Cleopatras Couch (RM 16.5) The revised hydraulic model for Cleopatras Couch was developed from eight cross sections and supplemental topography surveyed on August 24 and 25, 1995 (Figure 2.5). The five most downstream cross sections (XS 1 through XS5) were at the same locations as those from the 1991 survey. Three new cross sections (XS6 through XS8) were surveyed in the upstream pool and riffle to provide data with which to evaluate the hydraulic conditions that control the supply of sediment to the primary bar. Additionally, an un-monumented cross section was surveyed in the bend, approximately 500 feet downstream from the nose of the bar, to better define the downstream hydraulic control for the project reach. A detailed survey of the waters edge around the primary bar was also conducted to aid in locating the low flow hydraulic controls and to provide data with which to calibrate the model. The surveyed cross sections provided a core around which to build the hydraulic model. Based on the detailed water-surface profile that was surveyed on August 24, 1995 and other field observations, however, hydraulic controls occur at several points within the reach that were not adequately described by the surveyed cross sections. To account for local variations not described by the surveyed cross sections, additional cross sections were added to the model (Figure 3.1), The locations and shapes of the additional cross sections were estimated based on the detailed water-surface profile, the supplemental topography, features that are visible on available aerial photographs and detailed field observations taken during the survey. At flows less than about 10,000 cfs, the study reach in the vicinity of the bar consists of three separate flow paths, with the two main flow paths splitting around the primary bar and a smaller chute channel cutting across the bar surface (Figure 3,1). Because the hydraulic control for the two primary flow paths is at the downstream end of the reach, it was necessary to use an energy balance technique I instead of the automated split flow option in the HEC-2 model to determine the amount of flow in each 3.1 Mussetter Engineering, Inc.