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Table 1. Manning's `n' roughness values used in HEC -RAS model. <br />Land Use <br />Manning's n roughness values <br />I <br />Range' <br />Value Used <br />Creek channel <br />0.025 — 0.080 <br />0.050 <br />Creek overbank <br />0.040 — 0.100 <br />0,080 <br />Paved road <br />0.010 — 0.020 <br />0.015 <br />Dirt and gravel area <br />0.020-0.050 <br />0.030 <br />Water surface (Canals, ponds, etc) <br />0.010— 0.040 <br />0.025 <br />Grass <br />0.025 — 0.050 <br />0.040 <br />Trees and dense shrubs <br />0.035 — 0.200 <br />0.060 <br />Residential <br />0.050-0.100 <br />0.070 <br />Commercial <br />0.060-0.100 <br />0.080 <br />' Source: Chow, 1959' <br />Historical high-water marks and corresponding flows for Cf Creek anO <br />Cedar Gulch Creek were not <br />available to calibrate the model. Modeled inundation lim*vVere presented d�g an online meeting with <br />the County on October 28, 2020 and were determined t64ovldAXreasonable representation of observed <br />flow patterns and inundated areas. <br />4.1.4 Flow Data <br />Peak flows for Crystal Creek and Cedar Qulch C Were determined based on regional regression <br />equations developed by the USGS (Itin e`tL, 20b0�. These equations follow the format: <br />`ti 1 <br />Q = aAbl0cp/10d(CAN) - <br />Where: Q = Peak F } A= Drainage Area (DA) (sq mi) <br />P = PrOpitation (in)rt) * Y CAN = Canopy Cover (%) <br />a,b,c,d ari ,efficier4 (provided in Mastin et. al, 2016) <br />The USGS's StreamStats online incorporates published regression equations with digital elevation and <br />precipitation mapping to automate estimation of regression -based flow quantiles. WSE used StreamStats <br />to apply the regression equations and determine flow quantiles at key flow change locations. Parameters <br />used in the flow analysis are presented in Table 2. Peak flow results are presented in Table 3. The resulting <br />Crystal Creek 100 -year flow at the downstream model boundary is approximately twice the peak flow <br />values used in the effective Flood Insurance Study (FEMA 1981). <br />WATERSHED P 7 <br />�� SCIENCE & ENGINEERING <br />