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Regraded area S WS #2 = 0.36 ac with CN = 89 (newly graded, no <br />• vegetation -hydrologic soil group B-C) <br />BMP design = silt fence or sediment logs <br />Results: <br />Peak flow into sediment logs = 0.63 cfs <br />Peak flow out of sediment logs = 0.15 cfs <br />Peak Settleable Solids (SS) concentration into silt fence or sediment logs = 44.88 <br />mUL <br />Peak Settleable Solids (SS) concentration out of silt fence or sediment logs = 0.00 <br />ml/L <br />Revegetated Building Site without BMP <br />Design Parameters: <br />Precipitation: <br />Use 10-year, 24-hour storm precipitation depth = 1.8 in <br />Land Use: <br />Paved azea SWS #1 = 0.22 ac with CN = 98 (paved pazking lot) <br />Revegetated azea SWS #2 = 0.36 ac with CN = 63 (grass -hydrologic soil <br />group B-C) <br />Results: <br />Peak flow out = 0.30 cfs <br />Peak Settleable Solids (SS) concentration out of disturbed area = 0.00 m]/L <br />• The maximum runoff volume and peak runoff to the access road drainage ditch will occur <br />when the upgradient office building pad is regraded prior to revegetation. These <br />conditions were modeled to design the road drainage ditch. The road drainage ditch was <br />divided into two (2) different sections to adequately convey surface runoff. The upper <br />section (Section 1, average slope of 5.1 %) is 530 feet long, and the lower section <br />(Section 2, average slope of 13.0 %) is 543 long (Figure 1). Section 1 was modeled as an <br />erodible channel consisting of graded silts to cobbles when colloidal. Runoff velocities <br />in this section of drainage ditch (resulting from the ] 0-year, 24-hour precipitation event) <br />must be less than 4.0 feet per second (fps) to ensure that the ditch remains stable with no <br />channel scour or degradation during the peak flow event. Section 2 was originally <br />modeled using the same criteria. Since the peak runoff velocity exceeded 4.0 fps, the <br />lower section of drainage ditch was modeled as a riprapped channel. $oth channels were <br />modeled with a trapezoidal geometry as required for the SEDCADT"' model to produce <br />final results. However, please note that the bottom width of each section of channel is <br />only 0.5 feet, which in practicality produces channel geometry that is triangulaz in cross <br />section (Figures 2 and 3}. Both sections of road drainage ditch are designed to maintain a <br />minimum of 1.0 feet of freeboard during the peak flow generated from the 10-yeaz, 24- <br />hour runoff event. Following is the summary of road drainage ditch design parameters <br />and results: <br />• <br />