Laserfiche WebLink
Flatirons, Inc. — Surveying & Engineering <br />of stony and cobbly, medium - textured alluvium; and stony and cobbly, medium textured <br />till. The soils are assigned to Hydrologic Group `C'. <br />Five different Runoff Curve Numbers (RCN) were used to model the runoff. The first <br />RCN was modeled as an Industrial Site in a fully developed urban area which has an RCN <br />of 91 for soil type C. <br />The second RCN is 77 and assigned to the forested areas. The forests account for the <br />majority of the lands in the study. The RCN represents Woods with a Poor groundcover <br />for a hydrological soil group of C. <br />The third RCN was assigned to the roads and represents a gravel road. The RCN for a <br />gravel road and soil group C is 89. <br />The fourth RCN value, 79, was assigned to the outside faces of the TSF. The RCN <br />represents an Open Space with a fair grass cover. The Owner of the Union Mill has <br />committed to revegetating the outer slopes of the TSF. It was assumed that the outside face <br />of the TSF will be revegetated. <br />The fifth RCN value was assigned to impervious areas. A value of 98 was assigned to <br />impervious areas and was used for the roof of the mill building. <br />Copies of the spreadsheets and the NRCS runoff volume calculations sheets can be found <br />in Appendix B. <br />Hydraulic Criteria <br />The drainage facilities will be used to convey offsite runoff around the disturbed areas on <br />the site, or direct onsite runoff to the TSF. Open channels are designed to carry the 100 - <br />year flows with at least a 0.5 -foot freeboard. The open channels are checked for both <br />capacity and stability conditions. The side slopes of the engineered channels are either 3 to <br />1 or 2 to 1 for the depth of the channel. The channels are considered to be an excavated <br />earth channels that are straight, uniform and clean. When velocities in the channels exceed <br />5 feet per second, a riprap lining will be added. The Manning's "n" value for modeling the <br />earth -lined channels is 0.025 for capacity checks and 0.018 for stability checks. The <br />Manning's "n" value for modeling the riprap -lined channels is 0.045 for capacity checks <br />and 0.035 for stability checks <br />The Detention Basin Volume Estimating Workbook spreadsheet, version 2.32, distributed <br />by the Urban Drainage and Flood Control District (UDFCD) was used to calculate stage - <br />storage volume for the TSF. <br />Union Mill 6 March 5, 2014 <br />