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Home My WebLink AboutCIRCLE C ADULT RESIDENTIAL SERVICES - PDP190009 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORTCOLORADO CIVIL GROUP, INC. PRELIMINARY DRAINAGE REPORT FOR THE CIRCLE C - ADULT RESIDENTIAL TREATMENT SERVICES CITY OF FORT COLLINS JULY 2019 Circle C - Adult Residential Treatment Services Preliminary Drainage Report July 2019 5 Basin C includes a portion of an existing commercial parking lot which will provide access to the site and a portion of landscaped area. Runoff from Basin C flows via sheet flow to McHugh Street without detention. 2.3 On-Site Basin Description Basin OS-1 includes a portion of the nursing center property drains to on-site Basin B. The flows from this basin are accounted for in the storm drain sizing and are conveyed through the property to McHugh Street undetained. Basin OS-2 includes the remaining portion of the nursing center property that drains to the Circle C property drains to on-site Basin C. As no drainage facilities are proposed for Basin C, runoff from this basin passes through the site and enters McHugh Street undetained, which is consistent with the existing drainage pattern. Figure 2-1 presents the delineation of the off-site basins and Appendix B presents the hydrologic calculations for the basins. Circle C - Adult Residential Treatment Services Preliminary Drainage Report July 2019 8 Table 2-2: FCSCM Frequency Adjustment Factors Source: Table 3.2-3 Fort Collins Stormwater Criteria Manual, December 2018 The adjusted composite runoff coefficient value was determined utilizing the following equation. C=Cx*Cf Maximum adjusted composite runoff coefficient value is limited to 1.00. The existing condition represents the current project site with no improvements or changes. The proposed condition represents the project site with the proposed improvements. Basin categories and area computations can be found in Appendix B. Tables 2-3 and 2-4 below provide a summary of the existing and proposed basin characteristics. Table 2-3: Existing Condition Basin Summary Table 2-4: Proposed Condition Basin Summary 2, 5, 10 1.00 25 1.10 50 1.20 100 1.25 Storm Return Period (years) Frequency Adjustment Factor (Cf) Design Point 100-Year 2-Year 100-Year I100 Q2 Q100 (acres) (minutes) (in./hour) (in./hour) (ft3/s) (ft3/s) A 1 0.54 11.9 0.10 0.13 2.1 7.4 0.12 0.50 B 2 0.05 5.0 0.95 1.00 2.9 10.0 0.12 0.45 2-Year C2 Basin Area Rainfall Intensity Runoff 2-Year I2 Runoff Coefficients 100-Year C100 Basin I.D. Time of Concentration 100-Year 2-Year 100-Year I100 Q2 Q100 (acres) (minutes) (in./hour) (in./hour) (ft3/s) (ft3/s) A 0.12 8.6 0.34 0.43 2.4 8.4 0.10 0.44 B 0.41 5.0 0.86 1.00 2.9 10.0 1.00 4.08 C 0.06 5.0 0.74 0.92 2.9 10.0 0.12 0.51 Runoff Coefficients Rainfall Intensity Runoff 2-Year C2 100-Year C100 2-Year I2 Basin I.D. Basin Area Time of DesignConcentration Point Circle C - Adult Residential Treatment Services Preliminary Drainage Report July 2019 10 2.5 Water Quality Capture Volume Equations in Chapter 7 of the FCSCM were used for the calculation of Water Quality Capture Volume (WQCV) required for the Circle C - Adult Residential Treatment Services project and are presented below. The Water Quality Capture Volume is calculated based on the imperviousness of the project site and the drain time. Most of the new impervious area on the Circle C - Adult Residential Treatment Services project site will drain to the StormTech system. The most conservative drain time presented in the FCSCM, 40 hours, has been used in the sizing calculations for the Circle C - Adult Residential Treatment Services. The WQCV is calculated based on the imperviousness of the project site and the drain time. Most of the new impervious area on the Circle C - Adult Residential Treatment Services project site will drain to the StormTech system. The most conservative drain time presented in the FCSCM, 40 hours, has been used in the sizing calculations for the Circle C - Adult Residential Treatment Services. WQCV = a(0.91I 3-1.19I 2+0.78I) Equation 7-1 FCSCM Where: WQCV = Water Quality Capture Volume (watershed inches) a = Drain Time Coefficient (1.0 for a drain time of 40 hours) I = Imperviousness, %/100 (0.88 for Basin B) For the Circle C - Adult Residential Treatment Services, the resulting WQCV is 0.382 watershed inches. The FCSCM uses the WQCV above to find the basin storage volume based on the following equation: V = (WQCV/12)*A*1.2 Equation 7-2 FCSCM Where: V = Basin Design Volume (cubic feet) WQCV = Water Quality Capture Volume (watershed inches) Circle C - Adult Residential Treatment Services Preliminary Drainage Report July 2019 13 3.2.2 Step 2: Treat and Slowly Release Runoff After reducing the runoff from a site, the second step in stormwater quality management is to capture and slowly release a Water Quality Capture Volume (WQCV). WQCV facilities may include bioretention, extended detention basins, sand filters, constructed wetland ponds, and retention ponds. The StormTech detention system is designed to capture and release the WQCV for the Circle C - Adult Residential Treatment Services site. The WQCV drain time for the site is 40 hours. 3.2.3 Step 3: Stabilize Drainageways Although steps 1 and 2 help to minimize the effects of runoff on downstream drainageways, natural drainageways are often subject to bed and bank erosion due to increases in the frequency, rate, duration, and volume of runoff. Step 3 includes measures to prevent drainageway erosion. There are no drainageways on the Circle C - Adult Residential Treatment Services site. By implementing steps 1 and 2, the project site does its part to reduce drainageway erosion downstream. 3.2.4 Step 4: Implementation of Source Controls The final step in stormwater quality management is source control. Site specific needs such as material storage or other site operations require consideration of targeted source control Best Management Practices (BMPs). These BMPs are shown on the Erosion Control Plan (Appendix C) and explained in the Erosion Control Report (ECR) for the project site. 3.3 BMP Selection Considerations The following sections discuss the considerations for determining the best BMP or LID solution to implement for the project. 3.3.1 Soils The existing soil condition on the project site must be considered when designing BMPs. Soils with good permeability provide opportunities for infiltration of runoff Circle C - Adult Residential Treatment Services Preliminary Drainage Report July 2019 14 and are well-suited for infiltration based BMPs such as dry wells, permeable pavement, and grass swales. The soils on the Circle C - Adult Residential Treatment Services site are in Hydrologic Soil Group B, which is defined as soils having a moderate infiltration when thoroughly wet. For this reason, infiltration based BMPs were considered a good option for the project site. Soil information for the site may be found in Appendix A. 3.3.2 Watershed Size The contributing drainage area is an important consideration for the design of BMPs. For a small site, such as Circle C - Adult Residential Services Treatment, it is not feasible to design a detention pond that releases the WQCV over a 40-hour drain time due to the small orifices that would be required. Instead, it is recommended that small watersheds use filtering BMPs, such as the StormTech system designed for the project site. 3.3.3 Base Flows BMPs such as constructed wetlands ponds, retention ponds, and wetland channels require a base flow to prevent the BMPs from becoming dry and unable to support wetland vegetation. No base flow exists for the Circle C - Adult Residential Treatment Services site, so no BMPs that require a base flow were considered for the site. 3.4 BMP Design The StormTech chamber system utilizes infiltration to reduce the transportation of pollutants to downstream receiving waters. The system is equipped with an “isolator row” which consists of wrapping the row of chambers that takes the first flush of storm runoff with fabric. This row captures the sediment which can be removed and disposed of as part of the maintenance process. 3.5 Hydrologic Criteria Per City of Fort Collins criteria, the Rational Formula Method was used for the hydrologic analysis of the Circle C - Adult Residential Treatment Services project site. The rainfall intensities (IDF Data) from the FCSCM were used for the calculation of CIRCLE C ‐ ADULT RESIDENTIAL TREATMENT SERVICES TABLE B‐2 ‐ TIME OF CONCENTRATION Existing Condition 100 yr Velocity tt tc tc Check (acres) (feet) (%) (min) (min) feet (ft/ft) (feet) (feet/sec) (min) (min) (min) (min) A 0.10 0.13 0.54 100 1.68 15.7 15.3 338 0.008 0.100 Asphalt w/ concrete gutter 1.8 3.1 18.5 11.9 NO 11.9 B 0.95 1.00 0.05 52 5.52 1.1 0.8 0 0.000 0.000 0.0 0.0 5.0 YES 5.0 Proposed Condition 100 yr Velocity tt tt tc Check (acres) (feet) (%) (min) (min) feet (ft/ft) (feet) (feet/sec) (min) (min) (min) (min) A 0.34 0.43 0.12 33 3.90 5.2 4.6 399 0.008 0.090 Asphalt w/ concrete gutter 1.7 4.0 8.6 12.2 YES 8.6 B 0.86 1.00 0.41 52 2.00 2.6 1.1 81 0.005 0.130 Asphalt w/ concrete gutter 1.7 0.8 5.0 10.5 YES 5.0 C 0.74 0.92 0.06 38 11.20 1.9 0.9 27 0.008 0.100 Asphalt w/ concrete gutter 1.8 0.3 5.0 10.2 YES 5.0 Off‐site Basins 100 yr Velocity tt tt tc Check (acres) (feet) (%) (min) (min) feet (ft/ft) (feet) (feet/sec) (min) (min) (min) (min) OS1 0.46 0.58 0.40 25 25.00 2.0 1.7 125 0.010 0.010 grass swale 0.2 10.5 12.2 10.7 NO 10.7 OS2 0.95 1.00 0.04 57 6.14 1.2 0.8 27 0.008 0.090 Asphalt w/ concrete gutter 1.7 0.3 5.0 10.2 YES 5.0 Basin I.D. Runoff Coefficient Basin Parameters C2 C100 Area Overland Flow tc Check Length Hydraulic Final tc Length Radius Concentrated Flow Type of Land Surface Basin I.D. Runoff Coefficient Basin Parameters C2, C5 C100 Area tc Check 100 yr ‐ ti Slope Hydraulic Final tc Length Slope Radius 2yr ‐ ti Length Basin I.D. Runoff Coefficient Basin Parameters C2 Final tc tc Check C100 Area Overland Flow Length Slope 2yr ‐ ti 100 yr ‐ ti Length Slope 2yr ‐ ti 100 yr ‐ ti Slope Overland Flow Concentrated Flow Hydraulic Slope Radius Concentrated Flow Type of Land Surface Type of Land Surface