The URL can be used to link to this page

Home My WebLink AboutNIX FARM NATURAL RESOURCES FACILITY - FDP - FDP130023 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFINAL DRAINAGE AND EROSION CONTROL REPORT FOR Nix Farm Natural Areas Facility Major Amendment Prepared for: City of Fort Collins Operation Services 300 LaPorte Ave., Building B Fort Collins, CO 80521 (970) 221-6273 Prepared by: Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado 80550 (970) 674-3300 May 13, 2013 Job Number 1165-012-10 iii TABLE OF CONTENTS TABLE OF CONTENTS ............................................................................................................ iii 1. GENERAL LOCATION AND DESCRIPTION ................................................................ 1 1.1 Location ........................................................................................................................... 1 1.2 Description of Property ................................................................................................. 1 2. DRAINAGE BASINS AND SUB-BASINS .......................................................................... 2 2.1 Major Basin Description ................................................................................................ 2 2.2 Sub-Basin Description ................................................................................................... 2 3. DRAINAGE DESIGN CRITERIA ...................................................................................... 2 3.1 Regulations ...................................................................................................................... 2 3.2 Directly Connected Impervious Area (DCIA) Discussion ........................................ 23 3.3 Development Criteria Reference and Constraints ...................................................... 3 3.4 Hydrological Criteria ..................................................................................................... 4 3.5 Hydraulic Criteria .......................................................................................................... 4 4. DRAINAGE FACILITY DESIGN ....................................................................................... 4 4.1 General Concept ............................................................................................................. 4 4.2 Specific Details ................................................................................................................ 5 4.3 Drainage Summary ........................................................................................................ 5 5. CONCLUSIONS .................................................................................................................... 5 5.1 Compliance with Standards .......................................................................................... 5 5.2 Drainage Concept ........................................................................................................... 6 6. REFERENCES ...................................................................................................................... 6 APPENDIX A VIC MAP, DRAINAGE AND EROSION CONTROL PLAN, FIRM APPENDIX B HYDROLOGIC COMPUTATIONS APPENDIX C HYDRAULIC COMPUTATIONS (NA) APPENDIX D WATER QUALITY POND INFORMATION APPENDIX E MISC INFORMATION 1 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location Nix Farm Natural Areas Facility Major Amendment is a proposed office building for the City of Fort Collins Natural Resources Department at their existing Nix Farm Natural Area Campus. The site is located north of the Burlington Northern Railroad Tracks, west of Timberline Rd, south of the Cache La Poudre River, and east of Lemay Avenue. The project is located in a portion of Section 18, Township 7 North, Range 68 West of the Sixth Principal Meridian, in the City of Fort Collins, Larimer County, Colorado. A vicinity map is provided in Appendix A. 1.2 Description of Property The site contains approximately 27.48 acres; however, the proposed new office development will disturb approximately 0.7 acres. The site is zoned POL, Public Open Lands. The project will construct a two-story, 4000 sf office building located just west of the existing maintenance building. Sidewalks and additional parking will also be constructed. This drainage report will examine the runoff changes associated with increasing the existing impervious area only. The existing lot is already developed and equipped with an existing water quality pond in place. According to the “Soil Survey for Larimer County Area, Colorado” (UDSA) soils for onsite and offsite basins include: Kim loam (54), Satanta loam (95) & Stonehem loam (103). The existing site drains from NW to SE at approximately 1.3% with good ground cover. See Appendix in the “Final Drainage and Erosion Control Report Nix Farm Natural Areas Facility“, by JR Engineering (Nix Report) for soils map and information. The project is not with in any FEMA mapped floodplain. Please refer to Flood Insurance Rate Map (FIRM) Map Number 08069C0983H in Appendix A. 2 2. DRAINAGE BASINS AND SUB-BASINS 2.1 Major Basin Description The site is located in the Spring Creek Drainage Basin and contains approximately 27.48 acres. The Nix Report has previously studied this site. According to this approved report and the conceptual review comments for the project, no detention is required for the basin. 2.2 Sub-basin Description This report will re-examine two sub-basins previously defined by the Nix Report, sub- basins 102 & 103. The drainage patterns of these two basins will remain the same; however, the percent impervious values will increase due to the addition of roof, sidewalk and parking areas. 3. DRAINAGE DESIGN CRITERIA 3.1 Regulations This report was prepared to meet or exceed the “City of Fort Collins Storm Drainage Design Criteria Manual” specifications. Where applicable, the criteria established in the “Urban Storm Drainage Criteria Manual” (UDFCD), developed by the Denver Regional Council of Governments, has been used. 3.2 Directly Connected Impervious Area (DCIA) Discussion Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for receiving water protection that focuses on reducing runoff volumes, treating the water quality capture volume (WQCV), stabilizing drainageways and implementing long-term source controls. The Four Step Process applies to the management of smaller, frequently occurring events. Step 1: Employ Runoff Reduction Practices To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low Impact Development (LID) strategies, including Minimizing Directly Connected 3 Impervious Areas (MDCIA). Runoff for the site will be routed through vegetated areas through sheet flow thereby reducing runoff from impervious surfaces over permeable areas to slow runoff and increase the time of concentration and promote infiltration. Runoff will also flow through grassy swales which also slow runoff and promote infiltration. Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release Once runoff has been minimized, the remaining runoff shall be treated through the water quality pond located south of the project. Step 3: Stabilize Drainageways Natural Drainageways are subject to bed and bank erosion due to increases in frequency, duration, rate and volume of runoff during and following development. Because the site will drain to an existing water quality pond, bank stabilization is unnecessary with this project. Step 4: Implement Site Specific and Other Source Control BMPs Proactively controlling pollutants at their source by preventing pollution rather than removing contaminants once they have entered the stormwater system or receiving waters is important when protecting storm systems and receiving waters. This can be accomplished through site specific needs such as construction site runoff control, post- construction runoff control and pollution prevention / good housekeeping. It will be the responsibility of the contractor to develop a procedural best management practice for the site. 3.3 Development Criteria Reference and Constraints The Nix Report was completed on February 6, 2001 and revised June 8, 2001. This report outlines the drainage features for the entire 27.48 acre development. All calculations and design provided herein adhere to the approved overall drainage report. 4 3.4 Hydrological Criteria Runoff computations were prepared for the 2-year and 10-year minor and 100-year major storm frequency utilizing the rational method. All hydrologic calculations associated with the basins are included in Appendix B of this report. Due to modifications of surface type only, Sub-basin 102 will encompass 0.38 acres and is calculated to have 2-year, 10-year, & 100-year discharges of 0.4, 0.6, and 1.6 cfs, respectively. Sub-basin 103 will encompass 1.22 acres and is calculated to have 2- year, 10-year, & 100-year discharges of 1.3, 2.3, & 5.9 cfs, respectively. Both sub-basins 102 & 103 will still be routed through the existing water quality detention facility. The Nix Report weighted average percent impervious value of drainage Basins 102, 103 and 105 treated in the water quality pond is 19%. The weighted average percent impervious value of the revised drainage Basins 102, 103 and 105 treated in the water quality pond is 21%. The water quality storage volume was calculated to be 0.07 ac-ft, unchanged from the Nix Report. The volume is provided by the existing design and is enough capacity to account for the small increase in impervious areas made to sub-basins 102 & 103. Water quality volume was calculated using the method recommended in the “Urban Storm Drainage Criteria Manual”. All pond calculations are in Appendix D of this report. 3.5 Hydraulic Criteria No hydraulic structures were added or modified per this report; therefore, no hydraulic calculations have been provided. 4. DRAINAGE FACILITY DESIGN 4.1 General Concept The majority of the proposed development will be collected and conveyed to the proposed water quality pond on the southern most portion of the site where it will be allowed to settle before being released into the adjoining drainage easement to the southeast of the property. 5 4.2 Specific Details A summary of the drainage patterns within each basin is provided in the following paragraphs. Basin 102 includes the area west of the Maintenance Building, and east of the existing access road loop centerline. This runoff will be collected and routed through a proposed 12” CMP and existing 15” RCP culverts and conveyed to the existing water quality pond. Basin 103 includes the area north and east of the Maintenance Building bounded on the north and east by the access road loop centerline. This runoff will be collected and routed through a proposed 12” CMP and existing 12” CMP and 15” RCP culverts before being conveyed to the existing water quality pond. 4.3 Drainage Summary The existing water quality pond was originally designed to hold and treat a volume of 0.72 acre-ft. The water quality capture volume calculated for the additional impervious area continues to be 0.07 ac-ft. Therefore, the existing pond is still adequately sized to handle all runoff changes from sub-basins 102 & 103 and no modifications to the outlet structure are required. 5. CONCLUSIONS 5.1 Compliance with Standards All computations that have been completed within this report are in compliance with the City of Fort Collins Erosion Control Reference Manual for Construction Sites and the Storm Drainage Design Criteria Manual. 6 5.2 Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provide for stormwater quantity and quality treatment of proposed impervious areas. Conveyance elements have been designed to pass required flows and to minimize future maintenance. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit will be required. 6. REFERENCES 1. City of Fort Collins, “Storm Drainage Criteria Manual”, (SDCM), dated March, 1986. 2. Urban Drainage and Flood Control District, “Urban Storm Drainage Criteria Manual”, Volumes 1 and 2, dated June 2001, and Volume 3 dated September 1999. 3. JR Engineering, LLC, “Final Drainage and Erosion Control Report Nix Farm Natural Areas Facility”, dated February 6, 2001 (revised June 8, 2001). A APPENDIX A VIC MAP, DRAINAGE AND EROSION CONTROL PLAN, FIRM C APPENDIX C HYDRAULIC CALCULATIONS (NA)