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Home My WebLink AboutPROSPECT STATION II - PDP - PDP150021 - REPORTS - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT Prospect Station II Prepared for: Prospect Station LLC 605 S. College Ave., Suite A Fort Collins, CO 80524 (970) 219-3300 Prepared by: Interwest Consulting Group 1218 West Ash, Suite A Windsor, Colorado 80550 (970) 674-3300 February 3, 2016 Job Number 1240-140-00 iii TABLE OF CONTENTS TABLE OF CONTENTS ............................................................................................................ iii 1. GENERAL LOCATION AND DESCRIPTION ................................................................ 1 1.1 Location ........................................................................................................................... 1 1.2 Description of Property ................................................................................................. 1 1.3 Floodplain Submittal Requirements ............................................................................. 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 .......................................... 3 3.3 Development Criteria Reference and Constraints ...................................................... 4 3.4 Hydrological Criteria ..................................................................................................... 4 3.5 Hydraulic Criteria .......................................................................................................... 5 3.6 Floodplain Regulations Compliance ............................................................................. 5 3.7 Modifications of Criteria ............................................................................................... 5 4. DRAINAGE FACILITY DESIGN ....................................................................................... 6 4.1 General Concept ............................................................................................................. 6 4.2 Specific Details ................................................................................................................ 6 5. CONCLUSIONS .................................................................................................................... 7 5.1 Compliance with Standards .......................................................................................... 7 5.2 Drainage Concept ........................................................................................................... 7 6. REFERENCES ...................................................................................................................... 8 APPENDIX VICINITY MAP AND DRAINAGE PLAN .............................................................................. A HYDROLOGIC COMPUTATIONS .......................................................................................... B HYDRAULIC COMPUTATIONS (PROVIDED AT FINAL) ................................................. C WATER QUALITY/LID AND SNOUT® INFORMATION .................................................. D EXCERPTS FROM REFERENCE REPORTS......................................................................... E 1 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location The Prospect Station II development is located in Fort Collins. It is located in the Northeast Quarter of Section 23, Township 7 North, Range 69 West of the Sixth Principal Meridian, in the City of Fort Collins, Larimer County, Colorado. Please refer to the vicinity map in Appendix A. The project site is located at the southwest corner of West Prospect Road and the vacated Tamasag Drive right-of-way. The vacated Tamasag Drive bounds the property on the east, West Prospect Road on the north, Lot 2 of the Hause Subdivision on the west and Lots 3 and 11 of Griffin Plaza Subdivision on the south. The legal description of the site is Lot 1 Prospect Station Subdivision Second Filing being a replat of Lots 1 and 2, Griffin Plaza Subdivision and a portion of vacated Tamasag Drive. 1.2 Description of Property The property consists of 1.04 acres of land with 0.08 acres being dedicated to Prospect. The project consists of a new building replacing the existing building and a new parking and drive configuration. The northerly 0.2 acres drains northeast to Prospect with the remainder sheeting east and south at a range of approximately 0.5% to 1.5%. The land is currently occupied by an existing building and parking areas. There is no offsite flow contributing to the site. According to FEMA Panel 08069C0979H there are no mapped FEMA Floodways on this property. 1.3 Floodplain Submittal Requirements Because the project is not within any FEMA or City of Fort Collins mapped floodway, a Floodplain Submittal is not required and a “City of Fort Collins Floodplain Review Checklist for 50% Submittals” has not been included with this report. 2 2. DRAINAGE BASINS AND SUB-BASINS 2.1 Major Basin Description The proposed development lies within the Old Town (a small part of the northern portion of the site) and Spring Creek (the remainder of the site) Master Drainage Basins. Today, the entire site locally drains to Spring Creek via sheet flow or existing inlets and pipe in Prospect and BNSF ROW and not to the Old Town Basin. The development has also been accounted for in the design of the Mason Street Outfall (MSO) pond (showing the entire site in Spring Creek Basin) and accounted for in the detention provided behind the RR embankment. The site is considered to be basin 801 of the Griffin Plaza in the Mason Street Outfall report and a part of Spring Creek Basin 126 in the Spring Creek Master Plan. Outfall for this site is Spring Creek. Please refer to Appendix F for an explanation of the division of Spring Creek and Old Town basins. 2.2 Sub-basin Description The northern portion of the site that is included in the Old Town Master Drainage Basin drains to the north where it is intercepted by West Prospect Road and then piped back to Spring Creek. The remainder of the site (included in the Spring Creek Master Drainage Basin) drains south through adjacent Lots 3 and 11 of the Griffin Plaza Subdivision. There are no offsite flows associated with this site. 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 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 Impervious Areas (MDCIA). Runoff for the northern and eastern portions of 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 from the parking lot and building roof shall drain directly to a Type C inlet with sump that incorporates a SNOUT® water quality system from Best Management Products, Inc. device before draining into a CDOT #4 Water Quality Chamber that promotes further infiltration and sedimentation (similar to the bottom of a pervious pavement section). 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 CDOT #4 Water Quality Chamber and a sump with SNOUT® water quality system from Best Management Products, Inc. device. Both allow for settlement of sediments. 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. 4 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 runoff from this site has been routed to conform to the requirements of the City Stormwater Department. Water quality facilities are required for the new construction proposed on the site. Water quality will be met through the use of the CDOT #4 Water Quality Chamber that includes 1,290 cf of storage in the voids below the pavement and a Type C inlet with sump and SNOUT® water quality system from Best Management Products, Inc. device to prevent sands and silt from clogging the rock treatment chamber. Impervious area for the site has already been accounted for in the hydrology for Spring Creek (the BNRR pond); therefore, detention is not required for this development. Please refer to excerpts from the Mason Street Outfall report in Appendix E. 3.4 Hydrologic Criteria Runoff computations were prepared for the 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. Standard Form 8 (SF-8) provides time of concentration calculations for all sub- basins. Standard Form 9 (SF-9) provides a summary of the design flows for all Sub- basins and Design Points associated with this site. Water quality capture volume was determined using the method recommended in the “Urban Storm Drainage Criteria Manual”. This volume is achieved within the storage in the voids of the CDOT #4 Water Quality Chamber. All related information is located in Appendix D. 5 3.5 Hydraulic Criteria All hydraulic calculations will be presented in the final drainage report and prepared in accordance with the City of Fort Collins Drainage Criteria. All calculations will be included in Appendix C of this report. Hydraulic calculations will be presented in the final drainage report. 3.6 Floodplain Regulations Compliance The project is not within any FEMA or City of Fort Collins mapped floodway; therefore, Floodplain Regulations Compliance is not required. 3.7 Modifications of Criteria The site will meet the full water quality volume requirement. Basins C and D will not be captured for treatment but both basins continue to follow current conditions. The entire site’s required water quality capture volume is 1,288 cf and will be treated by the CDOT #4 Water Quality Chamber. The treated water and flow from basins C and D will eventually reach the MSO water quality pond via the proposed storm system or overland where it will be treated again. The MSO pond has ample volume for the site. Please refer to Appendix D for supporting information regarding water quality and the MSO pond. If the CDOT #4 Water Quality Chamber is completely saturated or overwhelmed, water will overtop into the 15” RCP bypass pipe in the Type C inlet and then be conveyed south in the proposed system that conveys water to the MSO pond (which is within the boundaries of the large BNRR detention pond). Note that the entire site today drains to the MSO pond. The new LID Criteria of No less than seventy five percent (75%) of any newly developed or redeveloped area, and any modification on a previously developed area for which a construction permit is required under City codes and regulations, must be treated using one or a combination of LID techniques is met. There is 34,642 sf of new impervious area. The new impervious area of Basins A and B treated in the LID method of CDOT #4 Water Quality Chamber is 27,248 sf, which is 79% of new impervious treated by LID. In 6 addition, a SNOUT® water quality system from Best Management Products, Inc. is proposed to provide additional water quality and pretreatment ahead of the rock chamber. Also, as mentioned in Section 3.7, the BNRR pond has ample volume for water quality where this water will eventually be routed to and additionally treated. 4. DRAINAGE FACILITY DESIGN 4.1 General Concept The majority of the proposed development will be collected and conveyed to the proposed storm drain system where it will be treated for water quality via a the CDOT #4 Water Quality Chamber and a sump and SNOUT® water quality system from Best Management Products, Inc. before being released into the proposed system that conveys water south to the BNRR pond. 4.2 Specific Details A summary of the drainage patterns within each basin is provided in the following paragraphs. Please refer to Appendix A for the drainage plan. Basin A is 0.59 acres and contains half of the roof (south portion) and the entire parking area. This basin is calculated to have a 10-year discharge of 2.3 cfs and a 100-year discharge of 5.8 cfs. Flows from Basin A are captured by the Type C sump inlet and the CDOT #4 Water Quality Chamber. A new storm system is proposed to release this flow into the BNRR pond. Currently, this area sheets to the pond. Basin B is 0.17 acres, contains half of the roof (north portion) and is calculated to have a 10-year discharge of 0.7 cfs and a 100-year discharge of 1.7 cfs. Flows from Basin B are captured by the proposed roof drains that direct flow to the Type C sump inlet and the CDOT #4 Water Quality Chamber before being released into the proposed storm system. This system then releases flow into the BNRR pond. Currently, this area sheets to the pond. Basin C is 0.19 acres and has a 10-year discharge of 0.5 cfs and 1.2 cfs. Flows from this basin will continue current conditions and sheet north to West Prospect Road where it 7 will be conveyed east via curb and gutter to the existing storm inlet in West Prospect Road. This inlet is part of the existing storm system that discharges to the BNRR pond. Basin D is 0.09 acres and is calculated to have a 10-year discharge of 0.4 cfs and a 100- year discharge of 0.9 cfs. This basin is the west half of the concrete drive built with Prospect Station I in 2013. Flows from this basin will continue current conditions and flow south through a curb cut and through Lot 11 of Griffin Plaza Subdivision to the BNRR pond as they do today The percent impervious weighted average of the entire site is 72% which is below the Mason Street Outfall and the BNRR model assumption of 80%; therefore, detention is not required. The voids in the rock water quality chamber will provide some detention simply by slowing the flow to the outlet. Please refer to Appendix E for excerpts from the Mason Street Outfall drainage report. 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 Storm Drainage Design Criteria Manual. 5.2 Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provides 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. 8 6. REFERENCES 1. City of Fort Collins, “Fort Collins Stormwater Criteria Manual Amendments to the Urban Drainage and Flood Control District Criteria Manual”, adopted December 2011. 2. Urban Drainage and Flood Control District, “Urban Storm Drainage Criteria Manual”, Volumes 1 and 2, dated June 2001, and Volume 3 dated November 2010. 3. Ayers Associates, “Alternative Analysis for the Design of the Mason Street Outfall”, dated November 2010. A APPENDIX A VICINITY MAP AND DRAINAGE PLAN B APPENDIX B HYDROLOGIC COMPUTATIONS SUMMARY DRAINAGE SUMMARY TABLE Design Tributary Area C (10) C (100) tc (10) tc (100) Q(10)tot Q(100)tot Sub-basin REMARKS Point (ac) (min) (min) (cfs) (cfs) A A 0.59 0.82 1.00 5.0 5.0 2.3 5.8 Area Draining to Underground Treatment and Inlet B B 0.17 0.86 1.00 5.0 5.0 0.7 1.7 Roof Area to Storm Sewer C C 0.19 0.56 0.70 7.2 5.9 0.5 1.2 Area Draining to W. Prospect D D 0.09 0.80 1.00 5.0 5.0 0.4 0.9 Area Draining to Curb Cut Page 7 Interwest Consulting Group RUNOFF COEFFICIENTS & % IMPERVIOUS LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/16/2016 Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria Recommended % Impervious from Urban Storm Drainage Criteria Manual Runoff % coefficient Impervious C Streets, parking lots (asphalt): 0.95 100 Sidewalks (concrete): 0.95 96 Roofs: 0.95 90 Porous Pavers: 0.50 40 Landscape Areas: 0.25 0 SUBBASIN TOTAL TOTAL PAVED ROOF SIDEWALK POROUS LANDSCAPE RUNOFF % DESIGNATION AREA AREA AREA AREA AREA PAVERS AREA COEFF. Impervious REMARKS (ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C) Proposed LOT 1.04 45,223 16,292 12,531 5,161 0 11,240 0.78 72 Existing LOT 1.04 45,223 16,821 4,667 3,616 0 20,118 0.64 54 Equations - Calculated C coefficients & % Impervious are area weighted C = Σ (Ci Ai) / At Ci = runoff coefficient for specific area, Ai Ai = areas of surface with runoff coefficient of Ci n = number of different surfaces to consider At = total area over which C is applicable; the sum of all Ai's 2-1-16 Flow.xls Interwest Consulting Group RUNOFF COEFFICIENTS & % IMPERVIOUS LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/1/2016 Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria Recommended % Impervious from Urban Storm Drainage Criteria Manual Runoff % coefficient Impervious C Streets, parking lots (asphalt): 0.95 100 Sidewalks (concrete): 0.95 96 Roofs: 0.95 90 Porous Pavers: 0.50 40 Landscape Areas: 0.25 0 SUBBASIN TOTAL TOTAL PAVED ROOF SIDEWALK POROUS LANDSCAPE RUNOFF % DESIGNATION AREA AREA AREA AREA AREA PAVERS AREA COEFF. Impervious REMARKS (ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C) A 0.59 25,543 13,085 6,022 1,534 0 4,902 0.82 78 Area Draining to Underground Treatment and Inlet B 0.17 7,617 0 6,509 98 0 1,010 0.86 78 Roof Area to Storm Sewer C 0.19 8,168 702 0 2,949 0 4,517 0.56 43 Area Draining to W. Prospect D 0.09 3,895 2,505 0 580 0 811 0.80 79 Area Draining to Curb Cut Total 1.04 45,224 16,292 12,531 5,161 0 11,240 0.78 72 Equations - Calculated C coefficients & % Impervious are area weighted C = Σ (Ci Ai) / At Ci = runoff coefficient for specific area, Ai Ai = areas of surface with runoff coefficient of Ci n = number of different surfaces to consider At = total area over which C is applicable; the sum of all Ai's 2-1-16 Flow.xls Interwest Consulting Group STANDARD FORM SF-2 TIME OF CONCENTRATION - 10 YR LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/1/2016 10-yr storm Cf = 1.00 SUB-BASIN INITIAL /OVERLAND TRAVEL TIME / GUTTER OR CHANNEL FLOW tc CHECK FINAL REMARKS DATA TIME (ti) (tt) (URBANIZED BASIN) tc DESIGN SUBBASIN(s) Area C Length Slope ti Length Slope n Vel. tt tc = Total L tc=(l/180)+10 PONIT (ac) (ft) (%) (min) (ft) (%) Manning (ft/s) (min) ti + tt (ft) (min) (min) (1) (2) (3) (4) (5) (6) (7) (8) rough. (9) (10) (11) (12) (13) (14) A A 0.59 0.82 17 2.0 1.7 230 1.3 0.016 2.3 1.67 3.4 247 11.4 5.0 B B 0.17 0.86 0 2.0 0.0 230 0.5 0.016 1.4 2.70 2.7 230 11.3 5.0 C C 0.19 0.56 38 2.0 4.9 245 0.8 0.016 1.8 2.27 7.2 283 11.6 7.2 D D 0.09 0.80 22 2.0 2.1 96 0.6 0.016 1.6 1.03 3.1 118 10.7 5.0 EQUATIONS: tc = ti + tt ti = [1.87 (1.1 - CCf ) L0.5 ] / S 1/3 tt = L/Vel. Velocity from Manning's Equation with R=0.1 (corresponds to Figure 3-3 of City of Fort Collins Design Manual) final tc = minimum of ti + tt and urbanized basin check min. tc = 5 min. due to limits of IDF curves 2-1-16 Flow.xls Interwest Consulting Group STANDARD FORM SF-2 TIME OF CONCENTRATION - 100 YR LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/1/2016 100-yr storm Cf = 1.25 SUB-BASIN INITIAL /OVERLAND TRAVEL TIME / GUTTER OR CHANNEL FLOW tc CHECK FINAL REMARKS DATA TIME (ti) (tt) (URBANIZED BASIN) tc DESIGN SUBBASIN(s) Area C C*Cf Length Slope ti Length Slope n Vel. tt tc = Total L tc=(l/180)+10 PONIT (ac) (ft) (%) (min) (ft) (%) Manning (ft/s) (min) ti + tt (ft) (min) (min) (1) (2) (3) (4) (5) (6) (7) (8) rough. (9) (10) (11) (12) (13) (14) A A 0.59 0.82 1.00 17 2.0 0.6 230 1.3 0.016 2.3 1.67 2.3 247 11.4 5.0 B B 0.17 0.86 1.00 0 2.0 0.0 230 0.5 0.016 1.4 2.70 2.7 230 11.3 5.0 C C 0.19 0.56 0.70 38 2.0 3.6 245 0.8 0.016 1.8 2.27 5.9 283 11.6 5.9 D D 0.09 0.80 1.00 22 2.0 0.7 96 0.6 0.016 1.6 1.03 1.7 118 10.7 5.0 EQUATIONS: tc = ti + tt ti = [1.87 (1.1 - CCf ) L 0.5 ] / S 1/3 tt = L/Vel. Velocity from Manning's Equation with R=0.1 (corresponds to Figure 3-3 of City of Fort Collins Design Manual) final tc = minimum of ti + tt and urbanized basin check min. tc = 5 min. due to limits of IDF curves 2-1-16 Flow.xls Interwest Consulting Group RATIONAL METHOD PEAK RUNOFF (City of Fort Collins, 10-Yr Storm) LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/1/2016 10 yr storm, Cf = 1.00 DIRECT RUNOFF CARRY OVER TOTAL REMARKS Design Tributary A C Cf tc i Q (10) from Q (10) Q(10)tot Sub-basin Design Point (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) A A 0.59 0.82 5.0 4.87 2.3 2.3 B B 0.17 0.86 5.0 4.87 0.7 0.7 C C 0.19 0.56 7.2 4.29 0.5 0.5 D D 0.09 0.80 5.0 4.87 0.4 0.4 Q = Cf C iA Q = peak discharge (cfs) C = runoff coefficient Cf = frequency adjustment factor i = rainfall intensity (in/hr) from City of Fort Collins IDF curve (4/16/99) A = drainage area (acres) i = 41.44 / (10+ tc) 0.7974 2-1-16 Flow.xls Interwest Consulting Group RATIONAL METHOD PEAK RUNOFF (City of Fort Collins, 100-Yr Storm) LOCATION: Prospect Station II PROJECT NO: 1068-069-01 COMPUTATIONS BY: es DATE: 2/1/2016 100 yr storm, Cf = 1.25 DIRECT RUNOFF CARRY OVER TOTAL REMARKS Des. Area A C Cf tc i Q (100) from Q (100) Q(100)tot Design Point Design. (ac) (min) (in/hr) (cfs) Point (cfs) (cfs) A A 0.59 1.00 5.0 9.95 5.8 5.8 B B 0.17 1.00 5.0 9.95 1.7 1.7 C C 0.19 0.70 5.9 9.33 1.2 1.2 D D 0.09 1.00 5.0 9.95 0.9 0.9 Q = C iA Q = peak discharge (cfs) C = runoff coefficient i = rainfall intensity (in/hr) from City of Fort Collins IDF curve (4/16/99) A = drainage area (acres) i = 84.682 / (10+ tc)0.7975 2-1-16 Flow.xls C APPENDIX C HYDRAULIC CALCULATIONS (PROVIDED AT FINAL) D APPENDIX D WATER QUALITY/LID AND SNOUT® INFORMATION E APPENDIX E EXCERPTS FROM REFERENCE REPORTS