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Home My WebLink AboutPOWERHOUSE 2 - FDP220015 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORT (2) FINAL DRAINAGE REPORT AND EROSION CONTROL REPORT POWERHOUSE 2 FORT COLLINS, COLORADO December 14, 2022 NORTHERNENGINEERING.COM 970.221.4158 FORT COLLINS GREELEY This Drainage Report is consciously provided as a PDF. Please consider the environment before printing this document in its entirety. When a hard copy is necessary, we recommend double-sided printing. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY COVER LETTER December 14, 2022 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80521 RE: FINAL DRAINAGE AND EROSION CONTROL REPORT FOR POWERHOUSE 2 Dear Staff: Northern Engineering is pleased to submit this Final Drainage and Erosion Control Report for your review. This report accompanies the combined Project Development Plan (PDP) submittal for the proposed Powerhouse 2 development. This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual (FCSCM) and serves to document the stormwater impacts associated with the proposed Powerhouse 2 development. We understand review by the City of Fort Collins is to assure general compliance with standardized criteria contained in the FCSCM. If you have any questions as you review this report, please feel free to contact us. Sincerely, NORTHERN ENGINEERING SERVICES, INC. CARLOS ORTIZ GARCIA AUSTIN SNOW, PE Project Engineer Project Engineer NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY TABLE OF CONTENTS TABLE OF CONTENTS GENERAL LOCATION AND DESCRIPTION .......................................................... 1 DRAIN BASINS AND SUB-BASINS ..................................................................... 6 DRAINAGE DESIGN CRITERIA .......................................................................... 6 DRAINAGE FACILITY DESIGN ........................................................................... 8 CONCLUSIONS ............................................................................................ 12 REFERENCES .............................................................................................. 13 TABLES AND FIGURES FIGURE 1 – VICINITY MAP .................................................................................................1 FIGURE 2 – AERIAL PHOTOGRAPH ...................................................................................2 FIGURE 3 – PROPOSED SITE PLAN ...................................................................................3 FIGURE 4 – AREA FLOODPLAIN MAPPING ........................................................................3 FIGURE 5 – FOUNDATION DETAIL ....................................................................................4 APPENDICES APPENDIX A – HYDROLOGIC COMPUTATIONS APPENDIX B – DETENTION POND COMPUTATION APPENDIX C – HYDRAULIC COMPUTATIONS APPENDIX D – EROSION CONTROL REPORT APPENDIX E – PRELIMINARY LID DESIGN INFORMATION APPENDIX F – SUPPORT DOCUMENTS, PREVIOUS STUDIES AND USDA SOILS REPORT MAP POCKET DR-A – EXISTING VS. PROPOSED IMPERVIOUS AREA – BASIN A DR1 – POWER HOUSE 2 DRAINAGE EXHIBIT; (NORTHERN ENGINEERING) NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 1 | 12 GENERAL LOCATION AND DESCRIPTION A. LOCATION Vicinity Map The Powerhouse 2 site is located in the Southwest Quarter of Section 1, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. The project site (refer to Figure 1) is bordered to the north by the Lake Canal, to the south by East Vine Drive to the east by Jerome Street, and to the west by N. College Avenue. B. DESCRIPTION OF PROPERTY The Powerhouse 2 site is comprised of roughly ±4.97 acres. The site is currently occupied by three buildings as well as concrete sidewalks, open areas, gravel parking areas, and ±48,482 square feet of asphalt. The existing imperviousness of the site is 54%. An exhibit for existing versus proposed imperviousness is provided in the Map Pocket section. The existing ground slopes with a mild to moderate grade (i.e., 0.5 – 2.0±%) Figure 1 – Vicinity Map NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 2 | 12 through the interior to the south across flat grades into East Vine Drive. The drainage continues through the Poudre River Whitewater Park and on to the Cache La Poudre River. A soils report (Project No. FC10223-125) was completed by CTL Thompson, Inc. on March 14, 2022. The report contains the results of a complete geotechnical subsurface exploration as well as pertinent geotechnical recommendations. There are two major drainageways close to the site. The Lake Canal is located on the north side of the site and the Cache la Poudre River is located approximately 550 feet south of the site. The proposed project site consists of one building. Other proposed improvements include concrete and permeable paver drive aisle, gravel parking, walkways, a courtyard, landscape, and open space areas. The proposed imperviousness of the site is 56%. An existing and proposed imperviousness exhibit is provided in the Map Pocket section. Figure 3 shows the proposed site plan. Figure 2 – Aerial Photograph NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 3 | 12 C. FLOODPLAIN The Powerhouse 2 project is located within a FEMA Special Flood Hazard Area of the Cache la Poudre River (Poudre River). Most of this area is designated as With Base Flood Elevation (BFE) or Depth in Zone AE on FIRM Panel 08069C0997G, effective June 17, 2008. The floodplain from the Whitewater Park Letter of Map Revision (LOMR) Case No. 20-08-0643P, effective June 25, 2021, supersedes the current FEMA RiskMap. An Additional LOMR became effective on November 19, 2021, Case No. 21-08-1189X, to correct the Flood Zone designations for some of the islands (Zone X Shaded) within the Whitewater Park LOMR. Zone X Shaded is due to the Figure 4 – Area Floodplain Mapping Figure 3 – Proposed Site Plan NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 4 | 12 500-yr floodplain. Most of the property falls inside the regulated 100-year floodplain. Chapter 10 of the City of Fort Collins Municipal Code regulates critical facilities within this Flood Hazard Zone. It should be noted FEMA, as part of RiskMap, is currently remapping the Poudre River. Anderson Consulting Engineers (ACE) will provide the updated Floodplain modeling, mapping, and report for the site when completed. It is assumed a portion of the project site will continue to be located within the updated 100- year floodplain. To ensure the proposed buildings have adequate flood protection, the finished floors and all ductwork, heating, ventilation, electrical systems, etc., will be elevated 24-inches above the effective Base Flood Elevation (BFE). This elevation is known as the Regulatory Flood Protection Elevation (RFPE). The RFPE = BFE + 24 inches. The solar panels and the electric vehicle charging components in the parking lot shall be installed above the RFPE or have floodproofing as required by Chapter 10 of the City of Fort Collins Municipal Code. A minimum of 24-inches of freeboard will be provided from base (100-year) flood elevation (BFE). This freeboard level will be applied to either the design of finished floor elevations or the minimum level of floodproofing measures The vertical datum utilized for site survey work is the City of Fort Collins Benchmark # 1-00 (Elevation=4968.74); NAVD 88. Furniture or site components shall be securely anchored with a system of 4" lag bolts epoxied into ground base concrete or stone with straps and/or chains secured to elements to prevent flotation. Anchoring details will be supplied at the time of the floodplain use permit. A floodplain use permit will be required for the building structure and each site construction element in the floodplain. The floodplain use permit for the building will be approved at the time of the building permit application. A floodplain use permit and no rise certification is required prior to performing any work within the floodplain (i.e., curb cut removal, landscaping). A post-construction elevation certificate including components like transformer, yards, EV. charging stations, solar panel components, elevator, HVAC equipment, emergency vehicle Figure 5 – Foundation Detail NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 5 | 12 access gate, other electrical and mechanical systems, etc. must be approved before a certificate of occupancy will be issued. All elevators installed below the RFPE in the project will comply with FEMA NFIP technical bulletin on June 4, 2019, and the 2021 IBC. An Emergency Response and Preparedness Plan (ERPP) will be required by the City of Fort Collins prior to a floodplain or building permit being issued. Outdoor classrooms and informal outdoor gathering space will be used for site tours and outdoor learning. These spaces will be constructed using large boulders and crusher-fine surfacing. Critical facilities are prohibited in the floodplain, including hazardous material, government services, essential services and at-risk population critical facilities. This includes schools and after school programs/day camps for grades K-12. Per the “Hazardous Material Storage and Handling of Flammable Materials” memo by Bryan Wilson provided in Appendix F, the quantities listed below and the removal of hydrogen storage from the site will remove the designation of a "Critical Facility," as defined by Chapter 10 of City Code, from the project. Material storage restrictions will be removed with the approval of the LOMR. The following outlines the handling and storage processes of hydrogen for research procedures anticipated at the proposed Powerhouse 2 facility: · Hydrogen produced by hydrolysis or similar method will be disposed of as produced via venting outdoors (as per any code requirements), burned outdoors in a non-visible flare, consumed by fuel cell or similar device. · Onsite storage of hydrogen will remain at or below quantities regulated by international fire safety code or "household levels." · Hydrogen fueled vehicles, such as automobiles, light and medium duty trucks, and transit buses, will be tested onsite and adhere to all fire safety codes and DOT regulations regarding hydrogen fueled vehicles. No vehicles will be stored outside overnight per the floatable materials requirements in Chapter 10 of City Code. · H2 storage tanks will remain at Powerhouse I until approval of the LOMR. Flammable Materials: · Acetylene 1 commercial cylinder at 300 or less std. cu. ft. volume. · Oxygen 1 commercial cylinder at 300 or less std. cu. ft. volume. Non-Reactive Gases: · Compressed Air 500 gal. tank · Compressed Nitrogen 2000 L. Cryogenic Tank, Liquid · Compressed Argon 2000 L. Cryogenic Tank, Liquid · Compressed Helium <20 - 300 std. cu ft, steel cylinders · Compressed Argon <20 - 300 std. cu ft, steel cylinders · Compressed CO2 <20 - 300 std. cu ft, steel cylinders The proposed Powerhouse 2 building is projected to house several types of organizations: NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 6 | 12 retail, private companies working in the energy industry, and Colorado State University (CSU) functions relating to sponsored research. The CSU space will be assigned to researchers to conduct sponsored research for industry and government (local, city, state and federal) sponsors. These spaces will not be used for traditional educational functions such as classroom instruction or laboratory instruction. While university employees (researchers, staff, and student employees) will be using the space they will not be engaged in instructional activities. Government services facilities include essential structures such as dormitories, offices, classrooms, and laboratories for public higher education facilities. Powerhouse 2 will not be used for dormitory housing. Nor will it have instructional classrooms and the laboratory and office spaces will be used solely for sponsored research activities. Given this definition and purposes, Powerhouse 2 should not be classified as a government services facility per Chapter 10 of City Code and the restrictions associated with this designation. Regarding Government Service Critical Facilities and Hazardous Material Critical Facilities, the applicant has provided further explanation of the planned uses that will occur prior to the Letter of Map Revision becoming effective. See Appendix F for letters from the applicant. DRAIN BASINS AND SUB-BASINS A. MAJOR BASIN DESCRIPTION Powerhouse 2 is located in the Cache la Poudre River Basin, a major tributary to the South Platte River. The river is located in Larimer and Weld Counties, with a small portion of the drainage basin extending into southern Wyoming. Passing through Fort Collins, the Poudre River corridor provides stormwater drainage from various contributing city drainage basins. Floodplain restrictions have limited the amount of building in the floodplain, resulting in lower development density. This lower density, combined with sensitivities to the natural environment, has resulted in a high concentration of parks and open spaces along the river corridors. B. SUB-BASIN DESCRIPTION The subject property historically drains overland through the interior to the south across flat grades. Runoff from the site has historically been routed to an existing pond/wetland on the south side of Vine Drive. The project site was a part of a previous drainage study in this area, "Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project" by Anderson Consulting Engineers, Inc., dated June 6, 2017. Within the drainage study, Powerhouse 2 is associated with Basin 9, with a contributing area of 4.68 acres and an assumed fully-developed imperviousness of 85%. DRAINAGE DESIGN CRITERIA A. REGULATIONS There are no optional provisions outside of the FCSCM proposed with the proposed project. B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS In June 2017, Anderson Consulting Engineers, Inc. (ACE), per the direction of the City of Fort Collins Stormwater staff, provided a Stormwater Management Model (SWMM) model for a fully- developed condition scenario to evaluate the size of the proposed project infrastructure for the NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 7 | 12 Poudre River Whitewater Park project. The objective of this modeling was to generate a conceptual drainage plan for future improvements. In the final modeling, Subbasins 8, 9, 10, and 11 are related to the site. ACE considered conceptual detention ponds for the privately owned properties located north of East Vine Drive (Subbasins 9 and 11) to detain and release fully-developed 100-year flows at a 2-year historical rate. For the Powerhouse 2 project, the corresponding basin is Subbasin 9 of the drainage study. An excerpt from the "Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project" drainage report is included in Appendix F. In order to bring Powerhouse 2 into compliance with the current land use code and stormwater criteria, 75% of all newly added impervious areas will be treated by Low Impact Development (LID) techniques. The remaining water quality will be provided in the proposed detention pond. C. HYDROLOGICAL CRITERIA The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure 3.4-1 of the FCSCM, serve as the source for all hydrologic computations associated with the proposed development. Tabulated data contained in Table 3.4-1 was utilized for Rational Method runoff calculations. The Rational Method was employed to compute localized stormwater runoff utilizing coefficients contained in Tables 3.2-1 and 3.2-2 of the FCSCM. Two separate design storms were utilized to address distinct drainage scenarios. The first event analyzed is the "Minor" or "Initial" storm, which has a two-year recurrence interval. The second event considered is the "Major" storm, which has a 100-year recurrence interval. No other assumptions or calculation methods were used for this development that is not referenced by the current City of Fort Collins criteria. D. HYDRAULIC CRITERIA As previously noted, the subject property maintains historical drainage patterns. All drainage facilities proposed with the project are designed in accordance with the criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual. As discussed above, the subject property is located in a 100-year floodplain. The proposed project does not propose to modify any natural drainageways. E. MODIFICATIONS OF CRITERIA Due to site topographic constraints, it is effectively impossible to achieve 12” of freeboard in the site’s stormwater detention facility (to the top of the emergency spillway). The project includes a variance for the first paragraph of Section 3.4 of the FCSCM which spells out the requirement for 12” of freeboard from the 100-year water surface elevation (WSE) to the top of the emergency spillway. 12” of freeboard between the 100-year WSE to the minimum opening elevations of buildings on-site is provided as required. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 8 | 12 F. STORM MANAGEMENT STRATEGY The overall stormwater management strategy employed with the Powerhouse 2 development utilizes the "Four Step Process" to minimize adverse impacts of urbanization on receiving waters. The following describes how the proposed development has incorporated each step. Step 1 – Employ Runoff Reduction Practices. The first consideration taken in trying to reduce the stormwater impacts of this development is the site selection itself and the selection of developable areas on the site. The Powerhouse 2 development aims to reduce runoff peaks, volumes, and pollutant loads from frequently occurring storm events (i.e., water quality (i.e., 80th percentile) and two-year storm events). Site constraints limit the possible LID techniques; however, rain gardens have been provided for this site as LID techniques for storm quality management. Step 2 – Implement Best Management Practices (BMPs) that provide a Water Quality Capture Volume (WQCV) with slow release. The efforts taken in Step 1 will help to minimize excess runoff from frequently occurring storm events; however, the development still generates additional stormwater runoff beyond historical conditions. The primary water quality treatment and volume control will occur in the detention pond. Step 3 – Stabilize Drainageways. As stated in Section I.A.3, the Lake Canal is located on the north side of the site; however, no changes are proposed to Lake Canal with this project. While this step may not seem applicable to the Powerhouse 2 development, the proposed project indirectly helps stabilize drainageways. Once again, site selection has a positive effect on stream stabilization. Step 4 – Implement Site Specific and Other Source Control BMPs. This step typically applies to industrial and commercial developments. DRAINAGE FACILITY DESIGN A. GENERAL CONCEPT The main objectives of the project drainage design are to maintain existing drainage patterns and ensure no adverse impacts to any adjacent properties. A total combined release rate of 16.77 cubic feet per second (cfs) has been determined for the proposed detention facility. This release rate has been grandfathered and agreed to by City staff. The methodology accounts for existing impervious areas on the site, which will be allowed to be released at the 100-year historical rate. There is a combined 1.32 acres of existing impervious area within the development site, which drains to East Vine Drive. The 100-year discharge from this impervious area is 13.11 cfs. There is a combined 3.66 acres of existing pervious area within the development site, which also drains to East Vine Drive. A 2-year discharge of 3.65 cfs has been calculated from this pervious area. The sum of "grandfathered" existing impervious areas discharging into East Vine Drive combined with 2-year existing pervious areas is 16.77 cfs, which is considered the allowable peak release rate for the site. We have subtracted the 100-year undetained discharge computed from Basins 5, 7, 8, 10, 11, and 12 (3.68 cfs total) for an allowable release rate of 13.09 cfs (16.77 cfs -3.68 cfs). However, the detention pond calculations are limited to a release rate of 11.5 cfs due to the capacity of the proposed 12" outlet pipe. The previous Conceptual Design from Anderson Consulting Engineers, Inc. included an 18" RCP outlet pipe. Upon checking the conceptual design, it was discovered that the pipe did not have NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 9 | 12 sufficient cover on the north flowline of Vine Drive; therefore, the pipe diameter was reduced to 2-12" HDPE pipes with a lower release rate of 8.5 cfs for Outfall A and a 12" DIP with a release rate of 3.0 cfs for Outfall B. We have used this 11.5 cfs value (8.5 cfs + 3.0 cfs) when sizing our detention pond. Onsite detention and water quality treatment for the Powerhouse 2 project will be provided within the detention pond and rain gardens. A temporary piezometer was installed approximately 25' north of the proposed detention pond. On July 29, 2022, the groundwater reading was at 5 feet below existing grade, which is approximately 1.2' below the proposed detention pond outlet. While it is desired to have more separation between the pond bottom and groundwater, the pond bottom has been elevated as much as possible and the outfall pipe is as shallow as it can go. An underdrain has been proposed in the detention pond to help mitigate the infiltration of water from the detention pond into the groundwater. Undulations have been added to the pond bottom, so most of the detention pond is 18” or more above the groundwater elevation, with more than half being 24” or more. LID treatment requirements will be met using rain gardens on the north and south sides of the project, thereby meeting the requirement of at least 75% of all newly added impervious areas. As mentioned above, Anderson Consulting Engineers, Inc. included East Vine Drive and the Powerhouse 2 site in their drainage study for the Poudre River Whitewater Project. Their Site Map can be found in Appendix F. That study dictated that flows from Vine, Jerome, and the parcel at the northeast corner of the Jerome/Vine intersection be conveyed along Vine to an inlet that would then convey flows south using the proposed 18 RCP and on to the downstream detention and water quality features. Our proposed drainage design follows this approach but uses a 2-12" HDPE pipeS, as noted above. B. SPECIFIC DETAILS The detention volume required for the site is 0.26 acre-feet. This volume was calculated using the FAA Method. City Code requires LID treatment for all projects. This project proposes meeting the requirement by using rain gardens as a LID technique. The rain gardens will capture the majority of the water quality volume (WQCV) for the entire site, providing a total volume of 3,139 cubic feet. This configuration will treat at least 75% of the new impervious area. The project is also including permeable pavers and gravel parking areas throughout the project. While these don't count towards the official LID calculation, they achieve similar outcomes and further enhance stormwater treatment on the site. The Powerhouse 2 site has been broken into onsite sub-basins for design purposes. Anticipated drainage patterns for proposed drainage basins are described below. Basins 1a to 1d Basin 1a to1d, approximately 2.41 acres, consists of building, asphalt, sidewalk, paver, gravel, open areas, and solar panel rooftops in the parking area. Runoff from the basin will sheet flow north into a swale along the north property line. The swale will discharge into the rain garden and then to Basin 6. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 10 | 12 Basin 2 Basin 2, approximately 0.61 acres, consists of asphalt, sidewalk, gravel, open areas, and solar panel rooftops in the parking lot. Runoff from the basin will sheet flow to a proposed curb opening located on the southeast side of the parking area and then to Basin 6. Basin 3a to 3c Basins 3a to 3c, approximately 0.45 acres, consists of a building, sidewalk, and open areas. Runoff from the basin will convey flows via sheet flow and roof leaders to a rain garden located on the south and then to Basin 6. Basins 4 Basin 4, approximately 0.29 acres, consists of asphalt, sidewalk, and landscape areas. Runoff from the basin will convey flows via sheet flow to a proposed Curb cut and then to Basin 6. Basins 5 & 7 Basin 5, approximately 0.16 acres, and Basin 7, approximately 0.011 acres, consists of sidewalk and landscape areas. Runoff from the basins will convey flows via sheetflow to a proposed area inlet located on East Vine Drive, then routed to the Whitewater Park detention pond. Basin 6 Basin 6, approximately 0.36 acres, consists of open space. This basin is the onsite detention pond. Basin 8 and Co1 Basin 8, approximately 0.12 acres, and Basin Co1, approximately 0.43 acres, consists of asphalt, sidewalks, and landscape. Runoff from the basins will drain via sheet flow to the curb and gutter along North College Avenue, then into an existing inlet in sump. The runoff will follow existing drainage patterns. Basin 9 Basin 9, approximately 0.14 acres, is an offsite basin and consists of open space. Runoff from the basin will be conveyed through the north rain garden and then through Basin 6 without being detained. Basins 10 and 11 Basin 10, approximately 0.03 acres, and Basin 11, approximately 0.05 acres, consist of open space. Runoff from the basins will drain via sheet flow north to Lake Canal. The areas are outside the limits of disturbance and will continue to drain to the ditch as they currently do. Basin 12 Basin 12, approximately 0.14 acres, consists of sidewalks and landscape. Runoff from the basin will drain via sheet flow east to Jerome Street and follow existing drainage patterns to a proposed area inlet located on East Vine Drive. Runoff from the basin will be routed to the Whitewater Park detention pond. Basins W8 & W10 Basin W8, approximately 0.63 acres, and Basin 10, approximately 1.50 acres, are part of the "Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project" Drainage Study. Runoff from the basins will follow the assumed fully-developed conceptual design path from the drainage study. The runoff will be routed to a proposed area inlet on East Vine Drive and then to the Whitewater Park detention pond. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 11 | 12 Basin W11 Basin W11, approximately 0.89 acres, is part of the "Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project" Drainage Study. Runoff from the basin will follow the assumed fully-developed conceptual design path from the drainage study and will be routed via storm drain to Whitewater Park detention pond in the future when the parcel is developed. A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of this report. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 12 | 12 C. LID SUMMARY Powerhouse 2 is providing a rain garden to treat a portion of the north area of the site that will see the highest runoff rate and contains portions of the building, concrete drive aisles, sidewalk, permeable pavers, gravel parking, landscape areas, and solar panel rooftops in a portion of the parking lot (Basin 1). Powerhouse 2 is providing a rain garden to treat a portion of the southwest area of the site, which contains a portion of the building, sidewalks, and open areas (Basin 3). Powerhouse 2 On-Site LID Treatment Project Summary Total Impervious Area 134,735 sf Target Treatment Percentage 75% Minimum area to be Treated by LID measures 101,051 sf Treated Impervious Areas by Treatment Method Rain Garden 86,629 Sf Gravel Paving 31,096 Sf Total Treated Impervious Areas 117,725 sf Percent Total Project Area Treated 87.4% CONCLUSIONS A. COMPLIANCE WITH STANDARDS The drainage design proposed with the Powerhouse 2 project complies with the City of Fort Collins Stormwater Criteria Manual. The drainage design proposed for this project complies with the "Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project" Drainage Study. The drainage design proposed with this project complies with the Cache la Poudre River Basin requirements. The proposed project complies with the regulation from Chapter 10 of the City Code. These includes: -The Building finished floors and all ductwork, heating, ventilation, electrical systems, etc., will be elevated 24-inches above the effective Base Flood Elevation (BFE). -Any items located in the floodplain that can float (e.g., picnic tables, bike racks, etc.) must be anchored. -An Emergency Response and Preparedness Plan (ERPP) will be required by the City of Fort Collins prior to a floodplain or building permit being issued -Critical facilities are prohibited in the floodplain, including hazardous material, government services, essential services and at-risk population critical facilities. this includes schools and after school programs/day camps for grades k-12. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY 13 | 12 REFERENCES 1. City of Fort Collins Landscape Design Guidelines for Stormwater and Detention Facilities, November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility Services. 2. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, adopted by Ordinance No. 174, 2011, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code. 3. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007. 4. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation Service, United States Department of Agriculture. 5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008. 6. Drainage, Water Quality and Erosion Control Report for the Poudre River Whitewater Park Project dated June 6, 2017, by Anderson Consulting Engineers, Inc. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: POWERHOUSE 2 FORT COLLINS | GREELEY APPENDIX APPENDIX A HYDROLOGIC COMPUTATIONS Tc2Tc10Tc100C2C10C100I2I10I100Q2Q10Q100Historic Basins:imp. areas lot 2 & 3 Imp. Areas lot 2 & 3 0.705.7 5.7 5.0 1.0 1.01.02.8 4.7 10.0 1.8 3.1 7.0perv. areas lot 2 & 3 Perv. Areas lot 2 & 3 0.4612.5 12.5 12.5 0.4 0.40.52.1 3.5 7.2 0.4 0.7 1.7imp. areas lot 4 Imp. Areas lot 4 0.625.0 5.0 5.0 0.9 0.91.02.9 4.9 10.0 1.7 2.8 6.1perv. areas lot 4 Perv. Areas lot 4 3.2011.6 11.6 9.6 0.5 0.50.62.1 3.6 7.9 3.3 5.6 15.4Developed Basins:1a 1a 0.055.0 5.0 5.0 1.0 1.01.02.9 4.9 10.0 0.1 0.2 0.51b 1b 0.085.0 5.0 5.0 1.0 1.01.02.9 4.9 10.0 0.2 0.4 0.81c 1c 0.485.0 5.0 5.0 1.0 1.01.02.9 4.9 10.0 1.3 2.2 4.81d 1d 1.8011.7 11.7 11.0 0.6 0.60.72.1 3.6 7.6 2.2 3.7 9.72 2 0.615.6 5.6 5.0 0.8 0.81.02.8 4.7 10.0 1.4 2.4 6.13a 3a 0.015.0 5.0 5.0 1.0 1.01.02.9 4.9 10.0 0.0 0.0 0.13b 3b 0.395.0 5.0 5.0 0.9 0.91.02.9 4.9 10.0 1.0 1.7 3.93c 3c 0.055.0 5.0 5.0 0.9 0.91.02.9 4.9 10.0 0.1 0.2 0.54 4 0.295.0 5.0 5.0 0.5 0.50.72.9 4.9 10.0 0.4 0.8 1.96 6 0.367.7 7.7 7.3 0.2 0.20.32.5 4.2 8.8 0.2 0.3 0.85 5 0.165.0 5.0 5.0 0.5 0.50.72.9 4.9 10.0 0.2 0.4 1.17 7 0.115.0 5.0 5.0 0.6 0.60.72.9 4.9 10.0 0.2 0.3 0.88 8 0.125.0 5.0 5.0 0.4 0.40.62.9 4.9 10.0 0.15 0.26 0.679 9 0.147.2 7.2 6.8 0.2 0.20.32.5 4.3 9.1 0.07 0.12 0.3110 10 0.035.0 5.0 5.0 0.2 0.20.32.9 4.9 10.0 0.02 0.03 0.0711 11 0.055.0 5.0 5.0 0.2 0.20.32.9 4.9 10.0 0.03 0.05 0.1312 12 0.145.0 5.0 5.0 0.6 0.60.72.9 4.9 10.0 0.23 0.40 1.01co1 Co1 0.435.0 5.0 5.0 0.7 0.70.92.9 4.9 10.0 0.91 1.56 3.98w8 W81.5 3.0 6.9w10 W102.8 5.6 13.2w11 W111.8 3.5 8.11a, 1b, 1c, 1d (North Rain Garden)2.41 11.5 11.5 9.8 0.7 0.7 0.8 2.1 3.6 7.9 3.43 5.84 15.843a, 3b, 3c (South Rain Garden)0.45 5.0 5.0 5.0 0.9 0.9 1.0 2.9 4.9 10.0 1.14 1.96 4.472, 4, 6 to Extended Detention Basin WQ1.26 5.1 5.1 5.0 0.6 0.6 0.7 2.9 4.9 10.0 2.12 3.62 9.25and 6 to Detention Pond4.12 11.5 11.5 9.8 0.7 0.7 0.8 2.1 3.6 7.9 5.87 10.00 27.11DEVELOPED DIRECT RUNOFF COMPUTATIONSIntensity (in/hr)Flow (cfs)Powerhouse 2C. OrtizDecember 13, 2022DesignPointBasinIntensity, I, from Fig. 3.4.1 Fort Collins Stormwater Manual.Rational Equation: Q = CiA (Equation 6-1 per MHFD)Area(acres)Runoff CTc (Min)Date:Fort CollinsProject:Location:Calc. By:Combined BasinsInformation taken from "Drainage, Water Quality and Erosion Control Report for Poudre River Whitewater Park"Information taken from "Drainage, Water Quality and Erosion Control Report for Poudre River Whitewater Park"Information taken from "Drainage, Water Quality and Erosion Control Report for Poudre River Whitewater Park"Basins Not Detain Project Title Date: Project Number Calcs By: City Basins 1 WQCV = Watershed inches of Runoff (inches)61% a = Runoff Volume Reduction (constant) i = Total imperviousness Ratio (i = Iwq/100)0.240 in A =4.12 ac V = 0.10 ac-ft V = 1.2 x Water Quality Design Volume (ac-ft) WQCV = Water Quality Capture Volume (inches) A = Watershed Area (acres) Provided (LID) Rain Gardens = Volume to be provided in the Pond = 0.03 Ac-ft dia (in) =13/16 number of columns=1.00 number of rows =2.00 number of holes =2.00 Area Per Row =0.52 square inches Total Outlet Area (in2) =1.04 <-- CALCULATED from total number of holes Circular Perforation Sizing 3139 cu. ft. 1160 cu. ft. Powerhouse II December 14, 2022 207-030 C. Ortiz Fort Collins 4299 cu. ft. Drain Time a = i = WQCV = Figure EDB-2 - Water Quality Capture Volume (WQCV), 80th Percentile Runoff Event 1, 2, 3, 4, and 6 DETENTION POND Dp1 0.231 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 00.10.20.30.40.50.60.70.80.91WQCV (watershed inches)Total Imperviousness Ratio (i = Iwq/100) Water Quality Capture Volume 6 hr 12 hr 24 hr 40 hr ()iii78.019.10.91aWQCV 23 +-= ()iii78.019.10.91aWQCV 23 +-= AV* 12 WQCV   = 40 hr Project: Date:12/13/2022 WQCV ELEVATION : (A) (B) (C) (D)Total Volume Detention = 1,159.99 0.03 Ac. Elevation Depth Area Volume Σ Volume Σ Volume Elev. (A1) = 4,958.50 (Sf.) (Cf.) (Cf.) (Ac.-ft.)Σ Volume (D1) = 239.44 Partial volume (C2) = 1,178.87 Outlet Elevation:4,958.11 1.00 Depth, partial volume = 0.50 0.39 239.44 239.44 0.01 >>>>>>>>> 4,958.50 1,226.92 High water level (HWL) = 4958.89 0.50 1,178.87 1,418.31 0.03 4,959.00 3,488.55 Water Depth = 0.78 Ft. 0.50 2,370.19 3,788.50 0.09 4,959.50 5,992.21 0.50 3,796.67 7,585.17 0.17 4,960.00 9,194.48 0.50 4,904.95 12,490.12 0.29 4,960.50 10,425.31 ACTUAL STORAGE : (A) (B) (C) (D)Total Volume Detention = 11,117.73 0.26 Ac. Elevation Depth Area Volume Σ Volume Σ Volume Elev. (A1) = 4,960.00 (Sf.) (Cf.) (Cf.) (Ac.-ft.)Σ Volume (D1) = 7,585.17 Partial volume (C2) = 4,904.95 Outlet Elevation:4,958.11 1.00 Depth, partial volume = 0.50 0.39 239.44 239.44 0.01 4,958.50 1,226.92 High water level (HWL) = 4960.36 0.50 1,178.87 1,418.31 0.03 Free board = 0.14 4,959.00 3,488.55 Top pond bank elevation = 4960.50 0.50 2,370.19 3,788.50 0.09 4,959.50 5,992.21 Water Depth = 2.25 Ft. Detention Req'd 0.50 3,796.67 7,585.17 0.17 >>>>>>>>> 4,960.00 9,194.48 0.50 4,904.95 12,490.12 0.29 4,960.50 10,425.31 Pond No.:1 Detention Pond Stage Storage Curve Pond Stage Storage Curve Project Number:207-030 Project Location:Fort Collins, Colorado Calculations By:C. Ortiz Powerhouse II 207-030 - D-pond_Capacity, Det. Pond-1, 12/13/2022, 10:04 AM Area Inlet Performance Curve: PowerHouse 2 - Design North Rain Garden Governing Equations: At low flow depths, the inlet will act like a weir governed by the following equation: * where P = 2(L + W) * where H corresponds to the depth of water above the flowline At higher flow depths, the inlet will act like an orifice governed by the following equation: * where A equals the open area of the inlet grate * where H corresponds to the depth of water above the centroid of the cross-sectional area (A) The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown. However, what is known, is that the stage-discharge curves of the weir equation and the orifice equation will cross at a certain flow depth. The two curves can be found below: If H > 1.792 (A/P), then the grate operates like an orifice; otherwise it operates like a weir. Input Parameters: Type of Grate: CDOT Type C Close Mesh Grate Double Shape Rectangular Length of Grate (ft): 6.75 Width of Grate (ft): 2.22 Open Area of Grate (ft2):17.01 Flowline Elevation (ft): 4961.000 Allowable Capacity: 50% Depth vs. Flow:(Too much Depth, Inlet will be at a 4:1 Slope) Depth Above Inlet (ft) Elevation (ft) Shallow Weir Flow (cfs) Orifice Flow (cfs) Actual Flow (cfs) 0.00 4961.00 0.00 0.00 0.00 0.10 4961.10 0.85 14.45 0.85 0.20 4961.20 2.41 20.44 2.41 0.30 4961.30 4.42 25.04 4.42 0.40 4961.40 6.81 28.91 6.81 0.50 4961.500 9.51 32.32 9.51 0.60 4961.60 12.51 35.41 12.51 0.70 4961.70 15.76 38.24 15.76 Q100 0.80 4961.80 19.26 40.88 19.26 0.90 4961.90 22.98 43.36 22.98 1.00 4962.000 26.91 45.71 26.91 Inlet at Design North Rain Garden flow of 15.70 cfs at the elevation 0.7 above the inlet elevation. 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00Discharge (cfs)Stage (ft) Stage - Discharge Curves Weir Flow Orifice Flow 5.10.3 HPQ= 5.0)2(67.0 gHAQ=