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HomeMy WebLinkAboutNORTHSIDE AZTLAN COMMUNITY CENTER - PDP - 54-05 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTa I i (C 4' / CI c+r. ! Q92 roco.r wo. a,• ncmw. .a.. `. ;4L F'�•�.J t Ai . � "' ..ertiv-n mV.Lau{.H m _ - . 1 a 2 J IlrO r : //`♦,♦ /r `I \�.* • � � .� � ..Hewn A ,jai ,,;, T, / \` +\' Tlti ��/{� ` ��' Yam✓ J \ \ \ Aft IA7 % Ito `'�`•.r �, �\ .`.mow �\� � / /' / /� %\j\s\ , n.•v ,�///'/i ��yi9x3d�8_ �A� u ye eeJ be m 3 E t F ® y aL% NO AM 3 I©¢ i s[[r 1 1 Of 1 r " 203�-25 No Text APPENDIX A VICINITY MAP A 7. REFERENCES 1. City of Fort Collins, "Storm Drainage Criteria Manual", (SDCM), dated March, 1986. 2. FEMA, Flood Insurance Rate Map, Community -Panel Number 080102 0004 C, Revised March 18, 1996. 3. Urban Drainage and Flood 'Control District, "Urban Storm Drainage Criteria Manual", Volumes 1 and 2, dated June 2001 and Volume 3, dated September 2001. it Bleu.property. Cost sharing in the pond construction will be needed from at least the privately owned lands contributing to this off site flow. The treated water will be released into the Poudre River in its existing configuration. Basin 1 will continue to flow to the river via existing storm piping. 5. EROSION CONTROL 5.1. General Concept Erosion control will be designed with the FCP plans for the project. 5.2. Specific Details Erosion control will be designed with the FCP plans for the project. 6. CONCLUSIONS 6.1. 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. Floodplain and floodway documentation is located in Appendix F of this report. A FEMA Elevation Certificate is not required for any improvements proposed with the project. 6.2. Drainage Concept The proposed drainage concepts presented in this report and on the construction plans provides for stormwater quality treatment of improved areas. Conveyance elements have been designed to pass required flows and to minimize future maintenance. The existing outfall into the Poudre River is proposed to remain (although substandard) because of the environmental issues with excavating into the old City dump. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit would be required. 6 between existing and proposed conditions for the 11 acres is relatively small (14 cfs). Regarding the sizing of the water quality pond, this project typically would only be responsible for providing water quality for the 6 acres that is being improved with the project (the new Aztlan and parking lot). Because all of the off site areas drain to this same point, it is impossible to split the flows from the proposed improvements and the existing streets, and other off site flows. It has been decided to provide a water quality pond that is sized to accept the entire 11 acres (in a developed condition) rather than creating a smaller water quality pond which in reality would not work because of the off site areas draining to it. Basin 3 includes Pine Street and Willow Street Rights of Way that drain through the Aztlan and United Way parking lots and to the water quality pond. The drainage plan assumes some additional pavement in these rights of way because of the "Downtown River District" street improvements project, which is in its preliminary design phase. From aerial photos, it appears there is a small area south of Willow that also drains to this area. This area and imperviousness was estimated from the photos. This area will drain to the pond; be treated, and continue into the Poudre River. Basin 4 includes the properties owned by Bas Blue Theatre and Jon Prouty (currently in the development review process at the City). This area presently is partially developed and historically drains overland to the United Way parking lot and swale. Because there is no opportunity to provide a new outfall from this site to the Poudre River, this development presently proposes to continue draining to the United Way parking lot and intends to contribute to the ovenizing of the water quality pond. In a developed condition, this area contributes an additional 5 cfs to the outfall location. Basin 5 includes the existing United Way building and parking lot. This area all drains to the northwest corner of the parking lot and into the existing swale. The water quality pond has been sized for this area. No changes from the existing conditions in this basin are proposed. 4.3. Drainage Summary Because this is a City project, all drainage facilities' maintenance will be the responsibility of the City. A water quality pond is proposed that will treat the proposed Aztlan site, as well as the Willow Street, Pine Street, the United Way, and the Prouty/Bas 5 a of how much water is coming off of each basin and what percentage of the water quality pond is attributed to each ownership. 4.2. Specific Flow Routing A summary of the drainage patterns within each basin is provided in the following paragraphs. Appendix B shows the existing and proposed runoff from each of these areas so that the existing and proposed conditions can be compared. Basin 1 is the located on the west side of the project. It presently drains to an 8" steel pipe under the railroad tracks, then to a 48" RCP to the Poudre River. This area presently drains the Aztlan skate park, which is paved. With the proposed design, a small amount - of roof area will drain to this area. The overall flow to the pipe is not increased. Since the impervious area is changing from asphalt to roof, and this drainage will continue to run down a relatively flat, grassed swale, the water quality of the runoff is improved and no further water quality treatment is proposed. Basin 2 includes the existing and proposed Aztlan and parking and is the largest drainage basin that has been analyzed. The entire site drains to the northeast via overland and curb i and gutter flow. On the east side of the parking lot, there will be an 8' wide bioswale to i treat the runoff from the site prior to entering the water quality pond. There are some minor sidewalk crossings that will overtop in larger storms. The water quality pond and outfall to the river deserve special attention. As shown on the "Existing Drainage Basins" map in Appendix A, there are presently 11 acres that drain to the existing swale behind the United Way building. This includes off site flows from Willow Street, Pine Street, the Prouty/Bas Bleu Property, and the existing United Way. This swale drains to a 15" storm pipe and it is likely that this pipe overtops to the river in any significant storm. The United Way building is 3.8' higher than the highpoint (overtopping point) of the trail, and about 8 feet above the 100-year Poudre River Floodplain, so it is safe from flooding from both the trail overtopping and the river. During the final design, the overtopping of the trail will be looked at to determine the approximate depth of the flows as they pass over the water quality pond and the trail. Typically, a large pipe would be added from the water quality pond to the river that is sized for the 100-year storm. Because this site was previously the City Dump, it has been decided to avoid excavating in this area and adding a new outfall pipe to the Poudre River. The existing overtopping of the trail will remain, and the difference in runoff 4 4. 3.2. Development Criteria Reference and Constraints The runoff from this site has been routed to conform to the requirements of the City Stormwater Department. Detention is not required for this site. It is proposed that no new storm pipes be installed into the river, and specified that excavation on the site be minimized because the site was a dump in the past. This project complies with the requirements of Chapter 10 of the City Code. Constraints on the site include the "no fill" requirements for the floodway and that all proposed buildings in the floodplain be elevated at least 2.0' above the 100-year Base Flood Elevation (BFE). 3.3. Hydrologic Criteria Runoff computations were prepared for the 2-year and 10-year minor and 100-year major storm frequencies utilizing the Rational Method. All hydrologic calculations associated with the basins are included in Appendix B of this report. Water quality volume was calculated using the method recommended in the "Urban Storm Drainage Criteria Manual". All water quality calculations are included in Appendix C of this report. 3.4. Hydraulic Criteria All hydraulic calculations within this report have been'prepared in accordance with the City of Fort Collins Drainage Criteria and are included in the Appendix B of this report. All basins will release into the Poudre River. DRAINAGE FACILITY DESIGN 4.1. General Concept All proposed flow patterns remain the same as existing flow patterns. The site is split into five drainage basins. Basin 1 includes a small area that presently drains to a small drainage pipe under the railroad tracks. Flows to this pipe have not been increased and the pipe will remain as is. Basins 2 through 5 all drain to the water quality pond. In actuality, these basins could have been combined into a single basin, but because of different ownership of the different basins, the areas have been split to give an accounting 3 this project. 2. DRAINAGE BASINS AND SUB -BASINS 2.1. Major Basin Description The proposed development lies within the Cache La Poudre Basin. The outfall point for this site is the Poudre River. Extended water quality detention is required for this site and is being provided. Runoff volume detention is not required in this basin. 2.2. Sub -basin Description This site was split into 5 sub -basins. The existing drainage patterns will remain with the proposed improvements. All basins drain from southwest to northeast and, with the exception of Basin 1, are collected in the proposed water quality pond. After being treated, runoff will be released into the Poudre River through the existing pipe under the Poudre Trail, and also over the trail and riverbank when the pipe reaches its capacity. This existing condition of stormwater running over the trail is proposed to remain because of special conditions in this area. This area was previously the City Dump and there are significant issues that could be encountered by excavating this area for a new outfall pipe. Refer to Section 4 of this report for a more detailed summary of on -site drainage patterns. 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), 1984, developed by the Denver Regional Council of Governments, has been used. Since this site is located in the Poudre River Floodplain, it is also subject to Chapter 10 of the City Code "Flood Prevention and Protection". None of the existing or proposed improvements on the site are in the floodplain or floodway, so nothing is subject to the freeboard requirements of the City Code. 2 1. GENERAL LOCATION AND DESCRIPTION 1.1. Location This project is located on the north side of Willow Street between Pine Street and College Avenue and is described as Aztlan Subdivision. This project is located in the Northwest Quarter of Section 12, Township 7 North, Range 69 West of the Sixth Principal Meridian, in the City of Fort Collins, Larimer County, Colorado. See the location map in Appendix A of this report. The Poudre River bounds the majority of the site on the north and runs through the east side of the project at Linden Street. 1.2. Description of Property Presently, the site consists of the Northside Aztlan Center (which will be replaced with a new facility), the United Way and their respective parking lots. The remaining portion of the site is Heritage Park. Existing drainage patterns on -site are to the north; into the Poudre River at approximately 0.5% to 1%. Existing vegetation consists mainly of sod in the areas that are not paved. Improvements proposed with this. project include a 40,000 square foot building (replacing the existing center) with associated drives, parking, and walks. The building will be a slab on grade foundation. Existing drainage patterns will be maintained with proposed runoff flowing to the north into a proposed water quality pond, then releasing into the Poudre River. The northeastern portion of this site is within the Poudre River Floodplain and Floodway (FEMA Panel 080102 0004 C, March 18, 1996). A LOMR is in progress and will likely be approved prior to the final submittal of this project. The LOMR case number is not known at this time. There is no City floodplain map for the Poudre River. None of the existing or.proposed improvements are in the floodplain. Refer to the "Grading Plan" and "Drainage and Erosion,Control Plan" for specific information related to the building elevations and floodplain location. A FEMA Elevation Certificate is not needed for any existing or proposed improvements related to this project. See Appendix F of this report for information related to the floodplain, a No -Rise Certification, and a Floodplain Use Permit will not be required for I TABLE OF CONTENTS TABLEOF CONTENTS............................................................................................................... iii 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location...................................................................................................................1 1.2 Description of Property............................................................................................1 2. DRAINAGE BASINS AND SUB -BASINS 2.1 Major Basin Description..........................................................................................2 2.2 Sub -Basin Description..............................................................................................2 3. DRAINAGE DESIGN CRITERIA 3.1 Regulations.............................................................::............................................... 2 3.2 Development Criteria Reference and Constraints....................................................3 3.3 Hydrologic Criteria..................................................................................................3 3.4 Hydraulic Criteria....................................................................................................3 4. DRAINAGE FACILITY DESIGN 4.1 General Concept.......................................................................................................3 4.2 Specific Flow Routing.............................................................................................4 4.3 Drainage Summary ........................................................................................:..........5 5. EROSION CONTROL 5.1 General Concept......................................................................................................6 5.2 Specific Details........................................................................................................6 6. CONCLUSIONS 6.1 Compliance with Standards....................................................................................6 6.2 Drainage Concept.....................................................................................................6 7. REFERENCES....................................................................................................................7 APPENDICES A Vicinity Map B Hydrologic and Hydraulic Computations C Water Quality Pond Sizing D Erosion Control Calculations - With Final Design E Figures and Tables F Floodplain and Floodway Documentation iii North Star --' design, inc. I'. ` December 21, 2005 I Basil Hamdan City of Fort Collins Stormwater I' 700 Wood Street Fort Collins, CO 80522-0580 RE: Preliminary Drainage Report for Aztlan Subdivision Dear Basil, I am pleased to submit for your review and approval, this Preliminary Drainage Report for Aztlan Subdivision. I certify that this report for the drainage design was prepared in accordance with the criteria in the City of Fort Collins Storm Drainage Manual. I appreciate your time and consideration in reviewing this submittal. Please call if you have any .questions. Sincerely, North Star Design, Inc. Prepared By: Michael Oberlander, P.E., L.S.I. I'M 700 Automation Drive, Unit I Windsor, Colorado 80550 970-686-6939 Phone 970-686-1 1 88 Fax PRELIMINARY DRAINAGE REPORT FOR AZTLAN SUBDIVISION Prepared for: CITY OF FORT COLLINS Prepared by: North Star Design 700 Automation Drive, Unit I Windsor, Colorado 80550 (970) 686-6939 December 21, 2005 Job Number 203-25