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Home My WebLink AboutCENTERPOINT PLAZA REPLAT - FDP - 35-01D - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFinal Drainage Report DMW Civil Engineers, Inc. Centerpoint Plaza First Replat 0805.00-CPR October 28, 2008 REFERENCES 1) Anderson Consulting Engineers, Inc., Spring Creek Master Drainage Plan, Baseline Hydrologic Analyses Volumes I & II, Fort Collins, Colorado, May 5, 2003. 2) Anderson Consulting Engineers, Inc., Hydrologic Modeling and Hydraulic Analyses for Spring Creek Downstream of Edora Park, Fort Collins, Colorado, October 21, 2005. 3) City of Fort Collins, Colorado, Storm Drainage Design Criteria and Construction Standards, Fort Collins, Colorado, May, 1997, revised 1999. 4) United States Department of Agriculture, Soil Survey of Larimer County Area, Colorado, December 1980. t5) Urban Drainage and Flood Control District, Urban Storm Drainage Criteria Manual, Wright Water Engineers, Denver, Colorado, April, 2008. 1 I 1 Page 6 of 6 I Final Drainage Report DMW Civil Engineers, Inc. Centerpoint Plaza First Replat 0805.00-CPR October 28, 2008 measures. In addition to silt fence, a temporary sediment pond will be utilized as ' a sediment basin for additional sediment control. 5.5 Water Quality ' Structural Best Management Practices (BMPs) are proposed for the Centerpoint Plaza First Replat site to improve water quality of runoff as recommended by the Urban Storm Drainage Criteria Manual. The detention pond incorporates an ' extended detention basin for water quality purposes. An extended detention basin (EDB) is a sedimentation basin which uses a much ' smaller outlet that extends the emptying time of the more frequently occurring runoff events to facilitate pollutant removal. Soluble pollutant removal can be somewhat enhanced by providing a small wetland marsh or ponding area in the basin's bottom to promote biological uptake. The basins are considered to be "dry" because they are designed not to have a significant permanent pool of water remaining between storm runoff events. However, EDB may develop t wetland vegetation and sometimes shallow pools in the bottom portions of the facilities. t The water quality pond for Centerpoint Plaza First Replat has been designed to provide 4704 cubic feet of water quality capture with a water surface elevation of 4909.17. Release of the captured water is controlled by a steel plate with one column of six 5/8" diameter circular perforations. The outlet structure itself and 36" diameter RCP outlet pipe convey flows in excess of the water quality capture volume north to Spring Creek. The pond will reach a maximum water surface elevation of 4910.50 to discharge these flows through the outlet structure. The parking lot serves as an emergency overflow and any excess runoff is conveyed off -site to the southeast towards Skunk Pond. VI. CONCLUSIONS 1 6.1 Compliance with Standards All drainage analyses have been performed according to the City of Fort Collins' Storm Drainage Design Criteria and Construction Standards manual and the ' Urban Storm Drainage Criteria Manual, Volume 3, by the Urban Drainage and Flood Control District. Page 5 of 6 F C I L L I Final Drainage Report Centerpoint Plaza First Replat 0805.00-CPR October 28, 2008 DMW Civil Engineers, Inc. Basin 6 (1.18 acres) is a combination of developed and undeveloped areas, but was modeled as fully developed. Runoff was modeled as flowing east to west, conveyed by pans, gutters and sheet flow to Basin 7 and then to the Pond. The peak 10 and 100-yr flows from Basin 6 are 4.9 and 11.7 cfs, respectively. Basin 7 (0.33 acres) is composed primarily of paving and roof. Runoff from Basins 6 and 7 is conveyed via sheet flow and gutters to an 8-foot wide concrete sidewalk culvert and into the Pond. The peak 10 and 100-yr flows from Basin 7 are 1.5 and 3.3 cfs, respectively. The combined peak 10 and 100-yr flows from Basins 6 and 7 are 6.2 and 14.7 cfs, respectively. Basin 8 (0.73 acres) is composed primarily of roof and paving. Runoff is conveyed by pans, sheet flow, and gutters to a 6-foot wide curb opening into the Pond. The peak 10 and 100-yr flows from Basin 8 are 2.6 and 6.6 cfs, respectively. Basin 9 (021 acres) is composed primarily of grassed slopes, and the Pond. Runoff is conveyed by sheet flow and pans, to the existing outlet structure, and then to Spring Creek. The peak 10 and 100-yr flows from Basin 9 are 0.4 and 1.0 cfs, respectively. The combined peak 10 and 100-yr flows from Basins 3 through 9 are 15.3 and 42.0 cfs, respectively. 5.3 Detention The drainage report for Phase 1 called for a Detention Pond which lies in Phase 2. The City of Fort Collins Stormwater department has indicated that detention is not required for this development providing that the proper drainage facilities exist to convey the undetained flows to either Spring Creek to the north or Skunk Pond far to the east. Due to the fact that a majority of the downstream facilities to Skunk Pond are inadequate and would require upgrading it was found to be more economical to convey undetained flows to Spring Creek to the north. There has been an amendment to the Phase 1 drainage plan to qualify these changes. This project reconfigures the approved Phase 1 pond and provides the same water quality capture volume, and complies with the original Phase 1 drainage report. 5.4 Erosion Control Plan and Criteria The erosion control plan presented here is intended to control rainfall erosion. The Erosion Control Reference Manual for Construction Sites (ECRM), City of Fort Collins, has been referenced for this erosion control plan. The proposed rainfall erosion control plan during construction will consist of temporary structural erosion control measures. Temporary sediment control. consisting of silt fencing will surround areas of the proposed construction. See the Grading and Erosion Control Plan for locations of proposed erosion control Page 4 of 6 Final Drainage Report Centerpoint Plaza First Replat 0805.00-CPR October 28, 2008 DMW Civil Engineers, Inc. coefficients were generated using Table 3-3 and 3-4 of the City of Fort Collins Storm Drainage Design Criteria and Construction Standards manual. 4.3 Hydraulic Criteria The computer program "Hydraflow" and spreadsheets from the Urban Storm Drainage Criteria Manual have been used to analyze the capacity of proposed pipes and swales for the site. Various spreadsheets using City of Fort Collins criteria have been used in the design of proposed detention ponds. V. DRAINAGE FACILITIES DESIGN 5.1 Proposed Site Development Development of the Centerpoint Plaza First Replat site will consist of a commercial building. The proposed site infrastructure includes paved parking areas with curb, gutter and sidewalk, utility service lines, and storm utilities. 5.2 Developed Drainage Patterns The developed site has been divided into nine (9) on -site and one (1) off -site drainage basins. Stormwater runoff will be conveyed overland to an onsite detention and water quality, pond and released into existing stormwater utilities at the northwest corner of the site. Basins 1 (0.09 acres) and 2 (0.12 acres) are comprised primarily of asphaltic paving. Runoff from Basins 1 and 2 sheet flows across the paving, parking lot, and along concrete gutters from northwest to southeast. Stormwater runoff follows existing flow patterns and is conveyed to the southeast. The peak 10 and 100-year flows from Basin 1 are 0.3 and 0.9 cfs, respectively. The peak 10 and 100-year flows from Basin 2 are 0.4 and 1.0 cfs, respectively. Basin 3 (1.30 acres) is comprised mostly of the existing building and parking lot, and includes existing landscaping, and proposed roof and parking. Runoff from Basin 3 follows existing flow patterns and flows from west to east. Runoff is conveyed in concrete trickle pans and gutters to Basin 4, and then to Basin 5. The peak 10 and 100-yr flows from Basin 3 are 4.1 and 11.5 cfs, respectively. Basin 4 (1.01 acres) is currently primarily grassed meadow, but was modeled as developed. Runoff was modeled as flowing east to west, conveyed by pans and gutters to Basin 5. The peak 10 and 100-yr flows from Basin 4 are 4.2 and 10.1 cfs, respectively. Basin 5 (0.09 acres) is composed primarily of a grassed swale. Runoff is conveyed to the south and into a storm drain pipe discharging in the Pond. Basin 5 also conveys runoff from Basins 3 and 4 to the storm pipe. The peak 10 and 100-yr flows from Basin 5 are 0.02 and 0.4 cfs, respectively. The combined peak 10 and 100-yr flows from Basins 3, 4, and 5 are 7.5 20.7 and cfs, respectively. Page 3 of 6 Final Drainage Report Centerpoint Plaza First Replat 0805.00-CPR October 28, 2008 DMW Civil Engineers, Inc. 2.2 Site Description The Centerpoint Plaza site is a total of approximately 2.34 acres of land. Phase 1 development was completed in May of 2006. The Phase 2 area is approximately 1.57 acres. Vegetation on Phase 2 consists of native weeds and grasses. Onsite soils are Hydrologic Group C according to the Natural Resources Conservation Service (NRCS). The site is zoned E (Employment) and is adjacent to industrial development. The Centerpoint Plaza First Replat site is within Flood Insurance Zone X, the areas determined to be outside the 500-year flood plain according to the FEMA Flood Insurance Rate Map, Community -Panel No. 08069C-0983G. III. DRAINAGE BASINS AND SUB -BASINS 3.1 Maior Basin Description The Centerpoint Plaza, First Replat site is within the Spring Creek Drainage Basin, as identified by the City of Fort Collins Master Drainage Plan, but not within any regulatory floodplains designated by either the City or FEMA. Spring Creek is a major watercourse that flows from Spring Canyon Dam at Horsetooth Reservoir to its confluence with the Poudre River. The Spring Creek drainage basin encompasses nearly nine square miles in central Fort Collins. The basin is dominated by residential development, but it also includes open space, parks and isolated areas of commercial and industrial development. 3.2 Sub -Basin Description Stormwater runoff from the Centerpoint Plaza First Replat site drains overland generally from northwest to southeast at slopes ranging from 1.0% to 2.0%. Stormwater runoff from most of the site is conveyed to a water quality pond and routed via storm pipes to Spring Creek, north of the site. Runoff not captured by the pond currently is conveyed from the site towards the southeast in a shallow swale adjacent to the railroad. IV. DRAINAGE DESIGN CRITERIA 4.1 Design Criteria and Restraints Drainage design criteria specified in the City of Fort Collins Storm Drainage Design Criteria and Construction Standards manual and the Urban Storm Drainage Criteria Manual, Volume 3 by the Urban Drainage and Flood Control District have been referenced in the preparation of this study. 4.2 Hydrologic Criteria The rational method has been used to estimate peak stormwater runoff from the project site. An initial 10-year and major 100-year design storms have been used to evaluate the proposed drainage system. Calculations made as part of this investigation, along with other supporting material, are contained in Appendix A. Rainfall intensity data for the rational method has been taken from rainfall intensity data tables contained in the City of Fort Collins Storm Drainage Design Criteria and Construction Standards manual (see Figure 3-1a). Composite runoff Page 2 of 6 Final Drainage Report for Centerpoint Plaza First Replat Fort Collins, Colorado October 28, 2008 I. INTRODUCTION 1.1 Oblective This study documents the results of a comprehensive hydrologic and hydraulic analysis of both pre and post -development conditions for the proposed Center Point Plaza, First Replat development. The construction plans which accompany this report are entitled Final Construction Plans for Centerpoint Plaza, First Replat prepared by DMW Civil Engineers. 1.2 Project History and Previous Studies A Master Drainage Plan for Centepoint Plaza could not be located. Stewart and Associates prepared a drainage study for Phase 1 of Center Point Plaza, dated February 14, 2003. - The neighboring property, owned by Larimer County, has had multiple drainage studies done in which the subject site is considered an offsite basin. But due to reasons that will be discussed more fully in this report, the runoff from this site will not enter Larimer County's property. 1.3 Mapping and Surveying Field survey information and topographic mapping with a contour interval of 1- foot was obtained by DMW Civil Engineers from Stewart and Associates, Fort Collins, Colorado. Additional topography outside of the project boundary was taken from City of Fort Collins Aerial Topography with a two (2) foot contour interval. 1.4 - Site Reconnaissance The project engineer performed site visits to the Centerpoint Plaza, First Replat site in Fort Collins in October, 2008. Existing drainage patterns and structures were observed for use in defining drainage basin boundaries and flow patterns. II. GENERAL LOCATION AND DESCRIPTION 2.1 Site Location I The site is located in the northwest quarter of Section 20, Township 7 North, Range 68 West of the 6th Principal Meridian in the City of Fort Collins, Colorado. The site is bounded by Timberline Road on the west, Midpoint Drive on the north, Railroad Right of Way on the north andindustrial development to the east (See Vicinity Map). i Final Drainage Report for Centerpoint Plaza First Replat EXHIBITS Exhibit 1 Centerpoint Plaza, First Replat Drainage Plan Exhibit 2 8.5"x11" FIRM, Community -Panel No. 08069C-0983G APPENDICES Appendix A Developed Hydrology Computations Appendix B Developed Hydraulic Computations Appendix C Detention & Water Quality Pond Calculations Appendix D Erosion Control Calculations n i Final Drainage Report for Centerpoint Plaza First Replat I TABLE OF CONTENTS VICINITY MAP Page No. INTRODUCTION 1.1 Objective..............................................................................................................1 1.2 Project History and Previous Studies...................................................................1 1.3 Mapping and Surveying.......................................................................................1 1.4 Site Reconnaissance...........................................................................................1 II SITE LOCATION AND DESCRIPTION 2.1 Site Location.....................................................................................................1 2.2 Site Description....................................................................................................2 III DRAINAGE BASINS AND SUB -BASINS 3.1 Major Basin Description.......................................................................................2 3.2 Sub -Basin Description.........................................................................................2 IV DRAINAGE DESIGN CRITERIA 4.1 Design Criteria and Constraints...........................................................................2 4.2 Hydrologic Criteria...............................................................................................2 4.3 Hydraulic Criteria.................................................................................................3 V DRAINAGE FACILITIES DESIGN 5.1 Proposed Site Development................................................................................3 5.2 Developed Drainage Patterns..............................................................................3 5.3 Detention.............................................................................................................4 5.4 Erosion Control....................................................................................................4 5.5 Water Quality.......................................................................................................5 VI CONCLUSIONS 6.1 Compliance with Standards.................................................................................5 IREFERENCES......................................................................................I ........6 I I 1_1 I 1 I 1 J I OMV�L0001 CIVILEERS October 28, 2008 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 Re: Final Drainage Report for Centerpoint Plaza, First Replat Fort Collins, Colorado Dear Staff: 3400 WEST 1 6T" STREET BUILDING 5, SUITE Z GREELEY, 00 80634 (970) 378-6935, FAX (970) 378-6997 WWW.DMWCIVILENGINEERS.COM DMW Civil Engineers is pleased to submit this drainage study for Centerpoint Plaza, First Replat for your review. This report complies with technical criteria set forth in the City of Fort Collins' Storm Drainage Design Criteria and Construction Standards manual and the Urban Storm Drainage Criteria Manual, Volume 3, by the Urban Drainage and Flood Control District. We acknowledge that the City of Fort Collins' review of this study is only for general conformance with submittal requirements, current design criteria and standard engineering principles and practices. =f w` If you should have any questions or comments as you review this report, please feel free to contact me at your convenience. Sincerely, DMW CIVIL ENGINEERS 33885 c'•, k; �S`S�ONAL ENG\� Jade P. Miiler, P.E. (CO #33885) Project Engineer I 1 1 1 1 1 I Final Drainage Report for Centerpoint Plaza First Replat Fort Collins, Colorado October 28, 2008 Prepared for: Kevin Frazier Frazier & Company Ventures, LLC 141 S. College Suite 101 Fort Collins, CO 80524 . Prepared by: �MI/1j::CIVILEERS A 0 b 0 0 0 0 0) DMW Civil Engineers, Inc. 3400 West 16t' Street Building 5, Suite Z Greeley, CO 80634 (970) 378-6935 Fax (970) 378-6997 www.dmwcivilengineers.com Project No: 0805.00-CPR