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Home My WebLink AboutBEST BUY EXPANSION MAJOR AMENDMENT AND REPLAT - 29 92B - SUBMITTAL DOCUMENTS - ROUND 3 - DRAINAGE REPORTBEST BUY P.U.D. ` FINAL DRAINAGE & EROSION CONTROL STUDY BEY ON D E N G IN E E R ING �-,fim 1 -1 7 7: NOCTE Final Drainage & BEYOND ENGINEERING Erosion Control Study Best Buy P. U.D. north end through (2)-18" RCP culverts. Other flow entering this basin is limited to the controlled detention pond release (QoR,=4.44 cfs) from Basin 200. The combined flows (Q,00 14.24 cfs) entering Basin 101 during the 100-year event exceed the capacity of the existing 21" RCP (Qcapa6 12.77 cfs). A comparison was made to determine the headwater requirements for 12.77 cfs and for 14.24 cfs to enter the existing 2 1 " RCP. The culvert calculations (refer to Appendix A) show that only 0.24' of additional head is required for a 1:1 inflow to outflow relationship to exist. The additional depth is negligible. Therefore, the current condition at the outfall is acceptable. Developed flows (Q,00 16.29 cfs) in Basin 200 (2.52 ac) sheet flow in a southeasterly direction and release to the detention pond through a curb cut along the east curb line. Off -site flows from Basins C, and Cz enter Basin 200 along its west boundary. Basins C, and CZ were first shown in the Final Drainage and Erosion Control Report for First Interstate Bank (Parsons & Associates, Inc, September 1992) as Basin C. This Basin was subdivided as a result of the proposed grading along the west curb line that created a high point north of the existing Best Buy P.U.D. parking lot entrance. The original basin was detained in a temporary pond situated south of the First Interstate Bank project (now First National Bank). The proposed Best. Buy P.U.D. grading will eliminate this pond. Considering this, the developed flows from Basin C will be detained in the on -site pond of Basin 200 and released at 0.87 cfs/acre. 6 NAFC0I 14\Documents\Word\Fc01141)mgRpt.doc NOLTE BEYOND ENGINEERING Final Drainage & Erosion Control Study Best Buy P. U.D. Developed flows in Basin OS-1 will flow overland to the west curb line of J.F.K. Parkway. These flows get picked up by an existing curb inlet and area inlet which are located in the first drive entrance south of the Site. The developed flow patterns and historic flow patterns are identical. Outside of the proposed landscaping improvements, the only other proposed improvements are two driveway connections to J.F.K. Parkway. The added imperviousness will have a negligible affect on the basins historic condition. 4.3 Detention Pond Design The detention pond shown in Basin 200 was sized using the FAA method (See Appendix B). According to the calculations, 0.65 ac-ft of storage is required. At elevation 5029.15, the proposed detention pond has a capacity of 0.72 ac-ft. The additional volume, 0.07 ac-ft, is required for water quality. The release rate, Q,, 4.44 cfs, was based on an allowable release rate of 0.87 cfs per developed tributary acre (5.10 acres). The areas directly north and south of the proposed entrance from J.F.K. Parkway, which are hydraulically connected by (2)-18" RCP culverts, do not function as a detention pond. Likewise, the area north of the entrance does not independently function as a detention pond that releases to the area situated south of the entrance. The existing stormwater flows from the north (i.e., from F.I.B. and "The Fountainhead") have already been detained. The area south of the J.F.K. Parkway entrance was split (i.e., wetlands area and detention pond) in order to separate the proposed undetained developed flow associated with the Best Buy P.U.D. store expansion from the existing detained flows. The proposed configuration applies the benefits associated with the water 7 NAFC0114\Documents\word\Fc01141)mgRpt.doc NC43 E 0 E Y O N D E N G I N E E R I N G Final Drainage & Erosion Control Study Best Buy P. U.D. quality directly to the proposed developed flow while the wetlands mitigate the existing flow. 4.4 Water Quality Criteria outlined in Volume 3 of the Manual were used to determine the required Water Quality Capture Volume (WQCV) (See Appendix B). The proposed detention pond is an "extended detention basin". The water quality storage volume is equal to 120 percent of the WQCV based on a 40-hour drain time. The WQCV is based on the tributary area draining to the detention pond as well as the percentage of the total area that is impervious. The calculated water quality storage volumes is 0.07 ac-ft. Outlet structure details are shown on Sht. C7. 8 NAFCOI 14\Documents\Word\Fc0I 14DmgRpt.doc NOISE BEY ON D E N G IN E E N INC 5.0 EROSION CONTROL Final Drainage & Erosion Control Study Best Buy P. U.D. 5.1 General Concept The Best Buy P.U.D. is in the Moderate to High Rainfall and Moderate Wind Erodibility zones per City of Fort Collins zone maps. Until the disturbed ground is vegetated, the potential exists for erosion problems during and after construction. In accordance with the City of Fort Collins Erosion Control Reference Manual for construction sites, the erosion control performance standard to be 96.0% during construction and 99.0% after construction. 5.2 Specific Detail The Site is situated such that, during construction, disturbed areas drain overland and exclusively to the proposed wetland areas and on -site detention pond. Therefore, erosion control measures such as silt fence and straw bale check dams are not necessary. The aforementioned low-lying areas will function as sediment traps during construction. These areas may or may not require regular maintenance depending on the conditions during the construction phase of development. Following completion of the overlot grading all areas that will not be paved shall be mulched and seeded and/or landscaped as shown on sheets L 1 and L2 of the Landscape plan and Wetland Mitigation Plan, respectively. All construction activities must also comply with the State of Colorado permitting process for Stormwater Discharges Associated with Construction Activity. A Colorado Department of Health NPDES Permit will 'be required before any construction grading can begin. 9 NAFC0114\Documents\Word\Fc01 ]4DmeRpt.doc NOLTE Final Drainage & BEY ON D E N G IN E E R INC Erosion Control Study Best Buy P. U.D. 6.0 CONCLUSIONS 6.1 Drainage Concept The proposed drainage concepts presented in this study and shown on the final drainage plans adequately provide for the conveyance and detention of developed runoff from the proposed development. 10 NAFCOI 14\Documents\Word\Fc0114DmeRpt.d0c Final Drainage & BEYOND ENG INEERING Erosion Control Study ' Best Buy P. U.D. REFERENCES 1. Storm Drainage Design Criteria and Construction Standards (Criteria), City of Fort Collins, Colorado (Revised January 1997). 2. Drainage Criteria Manual (Manual), Urban Drainage and Flood Control District, Wright - McLaughlin Engineers, Denver, Colorado, March 1969. 3. Final Drainage & Erosion Control Report for Best Buy P.U.D., Fort Collins, Colorado, prepared by Parsons & Associates, Inc., May 1992. 4. McClellands and Mail Creek Major Drainageway Plan Colorado, prepared by Cornell Consulting Company, December 1980. 5. Final Drainage Report for the Heat Special Improvement District, Colorado, prepared by Engineering Professionals, Inc., January 1985. 6.. Final Drainage & Erosion Control Report for First Interstate Bank Drive-thru Facility, Fort Collins, Colorado, prepared by Parsons & Associates, Inc., September 1992. It I I N AFC0114\Documents\Word\Fc0114DmgRpt.doc BEST BUY P.U.D. FINAL DRAINAGE & EROSION CONTROL STUDY for ..... � Wiest: .u�t 7075 Flying Cloud Drive Eden Prairie, MN 55344 10 Nolte Associates, Inc. 432 Link Lane Fort Collins, Colorado 80524 (970) 221-2400 May 3, 2000 Mr. Basil Hamdan City of Fort Collins Utility Services, Stormwater 700 Wood St. Fort Collins, CO 80521 RE: Best Buy P.U.D. Final Drainage & Erosion Control Study Dear Mr. Hamdan: We are pleased to submit the Best Buy P.U.D. "Final Drainage & Erosion Control Study" for your review and approval. This report was prepared in compliance with the technical criteria set forth in the Storm Drainage Design Criteria and Construction Standards (revised January 1997) for the City of Fort Collins. Sincerely, NOLTE and ASSOCIATES, Inc. Doug Paull, P.E. Project Manager GAFeissner\Fc0 114\W 0rd\Fc0114_DmgLtr.doc NOLTE BE Y ON D E N G IN E E R INC 1.0. INTRODUCTION .... Final Drainage & Erosion Control Study Best Buy P. U.D. TABLE OF CONTENTS PAGE 1.1 Site Location.........................................................................................................I 1.2 Existing Site Description......................................................................................I 1.3 Proposed Project Description................................................................................I 1.4 Previous Reports and Studies................................................................................2 2.0 METHODOLOGY...........................................................................................................3 2.1 Compliance with Standards......................:...........................................................3 2.2 Analytical Methods...............................................................................................3 3.0 HISTORIC DRAINAGE CONDITIONS ....... .................................................................. 4 3.1 ' Major Basin Description...................................................................:...................4 4.0 DEVELOPED DRAINAGE CONDITIONS....................................................................5 4.1 General Concept....................................................................................................5 4.2 Basin Descriptions................................................................................................5 4.3 Detention Pond Design.........................................................................................7 4.4 Water Quality........................................................................................................8 5.0 EROSION CONTROL...........................................................................................:.........9 5.1 General Concept.................................................................................................9 4.4 Specific Detail.......................................................................................................9 6.0 CONCLUSIONS.............................................................................................................10 6.1 Drainage Concept................................................................................................10 REFERENCES...............................................................................................................11 APPENDIX A - Developed Site Hydrology • Developed Flow Calculations • Hydraulic Calculations • Rip -Rap Calculations APPENDIX B - Detention Pond Design • Detention Pond Sizing Calculations + Water Quality Calculations APPENDIX C - Erosion Control Calculations APPENDIX D - Charts, Tables & Graphs BACK POCKET - Drainage and Erosion Control Plan (Sht. C6) i NAFC0I J 4\Documents\Word\Fc0114DmgRpt.doc NCU E BEY ON D E N G IN E E R ING 1.0 INTRODUCTION 1.1 Site Location Final Drainage & Erosion Control Study Best Buy P. U.D. The proposed Best Buy P.U.D. (Site) retail store expansion is located South of the First National Bank (FNB.); North of the Marketplace P.U.D. (Savers); West of J.F.K. Parkway and East of South College Avenue. More particularly, the Site (3.03f acres) is located in Section 36, Township 7 North, Range 69 West of the 6`h Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. 1.2 Existing Site Description In general, the northern portion of the Site slope's south and east along grades which are, on average, very flat. The area is covered by a 5 1 ' x 82' concrete slab and well -compacted soil. The terrain, in its existing state, is essentially impervious. In contrast, a north to south drainage way and surrounding undulating terrain easily distinguish the eastern region of the Site. The wetlands vegetation such as cattails and reeds indicate that sufficient base flow and overland flow exist (on a seasonal basis) to sustain the wetlands (See Sht. 2 of 8 for wetlands delineation). Surface runoff from the local tributary area and developed flows from the F.N.B. detention pond and Fountainhead P.U.D. Phase III facilitate the growth and development of this vegetation on a seasonal basis. The soil report that was prepared by Advanced Professional Engineering, Inc. (I1/15/99), indicates that on -site subsoil conditions are suitable for the proposed construction. 1.3 Proposed Project Description The proposed Best Buy retail store expansion will increase the area of the existing Best Buy retail store by nearly 33% (A«i:,. = 28, 520 ffz to Aprop. = 42,770 ft), add 81 parking spaces to the area north of the building and improve Site access with a connection to J.F.K. Parkway. 1 NAFC0 t 14\Documents\Word\Fc0114DmgRpt.doc NCUE BE Y ON D E N G IN E E R ING Final Drainage & Erosion Control Study Best Buy P. U.D. 1.4 Previous Report and Studies For the existing Best Buy retail store, Parsons and Associates, Inc. prepared a report entitled, Final Drainage & Erosion Control Report for Best Buy P. U.D. in May 1992. Cornell Consulting Company prepared the master drainage plan for the Site, entitled McClellands and Mail Creek Major Drainageway Plan, in December 1980. The Site is located in Reach 6D (Boardwalk Drive to the Larimer 42 Canal) of the McClellands Basin. Engineering Professionals, Inc. (EPI) prepared the Final Drainage Report for the Heart Special Improvement District in January 1985. This report requested that all future developers in Reach 6D above the Larimer #2 Canal and East of College Avenue detain 100 year developed flows and release at a rate of 0.87 cfs per tributary acre. 2 NAFC0114\Documents\Word\Fc0114DmgRpt.doc NOLTE BEY ON D E N G IN E E R ING 2.0 METHODOLOGY Final Drainage & Erosion Control Study Best Buy A U.D. 2.1 Compliance with Standards The following Final Drainage & Erosion Control Study was prepared in accordance with the design requirements and procedures set forth in the City of Fort Collins (City) Stormwater Drainage Design Criteria and Construction Standards (Criteria) (Revised January 1997) and Urban Storm Drainage Criteria Manual (Manual) by the Urban Drainage and Flood Control District. 2.2 Analytical Methods The Rational Method was used to calculate 10- and 100-year developed flows. The Rational Method is widely accepted for design problems involving small drainage areas (<160 acres) and short times of concentration. Mathematically, it relates peak discharge to the runoff coefficient, rainfall intensity, and drainage area. Runoff coefficients and rainfall intensity data were obtained from the Criteria. 3 NAFC0114\Documents\Word\Fc0114DmeRpt.doc NOLTE 9 E Y�O N D E N G I N E E R I N G Final Drainage & Erosion Control Study Best Buy P. U.D. 3.0 HISTORIC DRAINAGE BASINS 3.1 Major Basin Description The Site is located in Reach 61) of the McClellands Basin. Future development within this Reach was specifically addressed in the Final Drainage Report for the Heart Special Improvement District (EPI, January 1985). In this report, EPI requested that the City "require all future developers in this basin East of College Avenue above the Larimer #2 canal to design and construct facilities sized to detain the developed 100-year runoff volume and release at a rate of 0.87 cfs per tributary acre" (Refer to Figure 1, Appendix B). 4 NAFC0114\Documents\Word\Fc01 14DmeRpt.doc BEY ON D E N G IN E E N I N G Final Drainage & Erosion Control Study Best Buy P. U.D. G � 4.0 DEVELOPED DRAINAGE CONDITIONS 4.1 General Concept In general, developed flows from the proposed site improvements (e.g., retail store expansion, expanded parking lot and revised loading dock) will sheet flow in a southeasterly direction to the proposed on -site water quality/detention pond. Developed flows along the Site's East Side will follow a southerly route through proposed wetlands before they combine with detention pond outflow and exit at the southeast corner of the Site. 4.2 Basin Descriptions Developed flows in Basin 100 (0.63 ac) flow overland through landscaped area and wetlands (Refer to Sht. L2, Appendix B) to D.P. 100, which is situated at the basin's southern end. Existing fully developed off -site flows (Qin 6.10 cfs) from the Fountainhead P.U.D. enter this basin through a 14" x 23" HERCP along the basin's West boundary. The 6.10 cfs is a composite of three flows: 5.10 cfs from an inlet at the northeast corner of South College Avenue and Boardwalk Drive and two separate 2-year historic release rates (0.40 cfs and 0.50 cfs) from within "The Fountainhead". Refer to the Exhibit 1. Also, outflow (Q;,=1.29 cfs, 1.48 ac * 0.87 cfs/ac)) from the existing F.N.B. detention pond enters this basin along its north boundary. The wetlands mitigation plan and landscape plan for this basin and Basins 101 and 200 are shown on sheets L 1 and L2, respectively. Developed flows in basin 101 (0.27 ac) flow overland through landscaped area and wetlands (Refer to Sht. L2) to D.P. 101, which is situated in the basin's southern end. The flows previously described in Basin 100 enter Basin 101 at its III 5 N AFC0114\Documents\Word\Fc0114DmgRpt.doc