The URL can be used to link to this page

Home My WebLink AboutDOWNTOWN TRANSIT FACILITY - PDP - 9-00 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTNOLTE Final Drainage & B E Y O N D E N G I N E E R I N G Erosion Control Study Downtown Transit Center 5.0 EROSION CONTROL 5.1 General Concept The Downtown Transit Center site lies within the moderate Rainfall Erodibility Zone and the low Wind Erodibility Zone per the City of Fort Collins zone maps. The potential exists for erosion problems during and after construction until the disturbed ground is again vegetated. The Erosion Control Performance Standard (PS) during construction. for this project was computed to be 76.80 per the criteria in the City of Fort Collins Erosion Control Reference Manual for Construction Sites. The Effectiveness (EFF) of the proposed erosion control plan was calculated to be 90.35. The proposed erosion control methods meet the City of Fort Collins' requirements. Calculations can be found in the appendix. 5.2 Specific Details During overlot grading, the detention area will be excavated. This area will act as a sediment trap during construction. A gravel filter will be placed over the outlet pipe until sodding is completed. Silt fence will also be installed. After overlot grading has been completed, all disturbed areas not in a roadway area shall have straw hay mulch applied. The areas of the site that are to be paved shall �i have gravel mulch applied. After the utilities have been installed, the impervious areas shall be paved as soon as possible. The pervious areas will have sod installed. The erosion control measures can be seen on the drainage and erosion �I control plan in a pocket in the back of this report. �l 9 N:\FC0091\Doawents\dramage\Fc0091DmgRptdoc October 19, 2000 Mr. Basil Hamdan City of Fort Collins Water Utilities - Stormwater 700 Wood Street Fort Collins, Colorado 80522 p4C)= B E Y O N D E N G I N E E R I N G SUBJECT: Final Drainage and Erosion Control Study for the Downtown Transit Center Dear Mr. Hamdan: ' We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Study for the Downtown Transit Center. All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. We appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Sincerely, NOLTE ASSOCIATES, Inc. Prepared by: Storm Gewirt�EIT Project Engineer Cc: File FC0091 NOLTE ASSOCIATES, INC. 1901 SHARP POINT DRIVE, SUITE A FORT COLLINS, CO 80525 970.221.2400 TEL 970.221.2415 FAX WWW.NOLTE.COM Reviewed by: Thomas Ochi Project Mana, n:\fc0091\documents\drainage\cover letter.doc CHERRY ST. �rlLOW S> MAPLE ST. h'Sp N Q V O ¢ LAPORTE AVE. ell - MOUNTAIN AVE. VICINITY MAP NTS APPENDIX A Vicinity Map N007E Final Drainage & BE POND ENGINEERING Erosion Control Study Downtown Transit Center r 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. Erosion Control Reference Manual for Construction Sites, City of Fort Collins, Colorado, January 1991. 4. Final Drainage Report for Temporary Parking Lots Parsons &Associates, Inc., Fort Collins, Colorado, August 19, 1998. 5. Civic Center Parking Structure Final Drainage Report, Parson & Associates, Inc., Fort Collins, Colorado, (Revised January 1999). 12 N:\FC00911Documentsldrainage\Fc009lDmgRpLdoe z NUL=E Final Drainage & BEYOND ENGINEERING Erosion Control Study ' Downtown Transit Center 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. 11 I N:1FC00911Documents\dminage\Fc009llhngRpLdoc NO= Final Drainage & BEYOND ENGINEERING Erosion Control Study Downtown Transit Center All construction activities must 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. 10 N:\FC00911Domments\drainage1Fc0091DmgRpt.doc NC U E Final Drainage & BE Y O N D E N G I N E E R ING Erosion Control Study Downtown Transit Center vegetated areas. The Downtown Transit Center proposes to use a combination of sedimentation and straining as a means of water quality. Due to the size of the site, only a small amount of water quality storage is required. The Downtown Transit Center proposes to employ characteristics of different structural BW's for basic pollutant removal. The detention/water quality pond on the site is a shallow flat grassy area that allows for stormwater to sheet flow slowly, allowing pollutants to settle out before the runoff leaves the site. The flat grassy area also acts like a grass buffer that promotes filtration, infiltration and settling to reduce runoff pollutants. In addition, the detention/ water quality pond acts like an extended detention basin, allowing small particles to combine and settle. The pond proposes to utilize a water quality structure with an orifice plate to slow the release of runoff from the site, allowing more time for sediment to settle. See appendix and plans for more details. 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 volume is 0.041 ac-ft. At elevation 4979.94, 0.041 ac-ft of water quality has been provided for. Calculations can be found in the appendix. The 100-yr WSEL for the 0.09 ac-ft of required storage plus the 0.041 ac-ft of water quality is 4980.74. The proposed detention/water quality pond adequately provides for this amount. 8 N:1FC00911DommeNsWramge1Fc0091D rngRpLdoc N : LO= Final Drainage & BEYOND ENGINEERING Erosion Control Study Downtown Transit Center 4.3 Detention Pond Design As previously stated, the proposed development is not required to detain the runoff from the site. The capacity of the 15" pipe the site drains to is 11.45 cfs. The site (Basin 7) has a total flow of 12.93 cfs. The development proposes to release at the 10-yr flow and provide detention for the remainder of flow. The detention pond shown in Basin 7 was sized using the FAA method. According to the calculations, 0.09 ac-ft is required. Additional volume for water quality has also been provided. See appendix for calculations. The release rate Q1o=4.56 cfs, is based on the 10-yr flow from Basin 7. Emergency overflows from the detention pond will be directed out the north entrance of the site, at an elevation of 4981.86, onto Maple Street. 4.4 Water Quality Criteria outlined in Volume 3 of the Urban Storm Drainage Criteria Manual were used to determine the required Water Quality Capture Volume (WQCI). The water quality storage volume is equal to 120 percent of the WQCV based on a 40- hour drain time. The Urban Storm Drainage Criteria Manual suggests many mechanisms for treating stormwater runoff for water quality enhancement, two of which are sedimentation and straining. Sedimentation, which includes extended detention, allows smaller particles to combine into larger ones and settle. According to Urban Drainage, sedimentation is the primary pollutant removal mechanism for most structural BMV's. Straining is a method that uses grass to filter out pollutants. This happens when sheet flow is directed to flow slowly over 7 N:\FC0091\Documents\drainage\Fc0091DmgRptdoc Final Drainage & B E Y O N D E N G IN E E R ING Erosion Control Study Downtown Transit Center Basin 7 (1.38 ac) contains the main portion of the site involving improvements. Developed flows from this basin will travel northwest in curb and gutter to the detention/water quality pond in the northwest corner of the site. s 6 N:1FC0091\Documents\drainage1Fc009IDmgRpt.doc B E V O N D E N G IN BE RING Final Drainage & Erosion Control Study Downtown Transit Center 1 4.0 DEVELOPED DRAINAGE CONDITIONS 4.1 General Concept In general, developed flows from the proposed site will sheet flow in a northwesterly direction to the proposed on -site water quality/detention pond. Runoff from portions of the site will continue to flow as it has done historically. 4.2 Basin Descriptions Stormwater runoff from Basin 1 (0.40 ac) travels along the curb and gutter on Mason Street where it will enter a relocated inlet. Developed flows from Basin 2 (022 ac) will travel in the curb and gutter on Mason Street where it will enter a relocated inlet. Flow from Basins 1 and 2 have historically entered existing inlets along Mason Street. The alignment of Mason Street has changed resulting in the relocation of the inlets to the new curb and gutter. The northeast corner of the LaPorte/Mason intersection has been improved and the new inlet that will catch flows from Basin 1 has been installed. Basins 5 and 6 (0.08 ac and 0.52 ac, respectively) combine and travel east along existing the curb and gutter of LaPorte Street, where it will enter an existing inlet. Runoff from Basin 3 (0.17 ac) will flow in curb and gutter along Maple Street where it will combine with the developed flows from Basin 4 (0.74 ac). Basin 4 consists mostly of the back of the existing freight depot building, as well as parking and asphalt drive. This area has historically drained to the northeast corner of the site and will continue to do so. 1 5 N:1FC00911Documents\dminage\Fc009lDmgRptdoc r4C TE Final Drainage & BEYOND ENGIN BERING Erosion Control Study Downtown Transit Center 3.0 HISTORIC DRAINAGE BASINS 3.1 Major Basin Description As seen on the Fort Collins Stormwater Basin Map, the Downtown Transit Center lies within the Old Town Master Drainage Basin Plan. There is no major drainageway through the basin. Runoff travels through streets and several storm sewer systems where it eventually discharges to the Poudre River. Through discussions between the City of Fort Collins Stormwater Department and Nolte Associates, Inc., the Downtown Transit Center site is not required to provide detention. However, the site can not release more runoff than the existing storm sewer system can handle. Therefore detention has been designed for the difference. Water quality is also required. 4 N.\FC0091\Documents\dmamp\Fc009lDmgRptdoe NOLTE Final Drainage & Erosion Control Study BEYOND ENGINEERING Downtown Transit Center 2.0 METHODOLOGY 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 Stormwater Drainage Design Criteria and Construction Standards (Revised January 1997) and Urban Storm Drainage Criteria 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 Stormwater Drainage Design Criteria and Construction Standards. 3 N:\FC0091\DaumentsWrainage1Fc009lDmgRpt.doc NOLTE Final Drainage & Erosion Control Study BEYOND ENGINEERING Downtown Transit Center 1.4 Previous Report and Studies IParsons & Associates, Inc. prepared a report entitled, Final Drainage Report for Temporary Parking Lots, for the existing Downtown Transit Center site, in August ' 1998. ' The Civic Parking Structure Final Drainage Report, by Parson & Associates in October 1998, also makes reference to the Transit Center site. Both reports refer to the Downtown Transit Center site as Block 22 and they describe the current drainage patterns of the site. 2 N:\FC009lW40ementsWmlnage\FcOO91DMgRPLdoc I r% L7- IB E Y O N D E N G IN E E R ING 1 1.0 INTRODUCTION Final Drainage & Erosion Control Study Downtown Transit Center 1.1 Site Location The proposed Downtown Transit Center is located south of Maple Street, north of LaPorte Street and west of Mason Street. More particularly, the Site is located in the northeast '/a of Section 11, Township 7 North, Range 69 West of the 6111 Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. The site location can be seen in the Appendix. 1.2 Existing Site Description The Downtown Transit Center site contains approximately 3.5 acres and currently serves as a parking lot. It was designed to temporarily replace the parking that was lost during the construction of the parking garage located to the south of the site. The existing Freight Depot building is also located on this site, as well as a back alley. The historic "Anne's Grave" monument is located on the west side of the site. Currently, the site's topography generally slopes toward the center of the site, toward four existing area inlets. A gravel base covers the majority of the site. Empire Laboratories prepared a geotechnical engineering report in January 1995 for the Storm Drainage Department Office Building, which is the present Downtown Transit Center site. This soil report indicates that on -site subsoil consists of sandy lean clay underlain by well -graded gravel with silt, sand, cobbles and boulders. This study indicates the clays and gravels exhibit moderate to high bearing characteristics and non to low swell potential. 1.3 Proposed Project Description The proposed Downtown Transit Center will operate as a transit station for City of Fort Collins buses (Transfort). A bike shelter is also proposed. Renovations to 1 the existing Freight Depot building as well as the construction of new buildings are proposed for the future. The back alley is to remain unchanged. The proposed ' site can be seen on the drainage plan in the back of this report., 1 N:\FC0091WomnentsWrainW\FcOO9lDmgRpLdm NCCTE BEYOND ENGINEERING Final Drainage & Erosion Control Study Downtown Transit Center TABLE OF CONTENTS PAGE 1.0. INTRODUCTION........................................................................................................ I 1.1 Site Location..................................................................................................... 1 1.2 Existing -Site Description...................................................................................1 j 1.3 Proposed Project Description............................................................................1 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 1 4.1 General Concept ........................... ................................................................. 5 4.2 Basin Descriptions............................................................................................. 5 4.3 Detention Pond Design ...................................................................................... 7 4.4 Water Quality.................................................................................................... 7 5.0 EROSION CONTROL................................................................................................. 9 5.1 General Concept............................................................................................... 9 5.2 Specific Details................................................................................................. 9 6.0 CONCLUSIONS........................................................................................................11 6.1 Drainage Concept........................................................................................... 11 REFERENCES........................................................................................................... 12 APPENDIX A - Vicinity Map. APPENDIX B - Developed Site Hydrology • Developed Flow Calculations • Curb Cut & Inlet Analysis APPENDIX C - Detention Pond Design & Water Quality Design Is Orifice Sizing & Existing Pipe Capacity APPENDIX D - Erosion Control Calculations APPENDIX E - Charts, Tables & Graphs BACK POCKET - Drainage and Erosion Control Plan I( 1 N:\FC009100mmentsWmHWP\Fc009llhngRptdOe d - DOWNTOWN TRANSIT CENTER FINAL DRAINAGE & EROSION CONTROL STUDY for AllereLii 748 Whalers Way Building E, Suite 200 Fort Collins, Colorado 80525 m Nolte Associates, Inc. 1901 Sharp Point Drive, Suite A Fort Collins, Colorado 80525 (970) 221-2400 August 10, 2000