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HomeMy WebLinkAboutTHE OVERLOOK/SENECA CENTER AT ARAPAHOE FARM PUD - FINAL - 55-87P - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT (3) N N I I IFINAL DRAINAGE AND EROSION CONTROL REPORT I OVERLOOK/ SENECA CENTER P.U.D. I 111 U U Prepared for: Wheeler Commercial Property Services 1027 West Horsetooth Road, Suite 200 Fort Collins, CO 80526 Prepared by: 11 North Star Design 1200 Carousel Drive, Suite 201 Windsor, Colorado 80550 (970) 686-6939 MO September 14, 1999 Job Number 103-02 I N I U 1Nórth Star design U September 14, 1999 . Basil Hamdan City of Fort Collins Stormwater 700 Wood Street 141 Fort Collins, CO 80522-0580 RE: Final Drainage Report for Overlook/ Seneca Center P.U.D. 111 Dear Basil, I am pleased to submit for your review and approval, this Final Drainage Report for Overlook/ Seneca Center P.U.D. I certify that this report for the drainage design of Seneca Center was prepared in accordance with the criteria in the City of Fort Collins Storm Drainage Manual. 111 I appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. 111 Sincerely, Patricia Kroetch, P.E. I U U U 1200 Carousel Drive, Suite 201 Windsor, Colorado 80550 970 - 686 - 6939 Phone & Fax TABLE OF CONTENTS TABLE OF 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 1 2.2 Sub-Basin Description 2 3. DRAINAGE DESIGN CRITERIA 3.1 Regulations 2 RI 3.2 Development Criteria Reference and Constraints 2 3.3 Hydrologic Criteria 2 3.4 Hydraulic Criteria 3 II 4. DRAINAGE FACILITY DESIGN 4.1 General Concept 3 II 4.2 Specific Flow/Routing 3 4.3 Drainage Summary 5 1111 5. EROSION CONTROL 5.1 General Concept 5 111 5.2 Specific Details 5 6. CONCLUSIONS III 6.1 Compliance with Standards 6 6.2 Drainage Concept 6 PI 7. REFERENCES III APPENDICES A Vicinity Map B Hydrologic Computations ill C Hydraulic Calculations D Erosion Control Calculations E Figures and Tables leF Excerpts from Previous Reports iii III , III 1. GENERAL LOCATION AND DESCRIPTION II1.1 Location , ® This site is located within the Overlook/Gates development in Fort Collins, at the { Harmony corner of the intersection of Seneca Street and Road. The project is PI bounded to the northeast by the Harmony Road, to the northwest by the back lots of the Overlook at Woodridge, First Filing, and to the south by the old alignment of Harmony Road. 111 More precisely the proposed development is located in the Southwest Quarter of Section 34, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins; Larimer Count Colorado. The project site is shown p .1e on the Vicinity Map in Appendix A. II1.2 Description of Property The Overlook at Arapahoe P.U.D. site contains approximately 3.2 acres and is generally undeveloped at the present. The existing site is presently open ground covered with native grasses. The site slopes gradually to the southeast at approximately 2.0% to 3.0% and historically drains to an existing roadside ditch running along the south boundary of the site and to Harmony Road along the northeast boundary of the site. The project will consist of three buildings, a car wash and associated parking. One building will contain a convenience store, one will be a combined office and residential and the third building will contain retail or commercial, uses. The site will access off Harmony Road and Seneca Street. IN 2. DRAINAGE BASINS AND SUB-BASINS 111 2.1 Major Basin Description 7' The project site is located in the McClellands/Mail Creek Basin. It is more particularly detailed in the "Preliminary Master Drainage Study for Woodridge", December 1991, RBD Inc. 4 l 1 i 1111 2.2 Sub-basin Description For the most part, this site is delineated as Basin 7C on the "Preliminary Grading and Drainage Plan" for Woodridge, by RBD Inc. A portion of that plan is included in Appendix A. The site is split by the proposed new Seneca Street, which runs northeast to southwest in the eastern boundary of the site. As mentioned before, there is an existing roadside ditch running along the south property line of the site, but this will be removed with this development. PRI 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. 3.2 Development Criteria Reference and Constraints The criteria established by the "Preliminary Master Drainage Study for Woodridge", ® December 1991 by RBD Inc. and the "Final Drainage and Erosion Control Study for Arapahoe Farms Townhomes P.U.D.", July 15, 1994 by RBD Inc. were used for 1111 designing the drainage facilities for this site. This site was not planned to require on-site detention. Based on the "Preliminary Master 111 Drainage Study for Woodridge" and the "Final Drainage and Erosion Control Study for Arapahoe Farms Townhomes P.U.D.", the site drains undetained into the proposed storm drain beneath Harmony Road. The revised IDF curve compiled by the City of Fort Collins (implemented on March 26, 1999) was used in calculating the runoff from the site. 3.3 Hydrologic Criteria The Rational Method for determining surface runoff was only used for the project site. • 2 U iii ii The 10-year and 100-year storm event criteria and the revised IDF curve were used in calculating runoff values. 114 All l hydrologic calculations associated with the basins are included in Appendix B of this in liii report. Standard Form 8 (SF-8) provides time of concentration calculations for all sub- basins. Standard Form 9 (SF-9) provides a summary of the design flows for all Sub- basins and Design Points associated with this site. III 3.4 Hydraulic Criteria 4 All hydraulic calculations within this report have been ort Y p prepared in accordance with the City of Fort Collins Drainage Criteria and are also included in Appendix C of this report.c IR 4. DRAINAGE FACILITY DESIGN 4.1 General Concept The on-site runoff will drain via on-site swales and curb & gutter to inlets at low points, will be collected in storm pipe and conveyed to the existing 30" storm pipe at the southwest corner of the intersection of Seneca and Harmony, behind one of the proposed buildings. The proposed drainage and erosion control plan-is included in the back pocket of this report. 4.2 Specific Flow Routing Per the "Preliminary Grading and Drainage Plan" for Woodridge and the "Final Drainage r and Erosion Control Study" for Arapahoe Farms Townhomes P.U.D., it was proposed that 36.6 cfs be released from this site during the major storm into the existing 30" storm pipe. The drainage from the Harmony Road'A-street width along the northeast property line of this site was accounted for in the "Preliminary Master Drainage Study for Woodridge". Basically, all of the Harmony Road runoff coming from upstream of Seneca Street is collected in the existing 17" type `R' inlet (ST-3). This curb inlet was designed to capture a portion of the flows coming from the areas and streets upstream of Seneca Street. This stormwater is then routed to Detention Pond #2. The storm sewer plan and profile by RBD is included in the back pocket for reference. 3 • Basin 1 contains the north half of the north most building, a grassed swale (Swale A-A) and the back half of 7 lots in the Overlook First Filing. Runoff from this basin is collected in Swale A-A and is conveyed to an area inlet (Inlet #1). The water is then conveyed in an 18" storm pipe to Inlet#2. r.. Basin 2 contains the south half of the north most building, the east half of the southwest building, the west half of the convenience store, the car wash building, the refueling islands and the majority of the parking lot and drive areas. Runoff from this basin is collected in a 10' type `R' inlet (Inlet#2), is combined with runoff from other basins and conveyed in a 24" storm pipe to Storm Manhole#1. Basin 3 contains the west half of the southwest building, the parking area and driveways for the residential use and the back half of one lot in the Overlook First Filing. Runoff from this basin is conveyed in curb and gutter to a 5' type `R' inlet (Inlet #3). The water is then conveyed in an 18"storm pipe to Storm Manhole#2 and then to Inlet#2. Basin 4 contains the east half of the convenience store and a small green area behind the store. The runoff is conveyed in grass males to an area inlet (Inlet #4). The water then flows to Storm Manhole#1 where it combines with runoff from other basins and flows in the existing 30" storm pipe to Detention Pond#2. Basin 5 contains Seneca Street north of the high point. Seneca Street was designed by TST Consulting Engineers and the proposed grading for the street is being shown for reference. This basin receives runoff from Basin 6 and the combined runoff will flow into Harmony Road. All runoff in Harmony will flow to Inlet ST-3 (17' type `R') and eventually to Detention Pond#2. Basin 6 contains the entrance drive to the site from Seneca. The existing site grading constraints and the proposed grading on Seneca restrict the ability to collect runoff from this basin and convey it in the storm system. The runoff from this basin, therefore, is w " ,` conveyed in the curb and gutter to Seneca Street, to Harmony Road and into Inlet ST-3. Basin 7 contains a small strip of green area on the south boundary of the site. Runoff y a from this basin will flow south and off site. The amount of water generated by this basin is less than the historic flows to the south and will have minimal impact on the development adjacent to the south. _-- 4 The total runoff from this site in the 100 year event was calculated to be 34.69 cfs which is less than the allowable 36.6 cfs,release rate from this site. All runoff calculations were completed utilizing the revised rainfall criteria. —_ 4.3 Drainage Summary The storm conveyance system is primarily a sub-surface system. The project owner or subsequent owner will be responsible for the maintenance of all storm pipe and inlets on - the site and the City of Fort Collins will be responsible for maintenance of the existing storm drainage facilities located within the right-of-way. 5. EROSION CONTROL 5.1 General Concept • This site lies within the Moderate Rainfall and Wind Erodibility Zone per the City of Fort _T Collins Zone Maps. The potential exists for silt movement from the site and into the existing storm system during construction. Potential also exists for tracking of mud onto existing streets which could then wash into existing storm systems. 111 111 Per the City of Fort Collins Erosion Control Reference Manual for Construction Sites and related calculations in Appendix D, the erosion control performance standard for the I subject site is 80.9%. The erosion control plan gives a performance standard of 78.7%. The erosion control plan as designed does not achieve the performance standard required for the site but the erosion control plan as shown is adequate. All erosion control measures necessary to minimize sediment transport have been designed into this site. Additional measures implemented simply to achieve the desired performance standard would be redundant and add unnecessary costs to the project. The erosion control escrow amount is $5700. IN 5.2 Specific Details ^ To limit the amount of silt leavingthe site several erosion control measures shall { be implemented during construction. The inlets shall be protected with a gravel filters, the II swale around the building on the north shall have straw bales, and the south and east 5 I illi II 4 boundaries shall have silt fence installed. Vehicle tracking pads shall be installed at the x ...` entries to control the mud being tracked onto the existing pavement. During overlot IIIgrading, disturbed areas are to be kept in a roughened condition and watered to reduce wind erosion. 4 6. CONCLUSIONS 6.1 Compliance with Standards All computations that have been completed within this report are in compliance with the ii City of Fort Collins Erosibn Control Reference Manual for Construction Sites and the ,,-,,,„ ,.• Storm Drainage Design Criteria Manual. • r 6.2 Drainage Concept IIIThe proposed drainage concepts presented in this report and on the construction plans 4,. adequately provide for the conveyance and release of developed on-site flows into the proposed 30" RCP storm drain and into the existing curb & gutter on Harmony Road. No ,� on-site detention was proposed with this project, but the amount of stormwater leaving A the site does not exceed the allowable release for the site established with Woodridge and • Arapahoe Farms Townhomes P.U.D. The existing Detention Pond downstream at the aRu southeast corner of the Woodridge development has been designed and built to account for flows released from the Overlook/Seneca Center P.U.D. site. If groundwater is encountered at the time of construction, a Colorado Department of Health Construction Dewatering Permit will be required. P4,14, , ' , 4. -. . . it , r, 6 E 7. REFERENCES , 1. Storm Drainage Design Criteria and Construction Standards, by the City of Fort El ,,,:, Collins, Colorado, May 1984, revised January 1991. __:_ 2. Erosion Control Reference Manual for Construction Sites, by the City of Fort Collins,Colorado, January 1991. 3. Preliminary/Master Drainage Study for Woodridge, by RBD, Inc., Fort Collins, Colorado, December 1991. PI ,,, , 4. Final Drainage and Erosion Control Study for Arapahoe Farms Townhomes P.U.D., by RBD, Inc., Fort Collins, Colorado, July 15, 1994. y Y." 7 , i,. 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