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HomeMy WebLinkAboutCIVIC CENTER OFFICE BUILDING - PDP - 29-99 - REPORTS - DRAINAGE REPORTConstruction Sites. The EFF of the proposed erosion control plan was calculated to be 98.64 %. A copy of the calculations has been included in the Appendix. An erosion control escrow cost estimate of $2,099 is also included in the Erosion Control section of the Appendix. B. Specific Details Initially the Silt Fence must be place around the site. Then demolition and overlot grading will occur. Following the overlot grading the sediment pond will be installed. These will stay in place until final landscaping is installed. The sediment pond will then be ,replaced with the water quality feature following landscaping installation. 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 has been obtained such that construction grading can continue within this development. VII. CONCLUSIONS A. 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. B. Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provide for the transmission of developed on -site runoff to the existing and proposed drainage facilities. The combination of onsite street capacities in the curb and gutter and the onsite storm drain system will provide for the 10-year and the 100-year developed flows to reach proposed Howes OutFall. If, at the time of construction for some unforeseen reason, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit would be required. C. Stormwater Quality Concept The proposed design has addressed the water quality aspect of stormwater runoff. A water quality feature has been sized to accept onsite and offsite flows. Accepting 7 CIVIC CENTER OFFICE BUILDING �• e VINE_. ill/ 1 ■� _ .. 1n IllRIM ■■M■■■ MEMNON ..mom VICINITY MAP SCALE: 1 " = 2000' z VICINITY MAP I other off -site storm water from the west compensates for the small amount of onsite flow that does not go to the water quality pond. D. Erosion Control Concept ' The proposed erosion control concepts adequately provide for the control of wind and rainfall erosion from the Civic Center Office Building. Through the construction of the proposed erosion control concepts, the City of Fort Collins performance standard will be met. The proposed erosion control concepts presented in this report and shown on the erosion control plan are in compliance with the City of Fort Collins Erosion Control Criteria. VIH. REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, May 1984, interim revision January 1997. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins, Colorado, January 1991. 3. Old Town Master Drainage Basin Plan for the City of Fort Collins, By Recource n Consultants and Engineers Inc, January 7, 1993. rLl IV. STORMWATER QUALITY A. General Concept The Civic Center Office Building is anticipating construction beginning in spring 2000. Therefore, for this project, we have sought to find various Best Management Practices for the treatment of stormwater runoff. The Civic Center Office Building will be providing extended detention in the pond during and after construction. The pond feature will provide a mechanism for pollutants to be filtered out of the stormwater runoff before flows are directed to the Howes Street Outfall. B. Specific Details The pond will provide more than adequate volume for detention of the 100-year design event for on site flows as well as the off -site flows contributed to the site from the west. The pond will be equipped with an underdrain system that will provide filtration through a wetland -type growth media and an eighteen inch (18") thick sand bed to a network of drain piping which will convey the detained flows to the Howes Outfall. An area inlet is provided to convey overtopping flows from the pond directly to the Howes Outfall. VI. EROSION CONTROL A. General Concept This development lies within the Moderate Rainfall Erodibility Zone and the Moderate Wind Erodibility Zone per the City of Fort Collins zone maps. It is also anticipated that the project site improvements will be subject to both wind and rainfall erosion before new vegetation can take hold. The Erosion Control Performance Standard (PS) during construction for this project was computed to be 74.2 % 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 68.9 %. Though, this is below the Erosion Performance Standard required, it is not practical to do more to prevent erosion on a site of this small size. Therefore, we are requesting a variance to the erosion control standards for the site. The Erosion Control Performance Standard (PS) after construction for this project was computed to be 87.2 % per the criteria in the City of For Collins Reference Manual for 6 ■ of flows from sidewalks adjacent to the site on the north and east and those adjacent streets, will -flow through gutters to storm drain inlets. The inlet connected to a 36" storm drain system in Mason Street will intercept the flow from sub -basin 6, while a system of inlets at the intersection of Maple Street and the alley to the west ' of the site will intercept flows from sub -basins 7 and 8. Runoff from sub -basins 9 and 10 will flow overland to the north and east boundaries of sub -basin 1, where they will be conveyed by gutters to concrete sidewalk culverts directing the flows into sub -basin 1. Runoff from sub -basin 11, which comes off the roof of the building, will flow through a drainage pipe to be discharged into the extended detention pond located in sub -basin 1. Runoff from off -site sub -basin 12 will flow overland onto the site into sub -basin 1 and continue overland to the detention pond. Because the curb line of Mason Street adjacent to the site is being moved approximately 2.5 feet to the east, and because of the addition of the Howes Street Outfall inlets and regrading in LaPorte Avenue, it is proposed that all of the inlets on Mason Street except the inlet near the corner of Mason and Maple will be Iabandoned as they are unnecessary. There are presently three inlets along the West Side of Mason Street and two inlets just west of Mason Street on LaPorte Avenue. The Curb capacity of Laporte Avenue and Mason is represented by the calculation for basin 5. The street capacity for Mason Street during the Minor Storm is 2.84 cfs and for the Major Storm is 57.9 cfs. The calculated runoff for basin 5 to design point 1 is 2.69 cfs for the Minor Storm and 6.66 cfs for the Major Storm. This justifies the removal of these inlets. The existing 36-inch diameter pipe, which these inlets tie into, will remain intact to carry storm flows generated on the East Side of Mason near Laporte Avenue. The Howes Street Outfall design will include a large inlet system in Laporte Avenue that will accept flows from basin 5. In the event of flows to the site in excess of the 100-year design event, the site is graded such that excess flows will travel northward from the WQCF through the alley and into LaPorte Avenue. The proposed detention facility will accommodate flows far in excess of the 100-year storm as the 100-year surface elevation in the pond is 4979.23, the top of the pond grading is at 4981.0, and the majority of the parking area can be inundated to higher than 4982.0 over a large area with no damage to structures. In the event that the water surface rises above 4982.5 on the site, the water will flow south through the alley into Laporte Avenue and thence east. The City's consultant analyzing future hydrology including this basin has stated that the water surface elevation at the intersection of Mason and Laporte will not be expected to be above 4982.0, so excess flows from the project site will be conveyed away from the site even in such extreme situations. 5 D. Hydraulic Criteria All hydraulic calculations within this report have been prepared in accordance with s the City of Fort Collins Drainage Criteria and are also included in the Appendix. E. Variances from Criteria A variance is sought for a variance from the erosion control performance standard. The performance standard (PS) during construction is required to be 74.2 %; the j computed performance standard is 68.9%. The project has provided a sediment C basin and silt fence. In order to provide a higher PS gravel mulch would need to be used. This would be impractical for a site of this small size. IV. DRAINAGE FACILITY DESIGN A. General Concept The majority of on -site runoff produced by the proposed development of the Civic Center Office Building flows westerly to a water quality enhancement feature and will subsequently discharge into the Howes Street Outfall. A very small portion of the remaining runoff will drain into a curb inlet discharging to the 36" storm drain in Mason Street. It is also proposed that three existing inlets along Mason Street and two inlets in Laporte Avenue near Mason Street be removed. The inlet at design point 5 will be relocated to accommodate the revised curbline. The proposed drainage and erosion control plan is included in the back pocket of this report. B. Specific Details The Civic Center Office Building site has been broken down into twelve sub - basins. Runoff from off -site sub -basins 2 and 3 will flow overland across the alley on the West Side of the site and into on -site sub -basin 10 at the entrance to the parking lot. Runoff from sub -basins 4 and 5, which are comprised of flows from sidewalks on the west, south and east sides of the building and the adjoining streets, will flow through gutters to a system of inlets at the intersection of Laporte Avenue and the alley to the west of the site. This system of inlets is directly connected to the Howes Street Outfall. Runoff from sub -basins 6, 7 and 8, which are also comprised 4 Iis collected in a curb inlet. Basins 7 and 8 are off -site basins comprised of the south half of the Maple Street R.O.W. from which runoff is directed to the Howes IOutfall inlet system at the intersection of the alley and Maple. Basins 9 and 10 are on -site basins comprised mostly of the parking lot area and discharge to basin 1. Basin 11 consists of the building and discharges to Basin 1. Basin 12 consists of ` an asphalt alley and parking, the runoff from this area flows overland to Basin 1. The on -site portions of basins 4 and 5, with a combined area of approximately 10,340 square feet, were not distinguished from the off -site portions of those basins because their combined area is substantially less than that of the off -site basins 2 and 3, which contribute all flows to the project site and have a combined area of approximately 28,900 square feet. These sub -basins are shown on the Drainage and Erosion Control Plan in the back pocket of this report. The building finish floor elevation is set at 1.5 feet (4983.5) above the expected 100 year flood elevation (4982.0 feet). The 4982.0 flood elevation is an estimated maximum water surface elevation that has been stated in a letter from Glen Schleuter, Stormwater Development Review Manager, dated June 24, 1999. See the letter in the City Correspondence section of the Appendix. III. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for the subject site. B. Development Criteria Reference and Constraints The drainage criteria used herein is in accordance with the City of Fort Collins Storm Drainage Design Criteria and Construction Standard Manual. C. Hydrologic Criteria The Rational Method for determining surface runoff was used for the project site. The 10-year and 100-year storm event intensities, provided by the City of Fort Collins, were used in calculating runoff values. The City of Fort Collins is currently completing regional hydrologic modeling. These calculations and criteria are included in the Appendix of this report. 3 H. DRAINAGE BASINS A. Major Basin Description The proposed development lies within Basin 1 of the Old Town Master Drainage Basin Plan prepared by Resource Consultants & Engineers, Inc, December, 1992. Runoff from the site is routed to the west by means of curb & gutters, overland flow and underground pipes; Portions of the site are subject to flooding as stated in this Plan, however, this Plan is currently being updated by the City of Fort Collins to incorporate revised IDF criteria, storm drainage improvements that have been constructed subsequent to the Plan, and the proposed Howes Street Outfall. The revised flood plain is expected to be less than 4982.0 as stated in the June 24, 1999 letter from Glenn Schlueter to Jack Gianola (See Appendix). The updated Old Town Master Drainage Basin Plan is expected to be completed by September, 1999. The Howes Street Outfall Plans (by Icon Engineering, Inc.) include a proposed underground 6' by 12' reinforced concrete box culvert running north from Laporte along the western property line of the project site, across Maple and Cherry Streets, thence northeast to a proposed water quality pond and discharge to the Poudre River. The Howes Street Outfall is expected to be constructed and functional prior to construction of this project, and the majority of runoff from the site will be drained to this outfall. Construction for this project will not commence prior to the completion of the Howes Outfall Project. A very small portion of runoff will continue to be directed to the existing 36" storm sewer in Mason Street for subsequent discharge to the Poudre River. B. Sub -basin Description The Civic Center Office Building has been divided into 12 sub -basins, all of which are within the Old Town Drainage Basin. Basin 1 is an almost entirely landscaped area within the site including an extended -detention water quality feature (WQCV), which will accept flows from basins 2, 3, 9, 10, 11 and 12. Basins 2 and 3 are off - site basins comprised of existing alleyway and buildings from which runoff is directed to the northeast and southeast, respectively, onto the project site. Basin 4 combines flows from a narrow area adjacent to the southwest side of the building and from the south end of the alley, which are directed to the Howes Outfall inlet system at the intersection of the alley and Laporte. Basin 5 includes the narrow areas adjacent to the east and south sides of the building and the west half of Mason Street from the highpoint south to the centerline of Laporte and the north half of Laporte west to the Howes Outfall inlet system at the intersection of the alley and Laporte. Basin 6 includes the south half of the Maple Street R.O.W. from the highpoint east to the railroad track in Mason and the west half of the Mason Street R.O.W. from the highpoint north to the centerline of Maple Street where the flow 2 FINAL DRAINAGE AND EROSION CONTROL STUDY FOR CIVIC CENTER OFFICE BUILDING FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Civic Center Office Park is bounded by an alley on the west, Maple Street on the north, Mason Street to the east and by Laporte Avenue on the south. The site location can also be described as situated in Lots 1 through 21, Block 32, City of Fort Collins, Latimer County, Colorado. The site location can be seen on the second page of the Appendix. B. Description of Property The Civic Center Office Park contains approximately 2.20 acres, more or less. The property is being proposed for a municipal office building within the Downtown Zoning District and the Civic Center subdistrict. Existing Conditions Temporary construction offices at the southern end of the site, in use for the Larimer County Justice Center (currently under construction at the northeast corner of Mason and Laporte), and temporary parking lots over the northern portion of the site presently occupy the property. The topography of the site generally slopes from the property lines toward area inlets more or less centrally located on the property at approximately 1 to 1.5 percent. Proposed Conditions The proposed development consists of one office building located at the southeast corner of Mason and Laporte, associated parking on most of the remainder of the site, and some landscaped areas near the building and around the parking lot. The building will occupy approximately 24,200 square feet. The proposed site can be seen on the Drainage and Erosion Control Plan included in the back pocket of this report. i EROSION CONTROL 21 CITY CORRESPONDENCE 27 DATA 30 iv TABLE OF CONTENTS DESCRIPTION PAGE I. GENERAL LOCATION AND DESCRIPTION 1 A. Location 1 B. Description of Property 1 H. DRAINAGE BASINS 2 A. Major Basin Description 2 B. Sub -basin Description 2 III. DRAINAGE DESIGN CRITERIA 3 A. Regulations 3 B. Development Criteria Reference and Constraints 3 C. Hydrologic Criteria 4 D. Hydraulic Criteria 4 E. Variances from Criteria 4 IV. DRAINAGE FACILITY DESIGN 4 A. General Concept 4 B. Specific Details 4 V. STORMWATER QUALITY 6 A. General Concept 6 B. Specific Details 6 VI. EROSION CONTROL 6 A. General Concept 6 B. Specific Details 7 VII. CONCLUSIONS 7 A. Compliance with Standards 7 B. Drainage Concept 7 C. Stormwater Quality Concept 7 D. Erosion Control Concept 8 VIII. REFERENCES 8 APPENDIX VICINITY MAP 1 HYDROLOGY 3 WATER QUALITY POND 11 DESIGN OF INLETS AND STORM DRAIN 16 F 1 1 1 1 1 THE SEAR -BROWN GROUP FULLrSERVICE DESIGN PROFESSIONALS 209 SOUTH MELDRUM FORT COLLINS, COLORADO 80521-2603 970-482-5922 FAX:.970-482-6368 December 7, 1999 Mr. Basil Hamdan City of Fort Collins Water Utilities--Stormwater 235 Matthews Street Fort Collins, Colorado 80521 RE: Final Drainage and Erosion Control Study for Civic Center Office Park Dear Basil: We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Study for Civic Center Office Park. 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. Respectfully, The Sear -Brown Group Prepared by: Michael T. Carr, E.I.T. Project Engineer cc: File 875-001 City of Fort Collins NEW YORK • PENNSYLVANIA COLORADO • UTAH • WYOMING STANDARDS IN EXCELLENCE EQUAL OPPORTUNITY EMPLOYER L �I II li U II II U II Final Drainage and Erosion Control Study Civic Center Office Park Fort Collins, Colorado December, 1999 THE SEAR -BROWN GROUP Standards in Excellence