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HomeMy WebLinkAboutLAKEVIEW ON THE RISE, AFFORDABLE HOUSING - FDP - FDP170031 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTFinal Drainage and Erosion Control Report for Lakeview on the Rise Fort Collins, Colorado November 15, 2017 November 15, 2017 Mr. Wes Lamarque City of Fort Collins Water Utilities--Storm water 700 Wood Street Fort Collins, Colorado 80521 RE: Final Drainage and Erosion Control Report Lakeview on the Rise Dear Wes: We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Report for Lakeview on the Rise. 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, Aspen Engineering John Gooch, P.E. Principal TABLE OF CONTENTS DESCRIPTION PAGE I. GENERAL LOCATION AND DESCRIPTION 5 A. LOCATION 5 B. DESCRIPTION OF PROPERTY 5 II. DRAINAGE BASINS 6 A. MAJOR BASIN DESCRIPTION 6 B. EXISTING SUB-BASIN DESCRIPTION 6 C. PROPOSED SUB-BASIN DESCRIPTION 6 III. DRAINAGE DESIGN CRITERIA 8 A. REGULATIONS 8 B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS 8 C. HYDROLOGIC CRITERIA 8 D. HYDRAULIC CRITERIA 8 E. VARIANCES 9 IV. DRAINAGE FACILITY DESIGN 9 A. GENERAL CONCEPT 9 B. SPECIFIC DETAILS 10 C. DETENTION POND 17 D. STREET CAPACITIES 18 V. STORM WATER QUALITY 18 A. GENERAL CONCEPT & SPECIFIC DETAILS 18 VI. EROSION CONTROL 18 A. GENERAL CONCEPT 18 VII. CONCLUSIONS 19 A. COMPLIANCE WITH STANDARDS & STORMWATER 19 OPERATIONS/ MAINTENANCE PROCEDURE B. DRAINAGE CONCEPT 20 C. STORM WATER QUALITY 20 D. EROSION CONTROL CONCEPT 20 E. EROSION CONTROL ESCROW ESTIMATE 21 REFERENCES 21 APPENDIX PAGE VICINITY MAP A RATIONAL METHOD HYDROLOGY, POND & LID SIZING, OUTLET B STRUCTURE CALCULATIONS, ORIFICE SIZING, AND OVERFLOW WEIR SIZING STORM SEWER PIPE SIZING C INLET SIZING & STREET CAPACITY CALCULATIONS, CONCRETE D SIDEWALK CULVERT & CURB CUT SIZING SWALE SIZING E EROSION CONTROL & RIPRAP SIZING, EROSION CONTROL ESCROW F CALCULATION PROPOSED & EXISTING DRAINAGE BASIN EXHIBITS, EROSION CONTROL G PLAN, & EROSION CONTROL NOTES & DETAILS SHEET 5 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR LAKEVIEW ON THE RISE FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Lakeview on the Rise project is located east of and adjacent to South College Avenue (U.S. Highway 287) and northeast of the intersection of Triangle Drive and South College Avenue. The property is bounded to the west by South College Avenue, to the north by an existing irrigation ditch, and to the south by Robert Benson Reservoir, and to the east by Provincetown Filing Three subdivision. The project site can also be described as situated in the northwest quarter of Section 13, Township 6 North, Range 69 West of the 6th P.M., City of Fort Collins, County of Larimer, State of Colorado. The site (property boundary) comprises approximately 20.089 acres (see vicinity map in Appendix A). B. Description of Property The project site currently contains an existing 20.089-acre vacant field comprised of primarily dirt with native weeds and grasses and portions of South College Avenue. The site also contains one existing commercial building that is currently unoccupied and will be removed prior to any proposed construction activities. Robert Benson Reservoir and an associated wetland area bound the proposed site to the south. The existing site has topography which generally slopes from the northwest to the south-southeast at approximately 2.9%, with varying slopes from 2.0% to 5.0%. The proposed development consists of 17 apartment buildings, a Clubhouse with pool, and associated street, water, sanitary sewer, storm sewer, and private infrastructure improvements, as well as intermittent green spaces and outdoor spaces. 6 II. DRAINAGE BASINS A. Major Basin Description The Lakeview on the Rise project is located within the City of Fort Collins Fossil Creek Reservoir Drainage Basin. The Fossil Creek Reservoir Drainage Basin generally flows from the northwest to southeast and ultimately outlets into the Poudre River. B. Existing Sub-Basin Description The existing site consists primarily of dirt with intermittent grasses and weeds. All existing onsite drainage is conveyed from the northwest to the south-southeast ultimately collecting in Robert Benson Reservoir, located along the south side of the property. The existing subject site is contained within one existing sub-basin (EX1). Existing basin EX1 contains approximately 21.54 acres and was modeled with the basin having an existing ‘c’ value of 0.25. The calculated existing 2-year runoff from basin EX1 is 6.12 cfs. Please refer to the rational calculations in Appendix B. C. Proposed Sub-Basin Description The proposed development consists of 17 apartment buildings, a Clubhouse with pool, and associated street, water, sanitary sewer, storm sewer, and private infrastructure improvements, as well as intermittent green spaces and outdoor spaces. The majority of site runoff generated from the proposed site will be conveyed to two Rain Gardens, then to two proposed detention ponds, prior to being released into Robert Benson Reservoir. It is important to note that a small portion of the site (approximately 3.80 acres) along the western boundary consists of portions of South College Avenue and undeveloped land that is conveyed directly into Robert Benson Reservoir without detention or treatment, as historically occurs. All flows released into Robert Benson Reservoir will ultimately be conveyed to Fossil Creek Reservoir and into the Poudre River. The proposed public roads will be paved with asphalt, while the private drives and adjacent parking areas will be constructed of concrete or asphalt. Interior sidewalks will be constructed of concrete to serve pedestrians and provide access throughout the site. Runoff from the proposed site will be ultimately conveyed in two large spine storm drainage piping systems that will flow south towards the two proposed detention ponds (with upstream Rain Gardens). Runoff from the site will be conveyed to the Rain Gardens, where flow will be spread laterally through the basins. The Rain Gardens will provide approximately one (1) foot of ponding depth with base composed of bioretention sand media and gravel 7 over a 6” perforated underdrain system installed at the bottom of the Rain Garden. One foot of depth will be provided in the Rain Garden to meet the LID criteria. Upon the Rain Gardens filling up with runoff from the site, runoff will be filtered through the Rain Garden media and gravel and discharge into the proposed detention ponds via the 6” perforated pipes/underdrains. Inlets placed at each end of the Rain Gardens will also allow water to be conveyed south and into the detention ponds during major events, which will help improve flow to the detention ponds. An overflow spillway will also be constructed in each Rain Garden to allow excess runoff beyond the 1 foot of depth, to drain to the detention ponds. A water quality control volume of 0.053 acre-feet will also be provide within Pond 200 for the runoff from basins 24-28, which could not be routed to the Rain Garden. This volume will discharge through a water quality outlet plate. Per the Geotechnical report by Terracon, the site contains highly expansive clay materials, and as required, all roof drainage from the site and as much site area as possible, is required to be captured immediately by piping systems and conveyed away from the buildings and road infrastructure. Therefore, in meeting the requirements of the Geotechnical Report, large amounts of roof drainage piping/systems are being utilized for the site, as well as the proposed Rain Gardens to mitigate infiltration of runoff into the sub-grade, as required, and to provide LID treatment for approximately 88 percent of the total site, which includes the offsite area. It should be noted that as indicated in the LID table, 100 percent of the on-site basin impervious area is treated with the implementation of Rain Gardens and a water quality outlet structure. Runoff from one offsite inlet on US Hwy 287 will be conveyed to and pass through Pond 100. This more than makes up for the relatively small area of grass slope which will not be routed to the detention pond. Should the Rain Gardens fill up during major storm events, the Rain Gardens will overtop the south spillways and drain to the proposed detention ponds. The proposed detention ponds will be graded with relatively flat bottoms (0.5% to 2.0%) to allow for infiltration and pollutants to settle out of the runoff, prior to flows exiting the ponds via the proposed outlet structures and into Robert Benson Reservoir. Please refer to the grading plans for details. The proposed detention ponds will discharge the combined 2-year historic release rate (6.12 cfs) directly into Robert Benson Reservoir. Please refer to the pond outlet structure calculations in Appendix B. 8 III. DRAINAGE BASIN CRITERIA A. Regulations The drainage design for the subject site is required to meet the current City of Fort Collins Stormwater and Erosion Control Standards and requirements, as well as the requirements of the Geotechnical Report by Terracon. Therefore, the drainage design for the subject site has been designed in accordance with these standards and provides for the site drainage being conveyed to downstream Rain Gardens and detention ponds, as required and applicable. The proposed project will be required to provide water quality and on-site detention, prior to the 2-year historic release rate from the site being conveyed into Robert Benson Reservoir, as historically occurs. B. Development Criteria Reference and Constraints Drainage criteria and constraints required by the City of Fort Collins and Terracon will be achieved and provided by the Lakeview on the Rise project. The capacity of the system will be self contained and release to Robert Benson Reservoir at a rate not to exceed the 2-year historic release rate of 6.12 cfs for the entire 21.54 acres, which includes all attributing onsite and offsite basins, including the half-width of South College Avenue along the proposed site. C. Hydrologic Criteria The Rational Method for determining surface runoff was used for the project site. The 2-year and 100-year storm event intensities were used in calculating runoff values. The City of Fort Collins intensity duration frequency curves were used to obtain rainfall data for each storm specified. Detention and water quality pond sizing was calculated using the latest City of Fort Collins Criteria as well as Urban Drainage Flood Control District sizing methods. Rain Gardens sizing was calculated using the Design Procedure Form for Rain Garden Design spreadsheet developed by UDFCD. D. Hydraulic Criteria All hydraulic calculations within this report have been prepared in accordance with the City of Fort Collins Drainage Criteria and are also included in the Appendices. 9 Final storm sewer pipe/systems and inlet/curb cut sizing has been completed using UDSewer and UDInlet from UDFCD, as well as other orifice/inlet control sizing spreadsheets. Final Swale sizing has been completed during final design using AutoCad Civil 3D Hydraulic Software, while final erosion control/riprap sizing has been calculated using North American Green software and other riprap spreadsheet software. E. Variances No variances are being requested at this time. IV. DRAINAGE FACILITY DESIGN A. General Concept Twenty-nine proposed onsite drainage basins and one offsite basin have been created to analyze the drainage for the proposed development. Basins 1-15 drain ultimately to a proposed detention/water quality pond (Pond 100) located in the southwest corner of the site via overland flow, curb and gutter, and proposed storm sewer systems. The 100-year peak flow to be detained in the detention pond located in the southwest corner of the site will be approximately 46.4 cfs. Basins 16-29 drain ultimately to a second proposed detention/water quality pond (Pond 200) located in the southeast corner of the site via overland flow, curb and gutter, and proposed storm sewer systems. The 100-year peak flow to be detained in the detention pond located in the southeast corner of the site will be approximately 42.7 cfs. Basin OS1 is an offsite basin that consists of portions of South College Avenue that will be conveyed directly to Robert Benson Reservoir undetained. The total 100-year peak flow to be detained in the two proposed detention ponds located onsite will be approximately 89.1 cfs. Detention Pond 100 (west) and Detention Pond 200 (east) provide approximately 1.85 acre-feet and 1.27 acre-feet of storage, respectively, which provides for the combination of detention and water quality. Water quality is also provided within the proposed Rain Gardens, located upstream of each pond. A Water Quality Control Volume is also provided within Pond 200 for runoff not directed to the Rain Garden. 10 The allowable 2-year release rate for the site is 6.12 cfs. The release rates for Ponds 100 and 200 will be 1.0 cfs and 5.12 cfs, respectively. It's important to note that Pond 100 and Pond 200 have been sized to detain up to the 100-year event, while receiving the 100-year developed flow from 21.54 acres, with the calculated overall 'c' value for the site being 0.58, versus the overall combined existing 'c' value of 0.25. Please refer to the rational calculations in Appendix B and the Proposed and Existing Drainage Basin Exhibits in Appendix G for additional information. Refer to Appendices C & D for inlet sizing, concrete sidewalk culvert and curb cut sizing, and storm sewer system sizing. B. Specific Details Drainage to Detention Pond 100: The runoff from the following basins 1-15 will be collected in a proposed storm sewer system which will be ultimately be conveyed to a Detention Pond 100 located at the southwest corner of the site. Basin 1 Runoff in Basin 1 will be conveyed via overland flow and curb and gutter to a 5’ Type R inlet at design point 1. The 5’ Type R inlet discharges into a proposed 18” RCP storm sewer system at design point 1. Basin 2 Runoff in Basin 2 will be conveyed via roof drains, overland flow, and a grass lined swale to the proposed storm sewer system at design point 2. The roof drainage piping system captures flow from the basin and conveys runoff east and into the storm sewer main in Debra Drive. A small northwestern portion of basin 2 will be conveyed south and into basin 4, where a 15” Nyloplast Drain Basin with standard H-20 grate will capture this flow. Basin 3 Runoff in Basin 3 will be conveyed via overland flow, a grass lined swale and curb and gutter to a 5’ Type R inlet, located at the southwest corner of Debra Drive/Lorien Lane (design point 3). A small area along the north side of Building 14 will convey its runoff east in the grass lined swale, where a 12” Nyloplast Drain Basin with standard H-20 grate will capture the flow. The runoff will combine with roof drainage in basin 7. Basin 4 Runoff in Basin 4 will be conveyed via overland flow, curb and gutter, and roof drain piping. Runoff from the north side of Lorien Lane and the west half of Debra Drive flows to a 15’ Type R inlet (on grade) at design point 4. 11 The 15’ Type R inlet discharges east and into a proposed 18” RCP storm sewer system at design point 4. Runoff captured by the roof piping system will combine with roof drainage from basin 2 and will be conveyed east and into the 18” RCP in Debra Drive. Basin 5 Runoff in Basin 5 will be conveyed via roof drains, overland flow, and curb and gutter. Runoff in the east half of Debra Drive flows to a 10’ Type R inlet (on grade) at design point 5. The 10’ Type R inlet discharges into a proposed 18” RCP storm sewer system at design point 5. Runoff captured by the roof piping system in basin 5 will combine with a small portion of roof drainage from basins 16 and 23 and will be conveyed west and into the 18” RCP in Debra Drive. Basin 6 Runoff in Basin 6 will be conveyed via roof drains, overland flow, and a grass lined swale to the proposed storm sewer system. Runoff captured by the roof piping system in basin 6 will be conveyed southwest and into the 18” RCP in the private drive/parking lot. Runoff from the small green and hard areas on the north and east sides of the building will be conveyed northeast and east and out onto Lorien Lane and Debra Drive, while runoff from green and hard areas on the west and south sides of the building will be conveyed south, then east in the proposed grass-lined swale to design point 6, where a 2’ metal sidewalk culvert with concrete chase will convey water out to Debra Drive. Basin 7 Runoff in Basin 7 will be conveyed via roof drains, overland flow, and curb and gutter to a 5’ Type R inlet at design point 7. The 5’ Type R inlet discharges into a proposed 18” RCP storm sewer system. Runoff from the roof drainage system in basin 7 combines with a small portion of roof drainage from basin 3 and is conveyed southeast and into the 18” RCP in the private drive/parking lot. Basin 8 Runoff in Basin 8 will be conveyed via roof drains, overland flow, and a grass lined swale to a 24” diameter Nyloplast Drain Basin with standard H-20 grate at design point 8. The 24” diameter Nyloplast inlet discharges into a proposed 18” RCP storm sewer system at design point 8. Roof drainage from basin 8 is also piped to and will connect into the 24” Nyloplast drain basin/inlet. 12 Basin 9 Runoff in Basin 9 will be conveyed via roof drains, overland flow, a grass lined swale, and curb and gutter to two, 5’ Type R inlets. The 5’ Type R inlets discharge into a proposed 18” RCP storm sewer system at design point 9. Runoff captured by the roof piping system in basin 9 will combine with a small portion of roof drainage from basins 10 and 11 and will be conveyed southeast and into the 18” RCP at design point 10. Basin 10 Runoff in Basin 10 will be conveyed via roof drains, overland flow and curb and gutter to a 5’ Type R inlet. The 5’ Type R inlet discharges into a proposed storm sewer system at design point 10. Runoff captured by the roof piping system in basin 10 will combine with a small portion of roof drainage from basins 9 and 11 and will be conveyed and into the 18” RCP and 5’ Type inlet at design point 10. Basins 11 and 12 Runoff in Basins 11 and 12 will be conveyed via overland flow and curb and gutter to a 5’ Type R inlet at design point 12. The 5’ Type R inlet discharges into a proposed 18” RCP storm sewer system at design point 12. Runoff captured by roof drainage systems in basins 11 and 12 will be conveyed and discharge to the 5’ Type R Inlets at design points 10 and 12. Basin 13 Runoff in Basin 13 will be conveyed via overland flow, a grass lined swale, and curb and gutter to a 10’ Type R inlet at design point 13. Flow from the grass lined swale (from basin 6) will combine with runoff from a small portion of basin 13 and will pass east through a 2’ curb cut w/ metal sidewalk culvert and onto Debra Drive. Flow from the west half of Debra Drive (within Basin 13) will combine with the above flow and continue south down Debra Drive towards the 10’ Type R Inlet at design point 13. Any carry-over runoff will flow east and towards the 5’ Type R Inlet at design point 28. Basin 14 Runoff in Basin 14 will be conveyed via overland flow and a grass lined swale to the proposed detention pond (Pond100) located in the southwest portion of the site. A small northern portion of basin 14 will be conveyed to a proposed inlet, located in the northwest corner of basin 10, with the flow combining with the roof drain flow in this area. Basin 15 Runoff in Basin 15 will be conveyed via overland flow to the proposed Detention Pond 100, located in the southwest portion of the site. The basin includes the proposed Detention Pond 100. Runoff is released at a controlled 13 rate of 1.0 cfs, via a water quality control structure at design point 15, and into Robert Benson Reservoir. Drainage to Detention Pond 200: The runoff from the following basins 16-29 will be collected in a proposed storm sewer system which will be ultimately be conveyed to Detention Pond 200 located at the southeast corner of the site. Basin 16 Runoff in Basin 16 will be conveyed via roof drains, overland flow, and curb and gutter to a 5’ Type R inlet at design point 16. The 5’ Type R inlet discharges into a proposed 18” RCP storm sewer system at design point 16. An intermittent 2’ wide concrete chase will be constructed within the parking island in Basin 16 to convey runoff east towards design point 16. Roof drainage from the majority of basin 16 is conveyed east and is conveyed into the inlet at design point 16. A small portion of roof drainage runoff from basin 16 combines with a portion of roof drainage from basins 5 and 23 and is conveyed into the 18” RCP in Debra Drive. Basin 17 Runoff in Basin 17 will be conveyed via roof drains, overland flow, and a grass lined swale to a 24” diameter Nyloplast drain basin with standard H-20 grate at design point 17. The 24” diameter Nyloplast drain basin discharges into a proposed 15” HDPE storm sewer system at design point 17. Basin 18 Runoff in Basin 18 will be conveyed via roof drains, overland flow, and curb and gutter to a 5’ Type R inlet at design point 18. The 5’ Type R inlet discharges into a proposed 18” RCP. The roof drainage system from Building 10 (within basin 18) conveys runoff east and combines with roof drainage flow from basin 19 and is conveyed southeast and into the 15” Nyloplast Drain basin with standard H-20 grate at design point 19. Basin 19 Runoff in Basin 19 will be conveyed via overland flow, a grass lined swale, and curb and gutter to a 15” diameter Nyloplast drain basin with standard H- 20 grate ate design point 19. Flows from the curb and gutter will pass through a 2’ curb cut and join runoff collected from the grass lined swale as it passes to the 15” diameter Nyloplast drain basin. The 15” diameter Nyloplast drain basin discharges into a proposed 15” HDPE storm sewer system at design point 19. 14 Basin 20 Runoff in Basin 20 will be conveyed via roof drains, overland flow, a grass lined swale to a CDOT Type C inlet with close mesh grate at design point 20. The Type C Inlet discharges into a proposed 24” RCP storm sewer system at design point 20. Roof drainage from Building 8 will be piped around the building with the roof drainage piping system emptying into the 5’ Type R Inlet at design point 22. Basin 21 Runoff in Basin 21 will be conveyed via roof drains, overland flow and curb and gutter to a 10’ Type R inlet at design point 21. The 10’ Type R inlet discharges into a proposed 24” RCP. Roof drainage from the attributable portion of basin 21 (south side of Building 5) is conveyed south and into the 18” RCP storm sewer system, located southwest of design point 16. An intermittent 2’ wide concrete chase will be constructed within the parking island in Basin 21 to convey runoff south towards design point 21. Basin 22 Runoff in Basin 22 will be conveyed via overland flow and curb and gutter to a 5’ Type R inlet at design point 22. The 5’ Type R inlet discharges into a proposed 18” RCP storm sewer system at design point 22. An intermittent 4’ wide concrete chase will be constructed within the parking island in Basin 22 to convey runoff south towards design point 22. Basin 23 Runoff in Basin 23 will be conveyed via roof drains, overland flow, a grass lined swale and curb and gutter to a CDOT Type C inlet with close mesh grate at design point 23. Flows from the north parking lot area will flow southeast and will pass through a 2’ curb cut and join runoff collected from the grass lined swale as it flows south towards the CDOT Type C inlet. The Type C inlet discharges into a proposed 18” RCP storm sewer system at design point 23. Basin 24 Runoff in Basin 24 will be conveyed via roof drains, overland flow, a grass lined swale to a 15” diameter Nyloplast drain basin with standard H-20 grate. The 15” diameter Nyloplast drain basin discharges into a proposed 15” HDPE storm sewer system at design point 24. Basin 25 Runoff in Basin 25 will be conveyed via roof drains, overland flow, and grass lined swales to a CDOT Type C inlet with close mesh grate at design point 25. The Type C inlet discharges into a proposed 18” RCP storm sewer 15 system at design point 25. Roof Drainage runoff from basin 25 will be piped south and tie-in the Type C Inlet at design point 25. Basin 26 Runoff in Basin 26 will be conveyed via overland flow and grass lined swales to a CDOT Type C Inlet with close mesh grate at design point 26. The Type C Inlet discharges into a proposed 18” RCP storm sewer system at design point 26. Basin 27 Runoff in Basin 27 from the east half of Debra Drive and north half of Stoney Brook Road is conveyed via overland flow and curb and gutter to the proposed 5’ Type R inlet at design point 27. The 10’ Type R inlet discharges into a proposed 24” RCP storm sewer system at design point 27. Basin 28 Runoff in Basin 28 from the west half of Debra Drive and south half of Stoney Brook Road is conveyed via overland flow and curb and gutter to the proposed 10’ Type R Inlet (on grade and located in the bulb-out) and 5’ Type R inlet at design point 28. Runoff captured by the on-grade 10’ Type R inlet discharges southwest and into Pond 100, while carryover flow continues east down Stoney Brook Road to the 5’ Type R Inlet at design point 28. The 5’ Type R Inlet captures the remaining flow from basin 28 and discharges runoff south, via 24” RCP storm sewer, and into Pond 200. Basin 29 Runoff in Basin 29 will be conveyed via overland flow to the proposed Detention Pond 200, located in the southeast portion of the site. The basin includes the proposed Detention Pond 200. Runoff is released at a controlled rate of 5.12 cfs, via a water quality control structure at design point 29, and into Robert Benson Reservoir. Offsite Drainage The runoff from the following basin drains offsite. Basin OS1 Runoff in Basin OS1 will be conveyed via overland flow and curb and gutter to Robert Benson Reservoir south of the site. The flows in Basin OS1 represent a portion of South College Avenue and undeveloped land in the southwest corner of the site. Runoff in College Avenue will be intercepted by a 5’ Type R inlet and 10’ Type R inlet in the east flowline of US287. Flow is piped east from these inlets, where riprap pads will slow water down and mitigate erosion, prior to flows continuing east, un-detained, and into Robert Benson Reservoir, as historically occurs. 16 Overflow Conditions Basins 1-5: Should the inlets at the low points in these basins become obstructed and fail to collect the required drainage, the runoff will pass to Debra Drive and ultimately be conveyed to the detention pond located in the southwest corner of the site. Basins 6-12: Should the inlets at the low points in these basins become obstructed and fail to collect the required drainage, the runoff will pass through the proposed parking areas to Debra Drive and ultimately be conveyed to Detention Pond 100, located in the southwest corner of the site. Basin 13: Should the on-grade inlet at design point 13 in this basin become obstructed and fail to collect the required drainage, runoff will pass to the low point in Stoney Brook Drive and ultimately be conveyed to Detention Pond 200, located in the southeast corner of the site. Basins 14-15: Should the swale in basin 14 be obstructed, flows will pond, then spill south and into Detention Pond 100. Should the outlet structure in Pond 100 clog, runoff will pond to the spillway elevation, and will then be conveyed southeast over/through the overflow weir and into Robert Benson Reservoir. Basins 16-22: Should the inlets at the low points in these basins become obstructed and fail to collect the required drainage, the runoff will pass through the private drive (Rohan Rise), onto Stoney Brook Road, and will ultimately be conveyed to Detention Pond 200, located in the southeast corner of the site. Basin 23: Should the CDOT Type C inlet at the low point in this basin become obstructed and fail to collect the required drainage, the runoff will flow onto Stoney Brook Road and will ultimately be conveyed to Detention Pond 200, located in the southeast corner of the site. Basins 24-26: Should the inlets at the low points in these basins become obstructed and fail to collect the required drainage, the runoff will pass to the low point in Stoney Brook Drive and ultimately be conveyed to Detention Pond 200, located in the southeast corner of the site. Basins 27-28: Should the inlets at the low points in these basins located at the low point in Stoney Brook Drive become obstructed and fail to collect the required drainage, the runoff will overflow south and into Detention Pond 200, located in the southeast corner of the site. 17 C. Detention Pond Detention Pond 100 has been sized to provide water quality and detention for 8.57 acres of the total 21.54 acres of on-site and off-site areas. Detention Pond 100 will provide 1.85 acre-feet of available volume at an elevation of 5009.00. The Rain Garden will provide 0.12 acre-feet of available volume at an elevation of approximately 5009.00. Detention Pond 200 has been sized to provide water quality and detention for the 9.18 acres of the total 21.54 acres of on-site and off-site areas. Detention Pond 200 will provide 1.27 acre-feet of available volume at an elevation of 5008.80 (which includes approximately 0.05 ac-ft for water quality volume in the detention pond). The Rain Garden will provide 0.09 ac-ft for water quality at an elevation of approximately 5008.50. The proposed detention ponds will allow for detention up to the 100-year storm event for the development. The lowest outlet elevation of the Detention Pond 100 (invert at the front of the outlet structure) is approximately 5004.00. The calculated 100- year water surface elevation of Detention Pond 100 is approximately 5009.00 with a release rate of 1.0 cfs. The lowest outlet elevation of the Detention Pond 200 (invert at the front of the outlet structure) is approximately 5003.00. The calculated 100-year water surface elevation of Detention Pond 200 is approximately 5008.80 with a release rate of 5.12 cfs. The proposed detention pond outlet structures will discharge to 15” RCP pipes with orifice control plates at the entrance of the pipes. The orifice on Pond 100 will be 3.20-inches in diameter to restrict release to 1.0 cfs, while the orifice restriction for Pond 200 will be 7.06-inches in diameter to restrict release to 5.12 cfs. Therefore, the allowable 2-year historic release rate of 6.12 cfs into Robert Benson Reservoir will be adhered to, as historically occurs. Please refer to the Orifice Sizing calculations in Appendix B. An Emergency Spillway will be constructed along the south edges of Pond 100 and Pond 200. The proposed spillways on Pond 100 and Pond 200 will have overflow elevations of 5009.00 and 5008.80, respectively. The spillway on Pond 100 was designed to pass the Q100 = 46.4 cfs at a depth of 0.59 feet and width of 60 feet with 0.36 feet of freeboard. The spillway on Pond 200 was designed to pass the Q100 = 42.7 cfs at a depth of 0.17’ feet and width of 200 feet with approximately 0.33 feet of freeboard. North American Green C350 Erosion Control Fabric is also being provided on the spillways to mitigate erosion, in event the ponds overtops. 18 Groundwater elevations were provided by Terracon and measured 5 days after drilling and installing piezometers. In Pond 200 groundwater elevations of 4999.3, 4997.4 were measured within the footprint of the pond. The lowest point in Pond 200 at the outlet structure is 5003.0 which would be approximately 3.7 feet above groundwater. Terracon will be providing additional borings in Pond 100 and a third point in Pond 200 to verify conditions in Pond 200 and provide data for Pond 100. Results will be updated in the report as they become available. Should Pond 100 and Pond 200 overtop, runoff will be released south into Robert Benson Reservoir. Please refer to the Utility Plans for specific grading and overflow weir details. D. Street Capacities Street capacity calculations for public roads have been analyzed and calculated, with the results provided in Appendix D. V. STORM WATER QUALITY A. General Concept & Specific Details The water quality of stormwater runoff must be addressed on all final design utility plans. Therefore, Best Management Practices (BMP's) for the treatment of stormwater runoff for the subject site will include grass/ sod-lined swales, riprap pads at culvert discharge points, permanent erosion control fabric at curb cuts and swale entrances into the ponds, grass scrubbing across the bottom of the detention ponds during first flush conditions, and proposed Rain Gardens consisting of water quality catch basins located at the entrances to the detention ponds, which have been incorporated into the detention pond design. These proposed water quality features will provide a mechanism for pollutants to settle out of the stormwater runoff before flows are directed into the detention ponds and ultimately to Robert Benson Reservoir. The proposed water quality/ detention facilities have adequate capacity and will provide all of the required water quality and detention up to the 100-year event for the developed condition of the subject site (21.54 acres), prior to releasing flows to Robert Benson Reservoir. 19 VI. EROSION CONTROL A. General Concept Silt fence will be installed along the north, west, south, east and west property boundaries of the site to prevent sediment from leaving the site. In addition, silt fence is proposed around the perimeter of each block of buildings surrounded by curb and gutter to prevent sediment from entering proposed drive aisles and streets. A vehicle tracking pad will be placed at the site access point at US 287 and Lorien Lane. Rock Socks and gravel inlet filters will be placed at proposed inlet and dam locations, as shown in the erosion control plan, to mitigate the build-up of sediment in the proposed inlet and manhole structures. Straw wattle check dams may also be installed at 300' intervals in the proposed swales throughout the site, as may be applicable during construction, or specified by the inspector(s). The Erosion Control Escrow Amount ($78,770.00) has been calculated and can found in Appendix F. Please refer to Appendix G for the Erosion Control Plan and Erosion Control Notes and Detail Sheets. VII. CONCLUSIONS A. Compliance with Standards & Stormwater Operations/Maintenance Procedure All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. The City of Fort Collins Storm water Utility will not maintain the on-site storm drainage facilities within the subject site. The owners of the subject site will maintain their on-site storm drainage facilities on a regular basis. The following shall be implemented for the private stormwater improvements’ operations/ maintenance procedures for the project on an annual or bi-annual basis: 1) Storm Sewer Inlets, pipes and flared-end-sections, curb cuts and concrete sidewalk culverts/chases, structures, manholes, and the LID water quality catch basins shall be cleaned through the removal of debris and sediment from the associated items to allow for adequate drainage through the site to the proposed detention facilities. 2) Pond sedimentation/ silting shall be removed to allow for adequate drainage along the bottom of the ponds and to prevent ponding and silting in of the 20 pond bottoms. Grass scrubbing along the bottom of the ponds may be required to remove sediment and promote grass growth. 3) Re-vegetation through the use of Natural Seeding/ Sod shall be provided for disturbed areas and other permanent erosion controls shall be provided for areas where erosion has taken place and requires remediation back to the proposed condition shown in the plans. The Stormwater Operating/ Maintenance Procedures listed above are guidelines to the minimum procedures that shall be implemented for the site, with these and additional measures being utilized on an as-needed basis. B. Drainage Concept The proposed drainage concepts presented in this study and shown on the utility plans adequately provide for the transmission of developed on-site runoff to the proposed detention ponds (Pond 100 and Pond 200), located southwest and southeast corners of the subject site. The proposed storm sewer system will provide for the 100-year developed flows to reach the proposed water quality and detention ponds, prior to flows being released downstream in accordance with the 2-year historic release rate of 6.12 cfs. If groundwater is encountered at the time of construction, a Colorado Department of Health Construction Dewatering Permit will be required. C. Storm Water Quality The preliminary design has addressed the water quality aspect of stormwater runoff. The proposed grass-lined detention ponds and Rain Gardens will provide an opportunity for stormwater pollutants to filter out of the stormwater runoff before flows are directed downstream to Robert Benson Reservoir. Furthermore, the proposed grass-sod-lined swales, riprap pads at culvert outlets, permanent erosion control fabric and other erosion control devises that may be utilized for side-slopes and embankments, grass scrubbing across the bottom of the detention ponds during first flush conditions, and the water quality basins incorporated into the detention pond designs will provide additional mechanisms for pollutants to settle out of the stormwater runoff before flows are directed south and ultimately to Robert Benson Reservoir. D. Erosion Control Concept Proposed erosion control concepts will adequately provide for the control of wind and rainfall erosion from the proposed development. Through the construction of the 21 proposed erosion control concepts, the City of Fort Collins standards will be maintained. The proposed erosion control concepts presented in the final report and shown on the erosion control plan are in compliance with the City of Fort Collins Erosion Control Criteria. E. Erosion Control Escrow Estimate The Erosion Control Escrow Estimate for the subject property is approximately $78,770.00. Please refer to Appendix F for this calculation REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, May 1984, Revised January 1997, & Associated UDFCD Updates and New City of Fort Collins Stormwater Criteria manual, adopted January, 2012. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins, Colorado, January 1991, Revised January 1997, & Associated UDFCD Updates and New City of Fort Collins Stormwater Criteria manual, adopted January, 2012. 3. Urban Storm Drainage Criteria Manual Volume 1; Urban Drainage and Flood Control District, Denver, Colorado, June 2001. 4. Urban Storm Drainage Criteria Manual Volume 2; Urban Drainage and Flood Control District, Denver, Colorado, June 2001. 5. Urban Storm Drainage Criteria Manual Volume 3, Best Management Practices; Urban Drainage and Flood Control District, Denver, Colorado, August 2011. 22 APPENDIX 23 APPENDIX A 24 VICINITY MAP 25 APPENDIX B 26 RATIONAL METHOD HYDROLOGY, POND & LID SIZING, OUTLET STRUCTURE CALCULATIONS, ORIFICE SIZING, AND OVERFLOW WEIR SIZING 27 APPENDIX C 28 STORM SEWER PIPE SIZING 29 APPENDIX D 30 INLET & STREET CAPACITY CALCULATIONS, CONCRETE SIDEWALK CULVERT & CURB CUT SIZING 31 APPENDIX E 32 SWALE SIZING 33 APPENDIX F 34 EROSION CONTROL & RIPRAP SIZING, & EROSION CONTROL ESCROW ESTIMATE 35 APPENDIX G 36 PROPOSED & EXISTING DRAINAGE BASIN EXHIBITS, EROSION CONTROL PLAN, & EROSION CONTROL NOTES AND DETAILS SHEET