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HomeMy WebLinkAboutCROWNE AT OLD TOWNE NORTH - FDP - FDP170025 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTFinal Drainage and Erosion Control Report for Crowne at Old Town North Fort Collins, Colorado November 29, 2017 November 29, 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 Crowne at Old Town North Dear Wes: We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Report for Crowne at Old Town North. 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 7 C. PROPOSED SUB-BASIN DESCRIPTION 7 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 8 IV. DRAINAGE FACILITY DESIGN 9 A. GENERAL CONCEPT 9 B. SPECIFIC DETAILS 9 C. DETENTION POND 16 D. STREET CAPACITIES 16 V. STORM WATER QUALITY 16 A. GENERAL CONCEPT & SPECIFIC DETAILS 16 VI. EROSION CONTROL 16 A. GENERAL CONCEPT 16 VII. CONCLUSIONS 17 A. COMPLIANCE WITH STANDARDS & STORMWATER 17 OPERATIONS/ MAINTENANCE PROCEDURE B. DRAINAGE CONCEPT 18 C. STORM WATER QUALITY 18 D. EROSION CONTROL CONCEPT 18 E. EROSION CONTROL ESCROW ESTIMATE 19 REFERENCES 19 APPENDIX PAGE VICINITY MAP A RATIONAL METHOD HYDROLOGY B STORM SEWER PIPE SIZING C INLET & STREET CAPACITIES, CURB CUT & SIDEWALK CULVERT SIZING D SWALE SIZING, LID BASIN SIZING & POROUS PAVER SIZING E EROSION CONTROL & EROSION CONTROL ESCROW F PROPOSED AND EXISTING DRAINAGE BASIN EXHIBITS, EROSION CONTROL PLAN & EROSION CONTROL NOTES AND DETAILS SHEET G 5 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR CROWNE AT OLD TOWN NORTH FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Crowne at Old Town North project is located approximately 450 feet east of North College, along future Suniga Road. The property is bound to the west by a portion of Dry Creek and Jax Outdoors. The property is bound to the north by Conifer Street and to the south by future Suniga Road. The Aspen Heights development bounds the property to the east. The project site can also be described as situated in the southwest quarter of Section 1, Township 7 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 15.71 acres (see vicinity map Appendix A). B. Description of Property The project site currently is an existing 15.71-acre vacant field comprised of primarily dirt with native weeds and grasses. The site was formerly used as a materials staging area during the construction of the Aspen Heights development. Dry Creek runs along the west boundary of the proposed site. The majority of the existing site (east portion) has topography which generally slopes from the northwest to the south-southeast at approximately 0.5%, with varying slopes from 0.3% to 0.8%. The proposed development consists of 289 apartment units housed in five buildings, 15 townhome units housed in four buildings, a maintenance garage, 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 Crowne at Old Town North project lies within the City of Fort Collins Dry Creek Drainage Basin. The Dry Creek Drainage Basin generally flows from the northwest to southeast and ultimately outlets into the Poudre River. A.1 NECCO Regional Storm Sewer System In speaking with the City of Fort Collins Stormwater Department, the proposed development is not required to provide detention or water quality as both water quality and detention volume for the Crowne site is being provided in the NECCO regional detention pond. The NECCO regional pond is scheduled to be constructed at the beginning of 2018. The development is part of the NECCO Master Plan. NECCO Storm Sewer C1 travels along the southern boundary of the site, adjacent to Suniga Road. The storm sewer starts at the NECCO regional pond and heads west as a 66-in reinforced concrete pipe (RCP). Per the current NECCO materplan, the storm sewer is to transition to a 48-in storm sewer at the southeast corner of the site. However, per the City Stormwater’s request, the 66” RCP shall be extended further west and to the first private drive, that runs north-south, within the southeast corner of the Crowne at Old Town North project. The 66” RCP will transition down to a 48” RCP at the first private drive and will be extended west to Crowne’s western property boundary. It’s important to note that the NECCO masterplan improvements show the 48-in storm sewer making a 90-degree bend at the southwest corner of the Crowne site and traveling north along the existing Dry Creek alignment. Per discussions with City Stormwater, the 48” extension to the north will not be required at this time, as the intent of the 48” RCP is to serve the developed condition of properties to the west and not the Crowne site. Therefore, a storm sewer stub-out will be provided to the north (at Crowne’s southwest corner) to allow for runoff, from properties to the west and northwest, that flows in the existing open ditch (along Crowne’s western boundary) to be conveyed into the NECCO 48” storm system that runs west-to-east along the north side of Suniga Road. The flows from the attributable, adjacent, existing properties and from the proposed project site will be conveyed east and into the regional detention pond. The 66-in storm sewer was constructed with the Aspen Heights development. The extension of the 66-inch storm sewer and the transition to the 48-in storm sewer will be constructed with this project, as noted above. 7 The proposed site was broken into four major basins and included into the Dry Creek master plan model to insure that the proposed design would meet the design intents of the master plan. Basin Crowne-West represents basins 18-23. These basins will drain into the 48-in storm sewer at the southwest corner of the property. Basin Crowne-Main represents the majority of the proposed development (basins 1-17 and OS1). This basin discharges into the 66-in storm sewer in the southeast quadrant of the site. The modeling demonstrated that the proposed development can drain into the NECCO system at both design points. The hydraulic grade line in the 48-in storm sewer is reduced slightly from the master plan conditions. The basins draining to Suniga Road were also adjusted in the master plan model as well as the basins draining to Conifer Street. This was to insure that the minor change in basin area does not negatively impact the NECCO storm sewer system. These basins are described in more detailed in sections below. B. Existing Sub-Basin Description The existing site consists primarily of dirt with intermittent grasses and weeds. Dry Creek runs along the west side of the property. The property can be broken into two main existing drainage basins. The majority of the property consists of one basin (EX1). The site drains from the northwest corner of the site to the southeast corner of the site. This drainage continues south to two 14” x 23” HERCP. These culverts cross under Blue Spruce and ultimately to Dry Creek. A smaller basin (EX2) flows west to the Dry Creek channel that travels along the west side of the property. It’s important to note that a small portion of the site (approximately 2.59 acres) from Aspen Height’s basins OS6 and OS7 was modeled to drain into 5 inlets, located along the western boundary (bottom west side of retaining walls) of Aspen Heights. C. Proposed Sub-Basin Description The proposed development consists of 289 apartment units housed in five buildings, 15 townhome units housed in four 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 the runoff from the proposed 15.71+/- acre development will be routed to the NECCO storm sewer systems located along the south and southwest boundaries of the proposed site. The NECCO storm sewers will route the flows to the NECCO regional detention pond. The regional detention pond provides detention and water quality for the proposed development. The proposed public roads will be paved with asphalt, while the private drives and adjacent parking areas will be paved in concrete or asphalt. Interior sidewalks will be paved in concrete to serve pedestrians and provide access throughout the site. 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. Therefore, the drainage design for the subject site has been designed in accordance with these standards and provides for the site drainage. The proposed project is not required to provide water quality or on-site detention, as both water quality and detention are being provided in the NECCO Regional Detention pond. Previous masterplan modeling, by Ayres Associates, provided for an impervious ratio of 89% for the Crowne development. The calculated impervious ratio for the Crowne development (including offsite basins) is 76%. B. Development Criteria Reference and Constraints The criteria and constraints from the City of Fort Collins will be met. The proposed project is being routed to the NECCO storm sewer system. The capacity of the system was verified in the Dry Creek Master Plan EPA SWMM model. 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. 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 Appendix. Final storm sewer pipe 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 using AutoCadd Civil 3D Hydraulic Software, while final erosion control/riprap sizing has been calculated using North American Green software. Please refer to Appendices C-F for calculations for the above items. E. Variances No variances are being requested at this time. 9 IV. DRAINAGE FACILITY DESIGN A. General Concept Twenty-eight proposed onsite drainage basins and six offsite basins have been created to analyze the drainage for the proposed development. Basins 1-17 and OS1 drain ultimately to the southeast quadrant of the site via overland flow, curb and gutter, and proposed storm sewer systems. The drainage is ultimately collected in a proposed storm sewer which will connect into the 66-in NECCO storm sewer. The 100-year peak flow to the 66-in storm sewer at this location will be 56.98 cfs. Basins 18-23 drain to the west side of the property via overland flow, curb and gutter, and proposed storm sewer system. The basins ultimately drain to the 48-in NECCO storm sewer located in the southwest corner of the site. The 100-year peak flow to the 48-in storm sewer is 13.76 cfs. The NECCO storm sewer drains to the east to the NECCO Regional Detention Pond. The regional pond is anticipated to be constructed at the beginning of 2018. The offsite basins OS3-OS6 drain to future Suniga Road. Basin 28 and offsite basins OS2 drains to Conifer Street. The impacts of these flows to storm sewer systems on Conifer Street and Suniga Road were verified in Dry Creek master plan EPA SWMM model. Please refer to the rational calculations in Appendix B and the Existing and Proposed Drainage Basin Exhibits in Appendix G for additional information. Please also refer to Appendices C-F for storm sewer pipe, inlet and street capacity, swale, and erosion control calculations and sizing. B. Specific Details Drainage to the NECCO 66-inch Storm Sewer: The runoff from the following basins is collected in a storm sewer system which ultimately connected into the NECCO 66-in storm sewer located in the southeast quadrant of the site. These basins represent basin Crowne-Main in the Dry Creek EPA SWMM model. Basin Crowne-Main was modified to determine the impacts of the proposed development to the NECCO system. It’s important to note that Ayres Associates masterplan modeling provided an impervious ratio of 89% for the Crowne development area, versus the calculated 10 impervious ratio of 76% (including offsite, adjacent basins), as seen in the rational calculations of this report. The following is the conveyance summary for the onsite and offsite basins for the Crowne at Old Town North development: Basin 1 Basin 1 will convey its flow overland, through 2- 2’ curb cuts and in to the LID Basin at design point 1. A 4” SDR35 perforated PVC underdrain pipe system, located underneath the LID Basin, will convey runoff into the Neenah R2533 inlet at design point 1. Excess flows (above the 1’ ponding depth in the LID basin) will flow into the Neenah R2533 area inlet, which discharges to the 18”RCP. Basin 2 Basin 2 conveys runoff from north roof area and area north of the building to two 15” Nyloplast drain basins with standard H20 grates at design point 2. The inlets are located in the grass strip between the parking lot and building and will drain to the proposed 15” HDPE storm sewer pipes. Basin 3 Basin 3 will convey its flow overland across the parking lot and porous paver area, through 2- 3’ curb cuts and in to the LID Basin at design point 3. A 4” SDR 35 perforated PVC underdrain system will be installed under the porous paver areas and LID Basin to capture and convey runoff into the CDOT Type C inlet with close mesh grate at design point 3. Excess flows (above the 1’ ponding depth in the LID basin) will flow into the CDOT Type C inlet, which discharges to the 24”RCP. Basin 4 Basin 4 will convey its flow overland across the parking lot and porous paver area, through 1- 2’ curb cut and 1-3’ curb cut and in to the LID Basin at design point 4. A 4” SDR35 perforated PVC underdrain system will be installed under the porous paver areas and LID Basin to capture and convey runoff into the CDOT Type C inlet with close mesh grate at design point 4. Excess flows (above the 1’ ponding depth in the LID basin) will flow into the CDOT Type C inlet, which discharges to the 30” RCP. Basin 5 Basin 5 drains via overland flow to two low points at within Basin 5. A 12” Nyloplast drain basin with standard H-20 grate captures runoff from the front yard swale (along northside of townhome units) and discharges runoff into the 12” HDPE pipe, while a CDOT Type C inlet with close mesh grate at 11 design point 5 captures runoff from the remainder of the basin, and discharges into the proposed 18” RCP storm sewer system. Basin 6 Basin 6 drains via overland flow across the parking lot and porous paver areas to a low spot at design point 6, where a CDOT Type C inlet with close mesh grate captures the runoff and discharges to the 30” RCP. A 4” SDR35 perforated PVC underdrain system will be installed under the porous paver areas to capture and convey runoff into the CDOT Type C inlet with close mesh grate at design point 6. Basin 7 Basin 7 drains via overland flow, curb and gutter, and storm sewer to a low spot at design point 7, where a 5’ Type R Inlet on the north side of Lupine Drive captures runoff from basin 7, and discharges into the 18” RCP. An 18” Nyloplast drain basin with standard H-20 grate will also be installed in the green area to capture runoff from the yard area and roof drainage systems, and will discharge south via the 15” RCP. Basin 8 Basin 8 drains via overland flow, curb and gutter, and storm sewer to a low spot at design point 8, where a 5’ Type R Inlet on the south side of Lupine Drive captures runoff from basin 8, and discharges into the 18” RCP. A 15” Nyloplast drain basin with standard H-20 grate will also be installed in the green area to capture runoff from the yard area and roof drainage systems, and will discharge north via the 15” HDPE and into the 5’ Type R Inlet. Basin 9 Basin 9 drains overland and in to the LID Basin at design point 9. A 4” SDR 35 perforated PVC underdrain system will be installed under the LID Basin to capture and convey runoff into the Neenah R2533 inlet at design point 9. Excess flows (above the 1’ ponding depth in the LID basin) will flow into the Neenah R2533 inlet, which discharges to the 15”RCP. Basin 10 Basin 10 will convey its flow overland across the parking lot and porous paver area to a low point in the parking lot, where a CDOT Type C Inlet with close mesh grate, at design point 10, will capture the runoff and discharge into the 24” RCP. Two 4” SDR35 perforated PVC underdrain systems will be installed under the porous paver areas to capture and convey runoff into the CDOT Type C inlet with close mesh grate at design point 10. 12 Basin 11 Basin 11 will convey its flow overland across the parking lot and porous paver area, through a 2’ curb cut, and in to a 24” Nyloplast drain basin with standard H-20 grate at design point 11. Runoff captured by the inlet is then conveyed south in the 24” HDPE. Basin 12 Basin 12 will convey its flow overland, across the parking lot and porous paver areas, through 2- 2’ curb cuts and in to an inlet at design point 12. A 4” SDR 35 perforated PVC underdrain system will be installed under the porous paver areas to capture and convey runoff into the CDOT Type C Inlet with close mesh grate at design point 12. Excess flow will enter the CDOT Type C inlet, which discharges to the 24”RCP. Runoff from the roof areas within Basin 12 will be captured by 1-12” Nyloplast, and 1-15” Nyloplast drain basin with standard H-20 grates, with the inlets located between the building and pool area. The Nyloplast inlets will convey runoff north/northeast in the 12” and 15” HDPE pipes, which will discharge into the CDOT Type C Inlet with close mesh grate at design point 12. Basin 13 Basin 13 will convey its flow overland across the parking lot and porous paver area to a low point in the parking lot, where a CDOT Type C Inlet with close mesh grate, at design point 13, will capture the runoff and discharge into the 30” RCP. A 4” SDR35 perforated PVC underdrain system will be installed under the porous paver areas to capture and convey runoff into the CDOT Type C inlet with close mesh grate at design point 13. One paver underdrain will connect at the manhole downstream of the CDOT Type C inlet. Two 2’ wide curb cuts and three 2’ wide concrete chases with sidewalk culverts will be installed within Basin 13 to convey runoff in green spaces away from the buildings and out into the parking lot areas. Basin 14 Basin 14 will convey its flow overland across the parking lot and porous paver area, through a 3’ curb cut, and in to a CDOT Type C Inlet with close mesh grate at design point 14. Runoff captured by the inlet is then conveyed west in the 24” RCP. Basin 15 Basin 15 consists of the area that drains to the east half of Jerome Street. The runoff from this basin flows via overland flow and curb and gutter to a low point at design point 15, where a 10’ Type R Inlet captures runoff from Basin 15. An intermittent cobble swale, located north of the building in the large green area, will also convey runoff west, through a 2’ curb cut with 2’ 13 metal sidewalk culvert and into the curb and gutter along the east side of Jerome Street. Runoff from portions of the front yard area long the west side of the building will be captured by yard drain inlets and conveyed north in the proposed storm pipe that discharges runoff into the 10’ Type R Inlet at design point 15. Basin 16 & OS1 Basin 16 and OS1 consists of the area that drains to the west half of Jerome Street and small western portion of Lupine Drive. The runoff from these basins flows via overland flow and curb and gutter to a low point at design point 16, where a 5’ Type R Inlet captures runoff from Basins 16 and OS1. Runoff from portions of the front yard area long the east side of the buildings will be captured by yard drain inlets and conveyed north in the proposed storm pipe that discharges runoff into the 5’ Type R Inlet at design point 16. Basin 17 Basin 17 conveys runoff from north roof area and area north of the building to a swale, which conveys runoff east, through a 2’ sidewalk culvert and into a CDOT Type C Inlet with close mesh grate at design point 17. Runoff is then conveyed east in the 15” HDPE. Basin 24 Basin 24 is a small 0.03-acre (predominantly green-area with Q100=0.15 cfs) basin that drains to an existing area inlet located at the southeast corner of the development. The existing inlet was constructed with the Aspen Heights project and was one of five inlets that were installed along the west side of Aspen Heights to pick up runoff from approximately 2.59 acres of Crowne’s site. The existing inlet in Basin 24 will be raised to match proposed grade of the Crowne Development. Flow to this inlet is well below that allowed in the approved Aspen Heights Drainage Report. Basins 25 and 26 Basins 25 and 26 are both 0.02 acres (combined area of 0.04-acres) and drain east down Lupine Drive and ultimately to Aspen Heights. The major storm event runoff from these basins is 0.18 and 0.21 cfs, respectively, is negligible and is considered to have little effect on the existing storm system within Aspen Heights. Basin 27 Basin 27 is a small 0.11 acre (predominantly green-area with Q100=0.71 cfs) that drains via overland flow to two existing area inlets, located along the bottom of the retaining wall, which was constructed with Aspen Heights. The existing inlets will be raised to match proposed grade of the Crowne Development. It’s important to note that though the Aspen Heights Drainage 14 Report provided for and allows approximately 2.59 acres of Crowne’s site to flow to the existing inlets, only approximately 0.18 acres will be directed to the existing inlets, of which only 0.11 acres is from basin 27. Drainage to the NECCO 48-inch Storm Sewer: The runoff from the following basins is collected in a storm sewer system which ultimately connects into the NECCO 48-in storm sewer located along the western boundary of the site. These basins represent basin Crowne-West in the Dry Creek EPA SWMM model. Basin Crowne-West was modified to determine the impacts of the proposed development to the NECCO system. Basins 18, 19 & 21 Basins 18, 19 and 21 drain to curb cuts and enter an LID basin, located along the west boundary of the site. Runoff is conveyed to the LID basins via overland flow, curb and gutter, and 2’ curb cuts to the LID Basins. The LID basin will have 4” SDR 35 perforated PVC underdrain systems that will convey filtered runoff south and into the 48” NECCO storm sewer system, via the CDOT Type D Inlet at design point 23. Basin 20 Basin 20 will convey its flow overland, across the drive aisle, parking lot and porous paver area, drains to a 2 foot wide curb cut and enters a swale and LID basin, located along the west boundary of the site. A 4” SDR 35 perforated PVC underdrain system will be installed under the porous paver areas to capture and convey runoff into the CDOT Type D inlet and into the 48” NECCO storm sewer system, via the CDOT Type D Inlet at design point 23. It’s important to note that the existing ditch along the west side of the project (some of which extends west and offsite) will remain in-place and will continue to convey existing restricted flow/runoff from Jax and the adjacent properties to the west, south and into the proposed 48” RCP stub that extends northwest out of the CDOT Type D Inlet with close mesh grate at design point 23. Basins 22 and23 Basins 22 and 23 drain to a swale, located along the south and west boundary of the site. Runoff is conveyed to the swale via overland flow, curb and gutter, and 2’ curb cuts to the swale. Runoff in the swale will flow south and west and enter the 48” NECCO storm sewer system, via the CDOT Type D Inlet at design point 23. Offsite Drainage The runoff from the following basins drains offsite: 15 Basin 28 and OS2 Basin 28 and OS2 represent a portion of the project that drains to the north and onto Conifer Street. These basins are part of basin 26 in the Dry Creek EPA SWMM model. Basin 26 was modified to correctly include the runoff from these basins. This was done to insure that the impacts to the offsite system were minimal and meet the design intent of the NECCO storm sewer system. Runoff from basins 28 and OS2 will flow east down Conifer and will be captured by the existing inlet on the south side of Conifer Street. OS3 and OS4 Basins OS3 and OS4 represent a portion of the project that drains to the west along Suniga Road. These basins are part of basin 316 in the Dry Creek EPA SWMM model. Basin 316 was modified to correctly include the runoff from these basins. This was done to insure that the impacts to the offsite system were minimal and met the design intent of the NECCO storm sewer system. Should Suniga Road be extended west of the Crowne development by the City of Fort Collins, basins OS3 and OS4 will drain to proposed inlets at low points along the north and south sides of Suniga Road. In the interim and should the City not extend Suniga Road west of the Crowne development, a riprap diversion channel will be installed at the west end of Suniga Road (at Crowne’s western boundary) to convey runoff from OS3 and OS4 north and into a 30” RCP that is stubbed out from the 48” NECCO storm sewer. OS5 and OS6 Basins OS5 and OS6 represent a portion of the project that drains to the east along Suniga Road. These basins are part of basin 318 in the Dry Creek EPA SWMM model. Basin 318 was modified to correctly include the runoff from these basins. This was done to insure that the impacts to the offsite system were minimal and meet the design intent of the NECCO storm sewer system. Overflow Conditions Basins 1-6: Should the inlets at the low spots in these basins clog, the runoff will flow through the parking lot to the inlets at design points 3, 4, and 6. Basins 7 and 8: Should the 5’ Type R Inlets at design points 7 and 8 clog, storm runoff will spill east and west down Lupine Drive. Basin 9: Should the inlet in the LID Basin clog, storm runoff will spill east and into the existing inlet near the bottom of the retaining wall along Aspen Heights’ west boundary. 16 Basins 10-14: Should the inlets at the low spots in these basins clog, the runoff will flow through the parking lot to the inlets at design points basins 10, 13 and 14. Basins 15 and 16: Should the 5’ Type R Inlet at design point 16 and the 10’ Type R Inlet at design point 16 clog, storm runoff will spill east down Lupine Drive. Basin 17: Should the inlet in this basin clog, storm runoff will spill east onto Jerome Street. Basins 18-22: Should the curb cuts or underdrain within the LID basin clog, the runoff will spill south and drain in a swale to the Type D inlet in basin 23. Basin 23: Should the inlet in basin 23 clog, the runoff will spill south to Suniga Road. Basin 24: Should the existing inlet in basin 24 clog, the runoff will flow south to Suniga Road. Basin 27: Should the existing inlets in basin 27 clog, the runoff will flow north to the inlet in basin 5. C. Detention Pond The proposed development does not contain a detention pond. The flow is ultimately collected in the NECCO storm sewer system. Detention and water quality for the proposed Crowne development is provided in the NECCO Regional Detention Pond. 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 NECCO regional detention pond provides water quality for the proposed development. LID treatment will be provided for the Crowne development through the utilization of LID basins and porous paver areas. Final sizing calculations are provided in the Appendix. An LID summary table is provided on the Drainage Exhibit. LID 17 measures are provided for through the conveyance of approximately 51% (3.87 acres) of the proposed impervious area of the site being routed through the porous paver areas and LID basins. Because the area being routed to and through the porous pavers and LID basin exceeds the 50% requirement for LID treatment, the project exceeds that normally required for water quality and LID treatment. Porous pavers will be implemented to treat approximately 15% of the new impervious paved area (212,916SF, 4.89 AC). Please refer to Appendix E for LID calculations, as well as sheet C-012, which provides an additional LID table, as required by the City of Fort Collins Stormwater Department. VI. EROSION CONTROL A. General Concept The subject site lies within the Moderate Rainfall Erodibility Zone and the Moderate Wind Erodibility Zone per the City of Fort Collins zone maps. The potential exists for erosion problems during construction, but should be minimal after completion of proposed development. Silt fence has been installed along the north, west, south, and portions of the east side of the site to prevent sediment from leaving the site, while also providing as a barrier for adjacent prairie dog mitigation on-site. Vehicle tracking pads will also be placed at entrances/exits to the site, with the exact location provided during final design. Straw Wattles and gravel inlet filters will be placed at proposed inlet locations to mitigate the build-up of sediment in the proposed inlet and manhole structures. Straw wattle check dams will also be installed at 300' intervals in the proposed swales throughout the site, as may be applicable. The Erosion Control Escrow Amount ($71,321.00) has been calculated and can found in Appendix F. Please refer to Appendix F 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. 18 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 outlet control structures shall be cleaned through the removal of debris and sediment from the associated items to allow for adequate drainage through the site to the offsite regional detention facility 2) 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 NECCO regional detention pond. The proposed storm sewer system will provide for the 100-year developed flows. If groundwater is encountered at the time of construction, a Colorado Department of Health Construction Dewatering Permit will be required. C. Storm Water Quality Water quality for the site will be provided in the NECCO regional detention pond. LID measures have also been provided for the project and utilize LID basins and porous pavers on the site to provide water quality treatment of storm runoff. 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 proposed erosion control concepts, the City of Fort Collins standards will be met. The proposed erosion control concepts presented in the preliminary report and shown 19 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 $79,085.00. 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, March, 1969. 4. Urban Storm Drainage Criteria Manual Volume 2; Urban Drainage and Flood Control District, Denver, Colorado, March, 1969. 5. Urban Storm Drainage Criteria Manual Volume 3, Best Management Practices; Urban Drainage and Flood Control District, Denver, Colorado, September, 1999. 6. Overall Drainage Plans, Aspen Heights, Fort Collins; Owen Consulting Group, Inc., October 17, 2013. 7. North East College Corridor Outfall Plans; Ayres Associates, May 2009. 20 APPENDIX 21 APPENDIX A 22 VICINITY MAP 23 APPENDIX B 24 RATIONAL METHOD HYDROLOGY 25 APPENDIX C 26 STORM SEWER PIPE SIZING 27 APPENDIX D 28 INLET & STREET CAPACITIES, CURB CUT & SIDEWALK CULVERT SIZING 29 APPENDIX E 30 SWALE SIZING LID BASIN SIZING & POROUS PAVER SIZING 31 APPENDIX F 32 EROSION CONTROL & EROSION CONTROL ESCROW ESTIMATE 33 APPENDIX G 34 PROPOSED & EXISTING DRAINAGE BASIN EXHIBITS, EROSION CONTROL PLAN, & EROSION CONTROL NOTES AND DETAILS SHEET