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HomeMy WebLinkAboutOAKRIDGE CROSSING (AFFORDABLE SENIOR HOUSING) - PDP - PDP160009 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTFinal Drainage and Erosion Control Report for Oakridge Crossing Fort Collins, Colorado April 19, 2016 March 19, 2016 Mr. Dan Mogen City of Fort Collins Storm Water Dept. 700 Wood Street Fort Collins, Colorado 80521 RE: Final Drainage and Erosion Control Report for Oakridge Crossing Dear Dan: We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Report for the Oakridge Crossing. 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 Prepared by: Reviewed by: Tony Willkomm, P.E. John Gooch, P.E. Senior Project Engineer 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 III. DRAINAGE DESIGN CRITERIA 7 A. REGULATIONS 7 B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS 7 C. HYDROLOGIC CRITERIA 7 D. HYDRAULIC CRITERIA 7 E. VARIANCE 8 IV. DRAINAGE FACILITY DESIGN 8 A. GENERAL CONCEPT 8 B. SPECIFIC DETAILS 9 C. DETENTION PONDS 11 D. STREET CAPACITIES 11 V. STORM WATER QUALITY 11 A. GENERAL CONCEPT 11 B. SPECIFIC DETAILS 12 VI. EROSION CONTROL 13 A. GENERAL CONCEPT 13 VII. CONCLUSIONS 13 A. COMPLIANCE WITH STANDARDS 13 B. DRAINAGE CONCEPT 14 C. STORM WATER QUALITY 14 D. EROSION CONTROL CONCEPT 14 E. EROSION CONTROL ESCROW ESTIMATE 14 REFERENCES 15 APPENDIX PAGE VICINITY MAP A RATIONAL METHOD HYDROLOGY B POND AND RAIN GARDEN SIZING CALCULATIONS C STORM SEWER, CURB CUT, INLET, & SWALE SIZING D EROSION CONTROL, RIPRAP SIZING & EROSION ESCROW E PROPOSED DRAINAGE EXHIBIT & DRAINAGE REPORT EXCERPTS & EXISING BASIN EXHIBIT F 5 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE OAKRIDGE CROSSING FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Oakridge Crossing project is bounded to the north by Oakridge Business Park 11th Filing, to the east by Oakridge Business Park 23rd Filing, to the south by Oakridge Business Park 18th Filing, and to the west by McMurry Avenue. The project site can also be described as situated in the northeast quarter of Section 6, Township 6 North, Range 68 West of the 6th P.M., of the City of Fort Collins, County of Larimer, State of Colorado. The Oakridge Crossing building site is comprised of approximately 2.61 acres. The drainage analysis includes 2.61 acres of the onsite area which is a portion of the existing drainage basin 210 (7.51 acres) established with the Master Drainage Study For the Oakridge Business Park (Master Drainage Study). Please refer to the vicinity map in Appendix A for the site location. B. Description of Property The site is comprised of an approximate 32,746 +/-square foot building footprint along with associated parking lot, west entrance, northeast drive aisle access, private drive improvements and utility improvements. Previously, the Oakridge Crossing building site was an undeveloped lot, vegetated with sparse grass and weeds. Enhanced landscaping is established on adjoining lots to the east and south. At the northeast side the lot, an asphalt paved drive aisle and parking bay with one end-cap island is constructed. Two concrete curb and gutter islands are constructed straddling the north property boundary at the west end of the property and a concrete access drive is constructed at the approach from McMurry Avenue. The existing site topography of the Oakridge Crossing building site generally drains to the south and south east at 0.5% to 1% slope. The existing paved drive aisle and parking spaces to the north, drain to a concrete pan and flows east at a slope of 1%. 6 II. DRAINAGE BASINS A. Major Basin Description The site lies within the McClellands Drainage Basin, with site drainage previously accounted for in the Master Drainage Study for the Oakridge Business Park. The approved grading and drainage master plan and SWMM Model for the Master Drainage Study Oakridge Business Park (Master Plan) provided for the runoff from the entire basin being ultimately routed to the regional detention pond southeast of the project site. The proposed Oakridge Crossing building site will discharge runoff after being detained and treated for water quality on site. Proposed storm sewer will connect to existing storm sewer in McMurry Avenue which will discharge to a regional detention pond southeast of the site. The intended connection in the Master Plan was to storm sewer south of the site which would then discharge to an open swale. This intended connection point is no longer available with current development in place. The volume of site runoff will be in compliance with the Master Drainage Study for the Oakridge Business Park, September, 1990 by RBD Engineering Consultants. The proposed improvements meet the intent of approved Master Plan. The developed site runoff (22.86 cfs) is less than the prorated share of runoff (23.62 cfs) for the Master Plan Basin 210. The composite runoff coefficient for the site is 0.73 (C100 =0.91) versus 0.80 for Basin 210 in the Master Plan. B. Existing Sub-Basin Description The existing Oakridge Crossing building site is contained within existing sub-basin 210 of the Master Plan. Existing basin 210 contains approximately 7.51 acres and was modeled with the entire 7.51 acres having a runoff coefficient of 0.80 in the developed condition. All flow from the basin being allowed to flow un-detained to the existing detention pond. No water quality was provided in the regional detention pond. Water quality will be provided for on site for the proposed improvements. The developed conditions for the Oakridge Crossing building site has been evaluated with the current IDF curve data for City of Fort Collins to determine if on site detention is needed for the property. It was determined that no addition detention will be necessary on site. However, considering the limitation of using the existing storm sewer connection point in McMurry Avenue, site runoff will be restricted and on site detention will be provided to avoid over taxing the existing storm sewer which must connect with a 15” diameter storm pipe as shown on the Master Plan drainage exhibit. 7 The area south and east of the proposed building comprised of 0.47 acres will sheet flow off site as occurs historically. The existing 0.22 acres of paved drive aisle and parking lot at the north side of the site will drain off site to the east as it historically has. The balance of the site will be directed to the proposed on site storm sewer and discharge to the existing storm sewer in McMurry Avenue. No offsite areas contribute overland flow to or through the Oakridge Crossing building project site. Please refer to Appendix F of this report for the existing and proposed drainage basin exhibits for the subject site. III. DRAINAGE DESIGN CRITERIA A. Regulations Since the subject site has been previously modeled, the site is allowed to release un- detained to the existing regional detention pond in Master Plan basin 90, which has been sized to provide detention for the developed 100-year runoff from the subject site. LID improvements on site are required and have been provided for with this project. Please see the LID calculations in Appendix C, as well as the LID Table on sheet C-007, which provides the treatment methods and area calculations for LID treatment using a rain garden. In addition, a grass buffer will be used in Basin 1 where runoff cannot be directed to the rain garden. B. Development Criteria Reference and Constraints The criteria and constraints from the City of Fort Collins will be met. The calculated detention pond sizing using the prorated allowable runoff from Basin 210, and the developed runoff coefficient for the proposed development results in no additional detention required. Due to the capacity of the existing storm sewer connection point, the site runoff rate will be restricted and detained on site before discharging to the regional detention pond. 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. 8 The City of Fort Collins intensity duration frequency curves were used to obtain rainfall data for each storm specified. Detention pond sizing was calculated using FAA methods and water quality volume was calculated using the rain garden sizing spreadsheets developed by Denver Urban Drainage. Detention will be provided for the subject site in the existing regional detention pond (Master Plan basin 90) from the previous infrastructure improvements of the Oakridge Business Park. On site detention will be provided to reduce the peak runoff from the site entering the existing storm sewer system in McMurry Avenue. BMP’s and LID measures are being incorporated into the proposed drainage development plan, with specific details and calculations shown in Appendices C and F of this report. D. Hydraulic Criteria All hydraulic calculations, including final calculations and sizing for storm sewer, curb cuts, swales, and erosion control and riprap measures have been provided in accordance with the City of Fort Collins Drainage Criteria and can be found in the Appendix D and E. E. Variance No variances are being requested. IV. DRAINAGE FACILITY DESIGN A. General Concept A total of five drainage basins are delineated over the proposed Oakridge Crossing building site. One basin to the north, will drain 0.23 acres east, offsite as it historically occurs. Another basin south and east of the proposed building, will drain 0.47 acres through grass buffers to the south and east in the historic drainage pattern. Two drainage basins will convey drainage from 1.87 acres of the 2.61 acre site into a rain garden LID. A small basin will drain 0.05 acres to a proposed storm sewer inlet. The site drainage will be conveyed to the rain garden via curb and gutter, roof drains and proposed storm pipes. Site runoff will be intercepted by proposed storm sewer and flow through existing off site storm sewer and conveyed to the existing regional detention pond southeast of the project site. The existing regional detention Pond (Basin 90) provides storage and detention and was sized to receive the 100-year developed flow from the Oakridge Crossing building site (from the Master Plan). The calculated composite 'C' value for Oakridge Crossing building site is 0.73. Please refer to the rational calculations in 9 Appendix B, and the Drainage Basin Exhibit and Master Plan Drainage Exhibit in Appendix F for additional information. LID measures are provided for through the conveyance of runoff from approximately 77% of the new impervious area consisting of building roof area, pavement, and sidewalk, which will be routed through the rain garden for water quality treatment. Because the area being routed to and through the rain garden exceeds the 75% requirement for LID treatment of the total site impervious area, the project exceeds that normally required for water quality and LID treatment. It should be noted that the existing impervious area within Basin 5 is excluded from the LID calculations. This area will drain east, offsite as currently occurs. Please refer to Appendix C for LID calculations, as well as sheet C-007, which provides an additional LID table, as required by the City of Fort Collins Stormwater Department. The rain garden, shown and called out in the plans, provides approximately 2,452 ft3 of storage volume with a maximum of 1’ of ponding depth, prior to the basin overflowing into proposed CDOT Type C inlets which discharge to proposed storm sewer then to existing storm sewer and ultimately entering the existing detention pond. It’s important to note that 0.47 acres of the site runoff along the south and east sides of the proposed building will be landscaped with grass to enhance water quality treatment by filtering through the grass before going off site as historically occurs. Please refer to Appendix C and sheets C-009 and C-011 of the civil utility plans for details on the rain garden. B. Specific Details Basin 1 Basin 1 consists primarily of grass/landscaped areas, located south and east of the proposed building. Runoff from basin 1 will sheet flow away from the building and flow off site as historically occurs. Since this basin will be established with grass, the ground cover is expected to be better than the existing grass cover on the undeveloped lot. The basin combines with offsite flow and will ultimately drain to the regional detention pond. Basin 2 Basin 2 consists of the proposed building, parking lot and rain garden. Runoff from the building roof enters roof downspouts and connect with underground roof drains which discharge runoff north and west onto riprap pads within the rain garden LID basin. Landscaped areas next the building will drain through sidewalk drains and discharge to curb and gutter. The parking lot will sheet flow north and west to curb and gutter and drain concentrated runoff to four, 3’ curb cuts with riprap pads draining to the rain garden. The rain garden is designed to provide a ponding depth of approximately 1.00’ and allow pollutants and sediment to settle out of the runoff, 10 prior to overflowing into three CDOT Type C inlets to enter the proposed storm sewer. Eleven downspouts are consolidated to four, 12” HDPE underground roof drain pipes discharging to riprap pads in the rain garden. In the event of clogging, overflow from the downspouts will be provided and will drain through sidewalk drains. It should be noted that one 18” Nyloplast drain basin is used between the building and sidewalk to drain the interior corner northwest of the building. An 8” HDPE pipe will convey flow from the drain basin directly to the proposed CDOT Type C Inlet within the rain garden and will not receive water quality treatment in the rain garden. This area could not drain to the surface of the rain garden due to grading constraints. A proposed 12” storm sewer constructed in Basin 2, will receive flow from the 5’ Type R inlet in Basin 4. Overflow from the rain garden and runoff from Basin 2, Basin 3 and drainage from the rain garden underdrain pipe will enter three proposed CDOT Type C inlets. Proposed 15” HDPE pipe will convey flow south then west along the length of the pond. A 15” RCP will connect the pond discharge to the existing storm manhole in McMurry Avenue. Flow will combine with upstream runoff from an existing 18” storm pipe to the north. Combined flow will drain south in an existing 24” RCP and ultimately discharge to the regional detention pond. Basin 3 Basin 3 conveys runoff from part of the existing north drive aisle and the proposed parking bay at the northwest corner of the site. The paved area drains southeast to curb and gutter passes through a 1’ curb cut and riprap pad before entering the rain garden in Basin 2. Basin 4 Basin 4 conveys runoff from the proposed parking bay at the northeast corner of the site. The paved area drains southwest to curb and gutter and enters a proposed 5’ Type R inlet which drains to a 12” RCP. Runoff from this basin was not able to drain to the rain garden due to grading constraints and is conveyed off site in the proposed storm sewer. Basin 5 Basin 5 conveys runoff from the existing paved drive aisle and parking bays. Sheet flow is concentrated in a central concrete drainage pan and flows east off the site to the neighboring parking lots which drain south and east. This areas drainage pattern will not be modified with the proposed improvements. Two landscape islands will be constructed within two existing paved parking stalls which will increase the overall pervious area of Basin 5. However, the runoff coefficient for the basin is based on the existing condition and the basin will drain as historically occurs. This basin area was developed in a prior phase of the Oakridge Business Park and will ultimately drain to the regional detention pond. 11 C. Detention Ponds Although no detention is required for the subject site when comparing the Master Plan peak runoff allowable to the developed site peak runoff, detention is required to avoid exceeding the capacity of the existing storm system in McMurry Avenue. Since this discharge point varies from the Master Plan, the site runoff release rate will be restricted Detention will be provided within the rain garden area defined as Pond 1. Site runoff will be detained in Pond 1 to a 100-year water surface elevation of approximately 4969.0. Pond discharge will be restricted by and conveyed in a proposed 15” RCP. The pipe size is governed by the design in the Master Plan and the as built conditions. The release rate was determined by considering a maximum of 1 foot of ponding depth in the parking lot and the capacity of the 15” RCP operating under pressure flow conditions. The resulting release rate is 8.1cfs. Using the FAA method, the required pond volume is 0.15 Ac-Ft. The detention volume will be provided above the rain garden volume (elevation 4967.50). Pond volume calculations and storm sewer sizing calculations are provided in Appendix C and D respectively. Note that the detention for the site runoff is also provided for in the regional detention pond (Basin 90), which was constructed as outlined in the Master Plan. D. Street Capacities The street capacity calculations are not required for the subject site, as no new public street construction will be built with this project. However, curb cut capacities have been analyzed and calculations provided in Appendix D of this report. V. STORM WATER QUALITY A. General Concept The water quality of stormwater runoff must be addressed on all final design utility plans. Therefore, Best Management Practices for the treatment of stormwater runoff for the subject site will include grass-lined swales, a rain garden LID basin, and the utilization of the grass buffers along the south and east side of the site. These water quality features will provide a mechanism for pollutants to settle out of the stormwater runoff before flows are directed to the regional detention pond. 12 B. Specific Details Best Management Practices (BMP) for the treatment of storm water runoff has been incorporated into the final design for this project. This will include continued utilization of extended detention in the existing, offsite detention pond. The existing detention facility has adequate capacity and will continue to provide detention for the developed condition of the subject site, prior to releasing flows downstream. On site detention will restrict the site runoff to avoid exceeding the existing storm sewer capacity. LID measures are provided for through the conveyance of 1.87 acres over and through the rain garden LID basin. To further enhance water quality 0.47 acres will sheet flow over grass buffer area. The footprint of the rain garden is approximately 1,048 square feet, with a depth of approximately 1.0 foot, and provides approximately 2,452 cf of storage volume. It’s important to note that 77% of sites new impervious area is being conveyed over and through the rain garden which exceed the 75% minimum that is normally required for water quality and LID treatment. The rain garden, shown and called out in the civil utility plans, provides for approximately 1.0’ of ponding depth, prior to the basin overflowing to storm inlets and into the storm sewer and eventually into the existing detention pond. Approximately 1.5’ of ponding depth will occur over the proposed CDOT Type C inlets during the 100-yr storm event. The ponding depth will be less than 1 foot in the parking lot. Please refer to Appendices C and F, and sheets C-009 and C-011 of the civil utility plans, for details on the rain garden. Please also refer to the LID Table on sheet C-007 for area calculations and specific LID calculations, as required by the City of Fort Collins. 13 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 will be installed along the north, south, east, and west sides of the site to prevent sediment from leaving the site. A tracking pad will also be placed at west entrance/exit to the site. Gravel inlet filters will be placed at proposed curb cuts, and storm drain inlets. Riprap will also be utilized at concentration points (curb cuts and pipe outlets), as shown in the plans, to minimize erosion and increase soil stabilization. VII. CONCLUSIONS A. Compliance with Standards 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 outlet 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 proposed detention facility 2) Sedimentation/ silting shall be removed to allow for adequate drainage along the bottom of the swales and LID/PLD basin. Grass scrubbing along the bottom of the LID/PLD basin 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 14 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 on site detention pond and the existing detention pond, located south of the subject site. The combination of the existing and proposed storm conveyances will provide for the 100-year developed flows to reach the existing detention pond. The calculated release rate will pond to a depth not to exceed 1 foot in the parking lot during a 100-year storm event. The existing release rate of the existing pond will be adhered to. 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 final design has addressed the water quality aspect of stormwater runoff. The LID items presented in this report for the proposed site will provide an opportunity for stormwater pollutants to filter out of the stormwater runoff before flows are directed downstream. D. Erosion Control Concept Proposed erosion control concepts have been provided for and 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 performance standards 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. Erosion Control Escrow Estimate The Erosion Control Escrow Estimate for the subject property is approximately $17,648.00. Please refer to Appendix E for this calculation. 15 REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, May 1984, Revised January 1997. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins, Colorado, January 1991, Revised January 1997. 3. Master Drainage Study for the Oakridge Business Park, by RBD Engineering Consultants, Fort Collins, Colorado, September, 1990. 4. Overall Drainage Concept for The Overall Development Plan For Oakridge Business Park, by North Star Design , April, 2000. 16 APPENDIX 17 APPENDIX A VICINITY MAP 18 APPENDIX B RATIONAL METHOD HYDROLOGY 19 APPENDIX C POND AND RAIN GARDEN SIZING CALCULATIONS 20 APPENDIX D STORM SEWER, CURB CUT, INLET, & SWALE SIZING 21 APPENDIX E EROSION CONTROL & RIPRAP SIZING; EROSION ESCROW 22 APPENDIX F PROPOSED DRAINAGE EXHIBIT DRAINAGE REPORT EXCERPTS & EXISTING DRAINAGE EXHIBIT