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HomeMy WebLinkAbout221 E. MOUNTAIN AVENUE - BASIC DEVELOPMENT REVIEW - BDR160011 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFinal Drainage and Erosion Control Report for 221 East Mountain Avenue Fort Collins, Colorado May 3, 2016 May 3, 2016 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 for the 221 East Mountain Avenue Project Dear Wes: We are pleased to submit to you, for your review and approval, this Final Drainage and Erosion Control Report for the 221 East Mountain Avenue Project. 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 8 E. VARIANCE 8 IV. DRAINAGE FACILITY DESIGN 8 A. GENERAL CONCEPT 8 B. SPECIFIC DETAILS 9 C. DETENTION POND 11 D. STREET CAPACITIES 11 V. STORM WATER QUALITY 12 A. GENERAL CONCEPT 12 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 LID BASIN SIZING CALCULATIONS C CURB CUT & STORM PIPE SIZING D EROSION CONTROL ESCROW E PROPOSED DRAINAGE BASIN MAP & EXISTING DRAINAGE BASIN MAP F 5 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR 221 EAST MOUNTAIN AVENUE FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The 221 East Mountain Avenue Project (referred to as “221 Mountain” here-forward) is bounded to the north by Mountain Avenue, to the east by Mathews Street, to the south by a public alley and neighboring apartment building, and to the west by a multi-story public parking structure. The project site can also be described as situated in the northwest quarter of Section 12, Township 7 North, Range 69 West of the 6th P.M., City of Fort Collins, County of Larimer, Colorado. 221 Mountain site is currently occupied by a vacant building, (formerly Mountain View Tire Center) with a paved parking lot. The proposed use will be a four story office building surrounded by paved sidewalk, landscape features and improvements to the public alley, south of the lot. Proposed improvements are planned to occur over entire lot area of approximately 0.45 acres. B. Description of Property The 221 Mountain site is comprised of a four story building with enhanced sidewalks and landscape improvements along the East Mountain Avenue frontage and Mathews Street frontage. The public alley will be reconstructed with a center trickle pan and outfall curb and gutter along the north and south sides of the alley. Currently, the 221 Mountain site is occupied by an existing brick building with a north driveway access on E. Mountain Ave and an east driveway access on Mathews Street. The entire lot area around the building is paved with asphalt and concrete. The space between the west side of the building and the public parking structure is surfaced in gravel. The public alley south of the lot is asphalt paved. The existing parking lot slopes north and east away from the building at slopes of approximately 1% and 3%. A concrete curb is constructed along the south and east sides of the lot. Drainage is released at an opening near the northeast corner of the lot, at the east driveway access on Mathews Street and at the North driveway access on Mountain Avenue. The off-site, gravel surfaced area west of the building drains 6 from the south to north and drains across the existing sidewalk to curb and gutter on E. East Mountain Avenue. II. DRAINAGE BASINS A. Major Basin Description The 221 Mountain site lies within the City of Fort Collins Old Town Basin. The proposed development will be evaluated to determine the change in imperviousness between existing and proposed conditions, while also providing LID and BMP’s for the proposed development for any additional impervious areas, added by the proposed development. B. Existing Sub-Basin Description The existing 221 Mountain site is comprised of two basins: EX-1 and EX-2. EX-1 comprises the existing building majority of the parking lot area to the east. Existing basin EX-1 contains approximately 0.40 acres with a calculated 'c' value of 0.95. Existing runoff from the basin EX-1 is comprised of the concrete and asphalt parking lot and the building roof area which is discharged to the paved surface via a roof drain on the east face of the building. Runoff generally flows east through the driveway access and northeast to a parking lot curb opening at the north end of the lot. Flow drains over existing sidewalk and enters the curb and gutter in Mathews Street. Drainage in Mathews Street flows north to enter an existing 12-ft combination inlet. Existing Basin EX-2 contains approximately 0.04 acres with a calculated ‘c’-value of 0.95. The basin is comprised of concrete and asphalt parking along the north face of the existing building. Runoff drains north over existing sidewalk and through the existing driveway access to the flowline in Mountain Avenue. Flow is conveyed east in the curb and gutter to an existing 20-ft Type R inlet. The existing Basin EX-3 is the off-site area comprised of the public alley and half the right-of-way of Mathews Street. The alley drains east to the flowline of Mathews Street, then north to the existing 12’ combination inlet. The basin is comprised of the asphalt alley, asphalt street, concrete sidewalk and curb and gutter, and grass landscaping between the curb and sidewalk. Basin EX-3 has an existing ‘c’ value of 0.88. It should be noted that Basin EX-3 is existing public right-of-way and therefore outside the limits of the site. The majority of the site runoff currently is discharged 7 through paved areas with no treatment for water quality. Minimal filtering may occur in minor events were flow from the curb opening at the northeast corner of the lot passes over grass area between the curb and sidewalk before entering the 12-ft combination inlet. Please see the Existing Drainage Exhibit attached for the existing basin limits and the delineated pervious areas. III. DRAINAGE DESIGN CRITERIA A. Regulations Under the existing fully developed conditions the site is 100% impervious and the proposed developed condition will also be impervious within the property boundary. The site is allowed to release un-detained, as occurs currently under the existing condition. It was noted by the Stormwater Department the water quality treatment for 50% of the site is treated by the Udall Natural Area water treatment facility. However, additional LID improvements and BMP’s will be implemented to provide for treatment of storm water runoff for the subject site. Please see the LID calculations in Appendix C. B. Development Criteria Reference and Constraints The criteria and constraints from the City of Fort Collins will be met. 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 pond sizing is not required as the impervious area has been decreased with the proposed development. However, additional water quality for the subject site is being provided through the utilization of BMP and LID measures being incorporated into the proposed drainage development plan. Please refer to Appendix C for specific details and calculation. 8 D. Hydraulic Criteria All hydraulic calculations, including final calculations and sizing for curb cuts, and erosion control have been provided in accordance with the City of Fort Collins Drainage Criteria and can be found in the Appendix. E. Variance No variances are being requested for this project. IV. DRAINAGE FACILITY DESIGN A. General Concept Three drainage basins will provide conveyance of drainage for the 0.45 acre site. The majority of the site will occupied by the proposed building which will drain runoff to three roof drain locations, one to the north and two to the east. A relatively small area of sidewalk between the building and the property boundary will sheet flow north and east of the building. The runoff from the hardscape on site and within the right-of- way will sheet flow to the respective street flowline in Mountain Avenue and Mathews Street then drain to the existing inlets on each respective street. The runoff from approximately 48% of the building will be treated using rain gardens for LID treatment. The north roof drain will be piped underground and connect to the existing Type R inlet in Mountain Avenue. The north roof drain along the east face of the building will be piped to the existing combination inlet in Mathews Street. Roof drain overflow will enter a chase and drain to a rain garden for water quality treatment. The south roof drain, along the east side, will discharge all roof runoff to a proposed concrete chase and discharge to a rain garden for water quality treatment. Overflow scuppers at both east side roof drains will drain to chases which enter the rain gardens. Excess overflow from the rain gardens will drain in the concrete chases to the flowline of Mathews Street. Site runoff and right-of-way runoff will be treated, prior to entering the storm sewer, through the utilization of the proposed rain gardens and porous pavers with underdrain system for LID measures. The existing parking spaces on Mountain Avenue will be reconstructed with porous pavers to treat runoff from the north frontage of the building and the east bound lanes of the existing Mountain Avenue along the lot frontage. The proposed storm sewer pipe, chases, and erosion control measures have been sized and calculations are provided in the Appendices. 9 In comparing existing conditions to proposed conditions for the subject site, the c- values are 0.95 and 0.95, respectively. The imperviousness for existing conditions is 96%, compared to the 91% for proposed conditions. Therefore, there is a net decrease in impervious area for the development. Exhibits are provided in Appendix F showing the existing and proposed basin areas, and drainage patterns. The public alley south of the lot will be reconstructed with curb and gutter outfall curbs and a 4-ft concrete drainage pan in the centerline of the alley. Alley runoff will drain east to the flowline of Mathews Street then north to the existing combination inlet. It should be noted that a long narrow paved section between the building and the back of curb, approximately 200 SF, will sheet flow to the alley. This area will not be able to be captured or redirected away from the alley due to grading restraints. The alley impervious area will remain the same as the existing condition. The rain garden LID measures, shown and called out in the plans, are anticipated to provide for approximately 1 foot of ponding depth, prior to overflowing to concrete chases and discharging to the Mathews Street. A 4-inch PVC underdrain will connect the rain garden areas to a proposed 12” HDPE storm sewer that will discharge to the existing storm sewer inlet. Runoff from hardscaping along the north side of Basin 1 and Basin 3 will drain to porous pavers located in Mountain Avenue. The hardscape areas along the east side of the building that are upstream of the rain gardens, will sheet flow to the rain gardens. Please refer to sheets C-013 of the civil utility plans for details on the porous paver areas. Rain garden details and the roof drain chases will be provided on the Landscape plans. B. Specific Details Basins 1 Basin 1 conveys runoff from approximately 80% of the north half of the roof, to a roof drain located near the northeast corner of the building. The building roof drain will connect to a proposed 12” HDPE storm pipe and discharge to the existing 20’ Type R inlet in Mountain Avenue. The hardscape between the building and the north property boundary was included with Basin 1 to simplify the basin delineation. This will slightly increase the design flow calculated for the roof area. The hardscape area north of the building will sheet flow to the flowline of Mountain Avenue then flow east to the existing Type R inlet. It should be noted that the site flow will combine with runoff in the right-of-way. Runoff will sheet over the proposed LID/porous pavers before entering the storm sewer system. Basin 2 Basins 2 will convey runoff from the south half of the building to a roof drain at the east side of the building. Flow will discharge to a proposed 20” wide concrete sidewalk chase then drain into a rain garden through a side opening in the chase. When the garden is inundated, overflow from the rain garden will drain back to the 10 chase then drain to the flowline in Mathews Street, flow north in the curb and gutter, and discharge to the existing 12-ft combination inlet as historically occurs. The hardscape between the building and the north property boundary was included with Basin 2 to simplify the basin delineation. This will slightly increase the design flow calculated for the roof area. The hardscape area east of the building and west of the rain garden, will sheet flow to the rain garden. The balance of the hardscape will sheet flow to the flowline of Mathews Street then flow north to the existing 12-ft combination inlet as historically occurs. Basin 3 Basin 3 will convey runoff from the eastern most 20% of the north half the building. The roof runoff will be conveyed to a roof drain at the east side of the building. The roof drain will connect to a proposed 12” HDPE storm sewer and discharge to the existing 12-ft combination inlet in Mathews Street. Overflow from the roof drain will discharge from a scupper into a proposed 20” wide, concrete sidewalk chase. Flow from the chase will drain into a rain garden through a side opening in the chase. When the garden is inundated, overflow from the chase will drain back to the chase and drain to the flowline in Mathews Street. Runoff will flow north in the curb and gutter, then discharge to the existing 12-ft combination inlet as historically occurs. The hardscape area between the building and the property boundary was included with Basin 3 to simplify the basin delineation. This will slightly increase the design flow calculated for the roof area. The hardscape area north of the building will sheet flow to the flowline of Mountain Avenue and flow east to the existing Type R inlet. The hardscape area east of the building and west of the rain garden, will sheet flow to the rain garden. The balance of the hardscape area will sheet flow to the flowline of Mathews Street then flow north to the existing 12-ft combination inlet as historically occurs. It’s important to note that because the proposed development is decreasing imperviousness, and there is limited available space, the runoff LID measures are placed within the right-of-way of Mountain Avenue and Mathews Street. The proposed rain garden and porous paver LID measures will increase the treated impervious area and provide additional water quality for runoff from site Basins 1, 2 and 3, which does not historically occur. Basin OS-1 The off-site Basin OS-1 will convey runoff from the public alley in a proposed 4’ wide, concrete trickle pan and discharge to the flowline of Mathews Street. Flow will drain north to the existing inlet on Mathews Street as historically occurs. The existing paved alley will be removed and replaced with outfall curb and gutter on the north and south sides of the alley and asphalt pavement to the trickle pan in the center 11 of the alley. The basin characteristic for ‘c’ value and imperviousness remain the same as the existing condition. Basin OS-2 The off-site Basin OS-2 will convey runoff from the opening between the proposed building and the existing public parking structure to the west. This area is currently gravel surfaced and will remain a gravel surface after construction. This basin drains from south to north. Excess runoff from the basin will flow over the existing sidewalk along Mountain Avenue, enter the existing curb and gutter and drain to the existing Type R inlet in Mountain Avenue as historically occurs. The basin characteristics for ‘c’ value and imperviousness remain the same as the existing condition. Overflow Conditions Basins 1 Should the roof drain plug in Basin 1 an overflow drain will discharge to a scupper and flow on the surface to the flowline of Mountain Avenue and enter the existing Type R inlet. If the inlet clogs, water will pond up to the crest of the intersection and flow east in Mountain Avenue. Basins 2-3 Should the roof drains clog within basins 2 and 3 flow will drain to the concrete chases and drain to the rain garden which will overflow to the flowline of Mathews Street. If the inlet clogs, water will pond up to the crest of the intersection and flow east in Mountain Avenue. C. Detention Pond The site imperviousness will be decreased with the proposed improvements and will not require detention. The addition of LID and BMP measures will be implemented to improve the water quality of runoff. Proposed and existing impervious area will flow overland across porous pavers in the north parking spaces on Mountain Avenue. Runoff from hardscape area north of the proposed building will combine with off-site runoff and receive treatment by the porous pavers. East of the proposed building, runoff will be directed to rain gardens. These measures will be provided to enhance water quality of on-site and off-site runoff which does not historically occur. 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 and the impervious area has been 12 reduced by this development. However, curb cut capacities have been analyzed and calculations provided in the Appendices 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 rain gardens and porous pavers. In addition to the site BMP’s, water quality treatment for 50% of the site is treated by the Udall Natural Area water treatment facility. These water quality features will provide a mechanism for pollutants to settle out of the stormwater runoff before flows are directed off site ultimately discharging to the Cache La Poudre River, as historically occurs. B. Specific Details Best Management Practices (BMP) for the treatment of storm water runoff has been incorporated into the final design for this project. LID measures are provided for through the conveyance of approximately 0.21 acres (of the 0.45 acre total site area) through the rain garden which accounts for 47% of the total site. This is considering only Basin 2, since Basin 3 roof drainage will treat only the overflow runoff. The hardscape north of the building, 1,254 square feet of area (6.4% of the total site) is being conveyed to the porous paver LID in Mountain Avenue. A total of 53% of the total site area is treated, thus exceeding the City’s LID treatment of 50% of new impervious area. It’s important to note that because the area being routed over and through the rain garden and porous pavers exceeds the 50% requirement for LID treatment, and because imperviousness for the overall site has been reduced by the proposed development, the project exceeds that normally required for water quality and LID treatment. This treatment is beyond that already being provided for 50% of the site by the Udall Natural Area water treatment facility. Please refer to sheet C-013 of the civil utility plans, for details on the porous pavers. 13 VI. EROSION CONTROL A. General Concept 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 and east sides of the site to prevent sediment from leaving the site. Where silt fence is shown to cross hard surface areas, gravel inlet socks shall be utilized for these areas, as may be applicable during construction. A tracking pad will also be placed in the alley access at Mathews Street, gravel inlet filters will be placed at existing storm inlets and at the proposed sidewalk chases to mitigate the build-up of sediment and transport of debris into existing streets and downstream systems. 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 Stormwater 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, curb cuts and concrete sidewalk culverts/chases, structures, manholes, and the water quality/ outlet control structures (as applicable) 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 (as may be applicable) or downstream systems 2) Sedimentation/ silting shall be removed to allow for adequate drainage along the bottom of the swales and LID/ porous landscape detention basins. Grass scrubbing along the bottom of the LID/ porous landscape detention basins 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 rain garden LID’s and porous pavers and to adjacent streets and existing and proposed storm sewer systems. The proposed roof drains will flow by surface and storm sewer systems which will connect to the existing, storm inlets in Mountain Avenue and Mathews Street. The proposed storm sewer will provide for the 100-year developed flows to be conveyed to existing storm systems, while increasing water quality for the overall site. 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 rain garden LID and porous pavers with underdrain systems for the proposed measures provide for additional water quality and treatment of runoff, prior to flows being 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 $5,141. Please refer to Appendix E for this calculation. 15 VIII. 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. 16 APPENDIX 17 APPENDIX A VICINITY MAP 18 APPENDIX B RATIONAL METHOD HYDROLOGY 19 APPENDIX C LID BASIN SIZING CALCULATIONS 20 APPENDIX D CURB CUT & STORM PIPE SIZING 21 APPENDIX E EROSION CONTROL & EROSION ESCROW 22 APPENDIX F PROPOSED DRAINAGE BASIN MAP & EXISTING DRAINAGE BASIN MAP