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HomeMy WebLinkAboutWEST VINE STORAGE - PDP - PDP170016 - SUBMITTAL DOCUMENTS - ROUND 2 - EROSION CONTROL LETTER/REPORTPRELIMINARY STORMWATER & EROSION CONTROL REPORT FOR WEST VINE STORAGE 1050 WEST VINE DRIVE FORT COLLINS, COLORADO 80521 Project Number 2700-17 September 18, 2017 Prepared for: Hattman Associates Architecture & Planning 524 South Loomis Ave Fort Collins, CO 80521 Prepared by: Forbes Engineering 2908 Cherly Street Fort Collins, CO 80524 Phone: (970) 295-4874 TABLE OF CONTENTS PAGE CERTIFICATION 1 I. GENERAL LOCATION AND DESCRIPTION A. Site Location 2 B. Description of Property 2 C. Flood Hazards 2 II. DRAINAGE BASINS AND SUB-BASINS A. Major Basin Description 3 B. Sub-basin Description 3 III. DRAINAGE ANALYSIS CRITERIA A. Regulations 3 B. Hydrological Criteria 4 IV. DRAINAGE FACILITY & DESIGN A. General Proposed Concept 4 B. Specific Drainage Design Details 4-5 V. EROSION CONTROL DESIGN A. General Proposed Concept 5-6 B. Specific Erosion Control Details 6-7 VI. CONCLUSIONS 7-8 VII. REFERENCES 8 VIII. APPENDICES 9 1 CERTIFICATION: I hereby certify that this preliminary stormwater plan & report for the West Vine Storage site development located at 1050 West Vine Avenue was prepared by me and meets or exceeds the criteria in the City of Fort Collins Stormwater Management Manual. _______________________________________ Kevin Forbes Registered Professional Engineer State of Colorado No. 35265 2 I. GENERAL LOCATION AND DESCRIPTION A. Site Location & Project Description The West Vine Storage Development is located in the Northeast quarter of Section 14, Township 6 North, Range 69 West of the 6th Principal Meridian, County of Larimer, State of Colorado. The property lies at the northeast corner of West Vine Drive and North Shield Street intersection. See provided Vicinity Map in the Appendix. The project will consist of the construction of 14 storage buildings along with necessary concrete drive aisles and aprons. There will also be an office with an attached apartment. B. Description of Property The property area is approximately 183,264 square feet (4.21 acres). Required right-of-ways for North Shields Street and West Vine Drive have been previously dedicated with the original plat of the Synder Minor Subdivision. The site is currently zoned for mixed use and has two businesses and a single family residence on site (see sheet D1 in the report insert for existing conditions). The property is bordered by the Arthur Ditch to the north, by a single family residence to the east and by North Shields Street and West Vine Drive to the west and south. A gas station and convenience store is located at the southwest corner of the site which borders two sides of the property (see provided existing conditions map). Other than a very small portion of landscaping located along the common property boundary with the gas station, the site has a consistent & slight slope towards the north (approximately 1%). Currently there isn’t any detention on the site although it appears that because of the shallow grade there has been an uncertain amount of surface run-off that pools along the south embankment of the Arthur Ditch. The soil classification for the property consists of a Haplustolls-Baller-Rock (1-2% slopes) along the west side and a Stoneham loam (1-2% slopes) on the east portion. Both belong to the hydrologic soil group B, as assigned by the Natural Resources Conservation Service. Run-off is moderate and the hazards of wind and water erosion are slight to moderate. Depth to seasonal high water table is greater than 6 feet. 3 C. Flood Hazards The Site does not lie within any floodplain. No known drainage studies exist for this Site and there are no wetlands associated with the Site. II. DRAINAGE BASIN AND SUB-BASINS A. Major Drainage Basin The property is shown as being located within the Old Town drainage basin but drains into the Arthur Ditch which has diversion capabilities to drain into the Cache La Poudre basin. No adverse effects to Arthur Ditch or to the Cache La Poudre drainage basin are anticipated with this development. B. Existing Sub-Basins The existing drainage for the site is broken up into 2 sub-basins (see historic drainage sub-basin maps in the appendix). Sub-basin A contains approximately 60% of the site and is located in the western portion of the property. It is fairly flat with an average slope of 1% towards the north. The outfall is along the south side of Arthur Ditch. Sub-basin B lies on the east side of the property and also slopes northeast at an approximate grade of 1%. The outfall for Sub-basin B is also along the south side of and into Arthur Ditch. The difference between the historic and developed sub-basins concerns the areas of the sub-basins only with a larger portion directed towards and into Sub-basin A as a result of the new building roof lines. III. DRAINAGE ANALYSIS CRITERIA A. Regulations The analysis of the storm drainage system for this project was prepared in accordance with the criteria set forth in the City of Fort Collins Stormwater Criteria Manual and the Urban Drainage and Flood Control District’s Urban Storm Drainage Criteria Manual (USDCM). 4 B. Hydrological Criteria 1. Design Rainfall Intensity-Duration-Frequency Table for City of Fort Collins 2. Runoff Method Rational 3. Design Storms a. Minor 2-year b. Major 100-year IV. DRAINAGE FACILITY & DESIGN A. General Proposed Concept The proposed drainage concept is to utilize the natural grade of the site and to release into Arthur Ditch at the historic 2-year rate. Prior to entry into the ditch, stormwater will be directed via grass swales and concrete pans into the low impact drainage structures (rain gardens) for water pre-treatment. From there it will then enter either the west or east detention ponds (note, the east detention pond utilizes two rain gardens for pre-treatment). Each detention pond will have a water quality outlet structure (WQOS) that will be required to have a trash rack, water quality plate and an emergency over flow spillway. Each will also be required to have all of the necessary erosion controls at the outlet structure (i.e. rip-rap, ect.). Additional volume in the detention pond has been added to account for the water quality capture volume. The WQCV has been calculated at 1871 cubic feet for the west pond and 1453 cubic feet for the east pond. B. Specific Drainage Design Details As outlined in the proposed site improvements, both drainage basins will be paved for vehicle access. The pavement between storage buildings will have a shallow concrete gutter with a centerline grade of approximately 0.5%. This is steep enough to create good drainage but flat enough not to erode the centerline of the channel. 5 C. Four Step Process The four step process will be used to minimize adverse impacts. This will involve the following; Step 1. Employ Runoff Reduction Practices – Reduction of runoff peaks, volumes, and pollutant loads as the site is developed by implementing Low- Impact Development (LID) facilities including; • Increase time of concentration by routing flows along the drive aisles and increasing the length of the flow path. • Routing runoff from the proposed site into rain gardens to promote infiltration and biological uptake. • Routing flows through filter material in the rain gardens to increase time of concentration promote infiltration and provide initial water quality. Step 2. Implement BMP’s that provide a water quality capture volume with slow release – The efforts outlined in Step 1 will facilitate the reduction of runoff; however, this development will also require water quality. Runoff will be routed through the rain gardens and then to a pond that will provide detention prior to release into the Arthur Ditch to reduce the amount of runoff generated from the site through infiltration and also treat the runoff prior to releasing is from the site. Water quality extended detention for this site will occur within each pond. Step 3. Stabilize Streams – Acceptance from the Arthur Ditch Company to convey storm water discharge into Arthur Ditch has been formally addressed and will be finalized prior to construction. The south bank of Arthur Ditch will be stabilized using a type L rip-rap at both the end of pipe discharge points as well as along the ditch bank. Step 4. Implement Site Specific and Other Source Control BMPs – This step applies to covering storage/handling areas and spill containment and control. Vehicle maintenance and washing will be completed off-site to prevent potential spills of vehicle fluids. Spills will be contained immediately and cleaned up using appropriate absorption materials. 6 V. EROSION & SEDIMENT CONTROL A. General Proposed Concept Volumes and Areas Impacted The proposed development will disturb almost all of the site and a portion of the north side of West Vine Drive. This has been calculated at approximately 183,142 square feet (4.2 ac) on-site and approximately 11,294 square feet (0.26 ac) off-site. Some of the Site will be graded with topsoil & gravel stripped and stockpiled. The total quantity of cut from the detention ponds has been estimated to be 1972 cubic yards. Because the required amount of fill for the site improvements, no exported soil from the Site is anticipated. Erosion and Sediment Control Measures The erosion control measures for the site are shown on the Grading, Drainage and Erosion Control Plan (Sheet C1) & Erosion Control Details (Sheet C2). A vehicle tracking control device will be install at the entrance for the site prior to any construction. A sediment control fence will be installed along the full perimeter. Along with these erosion control measures, permanent seeding will be required along the disturbed slopes and fill areas south of the proposed buildings to protect the slope. A concrete washout area will be provided for on-site and located either at the west side or east side of the site. Also if needed, any and all stock piles for excess cut shall be maintained with a silt fence around its base. Other graded areas will be left in rough surface condition until seeded. Timing & Sequence of BMP Installation Timing sequence of BMP installation is as follows: • Install perimeter controls and construction VTC at site entrance. • Detention pond with outlet structures, outlet protection (rock filter), and downstream inlet protection (rock socks). 7 • Permanent seeding in detention areas. • Site Grading, subgrade preparation and foundation excavation. • Install base course and rough surface all exposed soil areas. • Utility installation & construction – (Domestic water service, sanitary sewer service, power, gas, ect.) • Footings & building pad foundation installation. • Paving and landscaping. • Removal of Erosion Control Measures. Stormwater Management Plan Administrator The owners of the property, shall act as the stormwater management plan administrator. Potential Pollutant Sources Disturbed and stored soils. Sediment generation due to disturbed areas (grading & clearing) is anticipated and will be controlled using the lower portion (flat area south of the barrow ditch) of the drainage cut-of swale as a sediment trap. The sediment trap will be cleaned out as required. Stored soils are not anticipated but if so then the upper portion of the site (southwest end) will be used to stock pile soil and or rock. The stock piles will be limited to 10’ maximum in height and will be required to have downstream toe protection using silt barriers. Vehicle tracking of sediment. As noted previously a VTC will be installed at the entrance to the site. This along West Vine Drive (see sheet 4.0 for location). Management of contaminated soils. In the case of a release of fuel or other chemicals, absorbent booms or earthen berms will be immediately constructed to contain the spill and prevent runoff to adjacent surface waters. 8 The contractor will be responsible for the cleanup of the released materials and for the removal of any contaminated soils. Loading & unloading operations. These operations will be executed on- site south of the proposed buildings. Out-door storage. No out-door chemical storage is anticipated for this project, out-door storage will be limited to building materials and stored near the construction site. Equipment maintenance and fueling. During construction, no on-site equipment fueling or equipment maintenance is anticipated. Significant particulate generating. Due to the sandy loam nature of the site soil, it is not anticipated to have a significant particulate generation, but if so the construction manager is to bring in a water truck to abate the dust generation. Routine maintenance activities & on-site waste management. Good site house-keeping procedures including routine cleanup of construction debris and trash removal will be regularly maintained. Concrete washing. A dedicated concrete washout area will be constructed prior to placement of any concrete. Dedicated asphalt or concrete batch plants. Neither of these construction activities is anticipated for this site. Non-industrial waste sources. Both an individual worker trash roll-off as well as portable toilets are anticipated to be installed on this site. The portable toilets shall be anchored and located away from the drainage swale. The location of the roll-off will be up to the contractor, probably set up near the east access drive where it can be easily emptied or removed. Dry Land Vegetation The percentage of vegetative ground cover prior to construction is estimated to be approximately 80 percent. Vegetation includes sparse native and non- native grasses. 9 Per the soil survey of Larimer County by the US Department of Agriculture Soil Conservation Service, the majority of the property is a Ascalon Loam (SCS 8). The Ascalon Loam is in Hydrologic group B. Run-off is medium and the hazards of wind and water erosion are moderate. Depth to seasonal high water table is greater than 6 feet. The seed mix shall be a dryland seed mix and shall conform to the City of Fort Collins standards and specifications. At 5 lbs. per 1000 square feet, the amount of permanent seeding has been estimated at 330 lbs. Mulching of bare soils will be required in areas of steep slopes (along the north side of the swale and along the south side of the building pads). These areas will be checked regularly and areas where it was lost or damage will be re-mulched within seven working days when needed. Detailed Sequence of Construction Activities Final Stabilization Final stabilization at this site includes: • Building and graveled areas • A grass-lined storm water cut-off swale • Re-vegetation of disturbed areas with dryland seed mix Note that no soil amendments are associated with the project and with the reseeding the re-vegetative density will be 85% of the original vegetative coverage. Maintenance The General Contractor (GC) will perform a stormwater inspection of the site at least once per week during construction activities as well as within 24 hours of a storm event. The inspection will be documented on an inspection form appropriate for the Site. As stormwater maintenance issues are identified, the SWMP administrator will direct GC and sub-contractor personnel to execute the appropriate maintenance in a timely fashion. Documentation of the following events will be generated as they occur: 10 • Records of spills, leaks, or overflows, including time and date, weather conditions, etc. • Implementation of specific items in the SWMP • Contacts with regulatory agencies and personnel Along with inspections; the lower end of the drainage swale and the silt fence will be cleaned of excessive sediment when necessary. Mulching of bare soils will be checked regularly and areas where it was lost or damage will be re-mulched within seven working days when needed. Cost Estimate The estimated cost of the erosion control measures is $5,942.00; the total erosion control escrow has been calculated at $8,913.00. A detailed spreadsheet is attached to this report. VI. CONCLUSIONS Stormwater Management Conclusion A high water (no build) line has been added to the drainage map that delineates the 100 year watershed elevation with a 1/2 foot freeboard. The closest building is approximately 20 feet away & 1.2 feet above this line and will not be affected by the 100 year storm water detention or run-off. The analysis of the storm drainage system for this project was completed in accordance with the criteria set forth in the City of Fort Collins Stormwater Criteria Manual and the USDCM. No adverse affects to surrounding properties are anticipated from the proposed improvement of this site. Erosion Control Conclusion The Erosion and Sediment Control Plan will be placed in the City of Fort Collins file for this project. The plan will fulfill the City of Fort Collins criteria for erosion control. It is understood that additional erosion control 11 measures may be needed if unforeseen erosion problems occur or if the submitted plan does not function as intended. The requirements of this plan shall run with the land and be the obligation of the land owner until such time as the plan is properly completed, modified or voided. Details for the erosion control measures can be found on sheets C4.0 and C4.1 of the construction plans. VII. REFERENCES 1. Urban Storm Drainage Criteria Manual Volumes 1 and 2, Urban Drainage and Flood Control District, June 2001, and Volume 3, September 1999. 2. City of Fort Collins Stormwater Criteria Manual, City of Fort Collins Stormwater Department, 2009. 3. Soil Survey of Larimer County Area, Colorado, U.S. Department of Agriculture, Soil Conservation Service and Forest Service, December, 1980. 12 VIII. APPENDICES 1. Vicinity Map Existing Site Conditions Existing Drainage Sub-basins 2. Proposed Site Development Layout Developed Drainage Sub-basins 3. Existing Stormwater Run-off Calculations Proposed Stormwater Run-off Calculations 4. Detention Pond Volume Calculations 5. Pipe Capacity Calculations 6. Erosion Control Worksheets 7. Tables & Figures 8. Erosion & Sediment Control Escrow Calculations 9. Developed Drainage Plan (insert) 10. Erosion Control Plans (insert)