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HomeMy WebLinkAboutHEARTHFIRE PUD, 1ST FILING - FINAL - 31-95D - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPage 13 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing VIL EROSION CONTROL SECURITY DEPOSIT: 3. The erosion control security deposit is reimbursable. VIII. VARIANCE FROM CITY STANDARDS A. Variance from City of Fort Collins requirements 1. There will be no requests for variances from City of Fort Collins Stormwater Utility Standards. IX. CONCLUSIONS A. Compliance With Standards 1. The grading and drainage design for Hearthfire P.U.D., First Filing is in compliance with the City of Fort Collins storm drainage design criteria. 2. The erosion control measures shown on the erosion control plan comply with the City of Fort Collins standards and generally accepted erosion control practices. B. Drainage Concept 1. The proposed drainage design for Hearthfire P.U.D. First Filing is effective for the control of storm runoff with a considerable reduction in potential downstream effects. X. REFERENCES 1. City of Fort Collins "Storm Drainage Design Criteria and Construction Standards'; May, 1984 2. City of Fort Collins "Erosion Control Reference Manual"; January, 1991 3. Preliminary Drainage and Erosion Control Report for Hearthfire at Richards Lake; Prepared by Merrick & Company; Project no. 15011782; Dated July 10,1996 Page 12 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing VL EROSION CONTROL: A. General Concept 1. Erosion control measures are specified on the Drainage and Erosion Control Plan. a. Maintenance of erosion control devices will remain the responsibility of the contractor and the owner until the project is complete. VL EROSION CONTROL: B. Specific Details 1. The following measures are specified on the Drainage and Erosion Control plan: a. Area inlet filters composed of haybales around all area inlets. b. Silt fence along the downstream property lines, particularly along Richards Lake. c. Gravel inlet filters at all storm sewer inlets. d. Haybale dikes within all swales. VIL EROSION CONTROL SECURITY DEPOSIT: An erosion control security deposit is required in accordance with City of Fort Collins policy (Chapter 7, Section C : SECURITY; page 7.23 of the City of Fort Collins Development Manual). In no instance shall the amount of the security be less than $1000.00. 1. According to current City of Fort Collins policy, the erosion control security deposit is figured based on the larger amount of 1.5 times the estimated cost of installing the approved erosion control measures or 1.5 times the cost to re -vegetate the anticipated area to be disturbed by construction activity. i The cost to install the proposed erosion control measures is approximately $10,500.00. Refer to the cost estimate attached in Appendix I. 1.5 times the cost to install the erosion control measures is $15,750.00 ii. Based on current data provided by the City of Fort Collins Stormwater Utility, and based on an actual anticipated net affected area which will be disturbed by construction activity (approximately 40 acres), we estimate that the cost to re -vegetate the disturbed area will be $20,000 ($ 500 per acre x 40 acres). 1.5 times the cost to re -vegetate the disturbed area is $30,000.00 The $500.00 per acre for re -seeding sites greater than 10 acres was quoted to us by City of Fort Collins Stormwater Utility personnel. 2. The erosion control security deposit amount required for Hearthfire P.U.D., First Filing will be $30,000.00 See the Erosion Control Security Deposit Requirements document located in Appendix H. Page 10 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing IV DRAINAGE FACILITY DESIGN B. Specific Details - Storm Sewer (continued) 11. The table below summarizes the pipe capacities of the various storm sewers. Mannings n for RCP and ADS N-12 pipe are 0.013 and 0.012 respectively. Profile Pipe Slope Mannings Capacity From To Street Diam. n ft. ft/ft cfs A 1.5 0.005 0.013 7.4 Inlet Inlet Hearthfire Way 2.0 0.005 0.012 17.3 Inlet Pond 1 B 1.5 0.000 0.013 NA Pond 1 Pond 1 Hearthfire Way C 2.0 0.02 0.012 34.6 Pond 1 Inlet Hearthfire Drive 2.0 0.04 0.013 45.2 Inlet Inlet 2.5 0.01 0.012 44.4 Inlet Pond 2 D 1.5 0.0833 0.012 32.8 Inlet MH Green Wing Ct. 1.5 0.02 0.012 34.6 MH Pond 2 E 1.5 0.07 0.012 30.1 Inlet MH Blue Heron Ct. 1.5 0.02 0.012 16.1 MH Pond 2 F 2.0 0.01 0.012 24.5 Inlet Pond 2 Hearthfire Place G 1.5 0.005 0.012 8.0 Inlet Pond 1 Town Center Dr. H 2.0 0.02 0.012 34.6 Pond 1 Inlet Town Center Dr. 2.5 0.01 0.013 41.0 Inlet Inlet 2.5 0.01 0.012 44.4 Inlet Pond 2 I 1.5 0.02 0.012 16.1 Inlet Lake Town Center Ct. J 1.5 0.04 0.012 49.0 Inlet Lake Barn Swallow Cir. K 1.5 0.01 0.013 10.5 Inlet Inlet Waxwing Lane 2.0 0.02 0.012 36.6 Inlet Lake C. Specific Details - Overflow Structures An 6' x 6' area inlet will act as the overflow structure from the wetland area on the north side of Hearthfire Drive (Pond 1). The structure will act like a 20 foot broadcrested weir because the inlet has 6" thick walls. The top of the inlet will control the water level in Pond 1 and maximize the storage in Pond 2. a. Top of area inlet elevation = 5108 ft. b. Pipe Diameter = 2.0 ft. c. Slope of Pipe = 0.02 ft/ft d. Pipe capacity (HW/D =1.0) = 34.6 cfs Page 9 Final Drainage and Erosion Control Report Hearthf re P.U.D., First Filing IV DRAINAGE FACILITY DESIGN B. Specific Details - Storm Sewer (continued) 10. Storm sewer (Profile K) will be installed at a low point on Barn Swallow Circle. The storm sewer consists of the following; a. 1 - 12' wide valley pan at the low point to convey stormwater across the street b. 1 - 10' Type R inlet c. 143 lineal feet of 24" ADS N-12 storm sewer which conveys stormwater to Richards Lake d. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer. Refer to the profile for riprap dimensions. 11. Storm sewer (Profile L) will be installed at a low point on Waxwing Lane. The storm sewer consists of the following; a. 2 - 10' Type R inlets b. Approximately 45 lineal feet of 18" RCP storm sewer between the inlets c. 163 lineal feet of 24" ADS N-12 storm sewer which conveys stormwater to Richards Lake d. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer. Page 8 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing IV DRAINAGE FACILITY DESIGN B. Specific Details - Storm Sewer (continued) 6. Storm sewer (Profile H) will be installed in continuous grade condition on Town Center Drive to intercept some of the flows and divert them into the on site wetland area (Pond 2). The storm sewer consists of the following; a. 2 - 10' Type R inlets b. 31.67 lineal feet of 18" RCP storm sewer connecting the inlets c. 203 lineal feet of 24" ADS N-12 storm sewer which conveys stormwater to the wetland area (Pond 2) d. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer. Refer to the profile for riprap dimensions. 7. Storm sewer (Profile I) will be installed at a low point on Town Center Drive at the west end of the First Filing. This storm sewer will convey street flows to Richard's Lake and act as the emergency overflow from the wetland area (Pond 1). The storm sewer consists of the following; . a. 195 lineal feet of 24" ADS N-12 from Pond 2 to the inlet on the north side of Town Center Drive b. 2 - 10' Type R inlets c. 31 .67 lineal feet of 18" RCP storm sewer between the inlets d. 103 lineal feet of 24" ADS N-12 storm sewer which conveys stormwater to Richard's Lake. e. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer. 8. Storm sewer (Profile n will be installed at a low point on Town Center Court. The storm sewer consists of the following; a. 1 - 5' Type R inlet b. 163 lineal feet of 24" ADS N-12 storm sewer which conveys stormwater to Richard's Lake c. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer. Refer to the profile for riprap dimensions. Page 7 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing IV DRAINAGE FACILITY DESIGN B. Specific Details - Storm Sewer (continued) 3. Two sidewalk culverts will be installed at a low point (Sta. 10+71.75) on Hearthfire Way. The culverts will convey the stormwater from the streets to the wetlands. Sidewalk culverts have been used because the proposed road elevation is essentially at grade. Installation of a storm sewer in this area is not considered feasible and would result in more disturbance of the wetlands. 4. Storm sewer (Profile C) will be installed at a low point on Hearthfire Drive. This storm sewer will convey street flows to Pond 2 and act as the emergency overflow from the offsite wetland area (Pond 1). The storm sewer consists of the following; a. A 6 x 6 area inlet on the north side of Hearthfire Drive which will control the water surface elevation in pond 1 b. 60 lineal feet of 18" ADS N-12 from Pond 1 to the inlet on the north side of Hearthfire Drive c. 2 - 10' Type R inlets d. 41.33 lineal feet of 24" RCP storm sewer between the inlets e. 183 lineal feet of 30" ADS N-12 storm sewer from the inlet on the south side to Pond 2 f. Riprap with a D50 of 12 inches will be installed at the outfall of the storm sewer into Pond 2 5. The following Cul-de-sacs will have a 5' Type "R" inlet installed at the low point of the cul- de-sac. The stormwater is conveyed to the wetlands in Tract A by ADS N-12 pipes and by grass lined swales. Refer to the storm sewer profiles provided with the Final Utility Plans for more information. a. Green Wing Court (Profile D) b. Blue Heron Court (Profile E) c. Hearthfire Place (Profile F) Page 6 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing IV DRAINAGE FACILITY DESIGN A. General Concept 1. The majority of the site (sub -basins A-B ) will be contributing stormwater to the two wetland / pond areas ( Pond 1 and 2) via a combination of: a. overland flow b. gutter flow c. storm sewer flow The grading of the site attempts to divert as much of the runoff into the wetland areas as possible. This grading concept will continue with future filings. 2. The remainder of the site (sub -basins C-F) will contribute stormwater directly to Richard's Lake or Douglas Road via the same methods of conveyance mentioned in section W.A.1. 3. Water quality will be addressed in the design of the outfalls of the storm sewer into Richard's Lake. Refer to the Wetlands Mitigation Report prepared by T.R. Boss Associates. 4. Emergency overflow structures will be designed to pass any stormwater that exceeds the storage capacity of the wetland / pond areas safely to Richard's Lake. 5. All swales that have slopes less than 2.0 percent will have 3' valley pans installed at the flow line of the swale. B. Specific Details - Storm Sewer 1. A Storm sewer (Profile A) will be installed at a low point (Centerline Sta. 4+33.74) on Hearthfire Way. The storm sewer consists of the following; a. 2 - 10' Type R inlets b. 41.33 lineal feet of 18" RCP storm sewer between the inlets c. 30 lineal feet of 24" ADS N-12 from the inlet on the west side of the Hearthfire Way to the offsite wetlands (Pond 1). 2. An 18" RCP culvert (Profile B) will act as an inverted siphon connecting the wetlands (Pond 1) on either side of Hearthfire Way. The culvert will have zero (0.00%) slope because it's purpose is to provide a means to maintain a consistent water level on both sides of the road. Water will only pass through the culvert if the water level on one side of the culvert is higher than the water level on the other side. Area inlets with trash racks will be constructed at each end of the culvert. Page 5 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing III DRAINAGE DESIGN CRITERIA A. Regulations 1. This final report and the Master Grading, Drainage and Erosion Control Plans for Hearthfire P.U.D., First Filing were prepared in accordance with the requirements of the current City of Fort Collins Storm Drainage Design Criteria and Erosion Control Criteria. B. Development Criteria Reference and Constraints 1. Douglas Road is planned as a minor arterial according to the recently adopted new City Street standards. Douglas Road is currently built to County Road standards and will not be constructed to full minor arterial standards adjacent to the north boundary of the project. 2. The existing wetland areas retain much of the stormwater that flows to them except in cases where the 100-year storm event is exceeded. Emergency overflow structures are necessary to convey the excess stormwater out of the wetland areas to Richard's Lake. 3. The design of the drainage infrastructure considers the development of future filings that will occur on this property in terms of their contribution to the drainage infrastructure. 4. East Larimer County (ELCO) water district has an underground storage tank along the eastern property line of the site. This tank is flushed occasionally according to ELCO personnel. Provisions will have to be made to convey the water from the tank through the site to Richard's Lake safely. C. Hydrologic Criteria 1. Runoff calculations at various design points are based on the "Rational" method. The 2, 10, and 100-year storms have been analyzed. 2. No detention is proposed with this subdivision because the existing wetland areas will act as retention ponds. 3. Emergency overflow structures have been designed to convey flows that exceed the capacity of the retention (wetland) areas. D. Hydraulic Criteria 1. Storm sewer inlet design is based on the inlet curves provided in the City of Fort Collins Drainage Criteria Manual. 2. Storm sewer design is based on Mannings Equation with Mannings coefficients as suggested in the City of Fort Collins Drainage Criteria Manual. Page 4 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing H DRAINAGE BASINS AND SUB -BASINS D. Developed Conditions 1 The developed site increases the contributing area to ponds 1 & 2 for the purpose of water quality. Proposed grading has eliminated sub -basin E and substantially reduced the size of sub -basin D. The table below summarizes the sub -basins, their total area and the immediate destination of the flow from them. Area Sub -basin acres Flow Destination A 53.17 Pond 1 B 73.05 Pond 2 C 27.13 Richard's Lake D 1.66 Richard's Lake via Richard's Lake PUD F 12.16 Douglas Road and east The total area of the sub -basins is 167.17 acres. 2. The basin delineation for future filings assumes that the design intent will be the same as for the First Filing. 3 The table below summarizes the peak flows from the developed sub -basins. Sub -basin F is unchanged from the historic conditions by the development of this filing or future filings. Sub -basin Q2 cfs cfs Q100 Notes A 14.4 53.1 Retained in Pond 1 B 76.9 271.3 Retained in Pond 2 C 36.7 129.7 Richards Lake D 3.9 13.9 Richards Lake F 8.3 30.5 Douglas Road Page 3 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing II DRAINAGE BASINS AND SUB -BASINS C. Historic Conditions 1. The basin can be divided into 6 historic sub -basins designated A-F. These basins are undeveloped with a few active oil wells located on them as well as some wetland areas. We are utilizing the "C" factor for sandy soils with an average slope because of the wetlands present for most of the basin. We have assumed a "C" factor of 0.45 for Sub -basin F which consists of large estate lots in Cherrywood Acres. The table below summarizes the sub - basins, their total area and the immediate destination of the flow from them. Area Sub -basin acres Flow Destination A 50.53 Pond 1 B 57.59 Pond 2 C 40.50 Richard's Lake D 10.64 Richard's Lake via Richard's Lake PUD E 13.75 Serramonte Highlands F 12.16 Douglas Road and east The total area of the sub -basins is 185.17 acres. Sub -basins A-E contribute stormwater to Richard's Lake. 2. The table below summarizes the peak flows from the historic sub -basins. Q2 Q100 Sub -basin cfs cfs Notes A 7.9 29.1 Retained in Pond 1 B 9.5 35.1 Retained in Pond 2 C 7.7 28.6 Richard's Lake D 1.9 7.4 Richard's Lake E 3.4 12.4 Serramonte Highlands F 8.3 30.5 Douglas Road Page 2 Final Drainage and Erosion Control Report Hearthftre P.U.D., First Filing II DRAINAGE BASINS AND SUB -BASINS A. Major Basin Description 1. The site is located in the Dry Creek Basin as delineated on the City of Fort Collins Stormwater Basin Map. 2. This portion of the Dry Creek Basin is partially developed with large single family lots and medium sized ranches. B. Sub -Basin Description 1. The site topography is best described as rolling. There are several ridges on the site which create several sub -basins. The site generally slopes from the northeast to the southwest at an average rate of 0.04 ft/ft (4.0%). 2. The basin is defined by Douglas Road to the north, County Road 13 to the west and Richard's Lake to the south. The eastern property line of the site is the eastern boundary of the basin. 3. There are two (2) existing wetlands / ponding areas on the site which intercept much of the runoff from the site and retain it. They are designated Pond 1 and Pond 2 on the Drainage and Erosion Control Plan, although are not necessarily presented as detention ponds. a. Pond 1 is crossed by the proposed access road (Hearthfire Way) from Douglas Road. b. Pond 2 is the wetland area that is within the limits of the First Filing. The stormwater that is intercepted by the wetlands either percolates into the soil or evaporates. An exception to this occurs when storm events exceed the 100-year event and / or the storage capacity of the low areas are exceeded. The stormwater eventually overflows into Richard's Lake. 4. There are some offsite contributions from the rear lots of Cherrywood Acres onto the site. The majority of Cherrywood Acres contributes runoff to a Swale on the south side of Douglas Road. 5. Richard's Lake is the ultimate destination of all runoff from the site including any water that exceeds the storage capacity of the wetland / pond areas. Page 1 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing L GENERAL LOCATION AND DESCRIPTION A. Property Location Hearthfire P.U.D., First Filing is located in the West one-half (1/2) of Section 30, Township 8 North, Range 68 West of the 6th P.M., Larimer County, Colorado. 2. More specifically, it is located on the on the south side of Douglas Road (County Road 54), approximately 1.5 miles east of the intersection of Douglas Road and State Highway 1. 3. The site is bounded on the west County Road 13, on the north by Douglas Road and Cherrywood Acres, on the east by Serramonte Highlands and Richards Lake P.U.D. and on the south by Richard's Lake. 4. Richard's Lake is located immediately south of Hearthfire P.U.D. Storm runoff from the site has historically entered Richard's Lake and will continue to do so with the development of Hearthfire P.U.D. a. Richard's Lake is owned and operated by The Water Supply and Storage Company. b. The Water Supply and Storage Company has indicated that they will accept undetained flows from Hearthfire P.U.D. in a manner similar to historic acceptance. i. A statement has been provided with the Hearthfire P.U.D, Fast Filing Utility Plans and Final Plat indicating the reservoir company's acceptance of undetained flows to Richard's Lake. ii. The Water Supply and Storage Company has provided a letter indicating their intent to accept undetained flows to Richards Lake. A copy of this letter is attached to this report and is found in Appendix IV. 5. Approval of Hearthfire P.U.D., First Filing by The Water Supply and Storage Company will be required. B. Description of the Property 1. Hearthfire P.U.D., First Filing is a proposed residential subdivision in the City of Fort Collins, Colorado. Hearthfire P.U.D., First Filing will consist of a total of 91 lots on approximately 66 acres. a. The overall subdivision will consist of approximately 146 single family homes on approximately 105 acres. 2. Hearthfire P.U.D. First Filing has a platted area of approximately 66 acres. This includes the wetlands. 3. The site is currently vacant and is covered with native vegetation and some wetlands. 4. There are some existing oil wells on the site. November 25, 1996 Project No: 1552-01-96 Basil Harridan City of Ft. Collins Storm Water Utility P.O. Box 580 Ft. Collins, Colorado 80524 Re: Hearthfire P.U.D., First Filing; Ft. Collins, Colorado Dear Basil, Enclosed please find the Final Drainage and Erosion Report and Plans for Hearthfire P.U.D., First Filing. The hydrology data and the hydraulic analysis presented in this report complies with the requirements of the City of Fort Collins Storm Drainage Criteria Manual; dated March, 1984, and the Erosion Control Reference Manual. If you have any questions or comments, please call me at 226-5334. Sincerely, Brian W. Shear Shear Engineering Corporation BWS / meo cc: Richards Lake Development Company 4836 S. College, Suite 12 Ft. Collins, CO 80525 (970) 226-5334 FAX (970) 282-031 1 FINAL DRAINAGE AND EROSION CONTROL REPORT for HEARTHFIRE P.U.D., FIRST FILING Ft. Collins, Colorado Prepared for: RICHARDS LAKE DEVELOPMENT COMPANY 5319 Paradise Lane Fort Collins, Colorado 80526 Prepared By: SHEAR ENGINEERING CORPORATION Project No: 1552-01-96 Date: November, 1996 INITIAL SUBMITTAL. 4836 S. College, Suite 12 Ft. Collins, CO 80525 (970) 226-5334 FAX (970) 282-031 1 Page 11 Final Drainage and Erosion Control Report Hearthfire P.U.D., First Filing IV DRAINAGE FACILITY DESIGN B. Specific Details - Overflow Structures (continued) 2. An 6' x 6' area inlet will act as the overflow structure from the wetland area on the north side of Town Center Drive. The structure will act like a 20 foot broadcrested weir because the inlet has 6" thick walls. The top of the inlet will control the water level in Pond 1 and maximize the storage in Pond 1. a. Top of area inlet elevation = 5090 ft. b. Outlet Pipe Diameter = 2.0 ft. c. Slope of Pipe = 0.019 ft/ft d. Pipe capacity (HW/D =1.0) = 33.8 cfs C. Specific Details - Cross Pans 1. Twelve (12') foot wide cross pans will be installed at the following low points to convey the stormwater into the wetlands or Richards Lake; a. Centerline Station 18+54.38 Hearthfire Drive. b. Centerline Station 1+95 .75 Barn Swallow Circle. c. Centerline Station 3+04.15 Snipe Lane. 2. The minimum slope on these valley pans is 0.006 ft/ft. 3. Sidewalk culverts will be placed at all low points with valley pans. 4. Where grade allows grassed swales with minimum slopes of 0.02 ft/ft (2.0%) will be constructed to convey the stormwater from the sidewalk culverts to Pond 2. 5. If the minimum slope cannot feasibly constructed, three (Y) foot valley pans will constructed. The valley pans will have a minimum slope of 0.005 ft/ft (0.5%). V. WATER QUALITY A. General Concept 1. Water quality measures are specified in the Water Quality and Wetland Mitigation Report prepared by Ted Boss, PH.D.. a. Maintenance of water quality will be the responsibility of the contractor and the owner until the project is complete. The Homeowners Association will: be responsible upon completion of the construction.