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HomeMy WebLinkAboutHEARTHFIRE (HOFFMAN) PUD - PRELIMINARY - 31-95A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTHoffman P.U.D. Preliminary Drainage Report during extremely high water events. C. DESIGN CRITERIA 1. References and Regulations a. "Richard's Lake Subdivision, First Filing Preliminary Drainage Plan", by Downing -Leach 10/86, was reviewed and utilized to assess offsite contributions to the proposed Hoffman P.U.D. site. No previous drainage studies forthe projectsite have been prepared per available information. No drainage study for the adjacent property to the north, was available at the time this preliminary report was prepared. b. This report and the analysis and design of the storm drainage facilities for the Hoffman P.U.D. site • are based upon regulations outlined in the City of Fort Collins "Storm Drainage Design Criteria and Construction Standards Manual" May, 1984 and Urban Drainage and Flood Control District's "Urban Storm Drainage Criteria Manual", 1989. C. The Dry Creek Master Plan by Gingery Associates, Inc., 1/80 was reviewed for this preliminary report, 2. Hydrologic Criteria a. The design rainfalls were determined using the "Rainfall - Intensity - Frequency for City of Fort Collins, Colorado" (Graph included in Appendix 1) from the City's Storm Drainage Design Criteria. Manual. b. The Rational Method was used for calculation of storm runoff associated with this development. Overland flow time of concentrations were calculated based on the "Overland Time Of Flow Curves" (included in Appendix 1) from UD&FCD's Drainage Criteria Manual. Travel times for channelized flow were calculated using the "Estimate Of Average Flow Velocity" (included in Appendix A) also from the UD&FCD's Criteria Manual. C. The City of Fort Collins Criteria specifies detention of the 100 year (major) storm and release at the 2 year historic rate. The detention volume requirement was determined using a storage inflow and release hydrograph. The detention volume provided was determined Merrick & Company Page 7 Hoffman P.U.D. Preliminary Drainage Report F. REFERENCES 1. City of Fort Collins, Colorado - Storm Drainage Design Criteria and Construction Standards, 1984 EDITION. 2. Urban Drainage and Flood Control District - Urban Storm Drainage Criteria Manual, 1990 Revisions.' 3. Dry Creek Drainageway Planning, prepared by Gingery Associates, Inc. January, 1980. 4. Larimer County Storm -water Management Manual, prepared by Resource Consultants, Inc., May, 1979 5. Master Drainage Plan and Study, Richards Lake Property, by Downing Leach, October, 1986 6. Wellington Quadrangle, Colorado, U.S.G.S. 7.5 Minute Series (Topographic), 1960, Photorevised 1978. 7. Report of a Preliminary Geotechnical Investigation for Richard's Lake P.U.D. Larimer County, Colorado prepared by. Empire Laboratories, Inc. November,1986. Merrick & Company Page 13 Hoffman P.U.D. Preliminary Drainage Report Included in the Appendices are calculations to support the swale, detention pond, street and inlet capacities, and stormdrain pipe designs. 3. Summary of Erosion Control Concept A majority of the site drains to the onsite wetland/detention pond. Rainfall erosion will be prevented by minimal surface grading along the shore of Richard's Lake. Silt fences will protect the onsite existing wetland and will help prevent wind erosion. Overlot grading will be followed by temporary seeding, utility installation and paving. Gravel filters and hay bales will protect street inlets and swales respectively. The proposed development erosion plan will provide protection in excess of the required effectiveness and with downstream impacts consistent with historic patterns. The plan meets the requirements of City of Fort Collins criteria. f Respectfully submitted, MERRICK & COMPANY Glen A. Tulk, PE Project Manager IMerrick & ompany Page 12 Hoffman P.U.D. Preliminary Drainage Report 2. Specific Details The Hoffman P.U.D. lies within Moderate to Low Rainfall and Wind Erodibility Zones as shown by City of Fort Collins maps. Soils on the site include the Fort Collins Loam, Kim loam, and the Nunn Clay as detailed in the 1986 Geotechnical Investigation for the Richard's Lake P.U.D. by Empire Laboratories, Inc. Erosion is a potential problem during construction and after until exposed soils are revegetated. Erosion control performance standards were calculated according to criteria presented in the City of Fort Collins Erosion Control Reference Manual for Construction Sites. Temporary seeding with mulch will be used for areas that are overlot graded. Permanent seeding with mulch will be used for areas that are overlot graded. Permanent seeding with mulch will follow installation of stormsewers and any swales. Gravel filters will be placed at all curb inlets immediately after installation. Straw bales will be placed in all erosion -prone swales. Outfall velocities will be at velocities low enough to prevent erosion, rip - rap or other protection will be provided to prevent scour and bank erosion. Wind erosion will be minimized by phased grading and construction, and placement of wind barriers and silt fences when cleared areas exceed the maximum 200 foot criteria as presented in the City of Fort Collins Erosion Control Reference Manual for Construction Sites. F. . CONCLUSIONS 1. Compliance with City Requirements The designs and calculations included in this report comply with the 1984 City of Fort Collins Storm Drainage Design Criteria. Hydrologic and hydraulic calculations were based on accepted practices for the Rational Method as contained in the City of Fort Collins Storm,Drainage Design Criteria and in the Urban Drainage and Flood Control District Criteria Manual. Calculations of the runoff rates and storm system for this project are included in Appendices 1 and 2 of this report. 2. Summary of Drainage Concept The existing site currently drains to the southwest to onsite existing low/wetland areas or to Richards Lake. The proposed development drainage plan will continue that pattern, with downstream impacts consistent with the historic flow rates and volumes and meet the requirements of City of Fort Collins criteria. A Drainage Plan has been prepared and is ,included in the back of this report. IMerrick & Company Page 11 Hoffman P.U.D. Preliminary Drainage Report The existing, historic runoff flows on the site drain to either the existing wetland or to Richard's Lake. The design proposed for the development of the Hoffman P.U.D. is to continue those drainage patterns. The City of Fort Collins requires detention of the Major (100 year) Storm flows with release at the 2 year historic rate. The Rational Method of detention sizing for the Hoffman P.U.D. gives a required detention volume of 10.2 acre feet for the entire site. Available storage volume above the wetland water level in Pond A is approximately 26 acre feet. The design proposed in this report directs runoff from 81.9 acres (Basins P2, P3, P4, OS1, and OS2) through the detention pond (Pond A). Topographic constraints prevent direction of Basin P1 to the detention pond; it will be undetained. The backyard runoff from the lots backing onto Richard's Lake will sheetflow, as it has historically, to the Lake. The streets and remaining lots in Basin P1 will flow in curb and gutter to inlets which will discharge directly to Richard's Lake. The design proposed in this report allows 24.5 acres of the Hoffman P.U.D. to drain undetained to Richard's Lake which is privately owned. A drainage easement agreement with the owners of Richard's Lake, the Water Supply and Storage Company will be obtained. The proposed Hoffman P.U.D. detention pond is designed to detain for the onsite 81.9 acres and to accept potential overflow releases from the offsite wetland to the north (Pond B). The existing outlet to offsite Pond B will be modified to control any emergency releases to Pond A. A drainage and construction easement agreement to modify the offsite pond B outlet will be obtained from the owners of that property. Final detention pond outlet designs meeting Fort Collins Storm Drainage Design and Construction Standards Criteria will be submitted with the Hoffman P.U.D. Final Drainage Report. I E. EROSION CONTROL 1. General Concept A majority of the site drains to the onsite wetland/detention pond. Rainfall erosion will be prevented by minimal disturbance of the lots along the shore of Richard's Lake. Silt fences will be placed to protect the onsite existing wetland and to prevent wind erosion. Overlot grading will be followed by temporary seeding, utility installation curb and gutter placement, and street, paving. Gravel filters and hay bales will protect street inlets and swales respectively. The proposed development erosion plan will provide protection in excess of the required effectiveness. Downstream impacts of runoff will be consistent with historic patterns. Merrick & Company Page 10 Hoffman P.U.D. Preliminary Drainage Report conveyed southward to Richard's Lake by a proposed swale along the west property line of the site which will be in an easement. The proposed 2 year runoff coefficient for Basin P1 is 0.45. The basin is anticipated to create a developed peak flow for the minor storm of 19.8 cfs and 76 cfs for the major storm. Inlet and pipe sizing will be presented with the final drainage plan. The inlets will be sized to maintain gutter flow within the allowable street capacity for the 2 and 100 year storms. Basin P2 consists of the northwestern quarter of the site. Basin P2 contains 28 acres and is proposed to be fully developed with residences and landscaped open space. With the highest density proposed residential development, the basin is proposed to have -a 2 year runoff coefficient of 0.50, and a 100 year coefficient of 0.70. Flows from basin P2 will surface flow in proposed curb and gutter to proposed inlets and storm sewers and swales to the wetland/detention pond (Pond A). Basin P2 is anticipated to create a peak minor storm flow of 30.8 cfs, and a major storm flow of 125 cfs. (Refer to Hydrologic calculations, Appendix 1). Basin P3 consists of 27.8 acres located in the center of the site. The basin will be developed as 28 multi -family townhomes on 4.8 acres, 6 single family residences on 2.2 acres at the southeast end of the wetland, and includes approximately 21 acres containing the wetland open space. The basin is proposed to have a 2 year runoff coefficient of 0.25, and a .100 year coefficient of 0.45. The flows generated in the developed portions of basin P3 will be directed by proposed curb and gutter to the wetland. The calculated 2 year flow generated by Basin P3 is 16.3 cfs; the 100 year storm generates 86.3 cfs. Basin P4 is the 21.7 acres bordering the north and eastern sides of the wetland (Pond A). This basin is proposed to be fully developed with single family residences and landscaped open space. The basin is calculated to have a 2 year runoff coefficient of 0.45. Flows from basin P4 will surface flow via proposed curb and gutter and to the wetland/detention pond (Pond A). Basin P4 will generate 17.8 cfs. in the minor storm, and 82 cfs in the major event. The 100 year peak onsite developed design flows to Richard's Lake (undetained from 24.5 acres) from Basin P1 is 76.4 cfs, the 2 year storm generates 19.8 cfs. , The onsite developed flows to the wetland/detention pond are 57.6 cfs for the minor storm, and 298 cfs for the major storm evipnt. The wetland collects drainage from 81.9 acres. Detailed summary of the minor and major storm rates are provided in Appendix 1. Also included are the relevant calculations for the offsite flows. II Merrick & Company Page 5 Hoffman P.U.D. Preliminary Drainage Report by planimeter of the proposed detention pond included in the tracts reserved for wetlands and open space. Spreadsheets for both calculations are included in the Appendix. d. The Initial (Minor) storm was calculated using the 2-year recurrence interval for Residential Land Use as required by the City of Fort Collins' Design Criteria. The Major storm was calculated using the 100-year recurrence interval. 3. Hydraulic Criteria a. The storm sewer pipe hydraulic capacities used for the system design are as outlined in UD&FCD's Drainage Criteria Manual Volume 2. Swale capacities and design depths were calculated using Manning's equation and follow procedures referred to in UD&FCD's Drainage Criteria Manual. Inlet capacities will be calculated following the City of Fort Collins Storm Drainage Design Criteria and follow procedures referred to in UD&FCD's Drainage Criteria Manual. D. DRAINAGE PLAN 1. General Concept ' This drainage design is based on the 2-year recurrence interval storm for the initial storm, and the 100-year recurrence interval storm for the major storm. Proposed site improvements will direct the offsite flows through the site by means of curb and gutter, stormsewers, and graded swales. The developed onsite flows will be directed away from the houses and into the streets or swales where they will flow to the proposed wetland/detention pond located in the center of the site. Detention releases will meet the restrictive criteria of the owners of Richard's Lake. 2. Specific Details The site has been divided into 4 primary onsite basins and 2 offsite basins. (See included Preliminary Drainage Plan). Primary basins are designated with a P, the offsite basins are labeled with OS. Basin P1 consists of the southwestern part of the site and contains 24.5 acres. The site grading plan shows lot slopes to the southwest with streets conveying. runoff to a low which will drain by curb inlets to Richard's Lake. Flows generated on the north -south portion of County Road 13 enter the roadside swale and are Merrick & Company Page 8 Hoffman P.U.D. Preliminary Drainage Report Total flows generated by Historic Basin H1 are 10.2cfs and 60 cfs for the 2 year and 100 year storms respectively. Historic basin H2 comprises the majority of the site and includes the central, closed basin area draining to the existing wetland/pond (Pond A). Basin H2 covers 59.4 acres. This sub -basin presently sheet flows to the wetland. Under normal conditions, the wetland/pond drains by percolation to the water table and evapotranspiration. During high water events, a graded channel on the south side allows the wetland to discharge to the southwest to Richard's Lake.. These highwater releases occur when water levels in Pond A exceed 5089.5 feet elevation. In like manner, Pond B located offsite to the north, overflows through a graded channel into Pond A during high water events when water levels exceed 5110 feet in the offsite pond (Pond B). Historic Basin H2 generates 16.6 cfs in the 2 year event, and 97 cfs during the major storm event. Historic basin H3 is a small (2.9 acre) area along the southeast property line of the site where existing natural slopes direct flows towards the Master Planned Richards Lake P.U.D. property to the southeast. These flows are directed to the southwest on that property and eventually reach Richard's Lake. The basin generates a 100 year storm flow of 3.9 cfs. Historic basin H4 is a small (566 acre) onsite area along the east property line of the site where the topographic high caused by the oil service road defines a drainage divide between the area draining northwestward toward wetland pond A and a gentle eastward sloping pasture area offsite towards the Master Planned Richard's Lake P.U.D. property to the southeast. Offsite flows enter Historic Basin H4 along the east property line as sheet flow from the Serramonte Highland Subdivision. A portion of these offsite flows are directed to the southeast across the Richard's Lake P.U.D. property, and a portion are directed by existing slopes to the wetland pond A. Basin H4 generates a 2 year storm flow of 2 cfs and a 100 year storm flow of 11.9 cfs and passes through respectively 0.2 cfs and 0.5 cfs of offsite flows. 3. Outfalls Downstream From Property The south portion of the existing site drains by overland flow and swale to the south into Richard's Lake. The northern 60% of the existing site drains to the existing onsite wetland -pond area which only releases flows to Richard's Lake Merrick & Company Page 6 Hoffman P.U.D. Preliminary Drainage Report ' central part of the proposed Hoffman P.U.D. site is presently developed as single family houses and landscaping at a density of I 0.5 units per acre. OS1 is 1.2 acres of the Serramonte Highlands Subdivision and a water supply tank site. OS1 is calculated to have a developed 2 year runoff coefficient of 0.25, and a 100 year runoff I coefficient of 0.45. The existing 100 year peak flow to the Hoffman P.U.D. site from Basin OS1 is calculated to be 3.5 cfs. This runoff flows to the southwest across an existing dirt track and enters the site along the east. An existing poorly defined swale conveys these flows to the south to Richard's Lake. b. Offsite basin OS2, 3.2 acres, is presently developed as farm yard and oil service area with small sheds, and small outbuildings. The 100 year peak flow from OS2 is calculated to be 10.6 cfs. The flows from this offsite basin currently enter the north side of the proposed Hoffman P.U.D. site as sheet flow and drain to the existing onsite wetland pond. C. There are no major drainage ways on the proposed development site. 2. Drainaue Patterns Throuah Proaerty The site can be divided into four historic sub -basins and 2 offsite historic basins. Onsite Historic basins are designated with an H, the offsite histofic basins are labeled with OH and OS. (See Appendix 1- Historic Basin Map). Historic Basin H1 contains 40.6 acres of pasture sloping to the south and southwest. The basin presently drains to Richard's Lake. The basin is defined on the north by a natural highpoint located about 250 feet south of the intersection of County Road 13 and Douglas Road. County Road 13 defines the western boundary of Historic basin H1. County Road 13 is superelevated at a point about 1200 feet south of Douglas Road. This superelevation causes a drainage divide at the west turn in the road, and prevents the crossing of flows generated west and north of the road. An existing roadside swale collects flow from the east 1/2 of County Road 13. This swale discharges into a pasture located on the west side of the site. The County Road 13 swale collects the gravel road runoff from an area of approximately 0.33 acres. Basin H1 is defined on the south west by the water level of Richard's Lake. The northeast flank of the basin boundary is created by an existing topographic divide which separates Richard's Lake from the onsite low, wetlands/pond area to the north (Pond A). Merrick & ompany Page 5 Hoffman P.U.D. Preliminary Drainage Report B. HISTORIC DRAINAGE 1. Overall Drainage Basin Description The site is located in the major historic Dry Creek Drainage Basin. The portion of the major drainage basin in which this project is located is presently partially developed. Drainage is generally from north to south and then southwest with major defined man-made channels fed by sheet runoff and local irrigation and roadside ditches. The basin slopes vary across the site from 12.6% to as low as 0.5%, with an average of approximately 4 percent. Elevations range from 5131 to 5062.5 which is the average water level in Richard's Lake. Present zoning for the basin calls for residential development. Jurisdictions in the drainage basin include unincorporated Larimer County, and the City of Fort Collins. The site lies entirely within the City of Fort Collins. The existing tributary sub -basin in which the site lies is defined by County Road 11 to the east, Richard's Lake on the south, County Road 13 on the west, and Douglas Road on the north. Runoff from landscaped rear yards of the Serramonte Highlands Subdivision to the east enter the site as sheetflow along a portion of the eastern border of the property. Douglas Road on the -north forms a man-made drainage divide preventing southward flows from the farmland north of the road. On the west side of the property, the eastern side of County Road 13 is superelevated where it turns west; this superelevation prevents flows from crossing from the north and west. The ditch along the east side of County Road 13 drains into the site at the location of the west bend in the road. No flows enter the site from the south. The site slopes to the south west at an average of 4%. An oil field service road located at the southeast corner of the site acts as a drainage divide diverting flows offsite to the southeast. The existing drainage pattern on the majority of the site is surface flow draining to the existing onsite wetland area. Approximately 40 acres of the south side of the site drains southwestward to Richard's Lake (Refer to Historic Drainage Basin Map in Appendix 1). The historic onsite 100 year storm flow to this wetland is calculated to be 97 cfs (2.4 cfs/ac). The historic onsite 100 year storm flow to Richard's Lake is calculated to be 68.4 cfs (1.5 cfs/ac) See Appendix 1 - Historic Runoff calculation). Offsite Basins a. The proposed project will be impacted by two offsite sub - IMerrick & Company Page 4 Hoffman P.U.D. Scale: V=1/2 Mile Vicinity Map Figure 1 Preliminary Drainage Report Merrick & Company Page 3 Hoffman P.U.D. Preliminary Drainage Report developed, and 43 zero lot line single family attached homes will also be developed. The wetlands will be set aside as a water quality open space. The permanent wetland presently covers 7.1 acres within a proposed 19.7 acre tract (Tract A). The gross density is proposed at 2 units per acre, 3.15 units per net acre. The developed site will have a 2 year composite runoff coefficient of 0.39. J Merrick & Company Page 2 IHoffman P.U.D. Preliminary Drainage Report A. INTRODUCTION This report analyzes the drainage impacts of the proposed Hoffman P.U. D. development. The design considerations for this project are to provide a drainage system capable of handling runoff generated on the proposed site as well as convey the existing runoff from offsite tributary areas. In order to accomplish this, the offsite and onsite drainage areas are studied as they are impacted by the proposed project. This report was prepared following guidelines and criteria presented in the "Fort Collins Storm Drainage Design Criteria and Construction Standards", 1984 and the "Urban Storm Drainage Criteria Manual" (USDCM), 1992. 1. Location The project is located in the West half of Section 20, Township 8North, Range 68 West, of the Sixth Principal Meridian, City of Fort Collins, Larimer County, State of Colorado. (see Figure 1, page 2 - Vicinity Map) The project site is bordered on the north by Gregory Road and single family homes located on lands in unincorporated Larimer County and on the southwest by Richards Lake. Developed single family homesites of the Serramonte Highlands subdivision, unincorporated Larimer county border the northeast property line, the undeveloped Richard's Lake Master Plan area lies adjacent the site to the southeast. County Road 13 borders the site on the northwest. 2. Property Description The project site contains 101.3 gross acres of which approximately 37.6 acres are to be open space and approximately 62.7 acres will be developed. The site has previously been farmed and used as grazing pasture. It contains 3 producing oil wells of the Ft. Collins Field which produces from the Cretaceous Muddy sandstone. The existing vegetation consists of mainly native grasses and weeds. The soils onsite are silty to sandy loam type. The north part of the site has moderate wind and rain erodability, the portion of the site adjacent to the lake has low erodibility for wind and rainfall. The site slopes to the southwest at an average of 4% with slopes ranging for 0.5% to 12% on the site. The south quarter of the site presently drains to Richard's Lake, the northern portion of the site drains to an existing "slough" or closed low spot which retains water and supports a wetland. The proposed development will consist of two parcels to be developed as 56 and 28 multi -family townhouse units each. 71 single family detached homesites will be Merrick & Company Page 1 Hoffman P.U.D. Preliminary Drainage Report TABLE OF CONTENTS PAGE INTRODUCTION................................................ 1 GENERAL LOCATION AND DESCRIPTION ........................... 1 Location ............................................... 1 Property Description ...................................... 1 Vicinity Map ............................................. 3 B. HISTORIC DRAINAGE ..................................... 4 Overall Basin Descriptions ............................ 4 Drainage Patterns Through the Property .................. 5 Outfalls Downstream from the Property .................. 6 C. DESIGN CRITERIA ........................................ 7 References and Regulations 7 Hydrologic Criteria ................................... 7 Hydraulic Criteria ................................... 8 D. DRAINAGE PLAN ......................................... 8 General Concept ........? ........................... 8 Specific Details ..................................... 8 E. EROSIONCONTROL PLAN ................................. 10 General Concept .................................... 10 Specific Details ..................................... 11 F. CONCLUSIONS .......................................... 11 Compliance with Standards ............................ 11 Summary of Drainage Concept ....................... 11 Summary of Erosion Control Concept ..................... 11 5. REFERENCES ........................................... 13 H. APPENDICES .............................................. Appendix 1 - Hydrology ............................... 14 Appendix 2 - Hydraulics ............................... 15 ' Appendix 3 - Erosion Control ............................ 16 Merrick & Company Page ii Hoffman P.U.D. Preliminary Drainage Report Prepared for: Hoffman P.U.D. Richard's Lake Development 1412 Richard's Lake Road Fort Collins, Colorado 80524 Contact: Mr. Bill Yunker or Mr. Craig Hash (303) 484-1921 Prepared by: MERRICK &COMPANY 2456 S. Peoria Street Aurora, Colorado 80014 Contact: Mr. Glen A. Tulk, PE Telephone: (303)-751-0741 Merrick & Company Job Number: 15011782 Merrick & Company Page i HOFFMAN P.U.D. PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT June 15, 1995 Presented to The City of Fort Collins Prepared by Merrick & Company 000 000 MERRICK Engineersfi Architects Menrck 8 Company Mail: PO Box 22026 / Denver, CO / 80222 / USA Delivery: 2450 S. Peoria St. / Aurora, CO / 80014 .................................................. .303/751-0741 / Fax 303/751-2581 June 19, 1995 Ref: 15011782 0 Mr. Basil Y. Hamdan Mr. Glen D. Schlueter Stormwater Utility City of Fort Collins 235 Mathews Street Fort Collins, Colorado 80522-0580 Dear Mr. Hamdan and Mr. Schlueter: Merrick & Company is pleased to submit for your review and comments the attached IPreliminary Drainage and Erosion Control Reportforthe Hoffman P.U.D. This preliminary report presents the basic drainage plan and designs for this proposed residential ' development. These designs and plans were prepared in accordance with the specifications and requirements presented in the Fort Collins Storm Drainage Design Criteria and Construction Standards Manual. IIf you have any questions regarding this report, please don't hesitate to call Glen Tulk at Merrick and Company at (303)-751-0741. Sincerely, Merrick & Company Glen A. Tulk, P.E. Project Manager ......................................................... An Employee -Owned Company ❑o❑ o0o MERRICK Engineers & Architects