Loading...
HomeMy WebLinkAboutHEARTHFIRE (HOFFMAN) PUD - PRELIMINARY (REVISED) - 31-95A - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTHoffman P.U.D. Preliminary Drainage Report I 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.' I3. Dry Creek Drainageway Planning, prepared by Gingery Associates, Inc. January, 1980. I4. Larimer County Storm -water Management Manual, prepared by Resource IConsultants, 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), 1965, Photorevised 1980. 7. Report of a Preliminary Geotechnical Investigation for Richard's Lake P.U.D. Larimer County, Colorado prepared by. Empire Laboratories, Inc. November, 1986. 8. Reviewer Comments, City of Fort Collins, Colorado - Stormwater Utility June, 1995 Merrick & Company Page 16 I AINFRA\15011782\PRELIMI \H FMNDRNG. N15 Hoffman P.U.D. Preliminary Drainage Report 1. Summary of Drainage Concept The existing site currently drains to the southwest to on -site 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. 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 on -site wetland/detention pond. Sediment control barriers will protect the wetlands from siltation. Disturbance of the wetlands will be minimal. Rainfall induced erosion into Richard's Lake will be prevented by minimal surface grading along the shore of the lake. Silt fences will protect the on -site 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 erosion control plan meets the requirements of City of Fort Collins criteria. Respectfully submitted, MERRICK & COMPANY Glen A. Tulk, PE Project Manager Merrick & Company Page 15 IAIN FRA%150117821PRELIM I U1 FMN DRNG. N 15 Hoffman P.U.D. Preliminary Drainage Report Richard's Lake. Silt fences will be placed to protect the on -site 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. 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 final 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. Merrick & Company Page 14 IAINFRAl150117021PRELIM 1 \HFM NDRNG. N 15 Hoffman P.U.D. Preliminary Drainage Report and Storage Company and compatible with the historic flow path. (Refer to Appendix 2 for outlet design). The detention release structure will have a water quality release outlet which will release the 2 year runoff over a 12 hour period as specified in the Urban Drainage Manual, Volume 3. The 2 year storm generates 179709 cubic feet of runoff in 180 minutes; the 12 hour water quality protection release rate is therefore 4.15 cfs. The outlet design of the 100 year release structure will deduct the water quality release so that the total 100 year release in maintained at 16.6 cfs. Wetland Management Off -site wetland/pond B to the north will experience minimal disruption from the development of the proposed Hoffman P.U.D. The wetland will continue to retain runoff generated in historic basin OH2, the existing topography surrounding Pond B provides 23.4 acre feet of storage capacity above the average water level of 5108 feet.. The proposed Hoffman P.U.D. will not reduce that off -site capacity. The proposed emergency outlet improvements for Pond B will allow the wetlands' water level to fluctuate as it has historically. The emergency outlet structure is designed to release to the southern, on -site Pond A only in the event Pond B water levels exceed 5110 feet. On -site wetland/pond A will be minimally disturbed by the proposed Hoffman P.U.D. development. Grading will slightly modify the northwest quarter of the wetland, and a detention outlet structure with emergency spillway will be constructed outside of the existing wetland on the southeast side of the topographic depression. Water quality and vegetation in the wetland will be protected by a proposed sediment trapping gravel berms located downstream of each stormwater pipe inlet to the wetland. These berms will slow inflowing storm runoff allowing sediments and adsorbed pollutants to settle out in sediment basins upstream of the wetland. Periodic maintenance of these sediment traps by the homeowners association will insure continued water quality in the wetland. Detention releases from the detention pond will utilize a water quality outlet structure that protects the Richard's Lake receiving waters from sediments and pollutants. (See detention release structure design detail sheet). E. EROSION CONTROL 1. General Conceit A majority of the site drains to the on -site wetland/detention pond. Rainfall erosion will be prevented by minimal disturbance of the lots along the shore of Merrick & Company Page 13 LAIN FRA\15011782\RRE LIM t \H FMNDRNG. N 15 II Hoffman P.U.D. Preliminary Drainage Report 1 1 1 1 1 1 1 0 1 1 1 1 —, 1 L I Street and Inlet Capacities Appendix 2 contains spreadsheets showing the allowable 2 and 100 year street flows for a variety of street slopes. Detailed inlet sizing will be presented with the final drainage report, preliminary sizings are included in the Appendix for key design points. All inlets will be sized to maintain street capacities within City criteria for the 2 and 100 year storms. LZI • u,- - �- -� •i The Hoffman P.U.D. site historically drained to either a privately owned lake or to an on -site wetland. The City of Fort Collins requires detention of the Major (100 year] Storm Provision to accept off -site flows discharging through the property is also necessary. Using the Rational Method as outlined in UD&FCD's Drainage Criteria Manual Volume 2 "Storage", a required detention volume of cubic feet was calculated for the Hoffman P.U.D. assuming a release rate from the detention/wetland of 16.6 cfs, the historical 2 year flow from Historical. Basin H2. This assumes that the undetained releases to Richard's Lake from developed Basin P1 are approved. The net increase flowing to Richard's Lake is: Historical 2 year 16.6 cfs 100 year 97.0 cfs Developed 16.6+22.3=38.9 cfs 97.0+85.3=182.3 cfs The proposed grading and existing topography of the Hoffman P.U.D. limit the areas that can effectively be drained through the proposed detention pond. The proposed grading directs developed flows to detention/wetland pond A from 81.4 acres of the site. However, the detention volume is calculated for the entire 105.5 acres of the Hoffman P.U.D. with release at the required 1.6.6 cfs historic 2 year runoff rate for the original 60.6 acres (Basins H2 and OS1) draining to the central wetland area on the site. The proposed 81.4 acres includes 1.6 acres of Historic basin H3, and 4.4 off -site acres. Due to the steep slopes along the northeast shore of Richard's Lake, Basin P1 will discharge to the Lake undetained. Easement to discharge these undetained flows and the detention pond releases will be obtained from the privately owned Richard's Lake. Off -site flows from off -site basin OH2 and any discharges from wetland/Pond B are not included in the detention sizing of Pond A; it is assumed that any future development of Basin OH2 will be detained on that site. Proposed Pond A is designed with 1 foot of freeboard and an emergency release structure which would only discharge if the water levels exceeded 5086.4 feet. The outfall will be designed to discharge at non -erosive flow velocities acceptable to Water Supply Merrick & Company Page 12 I:\INFRA\75011782\PRELIM1 \HFMNDRNG.N15 1 Hoffman P.U.D. Preliminary Drainage Report I wetland/detention pond (Pond A). Basin P2 is anticipated to create a peak minor storm flow of 15.7 cfs, and a major storm flow of 56.4 cfs. (Refer to Hydrologic calculations, Appendix 1). Flows in Basin P2 will be conveyed by street, curb, and gutter to proposed inlets, and then by storm sewer to the on -site wetland A. Basin P Basin P3 consists of 44.22 acres located in the center of the site. The basin will be developed as 28 multi -family townhomes on 4.8 acres, the rear yards of 21 single family residences surrounding the wetland, and includes approximately 27.8 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 22.9 cfs; the 100 year storm generates 137.3 cfs. Basin P4 Basin P4 is the 13.73 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 to proposed inlets and then by storm sewer to the wetland/detention pond (Pond A). Basin P4 will generate 9.3 cfs. in the minor storm, and 38.7 cfs in the major event. Basin P5 Basin P5 is the 2.84 acres across the drainage divide at the northwest corner of the site. The present site plan shows minor development of this small parcel. Drainage from Basin P5 presently sheet flows to the northwest. The 100 year peak on -site developed design flows to Richard's Lake (undetained from 25.40 acres) from Basin P1 is 85.3 cfs, the 2 year storm generates 22.3 cfs. The on -site developed flows to the wetland/detention pond area 43.7 cfs for the minor storm, and 187.8 cfs for the major storm event. The wetland collects drainage from 78.34 acres with the design presented here. Detailed summary of the minor and major storm rates are provided in Appendix 1. Also included are the relevant calculations for the off -site flows. Merrick & Company Page 11 hINFRA\15011782TRELIM1 WMNDRNG.N15 Hoffman P.U.D. Preliminary Drainage Report feature. Disturbance of this off -site wetland will be minimal since improvements will be at elevations above the established wetland. 2. Specific Details The site has been divided into 5 primary on -site basins and 2 off -site basins. (See included Preliminary Drainage Plan). Primary basins are designated with a P, the off -site basins are labeled with OS. For the purpose of calculating gutter and street flows and inlet capacities, developed sub -basins were delineated and designated with the primary basin name followed by a letter (A, B, C, etc as P1 A, etc.). MEMIall Basin P1 consists of the southwestern part of the site adjacent to Richard's Lake. Basin P1 contains 25.40 acres and is proposed for development as single family residences, landscaped lots, and streets. The site plan shows slopes to the southwest with sheetflow off the rear yards of the lots bordering Richard's Lake. Basin P1 includes a small basin (2 acres) of rear yards along the west property line which will contain a proposed swale to convey undetained flows to the south directly into Richard's Lake. The remainder of the basin discharges to Richard's Lake by sheet flow off year yards, or by storm sewer pipe. All of the flows generated in basin P1 will enter Richard's Lake undetained. 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 22.3 cfs and 85.3 cfs for the major storm. Developed street flow in Basin P1 will be directed by grading to proposed curb and gutter and then flow southwest to a proposed curb inlet which discharges to Richard's Lake. Preliminary inlet and pipe sizing is presented here, with the final design to be presented with the final drainage plan. The inlet will be sized to maintain gutter flow within the allowable street capacity for the 2 and 100 year storms. Basin P2 Basin P2 consists of the northwestern quarter of the site. Basin P2 contains 17.41 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 0.45, and a 100 year coefficient of 0.60. Flows from basin P2 will surface flow in proposed curb and gutter to proposed inlets and storm sewers and swales to the Merrick & Company Page 10 IAINFRA\15011782\PRELIM 1 \HFMN DRNG. N 15 Hoffman P.U.D. Preliminary Drainage Report volume requirement was determined using a storage inflow and release hydrograph. The detention volume provided was determined by planimeter of the proposed detention pond area in Tract A. The available volume was calculated as that above the elevation of the existing wetland average water level. 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 Conceit 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 off -site flows through the site by means of curb and gutter, stormsewers, and graded swales. The developed on - site 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 or to Richard's Lake. Disturbance and grading around Richard's Lake and its existing wetlands will be avoided. Disturbance and grading of the on -site wetlands (Pond A) will be the minimum necessary to provide the needed detention outlet structure and emergency spillway proposed for the Hoffman P.U.D. detention pond. Detention releases will meet the restrictive criteria of the owners of Richard's Lake, and will include a water quality outlet. Provision for pass through of any discharges from off -site wetland B in the unlikely event it overflows will be made. Improvement to the existing off -site Pond B/wetland outlet will be made in cooperation with the owners of this off -site ■ Merrick & Company Page 9 IAINFRA\75011782\PRELIM? V iFMNDRNG.N15 Hoffman P.U.D. Preliminary Drainage Report evaporation and percolation to the groundwater table and no flows enter the Hoffman P.U.D. 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 off -site contributions to the proposed Hoffman P.U.D. site. No previous drainage studies for the project site 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 Merrick & Company Page 8 IAIN FRA\150117821P RELIM 1 \H FMNDRNG. N15 Hoffman P.U.D. Preliminary Drainage Report drainage divide between the area draining northwestward toward wetland pond A and a gentle eastward sloping pasture area off -site towards the Master Planned Richard's Lake P.U.D. property to the southeast. Off -site flows enter Historic Basin H4 along the east property line as sheet flow from the Serramonte Highland Subdivision. A portion of these off -site 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 off -site flows. Historic basin OH2 Off -site historic basin OH2 is defined by the drainage divide caused by Douglas road on the north and County Road 13 on the west and includes the residences, farmland and oil field area to the north of the proposed development. Basin OH2 is somewhat developed with approximately 10 homes, barns and oil field sheds, tanks, and graded dirt roads. Basin OH2 covers 50.2 acres. Runoff generated in basin OH2 drains internally by sheet flow to an existing wetland/pond (Pond B). Under normal conditions, the wetland/pond drains by percolation to the water table and evapo-transpiration and does not discharge any flows off -site. During extremely high water events, a poorly graded channel on the southwest side of wetland/pond B would allow it to discharge to the Hoffman P.U.D. on -site wetland, Pond A. These high water releases could only occur when water levels in Pond B exceed 5110 feet elevation. The existing available storage volume in the Basin OH2 wetland/retention pond is 23.4 acre-feet at the 5110 foot contour, which is 7 times greater than the 100 year historic runoff of 3.2 acre feet. Historic basin OS1 ' Off -site historic basin OS1 is defined by a drainage divide in the landscaped yards of the Serramonte Highlands subdivision. Basin OS1 is developed with approximately 3 homes with landscaped yards. Runoff generated in basin OS1 drains by sheetflow to the southwest. This off -site runoff enters the Hoffman P.U.D. site along the northeast property line and eventually flows to the existing on-site/wetland/pond (Pond A). 3. Outfalls Downstream From Properly The southern 40% 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 on -site wetland -pond area which will only releases flows to Richard's Lake during extremely high water events. Off -site flows in basin OH2 presently flow to wetland/pond B. Under normal conditions, this wetland drains by ' Merrick & Company Page 7 I:IINFRA%150117821PRELIMI W FMNDRNG.N15 Hoffman P.U.D. Preliminary Drainage Report 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 on -site low, wetlands/pond area to the north (Pond A). 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 Historic basin H2 comprises the majority of the site and includes the central, closed basin area which drains internally 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 evapo-transpiration. During high water events, a graded channel on the south side of the wetland allows it to discharge to the southwest to Richard's Lake. These high water releases occur when water levels in Pond A exceed 5089.5 feet elevation. In like manner, Pond B located off -site to the north, overflows through a graded channel into Pond A during high water events when water levels exceed 5110 feet in the off -site 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 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 Historic basin H4 is a small (5.6 acre) on -site area along the east property line of the site where the topographic high caused by the oil service road defines a Merrick & Company Page 6 IAIN FRA115011782XPRELIM 1 \HFMNDRNG. N 15 Hoffman P.U.D. Preliminary Drainage Report The proposed project will be impacted by two off -site sub -basins. a. Off -site basin OS1, located immediately to the east of the central part of the proposed Hoffman P.U.D. site is presently developed as single family houses and landscaping at a density of 0.5 units per acre. OS1 is 1.6 acres of the Serramonte Highlands Subdivision. OS1 is calculated to have a developed 2 year runoff coefficient of 0.25, and a 100 year runoff coefficient of 0.45. The existing 100 year peak flow to the Hoffman P.U.D. site from Basin OS1 is calculated to be 4.6 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 southwest to the wetlands (Pond A). b. Off -site basin OS2, 1.4 acres, is presently developed as farm yard and oil service yard with some small sheds, and small outbuildings. The 100 year peak flow from OS2 is calculated to be 4.7 cfs. The flows rom this off -site basin currently enter the north side of the proposed Hoffman P.U.D. site as sheet flow and drain to the existing on -site wetland pond (Pond A). C. Off -site Historical OH2 contains 50.2 acres of farmland, residences, farm buildings, and oil field buildings located to the northeast of the proposed Hoffman P.U.D. site. Under normal conditions this basin does not discharge runoff to the south onto the proposed site. Basin OH2 has an existing wetland/pond with an average water level of 5108 feet elevation. At 5108 feet, the north, off -site wetland covers 6.7 acres. The highest closing contour for the 15.67 acre closed area containing this wetland/pond is 5111 feet. d. There are no major drainage ways on the proposed development site. 2. Drainage Patterns Through Property The site can be divided into four historic sub -basins and 2 off -site historic basins. On -site Historic basins are designated with an H, the off -site historic basins are labeled with OH. (See Figure 1, Appendix 1- Historic Basin Map). Historic Basin H1 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 high point located about 250 feet south of the intersection of County Road 13 and Douglas Road. County Road 13 defines the Merrick & Company Page 5 I:\INFRA\15011782\PRELIMI\HFMNDRNG. NI 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. Major basin drainage is generally from northeast to, southwest and then southeast with major defined man-made channels fed by sheet runoff and local irrigation and roadside ditches. The 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 on the north to 5062.5 feet which is the average water level in Richard's Lake. Present zoning for the site 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 (County Road 54) 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 half of County Road 13 is superelevated where it turns west; this superelevation prevents flows from crossing from the north and west. A 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 runoff from a small area off -site to the southeast and directing the majority of the flows generated in the southeast corner of the site to the on -site wetland (Refer to Historic basin map, Basin H2). The existing drainage pattern of the majority of the site is surface flow draining to the existing on -site wetland area. The historic on -site 100 year storm flow to this wetland is calculated to be 97 cfs (2.4 cfs/ac). Approximately 40 acres on the south side of the site drain southwestward into Richard's Lake (Refer to Historic Drainage Basin Map, Basin 1­11). The historic Ion -site 100 year storm flow to Richard's Lake is calculated to be 68.4 cfs (1.7 cfs/ac) See Appendix 1 - Historic Runoff calculation). I Merrick & Company Page 4 1AIN FRA\15011782\PRELIM 1 \HFMNDRN G. N 15 I Preliminary Drainage Report F C � W OR C � 17 •i/�•/ •c�CA27E� ••. ............. ••-••••• .U�.... Richard Lake HOFFMAN P.U.D. Site cr MA Da A/ ............. rndenmeie L NMEfPr. T`1. Lake- n ~ C DER ry RTLEAFFEPL c Z r$ ............ W _ Scale: 111=1/2 Mile STA WY LT Vicinity Map Figure 1 Merrick & Company Page 3 Hoffman P.U.D. Preliminary Drainage Report Phase 2 - 98 units of medium -density (7.84 D.U. per Acre) single family residential use (please refer to Parcel A on the Site plan). Phase 3 - low -density (29 units at 3.05 D.U. per acre) single residential use (Parcel C). Phase 4 - 25 single family residential units at low -density (1.26 D.U. per Acre) adjacent to the Serremonte Highlands subdivision (Parcel D). Phase 5 - medium -density (28 units, 5.71 D.U. per acre) patio homes located at the southeast end of the wetlands (Parcel E). Phase 6 - planned as medium -density 34 patio home units, 4.15 D.U. per acre, (please refer to Parcel B on the Site plan). The on -site wetlands will be set aside as a water quality open space. The permanent wetland presently covers approximately 12.6 acres within a proposed 23.5 acre tract (Tract A). The entire project 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. Merrick & Company Page 2 I:\INFRA\15011782\PRELIM I \HFMNDRNG. N I5 Hoffman 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 off -site tributary areas. In order to accomplish this, the off -site and on -site 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 30, Township 8 North, 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 Douglas Road and single family homes located on lands in unincorporated Larimer County and on the southwest by Richards Lake. County Road 13 borders the site on the northwest. 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. t2. Property Description ' The project site contains 105.5 gross acres of which approximately 31.141 acres are to be drainage tracts and open space and approximately 74.415 acres will be developed as streets, and as single and multi -family residences. The site has ' previously been farmed and used as grazing pasture. It contains 2 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 on -site 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 ' half of the site presently drains to Richard's Lake, the northern portion of the Hoffman P.U.D. site drains to an existing on -site closed topographic low spot t which retains water and supports a wetland. The proposed development will occur in 6 phases: Phase 1 - low -density (30 units, 2.28 D.U. per Acre) single family residential (please refer to Parcel F on the Site plan). Merrick & Company Page 1 111N FRA\15011782\PRELIM 1 \H FMN DRNG. N 15 Hoffman P.U.D. Preliminary Drainage Report TABLE OF CONTENTS PAGE INTRODUCTION.......................................................................................................................... 1 GENERAL LOCATION AND DESCRIPTION.............................................................................. 1 Location........................................................................................................................................ 1 PropertyDescription..................................................................................................................... 1 VicinityMap............................................................................................................................:1, 3 B. HISTORIC DRAINAGE.................................................................................................... 4 Overall Basin Descriptions................................................................................................ 4 Drainage Patterns Through the Property .......................................................................... Outfalls Downstream from the Property 5 7 ............................................................................ C. DESIGN CRITERIA........................................................................................................... 8 References and Regulations......................................................................................:...... 8 HydrologicCriteria............................................................................................................. 8 HydraulicCriteria............................................................................................................... 9 D. DRAINAGE PLAN............................................................................................................. 9 GeneralConcept............................................................................................................... 9 SpecificDetails................................................................................................................ 10 E. EROSION CONTROL PLAN.......................................................................................... 13 GeneralConcept............................................................................................................. 13 SpecificDetail......:........................................................................................................... 13 F. CONCLUSIONS..............................................................................................................14 Compliance with Standards ................................. :.......................................................... 14 Summary of Drainage Concept....................................................................................... 14 Summary of Erosion Control Concept............................................................................ 14 G. REFERENCES................................................................................................................16 H. APPENDICES Appendix 1- Hydrologic Calculations.............................................................................. 17 Appendix 2 - Hydraulic Calculations............................................................................... 18 Appendix 3 - Erosion Control Calculations..................................................................... 19 Merrick & Company Page 2 1AINFRA11 5011782%PRELIM1IHFMNORNG. N15 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 2450 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 LAIN FRAY15011782\PRELIM? \H FMN DRNG. N 15 HOFFMAN P.U.D. PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT August 3, 1995 Revised November 14, 1995 Presented to The City of Fort Collins Prepared by Merrick & Company ❑00 EIOEI MERRICK cooEngineers 6 Architects