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Home My WebLink AboutFOSSIL LAKE P.U.D. - PRELIMINARY - 33-01D1 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTN VI91 !TY MAP 1 =000 Page 6 Preliminary Drainage Report Fossil Lake P.U.D. REFERENCES 1. Larimer County Storm -Water Management Manual, Circa August 18, 1978. 2. Urban Drainage and Flood Control District Urban Storm Drainage Criteria Manual, Wright -McLaughlin Engineers, Denver, Colorado, March, 1969. 3. Larimer County Road Policy & Standards Manual, Circa August 24, 1972 Page 5 Preliminary Drainage Report Fossil Lake P.U.D. Upstream ends of all storm piping installed as part of the road construction will be protected by straw bales immediately following final street grading and slope mulching. All project mulching will be applied at a rate of 2 tons of blown straw per disturbed acre. Silt fencing will be placed at the toe of any fill slope when practicable, following mulching activities. Areas of cuts will require installation of straw bale dikes in flow lines as will be shown on the construction details. This activity should occur shortly following mulching. Installation of detention ponds should occurimmediately following utility and road construction activities. Protection of the outlet will be provided by a straw bale dike around the pipes upstream end as soon as grade and mulching allows (12 weeks or less). Preliminary grading within the multiple detention areas mentioned should occur immediately following the installation of utilities. Care will be taken to not disturb any other erosion control measures installed before this activity. Grading of the temporary detention/ water quality areas will be a field decision, though pipe inverts where required will dictate grading. Following the preliminary grading of the detention areas mulch will be placed in all areas of disturbance. - Areas used for storage of soils will need a silt fence around the downhill side of the stock pile to reduce impact to adjacent ground. Any temporary berm required as part of this construction activity will be protected from erosion by a layer of mulch when that berm is graded. Areas next to any wetland, if applicable, will require additional protection methods to ensure water quality. Beyond any methods identified and required above, straw bale walls ( a continuous length ) will be required at the upstream side of wetlands. Diversions or temporary sedimentation ponds will be required at the ends of such structures to exclude direct surface runoffs from entering those sensitive areas. Maintenance of these facilities should occur bi-weekly or following any rainfall of 3/4 inches in 24-hours or more according to which ever event dictates. Page 4 Preliminary Drainage Report Fossil Lake P.U.D. What is evident is that sites topographic conditions will not present itself as the principal element driving the storm drainage design. Rather the street layout and aesthetics will be the key to the ultimate site character. Four types of density are being proposed for the project, single family, large lots, rural lots and estate lots. Based on the LCS-WMM runoff coefficients were determined for each density to be 0.4, 0.4, 0.35 and 0.30 respectively. Next a multitude of typical basin geometries were averaged such as overland length, gutter flow length and expected street slopes. The following table will summarize those parameters. General Basin Parameters Designation "C" Value Overland Avg. (ft) Gutter Avg. (ft) TOC (2-yr) (min) Intensity (2-yr) (in/hr) Single Fam. 0.40 155.0 1150.0 20.6 1.81 Large Lots 1 0.40 155.0 1665.0 24.0 1.69 Rural Lots 0.35 170.0 550.0 18.2 1.94 Estate Lots 0.30 285.0 900.0 26.0 1.60 Note: all lots grades assumed at 2%, road grades assumed at 1.5% Flow rate for direct runoff per acre ranged from 0.72 cfs to 0.48 cfs per acre in the 2-year recurrence and 2.70 to 1.76 in the 100-year event, see the appendices of this report for details. These flows were applied to the basins designated and pipe sizes determined. All pipes within the project shall be RCP with a typical slope of 1.5%. Additionally, no detention is presumed as part of the sizing effort, just direct runoff without surcharging. IV. EROSION CONTROL Residential development within the Fossil Lake P.U.D. will require two different levels of water quality protection. First is that associated with the immediacy of the road and utility installations. The second is related to ongoing water quality issues for the downstream receiving waters. That first issue of construction erosion is being addressed in the following bullet list. Rather than developing a construction sequence that would be steadfast and rigorous, sequencing of logical methods with milestones is being proposed to allow for flexibility and judgement in the field. Certain activities will need to proceed others', however, that is self evident to field crews as site improvements move forward. This open sequence of erosion control methods will produce a more responsive system and can be reviewed by field observers of the project. Please reference the following list of methods and the associated construction milestones that accompany each. Page 3 Preliminary Drainage Report Fossil Lake P.U.D. Little offsite sheet flows currently enters the project, rather, the service ditches from Muskrat and Mail Creek which collect storm runoffs do traverse the development. Both will be augmented to account for existing and proposed flow scenarios. A thorough study of offshe hydrology will be completed prior to any final report to determine precisely what flow can be expected. Currently, offshe sheet flows have been assessed for this conceptual design effort. Hydrologic and Hydraulic Design Criteria and References Primary reference materials used for the development of this report include two sources the Latimer County Storm -Water Management Manual and the Urban Storm Drainage Manuals. Both are accepted as the basis of storm design in Larimer County and the City of Fort Collins. General Drainage Concept Grading on the site for this first blush concept focused on minimizing road cut and fill associated with construction activities. When the design concepts further evolve more details will allow for a fully developed project. Street runoff capacities throughout the project have been enhanced by the use of "rollover" curb and gutter. Some areas include attached, detached or no sidewalk relating to the rollover curb and gutter proposed. Arterial will be provided with vertical curb and gutter and detached sidewalks. Most rights -of -way ( ROW) within the project will be 60 feet wide. Arterial's adjacent to the project boundary will be provided with an additional 20 foot making a 50 foot total ROW on this property. Surface conveyance will be the basic method of controlling direct runoff. Detention is anticipated but, as mentioned details of allowable release rates are still considered negotiable. Area available for detention total near 45 acres which will not impede any future requirements. These areas include specific detention ponds along with areas over normal WSEL in proposed water amenities. Included in the street plans have been a minimum of rollover curb and gutter which will provide a better means of street flow, rather than the normal borrow ditches. Curb outlet locations shown are based on spots where street flows appear excessive. Details of outlet spacing, piping and 100-year street flow outlets are considered beyond the scope of this conceptual design. Drainage Design Specifics Basin sizes are small enough that the Rational Methods for internal design is an adequate means of analysis. Site plan changes and revisions are imminent thus the lack of detail at this point in the projects evolution. Page 2 Preliminary Drainage Report Fossil Lake P.U.D. II. HISTORIC DRAINAGE Based on the topography, historic drainage is easily explained as 90 percent of the site slope directly to the Reservoir. Ten percent of the site drains into a seep ditch on the northeast corner of the project which also flow into the reservoir. Using the rudimentary Rational Method for historic runoff rates roughly 50 cfs of direct runoff is developed in the 2-year storm event. This runoff is based on a "C" value of 0.25 and rainfall intensity of 0.74 in/hr with a time of concentration of 81 minutes. Since we are over the normal application area for the method it is generally understood to over estimate flows. Thus the preliminary numbers for the large historic basin is slightly less conservative as a release rate. In the existing land use no detention or water quality are being provided. This proposal will be including both as part of the design package. When final design is undertaken a detailed hydrologic model will be provided for both historic and developed conditions. Negotiations with North Poudre Irrigation are being planned to address what runoff release rates can be allowed. 3. DEVELOPED DRAINAGE A number of assumptions have been included in the development of this report. All would be considered as typical for the level of detail in a preliminary investigation. Developed basins are small enough to allow for the use of the Rational Method, and average basin parameters are being assumed. These averages are based on the preliminary data from proposed basin geometry. Typical land use - densities, four of them, have been broken out for the residential layouts being represented. Then a direct runoff per acre of land was determined for each density. Additionally, pipes within the project were sized in the most conservative manner assuming no detention being required. Sizes of RCP are based on Mannirig's Equation without the benefit of driving head being considered. You will see however that a substantial amount of open space is available when the details of the release rates are finalized. Pipes as shown are capable of passing the 100-year recurrence interval, again a conservative assumption. The Fossil Lake Reservoir the historic receiving waters will experience the direct runoff increases from the intensified land use. Developed Conditions Storm drainage for the purpose of this preliminary analysis, will focus on the area within the project boundary. Consideration of offsite issues will focus on the known canalized flows crossing the project area. Preliminary Drainage Report for Fossil Lake P.U.D. Southeast Corner of Larimer County Road #36 and #9 Larimer County, Colorado May 20, 1997 I. GENERAL LOCATION SITE DESCRIPTION General Location This report summarizes the results of a preliminary storm drainage investigation for the residential development known as Fossil Lake P.U.D. (FL-PUD). The project is located in the southeastern quadrant of Larimer County. Location by Section, Township and Range would be a part of Sections 9 and 16, Township 6 North, Range 68 West. Specifically, on the southeast corner of the intersection of Larimer County Roads #9 and #36 on the northeast end of Fossil Creek Reservoir. The Parcel lies outside the Urban Growth Area as defined by Larimer County and the City of Fort Collins. _ Adjacent to the Fossil Creek Reservoir is a metes and bounds which allow the perpetual access easement for the North Poudre Irrigation Company. This existing easement serves as the south property boundary and will be enhance by the great number of buffer areas being proposed. Site Description This site falls outside of any existing hydrologic master plan, thus Northern Engineering Services (NES) will be presenting a stand alone design to accommodate the project and all hydraulically connected offsite basins. Topographically the site (±465.2 acre) would be referred to as a "bowl " shape which encircles the Reservoir on the south boundary The surface slopes are gradual and run generally toward the Reservoir. The property was formally in a combination of non- agricultural rangeland and irrigated agricultural land use. Most of the service ditches within the project area will be preserved with only rerouting occurring. This surface shape and irrigations history lends itself to the use of surface runoff conveyance which will be the focus of the preliminary review. Preliminary Drainage Report for Fossil Lake P.U.D. Southeast Corner of Larimer County Road #36 and #9 Larimer County, Colorado May 20, 1997 I. GENERAL LOCATION SITE DESCRIPTION General Location This report summarizes the results of a preliminary storm drainage investigation for the residential development known as Fossil Lake P.U.D. (FL-PUD). The project is located in the southeastern quadrant of Larimer County. Location by Section, Township and Range would be a part of Sections 9 and 16, Township 6 North, Range 68 West. Specifically, on the southeast corner of the intersection of Larimer County Roads #9 and #36 on the northeast end of Fossil Creek Reservoir. The Parcel lies outside the Urban Growth Area as defined by Larimer County and the City of Fort Collins. _ Adjacent to the Fossil Creek Reservoir is a metes and bounds which allow the perpetual access easement for the North Poudre Irrigation Company. This existing easement serves as the south property boundary and will be enhance by the great number of buffer areas being proposed. Site Description This site falls outside of any existing hydrologic master plan, thus Northern Engineering Services (NES) will be presenting a stand alone design to accommodate the project and all hydraulically connected offsite basins. Topographically the site (±465.2 acre) would be referred to as a "bowl " shape which encircles the Reservoir on the south boundary The surface slopes are gradual and run generally toward the Reservoir. The property was formally in a combination of non- agricultural rangeland and irrigated agricultural land use. Most of the service ditches within the project area will be preserved with only rerouting occurring. This surface shape and irrigations history lends itself to the use of surface runoff conveyance which will be the focus of the preliminary review. Mr. Russ Legg, Planner Larimer County Planning Office Larimer County 200 West Oak Street Fort Collins, Colorado 80521 May 20, 1997 RE: Fossil Lake P.U.D. Larimer County, Colorado Project Number: 9714.00 Dear Mr. Legg, Northern Engineering is pleased to submit this Preliminary Drainage Report for Fossil Creek P.U.D. for your review. It represents a preliminary study of existing and proposed storm water characteristics of the project site. If you should have any questions or comments as you review this report, please feel free to contact me at your convenience. Respectfully, NORTHE GINEERING SERVICES, INC. Alex V. Evonitz III, Project Manager I hereby certify that this report for the preliminary prepared by me (under my supervision) for the owners th Larimer County Sto -Water Mana ment Manual. ��.,.��-IF Mike Falcon Jones, P State of Colorado, N 3 Professional design of Fossil Lake P.U.D. was 19&w exceed the criteria in the V- r y� W 25033 Al PRELIMINARY DRAINAGE REPORT FOR Fossil Lake P.U.D. Larimer County, Colorado Southeast Corner of Larimer County Roads #36 and #9 May 20, 1997 Prepared For: Stan Everitt Everitt Companies P.O. Box 2125 Fort Collins, Colorado 80522 Prepared By: Northern Engineering Services, Inc. 420 S. Howes, Suite 202 Fort Collins, Colorado 80521 Preliminary Drainage Report for Fossil Lake P.U.D. May 20, 1997 Prepared For: Everitt Companies P.O. Box 2125 Fort Collins, Colorado 80522 Prepared By: Northern Engineering Services, Inc. 420 South Howes, Suite 202 Fort Collins, Colorado 80521 (970)221-4158 Fax(970)221-4159 Project Number: 9714.00