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Home My WebLink AboutHARMONY RIDGE PUD, PHASE 2 (2ND FILING) - PRELIMINARY / FINAL - 49-95D - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFIGURE 1 2C F : Fcsl—'T 6i044.G,tS ou ..n" -MA EO&MUF Leigh & Whitehead Associates Inc. _ CONSULTING CIVIL ENGINEERS &, SURVEYORS 2720 EAST YAMPA STREET, SUITE I COLORADO SPRINGS, CO 80909-5061 ' ' The results of the analysis with supporting calculations demonstrating the pond's total capacity, inflow/outflow can be found in Appendix F. 3.3 Erosion Control Because the Cathy Fromme open space is downstream from the majority of the site, sediment control will be critical. Most of the area from this site contributing runoff to the open space will either be undisturbed or will pass through a water feature/quality ponds that will act as sediment ' trap, inlet filters or pond outlet pipe filters, in perimeter silt fence, or by straw bale check dams located in the channels and drainageways. A sediment trap is being installed at DP-17 and Basin D 1 for additional silt control. Long term erosion protection will include re -seeding of all disturbed areas, channel bed stabilization, and pipe outlet rip -rap. The onsite measures and the natural processes present in the existing drainageways should ensure no detrimental effects to water quality in the Burns Tributary. Erosion control calculations and the erosion control cost estimate can be found in Appendix G. '] LWA #99104.62 proposed inlets so that inlet capacity is not affected. The results of the Storm Sewer Analysis and Design with supporting calculations and model outputs are presented in Appendix C. 3.2.3 Inlet Analysis All of the proposed inlets will be CDOT Type R curb inlets. All of the inlets will be in sump conditions and will intercept the 100-year runoff. The results of the inlet analysis and design with supporting calculations are presented in Appendix D. 3.2.4 Rip -Rag and Swale Design Proposed swales were designed to prevent erosion caused by storm runoff. In cases where this could not be accomplished by grade control, appropriate rip -rap was used. Proposed swales were evaluated with a program which utilizes Manning's Equation to determine flow depth and velocity based on input design flows, bed slopes, channel cross section, and roughness coefficients. The effects of developed flows from this site on the two proposed storm drain system's rip -rap pads were analyzed to determine the size of the pipe outlet. A summary of the rip -rap requirements for this site and our analysis of the swale is presented in Appendix E. 3.2.5 Detention Pond Analysis The existing detention pond located southeast of this site was used to reduce developed flows that continue southerly within the existing channel. With an inflow of approximately 55 cfs, the pond will peak in about 30 minutes in a 100-year event. Our analysis of the detention pond indicates that 0.42 acre feet of active capacity is required. The 100-year ponding elevation was determined by the flowline elevation of the existing outlet pipe. Gl LWA #99104.62 this portion of the Burns Tributary, which is tributary to Fossil Creek. Although detention is not required in the site, storm water quality is an important consideration because of the Cathy Fromme open space the site discharges to. Storm runoff from Basin A will be collected at low points in the streets and routed through a storm drain system and outlet into a sediment basin. Runoff from this basin will be routed via swale to an existing drainageway that flows in a southerly direction under the existing irrigation canal. Basin B discharges to the existing pond located southeast of this site. Runoff from this basin is collected entirely from low points in the streets and conveyed directly to the existing pond via storm sewers. Basins C, D, E, F, G and H are small basins that sheetflow in a southerly direction. Sub -basin C1 in Harmony Ridge Filing No. 1 features a water feature/quality pond that will serve as sediment control and modified detention. The results of the Rational Method Hydrologic Analysis with the methodology of calculations is shown in Appendix A. 3.2.1 Street Capacity Street encroachment criteria for the streets was taken from Table 4-1 (minor storm) and Table 4-2 (major storm) of the SDDC. Capacity calculations for all of the streets to assure that the runoff did not exceed criteria and will function below the allowable capacities. The results of the Street Capacity Analysis can be found in Table 3 with supporting calculations presented Appendix B. 3.2.2 Storm Sewer Desien Storm Lines 1 and 2 will allow for most of the runoff from Basin A and B to be intercepted and directed to an existing drainageway or detention facility. The lines were analyzed with StormCad and pipes sized such that hydraulic grades remain at or below the flow line of 5 LWA #99104.62 utilized to determine the time of concentration. tC=t;+tt where tc is the time of concentration in minutes, t; is the initial or overland flow time in minutes, and t1 is the travel time in the gutter in minutes. The initial or overland flow time was calculated with the SDDC manual equation. t1 = [1.87(1.1 - CC &0.5hs)0.33 where L is the length of overland flow in feet (limited to a maximum of 500 feet), S is the average basin slope in percent, C is the composite runoff coefficient, and Cf is the storm frequency coefficient. The formula limits the product of CCf to 1.0 and when the product exceeds this value, 1.0 is used in its place. Gutter (or channel) travel times were determined by calculating the flow velocity within the conveyance element assuming a flow depth equivalent to a minor storm. The travel time was then determined by dividing the gutter flow length by the velocity. This procedure for computing time of concentration allows for overland flow as well as travel time for runoff collected in streets, gutters, channels, or ditches. After the peak runoff was calculated, design point runoff was calculated. This was done by combining all contributing areas upstream of a given design point. The time of concentration for the design point was taken as the greatest time of all the contributing sub -basins. 3.2 Drainage Plan Development The proposed drainage plan consists of a combination of overland flow and gutter flow. The runoff will sheetflow across landscaped yards and common areas, then concentrate at proposed street low points, drainage swales, or in a detention pond. Gutter flow in streets will be collected at low points via curb inlets and then conveyed to the channels or ponds via a storm sewer system. Sub -basins were delineated based on proposed grading. Final grading and basin delineation are shown on the Final Drainage and Erosion Control Plan sheets which can be found in the back of this report. This site lies within the study area of the Fossil Creek Basin Master Plan (Simons & Li). Per this master plan, no detention is required and we have confirmed that no detention is required within 4 LWA #99104.62 3.0 DEVELOPED CONDITIONS PLAN 3.1 Design Criteria The drainage system presented in this report has been developed in accordance with the criteria established by the City of Fort Collins Storm Drainage Design Criteria and Construction Standards Manual (SDDC) dated May 1984 and revised in January 1997. Where applicable, design guidelines and information were also obtained from the Denver Regional Council of Government Urban Storm Drainage Criteria Manual (USDCM). Developed condition hydrology was evaluated based on the 2-year and 100-year storm frequencies as dictated by Table 3-1 of the SDDC manual. Detention of developed flows from the north end of this site is not specifically required by the McClellands Basin Master Plan but is provided by this design. The southern portions of the development are within the Fossil Creek Basin in which there is no requirement for detention so historic runoff computations were not necessary for this site. Because of the limited size of the sub -basins on the site, the Rational Method was selected to calculate runoff. The Rational Method utilizes the SDDC manual equation. Q = CCfIA where Q is the flow in cfs, C is the runoff coefficient, Cf is the storm frequency coefficient, I is the rainfall intensity in inches per hour, and A is the total area of the basin in acres. The runoff coefficient, C was calculated from Table 3-3 of the SDDC manual based on the proposed developed condition land use. A composite runoff coefficient was calculated for each sub -basin based on the percentage of impervious surface (C = 0.95) and pervious surface (C = 0.20). Cf was taken from Table 3-4 of the SDDC manual and was determined to be 1.0 for the 2-year storm and 1.25 for the 100-year storm. The appropriate rainfall intensity was taken from the updated rainfall intensity duration curve in Figure 3-1 of the SDDC manual. To obtain the rainfall intensity, the time of concentration had to be determined. The following equation was 3 LWA #99104.62 2.0 HISTORIC CONDITIONS The site currently lies within the Fossil Creek Drainage Basin. The site has a variety of native grasses and shrubs with some cactus in the higher areas and some small volunteer Russian Olive trees in the wetter drainageways. Vegetation is moderately dense consisting mostly of dryland grasses and some cactus. Rocks of moderate and small sizes are present and the topsoil appears to be underlain by gravel and bedrock. The site drains in a southerly direction and discharges runoff to the Burns Tributary in the Cathy Fromme Prairie open space. An existing irrigation lateral flows east to west along the southern property limits of the site. This ditch is still in use by others and will remain in use after this development is completed. It is suspected that the ditch may leak and could contribute moisture to the downstream drainageway. 2 LW A #99104.62 Rva i:. �mr�a,tt 1.0 INTRODUCTION 1.1 Scope and Purpose: This report presents the results of a final drainage evaluation for the Harmony Ridge P.U.D. Filing No. 2. A hydrologic analysis of the proposed development plan was completed to determine the location and magnitude of the storm runoff. The hydrologic data was then used to evaluate conveyance and detention facilities based on City criteria. 1.2 Project Location and Description: The Harmony Ridge P.U.D. Filing No. 2 is a proposed townhome residential site. The property is located in the Northwest Quarter of Section 3, T6N, R69W of the 61h Principal Meridian, Larimer County, Colorado. The site is bounded on the north by Harmony Road, on the east by Harmony Ridge P.U.D. Filing No. 1, on the south by the Cathy Fromme Prairie, and on the west by unplatted vacant land. A vicinity map illustrating the project location is provided in Figure 1. The Harmony Ridge P.U.D. Filing No. 2 consists of approximately 14.12 acres. The townhome site has a net density of approximately 8.11 du/ac. The development will consist of 123 townhome units with garages. The townhomes will be two story buildings with walkout basements. 1.3 Previous Studies: The Final Drainage Report and the Final Report Addendum for the Harmony Ridge P.U.D. (TST, Inc., January 20, 1999 and June 20, 1999 respectively) were reviewed prior to the preparation of this report. Portions of the Preliminary Drainage Study for the Overlook at Arapaho Farms P.U.D. (RBD, Inc., 1996) and the Fossil Creek Drainage Basin M.D.P.S. were reviewed for information pertinent to this development. LWA #99104.62 J TABLE OF CONTENTS 1.0 Introduction Page 1.1 Scope and Purpose ........................................... 1 1.2 Project Location and Description ................................ 1 1.3 Previous Studies.............................................1 2.0 Historic Conditions ................................................ 2 3.0 Developed Conditions Plan .......................................... 3 3.1 Design Criteria..............................................3 3.2 Drainage Plan Development ................................... 4 3.2.1 Street Capacity ......................................... 5 3.2.2 Storm Sewer Design ................................... 5 3.2.3 Inlet Analysis ......................................... 6 3.2.4 Rip -Rap and Swale Design .............................. 6 3.2.5 Detention Pond Analysis ................................ 6 3.3 Erosion Control.............................................7 Fizur_es Figure 1 - Location Map Technical Appendices Appendix A - Rational Method Analysis Appendix B - Street Capacity Analysis Appendix C - Storm Sewer Design Appendix D - Inlet Analysis and Design Appendix E - Rip -Rap and Swale Design Appendix F - Detention Pond Analysis Appendix G - Erosion Control Calculations Sheets Drainage & Erosion Control Plans Leigh &� II Whitehead?Associates, Inc. ENGINEERS, SURVEYORS & PLANNERS January, 2000 Basil Hamden City of Fort Collins Stormwater Department P.O. Box 580 Fort Collins, CO 80522 RE: Harmony Ridge P.U.D. Dear Mr. Hamden: 2720 EAST YAMPA STREET, SUITE I COLORADO SPRINGS, CO 80909-5061 TELEPHONE 719-63&5179 / FAX 719-63G5199 In accordance with the requirements of the City of Fort Collins Subdivision Ordinance, a drainage report and plan has been prepared for the proposed Harmony Ridge development. This report has been prepared under the current City of Fort Collins Drainage Criteria. Five (5) complete copies of the drainage report and plan are hereby transmitted for your review and approval. If there are any questions or comments concerning this report, please contact the undersigned. Sincerely, Leonard A. Beasley Date Project Engineer DAMD I WIiMEAD FINAL DRAINAGE REPORT AND PLAN FOR HARMONY RIDGE P.U.D. FILING NO.2 January, 2000 Leigh p,Whitehead (-•Associates Inc. CONSULTING CIVIL ENGINEERS & SURVEYORS 2720 EAST YAMPA STREET, SUITE 1 COLORADO SPRINGS, CO 80909-5061 LWA Project No. 99104.62