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Home My WebLink AboutGLENMOOR DRIVE PUD - PRELIMINARY - 8-95A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT4962.00 4960.00 u) 4958.00 2 cM c 0 ca 4956.00 W 4954.00 4952.00 L- 800 �— Existing bed profile — Proposed bed profile Water surface profile - existing conditions — — — — Water surface profile - proposed conditions 127\ 1135 10810 1020 965 890 Glenmoor Drive 1365 i' X 900 1000 1100 1200 1300 1400 1500 Station (ft) Figure 2. Channel invert and water surface profile comparison. 'LA PORTE AVE UE -_= _ -- t 1 j T / i I uCUXiPIN e Z I nIIVON! I LFaNO ✓ICv _� —� �- •� • 1 / x/Lx :INO(i. O i ( CCYCIERi I , I 1 1 OAK _ r JJRS Scale: l" r--1260' a - I CT '1 i CT. J /Lily P,vk Lo1e - jm MULBERFy -�^�STRE T 10 LEGEND: 10 Drainage Subcatchment Channel Element Detention Pond Peakf low. Point ( see Table 2 ) Figure 1: S W M M NETWORK WEST PLUM STREET MASTER DRAINAGE BASIN PLAN 11 27 Z _ t: F LK dF 1= 1 M1T I HPOOER SCHOOL 0 a O fC P.Pf dvfPa Proposed Glenmoor Drive Subdivision 26 au rrzn , 40 "d-- ~La jZ W ,0; I PLACE - y Table 3. Comparison of Computed 1 00-Year Water Surface Elevations Adjacent to and Upstream of the Development. ............ .... .... ...... ... t Tk f Y..., C t d.1 urface::: evations Cross :SectionExisting .. .... - ....... -�;:, Conditions., ::.:� -ti : o'e ct Pond ons Difference 890 4958.05 4958.03 -0.02 965 4958.22 4958.04 -0.18 1020 4958.26 4958.03 -0.23 1080 4958.35 4958.38 0.03 1135 4958.41 4958.57 0.16 1270 4958.64 4958.83 0.19 1365 4959.23 4959.04 -0.19 1490 4959.44 4959.52 0.08 Table 2. Computed Peak Flows in Major Drainage Channel. keiurn Point L cation Element is .......... .... .. . Conditions* D V. [bom: L Glenmoor 38 2-year 66 66 Drive 1 00-year 273 273 M Skyline Driv . e 39 2-year 88 89 1 00-year 400 401 *Includes Glenmoor PUD IMr. Kint Glover Page Two March 20, 1995 Roughness coefficients (Manning's n values) and downstream starting water surface elevations ' are also the same as the original study. The increased detail used to represent existing conditions on the property resulted in a 0.05-foot increase in water surface elevation at Cross Section 1490 just upstream of the property. ' Developed conditions through the property were modeled by adjusting cross sections and adding encroachments to reflect the design configuration. The HEC-2 special culvert routine was used to model the extension of Glenmoor Drive across the drainage channel. Several ' design configurations were modeled, all consisting of an improved grass lined channel that follows the alignment of the existing drainage ditch. A Manning's n value of 0.035 was used for the ditch, consistent with City of Fort Collins drainage criteria. A final design was arrived at that ' consists of a trapezoidal channel with a 35 foot base width and 4:1 sideslopes. The channel invert is approximately one foot below existing grade at the downstream (east) property boundary, and follows a constant 0.5 percent grade through the property. Three 25H X 12'W reinforced concrete box culverts were used for the Glenmoor Drive road crossing. Several ' different culvert crossings were considered, consisting of both multiple reinforced concrete pipes and reinforced concrete box culverts. The analysis showed that the concrete box culverts would be necessary since the reinforced concrete pipe culverts would create unacceptable ' backwater upstream. No road overtopping was computed for the final design configuration. Although the City of Fort Collins drainage criteria allow 6-inch overtopping for the 100-year event, this would not be acceptable in this situation due to the resulting backwater upstream. Figure 2 shows existing and final design channel grades and computed 100-year water surface elevations. The computed 100-year water surface elevations are compared in Table 3. The results show that the final design has only minimal impacts to water surface elevations adjacent ' to and upstream of the property. The maximum rise adjacent to the property upstream of the Glenmoor Drive crossing is approximately 0.2 feet (cross sections 1135 and 1270). The rise just upstream of the property (cross section 1490) is 0.08 feet. Since no floodway has been ' developed for the West Plum drainage, encroachments which allow up to 0.5 ft rise in the water surface should be allowed. This analysis shows that the proposed improvements are well within these limits. I hope these results support your drainage plan. Please give us a call if you have any questions. Sincerely, rick, Ph.D., P. ice President. cc: Jeff Couch cAwo rki ng\g Iov0317.Itr 34-0254.00 AIRES ASSOCIATES Formerly Resource Consultants & Engineers (RCE) March 20, 1995 Mr. Kint Glover 2101 Lindenmeier Road Fort Collins, Colorado 80524 Re: Hydrologic and Hydraulic analysis to support drainage plan for proposed Glenmoor Drive Subdivision Dear Mr. Glover: This letter summarizes our investigations performed to support the drainage plan for the proposed Glenmoor Drive Subdivision. The proposed development is located in the West Plum Street Basin and will encroach into the 100-year floodplain. The investigations were performed to (1) determine if on -site detention would be required due to increased runoff from the . development, and (2) to evaluate the impact of the proposed floodplain encroachment on water surface elevations. The impact of the development on peak flows in the main drainage channel was analyzed using the SWMM hydrologic model for the West Plum Street Basin Master Drainage Plan. The model as developed for the master plan reflected full development of the basin. As part of a subsequent study to determine the impacts of the Glenmoor PUD on peak flows, the model was modified to reflect existing development in the basin. This model, with the effects of the 3-acre Glenmoor PUD included, formed the baseline condition for determining the impact of the Glenmoor Drive Subdivision. Figure 1 shows the complete basin model and identifies the location of the Glenmoor Drive Subdivision in the basin. Table 1 shows the changes in percent impervious area made to simulate the effect of the proposed development. The Glenmoor Drive Subdivision was assumed to develop 50 percent impervious area which resulted in weighted percent impervious areas of 31 and 26 percent for subbasins 16 and 24, respectively. Table 2 summarizes the computed peak flows for the 2- and 100-year events. The results show only a 1 cfs increase in peak flows for both events downstream of the development. This represents an insignificant impact on peak flows in the West Plum Street Basin, and therefore on -site detention is not necessary. Printouts of the model results are included for your review. A second study was performed to determine if the proposed encroachment into the floodplain will adversely affect water surface elevations adjacent to and upstream of the development. For this analysis, the HEC-2 water surface profile model developed for the West Plum Street Basin Master Drainage Plan was utilized. Six additional cross sections were added to the model to provide a more detailed description of hydraulic conditions through the property. The additional cross sections were developed using the February 8, 1995, topographic survey performed by Stewart & Associates, Inc. Where the cross sections extended off the Stewart & Associates map, topographic information from the City of Fort Collins 1"=100' scale topographic map of the area (dated May, 1974) was utilized. Plate 1 shows the cross section locations. Discharges in the model, representing the 100-year event under full basin development, are the same as those in the original master plan study. Owen Ayres & Associates, Inc. c:\working\glov03l7.ltr Engineers/Scientists/Surveyors 34-0254.00 3665 JFK Parkway, Building 2, Suite 300, P.O. Box 270460, Fort Collins, CO 80527 (303) 223-5556, Denver Metro (303) 572-1806, FAX (303) 223-5578 Printed on recycled paper