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HomeMy WebLinkAboutSILVERBERG PUD PRELIMINARY - 12 92A - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTPRELIMINARY DRAINAGE AND EROSION CONTROL REPORT FOR SILVERBERG PUD To Meet the Preliminary Submittal Requirements for the City of Fort Collins 256 West Mountain Avenue Fort Collins, Colorado 80521 AYRES ASSOCIATES P.O. Box 270460 Fort Collins, Colorado 80527 (970) 223-5556, FAX (970) 223-5578 Ayres Project No. 34-0388.00 SILVERI 1.TXT August 1997 A MS ASSOCIATES Table 3. Conveyance Elements and Peak Discharges. Gutter Connected To: Type Width or Dia. (ft) Length (ft) Slope (ft/ft) Side Slopes Manning's n Peak Discharge Left (h Right (H:V) 2-Year (cfs) 100-Year (cfs) 101 102 Channel w/ Overflow 0.5 290 0.024 50- 0 0.016 1.63 5.35 Overflow Section 5.0 290 0.024 50 20 0.020 102 103 Channel 2.0 93 0.006 50 50 0.016 1.55 5.3 103 104 Channel 2.0 126 0.006 50 50 0.016 1.96 6.96 104 105 Channel 2.0 88 0.006 50 50 0.016 6.01 23.53 105 106 Channel 2.0 107 0.006 50 50 0.016 7.09 27.46 106 200 Channel 0.5 103 0.006 50 50 0.016 8.24 31.61 107 109 Channel w/ Overflow 0.5 67 0.004 50 0 0.016 0.34 1.51 Overflow Section 5.0 67 0.004 50 20 0.016 108 109 Channel w/ Overflow 0.5 150 0.004 0 50 0.016 0.75 3.31 Overflow Section 5.0 150 0.004 20 50 0.016 109 110 Channel 2.0 200 0.006 50 50 0.016 0.95 4.43 111 110 Channel w/ Overflow 0.5 180 0.004 0 50 0.016 0.66 3.49 Overflow Section 5.0 180 0.004 20 50 0.016 110 104 Channel w/ Overflow 0.5 137 0.004 0 150 0.016 2.89 12.28 Overflow Section 5.0 137 0.004 20 150 0.016 112 113 Channel 2.0 161 0.004 50 50 0.016 0.67 2.61 113 104 lChannel 2.0 122 0.004 50 50 0.016 0.6 2.44 114 105 lChannel 2.0 142 0.006 50 50 0.016 1.32 4.45 115 106 lChannel 2.0 145 0.004 50 50 0.016 1.34 4.51 116 117 Channel 2.0 172 0.004 50 50 0.016 1.65 5.53 117 118 Channel 2.0 107 0.004 4 4 0.030 1.58 5.36 118 119 Channel 2.0 110 0.004 4 4 0.030 1.81 6.99 119 200 Channel 2.0 65 0.010 200 200 0.016 1.78 6.97 120 200 Channel 2.0 171 0.016 4 4 0.040 0.13 1.97 121 122 Channel 2.0 147 0.004 50 50 0.016 1.53 5.32 122 200 Channel 2.0 64 0.004 50 50 0.016 2.9 9.61 123 JChannel 2.0 2 0.004 4 4 0.04 7.65 16.71 124 123 Channel 2.0 140 0.004 4 4 0.040 0.39 7.99 125 123 Channel 2.0 174 0.004 4 1 4 1 0.040 0.35 1 7.43 126 123 Channel 2.0 462 0.002 4 4 0.040 0.07 1.42 127 Channel 2.0 156 0.003 4 4 0.040 1.27 3.99 200 123 Pond I - - - - - - 4. STORM DRAINAGE FACILITIES A major portion of drainage flows off of parking lots and street gutters through sidewalk or curb culverts. The culverts will allow runoff to enter drainage flowpaths or the detention pond. Drainage swales will have a 2-foot wide concrete trickle pan and 4 to 1 side slopes. They will contain the 100-year runoff with a minimum of 33 percent additional capacity. The detention pond, element 200, will be sized to contain the 100-year storm runoff from contributing subbasins. Runoff will enter the pond through a Swale and curb culverts. For preliminary calculations, the pond was modeled using a small pipe in place of a pond volume rating curve. SWMM will allow the pipe's capacity to pass through and the remainder of the runoff to pond behind the pipe, thus indicating the amount of required storage volume. A 1.5 foot diameter pipe was used for the 100-year event and a 1.0 foot 9 Ayres Associates diameter pipe for the 2-year event. The peak 100-year outflow was 7.11 cfs and the required storage volume of 0.88 ac-ft. The as -graded pond volume is 1.15 ac-ft, as shown by the computations included in the Appendix. We anticipate that the surplus pond volume will be used to accommodate reduced pond outflow during the early part of the storm. The 2-year peak outflow from the pond reached a value of 1.12 cfs. Including undetained flow, the amount of runoff in the natural drainageway (elements 123 and 127) is 16.7 cfs, which is less than the historic 100-year runoff of 17.40 cfs. The 2-year discharge leaving the site is 1.73 cfs versus 1.70 cfs for the historic condition. During the final design, the pond outlet structure will be refined to reduce the 2-year release to historic conditions. A volume rating curve which represents the final outlet structure and detention pond grading will be used for the final design. The outlet structure will consist of an ADS pipe with orifice to reduce the outlet flow rate during large storm events (e.g.,- the 100-year storm). A concrete weir will be built around the outlet to reduce the flow rate during small storm events. A 3-inch diameter PVC pipe will run through the weir to allow nuisance flows to pass and the for the detention pond to completely drain.. No buildings will be inundated with detained 100-year runoff. An overflow spillway will be located above the pond's outlet structure. The spillway would release water into the drainageway if the orifice clogged during the 100-year storm. 5. EROSION CONTROL PLAN 5.1 Overview Silverberg PUD will be a 7.5 net acre development on moderate erodibility soil. Overlot grading will begin in April, 1998, with building construction to begin in June, 1998 (Figure 6). Both proposed wind and water erosion control measures will be addressed in this plan; however, calculations will be completed only for the final submittal. 5.2 Wind Erosion Since construction will occur during the spring months, effective wind erosion plans must be established. On moderate erodibility soil, significant erosion could occur without steps taken to protect the soil. Those portions of all basins not covered with pavement, concrete, or buildings will have 2 tons/acre of straw mulch applied for water erosion control, which also meets the requirements for minimum wind erosion protection. All mulches will be properly anchored. 5.3 Rainfall Erosion Runoff leaves the site in two areas: -(I) overland flow across the south property line toward the natural drainageway, and (2) overland flow across the east property line toward the ditch along the frontage road. Measures will be taken to prevent sediment from leaving the site at these areas. 10 Ayres Associates PROJECT: Silverberg PUD SEQUENCE FOR 1998 ONLY COMPLETED BY: Chris Carlson DATE: 8/20/97 Indicate by use of a bar line or symbols when erosion control measures will be installed. Major modifications to an approved schedule may require submitting a new schedule for approval by the City Engineer. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. OVERLOT GRADING WIND EROSION CONTROL Soil Roughing Perimeter Barrier Vegetative Methods Mulching / Sealant RAINFALL EROSION CONTROL STRUCTURAL: Detention Pond Pond Outlet Filters Straw Barriers Silt Fence Barriers Sand Bags Bare Soil Preparation Contour Furrows Asphalt / Concrete Paving VEGETATIVE: Permanent Seed Planting Mulching / Sealant Temporary Seed Planting Sod Installation "7 Nettings / Mats / Blankets BUILDING CONSTRUCTION: STRUCTURES INSTALLED BY: Contractor VEGETATION/MULCHING CONTRACTOR: Contractor DATE SUBMITTED: 8/20/97 MAINTAINED BY: Contractor APPROVED BY CITY OF FORT COLLINS ON: Figure 6. Construction timetable. 11 Ayres Associates ' A silt fence, which will prevent sediment from flowing off the property, will be placed along the southern and eastern property lines. The swales that run to the detention pond ' (element 200) will contain straw bale dikes. These dikes will be no more than 100 feet apart and will be placed in accordance with City of Fort Collins Storm Drainage Design Criteria procedures. During overlot and final grading, the soil will be roughened and furrowed ' perpendicular to the prevailing winds (NW to SE). All areas not covered by asphalt, concrete, or foundations will have 2 tons/acre of straw mulch either crimped 4 or more inches into the soil or sprayed with a tackifier. After the detention pond outlet pipe is t completed, a gravel filter will be installed at the pipe's inlet. Gravel filters will also be installed at all curb culverts. The filters will remain in place until construction is complete. ' With proper precautions, damage to offsite areas from a greater than historic sediment load can be eliminated and the loss of sediment from the site minimized. Calculations for performance standards and effectiveness will be submitted with the final drainage and ' erosion control plan. ' 12 Ayres Associates AIRES ASSOCIATES September 2, 1997 Mr. Basil Hamdan City of Fort Collins Stormwater Utility 235 Mathews Street Fort Collins, Colorado 80524 Re: Silverberg PUD Dear Basil, Enclosed please find our "Preliminary Drainage and Erosion Control Report for Silverberg PUD." This report was prepared in accordance with the City of Fort Collins Storm Drainage Criteria as revised in May 1991. We appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Sincerely, a``a pt�fEje en Ayr & Associates, Inc. Scott Que P.E. Civil Engineer SQ:sp Enclosure Owen Ayres & Associates, Inc. Engineers/Scientists/Surveyors HAMD-9A. LTR 3665 JFK Parkway, Building 2, Suite 300, P.O. Box 270460, Fort Collins, CO 80527 34-0388.00 (970) 223-5556, Denver Metro (303) 572-1806, FAX (970) 223-5578 Printed on recycled paper 1. INTRODUCTION This report summarizes the storm drainage investigations performed for the proposed Silverberg PUD in Fort Collins, Colorado. A hydrologic analysis of the proposed development was completed to determine the magnitude and location of storm runoff, allowable release rates into existing drainageways, determine the required detention volume, and to determine downstream effects of the proposed development. This analysis and the drainage plan were prepared in accordance with City of Fort Collins Storm Drainage Design Criteria. Silverberg PUD is located at the southeast corner of East Prospect Road and Interstate 25 as shown in Figure 1. It consists of approximately 7.5 acres occupying a portion of the northeast quarter of Section 21 and the northwest quarter of Section 22, Township 7 North, Range 68 West, of the 6th Principal Meridian and lies within the Boxelder Creek drainage basin. _, 2. EXISTING DRAINAGE CONDITIONS Silverberg PUD lies between the frontage road and Interstate 25 at the southeast corner of East Prospect Road and Interstate 25. The property is bordered by State right-of-way on the west, East Prospect Road to the north, the frontage road to the east, and private agricultural land to the south. The property is currently covered with irrigated crops. Generally, runoff sheet flows to the southwest into a shallow natural depression that conveys runoff away from the site. This depression, which really can't be defined as a channel, continues south along the side of the interstate. Drainage flows south through fields and roadside ditches to the Cache la Poudre River. At this time, there are no drainage easements along the drainage path. A small portion of land drains southeasterly into a small ditch that parallels the frontage road. Existing runoff calculations were performed to determine the 2- and 100-year runoffs for the site using the Storm Water Management Model (SWMM, UDFCD 1985). A schematic showing the linkages of the subbasins and conveyance elements for the existing conditions is shown in Figure 2. The overall basin is shown in Figure 3. The total runoff leaving the site in the natural drainageway is 1.05 and 17.40 cfs during the 2- and 100-year storms, respectively. The total site runoff in the ditch along the frontage road is 0.65 and 4.00 cfs during the 2- and 100-year storms, respectively. These flows include contribution from offsite areas. SWMM output is shown in the Appendix. We also performed a hydrologic and hydraulic analysis for some of the area north of Prospect Road to determine the likelihood that flows from that area would overtop Prospect Road and flood Silverberg PUD. The tributary area north of concern extends north from Prospect and is bounded on the west side by Interstate 25, and on the east side by the south embankment of the Lake Canal. The north limits of the tributary area is also limited by the south embankment of the Lake Canal where it intersects the frontage road. We note that flows north of the frontage road and south of Lake Canal are likely to overtop the frontage road and pond along the north side of Prospect Road. This tributary area encompasses 26.6 acres, consisting. of agricultural land and roadways (frontage road, 1-25 on -ramp, and north lane of Prospect Road). A 100-year runoff of 15 cfs was computed using the rational method (computations are included in the Appendix for reference). Ayres Associates Mulberry St. IL Project Site • Prospect Rd. i PO i �@ i D j i 1 CD ti O \ O CD u Boxelder Sanitation'' ~� District Facilities a� i 01 $1 w Figure 1. Vicinity map. 2 Ayres Associates EXISTING CONDITIONS SWMM SCHEMATIC w 150 1 55 50 01 155 165 0 LEGEND 10 SUBBASIN LABEL 110 CONVEYANCE ELEMENT 9165 DESIGN POINT Figure 2. Existing conditions SWMM schematic. / EDGE OF ASPHALT Figure 3. Existing conditions overall basin. The single relief for ponding on the north side of Prospect Road is a 24-inch CMP crossing from north to south under the Prospect Road embankment approximately 230 feet east of the center of the intersection of Prospect Road and the 1-25 off/on ramps. The invert of the culvert is approximately 4.4 feet below the crown of Prospect Road at its lowest point (just east of the proposed Silverberg entrance). We used HY-8 to evaluate the culvert hydraulics, and determined that the entire 100-yr discharge of 15 cfs can pass through the culvert without causing overtopping of Prospect Road (computations are included in the Appendix). Consequently, no flood flows from the north side of Prospect were included in our site drainage analysis. 3. PROPOSED DRAINAGE CONDITIONS A detailed grading, drainage, and erosion control plan (enclosed in pocket) shows the building layout and storm drainage facilities that will be used for Silverberg PUD. The map contour interval is 1 foot. A schematic of the proposed conditions SWMM network is shown in.Figure 4. The overall basin is shown in Figure 5. The Silverberg property and adjacent State right-of-way were divided into 24 subbasins that contribute drainage to 2 separate outlets from the property. The SWMM model was used to determine the proposed condition 2- and 100-year peak discharges, detention pond volumes, and downstream effects. Subbasin constants are shown in Table 1, basin parameters in Table 2, and conveyance elements and runoff in Table 3. The various basin parameters were determined from our preliminary design. Specifically, the percent imperviousness for each basin was computed by making use of area computations available in our CAD software. In some cases, the percent impervious appears to be lower than expected, but the permeable area in parking lot islands and landscaping areas occupy more area than one would expect. The majority of the Silverberg property will drain southwesterly to a proposed detention pond at the property's southwest corner. The pond will detain storm runoff and release it into the existing drainageway at or below historic runoff rates. A portion of property near the east boundary will have undetained runoff flowing into the ditch along the frontage road. Runoff from subbasins 31, 32, 33, 34, 36, 37, 38, 39, 41, and 42 will sheet flow to street gutters or concrete pans and into the detention pond (SWMM element 200). Runoff from subbasins 35 and 40 will flow into a swale along the southern property line. The swale will convey the runoff to the detention pond. Runoff from subbasins 10, 11, and 12, which includes the area on Prospect Road at the site's entrance, will flow through a curb cut and into the parking lot system, eventually reaching the detention pond. A swale conveys runoff from subbasin 44 to the detention pond. Subbasin 43 drains directly into the detention pond. Subbasins 20, 21, 45, and 46 , consisting wholly of State ROW property, drain to a small swale just west of the detention pond (offsite of Silverberg) that flows south to the detention pond outlet. At this point, the swale flow combines with the pond outlet flows and leaves the project site. The runoff from subbasin 23 will flow undetained into the existing ditch along the frontage road. Since most of this subbasin is offsite property, the drainage patterns, runoff volumes, and flow rates will not be altered appreciably. Undetained runoff will be at or below historic rates. Ayres Associates as Proposed Conditions SWMM Schematic Figure'4. 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