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HomeMy WebLinkAboutCOTTONWOOD RIDGE SUBDIVISION - PRELIMINARY - 31-94 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTNo Text ft length. b. S = % slope of basin = 26/380 =6.8 % C. D= Length of Basin ft. = 380 ft d. Cf= Frequency Adjuster Table 3-4.= 1.00 Tc= 1.87(1.1-0.20(1.0))*(380)1/2 /(6.8)1/3 @ Tc= 17.3 min I= 2.0 in/hr Figure 3-1 e. Q=CfCIA A =1.9 ac measured area. Q = (1)(0.20)(2.0)(1.9) Q=0.8 cfs 2 year existing basin "D" 3. Existing conditions 100 year flow. a. Frequency Adjustment factor = 1.25 Tc= 16.3 min. I= 6.75 in/hr Q = (1.25)(0.20)(6.75)(1.9) Q = 3.2 cfs 4. Developed conditions 2 year flow. a. Length of Basin = 340 ft b. Area = 1.7 ac C. Slope = 26/340 = 7.6% d. C= 0.45 e. Cf=1.0 f. Tc = 11.4 min I = 2.2 in/hr g. Q=0.45*2.2*1.7 = 1.7 cfs 5. Developed conditions 100 year flow a. Cf= 1.25 all other the same as above. b. Tc = 9.4 min I = 7 in/hr off scale C. Q = 1.25*0.45*7*1.7 = 6.7 cfs 1. Composite Runoff Coefficient Use Area Existing C Developed C The Ridge Lots 2.1 ac 0.45 0.45 Ridge Greenbelt 1.3 ac 0.20 0.20 Cottonwood Lots 8.48 ac 0.20 0.45 From Area "B" 3.07 ac ---- 0.45 Cottonwood open 0.91 ac 0.20 0.20 Area 12.78 ac 15.86 ac Composite Runoff 0.241 0.415 2. Existing conditions 2 year flow. a. Tc=1.87((1.1-C*Cf))*D1/2/S1/3 or Fig 3-2 basins less than 500 ft length. b. S = % slope of basin = 68.8/ 1426 =4.8 % C. D= Length of Basin ft. = 1426 ft d. Cf= Frequency Adjuster Table 3-4. = 1.00 Tc = 1.87(l.1-0.241(1.0))*(1426)1/2 /(4.8)1/3 @ Tc= 36 min I= 1.3 in/hr Figure 3-1 e. Q=CfCIA A = 12.8 ac measured area. Q=(1)(0.241)(1.3)(12.8) Q=4.0 cfs 2 year existing basin "C" 3. Existing conditions 100 year flow. a. Frequency Adjustment factor = 1.25 Tc = 33.4 min. I = 3.87 in/hr Q= (1.25)(0.241)(3.87)(12.8) Q= 14.9 cfs 4. Developed conditions 2 year flow. a. Length of Basin = 1500 ft b. Area = 15.86 ac C. Slope = 65.5/1500 = 4.4% d. C= 0.415 e. Cf=1.0 f. Tc = 30.3 min I = 1.45 in/hr g. Q=0.415*1.45*15.86 = 9.5 cfs 5. Developed conditions 100 year flow a. Cf = 1.25 all other the same as above. b. Tc = 25.7 min I = 4.5 C. Q = 1.25 *0.415 *4.5 * 15.86 = 37.0 cfs 6. Irrigation Flow a. Base Flow b. Minor Storm Flow C. Major Storm Flow E. Point "D" southwest. 1. Runoff Coefficient Existing 0.2 Developed 0.45 2. Existing conditions 2 year flow. a. Tc=1.87((1.1-C*Cf))*D1/2/S1/3 or Fig 3-2 basins less than 500 f. Tc = 42 min I = 1.17 in/hr g. Q=0.45*1.17*10.9 = 5.7 cfs 4. Developed conditions 100 year flow a. Cf= 1.25 all other the same as above. b. Tc = 34.8 min I = 3.8 C. Q = 1.25*0.45*3.8*10.9 = 23.3 cfs C. Point "B" northeast corner. 1. Existing conditions 2 year flow. a. Tc=1.87((1.1-C*Cf))*D1/2/Sl/3 or Fig 3-2 basins less than 500 ft length. b. S = % slope of basin = 36.1 / 1743 = 2.1 % C. C = Rational Coefficient Table 3-2 Storm Drainage Design Criteria City of Fort Collins. = 0.2 undeveloped area. 0.45 developed areas. Existing developed areas of The Ridge 0.72 ac outside greenbelt. Composite runoff coefficient C = ((0.45 *0.72) + (0.2*(16.1-0.72)))/ 16.1 C= 0.211 d. D= Length of Basin ft. = 1743 ft e. Cf= Frequency Adjuster Table 3-4. = 1.00 Tc= 1.87(1.1-0211(1.0))*(1743)1/2 /(2.1)1/3 @ Tc= 54 min I= 1.0 in/hr Figure 3-1 f. Q=CfCIA A =16.1 ac measured area. Q=(0211)(1)(16.1) Q=3.4 cfs 2 year existing basin "B" 2. Existing conditions 100 year flow. a. Frequency Adjustment factor = 1.25 Tc= 51 min. I= 2.95 in/hr Q= 12.5 cfs 3. Developed conditions 2 year flow. a. Length of Basin = 935 ft b. Area = 9.6 ac C. Slope = 20.3/935 = 2.2 % d. C= 0.45 e. Cf=1.0 f. Tc = 28.6 min I = 1.5 in/hr g. Q=0.45*1.5*9.6 = 6.5 cfs 4. Developed conditions 100 year flow a. Cf= 1.25 all other the same as above. b. Tc = 23.6 min I = 4.75 C. Q = 1.25 *0.45 *4.75 *9.6 = 25.6 cfs D. Point "C" southwest. VII. Calculations: A. Calculation of existing flows 2 and 100 year at key points. 1. Point "A" southeast corner. 2. Point "B" northeast corner. 3. Point "C" ravine. 4. Point "D" south side 5. Review of the Fossil Creek Drainage Basin Master Drainageway Planning Study indicates that no areas on the property are subject to the 100 year floods. The Mail Creek drainage plan is under review and not available. B. Calculation of proposed Flows 2 and 100 year at key points. 1. Point "A" southeast corner. 2. Point "B" northeast corner. 3. Point "C" ravine. C. Location and of detention areas. D. Downstream conditions of drainageways. E. Proposed maintenance responsibilities for drainage facilities. VIII. Existing flows 2 year and 100 year at key points. Rational Method must be used Urban Storm Drainage 5.3.1 areas under 200 acres. A. Initial Storm 2 year, Major Storm 100 year B. Point "A" southeast corner. 1. Existing conditions 2 year flow. a. Tc=1.87((1.1-C*Cf))*Dl/2/Sl/3 or Fig 3-2 basins less than 500 ft length. b. S = % slope of basin = 12/ 1100 = 1.1 % C. C = Rational Coefficient Table 3-2 Storm Drainage Design Criteria City of Fort Collins. = 0.2 d. D = Length of Basin ft. = 1100 ft e. Cf= Frequency Adjuster Table 3-4. = 1.00 Tc=1.87(1.1-0.2(l.0))*(1100)1/2 /(1.1)1/3 @ Tc = 54 min I = 1.0 in/hr Figure 3-1 f. Q=CfCIA A = 10.7 ac measured area. Q=(0.2)(1)(10.7) Q=2.1 cfs 2 year existing basin "A" 2. Existing conditions 100 year flow. a. Frequency Adjustment factor = 1.25 Tc = 51 min. I = 2.95 in/hr Q= 7.9 cfs 3. Developed conditions 2 year flow. a. Length of Basin = 1200 ft b. Area = 10.9 ac C. Slope = 12/1200 = 1.0% d. C= 0.45 e. Cf=1.0 The proposed development area in the drainage Area "D" is reduced. The developed southeast cul-de-sac flows into drainage area "A". The peak flows because of development will increase. The drainage area is small. The rational nomograph does not address time of concentrations for areas with less than ten minutes time of concentration. The result is the calculated flows for the 100 year event are on the high side of actual. The downstream existing 18" RCP in Wooded Creek Court should handle calculated and therefore actual flows. Detention is not required in drainages that flow into Fossil Creek. VI. Wind and Water Erosion Empire Laboratories Inc. conducted a Preliminary Soils Report for the site with field exploration on April 28, 1994. Four test holes were drilled on the site to a depth of approximately 15 feet. Generally the report indicates there is 1/2 foot of silty topsoil over a lean clay with sand. The initial permeability of the topsoil is good with some permeability of the underlying layers. The existing vegetation and topsoil indicate that re-establishment of vegetation can be accomplished. The grading plan for the site attempts to limit the area of over lot grading and disturbance to existing vegetation. During irrigation season water will be available to the contractor for dust control from our headgates. The wind erosion for the site is rated as moderate. It is presently projected that the majority of construction will take place during the seasons when wind erosion is the least problem. Prior to the end of construction the disturbed area will be tilled perpendicular to the prevailing winds, seeded, mulched and crimped to reduce wind and water erosion. Rainfall erosion is a factor that can be addressed with the use of hay check dams, sediment basins, silt fences and vegetative erosion control. The phasing of the project is projected into three phases over three construction seasons. The total area disturbed at one time will be limited. The intent is to sell the developed lots to individual homeowners or builders. The result of the individual sales is that the total build out will take longer with a smaller percent of area disturbed at one time. Wind and water erosion on this project can be minimized by using these techniques. across South Shields. The normal flows are 4 cfs. The lateral is on a ridge line and stormwater without being redirected will not enter the ditch. Trilby Lateral - The headgate is 24 inches in diameter located on the west side of the Pleasant Valley and Lake Canal at the dam above the ravine. The lateral flows to the west into The Ridge and has a normal flow of 10 cfs. Applewood Lateral - The headgate is 1.5 feet x 1.4 feet located on the east side of the Pleasant Valley and Lake Canal at the dam above the ravine. The lateral flows to the southeast into a 14" diameter pipe on the north property line of the Laurie subdivision. The normal flow of is 4 cfs. Thayer Headgates - Four headgates are located along the Pleasant Valley and Lake Canal that serve to irrigate various portions of the Cottonwood Ridge site. The Ravine - The City has installed a new headgate at the top of the ravine in the last several years. The headgate allows water from the dam at the end of the Pleasant Valley and Lake Canal to enter the ravine. V . Proposed development The proposed development is 69 single family homes located on 34.3 acres. The development will be in accordance with current City of Fort Collin's standards and will include paved roads and sidewalks as well as other improvements. The proposed alignment of the drainage in Area "A" will remain basically the same. The developed area will be increased because drainage from the SE cul-de-sac will flow into the basin. A detention pond or settlement basin is presently proposed at the SE corner of the project. Flows into the Fossil Creek drainage are not required to be detained but some detention may be required to reduce erosion in the road ditch on the west side of South Shields. The proposed development of drainage Area "B" will reduce the area drained. The peak flows will increase as a result of development in the basin. A detention pond is proposed at the northeast corner of the project to accommodate the peak flows. The manmade wetlands that are next to Shields will be moved westward to accommodate the widening of the road. The drainage area in the proposed development in drainage Area "C" will increase. The drainage in the area west of the Pleasant Valley and Lake Canal irrigation ditch and the ravine will be directed through a small settlement basin to the existing west tributary to the ravine. Rip -rap lining of the tributary channel will be required. Existing drainage from this site into the Pleasant Valley and Lake Canal and Trilby Lateral will be nearly eliminated. The additional peak flow added to the ravine because of development is below the area of major erosion concern. The flow capacity of downstream structures at Wooded Creek Court and Hepplewhite Court need to be evaluated for both development flow and irrigation flow. The proposed development reduces the flow in the upper end of the ravine. The erosion I damage presently occurring in the ravine is not the responsibility of Cottonwood Ridge but has a been created by the City of Fort Collins and the Pleasant Valley and Lake Canal Company when they commenced the non-traditional delivery of water to the South Ridge Golf Course. Erosion control measures are required in the ravine. Erosion control is required with or without development. With development the value of the resulting damage increases. The City and Pleasant Valley and Lake Canal Company are responsible for measures to mitigate the damage. existing drainage from the proposed Westbury PUD also passes through the 15" RCP and the damaged CMP. Downstream of the 15" RCP and CMP the drainage is undefined as it passes south of the Front Range Community College then into Mail Creek. Drainage area "C" is located west of the Pleasant Valley and Lake Canal irrigation ditch and the ravine. The west boundary of drainage area "C" is roughly the mid lot line of lots 36 through 39 of The Ridge. The drainage historically has entered the ravine through a tributary on the west side. The southwest corner of the Cottonwood Ridge project drains away from the ravine. It was not considered to be sufficient and was not addressed. Present site conditions include developed land along Regency Drive, native grass in both The Ridge Greenbelt and the west field of Cottonwood Ridge and irrigated pasture south of the Trilby Lateral in Cottonwood Ridge. Drainage north of the Trilby Lateral sheet flows south in the greenbelt area of The Ridge and in the west field of Cottonwood Ridge prior to entering the Trilby Lateral. Historically the drainage continued a short distance south then entered the ravine via the west tributary. The majority of the drainage from The Ridge flows in a swale between lots 35 and 36, crosses the Trilby Lateral and continues south. That drainage never enters the Cottonwood Ridge site. The area south of the Trilby Lateral is irrigated pasture. The slope of the area changes from moderate to steep as it approaches the ravine. The ravine was created by the overflow of the Pleasant Valley and Lake Canal. There is a dam at the top of the ravine with five headgates. Normal operation of the ditch is to provide water to the various water users at the headgates. Until a few years ago the quantity of water flowing into the ravine from the Pleasant Valley and Lake Canal was limited to flushing the ditch and storm events. The irrigation company is now delivering city water to the South Ridge Golf Course via the ravine. A new headgate was installed in the dam at the top of the ravine to accomplish this. The result is that the rate of erosion has increased because of the increased water flow. A 48" RCP is installed under Wooded Creek Court where it crosses the ravine. Drainage area "D" is a small basin on the south side of the project. The drainage is tributary to the ravine with the confluence south of Wooded Creek Court in the Laurie Subdivision. The area is irrigated pasture with moderate to steep slopes. There is an 18" RCP installed under Wooded Creek Court. IV. Irrigation ditches. The Pleasant Valley and Lake Canal generally flows north to south. It enters the project from the north and ends at the top of the ravine 360 feet from the south property line. Normal flows in the canal are 15 cfs. Flood capacity of the canal is limited by upstream box culverts at Harmony Road. The actual capacity of the canal at Harmony road needs to be investigated. High water level is limited by height of wier at the top of the ravine. Smith lateral - The headgate is 15" diameter located 380 feet south of the project's north property line. The lateral follows a natural high point on the property that divides the Mail Creek and Fossil Creek drainage. The ditch runs just north of the out lot and then into a siphon Cottonwood Ridge Preliminary Drainage and Erosion Report June 5, 1994 I. Scope This report is a preliminary drainage report submitted to the City of Fort Collins. The report is to be used for review of the proposed Cottonwood Ridge Subdivision. The report addresses the feasibility, possible problems and solutions for drainage in the proposed development. The intent is not to size drainage structures or detention areas. II. Site description The site is located 1/4 to 1/2 mile south of Harmony Road on the west side of South Shields Street in Fort Collins Colorado. The project is located in the East 1/2 of the NE 1/4 of Section 3, Township 6 North, Range 69 West of the 6th PM. The site is presently zoned RLP which allows a standard subdivision or a planned unit development. Presently the site is two thirds irrigated pasture and one third native grasses. Existing buildings on the site include a house, a single width mobile home and a barn. Presently the area is being used as a residence and for pasture in a horse operation. III. Existing site conditions: Drainage area "A" is located in the southeast portion of the property. The boundary is defined by the centerline of South Shields on the east, the Smith lateral to the north, and both the Pleasant Valley and Lake Canal and the Applewood Lateral to the west. Presently the area is gently sloping irrigated pasture. The drainage from the site flows in the road ditch on the west side of South Shields. The flow from this basin is the majority of the flow to an 18" RCP under Wooded Creek Court at South Shields. The drainage continues along the westerly road ditch of Shields Street until it intersects The Burns Tributary. One of the driveway culverts along the South Shields road ditch south of the project site is damaged or nonexistent and minor runoff creates problems with the driveway. The slope of the road ditch prior to entering The Burns Tributary is steep. Drainage area "B" is located in the northeast portion of the project. The historic boundary is defined by the centerline of South Shields on the east, the Smith lateral to the south, and the extension of the Smith Lateral on the divide to the high point in the road on Regency Drive. The north side of the drainage is defined by the property line to the Pleasant Valley and Lake Canal then follows a divide to the high point on Regency Drive. Presently the area is gently sloping irrigated pasture below the irrigation ditches, dryland grass in the fields and greenbelt west of the irrigation ditch and developed residential along Regency Drive. Presently all runoff west of the Pleasant Valley Lake and canal flows into the canal. The drainage east of the Canal will flow into a 15 " RCP under high water conditions only. The invert in elevation of the pipe is higher than the surrounding ground. There is also a damaged CMP just north of the property line that appears to handle low flows. The combined effect of irrigated pasture, a road fill on South Shields Street and a culvert that is in the wrong location and too high have created a manmade wetland area in the lower portion of the drainage. The INDEX I. Scope II. Site description III. Existing site conditions IV. Irrigation ditches V. Proposed development VI. Wind and Water Erosion VIL Calculations VIII. Site Plan Cottonwood Ridge Preliminary Drainage and Erosion Report June 5, 1994 Prepared By: Eric Thayer 31741 RCR #35 Steamboat Springs, CO 80487 303-879-1578 June 5, 1994