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Home My WebLink AboutDrainage Reports - 04/21/2014Cigvf Ft Cdlins prow d Plans Approved By... Date Addendum to the Final Drainage Report For STORYBOOK P.D.P. Located at Cooper Slough Drainage Basin Fort Collins, Colorado Prepared for: ' Midtown Homes At Storybook LLC Original Report Prepared by: Stewart & Associates, Inc. 103 South Meldrum Street Fort Collins, CO 80521 Phone: (970) 482-9331 July 1",1999 Addendum by: Forbes Engineering LLC 2908 Cherly Street Fort Collins, CO 80524 Phone: (970) 232-6014 April 15`h, 2014 FORBES ENGINEERING CIVIL & STRUCTURAL ENGINEERING March 14, 2014 Mr. Glen Schlueter Storm Water Utility City of Fort Collins P.O. Box 580 Fort Collins; Colo. 80522 RE: Storybook 3rd Filing, Storm Water Drainage Letter - Mr. Schlueter: I have been asked to provide a narrative of the current and proposed conditions of the stormwater drainage system for the Storybook 3rd Filing. Currently there are two on -site ponds that detain the drainage flows from the completed Storybook I' Filing and the partial completed site from Storybook 2nd Filing. Stormwater flows from the west pond to the east pond where there is an existing water quality structure with a restrictor plate that allows an offsite release rate of 8.8 cubic feet per second. From there the offsite storm drainage system consists of a series of 48" stormwater manholes and 24" ADS pipes which transports the stormwater east to the Number 9 ditch. At the last manhole for this storm drainage system there is a 40 foot length of 24" ADS pipe stubbed out to the east and a 40 foot length of 24" ADS pipe stubbed to the north. The east stub is plugged at its end so the stormwater discharges out of the north stub where it then sheet flows to the ditch. The Larimer & Weld Irrigation Company has been receiving this flow since the ponds were constructed during the first two filing projects. They have provided conformation that they will continue to accept the stormwater discharge until the offsite pond construction on the Poudre School District property directly east of Storybook 31rd Filing is completed and a water quality structure is in place that, reduces the discharge into their ditch to 3 cubic feet per second. With Storybook I' Filing constructed and only a portion of the second filing completed, the flows are not reaching the 8.8 cfs that the watershed area is anticipated to produce during a 100 year event. Also, with the construction of 24 homes in the first phase of Storybook 3rd filing, the maximum volume of the existing ponds will not be compromised and the discharge rate of 8.8 cfs will not be exceeded. The existing stormwater system has the capacity to handle the stormwater flows and therefore will not creating any issue with the ditch. Proposed construction is to add,a concrete apron and erosion control riprap at the east end of the existing 24" ADS pipe to prevent any damage along the west bank of the ditch and to satisfy the ditch company until the offsite pond is constructed during phase two of the construction of Storybook 3rd Filing. Please call if I can be of further assistance in this matter, or if you have any questions. I can be reached at (970) 295-4874. Sincerely, FORBES ENGINEERINGE. Foo po REGis 0 G c35265 Forbes Colo. PE #3 650 ' 3 ' l� 1 n'e. „a FORBES ENGINEERING CIVIL & STRUCTURAL ENGINEERING January 28, 2014 Mr. Wes Lamarque Storm Water Utility City of Fort Collins P.O. Box 580 Fort Collins, Colo. 80222 RE: Storybook 3rd Filing, detention pond outlet structure Mr. Lamarque: This letter is in regards to the Storybook 3`d Filing's offsite detention pond located on Poudre Rl school property. The detention pond is to be located at the southwest comer of the school's. property approximately 100 feet east of the west property line. The pond will have 6 to 1 side slopes and the total volume has been estimated at 21,140 cubic yards (see attached site exhibit). The required detention volume has been calculated at 62,640 cubic feet using a release rate of 8.8 cubic feet per second from the Storybook Subdivision and an allowable discharge rate of 3 cubic feet per second out of the off -site pond. The high water elevation is at elevation 5006.2 and the corresponding depth at the outlet structure has been estimated at 5.9 feet. The outlet structure restrictor plate has been sized for a 36.9 square inch orifice to restrict discharge to 3 cubic feet per second (see attached outlet calculations). Please call if I can be of further assistance in this matter, or if you have any questions. I can be reached at (970) 295-4874. Sincerely, FORBES ENGINEERING �p 35265 d Kevin Forbes, Colo. PE #3526S% °, e President FORBES ENGINEERING STORYBOOK 3RD FILING FORT COLLINS, COLORADO STORMWATER CALCULATIONS ✓OB # 1810 CLIENT.- STEWARTS & ASSOC. DATE.- 112V14 BY. KEF Subject: Detention Pond Outlet Orifice Plate Detention Pond release point Relocated 24"0 ADS storm pipe connected to existing stormwater manhole along north side of Mountain Vista Drive, and ultimately releasing into #8 Reservoir Outlet Ditch Maximum Release Rate out of pond, Qout 3 cfs, per agreement with ditch company Maximum Release Rate into pond, Qin 8.8 cfs, Storybook detention pond controlled release rate Off —site Pond size 6 to 1 side slope, 14 foot depth at outlet Approximately 570,780 cubic feet = 21,140 cubic yards (see site exhibit) Required Detention Pond Volume 3 hour storm, (Qin — Qout)(3 hours) _ (5.9)(10,800 sec) = 62,640 cubic feet = 1.4 AC —FT High Waterline ® 5006.2 Contour Invert out ® Outlet = 5000.3 Depth ® Outlet = 5.9 feet Detention Pond Outlet Restrictor Plate Q = AV, A = Q/V, V = (.6)( (2)((32.2)(5.9) ) V = (•6)( (2)((32.2)(5.9) ) V = 11.7 fps A = 3.0/11.7 fps A = 0.257 ft2 = 36.94 in2 Use 6.9" diameter hole or 6.1 "x6.1 " Square Orifice HWL DETENTION POND A d- .. x 1/4" STEEL PLATE W/ ORIFICE AS INDICATED — BOLT TO CONCRETE W/(4) 3/8"0 STAINLESS STEEL BOLTS AND NUTS Inv out 24"0 ADS — Storm Drain 7 v 00 OO • Ln a U� 7 ++ O � 7 C d O m 7 > DOc ,III N mil I I/ 0 11 O T to N a O J 0 I= , III >102- cn,�\�I lill L 7 0 a n II� li "il IN s}uai.uasoa Aj!j!}n ,g 43!M II ADM—;o—}46!d g� pasodad I }uawasoa a6�u!DJp U J C N � C � �Z i W L_ y C lL. 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