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HomeMy WebLinkAboutDrainage Reports - 04/12/1978• n L-4 April 12, 1978 Dr. Charles Collopy 1200 East Elizabeth Fort Collins, Colorado 80521 South Foothills Plaza, P.U.D. Project No. 1301-011 Dear Dr. Collopy: W consulting engineers 4710 South College Ave. Fort Collins, Colorado 80521 Telephone (303) 226-2323 Presented herein is the Storm Drainage Report for South Foothills Plaza, P.U.D., located in Latimer County, Colorado. This report conforms with the Larimer County storm drainage policy. All calculations pertaining to this report are available upon request at our office. Please feel free to contact us with your questions or comments. Very truly yours, M 6 I, INC. elo G. McLaughlin tEnclosure es M.,�sSR',.E.-L.S. as stated : JMN/bf E STORM DRAINANGE STUDY FOR SOUTH FOOTHILLS PLAZA, P.U.D. INTRODUCTION The South Foothills Plaza is a proposed commercial planned unit development located in the southeast 1/4 of Section 35, Township 7 North, Range 69 West of the 6th P.M. in Larimer County. The'site is bounded on the east by South College Avenue (U.S. Hwy 287); on the West by the Colorado 6 Southern Railroad; on the north by pasture land, zoned tourist; and on the south by Wickes Lumber and the proposed Bob Wells Automotive Center. The site is presently used as pasture land with topography gently sloping between 1-3% in an easterly direction. There are no existing streams or lakes within the boundary of the proposed development. The New Mercer Canal meanders through the center of the property and flows in a southerly direction. PURPOSE AND SCOPE The purpose of this report is to analyze the effects of storm water runoff and to provide the developer with recommendations for the proper control and disposal of storm water runoff for South Foothills Plaza, P.U.D. The scope of this report includes a storm drainage analysis for flows generated by the development in both its histprical and its improved conditions. This analysis consists of the following items: 1. Storm runoff flows and their directions in the designated streets. 2. The size and minimum slope of storm piping required. 3. On -site storage alternatives. -1- 0 EXISTING STORM DRAINAGE There are no major drainage ways designated within the project limits by the Larimer-Weld Regional Storm Drainage Study of 1973. Off -site drainage contributing to the site from West of the C & S Railroad is restricted to a 20" steel pipe under the tracks. The New Mercer Canal intercepts minor flows from upstream within the site. The northeasterly area of the site discharges runoff directly to South College Avenue while all other areas drain to a low point which discharges flows under College Avenue through a 24" concrete pipe. The runoff is then discharged into the Larimer No. 2 Canal on the east side of College Avenue. See Exibit 1. DESIGN CRITERIA The Larimer County's drainage policy and the procedures presented in The Denver Regional Council of Governments (DRCOG) Urban Storm Drainage Criteria Manuals were used for analyzing the storm runoff. The amount of storm runoff for the development, individual basins, and the streets was determined by the Rational Method. This procedure equates the design peak flow to the area, amount of rainfall on that area, and a multiplier that is determined by the physical characteristics of the area under study. A runoff coefficient of 0.30 for rolling farmland terrain, as recommended by the Larimer County Road Policy and Standards Manual, was used to determine storm runoff on the site. The design of all drainage facilities presented in this report is based on the rainfall intensity duration curves developed by the City of Fort Collins Engineering Department. See Exhibit 2. Capacities of streets and culverts were determined by Manning's formula, which equates a constant, a roughness factor, the area of the channel, the hydraulic radius and the slope to the capacity of the street or pipe. -2- RECOMMENDATIONS AND CONCLUSIONS It is proposed to relocate the New Mercer Canal to the West side of the C & S Railroad tracks. In doing so, the existing 20" steel pipe across the tracks would be abandoned and upstream runoff would be intercepted by the relocated Canal and discharged into Mail Creek south of Harmony Road. Because the capacity of the existing pipe across College Avenue is not adequate to carry other than the present runoff from the site, it is recommended that the individual lots within the development detain a 100-year runoff on -site and discharge it at a 2-year undeveloped rate. By doing so, this will also relieve the developer of liability from downstream property owners. It is recommended that the pipe system at the intersection of Troutman Parkway and South College Avenue be sized to accommodate a 25 yr. runoff from South College Avenue. In the event of a 100-yr. storm, minor ponding will occur at the intersection. However, ponding will not impede traffic flow on S. College Avenue, and will not excede a 6" depth in the area on Troutman Parkway. The following are storm water detention methods which would be appropriate for individual lots within the site. Rooftop Storage: Horizontal rooftops can provide storage which will not inconvenience pedestrains or motorists. A rooftop detention facility is not unsightly because it is not visible. However, rooftop storage is not a problem -free method as there are problems of leakage, possible structural overloading, maintenance for removal of debris and ice and the possibility that heavy rainfalls will overflow the top of the roof if drains become blocked. Exhibit 4 depicts, conceptually, a rooftop detention system. Parking Lot Storage: Storage of storm water on parking lots is another -3-. means of storm water detention. The large area which is characteristic of many parking lots serving commercial complexes is a factor that makes such parking lots extremely feasible for storm water detention. Because of the ease of inspection and access, the maintenance and operation of parking lot detention facilities is low cost and easily performed. See Exhibit 5 for a conceptual parking lot detention system. Surface Depressions and Basins: This method of storm water storage consists of using grassed swales and landscaped depressions. The advantages of utilizing this method of detention are that the areas are asthetically pleasing and in the winter can be used for,storage of plowed snow adjacent to parking areas. See Exhibit 6:for a surface depression detention concept. The concepts submitted herein are for pictorial purposes only. Each site builder will be required by the developments' protective covenants to prepare an individual site drainage report and plans to be certified by a professional engineer. This drainage report will be subject to approval of the Design Review Committee for South Foothills Plaza, who will then forward the report and plans to the County Engineer for his review and approval. This drainage proposal will satisfy all hydraulic considerations as well as governmental regulations, in providing the proposed development with a safe and equitable storm drainage system. -4- W W o � q G H I N N al r4 O _ 1� H is N v1 v1 Ln w '-I N ri U L O p y v G H 0 I' W N N CD O, 1-1 � 00 G M G y d 00 N G E. d W U T O rl O M M M 00 M O O O O O M to H N Vim] U z ZU H W a Ix W w al L) co 1 w w o N z N � L H.] H WW H 11 Pa O O 'rl ra ca C7 H 00 �Y M H O ca ri H u N U co W O G O U y q y 9 N al 't7 - 7 rGl U n1 a0i 1 w d C7 A W x U W U' G G 4J w H v W l y O H q a) aj - W W O U u W W W 7 q G H p H cc N m > h y O a " 0 U rl al v v N M -�r 0 %O t4 N a ca O w 0 O E4 +i is jJ _ _ I I III II II--r /fynll I � t I��a y _ M 4c I I aJ ~ o. d/ _m x z v r, I I b II. 41 41 p..LO I lj�l .YLn wi I } I 500 ZI o %rC \ I I I •� Q wl = G. I I I cP1P d -i.R. 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C.nNGEPT • RCQOFTOP DETENTION EXH151T 4 FTOP USED TO DETAIN SECTION (NO SCALE) PLAN \NO SCALE) 1 PROPERTY WATER ROAC LINE ROOFTOP DETENTION DOWNSPOUT E GUTTER NOTES : i. WATER ' TO BE DISCHARGED AT THE NORMAL HISTORICAL RATE. ( 2 YR. STORM) ZTHE OUTLET AT ORYWELL SHALL PSE SPECIFICALLY DF-51CINED TO CONTROL D15PERSE RUN-OFF 50 THAT OCCUR EROSION DOES NOT i 0 v WA MWAM CLIENT_SOyTN COOTNI Ls PLAZA JOB No. 13o1. 011. F.B. pG. DSGN. L•G.m - DWN FM-C. CK. _CONCEPT PARKING LOT WE.TENTIO EXHIBIT S DETAINED WATER OUTLET PIPE STORM SEWER. ROAD PROPERTY LINE SECTION NO SCALE %r� x uuunuumunmm�m STORM SEWER t SLOPE �CURFS PROP. INLET ES: / I\ TO T HA.ARC,ED AAZ THE N RMAL RICAL RATE. R. STORM) -T/ NINCi5 IN CL1R65 MAY AL50 SED FOR DISCHARGE. PLAN NO SCALE 'yam 9 ergheers . CLIENT SourN Poerwl . a o. x JOB No. 1301-011 F.B. - PG. DSGN. L. G. N1. DWN P+. M. C. CK. • CONCEPT 0 SURFACE DEPRESSION EXHIBIT G WAT E Fi LEVEL PEAKS i i NAGE LE MIST LEET / ISCHARGE IPE SECTION NO SCALE U5E OF c- DETAIN PSERM5 TO WAT E K DRAINAGE FLOW __ DRAINAGE _ 5WALE, PIPE SYSTEM, OR STREET -DISCHARGE PIPE -- D , F 0 I AC,E ENTION FACILITY NOTES : IWATER TO 6E DISCHARG,ED AT THE NORMAL HISTORICAL RATE - ( 2 YR. STORM) 2 D15CHARGE OUTLET SHALL ME SPECIFICALLY DESIGNED TO CONTROL 6 DISPERSE RUN-OFF 50 THAT EROSION DOES NOT OCCUR. .a,.00u,a �,q,,,o . F DETENTION FOND CLIENT SOUTH FOOT,a, PLAZA NO SCALE - JOB No. 1301-0il FB. PG. DSGN LAX, OWN. R.G. CK.