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Home My WebLink About00072563i3 MUNICIPAL DE VEL OPMENT GR O UP 2551 Texas Ave. South, Ste. A • College Station, Texas 77840 •409 693-5359 • FAX- 409 693-4243 • E-mail: mdgcs agte. com Consulting Engineers, Surveyors, Planners and Environmental Consultants REVIEWED FOR October 15, 1999 COMPLIANCE NOV 12 1999 Jeff Tondre COLLEGE STATION City of College Station ENGI ERIN Development Services PO Box 9960 College Station, Texas 77842 RE: Drainage Letter - J.P. No. 6 Parking Improvements MDG Job No. 000576-3248 Dear Jeff: The above referenced project consist of relocating existing parking spaces and constructing a new 35 space parking lot located at the rear of the existing building. This parking lot will be constructed with standard concrete curb and gutter with an H.M.A.C. surface with pertinent base and subgrade. The project site is located at the southwest corner of the intersection of George Bush Drive and Anderson Street. Exhibit # 1- "Site Plan" is attached for clarification of the parking lot configuration and drainage pattern. The project site is located adjacent to the Wolf Pen Creek and contains floodplain and floodway as graphically depicted by F.E.M.A. F.I.R.M. Community Panel No. 48041 CO 144C (July 2, 1992). The site area, which is located on the College Station ISD property, consists of approximately 1.00 acres. Using the Rational Method (Q=CIA), the parking lot improvements will create the following runoff increases: RUNOFF CALCULATIONS Storm (Yr) Pre (cfs) Post (cfs) Diff. (cfs) Req'd Detention (ft3) 2 2.552 3.961 1.409 1269 5 3.103 4.817 1.714 1543 10 3.483 5.407 1.924 1732 25 3.977 6.175 2.198 1978 50 4.496 6.980 2.484 2236 100 4.694 7.288 2.594 2334 The policy of the City of College Station is to encourage the rapid conveyance of storm water through and out of the City within the primary system. Since this project site is located directly adjacent to the Wolf Pen Creek, being a primary system, no runoff will be detained on the site and the post-development runoff will discharge directly, without constriction, to the Wolf Pen Creek. This is done in order to keep from stacking peaks of the primary creek and the discharge of the site runoff. 000576-c.20-67(3248) Justification of no detention was made by the comparison of time on concentration, (TC) of both hydrographs. The approximate TC of Wolf Pen creek, to the project site, is approximately twenty minutes. This TC assumes and average velocity of 3.0 ft/sec and a travel distance of 3600 feet. The minimum TC of the site allowed without detention is ten minutes. With detention, the TC of the site increases to approximately twenty minutes. Therefore, detention on-site would increase the overall peak of Wolf Pen Creek. In conclusion, the basic design of the drainage improvements will include: • No on-site detention with direct discharge into Wolf Pen Creek; Discharge of parking lot runoff through multiple curb leave-outs with drainage flumes; • Implementation of standard erosion control devices. Please call me if you have any questions. Sincerely, North B. Bardell, Jr., P.E. C.E.O., Project Manager ba OF 0,000000, 6 % 9 A f6ARDELL, JR. 07~oY°JV,~U: . __..-.TWO°Y~•°~~ 60873 ~T g~~'~°°h` tae S 000576-c.20-67(3248) SECTION #2 (MINOR) - PRE-DEVELOPMENT GENERAL INFORMATION Description: Drainage area of site before development. Drainage Area (APRE) = 1.000 acres TIME OF CONCENTRATION GIVEN Maximum Travel Distance (DPRE) 310 ft Velocity of Runoff (VPRE) = 0.75 a/sec Coefficient of Runoff (CPRE) = 0.40 ' NOTE: Minimum Tc allowed= 10 min. 2 YEAR FREQUENCY RATE OF DISCHARGE (Q) Coefficient (e) = 0.806 Coefficient (b) = 65 Coefficient (d) = 8 5 YEAR FREQUENCY, RATE OF DISCHARGE (Q RESULT TC(PRE) = 6.9 min. Rainfall Intensity (12) = 6.327 in /h, X 2.55-2. Coefficient (e) = 0.785 Rainfall Intensity (I5) = 7.693 'n /n, Coefficient (b) = 76 Q6 1:103 cft Coefficient (d) = 8.5 10 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Coefficient (e) = 0.763 Rainfall Intensity (110) = 8.635 in /hr Coefficient b = 80 Q...... a*- 3 ds: Coefficient (d) = 8.5 25 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Coefficient (e) = 0.754 Rainfall Intensity (125) = 9.861 'n/,,, Coefficient (b) = 89 . ~s Coefficient (d) = 8.5 50 YEAR FREQUENCY, RATE OF DISCHARGE (Q Coefficient (e) = 0.745 Rainfall Intensity (150) = 11.148 'n/nr Coefficient b = 98 Q 46 Coefficient (d) = 8.5 100 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Coefficient (e) = 0.73 Rainfall Intensity (1100) 11.639 in /hr Coefficient (b) = 96 Qua. C94 eIs' Coefficient (d) = 8 000576-3248 Drainage Calculations. As SECTION #3 (MINOR) - POST-DEVELOPMENT GENERAL INFORMATION Description: Drainage area of site after development Drainage Area (A1) = 0.380 acres (impervious area) Drainage Area (A2) = 0.620 acres (landscaped area) TIME OF CONCENTRATION (T,.) GIVEN Maximum Travel Distance (D1) = 230 ft Velocity of Runoff (V1) = 3.10 ft/See Coefficient of Runoff (Cl) = 0.90 Maximum Travel Distance (D2) = 160 ft Velocity of Runoff (V2) = 1.00 ft/See Coefficient of Runoff (C2) = 0.45 RESULT Tc1 = 1.2 min. TC2 = 2.7 min. ""NOTE: Minimum Tc allowed = 10 min. TC(Posr) = 3.9 min. 2 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Rainfall Intensity (12) = 6.327'n/hr Coefficient (e) = 0.806 QAREA 1 = 2.182 cfs Coefficient (b) = 65 QAREA 2 = 1.780 cfs Coefficient (d) = 8 3:9G1 cfs: 5 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Rainfall Intensity (15) = 7.693'n/hr Coefficient (e) = 0.785 QAREA 1 = 2.653 cfs Coefficient (b) = 76 QAREA 2 = 2.164 cfs Coefficient (d) = 8.5 Q 4-8 il 10 YEAR FREQUENCY . RATE OF DISCHARGE (Q) Rainfall Intensity (110) = 8.635 'n /h, Coefficient (e) = 0.763 QAREA 1 = 2.978 cfs Coefficient (b) = 80 QAREA 2 = 2.429 cfs Coefficient (d) = 8.5 cfs 5 4'b7 25 YEAR FREQUENCY . RATE OF DISCHARGE (Q) Rainfall Intensity (125) = 9.861 in/hr Coefficient (e) = 0.754 QAREA 1 = 3.401 cfs Coefficient (b) = 89 QAREA 2 = 2.774 cfs Coefficient (d) = 8.5 xxx. g 6k175 cfs: 50 YEAR FREQUENCY . RATE OF DISCHARGE (Q) Rainfall Intensity (150) = 11.148 in /hr Coefficient (e) = 0.745 QAREA 1 = 3.844 cfs Coefficient (b) = 98 QAREA 2 = 3.136 cfs Coefficient (d) = 8.5 Q«,''. <«;:::::>:::;.<:>> 6.980 clsi; 100 YEAR FREQUENCY, RATE OF DISCHARGE (Q) Rainfall Intensity (1100) 11.639 in /hr Coefficient (e) = 0.73 QAREA 1 = 4.014 cfs Coefficient (b) = 96 QAREA 2 = 3.274 cfs Coefficient (d) = 8 t~= 7288 cft 000576-3248 Drainage Calculations. As SECTION #4 - HYDROGRAPHS Chart 4 HYDROGRAPHS Chart 4 SECTION #4 - HYDROGRAPHS Chart 5 FIYDROGRAPHS Chart 5 SECTION #4 - HYDROGRAPHS Chart 6 HYDROGRAPHS Chart 6 SECTION #4 - HYDROGRAPHS Chart 7 HYDROGRAPHS Chart 7 SECTION #4 - HYDROGRAPHS Chart 8 50-YEAR HYDROGRAPH Volume Conveyed' 2,236 ft3 7 f~. 6 ffi::' f• WOO 015 ~M, r 5 F{l i:n. Yf /rf r:•::y FF:?}~:i.?:!•'•f:ff :;r%/•ffigg ? +f tivf%f%J"'+{yYf f%ffr:i•':X.. 111. ':iii~•':~::i: f~: f:::i'i ::~:~i::::';'~.`::`:fi}i. ~~r~/ ff{ r1 f 44- u Me. NMI' f V] . •:::::F/ Q . !F/fii S: f . ff 3 ?f rr Iii..,. f~ ff. .f..... . :ff F.:.:•: .ifs...... . f j ~ ( 2 F:• i::i:::::: iiiii:~:::::.;:y;i:;ii:•: : f ?...:::::::::is~:::~':"":.i:.:viii:%.'ii:~`:~'`i:~>i rf~• f v { { 1 v w::::.,............ ' :v; iiiiwi'..:.....::.~:Y2T:i:!4i?:::i•i>ii:..;::•ii:??'t:~ii?i?isi:.:k::::::::;::::;;%;;Y;:~:;:::::::: { . 0 0 10 20 30 TIME O Post-Development D Pre-Development (min) HYDROGR.APHS Chart 8 SECTION #4 - HYDROGRAPHS Chart 9 HYDROGRAPHS Chart 9