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DRAINAGE REPORT DRAINAGE REPORT FOR KWIK KAR LUBE AND TUNE Prepared for: Kwik Industries, Inc. 4725 Nall Road Dallas, Texas 75244 1 * • * * . •• • • 44001 • '* 4 Brooks & Sparks, Inc. Project Number 594 -0181 November 2003 Table of Contents A. Project Location and Description 1 B. Existing Drainage Conditions 1 C. Design Criteria 1 D. Proposed Drainage System and Detention Facilities 3 E. Conclusion 3 Attachments Attachment A Vicinity Map Attachment B Detention Calculations Attachment C Drainage Area Map Attachment D Storm Sewer Plan DRAINAGE REPORT FOR KWIK KAR LUBE AND TUNE College Station, Texas A. Project Location and Description The subject site for the proposed Kwik Kar Lube and Tune is situated on Rock Prairie Road about 210 feet southeast of the intersection of Rock Prairie Road and FM 2154 (Wellborn Road) and is within the bounds of Lot 2, Block Two of the Edelweiss Business Center. The development adjacent to the site includes a large church, a gas station/convenient store, and a fast food restaurant. The project Vicinity Map is included in Attachment A. The total site area is 0.995 acres of land which is partially developed with a old floral shop building and a small parking lot that is currently unused. The floral shop building and parking lot are to remain. The proposed Kwik Kar Lube and Tune facilities will be constructed next to the existing building in the undeveloped, grass covered portion of the site. The project consists of underground utilities, a parking lot, a new Kwik Kar Lube and Tune building, a detention pond, and an outfall ditch. B. Existing Drainage Conditions Majority of the site currently free flows to the west/southwest side, or back portion, of the tract. On the site is an existing 2 -foot wide concrete flume that runs from the from the edge of the sidewalk in front of the existing florist building, southeast to the back of the site. This flume was constructed as part of the drainage /detention system for the floral shop built in 1999 and was to outfall into a detention pond at the back of the site; however, the pond was never constructed. There are no existing inlets or underground storm sewers on the site. The storm water sheet flows over the ground on the site at approximately a 1.2% slope. Since the drainage /detention features constructed in 1999 will be demolished with the proposed development, the existing, pre - development flow was derived assuming the proposed site is clear of any buildings or paving and is considered natural grasslands. C. Design Criteria The design criteria used to determine the required drainage system and detention volume followed the previously approved drainage report for the subject site titled "Drainage 1 Computations for Edelweiss Floral Shop" prepared in August, 1999 by Garrett Engineering. In addition, the City of College Station's "Drainage Policy and Design Standards" manual was used. The Rational Method was utilized to determine the peak discharge for the site's storm sewer system. The formula that represents the Rational Method is as follows: Q =CiA, where "Q" is the peak discharge rate in cubic feet /second, "C" is the run -off coefficient, "i" is the rainfall intensity in inches/hour, and "A" is the cumulative contributing area in acres. The detention facilities were designed for the ultimate 100 -year storm event. The rainfall intensity used from the Garrett Engineering report for the 100 -year event is 11.64 inches/hour (in/hr) for both the developed and undeveloped flow. For the 100 -year undeveloped flow, a run -off coefficient of 0.37 was assumed for the site. This coefficient is feasible since we assumed that the ground cover on the site was all grass with no existing development. In addition, the natural ground slope across the tract is approximately 1.2 %, with lower elevations at the back of the site. For the 100 -year developed flow, a coefficient of 0.76 was used This was calculated using the City of College Station formula: C = [C + C + + CXAX] / A, where "A" is the sum of the sub -areas (A A + + A and C C2, , C,, are the runoff coefficients for the sub - areas. This formula yields a more accurate coefficient to use in the rational equation. The total area used to determine the required detention volume is 0.862 acres, which includes only the area that flows through our tract and into the site's proposed drainage system. Approximately 0.133 acres is flowing away from the subject site into the adjacent access road. Utilizing these coefficients, the rainfall intensity of 11.64 in/hr, and an area of 0.862 acres, the 100 -year undeveloped flow is 3.71 cubic feet /second (cfs), and the 100 -year developed flow equals 7.63 cfs. Both the developed and undeveloped flows were used to calculate the storage volume required for detention to ensure that the post - development discharge rate did not exceed the pre - development rate. Using a series of equations in the Triangular Method, the required detention volume was calculated to be 0.42 acre -feet. Calculations for flow and detention volume are included in Attachment B. The storm water on the proposed site will be conveyed from the drainage areas to the detention pond by either the proposed underground storm sewer system or by sheet flow over the site directly into the pond. The tract was divided into three sub - drainage areas determined by the direction of storm water flow over the site. A portion of the tract drains away from the site onto adjacent access road and onto Rock Prairie Road and is not included in calculations for storm sewer sizing. Attachment C illustrates these drainage areas. 2 D. Proposed Drainage System and Detention Facilities The underground storm sewer system on the site was designed according to the drainage areas. Drainage Area A flows either into the proposed ditch along the frontage of the site or into the two inlets located in the front parking lot of the Kwik building. Drainage Area B will sheet flow directly into the detention pond. The underground storm sewer system is designed to carry the flow from Drainage Area A only. Twelve -inch lines will be used to convey this flow to the junction box constructed prior to the detention pond. A 15 -inch pipe will outfall the storm water from the junction box into the detention pond. To accommodate the 0.42 acre -feet of required storage volume, a detention pond will be constructed along the back perimeter of the site, as well as a ditch along the frontage of Rock Prairie Road. The proposed pond will have an average depth of 4.5 feet with 2.5:1 side slopes to acquire the maximum storage volume. The pond will grass lined with a flat bottom. The proposed grass lined, V- bottom ditch along the frontage will have depths ranging from 3.5 to 4 feet with side slopes of 2.25:1. Additional detention volume will be gained by using the northern corner of the Kwik parking lot to pond storm water. Underground storage will also be obtained by 105 -feet of 48 -inch HDPE pipe that will be constructed from the junction box on the southeast side of the site. By utilizing the proposed detention pond, the parking lot, and the underground storage pipe, all detention volume requirements are achieved. The storm water in the proposed detention pond will be control released through a 6 -inch restrictor pipe at the outfall of the pond. The pond will discharge into the proposed outfall ditch constructed within the 20 -foot public utility and drainage easement on the southeast corner of the subject tract. The outfall ditch will be grass lined and vary in depth from 4.5 feet to daylight, gradually getting shallower. At the end of the outfall ditch where the flow line meets the natural ground elevation, the storm water will sheet flow approximately 80 feet to an inlet which is connected to the public storm sewer system. The detention facilities and the storm sewer system are shown on the Storm Sewer Plan included in Attachment D. E. Conclusion The proposed storm sewer system and detention facilities for the Kwik Kar Lube and Tune site are sufficiently designed to control the storm water within the tract after full development is complete. The design is conservative to ensure the drainage system is adequate for any storm event 100 -years or less. In addition, the post- development discharge rate will not exceed the pre - development discharge rate with the proposed drainage facilities. 3 ATTACHMENT A Vicinity Map VICINITY MAP ■ • JQ C �`�•�'� FOB , p O � B $c <v `�I Cp Fs+ p� � P J' X 4 4 Cpl O • 2C' 04 F G CO �'9Op <9. ' • ,1 / 0 9 � ' Oy � � 9c� C A m q CO � " 9 � 1 � 1. J T CL 40 i r • ?O Q .g) P out � Shwood P P Park t � !7 0� OH O p Qtii� .'A, Pte � 2 s t. ® �,� , PROJECT cy G AP LOCATION 4 4 >\( '''4' GQ O J� O , Al P G ( B q� jitX j GQ s>" ATTACHMENT B Detention Calculations Detention Calculations for Kwik Kar Lube and Tune College Station, Texas Brooks & Sparks, Inc. Project No. 594 -0181 100 -Year Developed Flow (I) 100 -Year Undeveloped Flow (0) C= C1A1 +C2A2 +.... +CxAx A Impervious Area = 0.639 acres Assume C = 0.37 Natural Grasslands = 0.223 acres C = (0.90) *(0.639) + (0.35) *(0.223) 0.862 C =0.76 C =0.37 i = 11.64 in /hr i = 11.64 in /hr A = 0.862 acres A = 0.862 acres I = CiA O = CiA 1= (0.76) *(11.64) *(0.862) 0 = (0.37)* (11.64)* (0.862) I= 7.63 cfs 0 = 3.71 cfs Total Inflow Volume (V) V= 11.47 *(A /12) V = 11.47 * (0.862 / 12) V = 0.824 acre -feet Duration of Inflow (B) B = (43,560 * V) / (0.5 * I) B = (43,560 * 0.824) / (0.5 * 7.63) B = 9,408.50 seconds Required Storage Volume (S) S = [0.5 *B*(I - 0)] / 43,560 S = [0.5 * 9,408.50 * (7.63 - 3.71)1/ 43,560 S = 0.42 acre -feet I 1 ATTACHMENT C Drainage Area Map In O W W? 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