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HomeMy WebLinkAboutFactory DetentionJuly 3, 2007 Mr. Alan Gibbs, P.E. Senior Assistant City Engineer, Public Works City of College Station 3 00 Krenek Tap Road College Station, Texas 77842 Re: Factory Apartments Offsite Drainage Evaluation Klotz Project No. 0731.001.000 Dear Mr. Gibbs: k I o t z 4~~ a s s o c 1 a t e s Klotz Associates has reviewed the options for offsite detention for the Factory multi-family development in College Station, Texas. The project site is located on the eastern corner of College Main Street and Cherry Street in College Station, Texas. The proposed development consists of a multi-story building and a parking garage. The project was designed by Binkley & Barfield and plans were dated October 2006. The approved plans for the development included an underground detention system. As an option, Klotz Associates analyzed the option of providing offsite detention. The proposed detention location is within the existing drainage channel located approximately 900 feet downstream to the northeast of the site along Inlow Blvd. Our engineering evaluation included the following services: • Preliminary Analysis -This phase consists of a review of the detention requirement calculations that were prepared by Binkley and Barfield. Based on that volume that was calculated for the site; determine proposed geometry for the expanded downstream channel to provide in-line detention. • Detailed Analysis -In order to route the storm flow to the channel, an analysis of the storm system on the adjacent roadway was necessary. This analysis would show the impact of the additional flow on the system. Klotz Associates designed improvements to Cherry Street in 2002. Winstorm was used for that analysis and included analysis of the 10 year and 100 year storm event. The analysis of the storm sewer drainage system shows that the proposed offsite detention to be feasible. The drainage system of Cherry Street has sufficient capacity to convey the 100-year Mr. Alan Gibbs, P.E. July 3, 2007 Page 2 of 4 k I o t z -~~ a s s o c i a t e s storm event due to the requirement and use of minimum pipe sizes and limited local drainage areas. It appears that existing channel along Inlow Blvd. can be expanded to provide detention volume equal to that required for the site as shown in Binkley & Barfield's calculations. The calculations are outlined in more detail below. During the initial review of the project, the plans for Cherry Street dated August 2001 showed that the majority of the site drained to College Main. The site plan prepared by Binkley & Barfield, routes the majority of the flow through an underground detention facility and discharges into the Cherry Street drainage system. Original Design The following information was provided by Binkley & Barfield by utilizing the Rational Method. The formula used is Q = CiA in which Q equals the peak rate of runoff in cubic feet per second (cfs), C equals the dimensionless coefficient ofrunoff, i equals the average intensity of rainfall in inches per hour for a period of time equal to the time of concentration (TC) for the drainage area, A equals the area in acres contributing runoff, and TC equals the time of concentration in minutes. • Existing storm drainage peak flow: Q(lOyr) = 7.70 cfs, C = 0.65 A = 1.42 AC, TC = 10 min, i = 8.35 in/hr • Proposed storm drainage peak flow: Q(lOOyr) = 15.66 cfs, C = 0.75, A = 1.44 AC, TC = 10 min, i = 12.5 in/hr • Detained Detention Volume = 7,164 cubic feet (CF) • The proposed drainage system has a drainage area of 1.15 AC which will discharge 5.55 cfs through an underground detention system that discharges through an 18-in HDPE into an existing inlet on the west side of Tauber Street & Cherry Street. The remaining drainage is of 0.285 AC which will discharge 2.15 cfs is directly discharged into a drainage system along College Main. Based on the current storm water design guidelines and the calculation that 0.4 acres of the proposed development will discharge directly into the drainage system along College Main, a further analysis of the drainage system along Cherry Street was necessary. Utilizing Winstorm, the existing drainage system down Cherry Street to the ditch along Inlow Blvd was re-evaluated. Once the existing system was verified, an analysis of the proposed additional runoff from the Factory site was evaluated for the designed 10 yr storm event and evaluated the 100 yr storm event. See Exhibit 1. The 10 year storm event was the basis of the design of the Cherry Street Improvements. Mr. Alan Gibbs, P.E. July 3, 2007 Page 2 of 4 k I o t z -~~ a s s o c i a t e s storm event due to the requirement and use of minimum pipe sizes and limited local drainage areas. It appears that existing channel along Inlow Blvd. can be expanded to provide detention volume equal to that required for the site as shown in Binkley & Barfield's calculations. The calculations are outlined in more detail below. During the initial review of the project, the plans for Cherry Street dated August 2001 showed that the majority of the site drained to College Main. The site plan prepared by Binkley & Barfield, routes the majority of the flow through an underground detention facility and discharges into the Cherry Street drainage system. Original Design The following information was provided by Binkley & Barfield by utilizing the Rational Method. The formula used is Q = CiA in which Q equals the peak rate of runoff in cubic feet per second (cfs), C equals the dimensionless coefficient ofrunoff, i equals the average intensity of rainfall in inches per hour for a period of time equal to the time of concentration (TC) for the drainage area, A equals the area in acres contributing runoff, and TC equals the time of concentration in minutes. • Existing storm drainage peak flow: Q(lOyr) = 7.70 cfs, C = 0.65 A = 1.42 AC, TC = 10 min, i = 8.35 in/hr • Proposed storm drainage peak flow: Q(lOOyr) = 15.66 cfs, C = 0.75, A = 1.44 AC, TC = 10 min, i = 12.5 in/hr • Detained Detention Volume = 7,164 cubic feet (CF) • The proposed drainage system has a drainage area of 1.15 AC which will discharge 5.55 cfs through an underground detention system that discharges through an 18-in HDPE into an existing inlet on the west side of Tauber Street & Cherry Street. The remaining drainage is of 0.285 AC which will discharge 2.15 cfs is directly discharged into a drainage system along College Main. Based on the current storm water design guidelines and the calculation that 0.4 acres of the proposed development will discharge directly into the drainage system along College Main, a further analysis of the drainage system along Cherry Street was necessary. Utilizing Winstorm, the existing drainage system down Cherry Street to the ditch along Inlow Blvd was re-evaluated. Once the existing system was verified, an analysis of the proposed additional runoff from the Factory site was evaluated for the designed 10 yr storm event and evaluated the 100 yr storm event. See Exhibit 1. The 10 year storm event was the basis of the design of the Cherry Street Improvements. Mr. Alan Gibbs, P.E. July 3, 2007 Page 3 of 4 The following is a summary of the results of analysis: The Existing conditions for the site are as follows: k I o t z 4~· a s s o c i a t e s • Q (lOyr) = 4.49 cfs, TC= 10 min., C=0.50, i = 8.63 in/hr, A=l.04 AC. • Q (lOOyr) = 6.05 cfs, TC= 10 min., C=0.50, i = 11.64 in/hr, A=l.04 AC. The Proposed conditions for the site are as follows: • Q (lOyr) = 8.08 cfs, TC= 10 min., C=0.90, i = 8.63 in/hr, A=l.04 AC. • Q (lOOyr) = 10.89 cfs, TC= 10 min., C=0.90, i = 11.64 in/hr, A=l.04 AC. The 1 OOyr hydraulic grade line CHOL) was below the pavement elevations along Cherry and Tauber Street. See attached WINSTORM calculations. The HGL was 338.47 ft and the top of inlet is 34 7. 7 5. Therefore the 100 year flows off of the Factory Apartment site are contained and conveyed within the existing storm sewer system all the way to the open ditch along Inlow Blvd and detention could be provided in that open ditch. Detention in Inlow Ditch The ditch geometry along Inlow Blvd was evaluated. See Exhibit 3. Based on the location of the existing power poles, five areas could be utilized. Each area would decrease the side slopes from 2: 1 or less to approximately 4: 1 and yield approximately 2,500 CF of mitigation volume for a total increased volume of 12,5 00 CF. See Exhibit 4. Each area has a length of about 200 feet. The location of existing sanitary manholes needs to be verified and could affect the available volumes. Also, it appears that the City limits of College Station and Bryan are located along the ditch. This added detention will be useful to the entire upper drainage for all storm events. We understand that Cherry Street LP plans to excavate all the length of the ditch on the south side only. The side slopes would be hydromulched and stabilized similar to what is existing there today. This excavation would produce approximately 12,500 CF. Cherry Street LP would document the amount of excavation by measuring the amount of dirt hauled away in each dump truck by tape measurement. A minimum of 7 ,500 CF will be constructed. This excavation would be in excess of the volume needed for the Factory apartments. Radakor, LLC, Development GP of Cherry Street LP may use that excess detention volume for future projects in the area with the review and approval by the city.