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Letter to StaffKimley»>Horn Memorandum To: Danielle Singh, P.E. From: Tyler Lewis, P.E. Subject: THE BARRACKS DETENTION FACILITY MOO/FICA TIONS (OP) Date: November 10, 2014 This memorandum is to address the City of College Station Staffs comments on the above mentioned project as received by Kimley-Horn and Associates, Inc. in the letter dated August 19, 2014. The following information has been included in this submittal for your review. • 1-Full Size set of Construction Plans • Drainage Report Please find the written responses to comments on the following pages. kimley-horn.com 2800 South Texas Avenue, Suite 201, Bryan, TX 77802 979 775 9595 g Kimley»>Horn Page2 1 IYi Engineering Comments No. 1 -$ 1. We have received the updated preliminary plan since your original submittal. P13 ./I does not include 5 acres of open space, this is mostly designated for multi-family. -~ P14 is also unlikely to be developed with 5 acres of open space. P6 includes ~ commercial to be developed with the Barracks though this is shown as open space. i The submitted design includes the latest land plan received from Schultz Engineering 11/4/2014. P13 now includes 8.65 acres of townhome, 1.2 acres of) multifamily, and 3.66 acres will be left in existing conditions (requiring the future development to include site specific detention). It was communicated to KHA that 3.35 acres of P14 be designated as multifamily, which is now reflected in the weighted impervious calculations. The land use in PG has been revised to eliminate the original commercial portion and only have townhome (15.06 acres) and open space (0.70 acres). /2. On the proposed cond ition drainage area map, the Watershed Information table was not updated with the changes to this report. The watershed information table on the proposed condition drainage area map has been updated. 3. The City will not maintain the storm sewer pipes between Pond 1 and Pond 2 except within the ROW of General Parkway. Please show an additional junction box on the northeast side of General Parkway. An additional junction box has been added on the east side of General Parkway for each of the 36" lines to demark the public and private portions. 4. It appears that the weir for Pond 1 will not properly drain, with higher elevations immediately downstream of the weir. How do you propose to address this situation? This issue has been addressed per the original construction plans. The weir now freely drains. 5. Are the tables provided produced by SWMM? If not, please provide SWMM output for reference. The SWMM output tables are now included in the appendix of the report for each existing and proposed conditional run for the 2, 10, 25, 50, and 100 year events. 6. In Appendix D, Downstream 100 Year Hydraulic Conditions, please explain why the flow is decreasing as it moves downstream. For instance, at station 7431 the flow is less than half of the flow leaving the site wh ich is immed iately upstream. The SWMM model is an unsteady state model that accounts for storage in each link, based on its geometry. This storage attenuates the flow, similar to a detention pond, so peak flow can decrease as you move downstream. The amount of storage in a link will affect the amount of attenuation. 7. Please label rip rap downstream of Pond 4. 200 SY of TxDOT Type 'R' riprap is called out at the outfall of Pond 4. kimley-horn.com 2800 South Texas Avenue, Suite 201 , Bryan, TX 77802 979 775 9595 7 ' Kimley»>Horn Page 3 Where is the emergency spillway for Pond 2? Since Pond 2 only receives flow from the interconnect with Pond 1 and precipitation that falls directly into the pond. The plugged flow condition reaches a lower peak water surface elevation than in normal conditions. 9. On Sheet 8, Pond 3 discharge is listed as 3325.43 cfs. Please revise . The peak outflow from Pond 3 is 346.26 cfs. This has been revised on sheet 8. kimley-horn.com 2800 South Texas Avenue, Suite 201 , Bryan, TX 77802 979 775 9595