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HomeMy WebLinkAboutAddendum #1 for Detention System Report ADDENDUM #1 FOR DETENTION SYSTEM REPORT CASTLEGATE II SUBDIVISION, SECTION 200 COLLEGE STATION, TEXAS Reflecting As -Built Conditions following Construction OCTOBER 2011 Prepared by: Phillips Engineering 4490 Castlegate Drive College Station, Texas 77845 (979) 690 -3141 r•``�� TBPE Firm #13130 .F -rgst *. /\ •.,� .KEN?M;. s.. • : 85823 RAW. • • A / Olt o„ ADDENDUM #1 Revisions to Drainage Model resulting from In- the -Field Modifications During the construction phase of Section 200 in Castlegate II, three significant modifications were made to the detention system design that had potential for changing the results predicted by the HEC -HMS computer model previously described in the Drainage Report. These modifications were input into the model and re- analyzed to determine what effect they had on the system. The three changes made to the design were: 1. Increase in the size of Detention Pond A — Following discussions with the city staff, it was determined that the existing FEMA Floodplain lines had strong impacts on the ability to develop the land adjoining the original pond boundaries. To address this situation and facilitate future development of the property, the southeastern boundary of the detention pond and its corresponding Common Area Boundary was moved outward to the existing FEMA Floodplain line. The northwest boundary of the pond adjacent to the city park property was not changed. The resulting pond is approximately 0.40 acres larger than the original at elevation 312.0. This increased size produces an additional 1.42 ac -ft of storage or about 15% more than the original pond design. The change in shape also required one of the inflow pipes to be rerouted to align with future lot lines planned in the area adjacent to the pond. 2. The shape of the outlet control structure on the existing pond in Castlegate was modified from a step weir configuration to a straight wall configuration — This modification was necessary as a result of in- the -field constraints that were not accounted for in the original design. 3. Reduced excavation downstream of WS Phillips Parkway — The area immediately downstream of WS Phillips Parkway was originally planned for excavation in order to increase storage capacity available during peak flow periods. As construction neared completion, it became evident the additional excavation was not needed due to the pond enlargement described in #1 above. The area was cleared and shaped, but no significant excavation was performed except around the flume connecting the box culverts on WS Phillips with the existing pool in Castlegate. Summary of Results The Post - development flowrates were re- analyzed at the same point immediately downstream of the outlet control structure of the existing detention pond near Castlegate Drive. The revised results are shown below: Location Pre -Dev Post Dev Reduction Study Pt A2 Castlegate Drive 448.8 cfs 383.8 cfs 65.0 cfs (original design results) 448.8 cfs 405.3 cfs 43.5 cfs As -Built Original Max. WSEL — Mod. Castlegate Pond (peak 100 yr) = 309.30 309.10 Max. WSEL — Mod. Castlegate Pond (blocked) = 309.80 309.80 Top of Berm — Mod. Castlegate Pond = 310.50 310.50 Max. WSEL — Pond A (peak 100 yr) = 311.70 311.70 Max. WSEL — Pond A (blocked) = 314.70 * 314.70 Low Pt of WS Phillips Pkwy = 314.03 314.03 As demonstrated above, the three modifications made over the course of the construction project show a slight decrease in the peak flowrate released from the original design and a slight increase in the WSEL of the Castlegate Pond. The new WSEL is still more than 1 foot below the Top of Berm level in the pond as required by the BCS Design Guidelines. 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