09-12-11 Regular Meeting of La Porte City Council

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in Subwatershed A104A, Subwatershed A104A encompasses the area upstream of SH 146 for Taylor Bayou. Subwatershed A104A will be subdivided to compute peak flow rates along Taylor Bayou from its confluence with HCFC Unit A104 -12 -00 to SH 146. Peak runoff rates will be computed for the 10 %, 2 %, and 1% exceedance probability events. Post - project hydrology will be determined based on the proposed channel improvements. Peak flow impacts will be determined between pre- and post - project conditions. Task 4— Hydraulic Analysis The FEMA effective HEC -RAS model for Taylor Bayou, HCFC Unit A104- 00 -00, will be extended from it current study limits at Shore Acres Boulevard to SH 146. New surveyed channel cross section will be obtained in accordance with HCFCD requirements. The surveyed channel cross sections will be supplemented in the overbank areas with HCFCD HDAR Digital Elevation Model data. Hydraulic profiles will be computed for the 10 %, 2% and 1% exceedance probability event flows. Alternative design channel sections and channel profiles will be evaluated to contain the 1% exceedance probability water surface elevations within the channel banks along the project limits. The design channel sections will include channel transitions between existing and proposed sections. A preliminary schematic of the proposed channel improvements including channel transitions will be prepared in this task. Task 5 -- Mitigation Analysis A mitigation analysis will be performed to determine the required detention volume required to reduce post - project flows to pre - project flows from the proposed channel improvements. A detailed hydrologic and hydraulic analysis will be performed to determine the required storage and control structure for the mitigation basin to mitigate any adverse impacts to downstream flows. A preliminary schematic of the proposed mitigation basin and control structure will be prepared in this task. Task 6 — Proiect Evaluation_ A project evaluation will be performed to determine the feasibility of the proposed channel improvements along Taylor Bayou, FICK Unit A104- 00 -00. The project evaluation will include preparing a preliminary construction cost estimate and determining the potential flood reduction benefits to the Shady Rivers Subdivision. Flood reduction benefits may be quantified in terms of the number of homes removed from the 100 -year flood elevation (riverine flooding only). It is anticipated that the City of La Porte will provide finished floor elevations for homes within the Shady Rivers Subdivision. The Consultant may provide recommendations on other structural and non - structural flood reduction measures, if deemed applicable. Task 7 -- Study Report A study report will be prepared to document the technical findings and recommendations for the proposed channel improvements and mitigation pond. Five (5) copies of the draft and final study report will be submitted to the City of La Porte. The study report will be signed and signed by a Licensed Professional Engineer in the State of Texas. CIVilleCh Page 3 of 3 Engineering, Inc.