08-21-2002 Committee Meeting - Laserfiche WebLink

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Port of Houston Authority Wetland Mitigation Plan Excess water from the south wetland will flow into the north wetland, and excess water from the north wetland will flow into Taylor Bayou via the Harris County Drainage Ditch. The design is intended to create wetlands with fluctuating water levels resulting from variations in the amount of precipitation and the rate of evapotranspiration over time. Water balance models were developed for each of the created wetland cells to project pool levels during normal precipitation periods (based on 46 years of median monthly precipitation data). Using median rainfall and evapotranspiration data the model indicated that, once the marshes filled, wetland pool levels would fluctuate during the year, but would contain some water each month of the year. The model was also used to project pool size for years when rainfall was not considered normal (median). Actual rainfall and evapotranspiration data for the years 1997-2000 were used to evaluate potential changes in pool size with respect to rainfall amounts and temporal distribution. The annual data showed that during dryer than normal periods, pool size may fall to zero (no standing water) at least once during the month, and remain dry for up to 3 consecutive months. The model also indicated that pool size could fall to zero during wetter than normal years when rainfall in not evenly distributed over the year. The wetlands will be developed by constructing two earthen levees that will impound runoff from the watershed to the southeast of the Harris County Ditch. Levee wings extending from the primary levees will intercept and direct runoff from a larger portion of the watershed into the wetland reservoir area. Levee construction materials will be obtained from on -site excavations and/or off -site sources, based on need and geotechnical requirements. Levee heights are planned to allow approximately 1.5 feet of freeboard (above the spillway elevations) when the wetlands are at full pool. On -site excavations will have no significant impact (depth modification) to the proposed wetland design. Soils from outside the wetland footprint, and thin (0.2 foot) layers from within the footprint may be used. During construction, the upper 6 inches of soil (topsoil) will be removed and stockpiled from all areas where levees and excavations will be located. Topsoil will be redistributed over the levees and excavated areas to establish final elevations and ensure that the surface is covered with an optimum planting medium. This will Benchmark Ecological Services, Inc. June 2002 ��-i a5 -� sLi