Stormwater Issues

The initial project focuses on flood control through the construction of additional floodplain storage at historic Lake Henrietta, decreasing the meander in the slough between Lake Henrietta and Lake Munson, removing the sediment and trash delta in Lake Munson, and providing a "pop-off" to the north arm of Lake Munson for high flows. The removal of the 27-acre delta at the head of Lake Munson decreased water surface elevations in the upstream slough by 1.5 feet, based on watershed modeling. Flood control measures provide an opportunity for concurrent water quality improvements as the 30-acre Lake Henrietta facility is designed for sediment capture and trash removal. The slough reconstruction provides an opportunity to stabilize the banks, which currently contribute to the lake's sediment load, by decreasing the side slopes and using geotextiles to support the banks while the vegetation reestablishes. The Lake Henrietta facility substitutes for one of the facilities recommended in the 1991 NWFWMD plan. The facility will capture up to a 2-year return frequency storm event, but does reduce 100-year flood elevations up to one foot in the downstream slough. The improved maintenance through sediment capture and removal will assist as well in flood reductions. The wetlands adjacent to Lake Henrietta are being restored by placing a fixed structure in the slough to hold the water upstream and increase the wetland hydroperiod. In addition, discharge points to the wetlands through the west bank maintenance access will improve the hydration of the western wetlands, which was a non-structural recommendation of the NWFWMD. The slough was analyzed to determine which reaches were subject to scour. The improved slopes, decreased meander, and the use of soil stabilization materials will decrease the erosion and sedimentation currently plaguing the system. The stormwater model showed that the improvements, including the removal of the sediment delta, were so successful in lowering flood elevations that rock dams will be constructed in the slough to maintain the hydroperiod of the wetlands south of the Lake Henrietta control structure to Lake Munson. During removal of the sediment delta, the project design and permits called for the slough base flow to be diverted through an adjacent City borrow pit to the north arm of Lake Munson. Following construction, the ditch would remain to provide for additional discharge of high flows during storm events, which would also serve to flush the isolated north arm which currently has unexpectedly high nutrient levels. Citizen concerns for the cypress fringe along the north arm led the Board to authorize the construction firm to take advantage of our current drought conditions and attempt the delta removal without the diversion ditch. The lake was drawn down for the duration of the removal effort to assist in dewatering the material. The delta removal is expected to be complete by August 2000.