Res 2003-152 - Laserfiche WebLink

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receives water as drainage from the surrounding street and hardpan surfaces. The City of San Marcos is interested in installing a berm and water control structure near the existing culvert structure to capture runoff from the surrounding area and create a 0.5 to 1.0 acre emergent wetland to increase runoff retention time. Following construction, native aquatic plant species will be installed to help improve water quality by removing nutrients and depositing sediment prior to entering the San Marcos River. The second site is located south of Cheatham Street and east of the Rio Vista Park area within the Crooks Oxbow area (Enclosure 7, Photo 23). The existing Crooks Oxbow wetland area receives water from the San Marcos River via the diversion spillway located upstream from the Rio Vista Dam (Enclosure 7, Photo 16). The narrow oxbow area is hydrologically connected with the San Marcos River and harbors many exotic aquatic plants. The City of San Marcos is interested in widening the existing oxbow area, establishing native vegetation and inserting nesting boxes to enhance habitat and attract a voider diversity of migrating birds and wildlife. The existing wetland/oxbow area is approximately 1.0 acres in size and would require the removal of approximately 200,000 cubic feet of soil, selective thinning of existing vegetation, and installation of a berm and water control structure. Existing exotic vegetation would be removed and replaced with native wetland species such as bald cypress, bulrushes and pickerel weed. The third site is located within Purgatory Creek, approximately 1100 feet upstream from the confluence with the San Marcos River (Enclosure 7, Photo 24). The existing wetland area is currently maintained by the City of San Marcos, but owned by the Meeks family and will require a conservation easement as part of the final restoration plan. As part of the restoration effort, the existing wetland will be widened to create a 1.0 to 2.0 acre wetland complex. A berm and water control structure will be installed at the downstream end of the wetland complex. Native aquatic and terrestrial vegetation will be planted within and around the wetland complex. The use of this area as a wetland will improve water quality and in- stream habitat values within the San Marcos River by increasing retention time and decreasing the large amount of sediment that is deposited from Purgatory Creek. (9) Dam Removal & Modification: The Rio Vista Dam, built in the 1880s, was constructed as a cedar wood crib dam and later overlain with concrete (Enclosure 7, Photo 25). In April 1995, the Freese and Nichols Engineering firm inspected the dam and found that the right and left abutment walls show signs of extreme structural distress, the right approach wall and overflow sections are undermined, the left abutment wall is undermined and there is a concentrated seepage path around the right abutment of the overflow section. Rather than restore and stabilize the Rio Vista Dam, the City of San Marcos is interested in converting the existing structure into a natural riffle - waterfall complex. The existing Rio Vista Dam structure consists of a 107 -foot headwall, a right and left abutment wall, a right and left approach wall, and an eight -foot mid - channel chute surrounded by two concrete aprons. To create the proposed riffle - waterfall complex the concrete headwall and mid - channel chute will be removed and replaced with native stone boulders. The chute in the middle of the dam will be maintained as it is an important recreational aspect of the dam. The upper and lower concrete aprons will remain in place to deter river headcutting and provide a crib for installed boulders. Appropriate sized native stone boulders will also be placed above and below the concrete aprons to provide additional aquatic habitat and promote transition of the river into the riffle - waterfall complex. In ►w,