Res 2011-045 - Laserfiche WebLink

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Attachment A Introduction Restoration activities on the San Marcos River remain a high priority for the City of San Marcos (the "City ") and Texas State University ( "TxState "). As part of these activities, both parties are examining channel restoration that targets creation of habitat conditions suitable for the Texas Wild Rrice ( "TWR ") [Zizania texana]. Due to historical land use practices, the accumulation of fine sediments within the San Marcos River below the confluence of Sessoms Creek and Purgatory Creek is evident and the invasion of non - native aquatic plants within TWR habitats may be limiting potential habitat. As part of the City's restoration planning, one option being contemplated is the removal of unsuitable sediments and non - native plants to allow expansion of TWR habitat. Obiectives The objective of this Project is to utilize Ground Penetrating Radar (GPR) to map river bottom sediments within the San Marcos River in two key areas near the confluence with Sessoms Creek and Purgatory Creek. The mapping is intended primarily to define the characteristics of the river bottom sediments in the vicinity of these two tributaries as well as the volume of these sediments to aid in restoration design activities. Methods TxState will utilize a GPR unit with a systematic irregular sampling regime to delineate the river bottom substrate characteristics starting upstream of each confluence area and proceeding downstream approximately 100 meters. Based on the measured stratigraphy from the GPR, TxState will use a sediment core sampler to verify substrate characteristics. In those areas not suitable for the core sampler, a small trench will be dug to visually verify the substrate characteristics to allow a calibrated interpretation of the GPR signals. The sampling data will then be used by TxState to prepare both plan view and volumetric maps of each study site that identify suitable TWR substrate characteristics (sand to small gravel). Deliverables TxState will prepare plan view maps of each study site as well as volumetric maps that show the location and depth of river bottom substrates in two key areas near the confluence with Sessom Creek and Purgatory Creek. Existing aquatic vegetation maps will then be over laid to identify the distribution of non - native aquatic vegetation to show potential areas where non - native plant removal would provide suitable substrates for TWR. In addition, these maps will allow the