Highland Lakes Draft FONSI Highland Lakes Environmental Assessment
Appendix A Appendix B Appendix C
Draft Finding of No Significant Impact
Environmental Assessment Revisions to Mansfield (Marshall Ford) Dam and Lake Travis Water Control Manual, Colorado River, Colorado River Basin
Description of Action. The United States Army Corps of Engineers, Fort Worth District (USACE) and the Lower Colorado River Authority, the non-federal sponsor, are evaluating the potential environmental consequences resulting from implementing proposed revisions to the operations rules of Mansfield Dam (Marshall Ford) and Lake Travis in the Highland Lakes region of the Lower Colorado River Basin, Texas. The proposed operating rule revisions were analyzed in the Phase II Flood Damage Evaluation Project study completed in August 2012.
The initial Phase I Lower Colorado River Flood Damage Evaluation Project study, completed in July 2002, provided baseline hydrology, reservoir operations, and hydraulics throughout the Lower Colorado River Basin, Texas. In August 2005, USACE completed a Programmatic Environmental Impact Statement (PEIS) for Flood Damage Reduction and Ecosystem Restoration, Lower Colorado River Basin, Colorado River, Texas. The PEIS was prepared to establish existing conditions, identify direct and indirect impacts, and cumulative impacts to the environment as a result of implementing water resource projects in the Lower Colorado River Basin.
This Environmental Assessment (EA), which is tiered to the 2005 PEIS, addresses proposed revisions to the Mansfield (Marshall Ford) Dam and Lake Travis, Colorado River, Texas, Water Control Manual by identifying and evaluating impacts that would result from proposed adjustments to water stage elevations included in the new Water Control Manual. For the purposes of this EA, the terms Colorado River, Colorado River Basin, Lower Colorado River, and the Lower Colorado River Basin are used interchangeably due to variations in the aforementioned documents.
Anticipated Environmental Effects. The EA considers seven alternatives, including the no action. Under the Proposed Action plan, the existing operating rules would be modified to provide the maximum benefit of releases by changing the stage elevations at both the Austin and Bastrop gauges to match the actual control discharges noted in the 1979 Water Control Plan. This Proposed Action alternative would have the least impact to interests downstream from Mansfield Dam and would provide for moderate reductions in flood inundation damages upstream from Mansfield Dam, especially for the events with recurrence intervals in the 10- to 25-year range. The modifications to the operating plan would update the published control stages so the regulating discharges control releases as was originally intended. Water control stages would be as follows in the new Water Control Manual: Austin Station - 33.0 feet (ft), Bastrop Station - 27.2 ft, and Columbus Station – 35.5 ft. Implementation of the new operation rules eliminates the discrepancies found in the current manual.
No significant adverse direct, indirect, or cumulative impacts to the human and natural environment associated with implementation of the Proposed Action are identified. Minimal temporal impacts to vegetation in the 100-year floodplain during the 10- to 25-year flood events were identified. Additionally, there are moderate benefits to the flood damage area of Lake Travis for the 10- to 25-year flood events with a slight increase in inundation acreage downstream of Mansfield Dam to the Columbus gauge.
Conclusions. Based on a review of the information contained in this EA, it is determined that the implementation of the proposed action is not a major federal action, which would significantly affect the quality of the human environment within the meaning of Section 102(2)(c) of the National Environmental Policy Act of 1969, as amended. Therefore, the preparation of an Environmental Impact Statement is not required.
Charles H. Klinge
Colonel, US Army
Commanding
Highland Lakes Draft FONSI Highland Lakes Environmental Assessment
Appendix A Appendix B Appendix C