SC Robertson Dam (Lake Limestone) and DeCordova Bend Dam (Lake Granbury) Breach Analyses and Inundation Mappings
The Brazos River Authority (BRA) owns, operates and maintains a series of dams along the Brazos River. The dams have high-hazard classifications because of the consequences of dam failures, and because their sizes and hazard ratings are subject to State Dam Safety criteria. One of the dam safety criteria is that high-hazard dams must have written Emergency Action Plans with detailed supplemental breach inundation mappings. The BRA hired a preeminent hydrologic & hydraulic engineering firm to prepare the complicated breach analyses (using HEC-RAS hydraulic modeling) and detailed breach flood inundation mappings for S.C. Robertson Dam (Lake Limestone) and DeCordova Bend Dam (Lake Granbury) for “sunny-day” breach failure conditions and for Probable Maximum Flood (PMF) design flood conditions.
S.C. Robertson Dam has a 8,130 foot-long embankment section with a maximum fill height of 60 feet, a concrete gravity spillway with 5 radial gates and a stilling basin. DeCordova Bend Dam has a 1,400 foot-long compacted fill embankment section with a maximum fill height of 63 feet, a concrete Ambursen-type spillway with 16 radial gates and a stilling basin. Breach analyses and breach flood inundation mappings were performed using HEC-RAS hydraulic program.
Our firm calculated the breach parameters for both dams, the embankment sections, the gated spillway sections, the “sunny-day,” and for PMF design flood hydraulic conditions. We performed data research of the configurations of the two dams, including reviews of the construction as-built drawings and performed site visits to confirm field conditions and to confirm the locations and types of reasonable breach scenarios.
Construction of 7.3 miles of Southern Walnut Creek Bike Trail. The trail is aligned close to the Walnut Creek and Boggy Creek channels where ongoing channel instability and erosion could undermine the new trail. CPE designed Walnut Creek and Boggy Creek channel stabilization measures at four locations, emphasizing the use of green channel stabilization measures, rather than traditional “hard” measures, including 520 CY of gabions, 1,800 SY of soil retention blanket, 3,400 SY of geogrids, 2,840 CY of rock riprap, and 220 CY of large limestone blocks.
Doucet assisted in the development of a master plan within the Waller Creek corridor the Waller Creek District. The master plan’s project area was generally bound by: 12th Street on the north; Interstate Highway 35 on the east; Lady Bird Lake on the south; and Red River street and Trinity Street on the west. One goal of this project was to provide strategies and costs for the restoration of eroded and otherwise degraded sections of Waller Creek stream banks within the study area. Doucet was responsible for budget estimates for all of the creek corridor improvements. Improvements included adding trails, stairs, ramps, terraces, overlooks, path lighting and the removal of obstructions and dams. Doucet also presented the Creek Corridor Comparable Projects Report and Budget Estimate to City of Austin staff and the Technical Advisory Group for their review and comment. Doucet also performed field reconnaissance visits to confirm existing conditions and to identify locations and to make measurements of stream channel improvements.
Whitaker Field at Waller Creek Stream Bank Stabilization Doucet provided bank stabilization services for three of the most severely eroded areas along the west bank of Waller Creek within the University of Texas Whitaker Field property boundary as well as preparation of a Master Erosion Plan for the Waller Creek along Whitaker Field. The existing Waller Creek was characterized by bank erosion, channel bottom down cutting, fallen tree trunks and limbs, significant amount of debris, significant amount of trees, and frequent wastewater line crossings. The University of Texas was losing property and the eroded bank was a potential safety hazard for the athletic field users. To correct this, Doucet used stabilization techniques for the three most severely eroded areas that included placing limestone blocks, selected soil backfill, soil retention blankets and 2’ diameter rock riprap at the problem areas. The City’s Austin Clean Water Program (ACWP) has a wastewater improvement project along a portion of the study area. Doucet coordinated with the ACWP director and project engineer to insure a seamless transition between the two projects. Towards this goal, Doucet assisted the project engineer in creating a Master Erosion Plan which included reviewing of peak discharges, channel velocities and shear, as well as identifying additional bank stabilization areas and locations of grade control structures. The Master Erosion Plan was presented to the representatives from the UT Athletic Department and was well received.
Downtown Overlay District (DOD), Water Quality Master Plan and Regional Water Quality Pond Design The City of Georgetown developed a downtown master plan to provide a comprehensive development strategy for the central part of the City. The master plan boundary followed that of the Downtown Overlay District (DOD). The DOD, comprised of approximately 110 acres, consisted of varied land uses including commercial, institutional, and residential. As part of the downtown master plan and to provide a more holistic approach toward re-development within the DOD, the City investigated a “regional” approach to providing storm water quality management. Utilizing such an approach eliminated the need for individual water quality ponds on each re-developing parcel, allowing more flexibility in design and providing a more aesthetically pleasing framework for this historic area of the city.
The La Frontera Section I and II development (a 113-acre commercial retail center) is located at the Northwest corner of IH-35 and FM 1325 in Round Rock, Texas. Doucet designed a regional stormwater detention facility to protect downstream properties (structures within the floodplain), and a stormwater quality treatment facility to protect the northern Edwards’ Aquifer. In addition, three downstream road crossings were sub-standard and did not allow for safe passage in a 25-year storm event. Doucet performed a hydrologic analysis on this tributary of Lake Creek and designed culvert improvements for the three road crossings.
The Town Lake Park water quality wet pond is located in the northwest quadrant of the Palmer Events Center. The pond collects, treats, and detains stormwater runoff from approximately 18.3 acres of the site which includes the following areas: Palmer Community Events Center building (rooftop runoff), parking garage, parking and drive area near the Doherty Arts Center, the children’s outdoor arts center, the open meadow inside the large oval sidewalk, and the area in the immediate vicinity of the pond.