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.
The City of Austin owns and maintains at least 63 high hazard dam structures across the City. State and City dam safety criteria require these high hazard dams to be able to safely pass the Probable Maximum Flood (PMF) without failing. Our project team performed preliminary design assessments of the 63 dams to determine whether or not the dams were capable of safely passing the PMF, and if not, to perform preliminary designs of mitigation efforts to allow the dams to safely pass the PMF. We provided the following services: Assisted in developing the hydrologic models for the 63 dams as part of the preliminary design services Calculated times of concentration for contributing runoff and developed HEC HMS models. Delineated drainage boundaries for each of the 63 structures based on available information and site visits Performed “peer review” of the proposed FNI methodologies for preliminary risk assessment from a theoretical breach and the overall structure ranking Provided input to the methodologies including technical publications on calculating breach width and breach side slopes, flowchart diagram to depict various evaluation steps, and a “sensitivity analysis” for dam breach velocity related to attenuation length Prepared memorandums summarizing the results of the peer review Under final design services, assisted the project team with developing hydrologic and hydraulic models for Pond Dams 26, 220, 267, 337, 581, and 726
Scenic Brook Flood Control Project, Hydrologic Modeling & Dam Hazard Analysis The Scenic Brook neighborhood is located on the Scenic Brook Tributary to Williamson Creek in the City of Austin. The neighborhood has been plagued by frequent local flooding. The City hired a team to develop mitigation measures. As part of the solution, the team identified a stormwater detention pond up gradient from South Brook Drive to reduce peak flowrates entering the neighborhood. We provided the following: Developed the SCS TR-20 model for the existing and future condition for the 2-, 5-, 10-, 25-, and 100-year storms Coordinated with TCEQ staff on dam safety requirements. Coordinated with staff in adjusting detention pond outlet structures to meet flood control objectives and TCEQ requirements. The detention pond consisted of a concrete retaining wall embankment, and is approximately 16’ high with a flood storage volume of about 22 acre-feet. Developed dam safety analysis using the Probable Maximum Flood (PMF) storm event Prepared engineering report documenting the project approach and conclusions.
34th Street Reconstruction from West Ave to Shoal Creek Bridge Drainage Analysis and Storm Drain Improvements We provided preliminary and final designs of the storm drain improvements for the reconstruction of 34th Street from West Avenue to the Shoal Creek Bridge, a distance of approximately 2,170 ft. Our designs included the following: Design of approximately 1,950 LF of storm drain conduits (ranging from 18-inch RCP to 7’x4’ box culverts) A final design of the storm drain system improvements including coordination of horizontal and vertical alignments of the storm drain conduits with water and wastewater relocations, pavement reconstructions, and structural designs of the impact basin and retaining wall at Shoal Creek. Innovative combinations of storm drain conduit sizes, pipe classes, and alignments were used to minimize utility conflicts and relocations and to allow tunneling under Lamar Boulevard to install the 48” storm drain line without disrupting Lamar Boulevard traffic
Bull Creek Watershed Floodplain Modeling and Mapping The Bull Creek Watershed, including West Bull Creek, is a suburban watershed with approximately 31.5 square miles of drainage area and 34.25 stream miles to be studied. The main purpose of this project was to develop digital geo-referenced hydrologic and hydraulic models and floodplain maps for the Bull and West Bull Creek watersheds for both existing and fully-developed land-use conditions. Our responsibility for this project involved developing hydraulic data and floodplain mapping for selected tributaries of Bull Creek. We developed: Fully-annotated HEC-RAS models with all supportive GIS files such as cross-sections and GIS geodatabase for both existing and fully-developed land-use conditions Digital hydraulic data including tables, methodology descriptions and exhibits for Existing and Fully-Developed Land-use Conditions to incorporate into the Hydraulic Technical Support Data Notebooks (TSDN), Water surface elevation profiles at similar vertical and horizontal scale as the profiles in the effective FIS report for selected FEMA flood frequencies using RAS-PLOT for the detailed study tributaries only, Floodplain mapping Digital copies of the floodplains and BFEs for the detailed study tributaries for both existing and fully-developed land-use conditions
Over 175 homes located in the Crystal Brook neighborhood were threatened by frequent flooding due to floor elevations below the 100-year storm water surface elevations. We provided the following improvements: 16,600 LF of storm sewer lines (ranging from 18” RCP to 12’ x 7’ concrete box) 7,000 LF of channel improvements (ranging from 6’ wide to 100’ wide bottom) 5,700 ft. flood barrier system 2,000 LFof creek bank stabilization and restoration
Los Indios Culvert Upgrade and Channel Improvements The Los Indios subdivision and Los Indios Trail are located in northwest Austin. The Los Indios Trail roadway was overtopped by Rattan Creek during the 10-year and greater return period storm events putting seven (7) homes in the vicinity of Los Indios Trail at risk of flooding in the 100-year storm event. Using on-the-ground topographic survey, we developed alternatives to mitigate the culvert and local flooding problems and prepared construction plans for the proposed improvements.