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.