TPWD Bastrop Lake Dam Rehabilitation

Halff worked with the Texas Parks and Wildlife Department on the Buescher State Park Lake Dam Rehabilitation, as well as the reconstruction and slope stabilization of the dam. ​

Bastrop State Park is approximately 6,600 acres. The Dam and Lake were originally constructed in 1913 with nearby available borrowed material. Beginning in 2011, the park experienced natural calamities and the Bastrop Complex Wildfire struck the loblolly pinewood forest. The Bastrop State Park Dam embankment failed on May 25, 2015, during an intense rainstorm that caused extensive flooding throughout the Bastrop area. The dam overtopped, causing the dam’s earthen embankment to breach. This resulted in a rapid draining of the entire lake reservoir. The dam breach flood wave surged down Copperas Creek, overtopping Highway 71 and then finally flowing to the Colorado River. The rapid drawdown of the lake caused slope failures that threatened soil stability adjacent to the historical cabins.

Prior to the dam failure in 2015, Halff prepared a Preliminary Engineer Report (PER) developing options to fortify the earthen embankment and control seepage. After the dam and slope stabilization failure near historical cabins, Halff and TPWD began preparing slope stabilization design plans to protect Cabins 12 and 1. Protecting this historical cabin is a high priority for TPWD.  The slope failure was within 15 feet of the historical cabins and a maximum height of 35-foot-high vertical drop in consolidated sandy soils. The slope stabilization solutions included mechanically stabilized earth (MSE) embankments with native grasses and concrete retaining walls veneered with brown stone. This helped it blend in with the historical brown sand stones used throughout the park.​

Halff and TPWD began developing conceptual solutions to reconstruct Bastrop State Park Dam. The final dam design consisted of a structural fill earthen embankment with a bentonite slurry cut off wall to control seepage. During flood events, the dam is designed to be overtopped and stabilized on the downstream slope with stepped roller compacted concrete (RCC) with a stilling, dissipating the hydraulic jump and lowering flow velocities.

In 2018, Halff and TPWD conducted and prepared a dam assessment and developed a Preliminary Engineering Report (PER) that proposed a temporary short-term and long-term solution. The short-term solution proposed a rock rip rap design that could dissipate energy on the surface, as well as a subsurface aggregate drain system. Halff then prepared stabilization design plans for the short-term spillway repair. The project was constructed in 2018. The short-term design was intended to be temporary and easily excavatable once a long-term fix is implemented.

The long-term solution will rehabilitate the dam structure to meet TCEQ Dam Safety criteria. In 2019, Halff was retained to prepare dam rehabilitation improvements. The design plans included:

• Replacing the existing spillway with a 200-foot sharp crested weir to safely pass the 75% PMF
• Increasing the embankment crest
• Controlling seepage through the earthen embankment with a toe buttress containing an aggregate under drain system

The shoreline immediately upstream of the dam was experiencing erosion. TPWD requested Halff to stabilize the shoreline with areas where park users could fish and enjoy the lake. Permitting coordination with state and federal agencies included USACE for 404 Nationwide permitting, THC for historical and cultural resources, USFWS for threatened and endangered species (specifically the endangered Houston toad), and TCEQ Dam Safety. Halff is currently acting as the TPWD’s dam engineer of record and is assisting with construction phase services.