Basin-wide flood data will help community leadership reduce future damages

A five-year study finalized in March brought stakeholders from across the lower Brazos basin together to complete – and be awarded for – the first regional comprehensive basin-wide floodplain protection analysis available.

The final draft of the Lower Brazos Floodplain Study provides community leaders, emergency management officials, and many others access to data that will identify potential flood risks and determine flood hazards during significant storm events.

The study’s results showed that modeling the river from this perspective provided a better understanding of the elevations and flows on the Brazos River and the entire basin system that must be considered due to the complex system of overflows. Its comprehensive analysis earned the report a statewide award for engineering excellence.

The study wouldn’t have been possible without grant funding and the contributions of partners across the lower basin.

The project was funded through two grant awards from the Texas Water Development Board Flood Protection Grant Program along with funds from local participants and the Brazos River Authority. The total project cost was $1.7 million. Nine local stakeholders participated in the study, helping make the project possible, including, Brazoria County, Waller County, Washington County, Lake Jackson, Sandy Point, Sugar Land, Velasco Drainage District, Fort Bend County and Pecan Grove MUD. Numerous public meetings were held to help share information with residents and customers throughout the Lower Basin.

This study provides a more holistic basin-wide approach to understanding flood risks, said Aaron Abel, BRA water services manager. The study is important because it has the potential to help a vast amount of people, communities and industries along the lower basin, Abel said. The project to complete the work was even more impressive due to the large number of participants that came together to make this study work, he said.

Prior to this work, the major tributaries had never been included in planning documents. All former analysis of the mighty river was limited to county boundaries. To top it off, the former hydrologic and hydraulic models for the majority of the Brazos River—from Waller County to the Gulf of Mexico—were based on outdated analysis and studies.

The sheer size of the study makes it impressive, said Pamela Hannemann, BRA water resources planner.

“Jurisdictions will be able to use this in developing future plans for how they’re going to handle flooding,” she said.

The National Weather Service River Forecast Center and the U.S. Army Corps of Engineers will also be able to use the updated data moving forward in developing models, she said. “There are federal agencies that will use this not just our local participants at the local level,” Hannemann said.

The compilation of data was set to end about the time Hurricane Harvey in 2017 triggered catastrophic flooding across the Houston metropolitan area. Abel said they prolonged the study to include data reflected from Hurricane Harvey. The project was, however, far enough along that Hurricane Harvey was among the first instances in which the updated data was used in real time, Abel said.

The Lower Brazos River Floodplain Protection Planning Study addressed the public safety and welfare of those below the Hempstead gage by identifying existing flood risks, recommending flood protection measures and developing more accurate floodplain management tools.

No one dreamed record flood events in 2015 and 2016, plus Hurricane Harvey, would occur during the compilation of data, but the teams were able to use information from those events to gain better information, Hannemann said.

“It made the modeling better. It strengthened the analysis of the study,” she said.

The study was even more critical as counties in the Lower Brazos region represent some of the fastest growing areas in the country.

The study, performed by Halff Associates, LLC, received a silver medal in the 2019 American Council of Engineering Companies of Texas Engineering Excellence Awards. The Engineering Excellence Awards Competition has been held by the American Council of Engineering Companies of Texas since its inception in 1975. The awards are aimed at honoring and recognizing outstanding achievements within the engineering community. Judges from across the state selected nine gold medal and six silver medal-winning projects based on uniqueness, originality, technical, value to the engineering profession, complexity, and how successfully the project met the needs of the client.

Roughly $19.4 billion worth of 51,000 insurable structures and 140,000 people reside within the Lower Brazos River FEMA floodplain. Connected by the Brazos River are numerous communities, counties, districts, petrochemical industry and a thriving port.

The information compiled in this extensive study will help engineering professionals and communities through the identification of flood risks during significant storm events. The water surface elevations, flows, flow timing and inundation extents are also useful to floodplain administrators, emergency management operations personnel, levee operators and others in helping to protect the public. This model will help determine the impacts to the floodplain for future development and projected growth along the Brazos River.

“Without the grants, the study wouldn’t have happened,” Hannemann said.

The new model includes an evaluation of environmental resources and potential constraints, including stream impoundments, wetlands, contaminated soils, impaired water surfaces, groundwater resources, water management entities, groundwater wells, endangered species, critical habitat, cultural resources, oil and gas, prime farmland and other matters.

This study is considered a first step in developing a master plan for the lower Brazos River basin. It includes recommended steps to improve flood protection planning and response, including:

• Adopting the study results by levee improvement districts and communities.
• Updating the hydraulic model with post-Harvey topography.
• Extending the detailed hydrologic and hydraulic modeling to College Station.
• Developing a hydraulic model of the Navasota River.
• Modeling inflows/outflows and operations at the USACE reservoirs.
• Expanding the stream gauge network along the Lower Brazos River and develop additional rating curves.
• Incorporating modeling results into FEMA Flood Insurance Studies and Flood Insurance Rate Maps.
• Developing a Lower Brazos River Coalition to maintain the Lower Brazos River model and adopt similar development criteria where applicable.
• Engaging the National Weather Service to enhance flood-warning capabilities using the hydrologic and hydraulic models developed with this study.
• Engaging the USACE to further develop the hydraulic model and flood warning capabilities.
• Evaluating development impacts in the basin by considering projected growth and flow volumes.
• Identifying conservancy areas along the river.
• Updating hazard mitigation plans in the region.

To read more on the study, go to brazos.org.