A study commissioned by the Brazos River Authority (BRA) to better understand the effects of large rain events and provide comprehensive regional drainage criteria for the lower Brazos River watershed was recently completed. The study began in 2015 and was funded by a Texas Water Development Board (TWDB) Flood Protection Planning Grant.

The results of this model will not only provide local entities with updated insight into the Brazos River’s 100 & 500-year flood elevations and flood duration but will also help the BRA regulate releases from the eight dams along the Brazos River within the studied watershed. The floodplain elevations and dam release rates will help guide local leaders as they create policy for future land use and continue to refine emergency response systems.

Study Area

The entire Brazos River watershed within the studied area covers 40,000 square miles across 18 counties. The upper 30,000 square miles within the watershed are controlled by a system of eight dams built along the Brazos. While the lower 10,000 square miles of the watershed is not controlled by any structures and are therefore considered uncontrolled. Portions of the lower watershed were previously modeled, but a comprehensive model of the entire watershed has never been completed. The five counties affected by this study include Washington, Waller, Austin, Fort Bend, and Brazoria.

Initiation of the study is timely and significant because this five-county region has been one of the fastest-growing areas in the country for several years, and this region of the state has also experienced substantial flooding along the Brazos River over the last three years. (Memorial Day 2015, Tax Day 2016, Hurricane Harvey 2017).

What Next?

Per the requirements of the TWDB grant, the results of the model have been made available to all stakeholders as well as the public. A final report with concluding data and recommendations is due out in September of this year. Each entity affected by the results of the study will have the opportunity to accept the results or respond back to the BRA, challenging the findings.

There are two separate potential resulting impacts of the study. First, under current criteria, it is assumed that because the floodplain is so large, the displacement of water is insignificant and would not create negative impacts. Using the completed model, the loss of storage volume within the floodplain due to the creation of levee districts will now be quantifiable. Secondly, the 100-yr water-surface-elevation (WSEL) for the Brazos River could rise in certain areas along the lower Brazos River watershed. If a higher 100-yr WSEL is adopted by local regulatory authorities, impacts on local citizens and stakeholders may be significant. See the figure below.

Below is a summary of additional potential effects:

• Expanded Floodplain Boundary: Increases to the 100-yr WSEL will widen the floodplain boundary, effectively adding new properties within the lower Brazos River watershed to the      100-yr flood zone.
• Flood Insurance: FEMA insurance rates may rise significantly for those who are now in the newly mapped 100-yr floodplain.
• Levee Improvement Districts: Levees may not be high enough to meet FEMA criteria.
• Emergency Management Protocols: Warning systems and service area boundaries will need to be re-aligned to account for the increase in the mapped floodplain area.
• Infrastructure Improvements: Significant improvements will be needed to protect flood-prone areas.
• Levee District Creation and Floodplain Reclamation: Regulations regarding developments with floodplain reclamation in the Brazos River Watershed and the placement of levees will significantly need to change to account for the preservation of floodplain storage.

As responses to the model and study are made by the stakeholders involved, J|C will follow along and provide updates as necessary. If you would like to receive future updates please follow us on LinkedIn, or contact us if you would like to speak to an engineer about these regulations or how they may affect a specific project.