Evaluation of Subsidence Impacts in the Spring Creek Watershed
The purpose of this study is to evaluate the impact of subsidence on flood risk within the Spring Creek Watershed, an area along the northwestern border of Harris County. Spring Creek has a natural channel with limited conveyance and a larger natural floodplain than other streams in Harris County. The Spring Creek Watershed spans over 392 square miles, extending into portions of Harris, Grimes, Montgomery, and Waller counties in southeastern Texas. Portions of major transportation routes are located within the watershed, including Interstate 45 and Texas State Highway 249, which serve as important evacuation routes for the Houston Region. This watershed also encompasses portions of the recharge zone for the Chicot and Evangeline aquifer, a primary source of groundwater for Texas' Gulf Coast Region.
Historically, this area has been largely undeveloped; however, development has increased in recent decades. The estimated population growth in this area from 2020 to 2070 is 47%, with projected subsidence of over three feet near The Woodlands. This study provides information on the increase in flood risk attributable to projected subsidence and future development across the watershed up to the year 2070. Additionally, the findings from this study provide an important perspective on the impacts of subsidence related to inland flooding and the economic consequences.
*The study area for this project encompassed the entire Spring Creek watershed for hydrology and the mainstem of Spring Creek, from the Waller County line to its confluence with the West Fork of the San Jacinto River, for hydraulics and changes in flood risk.
About the Study
This study developed a method to incorporate subsidence into the existing terrain for two different conditions in 2070, utilizing a baseline model to serve as a benchmark for comparison:
The Hydrologic and Hydraulic (H&H) model HEC-RAS was used to quantify the impacts of subsidence on pluvial flood risk and economic consequences for each condition. The datasets used to build the baseline H&H model include topography, rainfall, streamflow, land use, and soil characteristics, which serve as the building blocks for simulating the movement and distribution of water within the watershed. A comparison of the H&H modeling results between the baseline and subsidence with future development condition was performed to evaluate the impact of subsidence on flood risk, including flood depth, floodplain acreage, water level, and velocity.
Deliverables
Conclusion
This study identified the following flood risks and economic impacts from projected subsidence with future development in the Spring Creek Watershed for a 100-year flood event:
- Expansion of the 100-year floodplain by over 1,700 acres
- Average increase in flood depth by 1 foot
- Additional 3,700 flooded structures, with the majority being residential
- An increase of flooded roadways by 40 miles
- Damages up to $560 million per 100-year flood event, with approximately $446 million attributed to direct loss (e.g., structure and contents)
- Potential for $71 million in losses from faulting within a 200-ft hazard band
Recommendations
Subsidence poses significant flood and economic risks in the Spring Creek watershed. To address these challenges, the following actions were recommended:
- Infrastructure and environment – Evaluate subsidiary impacts of subsidence, including bank erosion, land destabilization, agricultural and environmental damages, and datum shifts, to provide a more comprehensive economic analysis of subsidence impacts.
- Planning and policy – Implement comprehensive strategies to address factors driving subsidence, including minimizing groundwater extraction.
- Monitoring – Include regular measurements of the land surface across the watershed.
- Community awareness – Promote the understanding of subsidence impacts and varying community vulnerabilities.