Over the past 30 years, the average statistics show annual flood losses in the United States at about $8 billion and nearly 100 fatalities per year. Bandera County is in the Texas Hill Country region, where high-intensity rain rates and steep terrain frequently contribute to flash flooding (Caran and Baker, 1986).
Bandera County River Authority and Groundwater District (BCRAGD) was approved for and received cooperative agreement funding grants from the Texas Water Development Board (TWDB) to contract with a third-party Federal Contractor, the U.S. Geological Survey (USGS) for the development of two Flood Early Warning System (FEWS) for Bandera County, Tx. The FEWS’s were a 3-year data collection and development period each and included additional USGS stream gage installations for monitoring and the assembly of near real-time streamflow data. A FEWS initial study area was completed (May 2019) that encompassed a 23-mile reach of the Medina River from the headwater confluence of Winans Creek to English Crossing Road above Medina Lake. Similarly, a FEWS study was completed (August 2023) for a 10-mile river reach of the Sabinal River in western Bandera County from Vanderpool, TX, to Uvalde County at Utopia, TX, and included portions of the West Sabinal River above Utopia, TX.
The USGS developed a Flood Inundation Map (FIM) of both FEWS’s using a Hydrologic Engineering Center-River Analysis System (HEC-RAS) hydraulic simulation model (U.S. Army Corps of Engineers, 2016 a, b). A flood atlas, consisting of a library of flood-inundation maps for a range of streamflow conditions, was developed and included on the USGS Flood Inundation Mapping Program (FIMP) website. The Flood Inundation Maps (FIMS) depict estimates of the areal extent and depth of flooding corresponding to selected water levels (stages) based on the USGS streamflow-gaging station 08178880 Medina River at Bandera, Tx bridge Highway 173 and the USGS streamflow-gaging station 08197970 Sabinal River at Utopia, Tx bridge FM-1050.
These flood-inundation maps, in conjunction with the real-time stage data from the USGS streamflow gages are intended to help guide the public in taking individual safety precautions and intended to provide emergency management personnel with a tool to efficiently manage emergency flood operations and post flood recovery efforts.
The FEWS provides near real-time hydrologic data, available on the internet and many other social media web-based outlets. A user can view data on river stage, flow, or rainfall in real-time, directly from the streamflow-gaging station using the internet, and can quickly access the specific flood map corresponding to the present river stage conditions. The flood atlas consists of a set of digital flood-inundation extent polygons and water depth grid maps derived from the gage height (river stage value), providing the user with corresponding land-surface inundation estimates and digital map overlays. Pre-defined user-set thresholds can be established for each available hydrologic monitored condition at the streamflow-gaging station, providing the user with email or text alerts when conditions reach or exceed a user-pre-defined threshold. This allows for critical information to be provided to the user preceding significant flooding conditions.
