Flood Risk Studies

The San Antonio River Authority (River Authority) performs engineering studies of rivers and creeks to estimate flood risk. These studies are undertaken as part of the River Authority’s role as a Cooperating Technical Partner with the Federal Emergency Management Agency (FEMA) and in coordination with Bexar Regional Watershed Management and the floodplain administrators of the communities within Bexar, Wilson, Karnes, and Goliad counties and the San Antonio River Basin.

The engineering studies consist of three main steps:

1. Estimating how much water runs off when it rains and which direction it flows.

To do this, we rely on several datasets:

Rainfall estimates from the National Weather Service (Atlas 14, Volume 11)
Topographic data from the Texas Water Development Board (TNRIS)
Soil data from the Natural Resources Conservation Service (SSURGO)
Stream flow gauge data from the United States Geological Survey (Current Conditions) and from Bexar County and City of San Antonio
Land use and land cover data developed from aerial photography and land use information from the City of San Antonio and Bexar County
Dam volume and discharge data from owner records

The analysis of these data sets produces an estimate of flow at various points along each stream.

Even with this high quality data and hydrologic modeling, there is uncertainty in the estimates due in large part to not being able to predict where and how the rain will fall. The image on the left below shows the statistical average rainfall used to map floodplains while the image on the right shows the Memorial Day 2015 event. The colors indicate a difference in pattern. Every storm is unique, so the methods used to develop floodplain maps attempt to account for the probability of storms over time rather than predicting “the big one”.

2. Estimating how deep it gets

The depth of flow is determined by running the results of Step 1 through hydraulic models representing the shape of the streams. These models are developed using a combination of the topographic data from Step 1 and field surveys. A field crew measures the elevation and dimensions of bridges and culverts, and the data is then used in the computer model to determine how those structures impact or are impacted by the flow through the streams.

The hydraulic models produce estimates of depth and velocity of flood water in each stream.

3. Drawing the estimated level of flooding on a map

Floodplain maps are developed by intersecting the depth results from Step 2 and the topographic data from Step 1. This provides a representation of the extents of flooding based on the probability estimates of rainfall. The result is the Digital Flood Insurance Rate Map.

In addition to floodplain maps, the results from Step 2 are used to develop other mapping data to show flood risk. Some of these are available on the Risk MAP Viewer. These are also used by the River Authority and community floodplain administrators to identify areas of high flood risk, and the identified areas become the focus of mitigation planning.

Mapping Data	Description
Water surface elevation grids	Representations of the modeled water surface elevation for various flood frequencies, primarily the 10%, 4%, 2%, 1% and 0.2% annual chance flood events.
Flood depth grids	Representations of the flood water depths for various flood frequencies; primarily the 10%, 4%, 2%, 1% and 0.2% annual chance flood events.
Percent annual chance grids	Representations of the chance of flooding in any given year.
Percent 30-year chance grids	Representation of the percent chance of flooding at least one time during a 30-year period.

San Antonio River Authority Risk MAP Viewer

The engineering studies are organized by watershed. The map below shows which watersheds are currently being studied and their approximate study schedules.

The data and models developed during these studies are available through the Digital Data and Model Repository (D2MR) website.

Revisions to the effective floodplain maps may be requested through FEMA’s Letter of Map Revision (LOMR) process. The River Authority reviews revision requests in partnership with FEMA for studies within Bexar, Wilson, Karnes, and Goliad counties.