Flood Warning Inundation Mapping from NOAA

Katie Hogan

With the recent storms in the southwest United States, many of the regional lakes are at or above capacity, including in the North Texas Region. Water is being released from lakes at a higher rate than normal, which may potentially cause river flooding downstream. Municipalities and communities may need to warn residents and business owners in flood-prone areas in a timely manner for public safety. The National Oceanic and Atmospheric Administration’s National Weather Service has an interactive system that can help some communities approximate the depths and extents the river may rise. 

NWS uses USGS gauge data to determine flood stage levels along major streams. Based on stage levels at that gauge, the NWS produced depth grids within the vicinity of the gauge. These depth grids show where the stream would likely overflow and how deep water could potentially be in certain areas. For example, on June 20, 2015, the Elm Fork of the Trinity River was predicted to reach a stage of 12.1 feet with the estimated release rate from Lake Lewisville. The predicted stages are shown on the Hydrograph tab of the webpage, and have changed since that day.

To estimate the extents and magnitude of the inundation, use the Inundation Mapping tab and choose the 12 ft stage (or 443.4) layer on the left. The inundation layer will then appear on the map. The user has the capability to zoom in, scroll and identify approximate depths at different locations. The map also has the ability to show the current inundation depths and compare it to the highest forecasted depths. This tool can be useful in predicting flood prone areas and preventing loss of life during high flood stages.

How can FNI help?

Freese and Nichols can assist municipalities in translating these depth grids into ArcGIS so that it is more accessible and can be used to obtain more information. For instance, with a depth grid, surface, and FEMA data, our team can run the FEMA HAZUS program to estimate the dollar amount of damage and population impacted. A number of other analyses can be run depending on available data to determine the number of parcels impacted, types of properties, etc. For further inquiries, please contact Katie Hogan, P.E., at Katie.Hogan@freese.com.

What if I don’t have a gauge in my community?

Freese and Nichols can create depth grids to determine potential flooding extents based on other information, such as FEMA data or available floodplain information the City may have.