New Rainfall Data Affects Texas Dam Owners

Patrick Miles

First in a series

Newly published rainfall data could have significant effects for dam owners in Texas, including cities, counties, water districts and river authorities.

In January, the Texas Commission on Environmental Quality (TCEQ) released a new probable maximum precipitation (PMP) study for the entire state, plus adjacent regions that drain into Texas. This study updated the raw precipitation data on which dam analyses and studies are based. View the final report on the TCEQ Dam Safety page (search for “PMP”).

About the Rainfall Data

The standard practice for measuring precipitation for a watershed — how much water drains into a lake — is based on nationwide studies that date back to the 1970s and ’80s. Dam owners have used this watershed data to predict the probable maximum flood (PMF) in order to evaluate dam safety and determine design requirements.

Now, the TCEQ, which regulates dam safety in Texas, has updated that data. The data in its new PMP study is more geographically precise, accounts for weather patterns and topography specific to Texas, and incorporates the past 40 years of rainfall records. These new data more accurately predict the precipitation a watershed would actually experience.

How the New PMP Affects You

The updated data changes the amounts of rainfall that dams are required to accommodate. In many places in Texas, the PMP has been reduced compared to the previous data, while other areas have minor PMP increases.

That means that the costs of required dam upgrades may have increased or decreased. For example, a dam that was able to pass only 80 percent of the design storm using the old rainfall data may now be able to pass 100 percent of the storm, thus saving potential construction costs.

The changes will likely have greater effects on dams with small watersheds, such as small recreation lakes and stormwater ponds, than they will for large supply reservoirs.

Your Next Steps

Freese and Nichols’ team of engineers and hydrologists can help you determine how the new PMP data affects your dam and advise you on your next steps. To learn more, contact me at or 817-735-7236.

Check back next month as this series continues with more insight about the effects of the new rainfall data, or subscribe to this blog now to receive updates in your inbox.