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Rainfall Data Saves Dam Owner $100,000

Third in a series

As we've been discussing in this series, a new study from the Texas Commission on Environmental Quality (TCEQ) significantly affects many Texas dam owners. The study updates the probable maximum precipitation (PMP) data by making it more geographically precise, accounting for weather patterns specific to Texas, and incorporating the past 40 years of rainfall records.  (Download the study from the TCEQ Dam Safety page.) 

Dam owners use the watershed data to predict the probable maximum flood (PMF) in order to evaluate dam safety and determine design requirements. In many cases, the new data changes the amounts of rainfall that dams are required to accommodate.

Trends and Observations

The Freese and Nichols team has been at work analyzing dams for cities, counties, water districts and river authorities statewide. In general, we have observed that the largest PMP reductions are in smaller watersheds and for shorter critical durations. Dams in West Texas have particularly been affected.

Here is a sampling of five dams that we have analyzed, along with the PMP changes according to the updated TCEQ data:

  1. Far West Texas stormwater facility
    • Watershed: 30 acres
    • Critical PMP (1 hour): 50% reduction
  2. West Central Texas cooling water reservoir
    • Watershed: 350 square miles
    • Critical PMP (24 hours): 3% reduction
  3. Central Texas rural flood control reservoir
    • Watershed: 12 square miles
    • Critical PMP (12 hours): 3% reduction
  4. North Texas water supply reservoir
    • Watershed: 350 square miles
    • Critical PMP (24 hours): 9% increase
    • Design PMF increase: 2 feet
  5. Southeast Texas multipurpose reservoir
    • Watershed: 7,200 square miles
    • Average PMP (multiple durations): 3%-9% increase

Case Study

The first entry in that list is Dam 9 (pictured at top), owned by El Paso Water Utilities. This flood control dam lies in an urban environment at the base of a mountain. The reservoir is typically dry, but during a rain event, it quickly fills up with water rushing off the mountain. Dam 9 had been evaluated according to the previous rainfall data and was found to be inadequate to pass the required storm. So, the owner had planned to raise the dam by 2 feet.

This year, however, our team re-evaluated Dam 9 according to the new TCEQ data and found that the PMP for the watershed had decreased by 50 percent. We determined that the updated PMF would now be 1 foot below the existing dam crest and that raising the dam is no longer necessary. This re-evaluation has saved the owner approximately $100,000 in rehabilitation costs.

Your Next Steps

Freese and Nichols’ team of engineers and hydrologists can help you determine how the new PMP data affects your dam, re-evaluate your dam's specific requirements, and advise you on your options. To learn more, contact me at patrick.miles@freese.com or 817-735-7236.

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