The Harris County Flood Control District (HCFCD) issued its first and final report on Tropical Storm Beta today. The 24-page report found that only 25-30 homes in Harris County flooded from the storm, none in the Lake Houston Area. In most places, streams and bayous remained well within their banks. The report also says that HCFCD flood-reduction projects along Braes and Sims Bayous prevented the flooding of 930 structures. The combination of low rainfall intensities, frequent breaks in heavy rainfall, and dry grounds going into the event likely helped reduce the number of homes flooded, according to the report.
Understanding the Uniqueness of Storms
As with all such HCFCD reports, this one presents a treasure trove of statistics, graphs, charts and illustrations. Taken together, they help you understand the uniqueness of individual storms and how to compare them to each other.
Frequent Breaks in Storm Prevented More Severe Flooding
Tropical Storm Beta could have been much worse. The report says, “While Beta did produce heavy rainfall over the county, there were frequent breaks in the training rain bands.”
Most storm drains in the City of Houston are designed to handle 2″ per hour. The report continued, “The lower rainfall rates along with breaks in heavy rainfall allowed most creeks and bayous and internal drainage systems to effectively carry storm water capacities with limited flooding.”
2- to 10-Year Rains in Most Harder-Hit Areas
In most of the harder-hit areas, the storm dropped between 2- and 10-year rains. Those included Clear Creek, Brays Bayou, Keegans Bayou, Willow Waterhole, Buffalo Bayou, White Oak Bayou and Cypress Creek. One place along Sims Bayou recorded a 50-year rain. All other Harris County watersheds had generally less than a 50% (2-yr) rainfall exceedance probability, says the report. Those rainfall exceedance probabilities correspond to new Atlas-14 rates adopted after Harvey.
Intense Short-Duration Rains Cause Most Street Flooding
House flooding in Harris County correlates strongly to the intensity of short duration rainfall rates. In Tropical Storm Beta, short duration rainfall rates generally fell below intensities that lead to widespread and significant house flooding from:
- Overwhelming capacity of primary drainage systems or…
- Exceeding capacities of bayous and creeks.
The following chart compares the maximum rainfall amounts recorded in Harris County from Tropical Storm Imelda, Hurricane Harvey, Tropical Storm Allison, and Tropical Storm Beta for various time periods.
The chart above compares the maximum rainfall amounts recorded in Harris County from Tropical Storm Imelda, Hurricane Harvey, Tropical Storm Allison, and Tropical Storm Beta for various time periods.
Beta Rates Lower than Other Recent Major Storms
As the chart above shows, rainfall rates associated with Tropical Storm Beta fell significantly below other recent tropical storms that impacted Harris County. The lower rainfall rates were likely a function of a dry air mass wrapping into the circulation, and a general lack of organized and persistent thunderstorm activity.
Beta’s short duration rainfall rates remained slow enough that drainage systems could remain effective. Additionally, dry grounds leading into the event allowed the first few inches of rainfall to be absorbed, helping to reduce run-off.Jeff Lindner, Harris County Meteorologist
Dramatic NW to SE Rainfall Gradient
The maximum rainfall over 48 hours shows a dramatic gradient aligning from northwest to southeast. As you can see from the iso-map below, people in the Lake Houston Area received less than a third of the rainfall that people received on the southwest side of Houston.
Posted by Bob Rehak on 10/5/2020 based on information from Harris County Flood Control
1133 Days after Hurricane Harvey and 382 after Imelda