Wednesday, June 1, 2016

Torrents in Texas

If you watch the CoCoRaHs national precipitation map each day no doubt you have noticed that most of the highest amounts in the past two months - anywhere from 8-16 inches - have occurred in Texas. One of the locations impacted by the heavy rain (by no means the only one) is Houston. The Houston area has received an astounding amount of rain in the past 60 days, largely from two big events. One of these occurred in April, and the other within the past week. Now, any location receiving 12 to 16 inches of rain in 8, 12, or even 24 hours would experience some type of flooding. What is it about Houston that seems to make it susceptible to severe flooding?

One reason is geography. The landscape around Houston is criss-crossed by web of bayous. Bayous are slow-moving rivers or streams generally in flat, low-lying areas. There are often associated with marshes or wetlands and are a tributary to a larger body of water. Houston was founded the Buffalo Bayou, 52-mile long waterway that winds through Harris County, in 1836. The waterways are an integral part of the landscape supporting wildlife, recreational activities, and providing drainage.

Map of the bayous and drainage areas in Harris County, TX.
Credit: Harris County Flood Protection District.


Another reason - the Houston area is becoming a bowl. In the past 100 years the withdrawal of groundwater, oil, and gas has caused the land to sink. The situation is particularly critical in northwest Houston, where wells are tapping the groundwater to supply new residential areas. In the area northeast of two major reservoirs (Addicks and Barker) the land has dropped seven feet since 1906 and continues to do so.

Population increase, population density, and its attendant urbanization is another reason contributing to flood vulnerability. The natural landscape was once dominated by marshes, prairies, and wetlands which helped buffer floods. Much of that has now been replaced by impervious surfaces - buildings, roads, and other paved surfaces - that increase runoff.

The heaviest rain amounts during the heavy rain event of April 17-18 in the Houston area were found north and northwest of the city. The storms were slow-moving and often training over the same areas. The rainfall amounts were bad enough, but the rainfall rates were astounding. A gauge in Pattison, TX in Waller County which lies at the head of Cypress Creek measured 23.50 inches of rain in only 14.5 hours.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of April 17 (left) and April 18, 2016 (right).

Rainfall amounts (light blue) and flooding in Harris County for the April 17-19, 2016 storm.


The rain gauge at TX-MG-49 Magnolia 10.6 ENE
with 8.47 inches of rain on May 27.
Final total was 11.35". (via Facebook)
This was a particularly challenging storm for CoCoRaHS observers for several reasons. The heavy rainfall occurred overnight, and many were sleeping when their gauges started to overflow resulting in a loss of an actual measurement. Those who attempted to empty the gauge before it filled to capacity were thwarted by nearly continuous lightning which made it extremely dangerous to venture outside. It's not really possible to even estimate the amount of rain once the gauge overflows. The gauge holds from 11.3 to 12 inches (depending on if the tube and funnel are in place), so the most we know from the overflow gauges is that at least 11 inches of rain fell.










The most recent heavy rain event on May 26-27 repeated the April scenario from 6 weeks ago, with the heaviest rain north and northwest of Houston.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of May 27 (left) and May 28, 2016 (right).

Flood warnings are still in effect from this rain. CoCoRaHS observers in Waller County had the highest two-day rainfall amounts with amounts from 16 to 22 inches. The highest was reported at TX-WA-17 Brenham 9.9 N with a two-day total of 22.41 inches, and TX-WA-24 Brenham 0.7 E with 20.97 inches.

CoCoRaHS 48-hour precipitation amounts for the period ending the morning of May 28, 2016.
Only amounts in excess of 12 inches are shown.


The automated station (AWOS) at Brenham recorded 16.62 inches of rain on May 26, making it the wettest day in the city's history by more than six inches. The Brazos River in Texas surged to a record high 54.76 feet early this evening at Richmond, TX, northwest of Houston. The river is at  4.46 feet above the previous record (50.3 feet) set on October 21, 1994.

Hydrograph for the Brazos River as of 9:20 p.m. CDT June 1. The river reached a record 54.76 feet about 6:15 p.m.

The flooding from this latest storm has resulted in six fatalities and damage to hundreds of homes and buildings. Seven people lost their lives in the April flood. Most of these were in vehicles.

Not a river, but a flooded road in Fort Bend County (west of Harris County).
Note the swimming alligator in the lower right hand corner.
Credit: Fort Bend Sheriff Office via Twitter.


There's no rest for the weary, either. Thunderstorms have been frequent the past several days, and thunderstorms rolled through Harris County and surrounding areas today. The outlook for the next five days paints a wet picture for the Houston area.

Quantitative Precipitation Forecast (QPF) for the 5-day period ending 7:00 p.m. CDT Monday, June 6.


Flooding is the most frequent and dangerous natural hazard in the Houston area, but they are prepared to deal with it. The Harris County Flood Control District continuously monitors stream flow and precipitation to asses the flood potential. It also controls the releases from the reservoirs that hold the runoff from storms, maintains the infrastructure, and develops and implements flood damage reduction plans.You can learn more about the flooding issues in the Houston area on their web site  and view the real-time Harris County Flood Warning System at http://www.harriscountyfws.org/.

Screen capture of the Harris County Flood Warning System web page.

1 comment:

  1. Always interesting information Steve. Thank you.

    ReplyDelete