Breaking down Houston's recent flooding events
More than a quarter of city's 311 flood calls were outside floodplainApril 27, 2016 Updated: April 27, 2016 12:26pm
People and their pets are rescued from their homes near Nanes at Baltic in Houston, Texas, Wednesday, April 20, 2016.Thousands of people have been evacuated from their homes and major highways were closed after the rains that started Sunday overwhelmed Houston's bayous. Forecasters have issued another flash flood watch for Houston through Wednesday night. (Steve Gonzales/Houston Chronicle via AP) MANDATORY CREDIT
Much of the rainfall that drove the Houston region to a standstill last week has finally receded. It was enough rain to force business owners to close, schools to cancel and service roads to shut down.
It also resulted in hundreds of high-water rescues and several fatalities.
But last week’s storm was anything but routine.
Some parts of northwest Harris County and Houston saw up to 15 inches of rain in about 24 hours, with most of it falling in a 10-hour period.
According to data obtained from the National Weather Service, Houston hasn't seen rainfall like that since Tropical Storm Allison, which was on an entirely different level. Numerous factors make one storm different from another – and data shows that it’s hard to compare.
But let’s see how abnormal Houston’s most recent flooding events were.
The following GIFs show the pattern of rainfall for each storm within a 24-hour period.
The duration of each storm is similar, with last week's storm lasting slightly longer. Last week’s rainfall lasted about 19 hours, with peak rains pounding about six hours. The 2015 Memorial Day rainfall went for approximately 13 hours, with peak rains also lasting six hours.
How unusual are those stats?
The last time Houston saw rainfall like that was in July 2012, when 10 inches of rain fell in 12 hours. Aside from hurricanes, Houston has seen that kind of rainfall since at least the 1960s.
Next, let’s compare total rainfall.
This map shows how much rain fell within the same 24-hour time frame. It's easy to see that the two storms affected different areas of the region, though there was some overlap.
The key differences between the storms were the paths and how much rain fell in 24 hours. Last week’s storm heavily impacted the northwest parts of the county.
Caption: Some Houston residents said they believe Houston flooding is getting worse, concluding that a routine rainstorm shouldn't produce tropical storm like flooding. But last weeks storm was anything but routine. Last week, some parts of northwest Harris County and Houston saw up to 14 - 15 inches of rain in about a 24-hour period, with most of the rain falling in a 10-hour period. Last year, the Memorial Day flood saw similar numbers when the storm poured 12 inches in 10 hours. According to data obtain from the National Weather Service, Houston hasn't seen rainfall like that since Tropical Storm Allison.
The standard for most Houstonians when comparing floods is Tropical Storm Allison. Allison was unique because its path didn't follow the trajectory storms usually take — up from the Gulf, moving either northwest toward Fort Worth and Dallas or curved to the northeast.
Allison moved from the Gulf but through Harris County, then circled back before heading toward the East Coast. The storm's movement resulted in days of rain — up to 30 inches in some places.
But the Memorial Day storm from 2015 and last week's pounding showed that when they do occur, Houston has a problem.
"This should not have happened like this," said Phil Bedient, a civil and environmental engineer at Rice University who heads a research collaborative focused on severe storms and their effects."There is something terribly wrong here."Photo: John D. Harden, Houston Chronicle More than a quarter of 311 calls the city received in response to flooding came from areas outside the county’s floodplain. One striking thing to note is that more than a quarter of the 311 calls received in response to flooding during both recent events came from areas outside the county's floodplain. Translator To read this article in one of Houston's most-spoken languages, click on the button below.
Source : http://www.houstonchronicle.com/local/article/How-floods-compare-7330750.php