Final Report on Imelda Says 62% of Flooding Outside of 100-Year Floodplain

Harris County Flood Control District released its final report on Tropical Storm Imelda this morning. It’s a work of incredible scholarship. If you really want to know what made this storm different, this is a must read.

While Imelda was the fourth wettest storm in Texas history when measured by total rainfall, Imelda produced incredible short duration rainfall rates that exceeded Harvey in the 5-, 15-, and 30-minute time periods. And that is the key to understanding why more homes flooded in the 500-year floodplain than the 100-year floodplain.

Few Structures In Lake Houston Area Flooded from River

Says Jeff Lindner, Harris County meteorologist, “Much of the structure flooding that occurred in the Kingwood area was not a result of flooding from the river, but instead flooding of local drainage systems that were overwhelmed from the intense short duration rainfall rates. Harvey remains the flood of record along the West Fork of the San Jacinto River.”

Almost Two-Thirds of Flooded Homes Outside 100-year Floodplain

Similar to other recent flooding events in Harris County a large number of flooded homes were located outside the 1% (100-yr) floodplain. Of the 3,990 homes flooded from Imelda, 2,479 (62%) were outside the 1% (100-yr) floodplain. 1,511 (38%) of the flooded homes were located inside the 1% (100-yr) floodplain.

“While house flooding did occur from creeks and bayous overflowing their banks, a large majority of the house flooding was a result of intense short duration rainfall rates overwhelming the internal drainage capacities and this is shown by the large number of homes that flooded outside the 1% (100-yr) floodplain,” says Lindner.

Rainfall Totals Throughout San Jacinto Watershed

See the actual rainfall measurements for gages throughout the San Jacinto Watershed below. All characterizations of floods (i.e., 100-year) are based on NOAA’s new Atlas 14 data.

*Flooding of Local Tributaries: Green = 10-50 yr flood, Yellow = 50-100 yr, Red = 100-500 yr*, *based on high water marks.*

*1-Hour Peaks throughout county. Note blue bullseye over US59 and Beltway 8. That’s a 500-year intensity rate.*

*48 Hour Peaks throughout county. Notice extreme gradient from east to west. 28″ in northeast part of county and less than 2″ in the west.*

*Regional map. Parts of Liberty, Chambers and Jefferson Counties received a 1000-year rain. As bad as Imelda was, Harris County escaped the worst part of the storm.*

Details Provide Clues to Reasons for Flooding

The final Imelda report has thousands of statistics for just about every location in the county and every time period between 5 minutes and 48 hours. Scanning it may give you a feeling for why you flooded or why you did not.

Increasing Frequency of Extreme Storms

The report also contains a discussion the number of extreme storms that have hit this area recently. Some key stats:

Three of five wettest tropical cyclones in Texas history occurred in the last 20 years (Imelda, Harvey, Allison)
Among Texas storms, four of the top five occurred in southeast Texas (Imelda, Harvey, Allison, and Claudette).
Imelda is also the 5th wettest tropical cyclone ever in the 48 contiguous United States.
Six of the wettest tropical cyclones in US history have occurred in Texas.
Six of the 10 wettest occurred in the last 22 years.
Three of the 10 wettest occurred in the last three years (Imelda, Florence, Harvey).

The table below shows the top ten wettest tropical cyclones by location and year in the 48 contiguous states.

*Source: Harris County Flood Control final report on Imelda.*

For the full report, click here.

Implications of Report Speak to Need for Change

Yesterday, I posted about an engineering company that made conservative and questionable assumptions about rainfall totals and more in their drainage analysis for Woodridge Village.

When far more homes flood in the 500 year flood plain than the 100, it’s clear that our infrastructure is not equipped to handle the kind of storms we’re now getting.

We can no longer allow developers and engineering companies to bet on the best case scenarios when the worst case scenarios are becoming the norm. We must start building infrastructure to handle bigger storms. The old norms are failing us.

For starters, we need Montgomery County to:

Stop replacing wetlands with concrete.
Close that “beat the peak” loophole in its flood plain regulations.
Map the entire county instead of leaving gaps in flood maps that developers can exploit.
If a developer is not already in development, make them use NOAA’s new Atlas 14 rainfall data as the basis for flood calculations.
Remove the exception that if their plan was in development or approved before Jan. 1, 2019, they do not have to abide by Atlas 14.
We now have better information. Make them use it. Eliminate the “Beat the Clock” exception.
Rethink stormwater and sewer drain design. Be more aggressive to reflect current realities. As a region, we need to find smarter strategies.
Update the data behind its floodplain maps.
Adopt best practices for developers that require detention ponds to be built as each portion of a site is cleared, rather than waiting until the entire site is cleared.
Discourage building in 100-year floodplains by limiting the percentage of impervious cover and homes per acre, while increasing the amount of detention required and elevation beneath the homes.
Force developers who flood neighbors to compensate them for damages.
Enforce all of the above.
Enforce the regulations you already have on the books. In this regard, see page 28 of your own rules, regulations and requirements for new developments. Ask yourself if Perry Homes is being a good corporate citizen and complying with the sediment control regulations.
Until MoCo Commissioners take these steps, they will continue to knowingly inflict pain, suffering and damage on downstream neighbors and their own residents.

Posted by Bob Rehak on 11/11/2019

804 Days since Hurricane Harvey and 55 since Imelda

The conclusions expressed in this post represent my opinions on matters of public policy and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the great State of Texas.