New data shows the 100-year rainfall for this area has increased 4-5 inches since the NOAA study in 1961 or 2-3 inches since the USGS study in 2004. This is why flood mitigation and reducing sedimentation are so important. Basically, what we used to think of as a 100-year storm is now almost a 25-year storm.

NOAA Atlas 14, Volume 11: The New Go-By for Everything Related to Rainfall

Today, the Hydrometeorological Design Studies Center of the NOAA’s Office of Water Prediction released updated precipitation frequency estimates for Texas.

They are published as  NOAA Atlas 14 Volume 11: Precipitation-Frequency Atlas of the United States, Texas.

The new precipitation frequency estimates supersede the NOAA estimates published in 1961, 1964, and 1977, and the USGS estimates published in 2004. The new NOAA estimates include data from Harvey and all of the huge storms we have had since 1994 including Tropical Storm Allison, the Tax Day Flood and the Memorial Day Flood. Here’s what the 100-year/24-hour rainfall map looks like. Note that the Houston to Beaumont area is in the bulls-eye.

For a full scale map like the one above, download this PDF: tx100y24h rainfall intensity pdf.

To find precise figures for your location, go to the Precipitation Frequency Data Server – PFDS.

The data varies by location, so…

Next, review the rainfall table associated with that gage. Clicking on the other tabs or “print” brings up additional information.

Comparison to Previous Studies

From this data, we can see that – for the gage at the San Jacinto and US59 – the new, official 100-year rainfall is 17.3 inches in a 24-hour period.

Compare a previous dataset published. Look on page 58 for the 100-year/24 hour data from 1961. Twelve inches in 24-hours represented the old 100-year rainfall for our area for decades.

USGS also published a precipitation frequency study in 2004.  See the USGS Rainfall Maxima Guide for Texas (Warning: 40 meg PDF). I believe it became the basis for the current flood-plain maps redrawn after Tropical Storm Allison that were released in 2007. It shows the 24-hour, 100-year rainfall to be about 13 inches.

How New Data Will Be Used

What does it mean that the 100-year rainfall has increased 4-5 inches?

First and foremost, it means that all of the floodplain maps will be revised. One expert I talked to suspected that the new 100-year floodplain could be close to where the 500-year flood plain is now. However, that is far from certain and not official.

Floodplain Maps

The flood plain maps have not yet been redrawn, as Matt Zeve, Harris County Flood Control Director of Operations, discussed at the September meet of the Lake Houston Area Grass Roots Flood Prevention Initiative. The next step is for the County to process the new rainfall data in a new 2-D model that the Flood Control District has developed with new high-resolution LIDAR data. Contour internals in the new models will shrink from feet to inches. The LIDAR data also reflects new conditions in the watershed (developments, road expansions, siltation in ditches, etc.), so predictions should become much more accurate.

Insurance

Based on the new rainfall data, flood insurance rates could also change.

Construction

Finally, the new data will become crucial in city planning, construction and permitting. The City is already demanding that new construction be raised to two feet above the 500-year flood plain. Perhaps Mayor Turner had a hint of what the new numbers would show when he suggested the new construction standards.

Infrastructure

The larger rainfall totals also mean that cities must use larger storm drains and sewers in new developments. Everything will change.

For more information about the new data, review this quarterly newsletter from NOAA.

Posted on September 27, 2018 by Bob Rehak

395 Days since Hurricane Harvey