Saturday night, May 23rd, 2015, something happened in the Texas Hill Country that has never happened,at least not in recorded history.  It was called a 500 year flood and it sent the Blanco River to a crest of 40 feet, that's a full 30 feet above flood stage. 

We've seen the pictures and videos of the aftermath, we've heard the heart-wrenching stories.  All of this, the result of a horrendous downpour that lasted just a few hours.

A big portion of the state of Texas saw record breaking rainfall amounts in May.  In our neck of the woods, 15-20 inches of rain for the month was rather commonplace.  Downpours led to flash flooding and rising rivers caused roads and even some homes to be flooded.  However, nothing here compared to the devastation that happened in the Wimberley area.

But...will it happen here someday? Could this type of 500 year flood happen on the Angelina and/or Neches Rivers?  And if it did, how widespread would the devastation be?

I talked to C.S. Ross about this.  He is a Hydrologist with the National Weather Service in Shreveport.  The first thing I wanted to know was what kind of weather scenario in East Texas could produce a biblical-type flood. Ross quickly pointed out that less than 15 years, the Pineywoods was only about 100 miles away from experiencing a 500 year flood.

Remember Tropical Storm Allison?

In June 2001, it came ashore and moved inland to about the Lufkin area before making a U-turn and then stalling for awhile on top of Southeast Texas.  Over a 6 day period, southeast Texas received around 35-40 inches of rain.  Houston was brought to a standstill for several days.

According to Ross, a stalled tropical system over East Texas could easily bring 20-25 inch rainfall totals over 2-3 days and that would be enough to trigger a record-breaking flood scenario.

Let's use the Angelina and Neches Rivers as our examples.  The flood stage of the Angelina River at Highway 59 is 10 feet.  The flood stage of the Neches River near Diboll is 12 feet.  This compares well with the Blanco River which has a flood stage of 10 feet near the site of the most devastation.

The Blanco River rose upwards of 30 feet in just a few hours on the night of May 23rd.  The quick rise of the river was due in part to the topography of that area.  The hilly areas between Blanco and Wimberley quickly channel the runoff to the Blanco River.  The geography of Deep East Texas would not be conducive to such a steep rise in such a short period of time, but a 20-30 foot rise in the rivers over a 2-3 day period could be very realistic.

According to Ross, if the Angelina River at Highway 59 rose to a height of 30 feet (20 feet above flood stage) the river would crest to an elevation of 194.72 feet above sea level.  The inflow into Sam Rayburn would be on the order of nearly 200,000 cubic feet per second, or 1,500,000 gallons per second.  The south approaches to the bridge would be flooded on both lanes and the bridge would close.  The bridge itself would not be inundated, however, if the river rose to 40 feet as did the Blanco River, all bets are off.  The bridge would be covered and many of the establishments near the Angelina River would be destroyed.  Needless to say, many homes and pastures on the flood plain of the river would suffer the same consequences of those along the Blanco. Also, property beyond the Sam Rayburn dam would be in grave danger.

In his analysis of the Neches River, Ross said that a crest of 30 feet would bring the river to an elevation of 164.5 feet above sea level.  The discharge measurement would be on the order of near 500,000 cubic feet per second.  Much of the southern half of Diboll proper would flood. Once again, Highway 59 would be closed in that area as would Highway 94 between Hudson and Apple Springs.  Both bridges would be inundated, and if the crest was up to 40 feet, the devastation to businesses and homes would be unimaginable.

As far as recorded history is concerned, the highest crest for the Neches River at Diboll has been 21 feet while the Angelina River between Lufkin and Nacogdoches has gone as high as 26.5 feet.  Both of these levels were recorded during severe flooding in May of 1884.  The rains over the past several weeks in Deep East Texas have certainly put our local rivers over flood stage, but no where near major flood levels.

The map below is from Flood Maps and represents possible areas of flooding in a 500 year flood scenario.

Floodmap.net
Floodmap.net
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