Yesterday, I published on this website the 32-page Final Hurricane Harvey Flood Report from Harris County Flood Control. Today someone asked me, “What was the biggest surprise in it?” Talk about pressure! Harris County packed lots of meat into those 32 pages! I pondered the question all morning and connected the following dots.
Five Times the Average Flow of Niagra Falls
For me, the biggest surprises were the volume of water going over the Lake Houston Dam and where it came from. At the peak of the storm, the amount of water going over the dam exceeded the volume ofNiagra Falls on an average day by 5X. The final figures actually show more water than previously thought going over the spillway: 491,800 cubic feet per second (cfs) vs. 450,000 cfs previously estimated, an increase of almost 10 percent.
More Flow From East Fork than West
When you look at where all that water came from, there was another surprise. More came from the East Fork than the West! See page 12.
In the East Fork numbers, include Peach Creek, Caney Creek and Luce Bayou; In the West Fork numbers, include Spring Creek and Cypress Creek.
One Third of West Fork Flow Came From Lake Conroe Dam Release
Roughly 240,000 cfs came down the West Fork. Seventy-nine thousand cfs came from the release at the Lake Conroe Dam, according to SJRA estimates. So ONE THIRD of the water coming down the West Fork at its peak was from the release. That’s important for the following reason.
Previously, SJRA indicated the Conroe release was approximately 15 percent of all the water going into Lake Houston. While technically true, this observation clouds the picture of what happened on the West Fork. The West Fork sustained 2.5x more damage than the East Fork and the main body of the lake COMBINED. (See Page 14). Survivor interviews suggest that much of that damage did not happen until the release from Lake Conroe!
Conclusion: Multiple Mitigation Measures Needed
That extra 79,000 cfs underscores the need to:
- Temporarily lower the level of Lake Conroe during the peak of hurricane season
- Enhance the carrying capacity and velocity of the West Fork through dredging
- Add upstream retention that helps offset Lake Conroe releases
- Add flood gates to Lake Houston.
The last item would increase the release rate of the Lake Houston dam during a major storm. The additional discharge capacity of the ten gates proposed by Mayor Turner could easily equal the 80,000 cfs discharged from Lake Conroe, eliminating a bottleneck on the river.
Of course, if we get another Harvey, many people will flood. No surprise there. But these measures should help reduce the damage, and perhaps eliminate it when we have smaller events, such as the Tax and Memorial Day Floods of 2015 and 2016.
Posted 6/5/2018 by Bob Rehak
280 Days since Hurricane Harvey