Perhaps no flood-mitigation project has generated more interest in the Lake Houston Area recently than the addition of more flood gates to the Lake Houston Dam. In recent months, as engineers worked on the project’s benefit/cost ratio, information about the project became hard to find. That fueled rumors.
But Tuesday night, at the Kingwood Community Center, Mayor Pro Tem Dave Martin put many of those rumors to rest. He denied the project was on hold, reaffirmed the City’s commitment to the project, and outlined the three main issues that engineers are currently grappling with.
Issues Still Being Evaluated
The main issues include:
- Safety concerns about cutting into the concrete of a dam that’s almost 70 years old.
- Getting the benefit/cost ratio up.
- Finding a suitable alternative that significantly reduces flood risk within the budget.
Martin elaborated on each. Regarding:
Safety concerns – He described the risks of cutting into it to install crest gates. Among them, he said engineers worried about structural stability of the dam after construction. Accordingly, they are recommending significant reinforcement of the concrete. He also hinted that contractors might not bid on the project for fear of the potential liability.
Benefit/Cost Ratio – He said that the higher-than-expects costs on of some alternatives drove the BCR down, and that that was driving the exploration of additional alternatives. He did say, however, that FEMA allows including “social benefits” when the BCR is between .75 and 1.0. The inclusion of social benefits still must yield a BCR of 1.0 or greater. On a separate note, a federal employee told me that the Biden administration may change this policy. So significant uncertainty still exists re: calculation of the BCR.
Budget – He implied that some alternatives under consideration became non-starters because of high costs and inflation.
Alternatives Still Under Consideration
So, the search for a suitable alternative that meets all objectives continues. Among the options still in the running, Martin mentioned crest gates on the west side of the dam and adding a tainter gate to the earthen, eastern portion.
Martin shared a timetable that shows construction beginning in November. However, FEMA must approve the benefit/cost ratio before they release construction funds.
Background of Project
At the peak of Harvey, 425,000 cubic feet per second (CFS) went over the dam’s spillway. That’s five times the average flow of Niagra Falls. Floodwater backed up so far that it flooded thousands of homes and businesses. It also killed 13 people in the Lake Houston Area, 12 of them in one retirement center.
The release of 80,000 CFS from Lake Conroe contributed almost a fifth of the water going over the spillway. Lake Conroe gates can release 150,000 CFS while Lake Houston’s can release only 10,000 CFS. The disparity in release capacity caused many to ask whether more gates on Lake Houston could reduce flooding.
Martin pointed out that when water gets high enough in Lake Houston, it can escape over a 2,000-foot-wide spillway. However, more gates could play a role in a pre-release strategy.
Pre-releasing water from Lake Houston in advance of major storms, as the City does now, creates extra capacity in the lake so that it can absorb more water without flooding homes and businesses. This strategy (coupled with the seasonal lowering of Lake Conroe) has worked effectively since Harvey and prevented flooding on more than one occasion.
Time Vs. Release Capacity Vs. Water Preservation
However, right now, it takes so long to release water from Lake Houston that storms can sometimes veer away and miss us after the lowering starts. Thus, water could be wasted. But bigger gates would let dam operators release the same volume of water in less time, so operators would not have to start releasing water so far in advance. In other words they would have a higher degree of confidence that the the storm would not veer away and that release was worthwhile.
Martin reassured people that:
- Smaller (i.e., less costly) floodgates can lower Lake Houston sufficiently if given enough time
- The lake usually refills quickly
- Even if it doesn’t, the City can always call for the release of water from Lake Conroe.
We should know within a few months whether Black & Veatch, the engineering company leading the project, has succeeded in designing additional gates within the budget that meet all other objectives.
Staying on the schedule above will be ambitious. FEMA must approve the BCR before releasing money for construction.
Posted by Bob Rehak on April 21, 2022
1696 Days since Hurricane Harvey