Tag Archive for: spillway

Aerial Photos of Lake Houston Dam Dramatize Need for More Gates

Before Harvey hit, we knew tremendous rains were coming. But we could do little to prepare Lake Houston for the onslaught. The small gates you see in the photo below release a combined 10,000 cubic feet per second (CFS). That’s nothing compared to the 150,000 from the gates at the Lake Conroe Dam.

Lake Houston’s gates release a maximum of 10,000 CFS.
The gates at Lake Conroe can release water at up too 150,000 CFS...15X faster.

We Were Sunk

When Lake Conroe had to open its gates during Harvey, we were sunk. Literally. Had we had bigger, more modern gates on Lake Houston, we might have been able to lower the lake fast enough to avoid flooding thousands of homes.

11-Foot High Wall of Water Cascaded Over Spillway

Of course, Lake Houston also has a spillway. In fact, the spillway represents the primary way to shed water from the lake. The top of that spillway is at 42.38 feet.

The primary overflow mechanism on the Lake Houston Dam: a 3,160-foot long spillway.

But Harris County Flood Control District’s (HCFCD) final report on Hurricane Harvey stated that a record pool elevation of 53.1 ft was recorded at the Lake Houston Spillway.

Thus, at the peak of Harvey, a wall of water 11 feet high was flowing over that spillway.

HCFCD estimated that’s 5 times the average flow of Niagara Falls and that the flow rate would fill NRG Stadium in 3.5 minutes.

Ten More Gates Could Have Lowered Harvey Flood by 1.9 Feet

HCFCD commissioned a study by Frees & Nichols about what effect additional gates would have in the event of another Harvey. The study found that ten more gates could have lowered the level of the flood by up to 1.9 feet (about 23 inches). That would have saved thousands of homes from flooding in the Lake Houston Area.

Interestingly, the WAY more gates would prevent flooding was not through pre-release; Harvey would have refilled the lake in a matter of hours and the storm lasted days. Rather, additional gates would have widened the spillway area so more water could move over the dam every second. Think of it in these terms: twice the width, half the height. (That’s an over-simplified example of how the principle works; ten more gates would not actually double the width.)

Is Pre-Release Practical?

The Frees & Nichols study only considered one case – Harvey. For lesser floods, the gates could help make pre-release a viable strategy for the Lake Houston Area. At least in my opinion.

Here’s how.

  • More gate capacity could help offset the volume of water released from Lake Conroe, to reduce the risk of Conroe flooding Houston again.
  • More gate capacity could release more water in less time, thus reducing uncertainty when pre-releasing before a storm. That would allow officials to delay releasing water until they were sure they needed to. And that could save precious water in the event that a storm veers off in another direction at the last minute. We may know that a storm will cross the area. But it’s much harder to tell where the heaviest rainfall will occur. For instance, during Imelda, parts of the East Fork received more than 20 inches of rain while Lake Conroe received only two.

In the last year, the City prevented homes from flooding several times by pre-lowering the lake. But the small gate capacity meant that we had to start releasing water DAYS beforehand to make an appreciable difference in the lake level. That has to be nerve wracking for Public Works.

Where the Gate Project Stands

Earlier this year, the City of Houston secured a FEMA grant to design and construct more gates for the Lake Houston Dam. The two-phase grant covers design and construction. Each phase must be completed within 18 months, though extensions are possible. Currently, we are four months into the 18-month design phase. That means we should see more gates by mid-2022.

In the meantime, the photos below give you a feeling for the immensity of the project.

The height of the trees on the San Jacinto River below the dam gives you a feeling for the height of the dam.
Looking SE. Repairs are underway to the structures below the Lake Houston dam. Note the trees caught on top of it.
Looking west over the Lake Houston Dam which dates back to 1953.

Posted by Bob Rehak on 11/14/2019

807 Days since Hurricane Harvey