Lake Houston Dam

Mid-February Update on Lake Houston Gate Project

The preliminary engineering phase of the project to add more gates to Lake Houston is rapidly drawing to a close. Engineers should complete their final (preliminary) report on the Spillway Improvement project next month.

At the Coastal Water Authority board meeting on 2/10/21, Chris Mueller, PhD, PE, gave an update on the preliminary engineering report being prepared by Black & Veatch (BV) on the Lake Houston Spillway Improvement Project. Because the report is still in draft form, I won’t reproduce every slide here. Nor will I list exact flood-reduction benefits, which are still being refined. I will simply summarize some of the key points which seem unlikely to change at this point.

The final preliminary engineering report should be released next month. Before then, BV and subcontractors need to finish environmental surveys and their drainage analysis. They also need to put a sharper pencil to the Benefit/Cost Analysis, and prepare scope, schedule, and budget proposals for detailed engineering. The board will consider authorizing the next steps in its March 10 meeting.

Lake Houston Spillway photographed 1/1/2021.

Top-Ranked Alternatives to Date

BV’s top ranked alternatives to date include adding (in order of preference):

  1. 1000 feet of crest gates on the Spillway (see foreground in picture above).
  2. 1200 feet of crest gates on the Spillway.
  3. 6 new tainter gates to the existing gate structure.

In evaluating alternatives, BV had to balance competing priorities. They go beyond just reducing upstream flood-reduction benefits and downstream impacts. They include all the factors shown below.

From BV presentation to CWA on 2/10/2021

Metrics that engineers and partners will use to evaluate alternatives and success include:

From 2/10/2021 presentation to CWA Board. U/S and D/S stand for upstream and downstream.

According to the BV presentation, some positive benefit revealed by the work to date include:

Benefits Revealed To Date

  • Engineers found that the peak of floods from Buffalo Bayou would arrive downstream much faster than water from the Upper San Jacinto Watershed above the Lake Houston Spillway. This means the peaks would not arrive on top of each other. That’s good. It helps mitigate downstream impacts.
  • The recommended alternative would reveal a positive benefit/cost ratio and not adversely affect downstream structures.
  • The recommended alternative could turn a 100-year flood into a 25-year flood.

Engineers based the flood reduction benefits on a 100-year flood, not another Harvey.

How Final Decision Be Made

The engineers and partners will consider both cost and non-cost factors in making final recommendations.

Type of Crest Gate Recommended

BV is recommending an Obermeyer gate, though they have not detailed how it would be installed. Water Power and Dam Construction Magazine says, “The Obermeyer spillway gate system is most simply described as a row of steel gate panels supported on their downstream side by inflatable air bladders.” The air level in the bladders control water levels by raising or lowering the bottom-hinged steel panels.

This one-minute video shows how Obermeyer gates work.

Gate Project in Context

After Harvey, Lake Houston Area leaders recommended three main types of projects to reduce flooding. They included:

  • More upstream detention to reduce the amount of water coming downstream during floods.
  • Sediment removal/dredging to restore throughput and conveyance.
  • More gates on the dam to speed up output.

For the October 2020 update on this project, see this post.

Posted by Bob Rehak on 2/12/2021

1263 Days since Hurricane Harvey