9/29/24 – When talking about using the Lake Houston Dam to help control flooding, one often hears experts say, “That’s not a flood-control dam. That’s a water-supply dam.” So, what’s the difference? And why does it make a difference?

The purpose of the dam influences its design, operation and management. An insider familiar with the effort to add more gates to the Lake Houston Dam said last week that engineers at both Houston Public Works and the Coastal Water Authority have resisted trying to modify a water-supply dam for flood control.

This is not the only reason this project has taken so long to get off the ground. But it helps explain why new, higher capacity Lake Houston gates are just now going into final design – 2588 days after the storm that made the need abundantly clear.

To put that in perspective, 2588 days is almost twice the number of days that it took to win World War II.

Conflicting Purposes, Designs

The primary purpose of a flood-control dam is to reduce flood risk by controlling the flow of water downstream during heavy rains. They accomplish this by temporarily holding back the flow of water. They then release it later in a controlled fashion to smooth out peaks and reduce flood damage.

To mitigate flooding, flood-control dams:

• Often have lower water levels under normal conditions to accommodate sudden influxes of water.
• Have large spillways and gates to rapidly release water when needed.
• Are designed with a large storage capacity relative to the expected flood volumes.
• Are sometimes kept partially empty to ensure sufficient space for incoming floodwaters.
• Have more robust construction to withstand sudden large inflows and outflows.

On the other hand, the primary purpose of water-supply dams is to store water for human consumption (drinking, bathing, irrigation, industry, etc.).

To ensure consistent supply, a water-supply dam:

• Prioritizes a consistent water level to ensure a reliable supply of water throughout the year, even during droughts.
• Is usually kept at higher levels.
• Stores water over longer periods.
• Is managed to ensure sufficient supply throughout the year, with a focus on maximizing storage before dry seasons.

In summary, a flood-control dam is designed and managed to mitigate floods by managing excess water. However, a water-supply dam aims to store water for human use.

In practice, these extremes aren’t quite as mutually exclusive as the terms might imply.

Seasonal Management and Its Alternatives

Seasonal management strategies can help bridge the gap between the two. For instance, operators often manage both types of dams seasonally so they have more storage space during rainy seasons when flood risk spikes.

The SJRA adopted a seasonal lake-lowering strategy for several years after Harvey. But because of political pushback from Lake Conroe residents, the strategy was abandoned after several years.

SJRA now uses another hybrid strategy called “active storm management.” “Active Storm Management” seeks to manage lake levels by releasing water based on real-time weather forecasts to manage reservoir levels more effectively.

Other operators use a similar strategy known as FIRO (Forecast-Informed Reservoir Operations). Advanced forecasting models predict inflows and adjust dam releases preemptively.

Lake Houston, Lake Conroe Both Water-Supply Dams

Both Lake Conroe and Lake Houston are water-supply dams. But Lake Houston, built in the early 1950s, has a 3160-foot, fixed-height spillway with four small gates capable of releasing only 10,000 cubic feet per second (CFS) combined. Clearly, engineers prioritized consistent water level over flood mitigation when designing this dam. The small gates make it difficult and time consuming to release water before it reaches the level of the spillway.

According to the Houston Public Works Drinking Water Operations Group, Lake Houston supplies water to 2.2 million people.

Lake Conroe, built in 1973, has five large gates capable of releasing 150,000 CFS. It was designed as an alternate water supply for the City of Houston. The City financed its construction and owns two thirds of the water in the lake.

Lake Conroe has nothing comparable to the fixed-height spillway on Lake Houston.

3160-foot concrete and steel spillway on Lake Houston

During the peak of Harvey, an estimated 425,000 CFS went over the Lake Houston Spillway – 5 times the average flow of Niagara Falls.

In contrast, Lake Conroe released about 80,000 CFS from its gates during Harvey. So why the push to add more gates to Lake Houston?

Gates Key to Hybrid Strategy

Simple. Bigger gates are key to both water conservation and Active Storm Management. Right now, Lake Houston’s gates are so small that lowering the lake significantly can take days. During that time, storms can veer away.

That long lead time creates uncertainty that jeopardizes what Houston Public Works and the Coastal Water Authority see as their primary mission – providing water for 2.2 million people.

We just can’t create extra storage capacity in Lake Houston fast enough with the existing gates.

Why is Design Taking So Long?

But with more, larger gates, Lake Houston could release enough water in hours to create extra storage capacity. And operators would have confidence that water would not be wasted. So why are the gates taking so long?

Design of more gates for Lake Houston has just now started. Using Harvey as a starting point, we are now almost at twice the amount of time that it took to win World War II.

It’s hard to believe that if everyone agreed with the need for more flood gates, it would have taken this long to start design.

In that regard, I have heard of pushback from both Houston Public Works and Coastal Water Authority. The pushback had to do with the mission of the organizations: to supply water. They just didn’t want to risk wasting it in case we encountered drought.

But our two backup water sources (Lake Conroe and Lake Livingston) combined have 15 times the volume of Lake Houston. So the City probably won’t run short anytime soon.

Flood Mitigation Not In CWA Mission

The Coastal Water Authority, which is the City’s contract operator for Lake Houston, does not even mention “flooding” on its website. That’s right. CWA doesn’t mention the word in its mission statement, guiding principles, strategies, or tactics. They do, however, mention “pumps” 15 times on their Strategic-Plan page. That should give you some idea of their priorities: water supply.

Hopefully, Mayor John Whitmire’s recent appointment of former State Representative Dan Huberty to the Coastal Water Authority Board will help the CWA see Lake Houston from more than one perspective. Huberty has advocated for the gates since Harvey.

Posted by Bob Rehak on 9/29/24

2588 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.