How SJRA Set Lake Conroe Release Rates During May Storm

5/24/24 – The SJRA board reviewed a presentation yesterday about Lake Conroe release rates during the early May storm this year. Sadly, while some people flooded, far fewer flooded than in Harvey, despite the fact that the peak inflow to Lake Conroe was higher during this storm than it was in Harvey.

Better management of the temporary floodwater storage space between Lake Conroe’s normal lake level and flowage easement may have made the difference. But there may be opportunities to make an even bigger difference.

Dissecting a Disaster

In early May this year, the headwaters of Lake Conroe received half a year’s rainfall in a few days. It was extreme to say the least. At the peak of the inflow, dam operators were releasing almost 72,000 cubic feet per second, yet the lake level still rose.

That was the second highest release rate in the history of the San Jacinto River Authority (SJRA). Only during Hurricane Harvey did the SJRA release more – 79,000 CFS at the peak.

The peak inflow was greater during this event than during Hurricane Harvey, according to Bret Raley, the San Jacinto River Authority’s Lake Conroe Division Manager.

This time, hundreds of homes flooded downstream. Thousands more were threatened. But it wasn’t nearly the disaster that Harvey was.

Regardless people who flooded or nearly flooded want to know why the release rate was so high and whether it could have been lower. Raley’s after-action report to the SJRA board meeting yesterday provides some answers.

Presentation to SJRA Board on May 23, 2024

Here’s a link to Raley’s entire presentation to the SJRA Board.

You can watch it starting at 37 minutes and 40 seconds into this video. It lasts 15 minutes.

Or you can review the slides below and my summary.

Temporary Storage Provides Partial Buffer Against Flooding

Raley began with a recitation of SJRA’s top priorities re: operation of the Lake Conroe Dam.

He stated that “Our number one priority is … to preserve and protect the structural integrity of the dam. If we did not, the ramifications would be unimaginable.”

“I contend we are protecting public safety. How? Through a vigorous inspection and maintenance program; post-event assessments; and safe spillway operations.”

The dam, he says, provides a buffer against the full brunt of floodwaters. You’ll see later in the presentation, that at times, the inflow exceeded the outflow. Even though the Lake Conroe dam wasn’t designed for flood control, it still provides a modest buffer of temporary storage that helped reduce downstream impacts.

Only 15% of Upper Basin Protected by Dam

The upper San Jacinto River basin includes nine different watersheds that all drain to Lake Houston. Said Raley, “Ours is at the very, very top of what I sometimes refer to as a funnel. Tthe bottom of the funnel is Lake Houston, the Lake Conroe watershed is at the top.”

Raley continued, “The Lake Conroe watershed is only 450 square miles. But it’s also the only watershed in the entire 3000 square miles controlled by a gated structure. The other nine watersheds flow through seven different counties.”

When rain makes its way to Lake Houston depends on when and where the rain fell. In this event, the heaviest rain fell in the West Fork (Lake Conroe) and East Fork watersheds. It could have been much worse if heavy rains had also fallen in Lake, Spring, and Cypress Creek watersheds.

Permit Limits Storage Higher Than 201 Feet Above Sea Level

The Lake Conroe Dam’s operating permit imposes several limitations that operators must contend with.

Said Raley. “The dam was constructed pursuant to a permit from the State of Texas. It authorizes us to store up to an elevation of 201 feet above mean sea level permanently. We also have the authorization to store above 201 temporarily during storm flows.”

“The permit also requires us to pass the inflows once the lake level gets above 201. But any water released below 201 must be used against permitted water rights. So there must be a permittee with available water, and then that permittee can ask to use the water.”

“The City of Houston may have, from time to time, water that’s available, and if so, they can call on it.”

Storage, Mitigation and Pre-Release

Raley then discussed the three main things he wanted people to remember from his presentation. He summarizes them in this slide.

Raley says that water which rises above 201 feet above mean sea level (MSL) cannot be held back; the lake’s operation procedures mitigate peak downstream flows; and that if the City of Houston wants to lower Lake Conroe, the City must ask for the water out of the two thirds of the lake that it owns.

Raley never addressed why the SJRA discontinued its seasonal lowering program that was in effect for several years after Harvey. The existence of that program contradicted some of the points he made in his presentation. That leads one to wonder how firm the rules he describes really are in emergencies.

Water Must Be Released as it Starts to Rise

After introducing his key points, Raley goes into more detail on each.

Raley discussed the differences between water-supply and flood-control reservoirs. “Lake Conroe is a water supply reservoir,” he said. “The intent is to keep the reservoir as full as possible to maintain your supply. In a flood control reservoir, you want to maintain capacity. You want to maintain space that can accept inflows so that you can absorb that water before having to pass it.”

Normally, Only 18″ Between Top of Water and Top of Gates

The design of the tainter gaits on Lake Conroe is another constraint. They do not allow much room for water to rise.

The photo on the left shows a gate in the normal resting position with the lake at the normal level. Water can rise only 18 inches before overtopping the gate.

If that happens, operators lose control. So, as the water rises, they must elevate the gate to stay ahead of rising levels. Remember, storms often pack high winds that create waves. Water can be choppy, not placid as shown here.

“Our gates are not designed to be overtopped” said Raley. “So, we must raise them incrementally to maintain safe freeboard.” Freeboard is an engineering term that, in this context, means “safety margin.”

The second photo shows the downstream side of a gate. Sheet flow coming over the top could destroy this support structure says Raley. “They’re just not designed to withstand that kind of force.”

Six Feet of Buffer Space

A flowage easement lets the SJRA flood properties surrounding Lake Conroe during extreme events. “We’ve got six feet of buffer that we can use,” said Raley. “Our operating protocol helps balance lake-level rise upstream and outflows downstream.”

As the water in the lake rises, SJRA must raise the gates. Still, rising water indicates that more water is coming in than going out…at least temporarily.

In extreme events, SJRA could certainly use even more stormwater storage. (But that would require pre-releasing water. Raley describes the constraints on pre-release several slides further down.)

Areas north and east of Lake Conroe received about half a year’s rain in a few days, testing the limits of the system that Raley described. It wasn’t just the amount of rain. It was also the intensity.

The basin wide average for May 2nd, for instance, approached 7 inches.

The lake at at FM1375 received more than an inch (or close to) it in several 15-minute periods. At that same location, they also received three inches in one hour.

It’s also important to realize that SJRA came into this event coming off another heavy rain. Note how the solid black line below (at the left) was just starting to return to normal when it suddenly peaked.

Note also the steepness of the rise when the black line turns up. “In a ten hour period, we went from 8,000 to 72,000 cfs,” said Raley. “That is fast. Very fast.”

The backline represents the inflow and the blue line represents the outflow. The rapid rise caused operators to open the gates wide at first. However, they also reduced the outflow when the inflow dropped. The two curves follow each other.

Inflow Higher than Harvey

The peak inflow during that rapid rise exceeded the peak inflow rate during Harvey.

Note how peak inflows from Lake, Spring and Cypress Creeks on the west were fractions of the volume coming from Lake Conroe.

Pre-Release Capabilities Constrained

Raley finished his presentation with a discussion of pre-release as a possible mitigation measure. The headline below says it all. Gone are the days of seasonal lowering.

At 700 cfs, the permit limit, Raley says it would take weeks to lower the lake. But weather predictions are not accurate that far in advance.

And if the storm misses Lake Conroe, a big release from the lake could make flooding in Lake Houston worse – especially if the storm misses Lake Conroe to the south.

So the SJRA tosses the responsibility back to the City of Houston, They make the City call for the water (out of its share of the lake) if the City wants Conroe lowered

Conclusion and Insight

Raley’s presentation won’t make flood victims feel better. But it may help them understand how difficult managing floods can be.

The important thing to take away from this is that there’s room in Lake Conroe for temporary storage of stormwater. And releasing that water so that it reaches populated areas downstream before or after the peaks coming from other tributaries will be the key to minimizing flood damage.

For instance, releasing a smaller amount after a larger peak has already passed won’t flood any more homes.

Remember, flood peaks are not necessarily additive. You can’t automatically assume that all peaks will hit at the same time and pile on top of each other.

I’m sure more knowledge can be acquired from close examination of this event. For instance, one of the big differences between this storm and Harvey was the downstream representation on the SJRA board by Mark Micheletti.

But Micheletti is the only downstream representative on a seven-person board. lf that continues, you can bet no substantial changes will be made in the long-term operation and policies of Lake Conroe.

Another thing to look at is whether keeping Lake Conroe at slightly lower levels during wet seasons would give dam operators more temporary storage, and thus protect more homes and businesses both upstream and down.

Posted by Bob Rehak on 5/24/24

2460 Days since Hurricane Harvey