Can Cypress Creek Stormwater Detention Offset SJRA Releases from Lake Conroe?

12/6/24 – When you look at Harris County Flood Control District (HCFCD) spending figures, it’s clear they are making a major flood-mitigation investment along Cypress Creek. At the end of the third quarter this year, HCFCD had spent more than $190 million on Cypress Creek projects since Hurricane Harvey. That was enough to rank it second among all Harris County watersheds. Only Brays Bayou received more funding.

So I asked how much the county’s investment in stormwater detention basins along Cypress Creek could help offset future releases from Lake Conroe by the SJRA during flood events.

Of course, any detention upstream helps offset flooding downstream. So the question is really “Can additional detention offset SJRA releases significantly?”

“Not a 1:1 Storage Question”

Harris County Precinct 3 Senior Project Manager Eric Heppen, P.E., PMP, explained how a professional engineer would answer my seemingly innocent question. It’s far more complex than it looks.

“Significantly” masks several layers of complexity.

For instance, how:

• Much can Lake Conroe release compared to the total detention along Cypress Creek?
• Fast is the SJRA releasing from Lake Conroe?
• Long would the SJRA release last?
• Much does the water spread out before reaching Lake Houston?

And then there are the questions of rainfall distribution and the arrival time of peaks. Is it raining as hard along Cypress Creek as it is above Lake Conroe’s dam? What if the flood peaks don’t arrive simultaneously?

For all these reasons, said Heppen, “It’s never going to be a 1:1 storage question. From a Lake-Houston-Area perspective, you can’t say that an acre foot of water held back on Cypress Creek exactly equals an acre-foot of water released from Lake Conroe.” Then he went on to answer my question as best he could given the uncertainties involved.

Comparison of Total Stormwater Detention

Heppen began by pointing out that studies show Cypress Creek needs another 25,000 acre-feet of floodwater storage. “Our initial goal is to add another 12,000-14,000 acre feet,” he said.

Lake Conroe, on the other hand, has a surface area of 21,000 acres. And the SJRA has an easement that gives it the ability to raise the lake six feet. For comparison, that’s 126,000 acre feet of additional detention, roughly 10X more than the detention volume being added to Cypress Creek initially or 5X more than the desired 25,000 acre feet.

So additional Cypress detention, when complete, would equal 10-20% of the variable storage that SJRA has to work with at Lake Conroe. That can make a dent in Lake Houston Area flooding.

Comparison of Release Rates and Durations

However, Heppen did not stop there. He also calculated how long Cypress Creek’s additional detention could offset releases from Lake Conroe. “IF the dam releases at 6,000 cubic feet per second (CFS) then that is approximately an acre foot every 7 seconds or so,” said Heppen. “That would mean SJRA was releasing approximately 500 acre feet per hour.”

“So…very, very, very simplified, if Harris County adds 10,000 acre feet along Cypress, and Lake Conroe releases 6,000 CFS, then the additional detention along Cypress Creek could take the increased release rate for nearly 20 hours,” added Heppen.

Compare that to actual release rates during a large flood. Affidavits by SJRA engineers show that releases above 70,000 CFS from Lake Conroe lasted 26 hours during Harvey.

Shaving 6,000 CFS off those releases for 20 hours could well have saved many homes and businesses on the periphery of the flood.

Conclusion: Blunted Peaks

The initial 12,000 to 14,000 acre feet of stormwater detention being added to Cypress Creek could have blunted the peak of Harvey had it been there at the time. The desired 25,000 acre feet would have made an even greater impact.

Of course, flooding is all about timing, i.e., when peaks arrive. But if the Lake Conroe and Cypress Creek peaks arrived at the US59 bridge simultaneously, the Cypress Creek storage could have reduced the combined peak significantly in my opinion.

Additional upstream detention was one of the three main goals advocated by the Lake Houston Area Task Force after Harvey. And I, for one, am all for the improvements being made to Cypress Creek stormwater-detention capacity.

Posted by Bob Rehak on 12/6/2024

2656 Days since Hurricane Harvey