By Itself, Lake Conroe Discharge During Harvey Among Top Ten West Fork Floods of All Time

The roughly 80,000 cubic feet per second (CFS) Lake Conroe discharged at the peak of Harvey would have created the ninth largest flood in West Fork History – all by itself. Only eight floods ever had higher “discharge” rates AND those all included floodwaters from other tributaries, such as Lake Creek, Spring Creek, Cypress Creek and numerous drainage ditches.

Discharge is the volume of water flowing past a point on a stream. Discharge from a dam would be the volume flowing past the gates.

The discharge during Harvey is important because residents fighting the seasonal lowering of Lake Conroe claim the impact of the release on Humble and Kingwood was insignificant.

Question I Asked Flood Control

So I asked Jeff Lindner, Harris County Flood Control meteorologist, this simple question. “Assuming not a drop of rain fell in any other of the West Fork tributaries, where would that 80,000 CFS have ranked among the history of West Fork floods?” I then asked him to base his analysis on the West Fork gage at US59. It combines water from ALL upstream West Fork tributaries. It’s also the closest gage in continuous operation to the highly populated Humble/Kingwood corridor. That’s where the vast majority of Lake Houston Area damage occurred during Harvey.

Only eight floods have ever exceeded 80,000 CFS at US59.

They occurred on:

• 8/28/17 (Harvey)
• 10/18/94 (Previous Record Flood)
• 11/26/40
• 5/31/29
• 5/29/16 (Memorial Day)
• 11/15/98
• 5/7/35
• 4/21/16 (Tax Day)

Note: three of these events happened before the construction of Lake Conroe in 1973. All since the advent of Lake Conroe flooded West Fork structures.

One Statistic Addresses Host of Issues

This observation (ninth largest flood by itself) addresses a host of issues raised by Lake Conroe people about lowering the lake seasonally to create a buffer against downstream flooding. They contend that:

• In the grand scheme of things, the Lake Conroe release during Harvey had an insignificant impact on flooding in the Humble/Kingwood area.
• Kingwood always floods.
• Imelda proves that Kingwood will flood even when Conroe is not releasing water.
• There’s no need to create extra storage capacity to offset future floods.

What Records Really Show

On the contrary:

• The Conroe release comprised ONE THIRD of all the water coming down the West Fork between Humble and Kingwood during Harvey.
• Since Lake Conroe was built, West Fork flooding has exceeded the release rate during Harvey only five times – and one of those was Harvey itself. So Kingwood does NOT always flood.
• During Imelda, the discharge of the West Fork, Spring Creek and Cypress Creek put together was about 35,000 CFS at US59. That’s far less than half of the Harvey release from Lake Conroe alone. All of the headline-grabbing rains during Imelda happened on the East Fork. Claiming Imelda proves Kingwood will flood regardless of what Lake Conroe does is a “red herring” – a logical fallacy intended to deceive.
• We do need extra storage capacity. Harris County Flood Control is actively looking at alternative sites to create extra detention capacity upstream on Cypress Creek and other tributaries.

Will Two Feet Matter?

Lowering Lake Conroe two feet in advance of another Harvey will probably not make much of a difference. Likewise, lowering it two feet in advance of 1-inch rain would be unnecessary. The real value happens somewhere between those extremes in a 10-, 25-, 50- or perhaps even a 100-year storm. Somewhere along that spectrum, we will get enough rain to perhaps flood homes, but we’ll also have enough extra lake capacity to avoid actually flooding them.

We just don’t know where that point is right now. Jing Chen, the engineer with Harris County Flood Control managing the San Jacinto River Basin Study says that project will not be far enough along to model those scenarios until August of this year.

However, the SJRA feels the lower lake levels did help avoid flooding between Humble and Kingwood in May of last year. I concur. It’s also possible that the two foot lowering might create enough of a buffer to have avoided flooding many homes along the West Fork during the Tax and Memorial Day floods in 2016. Perhaps it wouldn’t have saved them all. But it might have avoided flooding many at the periphery of a flood.

Increasing Upstream Detention Capacity: A Proven Flood Mitigation Strategy

According to Matt Zeve, Deputy Executive Director of Harris County Flood Control, holding water upstream is a proven flood mitigation strategy. Engineers in the Houston area created upstream detention more than 80 years ago with the Barker and Addicks reservoirs. Braes Bayou now has five different detention areas; White Oak Bayou has five with another under construction.

Such detention areas collect water during a storm and then release it slowly after a flood passes.

The extra storage capacity created by lowering Lake Conroe 1-2 feet works the same way. The idea: to reduce the amount and/or rate of water released during a flood to help avoid downstream property damage.

SJRA Mission and Lake Conroe History Includes Flood Prevention

The enabling legislation of the SJRA mentions flood prevention three times. Moreover, the area now occupied by Lake Conroe was considered for floodwater detention as early as 1957, as this Master Plan for the SJRA that year shows – i.e., on the map below. Many of these proposed lakes/reservoirs serve a dual purpose according to Zeve. They help prevent floods AND supply water.

This map puts to rest another rumor spread by the Lake Conroe Association. They claim Lake Conroe was never conceived as a flood control reservoir; it is strictly a water supply reservoir. See the discussion of flood control starting on page 16 of the 1957 Master Plan. Then on page 27, see Exhibit A.

It shows the SJRA considered building 20 dams that year.

So far it has built only one: Lake Conroe.

Therefore, Lake Conroe is currently the only way we have to mitigate flooding until we put other mitigation measures in place.

Posted by Bob Rehak on 1/23/2020

877 Days since Hurricane Harvey

Note: A reader’s question on Facebook prompted this post. But I can’t find his name now. My apologies. No offense intended.