Where The Water Came From: May Flood Dissected

May 6, 2024, Monday 2 AM – Flooding in the Houston area during the first week of May 2024 caused quite a stir. It made national headlines most days last week. Twenty percent of the region’s annual rainfall fell in two or three days in several places upstream from Lake Houston.

Water rose quickly along the West and East Forks of the San Jacinto. Several homes flooded in Kingwood. But thousands more were on the verge of flooding when water finally started to recede yesterday.

Thankfully, rainfall Sunday in most places was on the low side of predictions. And at this hour, all gages continue to head downward with the exception of one or two far upstream to the northwest.

As last week wore on, people rode an emotional roller coaster from uncertainty to anxiety, fear, relief and, for some, anger. If lives weren’t destroyed, they were certainly upended. Many are searching for someone to blame for sleepless nights.

So let’s try to dissect what happened during this event. Then, let’s try to draw some conclusions and identify questions that remain to be answered.

How Much Rain Fell Where

Fortunately, the Harris County Flood Warning System, USGS National Water Dashboard, and SJRA have put a lot of tools at our disposal.

The first thing to check is how much rain fell where. HarrisCountyFWS.org makes that easy.

Go to the home page.
Under Site Selection/Sites by Agency, select All.
Under Rainfall Data, select Last 7 days.
Click the Watersheds option in the View tab (top left)

You should get a map that looks like this.

*From Harris County Flood Warning System, 5/5/24 at 10PM*

Note the extreme figures:

18.16″ and 17.8 inches that fell above Lake Conroe’s Dam
16.52 inches south of the Lake Conroe Dam
17″ in the Peach and Caney Creek Watersheds,
16″ in the East Fork Watershed
15.4″ in the Luce Bayou Watershed.

How Runoff Converged

To get a better feeling for how these watersheds connect, consult the map below without all the visual interruptions. I’ve circled the area that drains into Lake Houston in red.

From this you can see that the upper San Jacinto watershed draining into Lake Houston is immense.

The Texas Water Development Board provides some statistics. The size of the:

Entire river basin is 3976 square miles.
Area circled in red is 2828 square miles.
Area draining into Lake Conroe is 445 square miles.

So…

71% of the entire river basin drains through Lake Houston. Only 15.7% of the area draining into Lake Houston comes from above the Lake Conroe Dam.
Calculations based on TWDB data.

There are no other dams in the watershed. So, 84.3% of the area is beyond the control of the SJRA including everything on the East Fork.

But still, did SJRA save the water up too long and then release it in a giant pulse that swamped Humble, Kingwood and Atascocita? That’s a little harder to tell. We’ll come back to that later.

But USGS gives you a fast, simple way to learn where and when water peaked.

Peak Flows Compared from Different Tributaries

Go to the USGS National Water Dashboard. It’s a central repository for all gages that measure discharge rates.

Note: the term “discharge” applies not only to water released from a dam. Hydrologists also use the term to describe water flowing under a bridge. Discharge is measured in Cubic Feet Per Second (CFS).

Let’s look at how major streams in the watershed compared.

East Fork and Tributaries Above Lake Houston

*East Fork at FM1485 in New Caney peaked at 68,000 CFS on Friday, May 3, 2024 *6:45 PM* .*

*Luce Bayou peaked on Friday May 3 at 4PM at 12,100 CFS.*

*Caney Creek peaked at 18,900 CFS Friday May 3 at 12:15AM.*

*Peach Creek in Splendora peaked at 10,300 CFS on May 3 at 9:30 AM.*

Peak discharges are not directly additive because the peaks occurred at different times.

West Fork

I won’t post graphs for every tributary on the West Fork. But let’s look at two key points: below the Lake Conroe Dam and at US59.

That’s the only portion of the river system that SJRA controls. All other West Fork watersheds, such as Spring, Cypress, Little Cypress, Willow and Lake Creeks, enter the river below there. They all peaked at different times.

But by the time water got to US59, the combined peak was slightly higher – 82,700 CFS.

*West Fork at 59. Peak discharge of 82,700 CFS occurred on May 4 at 10AM.*

Main Stem of San Jacinto Below Lake Houston

And by the time all the water from the East and West Forks went over the Lake Houston dam, the San Jacinto downstream in Sheldon peaked at 157,000 CFS.

*San Jacinto River at Sheldon downstream from Lake Houston peaked at 157,000 CFS on Saturday, May 4 at 11:30 AM.*

Conclusions

Several things should be clear at this point.

The SJRA release from Lake Conroe wasn’t responsible for all the water flowing into Lake Houston.
SJRA doesn’t control any water on the East Fork where the worst home flooding apparently occurred. Official damage reports may take weeks.
The broken gate on the Lake Houston Dam (one of four with a combined release rate of 10,000 CFS) made no difference at the peak of the flood. By then, the 3160-foot-wide spillway was discharging a 5+ foot wall of water every second – more than 150,000 CFS, compared to 2,500.

Unanswered Questions

Having said that, I believe we definitely need to do an “after-action report” on this flood. Engineers need to answer questions, such as:

How much sooner should we have started pre-releasing water from Lake Houston to have made a difference?
Did SJRA wait too long to start releasing? Would an earlier release at a lower rate have made a difference?
Did the new SJRA strategy of throttling back releases every 2 hours help downstream? Or harm anyone upstream?
Why is it taking so long (almost 7 years) to figure out how to add more floodgates on Lake Houston?
Why don’t we have more upstream detention yet, one of the basic mitigation strategies identified after Harvey?
What was the role of Colony Ridge in the East Fork flooding? It covers an area 50% bigger than Manhattan and has virtually no detention basins holding water back from the East Fork. How did they pull that off?
When this flood recedes, will we see that sediment has once again reduced conveyance?
Why is the Lake Houston Area, which drains an area bigger than Harris County (and which has the most severe flooding in the county) getting so little help from Harris County? We’ve received only $39 million out of more than $1.9 billion spent since Harvey on flood mitigation. That’s 2%.

We need to start these conversations now.

Posted by Bob Rehak on 5/6/2024

2442 Days since Hurricane Harvey