Lowering Lake Conroe: How much is enough?

The president of the Lake Conroe Association has said the group will fight a reduction of their lake to 199 mean feet above sea level (msl) during September. Remember, SJRA normally maintains Lake Conroe at 201 msl.

“The fight can be avoided, and relief can be felt in the Lake Houston area, by accepting a more reasonable approach of temporarily lowering Lake Conroe by not more than 1 foot for flood control,” said Mike Bleier, at the bottom of his post: http://www.lakeconroeassociation.com/the-lca-will-lead-the-fight-against-lower-lake-levels/

Is Reducing the Lake Level an Extra Foot Worth the Fight?

Readers have asked, rightly so, “Is this worth the fight? How much would we really gain, especially considering that evaporation already reduces the lake  to 199.4 msl on average during September?”

Depending on your perspective (and how far from the lake you live), you might say:

“That extra foot is not worth the fight, because nature will already likely give us most of it.”

Or…

“If another massive release from the dam is necessary, that extra foot, in fact, every inch, will save more homes and/or give us more time to evacuate. We need every inch and every second we can get in an emergency, especially considering that dredging has not yet started.”

The Value of an Extra Foot of Buffer

The extra foot would undoubtedly save some homes on the margin of the flood, because flood gates would not have to open as wide or as long or even at all. However, it’s impossible to precisely calculate how many homes would be saved.  That’s because of all the sediment clogging the river and other unknowable factors such as rainfall distribution and duration, and ground saturation. However, it is possible to calculate how much time we could gain to evacuate safely.

How Much Time We Would Gain Before Flood Gates Had To Open?

Let’s compute it:

1. Lake Conroe covers 21,000 acres.
2. A one-foot reduction in its level equals 21,000 acre feet.
3. But flow and discharge rates are measured in cubic feet per second (CFS).
4. So let’s figure out what that would be, given a rainfall as intense as Harvey’s.
5. Converting acre feet to cubic feet…
6. There are 43,560 cubic feet in one acre foot.
7. 21,000 x 43,560 = a total of 914,760,000 cubic feet being debated
8. The maximum inflow rate to the lake during Harvey was 130,000 cfs.
9. 914,760,000 cf / 130,000 cfs inflow = 7,037 seconds
10. 7037 seconds = 117 minutes = approximately 2 hours
11. A one foot reduction would provide an extra two-hour buffer against a rainfall as intense as Harvey’s.
12. How much buffer is there normally?
13. Gates must open when water level increases 18 inches above 201 feet.
14. Without lowering level of lake, we have a 3-hour buffer before gates must open.
15. Lowering the lake level one foot means there’s a 5-hour buffer before gates must open.
16. Lowering the lake level two feet provides a 7-hour buffer before the gates must open.

Jace Houston, general manager of the SJRA, confirmed these calculations. He also stated that they were generally consistent with the rate of rise that SJRA saw in Lake Conroe during Harvey.

Finally, Houston pointed out that few storms are as intense as Harvey, and that with smaller rainfalls, you would gain proportionally more time. For instance, half the inflow rate would double the buffer time.

Those smaller storms have a much higher probability of occurring. However, a 20-inch rain falling over a two day period would be classified as yet another 500-year storm.

In fact, as I write these words, a tropical system is brewing in the Gulf. The amount of expected rainfall associated with it is 15-20 inches at this moment. If it actually rains 20 inches, that would be exactly half the amount Harvey dumped on IAH.

Keep in mind: that storm is NOT currently predicted to affect the Houston area. Forecasters believe it will make landfall between SE Louisiana and the Florida Panhandle. I’m using this only as a hypothetical example.

So, in our hypothetical example, let’s assume the inflow rate to Lake Conroe is exactly half of Harvey’s. That would give us twice as much time – 14 hours as opposed to seven to prepare. What would you need to do during that time if you were in an area that could flood?

• Move your belongings to a higher floor if you have one?
• Gather up your valuables, albums, computers, documents, medications, insurance policies, guns, etc.?
• Refill prescriptions if you’re low?
• Pack enough food and clothes for a week?
• Gas up your vehicle with thousands of other people?
• Pack up your vehicle?
• Collect your children, elderly relatives, pets and all their medications and valuables?
• Move additional vehicles to higher ground?
• Find a friend or hotel who lives on higher ground who is willing to take in your family?
• Investigate escape routes when the power may be off and trees may be blocking roads?
• Move all that toxic stuff under your sinks and in your garage up into your attic?
• Shut off your electricity?
• Warn friends, neighbors, and relatives?
• Let them know your evacuation plans?
• Find an ATM with money left in it?
• Beat the traffic out of town?

Sounds like a pretty full day to me! Granted, you might have a little more time because it takes water a while to get from Conroe to Lake Houston. But you might also have a little less time, because sand blocking the river could cause water here to rise faster, regardless of Lake Conroe releases.

So is that extra foot worth fighting for? In my opinion, yes.

It would let Lake Conroe absorb more water, decreasing the chances that they would have to open their flood gates. And if they had to open their flood gates, it would delay the opening, giving you more time to prepare to evacuate and get to higher ground.

Remember, if the rainfall rate was heavier, you would have even less time.

I didn’t have to evacuate during Harvey. The flood stopped one house away from me. So I would like to hear from those who WERE forced to evacuate? Do you think 7-14 hours would have helped? How would your life have changed if you had had that much time to prepare?

Posted May 25, 2018 by Bob Rehak

269 Days Since Hurricane Harvey