06//01/18 – In its April board meeting, the San Jacinto River Authority voted to seasonally and temporarily lower the level of Lake Conroe to reduce flood risk for downstream residents.

Several learned and respected people suggested an alternative strategy in response to pushback from the Lake Conroe Association. Their proposal goes something like this. “As soon as a named storm enters the Gulf, begin lowering the level of the lake.”

On the surface, it sounds like an appealing and fair compromise. Wait until a real threat appears rather than react pre-emptively to an anticipated threat. Both strategies seemingly attain the same objectives.

However, like all difficult decisions, this one contains hidden layers of complexity. Let me attempt to explain the relative pros and cons of each strategy so that you can understand the tradeoffs.

Temporary, Seasonal Lowering

Pros

• Slow, controlled release of one or two inches per day
• Rate is certain not to flood downstream residents
• Extra capacity gained in lake reduces flood risk during storm
• Avoids liability for SJRA
• Predictability – Lake Conroe residents know what to expect and can plan around it
• No coordination issues with Lake Houston, which has a different kind of dam

Cons

• May waste water if no storm appears
• Will inconvenience some boaters with shallow docks on Lake Conroe for six weeks or possibly more  during hurricane season

Wait and See Before Lowering

Pros

• Less risk of wasting water
• Does not inconvenience boaters or other Lake Conroe residents without imminent cause

Cons

• Flood threat may not come from the Gulf or be a named storm
• Difficult to release enough water at a SLOW rate to make a difference during a major storm
• Tropical storms can blow up near coast or traverse Gulf in a couple days
• Less reaction time would require faster release rate
• Faster release rate might flood downstream residents living close to river
• Lake Houston can only discharge 10,000 cubic feet per second through gates.
• Lowering Lake Conroe two feet at that rate would require approximately 2.5 days.
• Could erase excess capacity in downstream watershed, which would most likely fill up first if storm approaches from south
• Weather forecasts cannot accurately predict how much rain will fall or where it will fall within a watershed. Lake Conroe might get NO rain and Lake Houston might get more than it can handle – on top of a rapid release from the Conroe dam.
• TCEQ recommended against it

Discussion

The primary objective of lowering Lake Conroe is to reduce flood risk when it is highest for downstream residents. It would also provide an extra margin of safety for Lake Conroe residents, many of whom flooded during Harvey. From that standpoint,  the temporary seasonal lowering has the highest probability of success. Here’s why.

The temporary, seasonal lowering can be carried out at a rate of one or two inches per day as weather and downstream conditions permit. It’s a sure thing.

The wait-and-see strategy carries more risk (from the flood prevention point of view) because of the unpredictability of tropical storms. Sometimes they blow up near the coastline at the last minute. Storms can also easily cross the Gulf in three days, as Alberto did, or change course at the last minute, as Rita did. (Remember the mass evacuation of Houston that turned out not to be necessary?)

If it typically takes a hurricane three days to cross the Gulf, using the wait-and-see strategy requires reducing the level of Lake Conroe eight inches per day (instead of one or two inches per day) to achieve a two foot reduction. That might flood downstream residents in Montgomery and Harris Counties – especially if the storm approaches from the south and loads up the downstream watershed WHILE Lake Conroe is releasing.

Another problem with the wait-and-see strategy is this. What if the storm is not tropical in nature? What if it approaches from the north or west and still dumps eight to 10 inches of rain on us? Engineers would have even less time to release in such a case. The Tax Day Flood in 2016 dumped more than 16 inches of rain in parts of our area in just 12 hours – from thunderstorms that approached from the west with less than two days’ warning.

Lake Conroe can release water ten times more quickly than Lake Houston.

A two-foot lowering would not do much to protect us from another Harvey; it would provide only a few additional hours to evacuate. However, such a lowering would protect us from smaller storms, such as 10-, 25-  or 50-year events, and those are FAR more likely to occur.

Both strategies have a flaw from a precedent point of view. A letter from the TCEQ to the SJRA dated March 26, 2018, states that, “The general rule in this country is that the operator of a dam may permit floodwaters to pass through a dam in an amount equal to the inflow, but will be liable if any excess amount is discharged.”

Hmmmm. That puts the ball squarely back in the City of Houston’s court. It looks as though the City of Houston will have to rely on its right to draw water from Lake Conroe if it wants to lower the level of the lake during hurricane season.

According to Houston City Council Member Dave Martin, the City has not used its allotment since the drought in 2011/2012. The City’s allotment of 100,000 acre feet. Because the lake is approximate 20,000 acres in size, if the City used its full allotment, it could lower the level of the lake by five feet, far more than the 2-foot reduction the LCA is fighting.

All parties should keep in mind that neither strategy is permanent. Lake level reductions would only happen UNTIL other long-term mitigation measures become effective. Those include dredging the river and the installation of additional flood gates on Lake Houston. Dredging is designed to restore the river’s carrying capacity and velocity. Additional gates on Lake Houston would eliminate a downstream bottleneck by equalizing the discharge rates of the two dams on the river.

June 1, 2018 by Bob Rehak

276 Days since Hurricane Harvey