Tag Archive for: strategy

Two Top Geologists Suggest Mouth Bar Dredging Strategy

Two world-class geologists, Tim Garfield and R.D. Kissling, both of whom live in the Lake Houston Area, agreed (at ReduceFlooding.com’s request) to offer their opinions on what would be the best strategy for dredging near the San Jacinto West Fork Mouth Bar. Garfield and Kissling helped bring mouth bar issues to the attention of the public after Harvey. Both have followed various dredging programs in that area closely ever since.

Looking west across the mouth bar on 9/11/2020 while hovering over Kings Point. Suspended sediment shows that main current of river is between remainder of the above-water portion of the mouth bar and Atascocita Point on the upper left. Photo taken 9/11/2020.


The Army Corps of Engineers dredged from River Grove Park to Kings Harbor in 2018 and 2019, removing approximately 1.8 million cubic yards. After a contentious battle with the City, the Corps then agreed to remove another 500,000 cubic yards south of the Mouth Bar in the Spring and Summer of 2019. This year, the City of Houston started removing the portion of the mouth bar that remained above water; they are still working on it (see above). Recently, FEMA agreed to remove another million cubic yards. And waiting in the wings is another $30 million that can be applied to additional dredging. State Representative Dan Huberty secured that money in the last legislature.

However, none of the various parties involved has volunteered to share their thinking about objectives and strategies behind mouth bar dredging alternatives. That’s why I asked Garfield and Kissling to offer their thoughts on what constituted the best strategy. Both worked for one of the world’s largest oil companies at the very highest levels.

Old Bathymetric Maps No Longer Valid

The first thing they realized was that they didn’t have enough data to make informed recommendations. The last published bathymetric maps were based on surveys taken before Imelda and before the Corps’ mouth-bar dredging.

Gathering Own Data

So Garfield and Kissling gathered their own data – with sonic depth finders, GPS, and a 14-foot pole with depth markings. They started upstream of the mouth bar, where the Army Corps finished its Emergency West Fork Dredging program near Kings Harbor. And they worked their way downstream beyond FM1960 to the railroad bridge.

Scope of Garfield/Kissling survey

Found Underwater Plateau 20′ High and 3 Miles Long

They found an underwater wall approximately 20′ high where the Corps stopped its first dredging program near Kings Harbor. It extended downstream more than 3 miles.

Cross section of river channel shows a rise of almost 20 feet wall on the upstream side of the mouth bar near Kings Harbor and an even greater drop near FM1960. The result: a 3-mile long underwater plateau.

That wall, they say, “…constitutes a significant and abrupt hydraulic barrier that will likely exacerbate flooding and sedimentation.”

That wall is the leading edge of a 3-mile-long underwater plateau.

Note abrupt drop north of FM1960 Causeway.

Recommend Following Original Channel

The cross-section graph of the river bed above represents the deepest part of the river. On either side of that centerline, the riverbed rises to two or three feet below the surface of the water. The centerline closely follows the paleo (original) channel of the river before the Lake Houston dam was built.

Garfield and Kissling recommend dredging along the deepest path (see below). They reason that would save money.

Recommended and alternate routes identified by Garfield and Kissling that take advantage of deeper water.

“This might not only be the most beneficial dredging plan, but could also be the most cost effective as it leverages the paleo-channel as much as possible,” they say. “It harnesses nature, rather than fighting it.”

The geologists also identified a second possible route farther to the east but still south of the above-water portion of the stream mouth bar (labeled SMB in diagram above).

They caution that hydraulic modeling should be used to decide the best dredging plan. Political considerations drove initial mouth bar dredging rather than data. The Corps was authorized only to dredge an amount that it believed Harvey deposited. “We should be past the politics at this point and looking to get the most bang for our bucks,” say the geologists.

Whichever strategy the City settles on, Garfield and Kissling recommend excavating a channel, not a broad area, to get the best results for the dollars invested.

Objective: Re-establish Full Channel From Kings Harbor to Lake Houston

“This new channel should be no shallower, nor narrower than the upstream dredged channel at its end dredge location (450’ wide x 26’ deep),” say the geologists.

As a minimum, the future dredging plan should re-establish a continuous and down-stream deepening channel volume from where the Corps channel dredging ended to the 1960 bridge.

Tim Garfield and RD Kissling

This will help reduce sediment build up upstream from the plateau. By accelerating water through the blockage, it will let the river carry sediment farther out into the deeper portion of the lake. It will also reduce water backup that contributes to flooding.

Read Garfield and Kissling’s full study, Evaluating West Lake Houston Bathymetry, Dredging Status and Recommendations.

Recommendations Consistent With City’s Preliminary Findings

The City has been methodically surveying Lake Houston and is in the process of developing its own maps, objectives and strategies. Stephen Costello, the City’s flood czar said they are not finished with that effort yet. However, he also said that the preliminary information they obtained suggested that a route south of the mouth bar might be the most effective.

Posted by Bob Rehak on 9/25/2020 with thanks to Tim Garfield and RD Kissling

1123 Days after Hurricane Harvey

Pros and Cons of Two Alternative Strategies to Lower Lake Conroe

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

  • 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
  • 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

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


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.

Posted DATE, 2018 by Bob Rehak

Day XXX since Hurricane Harvey