Putting Mouth Bar Removal in Larger Context; Need for Maintenance Dredging

As mechanical dredging whittles down the part of the San Jacinto West Fork mouth bar that sticks up above water, it’s important to think about longer-term maintenance dredging. Remember three things:

  1. How the sand got there in the first place
  2. Why it will be redeposited over time
  3. What the consequences will be of not removing it periodically

How Sand Got There

Most movement of sediment happens during floods. Sand and silt washes downstream from two main sources: natural and man-made.

The natural sources include erosion from river banks and beds.

The man-made sources include the dirt that washes into your storm drain during a rain. They also include new developments and construction sites that disturb the soil without adequate safeguards like silt fencing. Finally, in our area, we also have an abundance of sand mines that pump and/or dump silty wastewater into rivers.

Why It Will Be Redeposited Over Time

So-called mouth bars are giant sand bars formed at the mouths of rivers. They form wherever a river enters an ocean, sea, estuary, lake, or reservoir. Whenever water slows, a river will deposit sediment. And it always slows when a moving body of water encounters a standing body of water. It’s a well understood geophysical process that occurs everywhere around the world. A prime example is the Mississippi delta.

Mouth bars are actually part of river delta formation. As they build up, they force a river to split.

Why Intervention Is Necessary In Populated Areas

As sediment builds up, if left alone, it will eventually choke the headwaters of the lake and form a flat swampy lowland. You can already see this beginning to happen on the East Fork San Jacinto.

Looking north at northern part of East Fork Mouth Bar, which has become vegetated. Note how it causes the river to split. The river is carving itself up into a maze of small channels. Photos taken 5/11/2020.
Here’s the area immediately below the shot above. It is near the entrance of Luce Bayou to the East Fork. This area went from 18 feet deep to 3 feet after Imelda according to boaters. Photo taken 3/5/2020. Note how the newest mouth bar is forming in one of the channels formed by the previous mouth bar which has become anchored by vegetation.
Looking west toward West Lake Houston Parkway Bridge and the West Fork of the San Jacinto. Note how this mouth bar also caused the river to split. Sediment is now being deposited on both sides, and will eventually cause another split without more dredging. Photo taken 5/11/2020.
Looking south toward FM1960 from north of the mouth bar. Notice how shallow this section of the river has become. The loss of conveyance contributes to flooding. Photo taken 5/11/2020.
From Harris County Flood Control District’s page on the on-going Kingwood Area Drainage Assessment.

Part of the reason for the buildup of sediment behind the West Fork mouth bar is that Ben’s Branch and another major drainage ditch have been dumping sediment into the river there. Luckily, HCFCD is removing sediment from these and other ditches. That will help reduce the problem in the river, but not eliminate it.

Need for Maintenance Dredging

Erosion is relentless. We can do many things to minimize it (preserve wetlands, use best management practices in sand mining and construction, etc.). However, as long as rain falls, we can’t eliminate it.

To my knowledge, until the emergency West Fork dredging program began in 2018, the upper San Jacinto had never been dredged since the Lake Houston Dam was built in 1953. That’s 65 years. Over that time, sediment build up turned into a $100+ million dredging program. And that doesn’t even include flood damages which likely total another BILLION dollars according to a City estimate. Imagine all the heartbreak and misery that could have been avoided had the City budgeted $2 million for dredging each year.

Dredgers keep nibbling away at the mouth bar like an ear or corn, removing one row at a time. Photo taken 5/11/2020. Unfortunately, the effort to remove the portion of the bar above water may make people think the problem is solved, but it won’t reconnect the upstream river channel with the lake.

Imagine the:

  • Flood losses we could have avoided
  • Recreational opportunities we could have realized
  • Reservoir capacity we could have preserved
  • Home values we could have multiplied.

For all these reasons, we need to start a serious dialog about maintenance dredging. Even if it’s not every year, we need it after every flood. Think of it as a yearly insurance premium against the next disaster.

The Army Corps estimated this bar immediately downstream from River Grove Park blocked 90% of the West Fork. In the three months before the Corps removed it, River Grove flooded six times. Since then River Grove has not flooded at all.

Posted by Bob Rehak on 5/22/2020

997 Days after Hurricane Harvey