How Much Will Dredging Another Million Cubic Yards Reduce the West Fork Mouth Bar Area?

Earlier this month, the City of Houston announced that FEMA would pay to dredge another million cubic yards of sediment from the West Fork Mouth Bar. What does that mean in practical terms? What are the objectives of the program? How wide and deep will they go? Neither the objectives, nor a dredging plan, have yet been released.

The official plan will hopefully rely on new survey work and hydraulic modeling. A survey boat has been seen on the lake for several weeks now.

Since January of this year, the City of Houston has been trying to reduce the above water portion of the mouth bar with mechanical dredging, a much slower process than hydraulic dredging. Note how shallow the water is in the foreground.

How to Play Armchair Engineer

In the meantime, since fall football is in doubt due to COVID, here’s a simple way to keep your armchair quarterbacking skills finely honed. What would you do if you were the project engineer or manager? Play what if and experiment with different scenarios.

  • Download and open Google Earth Pro. It’s free.
  • Zoom in on the West Fork Mouth Bar.
  • Select the measuring tool.
  • Click on the polygon tab.
  • Select square yards for the unit measurement for the areas you will define.
  • Now start second guessing the project engineers. Play “what if” by defining an area that you would like to see dredged.
  • Readjust the points that define the area by dragging them in, out, up or down.
  • Watch the total square yards recalculate as you move the points.

Examples of Different Scenarios

Here are some examples to show you what I’m talking about.

4.3 Million Square Yard Area, Roughly 8 inches Deep
Largest area. This scenario takes in everything between where the Corps stopped dredging in its Emergency West Fork Program and the FM1960 Bridge.

The scenario above takes in the mouth of Ben’s Branch, plus all the other drainage ditches that empty Fosters Mill, Kings Point, and Atascocita north of the FM1960 Bridge.

The scenario above covers 4.3 million square yards. But with a budget to dredge only 1 million cubic yards, you would divide 1/4.3 = 0.23 yards of depth. That’s less than a third of a yard. It works out to about 8 inches.

3 Million Square Yard Area, 1 Foot Deep
In the next scenario, I pulled the boundaries in so that the area equaled 3 million square yards.

This scenario is a little more intuitive. You’re dredging 1 million cubic yards across an area of 3 million square yards. Within this bounding box, you could reduce the level of sediment roughly a foot.

2 Million Square Yard Area, 18 inches Deep
If you reduced the area to be dredged to 2 million square yards, you could reduce the level of sediment by half a yard or 18 inches.
1 Million Square Yards, 3 Feet Deep
Path followed by the relict channel before Lake Houston was built.

If you reduced the area further, to 1 million square yards, you could dredge to 3 feet. With the five feet of depth already there, you could have an eight foot channel connecting the river and the lake. Make it narrower and you could even go deeper. And perhaps, just perhaps, keep sediment from accumulating so rapidly upstream of the FM1960 bridge.

Difficult Choices Ahead

As you can see, engineers have some difficult choices ahead. They must chose between unblocking channels and streams, or dredging a channel roughly the size of the upstream west fork all the way to the 1960 bridge.

Last year, the Corps reduced the 600-acre area between the mouth bar and Atascocita Point to an average depth of 5.5 feet. So we have a good head start. But there are other considerations:

The West Fork was roughly 22 feet deep where the Corps stopped dredging just west of the mouth bar. Because of scouring, it’s at least that deep where the west fork passes under FM1960.

As a result, water coming down the west fork hits an underwater mesa that still blocks off three quarters of the conveyance.

Prominent area geologists, such as Tim Garfield and RD Kissling, theorize that that wall traps sediment and is rapidly diminishing the value of previous dredging programs. They believe that the most important objective for this phase of dredging should be to “reconnect the West Fork with the Lake.”

But Advancing Delta Now Blocks Major Streams, Channels

Meanwhile, consider this, too. In 2014, Bens Branch and the drainage ditch that empties large parts of Fosters Mill and Kings Point had a clear path to the river. Today, both are blocked by the mouth bar and an advancing delta within the lake. Compare the two images below.

Google Earth view from 2014 shows steams could still easily connect with relict channel.
Today, however, an advancing delta within the lake blocks them.

These are some of the real world trade offs that engineers and project managers must deal with every day.

So to return to the football analogy, do you send your receivers wide or deep? Do you have them hug the sidelines or cut for the goal post?

Understand that this isn’t a game, however. It’s a struggle to return a community to prosperity.

What would you do? And why?

Posted by Bob Rehak on 8/31/2020

1098 Days since Hurricane Harvey