Why We Must Remove Mouth Bar on West Fork of San Jacinto
The U.S. Army Corps of Engineers originally solicited bids to dredge the West Fork of the San Jacinto from 59 to Lake Houston, a distance of 8 miles. At some point in the project, the Corps limited the scope to 2.1 miles – from River Grove Park to the West Lake Houston Parkway bridge – for reasons never made clear. Perhaps they ran out of money when they had to double the volume they were dredging in that 2.1 miles. Regardless, that meant leaving a huge sandbar in place at the mouth of the river (see below).
I have been concerned about that bar ever since they excluded it from the scope. I wasn't the only one. Two retired geologists, Tim Garfield and R.D. Kissling, approached me after my post about reduced scope. They have more than 50 years of experience with one of the world's most successful oil companies. Both are experts in river morphology and sedimentation.
They compiled this fascinating 28-page report about the mouth bar, which I helped them edit. It explains how the river is changing, why it's crucial to remove the mouth bar, and what will likely happen if we don't.
The goal of this presentation was to get the Corps to expand the scope of their current dredging project to include the bar. Why? Approximately half of all the damage that occurred in Kingwood occurred BEYOND where the Corp intends to stop dredging. If not removed, everything behind the mouth bar for miles upstream will be at greater risk of flooding.
Major Concerns From a Geologist's Perspective
If this blockage is left in place, it will, say Garfield and Kissling:
Cause the river to run UPHILL
Create, in effect, a partial dam
Slow water down, back it up and elevate the water surface
Increase flooding upstream
Increase the rate of sedimentation behind it
Cause the river to escape its banks, flood neighborhoods and damage or destroy infrastructure
How the River Is Changing
Since the late 1970s, a delta has been forming within the river and advancing toward where the river meets the lake. You can see the 2-mile advance most clearly in this summary slide that shows Kingwood in 1977 and 2017.
Evolution of mouth bar over 40 years. The full presentation contains many intermediate images.
When Friendswood designed Kingwood's drainage, it was based on a different reality. Note, for instance, what happened east of the West Lake Houston Parkway Bridge. A huge area has filled in, reducing the conveyance of the river.
The chart below shows the increasing rate of change in the Stream Mouth Bar (SMB) at the right in the photo above (the area outlined in white with the red arrow pointing to it).
Sudden exponential growth in mouth bar volume tells geologists that it has reached critical mass and is likely contributing in a major way to upstream flooding.
How Our Mouth Bar Contributes to Upstream Flooding
This next series of slides shows how and why the West Fork mouth bar affects flooding.
Before the Lake Houston Dam was built, water in the river dropped steadily in elevation from the site of today's Grand Parkway all the way to the coast.
Since the construction of the Lake Houston Dam, water continues to drop to US59, but then it levels out. That's because the dam backs up water that far.
A huge mouth bar grew up where the river enters the lake. As water slowed down and spread out, it deposits sediment. When the river is flowing at normal levels, water can find its way around the blockage without threatening neighborhoods.
However, during floods, the mouth bar acts as a partial dam. It creates a hydraulic jump that begins to back water up behind it. Note how the orange bar is higher on the left than on the right, relative to the blue line. That's because the stream mouth bar increases the height of water behind it during a flood.
If the mouth bar were removed, water would no longer back up behind it. The river could flow freely in a flood. The dotted line represents an estimate of how much a flood like Harvey could be lowered. If the reduction were four to six feet, that could make the difference between major and minor flooding.
In this profile, the horizontal scale is less compressed than in the charts above it, so you can visualize more easily how water is forced to flow uphill as it approaches the mouth bar. This forces water to flow uphill and the water surface to elevate behind the mouth bar, contributing to upstream flooding.
Options Going Forward
Ignoring the mouth bar and hoping it will go away is not an option.
It has nearly doubled in size in the last three years. It will force the river to flood more frequently and more extensively, causing more damage to houses and infrastructure.
I suspect that the reason the Corps did not handle this in the first place is because they were constrained by budget. So removing the mouth bar as part of a change order is not an option either.
It will likely cost far more to remove the mouth bar than it does to clear the 2.1 miles upstream.
That leaves two options recommended by the Army Corps' Deputy District Engineer for Programs and Project Management, Dr. Ed Russo (plus another that he didn't recommend).
Option 1: Russo helped draft a proposal request for consideration under Section 7001 of the Water Resources and Reform Development Act of 2014. The proposal requires a local, county, or state agency to put up a match for federal funds. Deadline is August 20. If approved by Congress this fall, the mouth bar could potentially be removed while Great Lakes still has dredging equipment on the river, saving the cost of another mobilization.
Option 2: Similar to 1, but without cost sharing. A partnership of local, county, state, or federal agencies can hire the Corps to be their public engineer and constructor for the project under an Interagency Agreement . This requires 100% funding by the Federal, state, county, or local agency. There is no cost share. Hmmmm. County Bond Referendum? The State's rainy day fund? Lots of possibilities. But they would probably take longer to work out.
Option 3: Go it alone.
Benefits of Removing the Mouth Bar
The Lower West Fork delta of the San Jacinto River is advancing development in size and shape. The West Fork mouth bar and surrounding shoal sediments are constraining in-bank flow conveyance capacity.
With no action to restore flow conveyance capacity within the river's banks, the evolving conditions will cause the river to rise out of its banks and extensively flood properties and critical infrastructure in the region. If addressed, flood risks to developments will be reduced and the river will have the conveyance capacity to pass flood flows and flush sediment that would otherwise reduce conveyance capacity.
Removing the mouth bar would reduce flood damages to properties regionally and provide for increased resilience to flooding of properties, transportation systems, water treatment systems, public/private utilities, emergency response facilities, petrochemical industries, and other critical infrastructure, in the West Fork, San Jacinto River Watershed, Harris, Montgomery, and Liberty Counties,TX, on the order of $200 Billion.
Given that petrochemical industries in the region produce a significant amount of the Nation's petroleum based energy products, reducing flood risks of these plants and its workers who reside in flood-prone areas, and providing for greater resiliency, is a National security benefit. The environmental benefit of providing for this project is reduced risks of water treatment plant and chemical spills due to flooding, which is a threat to human and environmental health and safety. The non-monetary benefits would include reduce risks to loss of life due to regional flooding, especially to residents with insufficient means.
Posted by Bob Rehak, R.D. Kissling and Tim Garfield on July 27, 2018
0 0 admin admin2018-07-27 17:56:072018-07-28 07:14:48Why We Must Remove Mouth Bar on West Fork of San Jacinto