Tag Archive for: difference map

New Difference Map Confirms Buildup of West Fork Sediment Around Mouth Bar, Underscores Need for Removal

A new “difference map” published by the Texas Water Development Board (TWDB) shows the rapid build up of sediment in the West Fork of the San Jacinto River where it meets Lake Houston. Difference maps show changes in sediment levels over time.

When Stephen Costello, Chief Resiliency Officer for the City of Houston, presented this map at the Kingwood Town Hall Meeting on October 9, it was to describe three potential phases of West Fork dredging..

Having had several days to review and discuss this map, several other things became clear. See the map and legend below.

West Fork Difference Map. Brown/red/orange/yellow/green areas represent decreases in sediment since last survey. Blue, violet, pink and white represent increases.

Conclusion #1:

The worst sediment buildup in the West Fork is near the mouth…exactly where retired Lake-Houston-area  geologists Tim Garfield and RD Kissling said it was when they raised the alarm about the mouth bar month’s ago.

Conclusion #2:

The problem is much worse than even they suspected. This clearly shows the extent of the West Fork’s mouth bar.  Like an iceberg, the part you see above water is only a small part of the bigger picture.  Vast new sediment deposits extend down to FM1960.

Conclusion #3:

Even Harvey-strength currents could not cut through the mouth bar. Therefore, smaller floods won’t be able to either.

Conclusion #4:

The mouth bar creates a sediment dam in the river that will exacerbate flooding. If the mouth bar area is not dredged, it will cause higher-than-normal flooding on smaller-than-normal rainfall.

Google Earth shows that the area between the mouth bar and FM1960 comprises 1700 acres. According to the difference map, this area received an average of approximately 2 feet of sediment. This means the channel and lake lost 3400 acre feet of capacity in this area.

Conclusion #5:

The mouth bar must be removed immediately. It and adjacent shoal areas get higher with every passing day and storm as water becomes shallower. If the mouth bar is left in place, it will slow the river, causing the area upstream (that is now being dredged) to fill more rapidly than normal with sediment.

Difference Map Proves Mouth Bar Must Go NOW

The mouth-bar area was the focus of the Corps’ survey efforts. They have known for months that it is the major problem. As they stated in Galveston at a meeting I attended, it is the primary driver for backwater and upstream flooding. Their value engineering report also states that in the event of another heavy rainfall, there is a “near certain likelihood” that wide-spread flooding will occur impacting even more homes than before Harvey, due to the river’s inability to pass high volumes of water. See Page 2 for the words engineers almost never use without cause.

We’ve been lucky so far this hurricane season. But minor floods earlier this year on March 28/29 and July 4 proved how serious this problem is. Had the City not lowered the level of Lake Houston for July 4, homes almost certainly would have flooded according to Houston City Council Member Dave Martin.

This Week’s Holdups

At a meeting in Austin last week, the City, Army Corps, FEMA, County and State of Texas all agreed in principle that the mouth bar needs to be removed. Further, if it can be added to the current dredging project, taxpayers can likely save $17 million or more in mobilization fees for a second, separate project at a later date.

Only two hurdles remain: a disposal site for the dredged material and an environmental survey.

A large disposal site is already under review that is much closer to the area of concern than either of the two current sites.

The proposed disposal site’s proximity could save taxpayers tens of millions of dollars more.

However, the environmental survey could delay the project beyond April when current dredging is expected to finish. If so, that could also increase flood risk.

Trees on the mouth bar initially led the Corps to exclude the bar from dredging for two reasons: one was environmental, the other legal. According to the Stafford Act, the enabling legislation for FEMA, FEMA funds cannot be spent to fix things that existed before Harvey. The existence of the trees proved that at least part of the mouth bar existed before Harvey.

The “Mouth Bar.” Note the trees on the right end.

Let’s Get Real

If the trees on the island raise an environmental concern, leave the part of the island that existed pre-Harvey. Dredge the channel and other parts of the island first.

This area includes 3-5+ft of Harvey derived sediment which should qualify for FEMA disaster recovery funds under the Stafford Act based on the new TWDB map.

The longer we wait to dredge this island, the more vegetation will grow on it, the more resistant to erosion it will be, and the more expensive it will be to dredge.

Are the Army Corps and FEMA trying to save an invasive species that USDA wants to eradicate?

Close examination from a boat showed the trees in question to be Chinese tallow trees, an invasive species that the USDA has been trying to eradicate from here to Florida. It’s actually poisonous to local animals. If this tree costs taxpayers tens of millions of dollars in delays and opportunity costs, it will become the new poster child for government waste and folly…especially if Humble, Kingwood and Atascocita flood in the meantime. This is one delay I certainly wouldn’t want my name associated with. That’s the wrong way to go down in history.

This tree on the mouth bar which initially caused the Army Corps to exclude the bar from dredging is a Chinese tallow, an invasive species which the USDA is trying to eradicate. Note the heart-shaped, aspen-like leaves – the tell-tale identifier.

Posted by Bob Rehak on October 14, 2018

411 Days since Hurricane Harvey