Local Geologists Develop Way to Estimate Volume of Sediment in Mouth Bar Due to Harvey

Two top geologists, now retired from one of the world’s leading oil companies, have developed a reliable and repeatable way to estimate the volume of sediment deposited in the West Fork mouth bar by Hurricane Harvey. They calculate that Harvey deposited at least 268,000 cubic yards of sediment in that area alone.

Stream mouth bar where the West Fork of the San Jacinto meets Lake Houston creates a sediment dam. It backs water up throughout the entire Hunble/Kingwood area during floods. Water must flow uphill approximately 40 feet to get over this hump.

No Pre-Harvey Measurements Hampered Dredging Program Approval

According to the Army Corps of Engineers and Stephen Costello, the City of Houston’s Chief Resiliency Officer, the lack of a reliable way to estimate the volume due to Harvey was a major stumbling block in funding the dredging effort. The City, FEMA and the Corps have reportedly been arguing about this for at least nine months. The issue has to do with the Stafford Act, FEMA’s enabling legislation. The Stafford Act prohibits FEMA from spending disaster relief funds on cleanup not related to the disaster in question.

Because the City of Houston had no reliable sedimentation survey taken immediately before Harvey, calculating the volume due to Harvey became problematic.

How a Mouth Bar Forms

A mouth bar forms at the mouth of a river where it meets still water (in our case, Lake Houston). As moving water encounters the still water, coarser sediment like sand is deposited. It begins building up and up until sand bars emerge above the surface.

The Insight that Led to a Reliable Way to Estimate Volume

Mainly sand comprises the mouth bar; sand moves only during major floods. The sand above water can only be deposited when floodwater is much higher than the top of the bar. That insight became the key to unlocking the mystery of how much sand Harvey deposited.

Everyone could see from satellite images how much the above-water portion of the bar had grown. But there was no way to tell how much the bar grew below water. Then in October of 2018, Costello presented a “difference map” that showed the sediment buildup between 2011 and 2018.

RD Kissling and Tim Garfield reasoned that if they could calculate the percentage of above-water growth during Harvey from satellite images, they could then apply that same percentage to the total volume of sand deposited below water between 2011 and 2018.

Calculation for visible “above water” growth of the mouth bar during Harvey.

But to determine the total volume added between 2011 and 2018, they first had to:

  • Digitize the difference map
  • Create polygons around the different colors
  • Calculate the area of the polygons
  • Multiply area times thickness for each
  • Add up the results.
Digitized difference map shows boundaries between areas of different thickness.
Multiplying the area of the polygons times the thickness from the difference map yielded volumes for each area.

Tetra Tech Delays May Push Project Past Deadline

According to Costello, the City hired a company called Tetra Tech in early January to calculate the Harvey volume. He said they would do that by harvesting and analyzing core samples. The City expected the results of their study by the end of January. But when I talked to Costello in mid-February, he said Tetra Tech still had not finished harvesting core samples and that he wasn’t expecting results of their analysis until the end of February or early March.

Pro Bono Effort Might Save Taxpayers $18 Million

Kissling and Garfield developed their methodology and donated their time to help save money and to get the mouth bar removed before the start of the next hurricane season. If the current dredging program can be extended before the end of April, taxpayers could save the cost of recommissioning all the equipment. That could total $18 million.

The two geologists reasoned that their methodology would give all parties a basis for allowing the dredging to continue. Then, if Tetra Tech came back later with a different figure, the contract could be adjusted up or down.

Kissling and Garfield emphasize that their methods are conservative, reliable and repeatable. For peer review and public comment, this presentation shows how they arrived at their estimates in a step by step fashion.

I am presenting it here to start a dialog that leads to additional dredging without incurring the cost of remobilizing the massive amount of equipment now on the river.

Estimate is Only a Fraction of What Needs to Be Removed

Kissling and Garfield emphasize that this is just a start. Even if all 268,000 cubic yards were removed, the river would still be up to 20 feet shallower than at the time of impoundment. They hope this leads to a broader discussion of additional dredging which could be financed through other sources, such as the county flood bond and Proposition A. Finally, they point out the need for maintenance dredging after major floods to keep the sediment buildup at sub-critical levels.

Said Garfield, “We are confident this estimate of Harvey-specific sedimentation on the mouth bar is reasonable and should be used to support FEMA funded continuation of dredging. However, removing that volume alone still won’t solve the problem, because the mouth bar is much bigger than that and it remains the largest restriction of flow conveyance to the lake.”

Garfield continued:

“At a minimum, to restore flow and reduce flood risk, a 300-400’ wide channel 20’-25’ deep needs to be dredged to connect the river from the upstream dredging now nearing completion, through the mouth bar, to the FM1960 bridge. That is at least 5 or 6 times as much sediment as our estimate of what FEMA can fund, but without that the recent dredging alone has not solved the flood risk problem in our area.”

Posted by Bob Rehak on February 24, 2019 with help from R.D. Kissling and Tim Garfield

544 Days after Hurricane Harvey