October 2020 Regional Flood Mitigation Update

Flood mitigation is a complex process with hundreds of moving parts, all dependent on each other. When you focus intensely on the pieces, periodically it helps to review how they fit together. Only when you do this can you see progress toward your goals. In that spirit, I created this presentation. It has a Kingwood slant, but really addresses the whole Lake Houston Area. Everything connects.

What Caused Flooding

First, let’s look at what caused the flooding during Harvey and other recent storms…apart from record rainfalls.

The rain fell on a landscape that was ill-prepared the handle it.

Look at flooding like a process engineer. There’s always a bottleneck in the system somewhere between the sky and Galveston Bay. Both local and regional issues contributed to the magnitude of the Harvey and Imelda disasters.

Underestimating Rainfall Meant Underpreparing

When we built communities, such as Kingwood, we underestimated the amount of rainfall we could get. After four so-called 500 year rains in five years, we’re now working with higher rainfall precipitation frequency estimates (called Atlas 14). For this area, the 100-year rain is about 30-40% higher than the previous standard. That means floodplains will soon expand when new flood maps are released. Some people in the 500-year floodplain will find themselves in the 100-year. And people in the 100-year may find themselves in the floodway of rivers and streams. Unless we do something.

Lake Conroe Release Coincided with Downstream Peaks

On top of record rainfalls, during Harvey, the SJRA released water from Lake Conroe at a time that coincided with downstream peaks from other tributaries. The release by itself would have created the ninth largest flood in West Fork history. It comprised about one third of all the water coming down the West Fork.

Population Growth in MoCo

On top of that, Montgomery County has seen tremendous population growth in the last decade. Conroe was the fastest growing city in America in 2017 and is still #6. The county itself is the second fastest growing in the region.

Bad Development Practices

Combine the rapid growth in impervious cover with bad development practices and you are laying the groundwork for disaster. The bad development practices include lack of detention, clearcutting huge areas before detention is installed, and calculating detention for the old rainfall standards knowing that higher standards will soon go into effect.

Out-of-Control Sand Mining

To pave the way for all that development, we’ve seen a huge increase in sand mining upstream in the last 25 years. Before 2011, unregistered, bootleg operators went unregulated. But even those who are registered don’t follow common-sense regulations common in other states, which protect the environment and downstream residents. For instance, they mine so close to the river that dikes frequently fail. As a result, they have contributed to a greater than than natural buildup of sediment in our rivers. After Harvey, the US Army Corps of Engineers found that the West Fork was 90% blocked just downstream from River Grove Park.

But the biggest blockage was where the West Fork meets Lake Houston. There, sediment drops out of suspension as the water slows. The sediment built up for decades, forming a mouth bar that backed water up during Harvey and contributed to the flooding of thousands of homes and businesses.

Flood Gate Disparity

Another factor contributing to flooding: Lake Conroe can release water 15 times faster than Lake Houston. That makes it difficult to lower lake levels in advance of a major storm. Storms have a nasty habit of veering away at the last minute. The small gates on Lake Houston mean you have to start lowering the lake far in advance. That raises the risk of wasting precious water. Larger gates would make managing lake levels easier.

Ditches, Streams Filling In

Finally, sediment built up in more places than the San Jacinto. It has also built up in the ditches and streams that lead to the river. This reduces convenance within neighborhoods. There’s less room to store water in streams, so it backs up into storm sewers and streets, and can eventually flood homes.

Four-Part Solution

Montgomery County has now adopted Atlas-14, the new rainfall standard. And residents are pushing commissioners to close loopholes that let developers get away without building detention. Residents are also working with the TCEQ and TACA to adopt better sand mining practices.

But what can we do to remedy the damage already done? Leaders in the Lake Houston Area identified a four-part strategy and are working with the SJRA, City of Houston, Montgomery County and Harris County Flood Control to implement it. Elements include:

• Increasing upstream detention to reduce the volume of water coming downstream during floods
• Dredging the river to remove sediment dams that back water up
• Increasing the number of gates on the Lake Houston Spillway to release water faster
• Restoring or expanding ditches to handle more water

Let’s look at each of those in a little more detail.

Upstream Detention

The Humble/Kingwood area sits at the tip of a funnel. 535 square miles in a seven-county region drain into Lake Houston.

The San Jacinto River Basin Master Drainage Study identified 16 potential detention projects to hold water back during major storms. However, they would cost roughly $3 billion to reduce damage by less than $1 billion. So that’s a stretch. FEMA evaluates projects based on their benefit/cost ratio.

The SJRA has already applied for a grant for a detention project on Caney Creek which empties into the east fork. But we also need something that can help offset releases from the Lake Conroe Dam on the West Fork. Next step: figure out the funding piece of the puzzle.

Dredging

We’ve already removed almost three million cubic yards of sediment from the West Fork. The third phase of West Fork dredging should wrap up shortly. But there’s still more work to do.

Luckily:

• Voters approved $10 million for dredging in the 2018 Flood Bond, which can be used for matching funds.
• FEMA has already agreed to dredge another million cubic yards
• Dan Huberty’s amendment to SB500 will provide $30 million for additional dredging

The slide below shows why we need additional dredging.

An 18-foot high underwater plateau exists between where the Army Corps stopped dredging on the West Fork and the FM1960 Bridge where scouring during Harvey reduced the channel depth.

The chart above shows the deepest part of the channel between those two areas. If left in place, this plateau will force flood water out of banks during floods. It will also trap sediment, negating the value of previous dredging efforts.

More Gates for Lake Houston

FEMA has conditionally approved up to approximately $50 million to increase the outflow capacity of Lake Houston. During Havey, the flow of water over the spillway was estimated at 11-13 feet. That’s reportedly higher than the flow over Niagra Falls.

The FEMA grant initially covers preliminary engineering and environmental surveys. If the benefit/cost ratio is positive, they will release the remainder of the funds for construction.

So far, engineers have identified five possible solutions. Each produces a different flood reduction benefit, but also comes with different costs and environmental issues.

The next step: to analyze the benefit/cost ratio of each and submit the results to FEMA.

We are six months into the first phase, scheduled to take a total of 18 months. Construction, if approved, will take another 18 months.

Ditch Restoration/Expansion

Harris County Flood Control District has studied area ditches for the last year or more. Results of their analyses are now complete.

In Kingwood, they identified nine projects and recommended two for immediate implementation that should improve drainage on a third tributary, Ben’s Branch.

Recommendations include expanding:

• Kingwood Diversion Ditch to take water out of the Ben’s Branch watershed and also improve drainage from surrounding subdivisions from Woodland Hills down to the West Fork.
• Taylor Gully – if a deal cannot be reached to purchase Woodridge Village and build a regional detention basin on it.

In addition, Flood Control and the City are looking at minor improvements to other ditches that back water up in places, for instance, at culverts under Kingwood Drive.

Goal: Restore 100-Year Level of Service

Engineers solving all these problems have a goal: to restore your home to what they call a 100-year level of service. That means if your home was built above the 100-year floodplain, it should not flood in a 100-year rain – based on new Atlas 14 standards. In places, sedimentation and upstream development have reduced the level of service to 2 years over time.

We need to restore the capacity of ditches, streams, the river and lake to handle 100-year rains. So we can get water from the sky to the Bay without it going through your living room.

While progress may feel painfully slow, many improvements have already been made.

For instance, in addition to dredging, Flood Control has divided Ben’s Branch up into five projects. Three are already complete:

• Excavation from Kingwood Drive to YMCA
• Excavation from County line to St. Martha school
• Clearing underbrush and deadfall from the natural portion of the stream.

A fourth project, to restore Ben’s Branch from Kingwood Drive north to Rocky Woods should start soon. And a fifth, to restore conveyance between the YMCA and the River, will follow.

How You Can Help

You can help by remaining engaged. Without public pressure, it’s easy for elected officials to ignore these problems and defer expensive solutions. Keep flood mitigation a high priority. Floods can strike at any time. We had four tropical systems barely miss us this year.

Posted by Bob Rehak on 10/27/2020

1155 Days since Hurricane Harvey and 404 since Imelda

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.