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Photo Essay: How “Backslope Interceptors” Reduce Erosion, Ditch Maintenance, Flood Risk

“Backslope interceptors” help prevent erosion that can clog drainage ditches and contribute to flooding. Most people have probably seen them, but never paid much attention to them. Nor do they understand can reduce ditch maintenance costs by lengthening maintenance intervals. This photo essay shows what a difference they can make. All three counties in the Lake Houston Area require them, but Liberty County doesn’t enforce its own regulations. So the visual differences are dramatic.

What Are They? How Do They Work?

We’ve all observed water flowing through drainage ditches. But how does it get into the ditch? Broadly speaking, it can get into the ditch by a) flowing down the banks or b) through pipes. Option A increases erosion. Option B decreases it. B also reduces flood risk and the long-term cost of ditch maintenance.

What is a backslope interceptor? Imagine a small ditch (or swale) parallel to but offset from the main ditch. The swale captures runoff and overland sheet flow before it gets to the main ditch. The swale then funnels the flow into pipes that run under the banks of the main ditch. Keeping large volumes of water off those banks reduces erosion which could otherwise quickly fill the ditch with dirt and reduce its carrying capacity. If erosion reduces carrying capacity enough, water can flood nearby homes and businesses. The illustration below shows how backslope interceptors work.

Real-Life Examples

On 3/3/2021, I flew over three counties: Harris, Montgomery and Liberty. The “with/without” photos below illustrate the difference that properly constructed backslope interceptors can make. I shot the first one over the new Artavia development in southern Montgomery County. Note how the backslope interceptors let the developer establish grass on the banks of the ditch despite construction still in progress.

Ditches WITH Backslope Interceptors
Artavia ditch in Montgomery County. Note series of backslope interceptors behind the maintenance roads that flank the ditch.
Drainage ditch in Atascocita in Harris County. Again, backslope interceptors let grass establish on the sides of ditches, reducing erosion.
Wider shot along same ditch.
Ditches WITHOUT Backslope Interceptors

The rest of these examples came from Colony Ridge in Liberty County.

Lack of backslope interceptors has led to severe erosion. Runoff goes straight down the banks of ditch and into the East Fork San Jacinto.
Close up of same Colony Ridge ditch.

Role in Establishing Grass

The next two photos show the role of backslope interceptors in establishing grass. By preventing bank erosion from sheet flow, the interceptors give grass time to establish and grow, reducing erosion even more.

Ditch in Artavia, a still-developing area in Montgomery County, where developer has recently hydromulched to establish grass.
Liberty County ditch in newly developing part of Colony Ridge, also recently hydromulched. Without backslope interceptors, hydromulch has washed into bottom of ditch and will eventually wash away, leading to more severe erosion.

How Enforcing Regulations Can Reduce Costs, Flooding

Ironically, Liberty County drainage regulations updated in 2019 require developers to install backslope interceptors and plant grass on the banks of drainage ditches.

Page 100 states: “Erosion Control: All drainage facilities must be designed and maintained in a manner which minimizes the potential for damage due to erosion. No bare earthen slopes will be allowed. [Emphasis added] Various slope treatments, including turf establishment, concrete slope paving, and rip- rap, are accepted. Flow velocities should be kept below permissible values for each type of slope treatment. Interceptor structures and backslope swale systems are required [Emphasis added] to prevent sheet flows from eroding the side slopes of open channels and detention facilities.”

Unfortunately, Liberty County does not enforce its own regulations.

When the developer eventually tries to turn Colony Ridge over to Liberty County, the county will inherit as massive maintenance burden because of non-compliance with these regulations. But even before then, the developer is creating rivers of mud that reduce the conveyance of ditches, and thus contribute to flooding nearby residents in Plum Grove.

This Colony Ridge drainage ditch in Liberty County is rapidly filling in. Residents use it for joy-riding in their ATVs, which further contributes to erosion.

The sediment also contributes to dredging and water purification costs for people downstream in Harris County.

Posted by Bob Rehak on 3/6/2021

1285 Days since Hurricane Harvey and 534 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.

Before-After Shots of Clean-Out: HCFCD Restoring Conveyance of Taylor Gully In Elm Grove

Harris County Flood Control District (HCFCD) is nearing completion of its project to clean out Taylor Gully. The project will restore the ditch’s conveyance through Elm Grove. The ditch had become clogged due, in large part, to erosion from months of clear-cutting and construction activities immediately upstream in the new Woodridge Village development.

After the Flood, but Before the Clean-Out

Below, several shots taken shortly after the May 7th flood.

Erosion on Woodridge Village property. Concrete culvert in background is entrance to Taylor Gully on Harris County side of Montgomery County Line.
Another shot of erosion leading to culvert, visible in upper right.
Looking north at same culvert from Harris County side of county line.
Flood debris carried downstream into Elm Grove portion of Taylor Gully
Shot taken at end of May looking south along Taylor Gully. Three weeks after the May 7 flood.

After the Clean-Out

What a difference some backhoes and bulldozers can make!

Looking south from same area today, but from opposite side of Gully. Photo courtesy of Jeff Miller.
Re-contoured backslope swale with new culvert. Photo courtesy of Jeff Miller.
Newly cleared Taylor Gulley Backslope Swale near the homes that flooded in North Kingwood Forest. Photo courtesy of Jeff Miller.
Brand new backslope interceptor structure and improved swale by HCFCD located just north of Creek Manor where it dead ends into Taylor Gulley. Photo courtesy of Jeff Miller.

These backslope interceptor swales reduce erosion, provide additional floodwater storage, and help prevent floodwaters from impacting structures.

One Month From Statistical Peak of Hurricane Season

Today is one month from the peak of hurricane season – September 11. Hundreds of people in Elm Grove and North Kingwood Forest will have an additional margin of safety thanks to HCFCD’s Taylor Gully project. Despite three months of near-perfect construction weather, Perry Homes’ contractors have only completed two of five planned detention ponds upstream. More on the construction status of Woodridge Village in my next post.

Posted by Bob Rehak on 8/11/2019 with photography from Jeff Miller

712 Days since Hurricane Harvey and 3 months + 4 days since the May 7th flood