“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.
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
Ditches WITHOUT Backslope Interceptors
The rest of these examples came from Colony Ridge in Liberty County.
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.
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.”
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.
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
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