City Completes Repairs on Tree Lane Bridge, But Concerns Remain
The City of Houston has completed repairs on the Tree Lane Bridge over Ben’s Branch…at least for now. The City partially concreted a wing wall and placed riprap in the stream bed to help reduce erosion and scour. However, the root cause of the damage remains. Upstream development without adequate detention is funneling more and more water into Ben’s Branch. The higher volume will continue to contribute to scouring and erosion in major storms unless detention ponds upstream are built and fixed.
Water Under the Bridge
As I struggled to understand what I was looking at, an online search revealed this excellent 196-page, well-illustrated document. It’s titled “AN ILLUSTRATED GUIDE FOR MONITORING AND PROTECTING BRIDGE WATERWAYS AGAINST SCOUR.” Robert Ettema, Tatsuaki Nakato, and Marian Muste from the College of Engineering at the University of Iowa produced it for the Iowa Department of Transportation.
It begins with a maxim: “Person who overlooks water under bridge will find bridge under water.”
The document points out that bridge engineers often assume that a stream channel will maintain its course and dimensions. But most channels adjust their alignment and shape in response to floods and land-use changes in their watersheds. That has happened on Ben’s Branch in recent years.
“Channel changes sometimes have severe consequences for bridge waterways,” say the authors.
Scour and Progressive Degradation
A review of the entire paper reveals two types of destructive forces at work on the Tree Lane Bridge.
Long-term scour is one. It occurs over a time scale of several years, and usually includes progressive degradation and lateral bank erosion due to channel widening or meander migration.
Progressive degradation is another. It features the almost permanent lowering of the river bed at a bridge site owing to changes in the watershed [e.g., head-cut progression (head-cutting), or human activities (e.g., channel straightening or urbanization)].
The area around the Tree Lane bridge supports has been badly eroding for years. A 12-inch water line that was once buried 5 feet below the stream bed is now at the water surface. See below.
Scour and erosion under and around bridges are constant problems for engineers. This bridge is a great example. Any time a structure interrupts the flow of floodwater, it creates turbulence (vortices in the flow) that can lead to erosion.
During Harvey, scour destabilized the Union Pacific and US59 bridges across the West Fork. Both had to be replaced for safety.
The Tree Lane Bridge doesn’t appear to be to that point yet. But the heavier load placed on Ben’s Branch by new upstream developments, such as Woodridge Village, Woodridge Forest, and Brooklyn Trails, may be pushing this bridge beyond its design capacity.
Concrete and Rip Rap Should Help in Short Term
To address these problems, the City repaired part of a concrete wing wall. They also placed riprap (boulders) downstream from the bridge and below a stormwater drain.
Riprap breaks up the flow of water and slows it down. This reduces erosion and scour.
…But Root Causes for Damage Remain
However, problems remain, both here and upstream.
- That 12-inch water line could be taken out in the next flood by a tree flowing downstream. If the flood lasts for several days as it did during Harvey, the loss of water would be a major inconvenience to the residents of Bear Branch.
- Inadequate detention upstream will continue to erode both the stream banks and bed at accelerating rates.
Until we can address the root causes of such damage, I fear that maintenance on this bridge will be a constant, long-term issue…despite the City’s best intentions.
Posted by Bob Rehak on 3/31/2020
945 Days since Hurricane Harvey
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.