Tree Lane Bridge erosion

CoH Public Works Kicks Off Tree Lane Bridge Rehab Project

City of Houston (COH) Public Works Department held a kick-off meeting with contractors and the Bear Branch Trail Association (BBTA) this morning for the Tree Lane Bridge Rehabilitation Project. Public Works and the contractors outlined plans for the project. They also discussed access through BBTA property and safety protocols.

Chris Bloch (left) and Lee Danner (center) of BBTA met with contractor and Houston Public Works representatives at bridge on 2/21/24.

Construction should begin by mid-March and take approximately 150 days.

Project Scope

The $909,000 project will involve:

  • Creating access areas for equipment
  • Removing all existing concrete under the bridge and along the sidewalls
  • Establishing a 2-foot deep by 32-foot-wide low-flow channel from 20 feet upstream to 20 feet downstream of the bridge
  • Rebuilding wing walls
  • Restoring outfalls for local drainage
  • Restoring the work site and replanting trees.

See the construction plans here.

Safety Warning

Contractors will meet with the Bear Branch Elementary School principal to understand normal drop-off and pick-up times. They will then work around those times to try to minimize traffic interference.

Regardless, the City urged parents to warn children to stay away from the construction zone, which will be on both sides of the bridge. Lots of heavy equipment will be maneuvering in tight spaces with limited visibility near Bear Branch Elementary. Observe all traffic warnings and flag men.

Reason for Project

Embankments under the bridge have degraded significantly in the last 10 years. This is largely because of jetting.

Jetting is caused when water backs up behind a bridge that is too small for the volume of water coming down a stream. Pressure upstream of the bridge forces turbulent water to shoot out the downstream side destroying anything in its way. See six images below taken 2/21/24.

Looking upstream at eastern side of bridge. Jetting has blown out concrete under the bridge.
Damaged storm sewer outfall.
Exposed abutment
Downcutting as floodwater tries to make more room for itself.
Jetting continues to erode a large basin south of the bridge.
Note how narrow creek is upstream (Left). Right = effect of jetting downstream.

It Wasn’t Always This Way

The erosion has started in the last ten years and accelerated with upstream development.

In general, developments increase the time of accumulation of runoff. Compared to a densely developed area, when rain falls on a forest, it trickles its way to the creek. But if it falls on concrete and rooftops, it rushes to the creeks through storm sewers.

For illustration purposes, the graph below shows the actual change along Brays Bayou.

time of accumulation
Change in time of accumulation in Brays Bayou as watershed developed over 85 years.

If development were sufficiently mitigated, post-development runoff would be no higher than pre-development. But given the amount of recent erosion, some development upstream from Tree Lane is likely insufficiently mitigated.

I’ve lived near the Tree Lane Bridge for 40 years. For the first 30, erosion was no problem. Now look at it. Compare the two satellite images below.

In 2014, you could barely see creek.
Same area in 2023.

Chris Bloch, an engineer, says that at the time of Imelda in 2019, the open space under the bridge was 500 square feet. The design approved by COH and Harris County Flood Control District will add approximately 64 square feet in the form of a low-flow channel. Contractors will add that where the stream has already downcut.

If the 500 SF estimate is accurate, another 64 SF would represent a 12.8% increase in the carrying capacity of the channel under the bridge.

Will It Be Enough Conveyance?

This will be the second time in three years that the city has repaired the bridge and its embankments.

The state of the bridge now is worse than it was after Imelda, BEFORE the last repair job. And no storms since Imelda have even came close to Imelda’s magnitude. That means insufficiently mitigated development may have played a role in building flood peaks faster and higher; and undermining the bridge.

So, will 12.8% be enough to eliminate more jetting? Only time will tell. Keep your fingers and toes crossed.

Sign Up for Project Updates

You can sign up for project updates by visiting Houston Public Works’ “Engage Houston” page for this project.

Posted by Bob Rehak on 2/21/24

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