The SJRA announced earlier this week that the Harris County Flood Control District will partner with the River Authority on a “sediment trap” pilot study for the East and West Forks of the San Jacinto. The two have hired engineering firm Freese & Nichols to conceptually design the traps and identify the optimal locations for them.
Finding the Right Combination of Factors
Most sediment traps are big holes dug in a river or channel though some are off to the side. As water passes them, velocity decreases. Suspended sediment and sand being pushed along the river bed fall into them.
Sediment traps vary by depth, width, length, shape (wide, long, triangular, etc.), and placement relative to the channel. And as this Army Corps study shows, modifying any one of those factors can greatly affect their efficiency. The challenge: to find the optimal shape, size and location.
The optimal length, for instance, depends on the speed of floodwaters and the settling rate of sediment particles. The trap needs to be long enough to give suspended sand time to fall out of suspension. Otherwise, sand just passes over the trap and continues downriver.
The hope: that the right type of traps placed in the right locations could help reduce flooding by reducing the amount of sediment migrating downriver and then blocking the mouths of each river branch.
Coming Out of HB1824 and River Basin Study
House Bill 1824, passed just last year, helps make sediment traps financially feasible. It allows the partners to remove material from the San Jacinto River and its tributaries to restore, maintain, or expand storm flow capacity without the need for state permitting or a royalty payment to the state.
Also note that the project will use data developed for the San Jacinto River Basin Master Drainage Study. It started in April of 2019 and is now about two-thirds complete.
Scope of Work Outlined
The scope of work outlined for Freese & Nichols includes, but is not limited to:
- Reviewing and comparing: current and historical LiDAR surveys and aerial photos to gauge sediment erosion, deposition and location
- Evaluating digital elevation maps to identify constrictions
- Identifying the ten areas with the largest volumes of sediment deposition, including the two largest at a sand mine.
- Reviewing FEMA floodplain, National Wetland Inventory, Texas Historical Commission and Texas Parks and Wildlife information for any problems related to each potential site.
- Core sampling of sand bars to determine silt content
- Ranking preliminary locations using the following: sediment deposition volume, potential sediment storage volume, proximity to existing roads, proximity to existing APO facilities.
- Selecting the four locations with the highest potential based on drainage area, sediment load, geology, road access, etc.
- Narrowing that to three locations in conjunction with SJRA and HCFCD
- Developing sediment trap conceptual solutions specific to each of the final three selected sites
- Estimating rate of sediment accumulation and clean-out intervals
- Estimating reduction of sediment accumulation due to trapping
- Submitting a final report.
The project does NOT extend into Lake Houston. Freese & Nichols will examine both the East and West Forks plus three miles upstream from the East and West Forks along certain tributaries. Tributaries would include, for instance, Lake Creek and Caney Creek. The idea is to intercept sediment before it can make its way into the lake.
The project timetable indicates completion in early fall of 2020, about the same time as the entire San Jacinto River Basin Study. Thus, any recommendations coming out of this project would not be constructed for this hurricane season. SJRA currently hopes to have the trap(s) installed by the end of 2022.
Pros and Cons of Sediment Traps
Proponents say sediment traps can reduce expensive dredging, restore fish habitat, reduce flood risk and more. However…
The scientific literature on sediment traps reveals mixed reviews. Many efforts fail, primarily because of lack of maintenance budgets and regular clean-outs. The SJRA and HCFCD plan to address that issue upfront by involving the sand mining industry. Notice that of the four location-ranking criteria bolded above, three favor proximity to sand mines:
- Sand mines have pits to store sediment.
- They have roads that lead to the river that can carry heavy equipment.
- Mines have cleaning, sorting and transportation facilities onsite to reduce transportation costs, and thus make material more attractive for resale.
Compensation for Sand Miners
A compensation agreement for sand miners who remove sediment from traps has not yet been worked out. However, Matt Barrett, SJRA’s project engineer, says, “The intended benefit to the APOs of participating in the public-private partnership would be that they could utilize or sell the material they remove from the trap(s). ”
Concerns of Environmental Groups
One of the concerns environmental groups have had about HB1824 (which began life as SB2126) is that it could potentially open the door to river mining and all of its risks.
River mining is outlawed in Europe, many other first-world nations, and even in some third world countries.
It tends to alter the gradient of rivers; cause upstream and downstream erosion; destroy private property along river banks; and undermine infrastructure such as bridges and pipelines.
Thus, the sediment traps raise a question of potential liability. If a pipeline or bridge is undermined or if property is eroded by the traps, who is responsible?
“Stability of the river is key to the long term safety and well-being of our community. We know that our public agencies have an important task. Before spending additional taxpayer dollars, don’t we want to make sure that projects won’t make problems worse AND won’t raise taxpayer costs?” said Jill Boullion, Executive Director of the Bayou Land Conservancy.
SJRA Response to Concerns
The SJRA has consistently denied that it would permit wholesale river mining. Most recently, Matt Barrett, the River Authority’s engineer for the sediment trap project said, “SJRA has no intention of engaging in or facilitating river mining in the San Jacinto River or any of its tributaries. The legislative language in HB1824 allows for the removal of material for the purpose of restoring, maintaining, or expanding the capacity to convey storm flows. Any projects undertaken by SJRA to remove or facilitate the removal of material from the San Jacinto River or its tributaries would be for this purpose.”
“River mining is not part of what we are doing. Only sediment in the trap will be removed,” he said.
Barrett is aware of potential hazards. “SJRA and its consultant are aware of the potential negative impacts that can be caused by trapping and removing sediment from a river or stream – changing the sediment balance – and prior to constructing any sediment trap or implementing removal of any material will perform analysis to ensure that any potential negative effects do not offset the positive.”
He sees the current design project as part of a pilot study that can scale up later. “One of the goals of this relatively small scale project is to serve as a “pilot” that gives us data on the real-world effectiveness of sand traps. If data indicates this is a viable flood risk mitigation solution, then additional funding and partnerships could be sought to expand the concept. We are excited to start this project and seek solutions to reduce flood risk.
For More Information
For more on how on sediment traps work, see this presentation found on the EPA site about a project in Michigan. It’s not directly analogous to south Texas because rainfall rates, soil types and gradients differ. But helps explain the theory of traps.
Click here to see Freese & Nichols’ full scope of work on the sediment trap project on the San Jacinto.
Posted by Bob Rehak on 4/3/2020
948 Days after Hurricane Harvey and 197 since Imelda