In March, the San Jacinto River Authority (SJRA) awarded engineering firm Freese & Nichols a $250,000 work order for “Conceptual Design” of sand traps. Then in June, SJRA applied for a $200,000 grant from the Texas Water Development Board (TWDB) for “Preliminary Design” of sand traps on the West Fork. What’s this all about? How do the projects relate? Are they worthwhile?
West Fork 90% Blocked After Harvey
After Harvey, the Army Corps documented that the West Fork had become 90% blocked by sediment in places. That contributed to the flooding of thousands of homes and businesses. It also triggered a massive dredging program that is still ongoing. Finally, it launched a search for solutions that stretched all the way to Austin (plus, interestingly, Kerrville and San Antonio). See more below.
SB1824 Opens Door for Sand Traps
House Bill 1824 was introduced by State Rep. Murr from Kerrville and sponsored by Senator Flores from San Antonio. Approved last year, it allows SJRA and Harris County Flood Control District (HCFCD) 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 royalty payments to the state.
SJRA and HCFCD hope to mitigate flooding by constructing one or more “sand traps.” Their plan calls for partnering with Aggregate Production Operations (APOs) in the vicinity of the sand trap(s) to clean them out periodically. It’s not yet clear whether APO’s would do this for a fee, or do it for the sand. One thing is clear, at this point, however. APO’s don’t want to go far. All the locations under consideration are next to sand mines.
Freese & Nichols Already Underway with Phase 1
The first phase of the project, Freese & Nichols’ conceptual design, is currently underway. It includes:
- Evaluating potential sand trap locations and trapping effectiveness
- Developing conceptual sand trap designs
- Determining downstream benefits of potential sand trap solutions
- Recommending which site(s) to carry forward into preliminary engineering.
Phase 2 of Pilot Goes Further
The goal: to move forward with preliminary engineering on two sites, with the likely construction of at least one. This small scale effort, involving only one or two sand traps, is intended to act as a “pilot” before a more costly, full-scale program.
Preliminary results in the first phase indicate that the sand traps will likely be located along the West Fork.
However, to move beyond conceptual design, even on the pilot, SJRA needs more money to supplement local match resources.
If successful, the TWDB grant application for preliminary design will cover:
- Environmental permitting investigation
- Preliminary land acquisition efforts
- Geotechnical investigation
- 30% design efforts.
Need for Sediment Control of Some Sort
To date, more than 2.3 million cubic yards of material have been removed from the West Fork, at a cost of more than $90 million. An additional $30 million has been dedicated to continue these efforts. SJRA hopes sand traps will help protect that investment.
It seems, though, that reducing sediment coming out of sand mines might be a simpler, less-risky, more cost-effective solution.
Long-term benefits beyond the initial sand trap development “pilot” project are anticipated to potentially extend beyond the immediate benefit area.
Benefits include potential reduction of sediment load entering Lake Houston.
SJRA can not yet quantify the level of flood mitigation provided by the sand trap(s). However, restoring or expanding storm flow capacity could potentially remove structures from the floodplain, they say. The conceptual design phase currently underway will attempt to evaluate downstream hydraulic benefits.
If this abridged application succeeds, SJRA will need to submit a more detailed application. TWDB won’t report results on that until late this year.
Freese & Nichols should report the results of their conceptual study this September.
SJRA anticipates it can complete the preliminary engineering study in 18 months. However, construction will take longer – up to 36 months.
If results of this pilot project indicate that sand traps are a feasible and effective solution, a larger program in various locations throughout the basin could be implemented.
Preliminary cost estimates will be developed as part of the conceptual design phase currently underway. So SJRA has not yet determined a benefit/cost ratio.
However, the cost of dredging has proved substantial. And the cost of flooding during Harvey proved astronomical. Reducing those costs just 1% could easily justify the cost of this project.
We have too many unknowns at this point to pass judgment.
- How much will the project cost?
- Will sand traps be effective?
- Who will maintain a trap when a sand mine goes out of business?
- What will the environmental impact be?
- Will the traps accelerate erosion and jeopardize infrastructure such as pipelines and bridges?
- Is this the opening volley in an effort to begin large-scale river mining?
Scientific literature and news reports on sand traps generally indicate mixed results.
I will withhold judgement until I learn more.
Posted by Bob Rehak on 7/9/2020
1045 Days after Hurricane Harvey