San Jacinto Flood Planning Group Releases Draft Recommendations
The Texas Water Development Board’s Region 6 San Jacinto Flood Planning Group has released the first draft of its recommendations. You can download the full 295-page Volume One document here (executive summary and all chapters). But the vast majority of the document focuses on methodology and research design. For convenience, I’ve extracted Chapter 5, the 35-pages that discuss recommendations, and summarized them below.
The draft recommendations include:
- Almost $200 million of additional studies, analysis, models and mapping
- $27.9 billion in projects.
The projects spread throughout the entire watershed. But here, I’ll focus on those in the northern portion of Harris and the southern portion of Montgomery Counties for brevity.
The Flood Planning Group recommends five projects in Halls Bayou totaling $99.65 million, all in collaboration with Harris County Flood Control District (HCFCD). They include:
- Channel conveyance Improvements on several tributaries
- Stormwater detention improvements near Hardy West
- Stormwater detention and channel conveyance improvements along the main stem.
These projects had a positive 1.46 Benefit/Cost Ratio PLUS additional community benefits hard to quantify. They would remove the floodplain from more than 3,000 structures and benefit more than 9,300 people. See pages 5-14 through 5-16.
White Oak Bayou
The Flood Planning Group recommends five channel improvement and detention basin projects for $120 million along White Oak Bayou. The flood planning group determined a benefit/cost ratio of .80 for these projects, meaning costs exceeded benefits. Regardless, they feel there are many community benefits that cannot be quantified. They include removing flood risk from seven miles of roads. See pages 5-15 through 5/18.
Greens Bayou would receive $120 million of improvements (construction costs only). They include projects in Fountainview Sections 1 & 2, Castlewood Sections 3 & 4, North Forest, Mid-Reach Greens, Parkland Estates, and Humble Road Place.
A bypass channel under the railroad that parallels US 59 could reduce upstream water surface elevations during extreme events. And a mitigation basin downstream would absorb any adverse impacts in Parkland Estates and Humble Road Place from the bypass channel.
The BCR for all Greens Bayou improvements equals 2.13, meaning benefits double costs. More than 20,000 individuals and 2,000 structures would benefit. See pages 5-18 through 5-20.
San Jacinto River
The Flood Planning Group recommends numerous projects associated with the East and West Forks of the San Jacinto River and their tributaries. It based these recommendations on the San Jacinto River Regional Watershed Master Drainage Plan and a 2018 LiDAR study. See pages 5-21 through 5-31.
Recommendations include channelizing part of Caney Creek and offsetting that with two dry-dam detention basins: one at FM1097 and the other at SH105. Together, they would store more than 40,000 acre feet of stormwater. That’s enough to hold a foot of stormwater falling across 62.5 square miles! Channelization would occur near the confluence of Caney Creek with the East Fork. That’s near Lake Houston and East End Parks. The projects would remove 42 miles of roadway and 2,422 structures from the 1% annual chance floodplain.
A 48-ft tall concrete dam would create a 1.60-mile-long earthen impoundment that captures runoff from Winters Bayou. The dry dam would have five reinforced 10×10 concrete culverts and twin 300′ backup spillways. It would cover almost 2,500 acres and hold 45,000 acre feet of floodwater. That’s enough to hold a foot of stormwater falling over 28.8 square miles.
Lake Creek would receive some channelization and two dry-dam detention basins holding 37,250 acre feet of storage, enough to hold a foot of stormwater falling over 58 square miles.
Recommendations also call for partial channelization and two dry-dam detention basins along Peach Creek.
- The Walker Detention basin would occupy 1,200 acres, hold 36,000 acre feet of stormwater, and cost $200 million.
- The SH105 Detention basin would occupy 3,000 acres, hold 36,000 acre feet, and cost $400 million.
- The total 72,000 acre feet of capacity would hold a foot of stormwater falling over 112.5 square miles.
This project would channelize 15.7 miles of stream at I-45 and through the Woodlands. It would also create two detention basins on Birch and Walnut Creek tributaries to help reduce flood risk downstream. Together, the projects would create more than 35,000 acre feet of floodwater storage capacity, enough to hold a foot of rain falling over 54.8 square miles. The report did not break out the costs.
The Flood Planning Group recommends widening and channelizing 5.7 miles of the West Fork near Highway 242. They would create 12,400 acre feet of mitigation storage by widening the river to 750 feet and creating a 2-foot bench above the stream bed. That would involve shaving down the floodplain to 2 feet above the waterline.
Farther downstream, in the Kingwood Area, they would also increase conveyance by widening a 5-mile-long stretch of the West Fork with 3,500-foot wide of benching. This project would require 923 acre-feet of mitigation storage
That would increase total floodwater storage in both locations by 13,423 feet – enough to hold a foot of rain falling across 20.9 square miles.
Is It Enough?
If all these detention basins get built, they could hold a foot of stormwater falling over 337.5 square miles upstream from Kingwood. That’s a lot. In conjunction with other strategies such as dredging and adding more floodgates to the Lake Houston dam, they should help reduce flood risk in the Lake Houston Area … if they aren’t negated elsewhere.
Other portions of the recommendations stress the need for additional strategies. They include but are not limited to:
- A regional approach to flood mitigation
- Floodplain preservation
- Natural solutions
- Minimum building setbacks
- More stringent building codes
- Better drainage regulations
- Uniform regulations across the watershed
- Adoption of standards for determining “no adverse impact”
Also note, that these recommendations would take decades to implement and that many would need to be implemented in a specific order. For instance, the State would need to build detention upstream before widening channels downstream. One helps mitigate the other. Without that, you could help people upstream, but hurt people downstream. That flies in the face of HCFCD principles.
To see the locations of all these streams and how much water they conveyed during Harvey, click here.
Posted by Bob Rehak on 8/8/22
1805 Days since Hurricane Harvey