October 2020 Gate Update
On Friday, October 16, 2020, stakeholders in the Lake Houston Spillway Improvement Project met at the dam to review gate alternatives and progress on the project. The main item of interest: a review of options still under consideration to increase the outflow during major storms, such as Harvey.
History of Project
Shortly after Harvey, representatives from the Lake Houston Area identified “increasing outflow” as one of the main strategies to reduce flooding. Harris County Flood Control District even conducted a pilot study. Flood Control included $20 million for the project in the 2018 bond fund (CI-028). With matching funds identified, grants were then written.
FEMA approved the project. And in April of this year, the clock started ticking on the first of two phases.
Phase One Includes…
Phase One includes preliminary engineering and environmental permitting. It should take 18 months and is on schedule at this point. A key deliverable for phase one is verification of the benefit/cost ratio. But, of course, to determine that, you need to know the cost.
Alternatives Still Under Consideration
FEMA allotted 18 months for Phase 1. We’re six months into that. Work to date has focused on determining the optimal alternative. Five remain:
- Expanding the existing spillway by adding new tainter gates
- Adding a new gated spillway within the east embankment
- Creating a new uncontrolled spillway within the east embankment
- Building crest gates within the east embankment
- Developing crest gates within the existing spillway
Tainter gates lift up from a radial arm. The tainter gates on Lake Conroe have 15X more release capacity than the lift gates on Lake Houston.
Crest gates flop down from a bottom hinge.
The east embankment is a solid earthen area 2800 feet long east of the spillway and existing gates. Water cannot get over it in a storm. By adding various structures in this area, engineers could widen the current spillway capacity, allowing release of more stormwater.
Benefit/Cost Ratio Must Be > 1.0
The lake-level reduction benefits of these gate alternatives during major floods range up to roughly 8x. The costs also vary by roughly 4x. Those are order-of-magnitude, back-of-the-envelope estimates and far from final. Much hard work remains to develop tighter costs and tighter estimates of flood-level reductions. The latter will determine flood-prevention savings in a storm. And the benefits divided by the costs will determine the benefit/cost ratio.
In FEMA’s eyes, the benefit/cost ratio must exceed 1.0 to justify the project. Said another way, it must produce more benefits than it costs.
FEMA allotted 18 months for phase 1. We’re six months into that phase with a year left. Project partners expect results of the alternatives analysis before the end of the year.
Given the ballpark costs of some of these gate alternatives, we will need very tight estimates of the benefits.
Potential Benefits include:
- Upstream flood risk reduction
- Reduced maintenance (debris management) for CWA
- Improved Water Quality (post storm)
Potential Impacts include:
- Increased scour and erosion potential to wetlands downstream
- Increased water surface elevations to structures downstream
Benefit Cost Ratio = (Net Present Value of Benefits)/(Project Costs)
Project Costs = (Capital costs) + (Net Present Value of Operations and Maintenance Costs)
Major Tasks Remaining in Phase 1
Barring surprises, the preliminary engineering report is due in February 2021 and environmental permitting should be complete by the Fall of 2021. Other tasks that must be completed by then include:
- Hydrologic modeling of flows into and out of Lake Houston using the latest Atlas 14 data
- Hydraulic modeling of Lake Houston, its Dam, and the San Jacinto River downstream of the dam to Galveston Bay
- Calibration of models to historic storms
- Examination of upstream benefits to residents/businesses removed from flood impacts
- Examination of downstream impacts associated with additional flow release scenarios
It’s important to understand that not everyone who flooded in the Lake Houston Area did so because of the lake level. Some on the periphery of the flood flooded because water backed up in streams leading to the lake. If you got two feet of water in your living room, it doesn’t automatically mean a lake level reduction of two feet would eliminate your flooding by itself.
Phase 2 Still Not Certain
Assuming all goes well in the planning, accounting, and conceptual validation, FEMA will make a go/no go decision on construction at the end of next year or the beginning of the following year. Construction should take another 18 months.
Funding for this project comes from FEMA, Texas Division of Emergency Management, City of Houston and Harris County Flood Control. Other stakeholders include the Coastal Water Authority, Harris County, Fort Bend County, Baytown, Deer Park, and other communities adjacent to Lake Houston.
Posted by Bob Rehak on 10/17/2020
1145 Days since Hurricane Harvey and 394 since Imelda