Preliminary Engineering Starts for Adding More Gates to Lake Houston Dam
In early April, the Coastal Water Authority (CWA) quietly finalized the scope of work for engineers working on adding more gates to the Lake Houston Dam. Engineering firm Black & Veatch’s contract was approved. And their work has now begun. Here’s what it involves.
Background: Why More Gates?
After Hurricane Harvey, a pilot study by HCFCD and Freese & Nichols showed additional gates could have helped lower floodwaters.
The current gates on Lake Houston’s dam have one fifteenth the discharge capacity of Lake Conroe’s – 10,000 cfs vs. 150,000 cfs.
Additional gates could help synchronize the release rates of the two dams and thus reduce flood risk. More/bigger gates could lower the Lake Houston faster in advance of a storm and add width to the spillway during a storm. Both help reduce flooding.
Avoiding Unnecessary Releases
Currently, it takes several days to lower Lake Houston enough to significantly reduce flood risk. During that time, approaching storms can veer away or dissipate. So a conservation angle exists here, too. More gates release water faster. That lets CWA wait until weather-forecast certainty is higher before lowering the lake. And that, in turn, helps avoid unnecessary discharges and conserve water.
18-Month Project Starting from April 8th
The addition of gates is a three-year project broken into two 18-month phases.
Phase 1 involves preliminary design of conceptual alternatives, selecting the “best” based on criteria described below, and permitting.
The clock for Phase 1 started ticking on April 8, 2020, the day Black and Veatch’s contract was approved. Phase 1 should conclude in September 2021.
Phase 2 involves final design and construction. Assuming all goes well, we could have more discharge capacity at the Lake Houston dam by March 2023 at the earliest. However, there will be an evaluation period between the two phases that could push the completion date out further. Also…
Phase 2 Depends on Outcome of Phase 1
One objective of Phase 1 is to prove up the concept, the budget, and the benefit/cost ratio.
Proceeding to construction in Phase 2 will depend on the outcome of Phase 1. In Phase 1, engineers will examine several possible designs to determine the most effective alternative. They will consider flood reduction benefits, downstream impacts, cost, environmental impact, constructibility and more.
Then FEMA will evaluate the benefit/cost ratio of the winning design to ensure it meets or beats initial projections in the grant request.
- If it does, FEMA will release money for Phase 2, the final design and construction.
- If it doesn’t, the whole project could die.
FEMA does not guarantee Phase 2 funding at this time.
What Happens Now?
The scope of work document reveals who will do what in the next 18 months on the Lake Houston Spillway Improvement Project (LHSIP).
Objective: To relieve upstream flooding by increased discharge capacity that supports pre-releases.
Modifications could include (but are not limited to):
- Additional crest gates on or adjacent to the existing dam or…
- New, as-yet-unspecified hydraulic structures that provide for releases elsewhere on the embankment
Unlike tainter gates which swing up from a radial arm, crest gates swing down from a bottom hinge.
The project will consider both upstream benefits and downstream impacts.
After defining alternatives and constraints, the contractor, Black & Veatch, will analyze the alternatives to quantify and compare costs and benefits of each configuration.
Five Major Tasks in Phase 1
Preliminary engineering involves five major tasks:
- Management plans
- Hydrology and Hydraulic Modelling
- Field Investigation
- Development of alternative concepts
Let’s look at each.
Black & Veatch will begin Phase 1 by developing project-, quality-, and risk-management plans.
Hydrology and hydraulics (H&H) studies will evaluate the ability of the various concepts to reduce upstream flooding and downstream impacts. Black & Veatch will develop H&H models that combine both the San Jacinto River and Buffalo Bayou basins to evaluate downstream impacts of any dam.
The combined model will extend all the way to Galveston Bay and evaluate design alternatives for up to nine events:
- 2-, 10-, 50-, 100-, and 500-year storms
- Extreme historic events (e.g. Harvey, Ike or Memorial Day), including at least one with storm-surge effects
- A hypothetical Probable Maximum Precipitation event.
The process includes collecting, reviewing, adjusting and validating existing models before performing simulations.
To save time, permitting will begin concurrently with design. The permitting schedule is aggressive and may spill over into Phase 2 as details are refined. Permitting includes (but is not limited to) coordination with federal, state and local agencies for:
- Environmental Assessment
- Environmental Impact Statement
- Wetland delineations
- Threatened and endangered (T&E) habitat assessment
- T&E species-specific surveys
- Migratory Bird Treaty Act (MBTA) surveys
- Freshwater Mussel survey
- Stormwater pollution prevention
- Clean Water Act
- Flood Plain Construction
The environmental team will also consider:
- Topography and Soils Construction Impacts
- Land Use
- Indirect and Cumulative Impacts
- Geology, Hydrology and Drainage
- Sediment Quality
- Air Quality
- Invasive Species
- Coastal Zone Management Wildlife and Endangered Species
- Essential Fish Habitat
- Existing Facilities and Utilities
- Noise Quality
- Traffic and Circulation
- Waters of the U.S., including Wetlands
- Environmental Justice
- Cultural Resources (historical and archaeological)
- Visual/ Aesthetic Appeal
- Water Quality
- Hazardous Materials
Black & Veatch will also conduct site surveys and a geotechnical investigation, complete with borings, to evaluate soil conditions, depth-to-water, permeability, and seepage control.
A bathymetry team will measure water depth and develop contour maps for an area that extends 500 feet upstream from the dam.
Preliminary Engineering/Conceptual Design
Finally, preliminary engineering will develop conceptual layouts and site plans for several alternatives.
This exercise will also evaluate areas of impact, site access and utilities, staging and borrow areas, dewatering extents, existing structure tie-in, general facilities layout, and downstream channel alignment.
These site plans will be used for costing and evaluating the feasibility for each alternative.
Criteria for Choosing Best Alternatives
The engineers will also develop an evaluation matrix that includes, but is not limited to:
- Ability to meet project goals
- Environmental clearance
- Construction costs, including any environmental mitigation
- Long-term operation and maintenance costs
- Benefit/cost analysis
- Risks in design, construction, and operation.
From all the feasible options, engineers will then chose the three best based on:
- Upstream impact
- Downstream impact
- Environmental impact
- Permitting requirements
Timing on Phase 2
Assuming we get to Phase 2, the second 18 months may not start immediately. FEMA will need time to evaluate Phase 1 results. And the CWA will need to develop bid specs, bid the job, select a winner, and develop a contract with a scope of work, just as they did for Phase 1. That could talk several months and push completion well into 2023.
For More Information
All that, just to figure out what to do! To read the full 27-page Scope of Work, click here. I will also post this document for future reference in the Reports page of this web site under a new tab titled Lake Houston Dam Spillway Improvement Project.
Posted by Bob Rehak on 5/1/2020
976 Days after Hurricane Harvey