City of Houston Mayor Pro Tem Dave Martin’s office has supplied ReduceFlooding.com with the Black & Veatch Engineering report on the recommended alternative for adding floodgates to Lake Houston. One key finding immediately jumped out at me that wasn’t in Martin’s press release last week. The recommended gates would have a release capacity that virtually equals the highest release rate of Lake Conroe during Hurricane Harvey.
The Lake Conroe release rate during Harvey maxed out at 79,000 cubic feet per second (CFS).
The eleven tainter gates recommended by Black & Veatch would have a release rate of 78,700 CFS.
New Possibilities, More Certainty
That opens up a world of possibilities. For instance, the City could wait to start releasing water until it knew water was coming downstream from Harvey.
Said Martin, “Once constructed, we can release with a moments notice which gives us great opportunities to coordinate release protocols with the SJRA!!”
Previously, Public Works has been reluctant to release water in advance of a storm because the release rate of the existing gates is so small. They have to start lowering the lake so far in advance of storms that a storm can veer away before it gets here. If it does, that means water has been wasted.
The recommended floodgates should provide much more certainty for operators and avoid waste.
Key Elements of Recommendation
Other key elements of the recommendation include:
Locating the floodgates in the earthen eastern portion of the dam near the old channel of the San Jacinto River.
Creating baffles and a dissipation basin downstream from the new gates to break up the flow and reduce water velocity
“Outdenting” the gates (i.e., building them in front of the current dam)
A bridge between the two parts of the earthen dam
Using tainter gates, the same type used at Lake Conroe.
A 3.5 year construction schedule.
The last point means that if construction started in January, the earliest completion date would be mid-2026.
Posted by Bob Rehak on 12/14/22 based on the Black & Veatch Report
1933 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2022/12/20220722-RJR_0779.jpg?fit=1200%2C800&ssl=18001200adminadmin2022-12-14 19:42:472022-12-14 19:45:58Recommended Floodgates Could Release at Rate of Lake Conroe During Harvey
According to minutes of the Coastal Water Authority (CWA) February 9 board meeting posted in March, work on the project to add more gates to Lake Houston was paused in January while the City of Houston updated the project’s benefit-cost ratio.
CWA Board Minutes Give High-Level Overview of Concerns, Status
Earlier, in December, the board learned that the project team was trying to get the benefit/cost ratio above 1.0, so benefits exceed costs.
At that time, the CWA hoped to receive the updated BCR later in January. But it still hadn’t happened by the February board meeting.
That leaves a lot of questions regarding this project.
History of Project
After Harvey, the Lake Houston Area Task Force identified adding additional gates to Lake Houston as one of three primary strategies to reduce flood risk in the Lake Houston Area. The idea: to equalize the discharge rates of the flood gates on Lake Houston and Lake Conroe. Conroe’s is 15X greater. That makes it difficult lower both lakes quickly in advance of approaching storms.
Back in October 2020, the engineers calculated that the upstream influence of the dam ended at approximately Lake Houston Parkway. But they never explained why. It would seem that if the influence extended upstream to US59 when the lake is at its normal level, that the influence should extend at least that far in a flood. However…
BCR Not Based on Harvey Damage
Much of the damage to the Humble/Kingwood Area during Harvey happened upstream of the West Lake Houston Parkway Bridge. It included:
$60 Million to Kingwood College
$70 million to Kingwood High School
$50 million to Kingwood County Club
283 homes in Barrington
218 homes in Kingwood Lakes
97 apartments in Kingwood Lakes
110 homes in Kings Forest
100% of businesses in Kingwood Town Center
225 homes in Kingwood Greens
30 homes in Deer Cove
3 Homes in Deer Ridge Estates
32 homes in Trailwood Village
An unknown number of homes in Forest Cove
78 townhomes in Forest Cove
All of the Big Box stores along 59
Homes and business north of Deerbrook mall
40% of all businesses in the Lake Houston Chamber
Humble ISD admin building
Destruction of US59 southbound lanes
Union Pacific Railroad Bridge
However, Black & Veatch does not base its benefit-cost ratio calculations on another Harvey. They’re basing it on 25- and 100-year storms. Almost all homes, businesses and infrastructure near the lake are already above those levels – at least based on pre-Atlas 14 standards. That may explain the difficulty and delays with benefit/cost ratio calculations and the multitude of scenarios examined.
Time, Uncertainty: More Factors to Consider in Cost and Risk Reduction
Each flood-risk reduction alternative would reduce lake levels by a different amount during a 100-year storm and therefore require its own BCR.
Other factors to consider: How much time do dam operators really need to lower Lake Houston? And how much uncertainty are they willing to live with?
Given the desire to preserve water, these are crucial considerations. If forecasters can reliably predict a need to lower the lake two days before a storm instead of one, operators may only need half the number of new gates. That could get the cost down to the point where the benefit-cost ratio needs to be.
Posted by Bob Rehak on 4/4/22 and updated on 4/5/22 with MAAPnext timetable
1679 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2021/07/Gates-Side-by-Side.jpg?fit=1200%2C400&ssl=14001200adminadmin2022-04-04 17:20:252022-04-05 13:47:16City of Houston Re-evaluating Benefit-Cost Ratio on Lake Houston Gates Project Alternatives
At one of the first large public meetings since Covid began, several hundred people crowded into the Kingwood Community Center last night. They came to see the City unveil floodgate and dredging plans for Lake Houston. Stephen Costello, PE, the City’s Chief Recovery Officer, addressed dredging. And Chris Mueller, PhD, PE, of engineering firm Black & Veatch discussed adding more floodgates to the Lake Houston Dam. Mayor Pro Tem Dave Martin coordinated the meeting.
Scope of Long-Range Dredging Plan Still in Development
A long-range dredging plan for Lake Houston is critical. We must understand where the sediment comes from, how fast it builds up, where it builds up, and the consequences of not removing it periodically.
Costello says the City is currently working with affected homeowner associations to discuss cost-sharing arrangements.
He also says that the City must identify a long-range site for depositing the spoils that is suitable for hydraulic dredging. He called the mechanical dredging now in progress “not sustainable.” Currently, the City is using Berry Madden’s property on the West Fork south of Kingwood’s River Grove Park to deposit the mechanical dredging spoils. That’s a long haul for barges on the East Fork.
Next Dredging Steps: Channel to East Fork and East Fork Itself
Contractors must next deepen the channel between the West and East Forks of the San Jacinto to move dredging equipment and spoils back and forth (see below).
From there, dredgers will move slightly north of where Luce Bayou (far right) enters the East Fork and begin dredging the East Fork mouth bar. See large circle above. The map shows that area grew shallower by up to nine feet between 2011 and 2018. Imelda, in September 2019, made it grow even shallower. Note the fresh deposits of sand in the photo below now poking up above the water.
Additional Floodgates for Lake Houston Dam
Chris Mueller of Black & Veatch then discussed the reasons for adding additional floodgates to Lake Houston, preliminary engineering findings, and an implementation schedule.
The primary objective: to increase the outflow capacity of the dam to reduce the risk of future flooding. However, he emphasized that reducing the risk for people upstream of the dam cannot have an adverse impact on people below it. See below.
He emphasized that Lake Houston is, first and foremost, a drinking water reservoir. He also emphasized that the dam is almost seventy years old and near the end of its useful life. Significant safety issues exist in working with such old concrete.
Calculating the Benefit/Cost Ratio of Additional Floodgates
Mueller then explained how FEMA calculates the benefit/cost ratio of additional floodgates.
On the benefit side, it considers: the reduction in water surface level; how many buildings and streets that will prevent from flooding; reduced societal impacts; and reduced impacts to business revenues. These are primarily damage costs avoided.
On the cost side of the equation, FEMA factors in construction costs and annual operation and maintenance costs.
The peak inflow to Lake Houston in a 100-year storm: 286,000 cubic feet per second (CFS), enough to fill the Astrodome in 3 minutes! However, during Harvey, SJRA estimated the peak inflow at 400,000 cfs.
Proposed Alternative Produces 11-Inch Benefit Nearest Dam
A hydrologic and hydraulic analysis conducted by Black & Veatch will help prove up the benefit/cost analysis. The San Jacinto Watershed (including Buffalo Bayou) includes flow from eight counties.
In evaluating about ten alternatives for adding floodgates, Black & Veatch considered both cost and non-cost factors listed below.
The company’s first choice was to install additional gates on the earthen portion of the dam on the east side. But environmental considerations there would have delayed the project by a decade or more.
So they decided to recommend a 1,000 feet of crest gates on the west side of the spillway instead. See example of crest gates in operation below.
Such gates would increase the discharge capacity to 45,000 cfs, more than four times the current capacity of 10,000 cfs. That’s still only about a third of the discharge capacity of the floodgates on Lake Conroe. But according to Martin, that would still be enough to lower the level of the lake 4 feet in 24 hours.
However, before floodgate construction can begin, engineers must evaluate:
Downstream impacts and how to mitigate them
Impact to the stability of the existing concrete dam
Back in the 1950s when the Lake Houston dam was built, engineers did not use rebar. So this will be a delicate operation. Contractors must cut 6 feet into the existing spillway; cap the remaining concrete with a slab; and install the crest gates on top of the slab.
Black & Veatch must also develop an operations protocol for new floodgates that maximizes upstream benefits and limits downstream impacts. Mueller shared this schedule with attendees.
Best-Case Project Timeline Shows Completion in 2024
A best-case scenario shows construction starting at the end of 2022 and finishing before the start of hurricane season in 2024. So, at least three more hurricane seasons to get through before seeing any benefit from additional gates.
On Thursday, July 8, Mayor Pro Tem Dave Martin will host a pubic meeting to discuss the status of adding more floodgates to the Lake Houston Dam. Preliminary engineering finished earlier this year. In March, the Coastal Water Authority board approved Black & Veatch to begin final engineering.
Need for More Gates
The Lake Houston Area Task Force identified more and higher capacity floodgates as a key element in the area’s flood-mitigation strategy. The current gates have one-fifteenth the capacity of those at the Lake Conroe Dam. That makes it difficult to shed water from Lake Houston before people flood if Lake Conroe opens its gates as it did during Harvey.
The main presentation by Black & Veatch, the project engineers, will be followed by a short Q&A session. The meeting will then transition into breakout sessions. Breakout tables will let residents engage with project management staff and engineers in small groups to ask more detailed questions.
The Lake Houston Dam Spillway project will increase the outflow capacity of the Lake Houston Dam. The project proposes installing new crest gates in the existing uncontrolled spillway. This will allow for a rapid decrease of water levels in Lake Houston in advance of storm events to prevent or reduce upstream flooding. Engineers estimate the recommended alternative could help about 35,000 residents and 5,000 structures. It’s important for people to understand that if they flooded from streams or channels far from the lake during Harvey, this may not help them.
A Federal Emergency Management Agency (FEMA) Hazard Mitigation Grant Program provides $4.3 million for engineering and positions the city to receive another $42.7 million for construction.
Posted by Bob Rehak on 7/6/2021 based on info provided by Dave Martin’s Office
1407 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/02/LakeHoustonDamDuringHarvey.jpg?fit=1500%2C968&ssl=19681500adminadmin2021-07-05 23:31:172021-07-06 10:16:41Reminder: Floodgate Meeting at Kingwood Community Center on Thursday, July 8
In its March 10th board meeting, the Coastal Water Authority (CWA) accepted the recommendation of a preliminary engineering report to add one thousand feet of crest gates to the uncontrolled spillway portion the Lake Houston Dam. The additional release capacity would let operators shed water faster before, during or after major storms to reduce the risk of flooding.
At the March meeting, directors also approved $4.4 million to begin Phase II of the project. Phase II calls for Black & Veatch to proceed to final engineering of the gates and a coffer dam to protect the work area during construction.
This morning, at its May board meeting, directors received an update on the progress of Phase II work to date and plans for the remainder of the project.
Start of Phase II Engineering Approved in March
In the March meeting, CWA approved funds to begin Phase 2 of the engineering which includes the final design of the selected alternative by Black & Veatch. The selected alternative was “crest gates” constructed on the uncontrolled spillway portion of the dam on its west side. (See below.)
Dam operators can raise or lower crest gates from a bottom hinge, much like the lid on a piano. When in the up position, gates hold water back. When lowered, they release water.
Also in March, CWA and Black & Veatch completed negotiation of the scope and fee for the final design. The key deliverables during Phase II will include:
Plans to modify the spillway to support the 1,000 linear feet of crest gates (in five 200-foot long sections)
Design of the cofferdam system to protect the work areas during construction
Preparation of a new gate operations plan for CWA Lake Houston Dam operators.
Director Douglas Walker moved to authorize the Executive Director to issue a contract amendment with Black & Veatch Inc. in the amount of $4,465,727.00 for “Phase 2 – Final Design of the Lake Houston Dam Spillway Improvement Project.” Director Giti Zarinkelk seconded the motion. The Motion carried unanimously.
In the May board meeting this morning, directors received an update on Phase II work to date and plans.
In April, the design team held a number of workshops and coordination meetings.
Black & Veatch also completed three weeks of field surveying of the existing spillway; that’s why CWA temporarily lowered Lake Houston during that period.
Support City of Houston in a public outreach meeting scheduled for June 17. The public outreach meeting will coincide with the public comment period for the permit application. CWA expects permitting to take nine months, i.e., through March of 2022.
Complete final design by the end of September 2022.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2021/05/20210512-Screen-Shot-2021-05-12-at-10.23.26-AM.jpg?fit=1200%2C670&ssl=16701200adminadmin2021-05-12 12:28:352021-05-12 15:19:42May 2021 Gate Project Update for 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.
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
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:
Hydrology and Hydraulic Modelling
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 Impact Statement
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
Indirect and Cumulative Impacts
Geology, Hydrology and Drainage
Coastal Zone Management Wildlife and Endangered Species
Essential Fish Habitat
Existing Facilities and Utilities
Traffic and Circulation
Waters of the U.S., including Wetlands
Cultural Resources (historical and archaeological)
Visual/ Aesthetic Appeal
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
Construction costs, including any environmental mitigation
Long-term operation and maintenance costs
Risks in design, construction, and operation.
From all the feasible options, engineers will then chose the three best based on:
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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/05/20191104-RJR_4790.jpg?fit=1200%2C800&ssl=18001200adminadmin2020-05-01 12:22:242020-05-01 15:57:06Preliminary Engineering Starts for Adding More Gates to Lake Houston Dam