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.
To see both presentations, click here. Or see the summaries below.
Dredging: About Half Done
In late 2019, the Army Corps finished hydraulic dredging in the area south of the West Fork mouth bar. Then in early 2020, the City of Houston began mechanical dredging to extend the effort. In terms of the estimated dollars designated for dredging, the effort is about halfway done.
The last two rows on the chart above are estimates because they depend on bids currently in progress and a long-range plan not yet complete. The need for a long-term plan and maintenance dredging were identified early on by the Army Corps so that any benefits of dredging were not immediately wiped out by future sedimentation.
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.
Posted by Bob Rehak on 7/9/2021
1410 Days since Hurricane Harvey