Second Outlet Channel and Gated Structure

Second Outlet Channel and Gated Structure
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Second Outlet Channel and Gated Structure

Located near SH 146, just north of the Clear Lake natural outlet at Kemah and Seabrook, the second outlet channel and gated structure were constructed as part of the 1986-authorized

Clear Creek Federal Project. This version of the project called for 14 miles of channel conveyance improvements from FM 1959 to Clear Lake, so the second outlet channel and gated structure were constructed to mitigate higher flows from those planned channel conveyance improvements.

The Clear Creek Federal Project was updated in 2012 and does not include the same breadth of channel conveyance improvements that necessitated the construction of the second outlet channel, but since it helps reduce area flood levels produced from rainfall runoff, the Flood Control District continues to operate the gated structure.

second outlet channel map

Click image to expand

The Clear Creek Federal Project was updated in 2012 and does not include the same breadth of channel conveyance improvements that necessitated the construction of the second outlet channel, but since it helps reduce area flood levels produced from rainfall runoff, the Flood Control District continues to operate the gated structure.

Second Outlet Channel Purpose: Constructed to ensure no adverse impact and Clear Lake water levels would not be increased from planned channel conveyance improvements from FM 1959 to Clear Lake. While the latest version of the Clear Creek Federal Project no longer includes those improvements, the second outlet channel does help reduce flood levels produced from the rainfall runoff.

Gated Structure Purpose: Since the construction of the second outlet channel meant that the salty Galveston Bay waters would be able to freely flow into the fresh Clear Lake waters, the gated structure within the second outlet channel was constructed to minimize the changes to the existing hydraulic and environmental conditions in Clear Lake after the completion of the second outlet channel. These conditions include salinity and sediment transport and the rate of tidal inflows from the bay.

FACT: The gated structure does not, nor was it constructed to, provide tidal, hurricane or storm surge protection to lakeside or Clear Creek communities. In other words, the gated structure is NOT a seawall. The gate structure’s wall was constructed at approximately the same height as the surrounding land, so it is too small to serve as a seawall.

Gates closed (under water) during normal conditions

Gates open (above water) during Tropical Storm Allison (2001)

Opening the Gates

Decisions to open or leave the gates closed are based on the National Weather Service rainfall forecast when the fronts are approaching Harris County, or a tropical system is in the Gulf. When rainfall begins in the Clear Creek watershed, Flood Control District staff monitors the rainfall and resulting water levels in the channels and Clear Lake using Flood Control District and U.S. Geological Survey field gages. The lake level rise is estimated using actual and predicted rainfall, actual water levels in Clear Lake and upstream, and tide forecasts.

The gates are opened for three primary reasons:
  1. On a monthly basis to confirm operational readiness.
  2.  As needed for repairs and upgrades.
  3. When Clear Lake is forecasted to rise four feet above the average water level from rainfall. This is typically when the National Weather Service forecasts a rainfall event of four to six inches, or more, depending on size and location over the watershed. For smaller rainfall events, the gates remain closed.

Since the gated structure became operational in 1998, the gates have been opened 52 times for storm events (as of August 2021). In total, the gates have only been open for 2% of the time – 1.7% for these storm events and 0.3% for operations and maintenance. The rest of the time, the gates are closed. Historically, when the gates were opened for storm events, a majority of the storms did not result in enough rainfall in the Clear Creek watershed to raise Clear Lake high enough to cause flooding. However, the eight times there was structural flooding from these storms, the gates were opened, which helped reduce the flood duration and flood levels.

Closing the gates

When the gates are opened for a rainfall event, they resume their normal position, closed, after Clear Lake water levels are less than 4 feet above the normal water level. Gates remain closed when:

  • Clear Lake has high tides or rising storm surge
  • Clear Lake is less than 4 feet above the normal water level
  • Clear Lake is expected to exceed 8.3 feet above the normal water level
  • Galveston Bay is expected to exceed 6 feet above the normal water level
  • Sustained winds exceed 55 mph

Operations and Maintenance

Maintenance is an essential duty to ensuring the environmental stability of Clear Lake. The Flood Control District has a team of personnel who are responsible for day-to-day operations and maintenance of the gated structure, as well as responsibility for opening and closing the gates before and after storm events. Since 2002, the City of Seabrook has served as operations back-up in the off chance that the Flood Control District could not fulfill its duties.

On average, it costs approximately $1,000 a month to operate and perform routine maintenance. The cost for major repairs, rehabilitations, and improvements has been about $2.8 million. Total expenditures have been about $3 million (actual, not current dollars).

Inspections of the gated structure occur annually, and surveys of the second outlet channel occur periodically to check for channel bottom scour and sedimentation.

Routine maintenance

History

1986: The U.S. Army Corps of Engineers and the local sponsors, the Harris County Flood Control District and Galveston County, signed an agreement to jointly fund and construct the Clear Creek Federal Project. This agreement included 14 miles of channel conveyance improvements from FM 1959 to Clear Lake, so to mitigate these higher flows, the plans called for the construction of the second outlet channel and the construction of the gated structure to ensure the environmental and hydraulic status quos were maintained in Clear Lake.

1989-1991: U.S. Army Corps of Engineers constructs the gated structure.

1996-1997: U.S. Army Corps of Engineers constructs the second outlet channel and rehabilitates the gated structure.

1998: Operations and maintenance of the second outlet channel and gated structure are transferred from the U.S. Army Corps of Engineers to the Flood Control District.

1999: U.S. Army Corps of Engineers initiates a general reevaluation study of the Clear Creek Federal Project.

2003: To make certain that the gates will open or close during a power failure, a trailer mounted backup diesel powered generator with a quick connect was installed in the new onsite operations building.

2010: Post-Hurricane Ike improvements performed on the gated structure, including the installation of an all-weather concrete access road, removal of any potential jersey barriers that could block access due to storm surge, concrete power pole installation, removal and overhaul of all six gate motors and bevel gears, corrosion protection and paint on all 6 gates, replacement of lower and upper stem covers on all 6 gates, cleaning of stems on all 6 gates, and replacement of all sacrificial cathodic protection anodes.

2012: U.S. Army Corps of Engineers produces the 2012 General Reevaluation Report, the latest authorized iteration of the Federal Project. This version of the project no longer called for the same breadth of channel conveyance improvements, but the second outlet channel continues to reduce flood levels from watershed rainfall runoff.

2021: The Flood Control District performs an evaluation to assess the performance and benefits of the second outlet channel and gated structure to understand whether they are working as originally intended by the U.S. Army Corps of Engineers. The evaluation found that the channel and gated structure are operating as designed, and no physical or operational modifications are recommended.

After construction of the gated structure (completed 1991) and prior to the construction of the second outlet channel (completed 1997)

Facts

Second Outlet Channel
Length: 5,130 feet (almost one mile)
Bottom: 15 feet below water surface, 120-140 feet wide
Top: 200-240 feet wide

Gated Structure
Length: 141 feet Gates: Six roller gates, each 20 feet wide by 21 feet high
Powered by: Electric motors
Power source: Reliant Energy (backup diesel-powered generator)

Operations
Normal condition: All six gates closed
Potential to flood: All six gates open (no partial opening)
Time needed to open/close: 1.5 hours for all six gates (opened in pairs)

Gate Operators
Primary: Harris County Flood Control District
Back-up: City of Seabrook Public Works Department

Gates Status Check

To check the status of the Clear Creek gates at any time, logon to the Flood Control District’s Flood Warning System at http://www.harriscountyfws.org/ and follow these steps: 

  1. Within the menu bar on the left side of the screen, scroll down to “Site by Location” under the “SITE SELECTIONS” section. 
  2. Click the downward arrow under “Site by Location” and select the first option: “100:100 Clear Lake 2nd Outlet @ SH 146.”  
  3. Mouse over the gage at SH 146 and select “More Information & alert signup” On this page, historical and real-time data on the Clear Lake and Galveston Bay elevations, rainfall amounts, and the gate status (open/closed) are available. You can also subscribe for real-time rainfall alerts on this page. 

Second Outlet Channel and Gated Structure in Clear Lake

PDF Presentation

Second Outlet Channel and Gated Structure in Clear Lake

Narrated recorded presentation

A200 Historic Evaluation Final Report

Clear Lake Second Outlet Channel and Gate operations evaluation