Bolivar Roads Gate System
Coastal Texas Study
The Threat
The Texas coast is extremely vulnerable to hurricanes and tropical storms that continually place our coastal communities and natural resources at risk. These powerful storms threaten lives and jeopardize the health and welfare of our nation’s economy and as well as our unique coastal environment. The purpose of this study was to identify the most vulnerable areas of the coast and determine how to best mitigate the impact of future storms.
We’ve used computer-generated storm models to capture the threat of flooding to our coastal zones. Based on our analyses, hurricanes heading our way push enormous amounts of water through the 2-mile wide Bolivar Roads inlet into the Galveston Bay, flooding the coast as well as the mainland.
Click on the map to zoom in and explore the flooding potential in your community. Click and drag the slider to see the potential flooding With the project and Without the project.
The Recommended Plan
To address the coastal storm surge threat, we’ve developed a multiple-lines-of-defense strategy for the coast. Rather than just focusing on a single feature, we’ve developed a broader, more holistic solution that combines man-made protection elements with coastal restoration projects that work in conjunction with each other to protect the coast. As a first line of defense, the Recommended Plan outlines a Bolivar Roads Gate System made up of a combination of sector gates, vertical lift gates, and shallow water environmental gates that tie-in to the shoreline with combi-walls and levees (see examples of these below).
The gates are intended to remain open year-round to maintain navigation and existing tidal flow between the Gulf and the Bay. The gates will be closed when a storm surge event threatens the Galveston Bay region. When closed, the Bolivar Roads Gate system significantly reduces risks to the region by reducing surge into the Bay by 30-60% depending on a hurricane’s track and intensity. Throughout the year, when the gates are open, our analysis shows that less than 10% of the tidal exchange into and out of the Bay is impacted. The gates also will be closed for needed maintenance a few times each year.
Click on the map to explore the Bolivar Roads Gate System. Use the layer list on the right side of the map to open and close the sector gates and to explore the storm models.
Floating Sector Gates
Currently, at the heart of the Bolivar Roads Gate System lies a pair of gates spanning the Houston Ship Channel. To improve navigation safety, enhance reliability, and reduce project cost, the Recommended Plan calls for the dredging of a second navigation channel to the east of the existing channel, and the construction of three man-made islands to anchor and house two pairs of large fan-shaped, hollow floating sector gates. The channel openings will be 650 feet wide by 60 feet deep (the current channel is 530 feet wide by 48 feet deep).
Normally, each gate will sit in a dry dock (underwater chamber) in its respective island, but when a storm is approaching, the dry docks will be flooded, allowing the gates to swing out and float into place. Water will then be pumped inside the hollow gates, which will cause them to sink down to the bottom and close off the channel.
Each gate will be 82 feet tall, which is almost eight stories high. When closed, 60 feet of the gates will be hidden below the water, with only 22 feet of the gates visible above the water. Once the storm has passed, the water will be pumped out of the gates, and they will be floated back into the artificial islands for storage.
To the right and left of the large gates, the Recommended Plan includes two pairs of smaller sector gates to allow for smaller vessels, such as commercial fishing vessels and recreational boats, to safely move into and out of the Bay. These gates will have 125-foot openings and be approximately 62 feet high (with a depth of 40 feet). When the gates are open, they will be stored in a gate bay (underwater chamber).
Vertical Lift Gates
To fill the gaps, the Recommended Plan calls for a series of 15 Vertical Lift Gates – each 300 feet wide and spanning more than 4,500 feet (the size of more than 12 football fields). These gates will remain open except in the event of a storm.
The gates will be leaf-shaped (elliptical) and will open using hydraulic cylinders with a long piston attached to the oval-shaped towers.
These gates will be suspended between individual concrete towers rising more than 100 feet into the air (a little more than 9 stories high).
The size of the individual gates will vary depending on the water depth, as it is deeper closer to Galveston Island (-40 feet) and shallower as you move towards Bolivar Peninsula (-20 feet). An access bridge will be built on the Bay side of these structures to allow maintenance crews access for repairs.
Shallow Water Environmental Gates
On the Bolivar Peninsula side of the inlet, some areas are as shallow as five feet deep. In these instances, vertical lift gates will not work. In addition to this constraint, we also knew that small marine animals use these shallow waters to move in and out of the bay. So we had to come up with a design that will work from an engineering perspective, and avoid and minimize impacts to these critical species. The study team designed a new type of gate referred to as “Shallow Water Environmental Gates” or SWEGs.
These SWEGs are large box-shaped culverts, with vertical sliding gates inside, that drop down to close. There are six boxes per SWEG, so when measured across they are each 16 feet wide and 16 feet long. Sixteen SWEGS are needed to cross the 1500-foot shallow water section on the east side of the inlet. An access road will be built across the top of the SWEGs to give operators access for maintenance and repairs. The road will be made from stainless steel industrial grating which will allow sunlight to pass through into the water below, which is imperative for marine life.
Tie-In Structures
To fill in the gaps and create seamless protection across the inlet, there must be a connector from the gate system to the shore on the Bolivar Peninsula side of the inlet. This will be a 22-foot-high combi-wall that will run in front of the Fort Travis area for 5,000 feet.
A combi-wall is a type of floodwall utilized in deep waters made from a combination of deeply embedded heavy columns or piles and partially embedded lighter piles that offers high resistance in the face of forcings from surge and waves.
The combi-wall will connect to a levee system, which is an earthen structure that has a trapezoid shape and is typically covered with native grasses. The levee will run northwest to Hwy 87, turn and run along the highway, and then turn south and tie in to the beach and dune system running along the front of Bolivar Peninsula. The levee system will be approximately 3 miles long.
Anchorage Areas
Just inside the north jetty on the Gulf-side of the inlet, there are three designated anchoring areas for large ships to drop anchor and wait for provisions, personnel, and approvals to move into the channel.
The Recommended Plan will impact these areas (Anchorage B will be unusable, and Anchorage A will be moderately impacted), leading us to explore mitigation opportunities to restore anchorage capacity elsewhere. This was done by coordinating our efforts with stakeholders (pilots, ports, the Coast Guard), and evaluating the space itself. We will assure these large ships have the necessary amount of space to swing around on their anchors.
Click on the map to explore the Anchorage impacts and the mitigation options in the Recommended Plan.
