The Bolivar Roads Surge Barrier

Abstract

In response to the devastation caused by Hurricane Ike to the Galveston Bay Area, Texas, United States in September 2008, several proposals emerged to protect the region against future storm surges. One of them is a coastal spine called the ’Ike Dike’. A challenge for this 160 km [100 mi] long flood protection is to cross Bolivar Roads, a 2.8 km [1.7 mi] wide channel between Galveston Island and Bolivar Peninsula. A storm surge barrier is required to close the coastal spine and prevent hurricane surges in the Gulf of Mexico from penetrating into the Galveston Bay. This thesis, based on a global-to-detailed approach, presents the design process and a feasible design for a storm surge barrier at this specific site. The design process starts with drafting a design framework consisting of a program of requirements and boundary conditions. According to the requirements Bolivar Roads is divided in two sections: a navigational section spanning the deeper section of the waterway allowing the passage of vessels and an environmental section which aims to preserve the Galveston Bay ecosystem. The objective of this thesis is to make a conceptual design for a barrier along the environmental section to satisfy two requirements. Firstly to enable sufficient water exchange between the Gulf of Mexico and the Galveston Bay through keeping at least 60% of the original flow area open, and secondly to sufficiently reduce the effect of storm surges with an estimated probability of occurrence of once in ten thousand years. The first design step considers the barrier as a system. It reveals that the large retention capacity of the Galveston Bay brings an opportunity to construct the barrier more cost-effective: it does not have to be fully retaining. A closed barrier with a continuous retaining height of only 0.1 m [0.3 ft] above mean sea level reduces the storm surge sufficiently. With this retaining height along both the navigational and environmental sections the barrier is vastly overflown during storms, but the Bay’s retention capacity ensures the flood hazard along the Galveston Bay shores to remain acceptable. Knowing the required retaining height and having a sense of the local conditions, several barrier types for the environmental section are evaluated. A shallow founded caisson barrier meets the drafted design criteria best. During this second design step a preliminary design for a caisson barrier is drafted to find the required main dimensions. The last part of this design step simulates the barrier on its final location through a settlement calculation. This reveals that the subsoil under the caissons is not able to bear the weight at all: the soil settles up to 3.7 m [12.1 ft] in depth, which is unacceptable. An alternative foundation method is therefore required and proposed. In the third and last design step measures are investigated to improve the soil friction capacity and to deal with the settlements. Four alternatives for a foundation are drafted: 1) a shallow foundation using vertical drainage as soil improvement, 2) a shallow foundation with vacuum preloading as soil improvement, 3) a deep foundation through steel tubular piles and 4) a shallow foundation through replacing the entire weak clay stratum with sand. The sum of foundations costs and caisson construction costs for these alternatives are estimated at $675, $800, $823 and $897 million respectively. For this caisson barrier design, a shallow foundation with vertical drainage as soil improvement is advised. This third design step provided an important insight: the drafted design criteria in the second design step, on which the choice for a caisson barrier was based, were unbalanced. There should have been more emphasis on the foundation as it is a large portion of the total costs for the environmental barrier (± 40%). It subsequently results in a two-fold recommendation. If the decision is to continue the caisson barrier design it is advised to apply vertical drainage. Alternatively, the suggestion is to reassess the barrier types in design step 2 while taking into account that the more expensive deep foundation method is likely more appropriate for Bolivar Roads. The report proceeds with the construction method for a caisson barrier with vacuum preloading soil improvement. Despite its relatively high costs, further investigating the application of this method is preferred as it has not yet been applied before for a large, shallow founded structure like a storm surge barrier. The construction method shows the consecutive activities up until the completion of the structure, together with a cost estimate. The cost estimate for the complete storm surge barrier in Bolivar Roads ranges from $2.7 to $4.0 billion. This includes the navigational section. In this thesis the focus lies on foundation design. For further design it is recommended to focus on the barrier ?doors, the bed protection and the adaptability to uncertain sea level rise. These are, next to the foundation aspects, important cost drivers for the storm surge barrier. For a complete integrated design the construction method should also be fully included in the design process, as for example the construction dock is linked to the optimal barrier type.