# Probabilistic design of the Land Barrier on the Bolivar Peninsula, Texas

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## Abstract

The Houston Metropolitan area is a hurricane prone area and vulnerable to flooding by large storm surges caused by these extreme storm events. The area is a major economic center in the State of Texas and even the whole United States. Protection of the area against flooding is extremely necessary to protect the economy, community and environment. Hurricane Ike (2008) is one of the many hurricanes ever to hit the coast of Texas and was a catalyst of the development of several flood protection strategies, such as the Ike Dike. In this report the Coastal Spine Strategy has been used as a reference strategy. The basic idea of this strategy is to shorten the coastline by implementing a moveable storm surge barrier at Bolivar Roads and Land Barriers at Galveston Island and the Bolivar Peninsula, which will reduce the inflow of water towards the Galveston Bay. The objective is to obtain a probabilistic design of the Land Barrier on the Bolivar Peninsula taking the overtopping failure mechanism into account. The possibilities of overtopping resilience are also explored. By designing probabilistically insight is gained on the uncertainties of the system and the failure probability of the design. The approach to obtain the objective is first describing the requirements. The most important one is the sufficient reduction of the inflow of water towards the Galveston Bay. Important boundary conditions are sea level rise, storm surge height and wave height. Three alternatives have been developed for the Land Barrier: ‘Natural’ approach, ‘Low’ design and ‘In between’. The first alternative is the highest, has little overtopping and the inner slope could be executed with a high performance turf mat. The second alternative, ‘Low’ design, has the lowest height. The overtopping discharge is extreme (>700 l/s/m) and therefore needs a stronger inner slope, which is executed with Elastocoast and a sandy cover layer. The third alternative is a compromise between those two alternatives. The ‘Low’ design was favored in consult with researchers in the area, since this alternative is assumed to fit the best in the flat environment. The design mostly deviates from a standard levee design at the inner slope due to the extreme discharges. Asphalt and Elastocoast are highly erosional resistant and cost-efficient, where Elastocoast is the most environmental friendly one and is therefore selected for the inner slope revetment. The Elastocoast revetment is assumed to withstand the erosional force, but the top layer could slide down the slope, exposing the subsoil. Also, behind the structure the excessive amount of water flows over unprotected soil leaving an erosion hole behind, which could undermine the structure if it becomes too deep. These two failure mechanisms have been expressed in limit states. The stilts of the houses create a weak spot if these interfere with the revetment on the slope, which is a point of attention during construction. It was not possible to make a reliable limit state for this mechanism. Another limit state is related to the ability to limit inflow towards the bay to prevent Kemah from flooding. These limit states are used in the probabilistic model to calculate the failure probability. The requirement is stated as a conditional failure probability given a once per 100-yr storm occurs. The base design is a levee with the highest point at MSL 5.9 meter. The outer slope, with a gradient of 1/4, and the inner slope, with a gradient of 1/6, is protected with an Elastocoast revetment. Behind the levee there is bed protection such that the scour hole will not undermine the structure and energy can be dissipated. Resilience is expressed with an additional conditional probability of failure given that a 500-yr storm occurs. Three resiliency measures are added to the design, namely; purely heightening, a milder slope with a small heightening to 500-yr storm surge level (MSL + 6.7 m) and a combination of various measures. A fragility curve is a nice tool to present failure probabilities given a certain storm surge height. It is interesting that the fragility curve is shifted to the right when resilience is added to the design. The first conditional probability of failure (for the 100-yr storm) becomes unnecessary. The total failure probability is reduced when the measures are added to the design. This means that resilience is an effective method to reduce the total failure probability. It is possible to design a low Land Barrier where overtopping discharges are very high. The inner slope needs to be protected with a revetment and bed protection needs to be applied to prevent scour to undermine the structure. This design can be improved if research is conducted on the real flow behavior behind the levee after the transition to the horizontal. Also, the actual strength of Elastocoast against overtopping discharges needs to be found in combination with the sliding induced by the force of the water over the layer. Investing in research on this topic can reduce the total costs of the Land Barrier. Next to that, an optimization can also result in an improved design. Applying a berm and milder slope to reduce overtopping discharge could give a better solution. A simplified 1D model, based on a rigid-column approximation, showed that the contributions of the flow through Bolivar Roads and the overland flow are comparable in the current situation during a 100-yr storm. Large storm surges result in a more dominant overland flow. The Coastal Spine will limit the inflow towards the bay the most compered to four other scenarios. But the Coastal Spine is not going to keep sea level rise out of Galveston Bay and the wind during hurricanes will push the water level towards the shoreline of the bay. It is advised to raise awareness of the vulnerability against flooding and heighten the existing structures (flood walls, quay walls, levees) in time. The simplified model can also be improved with a more complex model where bathymetry, wind speed and direction is included. The assumption to model a Land Barrier as a weir holds. Resilience is a measure to improve the reliability of a structure. There are some investments needed to upgrade the inner slope for example, but the total probability of failure is reduced and therefore improve the design. It is assumed that a resilient structure has a milder slope for the fragility curve, but measures to truly milden this slope were not found in this research. A cost-benefit analysis on adding resilience would be a next step for investigation as well as finding true measures to milden the slope of the fragility curve.