New Orleans storm surge barrier

Abstract

After New Orleans was struck by hurricane Katrina on the 29th of August a consortium including Iv-Infra was in a tender, which involved the design of a storm surge barrier at the east side of the city of New Orleans. This consortium designed a piled-supported structure including a concrete superstructure. The superstructure consists of a perforated inclined wall in front of a vertical backwall. The inclined wall has an energy dissipating function and the vertical backwall does have a water retaining function. However, there is no standard theory to determine the optimal configuration of the superstructure having such a configuration. To find the optimal configuration total insight in the occurring hydraulic processes in front of and inside the superstructure is a requisite. In this research an analytical analysis has been performed for the water movement in the basin of the superstructure; for the wave run-up and wave overtopping over the perforated inclined wall; for the emptying process of the basin of the superstructure; for the wave induced inflow through the gap at the bottom of the inclined wall and for the length spreading effect, which is the spreading of the extreme overtopping rates in longitudinal direction of the basin. In the analytical research a reduction factor was found for the influence of the gap in the inclined wall on the wave run-up and wave overtopping. The reduction factor is equal to the reflection coefficient and has to be implemented in the formula for a non-perforated inclined wall. In this analytical research it was also stated that the inclined wall did not influence the water movement in a different way than a vertical wall should do. The water level fluctuation would resonate for a basin width of 0,5L. Physical model tests have been performed to solve the encountered uncertainties, which were encountered during the analytical research. Besides solving the encountered uncertainties verifying the analytical results was an objective of the physical model testing. From the physical model testing it has become clear that the gap has no influence on the wave run-up, however it influences the wave overtopping. An explanation for this phenomenon is that the overtopping time decreases when a gap is implemented in the inclined wall. By comparing and combining the results obtained by the analytical research and the physical model testing the hydraulic processes have become clear and a numerical model combining these different hydraulic processes has been constructed. The numerical model provides one a tool to calculate the overtopping volumes over the vertical backwall; the outflow through the gap and the overtopping volumes over the inclined wall. It is not possible to perform an automatic optimization with the model. One has to check different configurations whether they fulfil the imposed criteria or not. The most important criteria are that the structure has to be as low as possible and that the overtopping discharge over the vertical backwall is not allowed to be larger than 0,1l/s/m. A configuration consisting of an 1:1 sloped inclined wall with height of 8m and a gap of 1m at the bottom of the wall and a vertical backwall with the same height with a distance of 7m from the inclined wall based on still water level is the result of using the numerical model. The water movement in the basin and the wave induced inflow are not included in the numerical model. However, from the analytical research it is known that the water level fluctuation for a basin width of 7m does not resonate. Therefore the water movement is included in the determination of the configuration.