ABSTRACT

Dynamic pressures caused by breaking water waves on a recurved model seawall have been investigated. The study was conducted in a two-dimensional wave flume using a model recurred seawall fixed with transducers to measure wave pressures. The magnitude, location and distribution of dynamic pressures were determined from the experiment.

The shock and secondary pressures were found to be dependent on water depth, breaking wave height and the size of the air pocket. The transducers nearest the still water level recorded the maximum pressure. The secondary pressures on the transducers near the recurred section of the wall consistently showed negative pressures. The study also investigated the validity of several wall shock pressure formulas used to predict the shock pressure. The maximum shock pressures on the recurred wall were found to be less than those experienced on a vertical seawall.

INTRODUCTION

As a breaking wave contacts the face of a seawall, it causes an initial shock (or impact) pressure of large magnitude and short duration immediately followed by a secondary (or surge) pressure of lesser magnitude and longer duration. There is no single formula used for determining the shock pressures, hence there are no recurred sea walls subjected to these dynamic pressures. It is assumed that the dynamic pressures on a recurred wall follow existing vertical wall pressure formulas.

The purpose of this study was to analyze the dynamic pressures on a recurred seawall caused by breaking waves. This research was conducted with the following specific objectives:

  1. To examine the magnitude of the shock and secondary pressures on a recurred wall,

  2. To estahlish the location of the maximum dynamic pressure, and

  3. To examine the validity of theoretical and empirical formulas.

Impact pressures have been studied extensively with regard to vertical walls, but it has been only in the last decade that research has been conducted with recurred walls. It is extremely difficult to formulate analytical solutions to the problem of breaking waves on a seawall. Therefore it was necessary to rely on scale model test. The data obtained for the dynamic pressures in this study were compared with the results of other s only by use of their theoretical formulas.

The research was conducted by experiment in a two-dimensional wave flume, using a model recurred seawall fixed with transducers to measure the dynamic pressures. Twenty four tests were conducted with wave heights ranging from 0.58 to 0.98 m for wave periods ranging from 1.5 to 2.25 secs.

STUDY REVIEW

Table 1 shows the formulas derived by researchers to determine the magnitude of impact pressures on vertical walls.

Bagnold (1939) theorized that the short duration shock pressures resulted from the rapid compression of thin air pocket trapped between the face of a breaking wave and the wall. The most widely used formula for predicting shock pressure on vertical walls due to breaking waves was derived by Minikin (1950).

The maximum force acted at the SWL and then decreased parabolically from SWL to zero at a distance of 1/2 the breaker height above and below the SWL.

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