In the case of coastal dikes and seawalls, since big waves break before reaching these structures, the frequency in which they are broken directly by strong wave force is low. On the other hand, with continuous impact of middle waves, scour arises in these fronts. Before long, incident waves infiltrate into these bodies, backfilling materials flow out, and caves in these bodies become big. Owing to these phenomena, the frequency of destruction of these dikes and seawalls becomes high. Therefore, the development of methods for predicting scour rates in these fronts and sand outflow rates of backfilling materials is useful. Regarding scour rates in these fronts, there are already some accurate methods that specialists can use under various conditions. However, for sand outflow rates of backfilling materials, there are some prediction methods limited to simple types of dikes and seawalls. In this research, the authors develop a numerical model that calculates the flow velocity and pressure in the dike or seawall with arbitrary form using "CADMAS-SURF." The model can predict the time change in the sand outflow rate and the development of the cave. First, for a sand outflow rate simulation, the authors propose empirical equations to modify the pressure calculated by CADMAS-SURF using hydraulic experiment data. Then, they confirm the practical feasibility of the numerical model by applying it to field cases on sand outflow damage in Japan and Thailand.
In very shallow areas, the frequency by which coastal structures (like dikes and seawalls) are directly broken by large wave forces is low because large waves are broken in deeper areas. The main cause for such destruction is ground scour in front of the structures and outflow of backfilling materials by middle-scale waves; therefore, the scour and the outflow should be considered when designing a coastal structure in a very shallow area. In this paper, a numerical model consisting of CADMAS-SURF, which can calculate fluid motion in porous media, and empirical equations for simulating the outflow phenomena are introduced; thereafter, practical calculations on field cases in Thailand and Japan are demonstrated. Additionally, since the effects of wave periods and water depth to the outflow rate have never been clarified, hydraulic model experiments, empirical calculations using an existing formula, and numerical simulations are performed in order to examine these effects on the outflow rate. The simulated results using the numerical model align well with the experimental results. Moreover, both results show that the outflow rate is proportional to the wave period and inversely proportional to water depth.
There are many cases where a coastal dike or a seawall constructed on a very shallow area was broken by a wave of a smaller height than the designed height. In many of these cases, the reason of the destruction was the suction phenomena. This phenomenon happens when waves reaches the front of the dike or the seawall, the wave pressure scours the front soil of the structure until it reaches the lowest edge of the structure followed by sucking of backfilling material of the structure. In this research, the authors proposed the numerical model which can calculate the suction rate with elapsed time by applying the pore water pressure and the flow velocity inside a dike or a seawall using “CADMAS-SURF”. The authors proposed three coefficients for improving the calculated pressure from CADMAS-SURF. The reliability of the model was confirmed by reproducing the suction phenomena in Hirono Coast of Japan.
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