Abstract
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.
Highlights
Many coastal dikes and seawalls constructed in very shallow areas in our surveys were seen to be damaged or destroyed by waves smaller than the designed waves
Silarom et al [9] proposed a numerical model consisting of CADMAS-SURF (Super Roller Flume Computer-Aided Design of Maritime Structure) and empirical equations for calculating the time change of the outflow rates from coastal dikes and seawalls with arbitrary shapes
The reason for using two stages was that the calculated outflow volume (≈3.7 m3/m) was large compared to the total amount of the backfilling materials (≈7 m3/m); coefficient Cd in front of the seawall could be changed during the simulation when the parameter d/dmax was smaller than 0.14
Summary
Many coastal dikes and seawalls constructed in very shallow areas in our surveys were seen to be damaged or destroyed by waves smaller than the designed waves. The results show that shear resistance decreases when the sand layer thickness in front of the structure becomes thinner and the outflow rate increases when the median grain size becomes finer Due to these results, the authors proposed an empirical method for predicting the failure of a dike or a seawall induced by outflow. Ioroi and Yamamoto [4] performed many experiments in order to determine the relationship between the outflow rate and the pore water pressure, the return flow velocity, and the median grain size of backfilling materials They proposed an empirical outflow formula that could consider the effects of the median grain size, the sand layer thickness at the front of the structure, the maximum pore water pressure, and the maximum return flow velocity. A hydraulic model, the numerical model of Silarom et al [9], and the empirical formula of Ioroi et al [10] were used
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