Numerical Model for Predicting the Sand Outflow Rate of Backfill Materials From a Coastal Dike

Abstract

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.

The Reproduction Ability of a Numerical Model for Simulating the Outflow Rate of Backfilling Materials from a Coastal Structure

“…In summary, when the inflow rate is very small compared to the outflow rate, the flow velocity of Equation (3) can be specified as the return flow velocity, and thus the outflow rate formula can be expressed as shown in Equation (6). For the proportional coefficient, β, Silarom et al [9] proposed an empirical equation, Equation 7, to calculate the coefficient by using experiments and simulation tests.…”

“…The D-F method can adequately consider the effects of pressure attenuation due to either precast concrete armor units (e.g., hollow tetrahedrons, tetrapods, and hexapods), spherical or cubic concrete blocks, stones, or pebbles. By using this model, the time history of the pressure and velocity inside the backfilling materials of a structure can be calculated and then used for the outflow rate calculation; however, the calculated pressure distribution inside a covered coastal dike (the front slope and a crown part are covered by concrete, and a leeward slope is covered by asphalt or concrete) and a covered seawall (the front slope and the crown part are covered by concrete) filled by very fine materials seems to be overestimated because this model cannot sufficiently consider the effects of fine grains (D 50 < 1 mm) [9,10]. The reasons for this limitation are as follows:…”

“…To handle this problem, Silarom et al [9] studied the pore water pressure distribution inside a dike model and the changing of the pore water pressure for the dimensions of a cave by using pore pressure meters installed in the dike. The positions of the pore pressure meters include a dimensionless inner horizontal length (the inner horizontal length/the horizontal length at the lowest edge of the dike) of 0, 0.067, 0.2, and 0.677.…”

“…These models are limited by the shapes of structures. Following this, 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.…”

“…Thus, some experiments were performed in order to examine the effects of these parameters on the outflow. In the present examinations, a hydraulic model, the numerical model of Silarom et al [9], and the empirical formula of Ioroi et al [10] were used.…”