Physical model tests with movable bed in front of seadikes

Abstract

Coastal erosion is a serious problem in several coastal areas in Vietnam. One of these areas is the district of Hai Hau, located in Nam Dinh province. This district suffers from severe coastal erosion, the coastline has been retreating for more than 100 years. Efforts have been made to stop this erosion which resulted in the construction of strong seadikes along almost the Hai Hau coastline. These seawalls are placed relatively close to the waterline. Therefore, during extreme conditions the waterlevels in front of the dike become high. Field observations have shows that during these extreme conditions scour holes are formed in front of these dikes. Since little is know about the influence of the geometrical dike properties on the scour development wave flume test have been carried out with a movable bed to gain more insight in this problem. These tests have been carried out in the Holland wave flume of the Hanoi Water Resources University. In addition the tests were simulated in the computational model DUROSTA. This model is developed to determine dune erosion under storm conditions. The model is extended so that it can include hard structures such as seadikes. The geometrical dike properties that are tested are: Dikeslopes. - Initial slopes in the seabed near the dikes. - Cylinders placed at the toe of the dikes. - Crownwalls placed on top of the dikes. The most important results following from this series of test are the influence of the reflection coefficient of the scour behavior. In the wave flume tests the two properties that are of main positive influence on the reflection coefficient are: Increasing dike slope. - Increasing wave period. - Increasing crownwall height. - An increasing reflection coefficient does not give larger scour depths as is to be expected from known literature about scour near seawalls. The area and length of erosion however does become larger. The influence of crownwalls on the reflections coefficient is governing over the influence of increasing wave period on the reflection coefficient. An other test properties that is of influence for a larger area of erosion is the initial sloping seabed. The modeling of the wave flume tests with DUROSTA give the following results: The performance of DUROSTA after 2 hours is quite accurate with the measured field experiments, DUROSTA underestimates the scour depth with an average of 4%. The average underestimation of the scour depth in DUROSTA after 4 hours of testing is 19%. In DUROSTA the influence of the wave period on the scour development is governing over the influence of increasing crownwall height.Civil Engineering and Geoscience