Topic 2.2 Project 4 SASME Book of Abstracts DHI 1 Morphological Modelling Around Detached Breakwaters Julio A. Zyserman, Ida Brøker, Kim Jørgensen, Erik D. Christensen and Jacob Hjelmager Jensen Danish Hydraulic Institute, Agern Allé 5, DK-2970 Horshølm, Denmark email: jaz@dhi.dk Introduction The morphological response behind a single detached breakwater has been investigated by application of DHI’s 2DH coastal area morphological modelling system (Johnson et al., 1994). The influence of the geometry of the structure on the morphological response was initially investigated by systematically varying the length of the surface-piercing breakwater and its distance to the coast. Afterwards, the morphological response to a single submerged breakwater was analysed by changing the submergence of the structure’s crest while keeping all other geometrical properties unchanged. The effect of diffraction and directional spreading on the simulated response has also been investigated. A quasi-3D description of sediment transport (Elfrink et al., 1996) has been incorporated in the sediment transport module of the morphological modelling system. Preliminary results are shown. Analysis of Surface-Piercing Breakwaters The morphological response was investigated through a series of 8 tests in which the length L and the distance of the structure to the coast X were varied according to Table 1 below. X 80 is the distance from the shoreline within which 80% of the total littoral transport takes place, and can be interpreted as a measure of the width of the surf zone. Table 1. Definition of test cases (X 80 = 240m) Test X (m) L (m) X/X 80 L/X 80 L/X Response KM1 120 312 0.50 1.30 2.60 Tombolo KM2 240 312 1.00 1.30 1.30 Tombolo KM3 360 312 1.50 1.30 0.87 Transitional KM4 480 312 2.00 1.30 0.65 Salient KM5 600 312 2.50 1.30 0.52 Salient KM6 360 192 1.50 0.80 0.53 Salient KM7 360 432 1.50 1.80 1.20 Tombolo KM8 360 552 1.50 2.30 1.53 Tombolo Irregular unidirectional waves were applied in all the tests, with root-mean-square height H rms = 2m, peak period T p = 8s and direction of propagation MWD = 10° at a water depth of 10m. The initial bathymetry consisted in all tests of a plane beach with slope 1:50. The sediment was characterised by d 50 = 0.20mm and σ g = (d 84 /d 16 ) 0.5 = 1.4. The morphological evolution was simulated for a minimum period of 9 days for each test. The predicted planform was either classified as tombolo, salient or intermediate, and the results compared to empirical formulas found in the literature. It was concluded that the morphological response predicted by the morphological modelling system is in good agreement with the predictions from empirical criteria, see Zyserman et al. (1998) for more details. The predicted type of response has also been listed in Table 1. The dependence of the morphological response on the geometrical characteristics of the structure was investigated through the total volume of sediment trapped by the structure after 9 days. The results are plotted in Figure 1 as a function of the distance to the coast for structures of constant length (tests KM1 to KM5) and in Figure 2 as a function of the length of the structure (tests KM3, KM6, KM7 and KM8). The labels correspond to the value of the L/X ratio for the particular test.