Beach Morphology behind a Single Impermeable Submerged Breakwater under Obliquely Incident Waves
ROSHAN SUMINDA RANASINGHE and SHINJI SATO
sumrosh77@yahoo.com


Small-scale physical model tests were conducted to investigate the morphodynamics of sand beaches of uniform grain size, with a single impermeable submerged breakwater under obliquely incident waves. Primary objective was to assess the performance of a single submerged breakwater in a littoral drift beach under relatively long period waves. Different patterns and mechanism of the currents behind these structures were identified on both fixed and movable beds using particle tracking velocimetry (PTV) technique.

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Fig.1: Current field in the vicinity of a submerged breakwater using PTV on fixed bed

The physical model tests showed consistent results for the beach morphology and the subsequent shoreline response with the geometry and the position of the submerged breakwater under the given wave condition.

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Fig.2: Bottom topography and current field (b) after 60 minutes and (c) after 150 minutes

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Fig.3: Definition sketch for submerged breakwaters

Submerged breakwaters found to be performing well under obliquely incident waves, as long as the longshore sand transport is bypassed with artificial beach nourishment.

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Fig.4: Bottom topography at the (a) initial sand nourishment and (b) after 120 minutes

Attempts have been made to formulate relationships between shoreline evolutions and parameters associated with waves and structures. All the cases were analyzed the following relationship was derived in general for all submerged conditions. Figure 5 shows the relationship between (YB-Ya)/Ls and Ls/YB at all submergences. This results the data in the form:
(YB-Ya/Ls)=0.7417(Ls/YB)-1.521 This yields to an interesting result that (YB-Ya)/YB=1 (that is there is no sinuosity) when YB/Ls=1.77; in other words if the breakwater is located a distance offshore greater than 1.77 times its length, it will not have any positive effect (advancement) on the shoreline on average for all the submerged conditions.

fig5
Fig.5: Dimensionless salient length vs. relative length of the structure

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