Deformation of Breakwater Roundheads under Construction

Deformation of Breakwater Roundheads under Construction

Curto Balbela, V.

Verhagen, H.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2014-04-04

Breakwaters under construction are prone to undesired deformation because the breakwater core is not designed to withstand severe or moderate wave loads. The reshaping mechanisms for a complete (finished) breakwater have been comprehensively studied; however, few research studies have been undertaken to analyze the deformation mechanisms of the breakwater core (or a breakwater under construction). It is imperative for the contractor to understand how the reshaping takes place because reshaping requires construction schedules to be adjusted as well as additional material and therefore, additional costs. The present study focusses on the deformation rubble mound breakwater roundheads undergo during the construction phase. Mulders (2010) performed physical model tests for breakwaters under construction exposed to wave attack. To investigate the deformation processes, twelve scaled physical model tests were performed by varying the wave height (8cm for low and 10cm for high wave heights), wave angle (0°, 30°, and 45°), and material grading (fg= 1.3 for narrow and fg=6 for wide grading). The data from Mulders’ tests was used in this study to analyze the breakwater deformation, with emphasis on the roundheads, via volumetric changes (initial and final test conditions) for a given control volume. The control volumes included the overwash, the overall breakwater, and the roundheads using radial and angular sectioning methods. The radial sections depicted the cross-shore transport while the angular sections illustrated the longshore sediment transport at the roundheads. As expected, the overwash volumes were proportional to wave load and angle of wave attack. Regarding the breakwater overall behavior, wide graded material caused a volume increase due to the transverse movement of the coarse fractions while the narrow graded material (fine fraction) decreased in volume thus, increasing the packing density. This sorting mechanism was also noticeably present in the roundheads for the radial sections. For the roundheads, distinctive percentage losses (sectors with low resistance to wave loading) were clearly present in the radial sectors; thus, transverse (radially outward) sediment transport was the dominant deformation mechanism over longshore transport. Longshore transport did contribute to roundhead deformation; however, no maximum losses were found because the volume losses were nearly equal throughout the angular sectors. As a result, the angle of wave incidence did not affect the volume losses in the angular direction. With regards to material sorting at the roundheads, wide graded material caused a volume increase at the outer radii due to the transverse movement of the coarsest fraction potentially reducing the packing density. On the other hand, grading did not influence the roundhead negative percent changes in the radial direction; thus, grading appeared to be indifferent to volume losses but relevant for volume gains.

breakwater
roundhead
core
construction
deformation
grading
wave obliquity
reshaping

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Student theses

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(c) 2014 Curto Balbela, V.