Damage measurements and images on low-crested breakwaters
Description
Low-crested and submerged structures (LCS) help stabilize beach nourishment and shorelines to minimize visual and water circulation impacts. In an era of climate change, it is essential to understand LCS resilience. There are gaps in the understanding of damage progression in LCS. This study's objectives were to i) explore the importance of LCS stability variables, ii) extend the existing datasets with a data collection campaign, and iii) formulate a damage progression model on an extended dataset. A scale model testing programme added 124 new data points and confirms the importance of relative crest height, period, increased vulnerability of the seaward slope, and crest and damage progression. The proposed model predicted damage similar to the pooled data and offered insights into the importance of i) seaward slope erosion, ii) drag and lift forces, iii) Shield’s stress relationships to relative depth, stone size, Reynolds number and gradation, and iv) non-linearity of damage progression.
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The specific density of the crushed angular quarry stones was determined from four samples to be 2422 kg/m3. Porosity was estimated from six samples for the two classes of stones, green stones A (red stones B), to be 0.431 (0.472). Gradation surveys on A and B determined Dn50 (M50) of 18 and 29 mm (15 and 29 grams) with D85:D15 of 1.25 and 1.6, respectively. A pair of smaller (18mm) and larger (29mm) stone sizes test sections were built adjoining in the flume, hand-packed and the slopes checked with a template of the various hc/h required, similar to Garcia and Kobayashi (2014) (figure 2). A buffer of 1 stone diameter was used to minimize edge and wall effects and is consistent with other physical model studies (Ranasinghe et al., 2009). An Omey ® flume 9.0 meters long x 0.6 meters wide and 0.6 meters deep was used for the experiments (figure 2). There was a wave attenuator at the end of the flume to reduce the reflected waves. A series of tests with a cross-section crest width of 0.10 meter (~4 or more stone widths), side-slopes of 1:1.5 and 1:2.0, constant water depth (h) of 0.10 meter and hc/h from 1.2 to 0.5 were examined. A Brettschneider spectrum generator was used throughout the experiments with an array of two resistance-type wave gauges to measure waves at the deep-water and the toe of the structures. Photogrammetry is used here because of access to technology. Calibration points or ground control points were used after draining the flume after each run. Firstly, the damage was measured by first determining the average erosion cross-section area and then equating it to the damage number (S). Photographs were taken after each test interval (with eight control points) and subjected to digital terrain modelling using Pix4D. The errors in erosion measurements were estimated to be less than 1mm from the digital terrain model error report. Three unique deep-water wave conditions were explored from 0.043 to 0.071 meters and resulted in three unique nearshore depth-limited wave conditions. Transitional and shallow water depth limited wave conditions with h/L ratios of 0.16 were determined for all three nearshore waves. Hs exceeded the breaking criterion (Hs>Hb), with wave heights of 0.09, 0.0616 and 0.052 meters in 0.1 meters of water depth, respectively. Nearshore waves were determined to be depth-limited waves. Viscosity effects in scale model experiments are a concern and were avoided herein. This study was limited to homogenous LCS and used a Re criterion of >10,000.