Document Type : Research Paper
Authors
1 Assistant Professor, Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran
2 Ph.D. in Hydraulic Structures, Sahand University of Technology, Tabriz, Iran
3 Associate Professor, Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran
Abstract
Introduction
The construction of various structures for coastal protection is one of the strategies for dissipating wave energy in coastal areas. Permeable and floating protective structures are among the new coastal protection designs proposed to address the shortcomings of previous structures. In the design and construction of such structures, piles and pipes are used as their foundations. Evaluating the pattern and extent of erosion around piles caused by wave interaction is essential for designing such structures. Therefore, in addition to structural design, hydrodynamic loading on the coastal protection structure and its foundation due to wave impact must be considered as a key criterion for foundation design in shallow waters. In coastal areas, waves undergo various processes such as wave refraction, wave shoaling, and wave breaking, which result from nonlinear interactions with the seabed. Moreover, energy transfer between different wave frequency components during the breaking process is highly nonlinear. Understanding the hydrodynamics involved is considerably more complex compared to breaking waves in deep waters. This complexity becomes particularly significant when wave breaking occurs exactly at the pile foundation, as the stability of permeable coastal structures depends on the stability of their foundations against erosion caused by marine currents.
Materials and methods
To evaluate the scour depth around cylindrical piles of coastal protection structures under impact pressure caused by wave breaking, an experimental study was designed. The experiment aimed to analyze how variations in wave characteristics such as wave height and period affect the research objectives. It is important to note that this study focused on breaking waves impacting the structure. The experiments were conducted in a two-dimensional wave channel at the Coastal Engineering Laboratory of the Soil Conservation and Watershed Management Research Institute. To create shallow water conditions and ensure wave breaking at the pile location, as well as to assess the resulting scour depth, a sloped surface and sediment reservoir were constructed in the central part of the main channel. The sediment reservoir, with a depth of 0.35 meters, was installed upstream of the sloped surface and filled with sand sediments. A cylindrical pile was placed at the center of the sediment reservoir. The wave channel was filled with water to a depth of 0.4 to 0.5 meters, and waves with varying heights and periods were generated using a wave paddle system. Trial and error was used to determine the exact location of wave breaking in relation to the pile. A total of 34 experiments were conducted under initial water depths ranging from 0.4 to 0.5 meters. Wave heights ranged from 0.05 to 0.14 meters, with wave periods between 2 to 7 seconds. After each experiment, the scour depth at the pile location was measured.
Results and discussion
In this study, wave characteristics, including wave height, breaking wave velocity, wavelength at the breaking point, breaker crest distance from the structure, and water depth at the breaking zone, as well as the maximum scour depth (S), were measured. Based on the measured values, the Keulegan–Carpenter number (KC) and the S/D ratio were determined. The results showed that as the KC number increases, the S/D ratio and scour depth also increase. Specifically, with increasing KC, the length of lateral vortices along the sides and behind the base of the cylindrical structure extends, exposing a larger portion of the seabed to scour and erosion. A regression analysis was established between these parameters, and the findings were compared with previous studies. A comparison of 34 experiments in the present study with the results of Sumer et al. (1992) indicated that the scour depth under wave-breaking conditions near the coastal protection structure was approximately 1.14 to 8.46 times greater than the scour depth caused by regular waves passing through the structure’s vicinity. On average, the scour depth due to wave breaking at the pile location was 2.4 times greater than that under normal wave conditions (Sumer et al., 1992). Additionally, comparing the results of this study with Dogan (2021) revealed that the scour depth under wave-breaking conditions near the coastal protection structure was approximately 1.31 to 2.85 times greater than the scour depth caused by regular waves. On average, the scour depth due to wave breaking at the pile location was 2.03 times greater than under normal wave conditions (Dogan, 2021).
Conclusions
In general, when coastal currents interact with waves, if the waves do not break, the depth of scour decreases. However, if the wave in question propagates in the direction of the coastal current and breaks, the increased turbulence and resulting shear stress near the seabed at the breaking zone lead to an increase in scour depth. If wave breaking occurs at the site of the coastal structure, the extent of erosion and scour will exceed the values reported in previous studies. The results of the present study indicate that wave breaking causes approximately a 2.4-fold increase in scour depth compared to the passage of regular, non-breaking sea waves around a cylindrical pile structure. Therefore, marine structure designers must carefully consider this issue. An inaccurate estimation of the scour depth at the foundation of coastal protection structures based solely on the theory of regular wave passage near a cylindrical pile may lead to incorrect foundation depth calculations, affecting the overall stability of the structure. If the findings of this study are not taken into account, instability conditions could arise for the marine structure, especially when wave breaking occurs near the foundation.
Keywords
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