Investigating Wave Transmission over Submerged Reefs in front of Sandy Beaches as a Nature-Based Coastal Protection Measure

Abstract

The functionality of existing coastal defence structures designed for depth limited wave conditions are at risk due to climate change. Sea level rise could result in stronger wave loads acting on the shore thus reducing the effectiveness of existing coastal structures. Therefore climate adaptation measures are required to ensure that interventions to protect the coastline do not become obsolete. A viable option is to reduce the wave loads before they arrive to the shore. This can be achieved through either increasing the foreshore via sand nourishments or through the construction of offshore breakwaters to attenuate wave energy. In recent times, sand nourishment has become increasing popular as a soft engineering solution to preserve coastlines. However sand nourishments are not permanent and more sediment has to be deposited at regular intervals depending on the rate of erosion at the coasts. Costs of sand nourishments may also rise to unsustainable levels if sea level rise result in elevated rates of erosion. The construction of artificial reefs in front of nourished beaches brings a twofold benefit: they promote marine life and biodiversity whilst simultaneously acting as submerged breakwaters to dissipate wave energy before it reaches the coast. The effective design of a dual nature based system consisting of a nourished beach and artificial reefs require proper and accurate understanding of the hydrodynamic processes that occur at the beach and in the coastal waters. It would then be critical to know how the waves would interact with the artificial reefs and subsequently the beach. This study built on previous research to expand the knowledge of the behaviour of waves and their interaction with artificial reefs. Through physical modelling tests, the study investigated whether the distance between the artificial reef and the beach would affect the extent of wave energy dissipation by such structures. This would be quantified by the wave transmission coefficient. The influence of this same distance on structure induced wave setup between the reef and beach will also be studied. Physical modelling tests were conducted in a wave flume , with a submerged artificial reef tested at three positions relative to a nourished beach, located at distances of 4.80 m, 9.20 m, and 13.70 m from the shoreline. A total of 132 test cases were carried out, varying the significant wave height (𝐻𝑚0 = 8– 20 cm), wave steepness (0.015 ≤ 𝑠𝑚−1,0 ≤ 0.040), and relative freeboard (−1.875 ≤ 𝑅𝑐/𝐻𝑚0 ≤ 0.625). Wave transmission and structure-induced wave setup were analysed and compared with existing empirical formulas. The combined use of a permeable low crested structure with the distance between the structure and the beach as a variable, to study wave transmission and structure-induced wave setup, represents a configuration which has not been clearly addressed in existing literature. This study has thus provided novel insights into this area. The results indicate that, within the tested range of conditions, the distance between the artificial reef and the beach does not have a significant influence on the wave transmission coefficient 𝐾𝑡 nor on the structure-induced wave setup 𝛿/𝐻𝑚0. The relative freeboard 𝑅𝑐/𝐻𝑚0 was identified as the dominant parameter governing wave transmission. It was concluded that within the tested conditions, designers of artificial reefs placed in front of nourished beaches need not place significant attention on the reef-to-shore distance as a primary design parameter. Instead, greater focus should be placed on the local freeboard, wave climate, and reef geometry, which are the parameters shown to govern the hydrodynamic effectiveness of such structures.