Gravel beaches are often made artificially to protect coastal beaches from erosion as they are of high permeability and excellent ability in dissipating wave energy. In this study, a laboratory experiment has been undertaken to investigate physical processes of gravel beach profile under different irregular waves. The initial beach slope is 1:7 and the scaled gravel diameter D50 is 2 mm. The laboratory data on the gravel beach profile under different wave conditions were collected and then analyzed. It is found that: the equilibrium beach profiles observed are reflective and stepped profiles; the beach surface above the average low tidal level is net siltation and underwater found to be eroded obviously, while the sediment in the upper beach surface is shown to accumulate continuously; a combination of different water levels and waves has been shown to result in different transport rates of the gravel beach.
The coastlines of many countries around the world confront risk of erosion. At present, there are mainly two ways to prevent coastal erosion. The first is the ‘hard’ protection approach, such as building underwater embankment or concrete seawall along coast. The benefit of this method is to directly protect the coastline from retreating any more, however the disadvantage is that the coastline landscape is greatly affected. The second method is the ‘soft’ coastal protection approach, such as beach nourishment with sand, which can create a beautiful beach with the beach sediment property unchanged. Beach nourishment is usually adopted with the combination use of building hard coastal structures (seawalls, groins, breakwaters, etc.), but scouring nearby the structure toes commonly occurs due to sharp change of local hydrodynamics. Hypothetical we purely use sand to nourish the beach, then an extremely large amount of sand is needed to this end and local governments perhaps can not afford it (Pierluigi, 2003). Alternative ‘soft’ coastal protection is to use gravel sediment instead of sand as the nourishing sediments on the beach. The gravel beach, generally found near the bedrock coast, is a typical coastal morphology formed under the action of waves. Though not widely distributed in the world's coastal zones, yet they are extremely stable coastal morphological type. Gravel is defined as sediment with a diameter between 2 and 60 mm by the category of Udden-Wentworth (Udden, 1914; Wentworth, 1922), and its shape is uneven (Carter, 1988; King, 1972; Zenkovich, 1967). Generally speaking, there are four feasible sources of material in a gravel beach: erosion from sea cliffs, river input, seabed erosion and longshore transport. Wave breaking occurs when waves reach shore, which can lift coarse-grained seabed sediments and push them toward shore. When reaching the bottom of a beach slope, large-particle sediments accumulate at the water edge due to the decay of wave energy, while smaller ones continue to be transported to the beach; however, under the condition of storm surge, the large particulate sediment is quickly lifted and run to the top of the gravel beach, forming a tall gravel embankment and a morphological system with alternating grooves on the top of the gravel embankment. In addition, due to the limited scope of the ordinary wave and tide current, the upper part of the high-water line of the spring tide formed a steep ridge; after a storm, intertidal gravel changes as it is moved back and forth by wave currents (Carr, 1971, 1969). Due to the unique characteristics of gravel sediment, such as hydraulic roughness and permeability (Kobayashi, 1989; Van, 2000), the ability to dissipate a lot of wave energy naturally(Pierluigi, 2003), gravel beach is also an important form of coastal natural defense (López, 2018; Poate, 2013). Therefore, beach managers tend to use coarse sand or gravel as a substitute for sand to regenerate eroded beaches, beach nourishment with coarse-grained material or gravel is thus becoming more and more frequent (Mason, 2007). Previous practice has shown that gravel beach has a very positive effect in coastal protection work, however, there are few studies on the profile morphology of gravel beach (López, 2016). In the preliminary work, according to the Dean (1977) equilibrium profile principle and the profile analogy method (Cai, 2015), we carried out profile design for a gravel beach restoration project, and determined design parameters such as median grain size (D50=20 mm), beach slope (1:7), front elevation of beach berm (+3.8 m) and width of beach berm (10 m) (Fig.1). In this paper, a laboratory physical experiment has been undertaken to investigate the shape stability of the profile, and to study the shape evolution processes of gravel beach profile under different irregular waves and water levels.
