# Monitoring of Recovery Process at Yeongildae Beach, South Korea, Using a Video System
## 1 Introduction  
Beaches are important for nearby communities, as they provide areas for recreational activities. They are sources of tourism, and thus well-groomed beaches with a nice coastal environment attract many visitors, even from abroad. Therefore, maintaining these beaches in good condition is necessary to improve their value. Apart from economic reasons, beaches are also environmentally important because they are buffer zones between the ocean and land, protecting on-land facilities by mitigating wave energy. Because of this buffer role, beaches themselves are exposed to attacks of extreme waves under storms such as tropical cyclones. When the wave energy is released in the surf zone due to the breaking of waves, sediments in the beach face and the nearshore seabed are dynamicallytransported. Therefore, one of the serious types of damage as a result of storm attacks is beach erosion. Under energetic wave conditions, nearshore sediments can move offshore rapidly due to wave-induced underwater currents [1], and the shoreline can significantly retreat, even for a short time of hours or days [2; 3]. The volume of erosion is directly related to the maximum wave height during the storm, as the near-bed shear stress initiates sediment motion proportional to the square of the wave height, and more sediment moves under higher shear stress once it exceeds a critical value. In addition, the wave period that contributes to the power of storm waves and the wave direction that influences the distribution of wave energy along the shore are important factors in the amount of eroded sediment. In addition to the wave parameters, the duration of a storm [4] or a sequence of consecutive storms [5; 6] could play a significant role in the erosional process. Once a storm event ends, onshore sediment motion prevails, under mild wave conditions, due to wave nonlinearity such as skewness and asymmetry [7], which results in the natural process of shoreline recovery following storm events. Therefore, studies on the recovery process after storms are as important as studies on the erosional process during storms [8]. The time scales of the post-storm nearshore morphological recovery are varying, from days [5; 8] to months [9]. However, large storms can cause rapid erosion locally, from which recovery may take many years or even decades if the impacts are sufficiently large [10]. In extreme cases, the damage can be irreversible if the coastal structures that are built to protect the shore are destroyed by the storms [11]. In the recovery process, the interval between multiple storms is important, as it is inversely related to the storm damage. If the interval between two consecutive storms is less than the recovery time of the beach, the next storm could cause greater damage to the beach [12]. For example, storm clusters with small return periods could impose similar erosion impacts to a single storm that had a much longer return period (i.e., much greater storm). If a beach reached equilibrium after the attack of the first storm, additional erosion would occur only when the intensity of the following storms exceeded that of the first one [5]. Therefore, the recovery after storm events is a complex process, not only depending on the post-storm recovery time but also depending on the intensity of the following storms. The studies on beach recovery after storms have used various approaches. One of the traditional methods is the direct measurement of the elevation and water depth along the lines set perpendicular to the coastline, which is called profiling. The profiling would be useful for long-term monitoring for beach processes with a low initial cost [13]. The profile data could be also used to validate simulation results calculated from numerical models [14]. The model approach is usefully applied for the prediction of beach recovery for either short-term or long-term processes.