ABSTRACT

To determine the mechanism of gravel profile deformation, the change in the spatiotemporal beach profile is investigated by hydraulic experiments. The temporal profiles are obtained by image sampling, and the groundwater flow is analyzed by using dye. By utilizing these results, various experimental scenarios are investigated to reproduce ordinary morphological deformations such as berm formation, collapse, movement, and growth, under accretive wave conditions. As a result, it is observed that the wave interacts in a complicated manner with the downwash and profile, and that the change of the break-point bar, amount of moveable sediment, and bathymetry lead to variable morphological change.

INTRODUCTION

Gravel beaches are primarily located in rough wave environments. The frequent occurrence of storm waves has the direct consequence of beach erosion and rapidly deforms the beach within a relatively short amount of time. Littoral drift, and infragravity and tidal waves, are also important factors contributing to beach erosion (Katoh and Yanagishima, 1992; Mizuguchi and Seki, 2015). However, the overall amount of external energy is predominant in wind waves and swells. This fact becomes more apparent when the target beach consists of gravel and is located in a particular environment, where it is subjected to microtidal or low mesotidal conditions. Consequently, the incident wave and sediment transport in the cross-shore direction are treated as the most influential factors affecting the occurrence of dynamic changes in the morphology of the beach, and determine the relationship between the wave information and the deformation of the profile, which is one of the main issues that has been investigated by previous studies (Dean, 1973; Sunamura and Horikawa, 1974; Hattori and Kawamata, 1980). These issues are still being introduced in the Coastal Engineering Manual (2008), and the Technical Standards and Commentaries for Port and Harbour Facilities in Japan (2009). Sunamura and Horikawa (1974) proposed the classification of the beach profile by using the wave height, wave period, slope of the initial profile, and size of the sediment grain. The use of this discriminant equation facilitates the categorization of the beach profile as an erosive or accretive type, under given wave and beach conditions. Hattori and Kawamata (1980) solved the problems in the application of grain size by utilizing the sediment's fall velocity based on the balance of an external force acting on the sediment particle between the stirring power of the suspending sand and the resisting power of the fall velocity. Dean (1973) also mentioned the importance of the dimensionless fall-time parameter, which consists of the fall velocity, wave height, and wave period. However, these studies could not explain the process of morphological change and the variable of morphodynamic deformation, which occur on the beach face due to using the fragmented wave information and profile slope.

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