Abstract
Beach topography quickly changes in response to the action of storm waves, resulting in erosion of the foreshore with accretion under a calm wave condition after a storm. These seasonal beach changes may occur on beaches with protective measures or artificial beaches produced by beach nourishment. On these beaches, the shore protection function of a sandy beach is reduced when a trough is formed immediately offshore of the shoreline and the foreshore slope increases, indicating the importance of the study on topographic changes. Moreover, the time required for a beach recovery in response to wave conditions has not been sufficiently studied, along with the 3-D topographic changes associated with beach cycles. In this study, we aim to investigate these issues using the Narrow Multi-Beam survey data, wave data, and seabed materials data, taking the Chigasaki coast as an example. It was found that a seabed shallower than 2 and 3 m depths was eroded by rapid offshore sand transport during a storm event with the deposition of sand in a zone between 3 and 5 m depths, and then the beach recovered within 1-2 years after the storm. It was also confirmed that a bar and trough disappeared in 1-2 months under the conditions of HE = 0.5 m, TE = 8 s, and H/L = 0.005 when the crown depth of the bar was smaller than approximately 2 m. Thus, the topography after the storm waves recovers within several months or 1-2 years depending on wave conditions and the crown depth of the bar.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/W_P_3p_xd8U
Highlights
Beach topography quickly changes in response to the action of storm waves on natural beaches, resulting in erosion of the foreshore with accretion under a calm wave condition after a storm. Davis and Fitzgerald (2004) clarified these beach cycles; when wave conditions are about equal to or less than the average energy conditions, such as a swell wave with a small wave height and a period of 8-12 s, an accretionary beach is formed with other conditions of an erosive or storm beach
Over a period that ranges from as short as a week to as long as three months, the ridge repairs the beach, resulting in an accretional profile. These seasonal beach changes may occur on beaches with protective measures or artificial beaches produced by beach nourishment
Topographic changes in terms of beach cycles, i.e., accretion caused by storm waves and subsequent beach recovery after storm events, were investigated using the data sets of wave observation, the Narrow Multi-Beam (NMB) survey, and grain size distribution, taking four typhoons that hit the Chigasaki coast as examples
Summary
Beach topography quickly changes in response to the action of storm waves on natural beaches, resulting in erosion of the foreshore with accretion under a calm wave condition after a storm. Davis and Fitzgerald (2004) clarified these beach cycles; when wave conditions are about equal to or less than the average energy conditions, such as a swell wave with a small wave height (generally < 1 m) and a period of 8-12 s, an accretionary beach is formed with other conditions of an erosive or storm beach. These seasonal beach changes may occur on beaches with protective measures or artificial beaches produced by beach nourishment On these beaches, the shore protection function of a sandy beach is reduced when a trough is formed immediately offshore of the shoreline and the foreshore slope increases because of the increase in the wave run-up height, indicating the importance of the study on nearshore topographic changes. The response time required for the beach cycles has not been sufficiently studied, along with the 3-D topographic changes associated with the beach cycles In this study, these issues were investigated using the Narrow Multi-Beam (NMB) survey data, wave data, and seabed materials data before and after storm events, taking the Chigasaki coast facing Sagami Bay in Japan as an example (Fig. 1). The topographic changes after storm events associated with typhoons were quantitatively analyzed using the NMB survey and wave observation data, together with the seabed material data
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