Introductory Chapter: Morphodynamic Model for Predicting Beach Changes Based on Bagnold’s Concept and Its Applications

The morphology of sand dunes formed on the Maisaka and Hamamatsu coasts facing the Enshu-nada Sea was investigated by field observation. The development of sand dunes was numerically predicted using a model predicting the effect of both waves and windblown sand. In the field observation, the development of sand dunes with a rhythmic shape similar to a sand spit was observed in the backshore area. It was found that pine trees died in the area with a narrow sand dune owing to wave run-up and the dispersion of salinity during storm wave conditions. In the numerical simulation, the formation of sand dunes was successfully reproduced under the condition that the predominant wind blew at a large angle relative to the direction normal to the shoreline.

Section: Modelmentioning

confidence: 99%

Prediction of Topographic Changes on Enshu-Nada Coast Considering Effect of Both Waves and Windblown Sand

Yokota,

Uda,

Kobayashi

et al. 2023

“…Regarding the prediction of shoreline changes of a cay, Miyahara et al (2013) numerically simulated the deformation of a circular island composed of sand using the BG model (a model for predicting 3-D beach changes based on Bagnold's concept) (Uda et al, 2018). They employed the longshore sand transport formula with Ozasa and Brampton's term (1980) as the fundamental equation in which the additional effect due to the longshore inclination in breaker height is included, and the wave field around the island was calculated using the energy balance equation for irregular waves.…”

Section: Introductionmentioning

confidence: 99%

Shoreline Variation of an Island in Response to Change in Wave Direction

Uda,

Noshi,

Yokota

et al. 2023

We selected Komaka Island located 3 km offshore of Chinen Peninsula in Okinawa and surrounded by coral reef as a study site, and we conducted field observation of the shoreline changes on this coral cay on 13 July 2019, and the shoreline changes were investigated using satellite images. The shape of the sandbar behind the island was classified into one of three types (circular, slender oval, and elongated oval). Then, the changes in the sandbar were numerically simulated using the modified BG model (a model for predicting 3-D beach changes based on Bagnold’s concept). In this model, waves were assumed to be incident from every direction so as to focus at the center of the circular island. Under this condition, the distribution of the wave diffraction coefficient Kd was assumed, taking the wave-sheltering effect of the cay into account, and longshore sand transport was assumed to be induced owing to the change in Kd in the direction normal to the wave direction. The results of numerical simulation explained well the deformation of the sandbar on Komaka Island.

“…It is necessary, therefore, to predict these beach changes at the planning stage of an artificial beach. In this study, these topographic changes due to high waves and subsequent beach recovery under calm wave conditions were calculated using the BG model (a model for predicting 3-D beach changes based on Bagnold's concept; Uda et al, 2018), taking the Chigasaki coast in Kanagawa Prefecture, Japan, as an example.…”

Section: Introductionmentioning

confidence: 99%

Topographic Response to High Waves and Subsequent Beach Recovery on Chigasaki Coast

Tamura,

Ishikawa,

Uda

et al. 2023

Beach topography quickly responds to the action of storm waves, resulting in foreshore erosion and accretion under calm wave conditions after a storm. Field observations were carried out on the Chigasaki coast to investigate these beach changes. It was found that the seabed shallower than 3 m depth was rapidly eroded by offshore sand transport during a storm event with the deposition of sand in a zone at depths between 3 and 5 m, and then the beach recovered within 1–2 years after the storm. Topographic changes immediately after the storm and subsequent recovery under calm wave conditions were calculated using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept). Given the equilibrium slope of fine sand d1 and medium-size sand d2 to be 1/120 during high waves, the erosion of the foreshore zone was numerically reproduced. Moreover, the recovery of the beach topography to a gentle slope under calm wave conditions after the storm was successfully reproduced.