Backshore coarsening processes triggered by wave‐induced sand transport: the critical role of storm events

Abstract

Spatial backshore processes were investigated through fi eld observations of topography and median sand grain size at a sandy beach facing the Pacifi c Ocean in Japan. A comparison of the backshore profi le and cross-shore distribution of the median sand grain size in 1999 and 2004 revealed an unusual sedimentary process in which sand was coarsened in a depositional area in the 5-year period, although sediment is generally coarsened in erosional areas. In support of these observations, monthly spatial fi eld analyses carried out in 2004 demonstrated a remarkable backshore coarsening process triggered by sedimentation in the seaward part of the backshore during a storm event. In order to elucidate mechanisms involved in the backshore coarsening process, thresholds of movable sand grain size under wave and wind actions (a uniform parameter for both these cases) in the onshore and offshore directions were estimated using wave, tide, and wind data. The cross-shore distributions of the estimated thresholds provided reasonable values and demonstrated a coarsening mechanism involving the intermediate zone around the shoreline under alternating wave and wind actions as a result of which coarse sand was transported toward the seaward part of the backshore by large waves during storms and then toward the landward part by strong onshore winds. The 5-year backshore coarsening is most certainly explained by repetition of short-term coarsening mechanisms caused by wave-induced sand transport occurring from the nearshore to the intermediate zone.

Projections of Future Beach Loss in Japan Due to Sea-Level Rise and Uncertainties in Projected Beach Loss

“…Dataset on the sediment size and beach slope do not exist for the entire Japanese coastline, and even if they exist, they change tempo-spatially depending on wave conditions even at the same beach [Udo and Mano, 2010]. Udo and Takeda [2014] shows that the median sediment size dso ranges from 0.2 to 0.6mm at most Japanese beaches with a median value of approximately 0.3 mm, based on a review of 240 studies in the literature (Fig.…”

Projections of Future Beach Loss due to Sea Level Rise for Sandy Beaches along Thailand's Coastlines

“…The beach slope, grain size and the beach width used in these projections were measured in different years and seasons. These parameters have temporal variation and the beach loss can be sensitive with varying sediment sizes (Udo and Mano, 2010). In addition, there is an uncertainty regarding the DoC.…”

Coastal geomorphology

“…In summerautumn, typhoon landfalls initiate offshore transport and the loss of sediment to the heads of submarine canyons. Also in Japan, Udo and Mano (2010) show how coarsening of backshore beach sediments in an otherwise depositional setting over a five-year period can be attributed to repetitive short-term transport of coarse sand under the combined influence of storm waves and strong onshore winds. In a meso-tidal deltaic setting, Tamura et al (2010) analyse multiyear surveys of beach morphology and sedimentology and show that monsoon-influenced changes in beach morphology are also mediated by variations in sediment supply.…”