High skewness and leptokurtosis in tidal flat sediments of bottle-neck shaped bays under a monsoon climate: A South Korean case study

While mean grain size (Mz) and sorting (SD) are commonly used to characterize sediment grain size distribution and understand transport and deposition processes, skewness (Sk) and kurtosis (KG) have been less frequently applied. This study proposes that Sk and KG can be valuable tools for understanding sedimentary processes, particularly in tide-dominated Korean tidal flats influenced by the Asian monsoon climate. Our analysis of Sk and KG from bottleneck-shaped bay and estuarine tidal flat sediments reveals a trend of very high values (Sk > 2, KG > 10) in medium to very fine sands (Mz: 1~4ø) deposited within Korean coastal bays with narrow entrances. These values indicate extremely fine-skewed and leptokurtic grain size distributions. We propose that this phenomenon is driven by strong wavy weather events, such as monsoonal summer typhoons or winter storms, and bidirectional tidal flow through the bottleneck-shaped bay entrance, which we term the 'Monsoonal Bay Tidal Winnowing (MBTW)' process. The MBTW occurs through a sequence of intense erosion and resuspension of tidal flat surface sediments in the wavy inner bay, transportation of resuspended sediments to the outer bay during ebb flow, selective deposition of coarser grains in the deep outer bay, reentry of finer sediments to the inner bay, and consequent redeposition of fine-skewed and leptokurtic sediments on the tidal flats. This study advocates for the combined use of both frequently applied parameters (Mz and SD) and the less commonly used Sk and KG for a more comprehensive interpretation of sedimentation processes in coastal environments.