C123Proposing evaluation of sediment produced by coral reefs and sustainable utilization

In Majuro Atoll, anthropogenic water pollution is causing concerns about a decline in sediment production by corals and foraminifera. Furthermore, the construction of upright coastal dikes and the reflected waves they create prevent sediment from accumulating on the shoreline and likely causes it to be washed offshore. These phenomena are leading to the loss of the natural functions that have formed and maintained the atoll's landmass over the years. This could lead to current coastal erosion and beach barrenness, and could make the unique landform mechanisms of atolls unsustainable in the long term.

In considering countermeasures against sea level rise and intensifying waves due to climate change, we believe that rather than relying solely on engineered coastal dikes and seawalls, maintaining healthy sediment production, transport, and deposition mechanisms in coral reef ecosystems and utilizing them sustainably is a necessary and effective adaptation strategy for the coastal areas of atolls and other islands formed by coral reef ecosystems.
Therefore, using Majuro Atoll as a model, this study aims to understand the spatial distribution and historical deposition of gravel and sediment produced by foraminifera and corals, and to identify methods for their sustainable utilization and conservation.

The deposition status of sediment production will be understood based on previously acquired topographical data (DEM data) for the land and shallow waters of Majuro Atoll, as well as local seafloor topography data measured on-site. In addition, the deposition rate of sediment production within a specific area will be estimated through boring surveys of the sediment deposits. Based on these results and the results of surveys on overseas conservation measures, we will propose effective utilization and conservation methods for the sediment produced by natural ecosystems on the island, taking into account the opinions of stakeholders.

Healthy coral reefs produce large amounts of gravel and sand due to corals and foraminifera. In addition to estimating sediment production in various locations on Majuro, we will estimate the annual sediment accumulation volume in the sand aprons formed between the islands, focusing on northern Majuro, where sediment production is thought to be particularly active. From these results, we will estimate the annual sediment production volume. Assuming that sediment use below this volume is sustainable while preventing impact on the atoll's topography, we will consider using this volume for beach nourishment.

In a previously published survey (Fujita et al., 2009) conducted near Eneko Island in northern Majuro Atoll, the sediment production volume by foraminifera at this location was reported as follows. Based on these results, we assumed a strip of land measuring 1 m wide and 40-70 m long perpendicular to the coastline, and estimated the annual sediment production volume by foraminifera in this area to be approximately 1.24 m³.

The amount of sediment deposited on the sand aprons formed on the lagoon side between the islands north of Majuro was estimated based on shallow water topography data (DEM) published by the USGS. As a result, it was estimated that approximately 100,000 m³ of sediment had accumulated on one sand apron compared to the surrounding topography.