**6. Simulation of formation of slender sand bar on Kutsuo coast**

To model the accumulation of sand on the tidal flat due to wave action and the formation of a slender sand bar, a point source in a single mesh (5 m×5 m) was assumed. The intensity of the point source was determined by trial and error, and it was set to 3.75×104 m3/yr at the point (*x*, *y*) = (200 m, 0 m). In addition, an upper limit of 0.5 m was assumed as the elevation of the sandy island. When the elevation of the island reached this height during the calculation, the wave energy was set to 0.

Given the simplified initial topography and the conditions of the annual energy-mean waves of the Kutsuo coast (significant wave height *Hi* of 0.4 m and wave period *T* of 4 s), and assuming wave incidence from the direction normal to the coastline, the beach changes were predicted. The depth of closure was given by *hc* = 2.5*H* (*H*: wave height at a local point). The water depth of the initial bottom of the tidal flat was assumed to be 2 m. This initial flat bottom was assumed to be a solid bed, and a sandy beach with a slope of 1/10 was set at the landward end of the flat bottom. The berm height was assumed to be 0.5 m, and angles of the equilibrium slope and repose slope were set as 1/10 and 1/2, respectively. The calculation domain was divided by *x* = *y* = 5 m intervals in the cross-shore and longshore directions, respectively, and a calculation for up to 5000 hr (5×104 steps) was carried out using time intervals of *t* = 0.1 hr. Table 2 shows the calculation conditions.
