**3. Movable-bed experiment on elongation of a sand spit**

A movable-bed experiment was carried out using a plane-wave tank of 16 m width and 21 m length (Uda & Yamamoto, 1992). A model beach was made of sand with *d*50 = 0.28 mm. A sandy beach was established as the source of sand in the right half of the plane basin and conditions were set up such that leftward longshore sand transport developed. In Case 1, a shallow seabed with a water depth of 5 cm was formed in the left half of the wave basin and an offshore bed was formed with a steep slope of 1/5. In Case 2, a steep slope of 1/5 was produced instead of a shallow sea where the sand spit was formed. The angle between the direction normal to initial shoreline and the wave direction was 20° in order for sufficient longshore sand transport to occur. The elevation of the flat surface on the land was assumed to be 10 cm above mean sea level. Regular waves with *H*0' = 4.6 cm and *T* = 1.27 s incident to the model beach were generated for 8 hr. When a shallow sea exists, incident waves break immediately offshore of the shallow seabed, resulting in the rapid decay of waves on the shallow seabed. Because of this effect, sand is deposited near the marginal line between the shallow flat seabed and the steep offshore slope, resulting in the rapid elongation of a sand spit.

Figures 1(a)-1(c) show the initial bathymetry and the beach topography after wave generation for 1 and 8 hr in Case 1, respectively. Here, the arrows in Figs. 1(a) and 1(d) show the breaking point (the tip of the arrows), the breaker height (the length of the arrows), and the wave directionat the breaking point (the direction of the arrows) measured immediately after the start of wave generation. Because the shoreline had a discontinuity due to a sudden change in the shoreline direction between the sand supply zone and the shallow seabedwhere sand was deposited, a straight sand spit extended from the boundary, and a slender sand spit extended along the marginal line between the shallow seabed and the steepoffshore slope over time. After 8 hr, the sand spit had reached the left boundary while forming a barrier island, and the width of the barrier island expanded upcoast from the left boundary because of the continuous sand supply.

Figures 1(d) and 1(e) show the initial bathymetry and the beach topography after wave generation for 8 hr in Case 2 with a steep offshore slope. The water depth in the zone where sand was deposited was large; thus, sand was deposited while forming a steep slope.

Because this steep slope reaches a great depth, a cuspate foreland was formed without the development of a sand spit. These experimental results were used for validating the improved BG model.

BG Model Based on Bagnold's Concept and Its Application to Analysis of Elongation of Sand Spit and Shore – Normal Sand Bar 345

**Figure 1.** Experimental results for development of sand spit on a coast with abrupt change in coastline orientation (Uda & Yamamoto, 1992).
