*5.2. Formation of a Sand Spit on a Coast with Abrupt Change in Shoreline Configuration*

A sand spit has formed at a location with an abrupt change in shoreline configuration south of the Balili River delta, enclosing a lagoon behind the sand spit, as shown in Figure 3. Such a formation of a sand spit can be numerically predicted using the BG model [4,12]. San-nami et al. [4] carried out a numerical simulation of the elongation of a sand spit with a model case of 1/100 on a seabed with different water depths and slopes around the location where the shoreline configuration abruptly changes. In their study, the water depth where a sand spit is formed were altered to 5, 10, 15, and 20 cm at various seabed slopes of 1/50, 1/40, 1/30, and 1/20. The incident wave height was *Hi* = 4.6 cm, and the wave period *T* = 1.27 s. A beach model was produced, as shown in Figure 28a, with incident waves from the *Y*-axis, so the wave incidence angle normal to the upcoast shoreline was to be 20◦. The depth of closure was given as *h*c = 2.5 *H*, where *H* is the wave height at a point. The berm height and equilibrium slope of sand were assumed as 5 cm and 1/5, respectively, based on the experimental results along with the angle of repose of sand of 1/2. The calculation domain was discretized by meshes of 20 cm, and the 8 h of calculation (8 × 104 steps) was carried out using the time intervals of Δ*t* = 10−<sup>4</sup> h. Table 2 shows the calculation conditions.



**Figure 28.** Elongation of sand spit and formation of barrier island [4]. (**a**) Initial topography, (**b**) A sand spit started to elongate from the corner owing to successive deposition of sand supplied by leftward longshore sand transport, (**c**) The tip of the sand spit attached opposite shore, forming a barrier island.

Figure 28b,c shows the results for the condition under which a sand spit is formed on a coast of the seabed slope of 1/20. When waves were incident from the *Y*-axis where the coastline direction markedly changes, similarly to the area south of the Balili River, leftward longshore sand transport occurred, and a sand spit started to elongate from the location where the coastline abruptly changes. After two hours of wave generation, a sand spit elongated very close to the opposite shore, enclosing a lagoon behind the sand spit. The elongation of a sand spit and the formation of a lagoon behind the sand spit may account for the observation results qualitatively. Thus, the formation of a sand spit south of the Balili River can be explained by the mechanism shown by San-nami et al. [4].
