*6.1. Estimation of the Rate of Longshore Sand Transport*

Although a sand spit extended south of Narvacan in recent years, many groins have been constructed upcoast of Narvacan, and smooth movement of sand by southward longshore sand transport was blocked by these groins, resulting in the shoreline recession at Narvacan. Here, the change in sand volume can be estimated from the shoreline changes, as shown in Figure 25, assuming that sand supply from upcoast became negligibly small because of the blockage by groins constructed upcoast.

First, the planar area in the shoreline recession zone between B' and C in Figure 25 between October 2003 and February 2015 is calculated to be 2.1 × 105 m2. The rate of change in the total volume of sand in the area between B' and C can be calculated by multiplying this planar area by the characteristic height of beach changes (*h*), which is a correlation factor when the shoreline change is transformed into the change in crosssectional area of the beach, and dividing by the period between 25 October 2003 and 28 February 2015. Usually, *h* can be determined from the bathymetric survey data, but in this case, it was difficult to employ this method because of the lack of bathymetric survey data. Therefore, *h* was calculated using empirical relationships *h* = (1.0–1.3) *h*<sup>c</sup> and *h*<sup>R</sup> = 0.31 *h*<sup>c</sup> among *h*, *h*c, and *h*<sup>R</sup> [16]. In this case, the berm height *h*<sup>R</sup> is given as 2.0 m owing to the field observation [11].

Since the mean value of *h* is determined to be 7.5 m, the rate of change in the total volume of sand in the area between B' and C became 1.3 × 105 m3/yr. North of Narvacan, southward longshore sand transport decreased because of the construction of groins. Assuming that the southward longshore sand transport at point B' is negligible, the rate of longshore sand transport through point C, which is the nodal point between the erosion and accretion areas, can be evaluated to be 1.3 × <sup>10</sup><sup>5</sup> <sup>m</sup>3/yr.
