**3.2 Result**

Kimigahama Beach had undergone significant beach changes caused by anthropogenic interventions in the past. In 1963, a natural sandy beach of 100 m width was extended alongshore without any protective measures between the rocky headland and Point Inubo, as shown in **Figure 4**. Then, a seawall was constructed near the coastline without preserving a sufficiently wide sandy beach as a buffer zone and the foreshore was excavated to raise the elevation of the flat land behind the seawall.

These interventions resulted in a reduction in the volume of the sandy beach, causing beach erosion. As a measure against erosion caused by the artificial activity, two artificial reefs (submerged breakwaters) with a length of 50 m and a crown height of 2 m below MSL were constructed offshore of the shoreline until 2012.

After the installation of these artificial reefs, the shoreline locally advanced to form a salient behind these artificial reefs because of their wave-sheltering effect, whereas the shoreline retreated on both sides, and wave overtopping the seawall became significant at the site designated by an arrow in **Figure 4b** immediately south of the south artificial reef. Numerous concrete blocks had to be placed to prevent waves from overtopping the seawall. On the other hand, because of sand deposition behind the south artificial reef, the amount of windblown sand on the widened beaches increased and sand was not only deposited in front of the seawall but also transported up to the hinterland.

**Figure 5** shows the longitudinal profiles along transect Nos. 1–5. Along transect No. 1 at the north end, the shoreline is completely covered by a gently sloping seawall with no foreshore left in front of the seawall (**Figure 6**). A walkway with an elevation of 5.5 m above MSL has been constructed immediately landward of the crown of the seawall, and a sand dune with a slope of 1/6.5 is formed landward of this walkway (**Figure 7**). Transect No. 2 crosses the north artificial reef and the beach is wide because of the deposition of sand caused by the wave-sheltering effect of the north artificial reef. In this area, a walkway with an elevation of +5.5 m runs along the shoreline, and a vegetation zone extends a 20 m wide on the seaward side of the walkway and a foreshore with a slope of 1/7 exists near the shoreline. On the other hand, on the landward side of the walkway, a low sand dune with an elevation of +6.0 m is formed (**Figures 8** and **9**).

#### **Figure 5.**

*Longitudinal profiles along transect Nos. (a) Transect No. 1, (b) Transect No. 2, (c) Transect No. 3, (d) Transect No. 4, (e) Transect No. 5.*

*Numerical Simulation on Sand Accumulation behind Artificial Reefs and Enhancement… DOI: http://dx.doi.org/10.5772/intechopen.107014*

#### **Figure 6.** *Transect No. 1 (near the shoreline).*

#### **Figure 7.** *Transect No. 1 (hinterland).*

Transect No. 3 crosses the south opening of the north artificial reef. At this site, a gently sloping seawall has been constructed near the shoreline and no foreshore exists, as shown in **Figure 10**. Moreover, a walkway with an elevation of +5.5 m has been constructed, and landward of this, a flat land, which was artificially constructed by reclamation using beach sand, extends. Transect No. 4 crosses the north end of the south artificial reef and a walkway with an elevation of +6 m runs in the south–north direction, and a wide foreshore is formed owing to the wave-sheltering effect of the south artificial reef (**Figure 11**). There is a break in the slope at 2 m height in this longitudinal profile, and the foreshore slope of 1/10 and the backshore slope of 1/5 are separated by this break in the slope, which were formed by waves and windblown sand, respectively. Windblown sand was transported from the shoreline up to the walkway along this transect (**Figure 12**). Landward of this walkway, flat land with an elevation of approximately +6 m is formed (**Figure 13**). The longitudinal profile with the combination of a flat land of +6 m height and a steep slope to the shoreline cannot be formed under the natural conditions, implying that this flat land landward of the seawall was artificially formed.

**Figure 8.** *Transect No. 2 (backshore).*

**Figure 9.** *Transect No. 2 (walkway).*

**Figure 10.** *Transect No. 3.*

*Numerical Simulation on Sand Accumulation behind Artificial Reefs and Enhancement… DOI: http://dx.doi.org/10.5772/intechopen.107014*

**Figure 11.** *Transect No. 4 (foreshore).*

**Figure 12.** *Transect No. 4.*

**Figure 13.** *Transect No. 4.*

Since the deposition of windblown sand in front of the seawall along transect No. 4 has occurred since 2012 after the construction of the artificial reefs, the rate of windblown sand deposition can be estimated to be 2.5 m3 /m/yr, because the change in the cross-sectional area in the seaward area of the seawall is 18 m2 . Transect No. 5 crosses the seawall near the south end of the beach. As shown in **Figure 14**, the coastline is protected by numerous concrete blocks from waves overtopping the seawall.

**Figure 15** shows the bathymetry around the two artificial reefs measured in 2003. These artificial reefs were constructed at a depth of approximately 4 m. The area between the north artificial reef and the rocky headland is covered by exposed rocks, as designated by complicated contour lines. The area between the south artificial reef and Point Inubo is also covered with exposed rocks, similar to the north artificial reef. In short, the areas between the rocky headland and 450 m south of it, and between Point Inubo and 400 m north of it, as shown by dotted lines in **Figure 15**, are covered by exposed rocks, and sandy beach only extends within the area of the alongshore length of approximately 1.1 km including the artificial reefs.

**Figure 14.** *Transect No. 5 (6 June 2017).*

*Numerical Simulation on Sand Accumulation behind Artificial Reefs and Enhancement… DOI: http://dx.doi.org/10.5772/intechopen.107014*

Taking these conditions into account, it is clear that this beach has a closed system of littoral drift, and sand transport occurred from outside the wave-shelter zone of the artificial reefs to the inside because of the construction of artificial reefs on such a coast. In the landward area of the south artificial reef, a wide flat seabed of 3 m depth extends and a steep foreshore slope extends in the zone with a depth smaller than approximately 1 m. The alongshore movement of sand in a narrow band in front of the seawall was triggered by the construction of artificial reefs, resulting in the formation of the present beach topography.
