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Salients were formed in the lee of two artificial reefs (submerged breakwaters) constructed on Kimigahama Beach in Chiba Prefecture, Japan, owing to the wave-sheltering effect of the reefs, and then, a significant amount of fine sand was transported inland from the salients by wind action. In this study, not only shoreline changes after the installation of the two artificial reefs but also beach changes caused by windblown sand were predicted using a model, in which the BG model (a model for predicting three-dimensional beach changes due to waves based on Bagnold’s concept) is combined with a cellular automaton method. Reproduction calculation was carried out on the basis of field data. Beach changes after the artificial reefs were removed were also predicted and the effect of beach nourishment was investigated. It was concluded that landward sand transport by wind is accelerated when wave-sheltering structures such as an artificial reef are constructed on a coast composed of fine sand, and such an effect can be successfully predicted by using the present model.
Coastal Engineering Laboratory Co., Ltd., Tokyo, Japan
Takaaki Uda
Public Works Research Center, Tokyo, Japan
Yasuhito Noshi
Nihon University, Funabashi, Japan
*Address all correspondence to: ctr.ty.725@gmail.com
1. Introduction
On a coast subject to strong wind action, foreshore sand may be transported to the hinterland, causing damage to the houses and coastal roads along the coastline. When a detached breakwater or an artificial reef (submerged breakwater) is constructed as a measure against beach erosion on such a coast, sand accumulates in the lee of the structure owing to its wave-sheltering effect, forming a salient [1]. In Japan, detached breakwaters have been widely used as a shore protection measure against beach erosion. Moreover, artificial reefs have often been constructed as a measure instead of detached breakwaters in recent years because they are submerged and thus do not block the ocean view. An artificial reef has the similar wave-dissipating effect as a detached breakwater; thus, sand deposition occurs in the lee of the constructed structure. For example, on Kimigahama Beach in Chiba Prefecture, Japan, salients were formed after the construction of two artificial reefs owing to the wave-sheltering effect of the reefs. Then, a significant amount of fine sand was transported inland from the foreshore by wind.
Thus, in this area, topographic changes due to the actions of waves and wind simultaneously occur on coast after artificial reefs were constructed, and therefore, the prediction of the beach changes is required to consider the effective measures to maintain the shoreline and reduce windblown sand to the hinterland. In the previous studies regarding the sand transport due to waves, a number of models predicting the beach changes have been proposed [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Moreover, regarding windblown sand, many studies have been carried out for more than half a century, and not only many formulae of windblown sand transport but also models for predicting topographic changes have been proposed [12, 13, 14, 15, 16, 17, 18, 19, 20]. However, there are few studies to predict topographic changes while taking the combined effects of waves and wind into account. Yokota et al. [21] developed a model for predicting these beach changes, in which the BG model (a model for predicting 3-D beach changes due to waves based on Bagnold’s concept) [22, 23, 24] is combined with a cellular automaton method [20]. In this study, this model was tried to apply to the prediction of beach changes on Kimigahama Beach, which is of importance in practical coastal engineering.
In Yokota et al. [21], the BG model was employed, which is a model based on the concept of the equilibrium slope and is derived by an energetics approach [25, 26], and there are eight types [22]. Among them, Type 8 is a model taking the effects of both wave action and nearshore currents into account ([22], Fujiwara et al. [27]). However, In this study, the simplest model of Type 1 BG model was used, which uses a simple sediment transport equation expressed by the wave energy flux at the breaking point, and does not calculate the nearshore currents field, so the calculation load is small [22]. Beach changes caused by wave action in the lee of a detached breakwater have already been successfully predicted when the Type 1 BG model is employed [22].
In general, when artificial reefs are installed near the shoreline, not only shoreward current but also rip current generated by forced wave breaking on the reef can strongly affect the deformation of the beach. In this case, it is necessary to include this effect in the prediction [22, 27]. However, the artificial reef on Kimigahama Beach has a large offshore distance of 300 m or more, so this effect is considered to be small, and the effect of the artificial reef on the beach can be regarded as similar to that of the detached breakwater. Based on this, in this study, the coefficient of wave height transmission of the artificial reef is given and calculated as a breakwater.
The model of topographic changes due to windblown sand used by Yokota et al. [21] is based on the cellular automaton method (Katsuki et al. [20]). This model has a relatively small calculation load compared with the other models and can perform calculations. Here, this model is used for predicting beach changes when artificial reefs are constructed on a coast subject to both waves and wind on Kimigahama Beach. Reproduction calculation was first carried out on the basis of the field data. Then, beach changes were predicted after the artificial reefs were removed to avoid excess sand accumulation behind the artificial reefs and erosion on nearby beaches. Moreover, the effect of beach nourishment around the artificial reefs was investigated. It was concluded that landward sand transport by wind is accelerated when wave-sheltering structures such as an artificial reef are constructed on a coast composed of fine sand.
Beach deformation owing to the combined actions of waves and wind after the construction of two artificial reefs on Kimigahama Beach was investigated. The study area is located near the east end of Honshu Island and faces the Pacific Ocean, as shown in Figure 1, which is a pocket beach of 1070 m length bounded by a rocky headland on the north end and Point Inubo on the south end. At present, two artificial reefs of 50 m in width and 200 m in length with a crown height of 2.0 m below the mean sea level (MSL) are constructed in the central part of the study area. A shallow zone covered with exposed rocks extends in the vicinity of the rocky headland and Point Inubo, whereas a sandy beach composed of fine and medium-size sand extends along the shoreline with salients being formed in the lee of the artificial reefs.
Figure 1.
Location of Kimigahama Beach at east end of Honshu Island in Japan, and alignment of transects of Nos.
The wind and wave conditions of this area are as follows. The wind conditions of the area predicted by NEDO Neo Winds [28] are shown in Figure 2. The predominant wind direction of the study area ranges between NE and NNE in winter and is SSW in summer. In Figure 2, the direction of the mean shoreline at transect No. 3 located at the central part of the pocket beach shown in Figure 1 is indicated by a dotted line. Because there is no predominant wind direction at the central part of the pocket beach between the rocky headland and Point Inubo, wind can be assumed to blow from the direction normal to the shoreline.
Figure 2.
Probability of occurrence of wind direction and mean shoreline orientation.
We referred to the forecasting results offshore of Chiba Prefecture predicted by the Japan Meteorological Agency [29] for the wave characteristics in this area. Figure 3 shows the probability of occurrence of waves in each direction: the predominant wave direction is E. As for the wave conditions of this coast, an average significant wave height is 1.9 m with a wave period of 8.6 s, and a tidal range is approximately 1.5 m.
Figure 3.
Probability of occurrence of wave direction offshore of the study area.
First, the shoreline changes on Kimigahama Beach were investigated using the aerial photographs taken in 1963 and 2018, as shown in Figure 4. The shoreline changes before and after the construction of the artificial reef can be clarified using these aerial photographs. The longitudinal profiles along transect Nos. 1–5 shown in Figure 4 were measured using an RTK-GNSS on 9 July 2019. Of the survey lines, transect Nos. 2 and 4 were aligned across the salient formed after the construction of the artificial reef, and transect Nos. 1, 3, and 5 were aligned on both sides of the salient. Then, the beach topography along each transect was compared. The conditions of the coast were also observed on 14 June 2022 and foreshore and backshore materials were sampled along transect No. 4. Furthermore, beach topography around the artificial reefs was investigated in detail using a bathymetric survey map prepared in 2002 by the Chiba Prefectural Government.
Figure 4.
Aerial photographs taken in 1963 and 2018. (a) 26 May 1963 and (b) 16 May 2018.
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).
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 10.
Transect No. 3.
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 14.
Transect No. 5 (6 June 2017).
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 15.
Bathymetry of Kimigahama Beach measured in 2003 by Chiba Prefectural Government.
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.
Topographic changes including the accumulation of sand in the shoreward zones of the artificial reefs caused by waves and the landward transport of windblown sand from the salients after the installation of the artificial reefs were predicted using a model for predicting 3-D beach changes under combined actions of waves and wind [21]. With this model, the beach changes caused by waves occurring in the depth zone between the depth of closure (hc) and the berm height (hR) are predicted using the BG model [22], whereas topographic changes caused by windblown sand in the area landward of a berm top are predicted by a cellular automaton method.
The BG model is based on following concepts: (1) the contour line becomes orthogonal to the wave direction at any point at the final stage, similarly, (2) the local beach slope coincides with the equilibrium slope at any point, and (3) a restoring force is generated in response to the deviation from the statically stable condition, and sand transport occurs owing to this restoring force [22].
In the calculation of windblown sand by the cellular automaton method, the two most important processes, saltation and avalanche, are taken into account [20]. Two-dimensional meshes were taken on Cartesian coordinates (x, y), and the elevation at the mesh point was set as z (x, y, t). The mesh size is assumed to be sufficiently larger than the size of the sand particles. The saltation distance Ls was defined using Eq. (1) on the basis of the observation results obtained by Andreotti et al. [30] and is the simplest polynomial expression that can be used to evaluate the obtained results of the sand flux on a sand dune including multiphase flow.
LS=L1.0+b1Zh−b2Zh2E1
b1 and b2 are the coefficients to control sand transport flux, expressed by the product of Ls defined using Eq. (1) and the mass of moving sand. L is a reference distance (here, we choose 1.0 m), and h is the reference height (here, the berm height).
Eq. (1) shows that the higher the elevation where sand particles are deposited, the longer the distance that the sand particles are transported by wind, but Ls has a limit and the sand flux after the maximum value is reached is regarded as a constant, and the decreasing functional form is not employed. When there is an obstacle in the field, saltation is assumed not to occur, because a vortex is formed behind an obstacle owing to the separation of the flow [31]. Originally, the sand flux is given by the product of the mass of moving sand and the saltation distance, and the sand flux can be expressed by Eq. (1) when the wind velocity is constant, assuming that the mass of moving sand is constant. When the wind velocity changes, the coefficient of Eq. (1) can be changed depending on the wind velocity.
To combine the BG model and the cellular automaton method, the calculation domains were separated at the location of the berm, assigning the landward region of the berm as the domain of windblown sand. The rate of windblown sand is assumed to attain equilibrium at a location distant from the starting point for the approach run in the downwind direction. Here, the condition for the windblown sand to occur was defined, as shown in Figure 16, depending on the backshore width, assuming that the minimum approach run is 10 m [32]. No windblown sand is transported when the beach width B is smaller than the foreshore width w’; the mass of moving sand (q) attributable to windblown sand was given by the value multiplied by the coefficient μ shown by Eq. (2) when the backshore width w is smaller than 10 m. When w is greater than 10 m, μ is unity.
Figure 16.
Schematic view of condition for windblown sand to occur. (a) No windblown sand (B < w’). (b) Windblown sand (0 < w < 10 m). (c) Windblown sand (10 m < w).
Topographic changes around the artificial reefs built on Kimigahama Beach under the combined actions of waves and wind were calculated using a model in which the BG model was combined with a cellular automaton method for predicting the amount of windblown sand. Table 1 shows the calculation conditions. The grain size of the seabed material was set to be 0.2 mm from the grain size measured on the backshore along transect No. 4. In the bathymetric chart in Figure 4, no topographic changes can be seen offshore of the opening of the artificial reefs, so the depth of closure due to waves was set to 5.0 m. The equilibrium slope was assumed to be 1/10 from the foreshore slope along transect No. 4, as shown in Figure 5. The berm height was given by the height of the break in the slope of 2 m. Since the predominant wave direction is E, as shown in Figure 3, wave direction was set to be E. The coefficients b1 and b2 and the mass of moving sand q were assumed to be b1 = 10, b2 = 0.5, and q = 2.5 × 10−6 m3/m2/step on the basis of the rate of windblown sand of 2.5 m3/m/yr measured along transect No. 4. In the numerical simulation, the topographic changes were predicted in Cases 1 and 2 without/with windblown sand, respectively.
Grain size of sand particle d50 (mm)
0.2
Equilibrium slope
1/10
Incident wave conditions
Incident wave height (m)
1.0
Wave direction α (deg)
15.0
Water level above MSL (m)
0.0
Depth range of beach changes
Depth of closure hc (m)
5.0
Berm height hR (m)
2.0
Coefficient of cross-shore and longshore sand transport Ky = A/√d50 = 0.2/√0.2 =0.45 [33]
0.45
Depth distribution of longshore sand transport ε(Z) = 1/(hc + hR) (m−1)
The initial topography for the calculation was assumed to have a foreshore slope of 1/10 and a 1/50 slope in the offshore zone deeper than 1 m. Then, waves were incident in this assumed initial topography for a sufficiently long time (10 years) to obtain the beach topography before the installation of the artificial reefs. By this calculation, we carried out the matching of beach topography to the given wave conditions. Figure 17 shows the result. Then, two artificial reefs and a seawall with a height of 6 m were installed along the shoreline. Figure 18 is thus the obtained initial topography. Since the seabed with a depth equal to or larger than 2 m is covered with exposed rocks, solid bed was assumed in these areas.
Figure 17.
Beach topography adjusted to the given wave conditions.
Figure 18.
Initial topography.
Figure 19 shows the results of the calculation in Case 1, that is, under waves without the wind effect, after the installation of the artificial reefs. The shoreline advanced owing to the alongshore sand transport toward the lee of the two artificial reefs with an increase in the beach width. Moreover, two salients with a berm height of +2 m were formed behind the artificial reefs.
Figure 19.
Predicted topography only under wave action.
Then, the calculation in Case 2, that is, with the wind effect, was carried out, given the same topography as that in Case 1. The beach topography and topographic changes relative to the initial topography in Case 2 are shown in Figure 20. It is seen that the beach width increased behind the artificial reefs, and sand was transported by wind from the foreshore to inland causing the deposition of sand in front of the seawall. Furthermore, part of the windblown sand was transported into the hinterland over the seawall.
Figure 20.
Beach topography and topographic changes in Case 2 with wind action. (a) Predicted topography. (b) Topographic changes relative to initial topography.
Figure 21 shows for comparison of the beach profiles in Cases 1 and 2 along the transect across X = 920 m, as shown by dotted lines in Figures 19 and 20. In Case 1, only a flat, plane beach with an elevation of +2 m was formed. In Case 2, however, sand was transported landward by wind, forming a steep slope of 1/7.5 in front of the seawall up to the crown height of +6.0 m of the seawall, whereas the shoreline has retreated because part of the beach sand was transported landward by wind. This result well explains the formation of a 1/5 slope (Figures 5d and 9) from the shoreline to the top of the seawall, which was observed in the field. It is concluded that installing a wave-dissipating structure, such as an artificial reef, on a coast composed of fine and medium-size sand induces the concentrated deposition of sand in the lee of the structure and the decrease in sand volume and the shoreline recession in the entire area.
Figure 21.
Longitudinal profiles along transect across X = 920 m in Cases 1 and 2.
5.3 Prediction of beach changes
In Case 3, topographic changes were predicted under the condition that beach nourishment was carried out using sand of the same grain size as that at the present coast at a location where the beach width decreased after the construction of the artificial reefs. Figure 22a and b show the initial topography in Case 3 and the change in beach elevation under the condition with/without beach nourishment using sand with a volume of 17,000 m3 behind the openings of the two artificial reefs.
Figure 22.
Beach topography and topographic changes in Case 3 with beach nourishment. (a) Initial topography. (b) Change in beach elevation relative to initial topography.
Figure 23a and b show the results of the prediction and the topographic changes relative to the initial topography in Case 3. In this case, alongshore sand transport continues to occur from the nearby beach to the lee of the artificial reefs, and the salients also continued to develop in the lee of the artificial reefs. Therefore, part of beach nourishment sand was transported over the seawall to the hinterland owing to the wind effect. Because part of beach nourishment sand was transported away to the hinterland, the effect of beach nourishment is minimal in increasing the beach width.
Figure 23.
Predicted topography and topographic changes in Case 3 with beach nourishment. (a) Predicted topography in Case 3. (b) Topographic changes in Case 3 relative to initial topography.
In Case 4, the artificial reefs were removed to investigate their adverse effects using the bathymetry shown in Figure 22a as the initial bathymetry. Figure 24a and b show the results of the calculation in Case 4 when artificial reefs were removed, and the change in topography relative to the initial topography. Owing to the removal of artificial reefs, salients that formed in the lee of the artificial reefs disappeared, and nourishment sand was transported to the entire pocket beach, resulting in the sand deposition in the entire area except behind the artificial reefs. Furthermore, since the sand deposition was no longer concentrated behind the artificial reefs owing to the removal of artificial reefs, the amount of sand blown over the seawall to the hinterland has greatly decreased.
Figure 24.
Calculation results in Case 4 after the removal of the artificial reefs. (a) Predicted topography in Case 4. (b) Topographic changes in Case 4 relative to initial topography.
In general, when a detached breakwater or submerged breakwater is constructed on a coast composed of fine and medium-size sand, sand accumulates in the lee of the structure owing to the wave-sheltering effect, forming a salient behind the structure. Owing to this sand deposition effect, these structures have been widely used in Japan as a measure against beach erosion. However, the beach formed by this accretive effect of waves is also subject to wind action, resulting in a significant amount of windblown sand. On a sandy beach widened in the lee of the structure, therefore, the amount of sand blown to the hinterland is also increased, causing loss of foreshore sand. Accordingly, on such a coast, even though beach nourishment is carried out, nourishment sand is transported to the hinterland, devaluating the effect of beach nourishment, and causing another sand maintenance problem. When considering shore protection measures against beach erosion on a coast composed of fine and medium-size sand, the effect of windblown sand must be taken into account. In such a case, the present model can be useful for predicting future beach changes.
Beach changes after the construction of artificial reefs and the increase in the amount of sand blown from the foreshore in the lee of the artificial reefs to inland were investigated, taking Kimigahama Beach in Chiba Prefecture, Japan, as an example. Then, a numerical simulation of these topographic changes was carried out using the combination of the BG model for predicting beach changes caused by waves and a cellular automaton method for predicting the windblown sand. In the field observation, it was found that salients were formed after the construction of two artificial reefs and the amount of sand blown from the widened foreshore in the lee of the artificial reefs increased. Not only was fine sand transported in front of the seawall but also part of sand was further transported inland. The formation of salients in the lee of the artificial reefs and the deposition of windblown sand in the hinterland as observed in the field were numerically predicted well.
As an application of the prediction of beach changes, the effect of beach nourishment at the opening of the artificial reefs was predicted using the same model. When beach nourishment was carried out while maintaining the present condition of the reefs as they are, sand was further transported inland by wind. Instead, if artificial reefs were removed and then beach nourishment was carried out, nourishment sand was distributed on the entire sandy beach, and loss of sand toward the hinterland became minimal. From this, the construction of a facility with a wave dissipating function such as a detached breakwater or an artificial reef in the nearshore zone on a coast composed fine and medium-size sand must be carefully carried out. It is important for a coastal engineer to sufficiently consider not only the effect of the movement of sand due to waves but also the management of windblown sand when shore protection facilities are constructed on a coast composed of fine and medium-size sand.
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Written By
Takuya Yokota, Takaaki Uda and Yasuhito Noshi
Submitted: 14 July 2022Reviewed: 09 August 2022Published: 18 September 2022
Open access peer-reviewed chapter
