**Abstract**

The long-term beach changes along the Kaike coast (Japan) have been investigated. Being a major source of sand to this coast, the Hino River has supplied a large amount of sand during the extensive mining of iron sand in the past, resulting in the shoreline advance. However, the stoppage of the sand mining has caused a marked decrease in the sand supply, resulting in a rapid shoreline recession around the river delta. The beach changes triggered by these human activities have been investigated using an old geographical map and aerial photographs taken between 1947 and 2005. Then, the beach changes have been reproduced using the contour-line-change model considering the change in grain size of the beach sediments. Bathymetric data have been analyzed in order to evaluate the longshore sand transport and the fluvial sand supply from the Hino River. The measured and predicted three-dimensional beach changes were in good agreement and the effectiveness of the contour-line-change model for predicting long-term beach changes was confirmed by this case history.

**Keywords:** beach changes, contour-line-change model, river delta, longshore sand transport, Kaike coast

### **1. Introduction**

The Yumigahama Peninsula located in the western part of Tottori Prefecture, Japan, has developed as a tombolo owing to the wave-sheltering effect of the Shimane Peninsula with a rich sand supply from the Hino River. The 18-km-long coastline between the Yodoe fishing port and Sakai Port located at both the ends of the peninsula is called the Kaike coast (**Figure 1**). In the Hino River basin, a largescale mining of iron sand had been carried out in the nineteenth and early twentieth centuries. A huge amount of sand was abandoned in the river basin and such a sand was transported toward the coast through the Hino River. With the cessation of the iron sand mining, however, the sand discharge from the Hino River markedly decreased, causing a strong erosion around the river mouth. The erosion on the Kaike coast was severe from the early 1930s to immediately after World War II. At first, groins were constructed as a measure against erosion, and the construction of detached breakwaters began in 1971. Behind 12 detached breakwaters constructed by 1982, cuspate forelands were well developed [1–3]. Although these detached

1970s, and the erosion area has been expanding. The exhaustion of sand supply by northward longshore sand transport from the sea cliffs, formation of a wave-shelter zone at Katakai fishing port breakwater, and ground subsidence caused by the pumping-up of underground water were the major causes of the shoreline recession. The arc-shaped shoreline of Kujukuri Beach was reproduced given these con-

*A Long-Term Prediction of Beach Changes around River Delta using Contour-Line-Change Model*

Upon these previous studies, the long-term topographic changes of the Kaike coast in a 70-year period have been analyzed in this study, including the period when extensive mining of iron sand has been carried out. Then, given these conditions, the process of the reduction in the size of the Hino River delta over 70 years was reproduced using the contour-line-change model considering the change in grain size of the seabed material, and the applicability of the model was validated.

**2. Long-term shoreline changes along Yumigahama Peninsula and**

The shoreline configurations in the Yumigahama Peninsula were determined from an old geographical map produced in 1899 and the aerial photographs taken in 1947, 1967, and 1973. The shoreline changes in 74 years were investigated between 1899, when rich sediment was supplied from the Hino River during the period of iron sand mining, and 1973, when the construction of detached breakwaters began. In the analysis of the shoreline changes, the correction owing to the changes in tide level was made using the tide level when the aerial photograph was taken and the mean foreshore slope of 1/15. The origin of the coordinate (*x* = 0 km) was set at the left bank of the Hino River and the alongshore distance *x* was taken westward

**Figure 2** shows the shoreline changes between 1899 and 1947. The shoreline markedly retreated in an area between *x* = 1 and *x* = 2 km around the Hino River mouth with the decline of mining of iron sand, and the shoreline receded by a maximum of 250 m at the river mouth. On the other hand, in the areas westwards and eastwards of *x* = 3 km, the shoreline advanced of 100 m in parallel at a rate of 2 m/year, except that in the vicinity of Yodoe fishing port. Similarly, **Figure 3** shows the shoreline changes with reference to shoreline in 1947 until 1967 and 1973, after that two detached breakwaters were constructed. Since a seawall and groins have been constructed in an area between *x* = 0 and *x* = 1.5 km westwards of the Hino River until 1967, the shoreline recession was prevented. However, excluding this area, the shoreline retreated westwards of the protected area. The eroded sand was transported westwards of *x* = 5 km, resulting in the shoreline advance. Until 1973, the area where the shoreline markedly receded expanded westwards, and the area where the shoreline advanced until 1967 was eroded. Here, the shoreline advance at two locations near *x* = 1 km was due to the formation of cuspate forelands behind

ditions. Furthermore, San-nami et al. [20] have investigated the long-term topographic changes since 1968 along the entire Shizuoka and Shimizu coasts including a 17 km stretch extending between the Abe River and the Mihonomatsubara sand spit. The beach erosion of these coasts was triggered by the decrease in sediment supply from the Abe River due to excessive riverbed mining until 1967. After 1982/1983, the natural sand supply from the river increased and accretion occurred on these coasts. Measured topographic changes were reproduced using the model. Not only the movement of the sand body but also the shoreline and bathy-

metric changes were numerically reproduced.

*DOI: http://dx.doi.org/10.5772/intechopen.85207*

**longshore sand transport**

along the coastline (**Figure 1**).

the detached breakwaters constructed by 1973.

**65**

**2.1 Shoreline changes**

#### **Figure 1.**

*Location of Yumigahama Peninsula in Tottori Prefecture, Japan.*

breakwaters and cuspate forelands have been stably maintained, fine-grained sand was locally deposited, natural sandy beaches rapidly disappeared, and the coastline was covered by artificial structures in the entire Yumigahama Peninsula [4].

In predicting the beach changes triggered by the imbalance in longshore sand transport on a coast such as the Kaike coast, a long-term prediction in an extensive area is required [5]. In such a case, the time scale changes from years to decades and the calculation domain reaches even up to 10–100 km. The N-line model can be applicable to the prediction of such beach changes [6–12]. We have developed the contour-line-change model considering the changes in grain size of the seabed materials as a type of N-line model to predict long-term beach changes including the prediction of changes in grain size [13, 14]. The model has applied to the longterm prediction of beach changes around a river mouth delta and those along an arc-shaped shoreline in previous papers.

Uda et al. [15] have first shown a predictive model of the three-dimensional development and deformation of a river mouth delta. This model enabled the prediction of not only the shoreline change but also the three-dimensional, longterm topographic changes around a river mouth, and offshore sand transport can be taken into account when the sea bottom slope exceeds a critical value given by the angle of repose of sand. Furuike et al. [16] have applied the model to the Enshunada coast. The shoreline changes between the Magome river mouth and Imagireguchi jetty have been investigated where beach changes triggered by the decrease in fluvial sand supply from the Tenryu River occur. It was observed that the shoreline retreated downcoast of the Magome river mouth, and the shoreline advanced in parallel further downcoast, while maintaining the curvature in the shoreline configuration. Miyahara et al. [17, 18] have investigated the long-term evolution of the Tenryu River delta associated with sand bypassing at several dams. When the sediment yield from the river was artificially increased, the supplied sediment was mainly deposited around the river mouth, but it took a longer time for a sandy beach far from the river mouth to recover. Given the annual discharge of sediment with three grain sizes, the recovery of the delta topography and the effect of nourishment on the nearby coast have been predicted. San-nami et al. [19] have analyzed the beach changes of Kujukuri Beach with a 60 km length located in Boso Peninsula. On south Kujukuri Beach, severe beach erosion has occurred since the

*A Long-Term Prediction of Beach Changes around River Delta using Contour-Line-Change Model DOI: http://dx.doi.org/10.5772/intechopen.85207*

1970s, and the erosion area has been expanding. The exhaustion of sand supply by northward longshore sand transport from the sea cliffs, formation of a wave-shelter zone at Katakai fishing port breakwater, and ground subsidence caused by the pumping-up of underground water were the major causes of the shoreline recession. The arc-shaped shoreline of Kujukuri Beach was reproduced given these conditions. Furthermore, San-nami et al. [20] have investigated the long-term topographic changes since 1968 along the entire Shizuoka and Shimizu coasts including a 17 km stretch extending between the Abe River and the Mihonomatsubara sand spit. The beach erosion of these coasts was triggered by the decrease in sediment supply from the Abe River due to excessive riverbed mining until 1967. After 1982/1983, the natural sand supply from the river increased and accretion occurred on these coasts. Measured topographic changes were reproduced using the model. Not only the movement of the sand body but also the shoreline and bathymetric changes were numerically reproduced.

Upon these previous studies, the long-term topographic changes of the Kaike coast in a 70-year period have been analyzed in this study, including the period when extensive mining of iron sand has been carried out. Then, given these conditions, the process of the reduction in the size of the Hino River delta over 70 years was reproduced using the contour-line-change model considering the change in grain size of the seabed material, and the applicability of the model was validated.
