change at the seawall located south of Oarai Harbor, Ibaraki Prefecture, 

 Japan. A location map is given in Figure 1. Shoreline change at this site 

 has been extensively documented and numerically modeled (Kraus, Hanson, and 

 Harikai 1985). There are two seawalls on this north-south oriented sandy 

 beach facing the Pacific Ocean. The north seawall is a continuous massive 

 concrete wall 2 km (1.24 miles) long and 5 m (16.4 ft) high from base to 

 crown. Portions of the north seawall at Oarai-are shown in Figures 2 and 3. 

 The face of the north seawall is mildly curved outward and armor blocks have 

 sometimes been placed at the foot of the wall when the beach eroded. The 

 south seawall is similarly constructed and 800 m (0.5 miles) long. Beach 

 change at the north seawall has mainly been studied. 



28. when the shoreline reaches the seawall, the local beach slope 

 becomes slightly steeper than the typical nearshore slope on this coast (which 

 itself varies between approximately 1/50 and 1/70 from the beach face to the 

 wave breaker line). The change in beach slope is mild and appears to be neg- 

 ligible for purposes of applying the shoreline model. No drastic alteration 

 in beach characteristics occurs and the beach is exposed at low tide (Fig- 

 ure 3). At high tide, when the shoreline has receded to the seawall, broken 

 waves slap against the face of the wall. 



29. Although the shoreline may reach the seawall at some location, it 

 has been inferred on the basis of the observed and modeled long-term shoreline 

 change that sand moves alongshore through the area to be deposited adjacent to 

 a large groin at Oarai Harbor (Kraus, Harikai, and Kubota 1981; Mizumura 1982; 

 Kraus and Harikai 1983; Kraus, Hanson, and Harikai 1985). Since alongshore 

 variations in the slope of the beach in front of the seawall are small, the 

 seawall does not appreciably alter the pattern of wave breaking. A surf zone 

 usually exists in front of the seawall and the capacity for waves to move sand 

 alongshore is retained. Sand is transported in the direction of the wave- 

 induced longshore current, and the beach in front of the seawall has been ob- 

 served to periodically erode and recover. 



Idealized Seawall Boundary Condition 



30. From the observations described above, Hanson and Kraus (1985) de- 

 veloped the concept of the idealized functioning of a seawall for use with the 

 shoreline model. They concluded that once the shoreline reaches a seawall at 



11 



