Observation of Wave Set-Up Height in a River Mouth

Field observation data of water level at three river mouths are collected and analyzed in order to find quantitative relationship between wave set-up height at a river entrance and wave characteristics. At a relatively bigger river mouth which is protected by two jetties, wave set-up cannot be observed in the river entrance, whereas at smaller river mouths, distinct water leve l rise above tidal elevation can be observed during the period high waves. An empirical expression is obtained for the relationship between wave set-up height and deep water wave height, which shows slight difference from a previous expression for wave set-up at the shoreline on a sloping beach. Introduction There have been a lot of observations of wave set-up caused by breaking waves, though, most of them have been limited to laboratory flume experiments ( e.g., Bowen et al., 1968; Battjes and Janssen, 1978 ). The reason of the scarcity of field observations might be its considerable difficulty in the surf zone, especially during high waves. In recent years, several field observations have been reported on wave set-up height in a river entrance ( Hanslow and Nielsen, 1992 ; Hanslow et al., 1996 ; Santoso et al., 1998 ; Tanaka and Shuto, 1992 ). Among these, Hanslow and Nielsen (1992) obtained very little set-up in a river entrance in Australia, whereas Tanaka and Shuto (1992) found that the wave set-up height measured at the Nanakita River mouth attained to 10-20% of the deep water wave height. The difference 1 Professor, Department of Civil Engineering, Tohoku University, 06 Aoba, Sendai 980-8579, Japan. Phone & Fax:(+81)-22-217-7451, e-mail: tanaka@ tsunami2.civil. tohoku.ac. jp 2 Professor, Department of Civil Engineering, Nihon University, Nakagawara, Tokusada, Tamura-machi, Koriyama 963-8642, Japan. 3 Civil Engineer, Construction Department, OBAYASHI Co. Ltd., 2-15-2 Kounan, Minato-ku, Tokyo 108-8502, Japan. between the results from these groups indicates that the occurrence of wave set-up has distinct dependence on the geometry of river entrance. In the present study, the measurement and analysis of wave set-up height is carried out at three river mouths with different geometry in Japan and the height is formulated in terms of the deep water wave quantities. Study Area and Data Collection Measured time-variations of water level at three river mouths ( see Table 1 and Figure 1 ) are collected and analyzed. Rivers in Japan are classified into class A and B according their dimensions and its importance. The former are governed by the national government, whereas the latter by prefectural government. The Natori River in Table 1 belongs to the former category and, subsequently, has larger catchment area as compared with other two rivers investigated in the present study. There are two jetties at the river entrance as depicted in Figure 1 to protect sediment intrusion into the mouth by waves. The water level variation in the mouth is measured every one hour by the Sendai Construction Office, Ministry of Construction. More detail of the study area can be found elsewhere ( Tanaka et al., 1996 ; Tanaka et al., 1997 ). On the other hand, the Nanakita River is classified into class B river as shown in Table 1. It is located about 9km north to the Natori River as seen Figure 1 and there is a jetty on the left-hand side of the mouth which limits the northward migration of the mouth, while the movement towards to the south is not limited. A comprehensive investigation on river mouth topography change has already been made one of the authors ( Tanaka et al., 1996 ; Tanaka and Ito, 1996 ). The water level variation in the river entrance is measured by means of an automatic water level gauge shown in Photo 1 every 5 minutes. The Natsui River is also rank B river, though, its has about three times larger catchment area as compared with the Nanakita River as seen in Table 1. Another distinct difference from other river mouths in Table 1 is the absence of a jetty at the river mouth. A water level gauge has already been installed at Suga and Yokokawa Stations in Figure 1 by the Fukushima Prefectural Government and the data has been being obtained every three hours. Table 1 Characteristics of study area. class catchment area (km) length of river (km) number of jetties Natori River A 984 55 two Nanakita River B 229 45 one Natsui River B 749 67 none Figure 1 Location map of observation sites. Photo 1 Measuring station of water level at the Nanakita River mouth. JAPAN SENDAI . IWAKI