Wind Wave Observation at Kenzaki Lighthouse at the Mouth of Tokyo Bay (Preliminary Report)

1. In connection with the study of storm surge in Tokyo Bay, wave observations have been made on the coast neighbouring the Kenzaki lighthouse at the mouth of Tokyo Bay. The main object of the observation was to establish the empirical relation between the wave height at the mouth of the bay and the sea level rise at the head of the bay. The observation has been made since April 20, 1961. The variation in wave height and period (see Fig. 9 and Table 1 in the text) during the last 12 months and several records of waves at the time of typhoons have been studied. 2. The wave recorder consists of a sensing part elected in the sea and a recorder installed in the lighthouse. The two parts are connected by a 3-conductor electric cable. The sea water flows in and out through a hole at the bottom of the iron pipe of the sensing part, and makes the metal bellows at the top of the iron pipe change its length. The motion of the bellows is transformed into an electric signal by means of a lowtorque potentiometer, nylon string connecting the bellows and a pulley on the axis of the potentiometer. Long-period motions of the sea level are eliminated by a slow-leak capillary tube provided between the iron pipe and the bellows. The recorder is a pen galvanometer of sensitivity 1mm/25μA and a revolving drum wound with smoked paper. This combination gives the joint sensitivity of 1 mm deflection on the record for the change of sea level of 17 cm. The drum revolves once in an hour and shifts horizontally by 12mm per revolution. The recording speed is 30mm/min for the first half revolution and 6 mm/min for the record half revolution of the drum. The alternation of the recording speed is made automatically by a special gear train, as shown in Fig. 8, driven by a 5 W Warren motor. 3. During the period of observation there was no damage due to storm surges along Tokyo Bay. A part of the wave record during the second Muroto typhoon of September 16, which caused a medium surge in Tokyo Bay, is shown in Fig. 10 in the text. Spectral analyses were made of the wave records of 4 min length taken at 6-hour intervals during the period from Sept. 13, 0800 to Sept. 16, 0700. During this period the sea ran gradually heavy. Gradual increase of the predominating wave period with time can be seen from Fig. 12. In Fig. 13 the relation between the period and wave height is given. 4. The relation between the wave height observed at Kenzaki and the amount of tidal anomally at the bay head is shown in Figs. 17a and 17b for two cases. Fig. 17a shows the case of May 12, 1961, when a southerly wind caused a slight surge in Tokyo Bay. Fig. 17b shows the case of a northerly wind which blew on Oct. 8-9, 1961. In both cases there seems to be a positive relation between the wave height and the tidal anomaly, regardless of the direction of winds, although in the case of the northerly wind the plotted points are somewhat scattered. Definite results need further observations