Coastal survey by video-image
Yasutoshi Tsuji, Oshiaki Takahashi, Minoru Komiya
Asia Air Survey Co.
2-16, Tsurumaki 5, Setagaya-ku, Tokyo, Japan
Introduction
High waves surging on to the coasts are causing great erosion. To prevent this, breakwaters, offshore embankments, and man-made reefs have been set up. Information concerning these waves and the state of the flow is indispensable for the planning and design for construction and for the management and maintenance after construction.
However, it is dangerous to investigate the waves and the state of flow of the sea during high waves because of many ensuing difficulties. On the other hand, there is a limit to the value of photographs taken from an airplane because of the lack of continuity. Consequently, we developed a new method, a video-image taken from a helicopter, for investigating the waves and the state of flow.
Objectives of the survey
The objectives of the survey are to obtain
- A one-minute flow-direction and flow-velocity chart (vector diagram) and
- A wave-peak-line variation diagram at 3-second intervals from the continuously obtained videos.
Contents of the survey
- The design and manufacture of floats for Measuring he flow-Direction and the Flow-Velocity.
A float must be used to obtain the flow-direction and flow-velocity char and it must satisfy the following conditions.
- Because a float is used during high waves when a ship cannot be used, it must be an airdrops type.
- A float must b identifiable on a video screen.
- A float must move with the current, not be driven by wind.
- Since the area of the sea o be investigated is within 500 m x 600 m and the video screen is made up of 525 scanning lines, the width of a float must be at least more than the width covered by one scanning line.
We designed and produced a float, which satisfies hese conditions. Fig. 1 shows the appearance of the float.
Fig.1 Structure of the float for investigation
- Determination of the Reference Points
In order to calculate the positions of the float and the wave-peak lines, reference points were set up on the sea and on the land before photographing.
- Airdrop of the Float
During high waves when a ship could not be used, the float was dropped from a special helicopter for this purpose a approximately 5 meters above the sea surface.
- Video Pictures
A helicopter to take the video pictures flew at a ground speed close to hovering at an altitude of approximately 1,000 metros over the area o be surveyed. The videos were taken for about one hour. As the video camera was placed on a special vibration-proof stand outside the helicopter, very stable pictures were obtained. The video system was completely remote-controlled from inside the helicopter.
