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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
  1. A one-minute flow-direction and flow-velocity chart (vector diagram) and


  2. A wave-peak-line variation diagram at 3-second intervals from the continuously obtained videos.
Contents of the survey
  1. The design and manufacture of floats for Measuring he flow-Direction and the Flow-Velocity.


    2. A float must be used to obtain the flow-direction and flow-velocity char and it must satisfy the following conditions.

    1. Because a float is used during high waves when a ship cannot be used, it must be an airdrops type.


    2. A float must b identifiable on a video screen.


    3. A float must move with the current, not be driven by wind.


    4. 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

  2. Determination of the Reference Points


    3. 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.

  3. Airdrop of the Float


    4. 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.

  4. Video Pictures


    5. 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.
Measurement and analyses
  1. Analyses of the Flow-Direction and the Flow-Velocity (Vectors)


    2. The measurements of the flow-directions and the flow-velocities do not cause problem if the positions between the float and the reference points are clear like those n the photograph. However, since the resolution of video pictures is inferior to that of phonographs, a hard copy of he video pictures enhanced by a special device was used for measuring.

    The pictures for obtaining the position of the float were taken every 10 seconds and then they were expressed in one minute units. The results of the analysis are shown in fig. 2.




    Fig. 2 Vector Chart of the Periphery of Man-made Reefs

  2. Wave-peak-line Analyses


    3. We analyzed how the wave-peak-lines linking the peaks of the waves change at the periphery of man-made reefs from the continuous video pictures. We used the enhanced hard copies made by the special device in he same way as in the analyses of the flow-directions and flow-velocities. The positions of the wave-peaks were obtained at one-second intervals and then they were expressed in 3-second units. Fig. 3 shows the results of the analysis.




    Fig. 3 The Change Chart of the Wave-peak-lines

  3. Mesh Vector Charts


    4. In a conventional vector chart the positions of floats are linked at equal time intervals as mentioned in Item 4-1. Since such a chart only shows he positions the float passed, it was difficult to now the conditions of the entire survey area. These charts are not realistic because a number of floats must be air dropped to cover this defect.

    Therefore, we divided the areas to be enhanced into mesh of 30 m x 30 m and made a mesh vector chart in which each mesh is represented by one vector. As for a mesh for which no observed data is available, vector direction and size are estimated using data of adjacent mesh. The results of the analyses are shown in Fig. 4.




    Fig.4 Mesh vector chart
Conclusion
As described above, we fond that the waves and states of flow can be surveyed by the video pictures instead of by photographs. It was also recognized that this survey could be performed even during high waves. In addition, the survey confirmed the following advantageous; the use of the video pictures can not only obtain detailed information of several seconds, but also continuous information of around two hours (flying-possible time of a helicopter).

Finally, because of the remarkable improvement of video instruments, the same analyses, described in 4-1 and 4-2, can now be performed on the video pictures of cathode ray tubes instead of an hard copies. The personal computer used in the system is shown in Fig. 5


Fig.5 Personal Computer System