Thanks for your reply.. It gives me some idea.
Later, I discover that the contour problem appears when,
The contour interval was 0.5m. If the depth falls at 4.0/ 4.5 / 5.0 / 5.5m... there will be contour problem.
If I change it to 4.000001m / 4.5000001m / 5.0000001m... The problem solved. The contour match the color coded elevation exactly.
Later, I discover that the contour problem appears when,
The contour interval was 0.5m. If the depth falls at 4.0/ 4.5 / 5.0 / 5.5m... there will be contour problem.
If I change it to 4.000001m / 4.5000001m / 5.0000001m... The problem solved. The contour match the color coded elevation exactly.