First, for those people who don’t know what scend is, here is a definition:
Scend is the distance from sea level to the bottom of the trough of a wave. The trough can be taken as being equal to the height of the wave above sea level, or more simply as half the height of the wave. This means that, with a 3m sea running over a bar, the depth available to you may be 1.5m less than you would have in a flat sea.
So how does a navigator deal with it? You need to think of the charted level of the sea, the average height of the waves and swells combined. To this you add your calculation of the relevant state of tide and estimate the scend with the above definition in mind. Call that ‘A’.
‘B’ is the depth of the keel and the amount of scend. Subtract ‘B’ from ‘A’ to get ‘C’. If ‘C’ is negative there isn’t enough water. Even if it is positive there has to be a reasonable safety margin.
Then, depending how frightened you are, add a second safety margin. Call that ‘D’.