hi sparki
do you mean a beach close to a port?
the main things i can think of that might increase the wave height due to a modified groyne:
- does the groyne/wall now cut off the angle of approach of seas/swell as you observe it, compared to before, eg. if it is 100s of meters long - not sure it does if I'm thinking of the right one, plus there is some refraction effects if the swell is very south - but it is in Cockburn sound just about...
- has the groyne/wall been made more rough (sorry, basic term only) eg. larger rocks, bigger voids between them, and so it is absorbing more wave energy and not reflecting it as it may have used to (reflected wave superimposing on the approaching waves and making it feel bigger ie. wedges). I have no idea.
- has the offshore bathymetry change drastically due to the structure or incidental to the structure. I don't think that would be the case off the freo coastline from my observations
I have a mate who is a coastal eng that grew up surfing around there and sandtrax etc during the right wintery conditions, so i'll ask him when i see him next as well...
oh, and to fit in...
if groynes are a sore point, I find rubbing it helps...