Back to scallopes do you find them in higher densities between rocks /slabs in gullies and small patches of gritty sand than in the open areas?the last lot I found were in groups not more than a foot apart in quite shallow water another spot they were in 2-3 foot of water on low water you could of got some. Wadeing in !i think they favour a gritty sea bed with good tidal flow ?
I think that you are right about the tidal flow although they can be found on all types of seabed, from mud to broken rock. They are probably easier to spot in coarse sand and gravel and the ones that you sea are only a small percentage, many others are completely buried.
They are less common where regularly affected by swell although there are some beds on the West coast and plenty on the West of Sark. Sometimes it seems to make little sense where they want to be as beds can start or stop for no apparent reason.