Sand ripples on the beach at Rhossili

It was suggested to me a couple of weeks ago that the ripple marks in the altostratus clouds that have attracted my attention are formed in the same way as ripple marks in the sand. I got thinking about it again on the beach at Rhossili. First we crossed a fresh water stream flowing into the sea. The water flowed over some pebbles and then formed the characteristic ripple pattern shown in the top photograph. The sand on the bottom of the stream was likewise rippled. It occured to me that the pebbles had probably made the flow of the water turn turbulent. There is certainly turbulence in sea water - witness the white water. There are the competing flows of water in and out. But why only ripple marks in some places? An initial look at Internet sources suggests I've got some more observations to make - perhaps measuring the wavelengths of the ripples and looking at the cross-sections. My current working hypothesis for the altostratus cloud is going to be turbulent airflow.