Megaripples, ridges and runnels on intertidal flats of the Wash, England

Observations have been made of parts of the channels and the outer portions of the sand banks in the Wash using 1 : 10 000 air photographs and brief ground surveys at low spring tides. Two principal structures are found. In the channels megaripples 0.3–0.6 m high and of 10–15 m modal spacing are dominantly flood orientated. They do not reverse during ebb tide. On the outer parts of banks low ridges of 0.5 m height and 50-100 m spacing are interpreted as wave-formed ridge and runnel structure. They contain wave-rippled mud patches in the runnels and although they have shorewards asymmetry do not appear to migrate to any great extent. It is suggested that only in sheltered areas do the Wash intertidal flats show a lower mudflat subfacies; the norm is outer bank sand flats with ridge and runnel structure eventually overlain by Arenicola sand-flats during progradation.