From Coastal Wiki
Definition of Runnel:
Depression between intertidal bars or between the dry beach and the upper intertidal bar (beach ridge).
This is the common definition for Runnel, other definitions can be discussed in the article
After high tide, the depression is filled with water, which is gradually discharged to the sea during falling tide via a channel (a so-called rip channel) that cuts through the beach ridge. The runnel bed is often covered with wave ripples. The rip channel bed is often covered with current ripples. Small antidunes than can often been observed in the shallowest part of the rip channel where the flow velocity is highest.