Updrift Migration of Tidal Inlets

Three mechanisms are responsible for tidal inlet migration in an updrift direction (counter to net longshore transport): (1) attachment of distal ebb tide delta bars to the downdrift barrier spit; (2) storm-induced breaching and subsequent stabilization to form a new inlet; and (3) ebb tide discharge around a channel bend creating a three-dimensional flow pattern which erodes the outer channel bank and accretes on the inner channel bank. The last two mechanisms can result in either updrift or downdrift inlet migration, depending on channel geometry in the bay and barrier beach configuration. The last mechanism is discussed here for the first time. Analysis of historical charts and aerial photographs, combined with an historical storm synthesis, shows that all three mechanisms are active at a natural tidal inlet along a sandy coast (Nauset Inlet, Cape Cod, MA). On a time scale of 10 years, these mechanisms were effective in producing an updrift migration of more than 2 km. Initiation of updrift migration coincided with a marked increase in storm frequency perturbing the historically stable inlet position. Subsequent updrift migration resulted from ebb-delta bypassing and channel bend flows.