Along Cape Cod’s ever-changing shoreline, erosion and movement of sand are part of a natural process that helps to sustain beaches, dunes and marshes. Even unexpected shoreline changes such as the overwash at Duck Harbor Beach demonstrate that the shoreline is functioning to sustain coastal resources.
The Duck Harbor Beach overwash is an observable example of barrier beach dynamics. Barrier dunes maintain their form and function, not by rigidly resisting the onslaughts of rising seas and storm waves, but by strategic retreat, migrating landward or “rolling over” to maintain both seaward-facing beaches and back-barrier wetlands.
“Barrier” dunes that occur just above the reach of normal tides are occasionally overwashed by storm tides and waves. Most overwashed sand remains within the barrier-beach and marsh system and can be either reworked into new dunes, or can supply much-needed sediment to back-barrier marshes so that they too can keep up with rising sea level.
The Duck Harbor Beach overwash is linked to peak Cape Cod Bay high tides. Flooding seawater inundated back barrier wetlands and, without a pathway to recede, ponded water caused salt-kill of wetland vegetation in the Bound Brook Island area. As sea level rises, future overtopping events could occur. Cape Cod National Seashore will continue to coordinate monitoring of tidal conditions and landside changes associated with the overwash events.
Even if overwash events continue, it is unlikely that a permanent inlet into historic “Duck Harbor” (a sub-basin of the Herring River system) will be re-established. Duck Harbor is no longer an open water body, and is now filled with peaty sediment that impedes water flow. Any breach of the line of barrier dunes will likely be temporary. When the original inlet was diked in 1909, tidal flow into shoaled Duck Harbor was already negligible, and the harbor reportedly un-navigable. This is not to say that a very large storm couldn’t overtop the dune system here, as well as near Ryder Beach (Truro) and even Powers Landing, and temporarily flood the wetlands behind with seawater, a potential recognized by federal flood insurance maps.
Fortunately, any potential flooding effects of the overwash at Duck Harbor will be significantly lessened with the implementation of the Herring River Restoration Project. Re-establishment of historic marsh channels and the new bridge planned at Chequessett Neck Road will enable floodwaters to recede and discharge into Wellfleet Harbor much more quickly than under existing diked conditions. Likewise, measures such as elevation of roads that will be put in place to protect structures from the effects of tidal restoration will also provide greater flood protection compared to current conditions.
