Cape Cod’s coastal history is closely tied to the transport of sand along the shoreline to barrier beaches, islands, and how inlets affect that transport. Each year during hurricane season and on into winter — notorious for our more common nor’easters — coastal residents and geologists alike ponder the possibility: Will a storm move enough water to cut a new inlet, or enough sand to close an existing one? Storm events can cause significant transformation of the shoreline, however they are overlain on the background of daily “normal” change. As waves are typically not perfectly perpendicular to the shoreline, they usually strike the beach at an angle with a portion of the energy moving parallel to the shoreline. Sediments are transported along the shoreline with net movement in a single shore-parallel direction. This Longshore Sediment Transport (LST) acts like a shallow river flowing parallel to the shoreline, so erosion at one spot provides material for beaches and dunes downdrift of that position. For more details click here.
Get a birds eye perspective on these changes, thanks to a tool called Time Machine, developed by Google Earth Engine and Carnegie Mellon University’s CREATE Lab. Working with Sea Grant, The Barnstable County Cape Cod Cooperative Extension’s Coastal Processes team has used Time Machine to create a virtual tour of Cape Cod’s sandy shoreline, revealing just how dynamic they are.
Longshore Sediment Transport (LST) is often called a “river of sand” moving along the coastline. The flow of sand along the Atlantic shoreline from Truro into Provincetown, past Herring Cove and Wood End, and all the way to Long Point is a textbook case of LST.
EASTHAM / ORLEANS
Nauset is a dynamic and curious inlet. While it may appear that sand is moving towards the north, there are mechanisms that can cause inlets to migrate counter to the net longshore transport: movement of sand from the outgoing tide toward a downdrift barrier spit; a breach from a storm forming a new inlet updrift from the original inlet; and tidal flow around a bend in the inlet, causing the outer bank to erode and moving that sand (accreting) to the inner channel bank. This inlet has documented all three.
The flow of sand is all the way from Wellfleet to its ultimate deposition on Monomoy. You can see the barrier beach turn into a barrier island and then meld onto mainland Chatham, greatly expanding the beach at the Chatham Lighthouse. Learn more about how sea level rise is affecting barrier beaching by clicking here by clicking here.
BARNSTABLE / YARMOUTH / DENNIS
See the flow of sand from Chapin Beach in Dennis, Sandy Neck in Barnstable, and parts of Yarmouth towards Barnstable Harbor. While some of the material does enter the estuary (Barnstable Harbor) much of the sand is carried offshore by the ebbing tidal currents. Erosion of Chapin Beach is pronounced in the last few images.
BARNSTABLE / YARMOUTH
There have been accelerated changes along the eastern side of the inlet to Lewis Bay, specifically the western side of Great Island and Egg Island. The inlet separating these two land masses has been filling in during recent years, possibly leading to less flushing of the bay and less access for boats. The expansion of the sand spit has restricted the entrance to Lewis Bay about 500 linear feet.
Sediment transport has been effectively blocked by the Cape Cod Canal which acts as a littoral cell boundary. A littoral cell is a section of coastline that contains sediment sources and transport paths. There is typically minimal sediment exchange with other cells. Whatever material was historically transported eastward is now being impounded at the jetty or transported offshore. Town Neck Beach and the entrance to Sandwich Harbor have been eroding rapidly, at least in part due to the lack of sediment supply.