Signs of Erosion
Shorelines are inherently dynamic, and not all eroding shores need to be protected or stabilized. However, you may want to address cases of chronic and intense erosion or where valuable habitat or built infrastructure are at risk. To determine whether a shoreline is eroding and whether that erosion is of concern, ask yourself the following questions:
Is there scarping or trees falling into the water?
Exposed large root systems during low tide or mature trees close to the waterline can indicate active erosion in areas that were once stable.
Is there evidence of undermining?
During low tide, currents and waves may cause erosion that is not easily visible. Undermining will quickly result in the further loss of sediment.
Are large portions of bank eroding?
Large breakaway pieces of land indicate a very unstable shoreline and high risk of shoreline loss.
Is land loss apparent when comparing historical images?
Comparing historic aerials and current Google earth photos of shorelines during similar tidal phases can show how much land has been lost or gained.
Causes of Erosion
Erosion can occur for many reasons, including changes in waves and currents, increased runoff, and disturbances such as dredging or construction.
Failed or failing structure
The washout of sediment from behind a bulkhead or revetment is a sign of failed or compromised structure. Unless addressed, adjacent shorelines will continue to erode.
Wind driven waves (fetch dependent)
Fetch is an uninterrupted stretch of surface of water that the forces of wind act upon. Wave sizes are directly related to fetch length—longer fetch can produce larger waves against the shore. Fetch can be eight miles long on the Hudson River.
Wakes and proximity to navigation channels
Boat traffic can generate wakes several times larger than those caused by wind. Predicting wake sizes is vital for the proper design of any shore stabilization method. Observing the wakes during peak traffic provides insight into resulting wake energy on the shore.
Currents running parallel to the shore, both from the ebb and flood of tides and downstream flow can erode the shoreline. The floating debris it carries can exacerbate the erosion.
Very often severe erosion will take place where built structure meets the natural shoreline, such as with the flanking end of this bulkhead. The reflection of the waves on the structure can cause erosion both at the ends of the structure and at the toe.
Large pieces of floating ice—moved by currents, tides, and wind—can scour the shoreline. The impacts of ice can be difficult to predict, but the impacts can be significant!