Hawaii County

About Beach and Coastal Erosion

• Beach Erosion
• Coastal Erosion

Beach erosion and coastal erosion are not the same, but they are related. Beach erosion is a reduction in the amount of sand a particular beach has. On a global level, sea level rise causes beach erosion. But beaches also erode (and expand) on a seasonal basis.

Beaches get sand from both the ocean and the land. Larger waves move sand from the coastal sand dunes off into the ocean. This raises the seafloor, flattens the overall profile of the beach, and, therefore, causes waves to break further offshore. This, in turn, minimizes the waves' impact on coastal lands. Beaches recover from these seasonal shifts when the waves move the sand back onto the beach and the winds blow the deposited sand into dunes. These dunes will store the land-based sand until the next large wave event.

Coastal erosion occurs when the beach migrates toward the land in order to compensate for beach erosion as it tries to maintain a constant supply of sand (see the right side of the photo). If sand is not available to a beach, such as when a wall is built to protect the land, the land is stabilized, however beach erosion will occur (see left side of photo).

Installing a seawall or revetment (i.e., hardening a shoreline) interferes with the natural cycle of beach erosion. Rather than pulling sand from a landward supply in order to promote waves breaking further off-shore during the seasonal high wave period, the seawall or revetment prevents this natural phenomena from occurring. Thus, the land itself begins to erode.

Therefore, it is tragically ironic seawalls or revetments have been installed to prevent coastal erosion, but their very presence exacerbates the very problem they were supposed to resolve.