Beaches need sand
Healthy beaches have abundant sand. When sand supply is restricted, beaches will erode. Beach management, then, is actually "sand management."
Beaches get sand from both the ocean and the land. Ocean currents can move sand along the coast to build beaches. Dunes and other landward sand deposits give sand to a beach in response to the forces of wind and waves.
High waves will cause a beach will change its shape (or profile). To absorb the additional wave energy, beaches and dunes will give-up sand to the waves which carry it seaward and drop it on the bottom. This raises the seafloor and flattens the overall profile of the beach. Waves then shoal and break further offshore, minimizing their erosive effects.
This typically happens is response to seasonal shifts in wave energy. Beaches recover from these natural changes when smaller waves move the sand back onto the beach and winds blow it into the dunes to be captured by coastal vegetation.
Coastal erosion and beach erosion are different
Coastal lands may experience long-term erosion under some conditions. For instance, if sea level is rising, the beach must eventually migrate landward or drown. This causes coastal land behind the beach to erode. Also, if the amount of sand from the seaward side is reduced, a beach will erode the land behind it to maintain a constant sand supply. This creates a condition called coastal erosion.
Beaches on eroding coasts still undergo seasonal profile adjustments, but they slowly shift their position landward as the land erodes.
Hardening a shoreline can interfere with necessary profile adjustments because the dune can no longer share its sand with the beach. As a retreating beach encounters a seawall or revetment it can no longer draw upon a landward sand supply and it begins to erode.
Beach erosion leads to narrowing, and soon, beach loss. Much of Hawaii's beach loss could have been avoided if houses were not built so close to the water. The law presently allows homes 40 feet from the shoreline. On coasts experiencing chronic erosion, this is too close, and leads to hardening in order to protect houses from the waves.Beach Fact: The University of Hawaii monitors over 80 beaches around the state to better understand the process of seasonal profile adjustment.
Beaches are threatened anytime their supply of available sand is reduced. In addition to shoreline hardening, other processes contribute to erosion: sea-level rise, sand mining, channel dredging, dune grading, reef degradation, and others.Beach Fact: Seawalls may stop coastal erosion, but on chronically eroding shores, hardening leads to beach erosion.
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