Beach Formation and Sediment Composition
Beach Formation
A beach is a deposit of unconsolidated sediments, ranging from sand to gravel (excluding mud, typically found in alluvial plains or mangrove coasts). It extends from the base of the dune or vegetation line to a depth where sediment movement ceases. This depth varies depending on beach bathymetry, geomorphology, and wave action.
Sediment Composition
Beach sediments vary in composition depending on their source:
- Lithogenic/Terrigenous Sediments: Originate from the Earth’s crust. These include silicates (light and dark), mica, and dark minerals (especially iron and magnesium). Rivers transport these sediments from inland to the coast, so terrigenous beaches are often found near river mouths.
- Biogenic Sediments: Derived from the remains of marine organisms’ calcium carbonate hard parts. These sediments form in situ, meaning they don’t travel far and are usually found near their source.
- Mixed Sediments: Beaches with both terrigenous and biogenic sediment sources.
Beach Shaping by Wave Action
Wave action is the primary force shaping beaches, though wind and human activity also play a role. A steep beach slope indicates a summer profile, with sediment accumulating on and behind the beach, forming a berm. A gentle, almost flat slope indicates a winter profile, where most sediment has been transported offshore, forming a pit and bar. Wave force is typically stronger in winter than summer, leading to more significant changes during the colder months.
Beach Formation Requirements
Beach formation requires specific conditions:
- A geomorphic area that allows sediment accumulation.
- Sediment accretion must exceed erosion. If erosion is greater, permanent accumulation won’t occur.
Every beach has a natural cycle of accretion and erosion. However, introducing new variables can disrupt this balance, exceeding the beach’s geomorphic limit and potentially leading to beach loss.