Coastal deposition is the laying down of material on the coast by the sea. It occurs when waves lose energy or when large inputs of sediment are made into the coastal system - perhaps due to the arrival of fluvial sediment at a river estuary. Wave refraction in bays also encourages deposition due to the dispersal of wave energy. Lower-frequency constructive waves often contribute to deposition due to their strong swash, moving beach material inland. Depositional landforms can be highly vulnerable to erosion during extreme storm events unless vegetation colonisation has taken place. Plant roots can help anchor sediments, making them more resistant to the action of destructive waves.