UNIT 4: DEPOSITION PROCESSES ON A COAST
Page 6 UNIT 4: DEPOSITION PROCESSES ON A COAST HOW BEACHES AND DUNES FORM Beach are built by CONSTRUCTIVE WAVES - gentle waves that push sand up onto the beach building up a feature called a BERM at the shore line - wind then blows the sand inland where it is trapped by vegetation in the dunes. As long as the weather is calm, the waves will continue to push sand up onto the beach & the beach will grow as incoming sand from the sea is available. Sand cycle animation; http://www. duedall. fit
THE BEACH SAND CYCLE IN CALM WEATHER - In periods of relatively calm weather the waves are CONSTRUCTIVE swash is more powerful than backwash and the beaches builds up as sand is deposited. IN STORMY WEATHER - In periods of stormy weather with DESTRUCTIVE waves (backwash more powerful than swash)the beach is eroded and the sand moves out to an offshore sandbar.c A typical Perth beach in mid-summer Wind deflates the beach blowing sand inland Constructive waves deposit sand on the berm BERM SAND ROCKS Sand moves toward thebeach WINTER SUMMER THE BEACH SAND CYCLE A typical Perth beach in mid-winter
IN STORMY WEATHER - In periods of stormy weather with DESTRUCTIVE waves (backwash more powerful than swash)the beach is eroded and the sand moves out to an offshore sandbar. A typical Perth beach in mid-winter GALE FORCE WINDS Dunes absorb the power of the waves SAND ROCKS Sand is deposited at an offshore bar THE RESULTS OF LONGSHORE DRIFT - SPITS
Moving sand along a beach spreads out into the sea where the land bends Look at this animation to see how a spit forms: FORMATION OF A SPIT Points A, B, and C show the progression of the spit's formation. The curved ends are explained in point 6. 1. Fetch - direction of prevailing winds (and therefore waves) in approaching the beach. 2. Direction of Long-Shore Drift. 3. Salt Marsh 4. Original Coastline 5. River Estuary 6. Short term changes in wind and wave direction causing the curved ends to the spit. LONGSHORE DRIFT SALT MARSH View this animation of a baymouth bar
Tombolo Development Tombolo - Santa Cruz, CA COASTAL SAND DUNES Look at this website Sand movement on a dune
These occur where sand is being constantly supplied by onshore winds that blow across sandy beaches. The most common dunes on the coast are parallel dunes. They occur with strong onshore winds & lots of fine sand. These dunes are useful energy absorbers in that during storms they act as a buffer protection zone from serious coastal erosion during winter storms. If vegetation is removed from the dunes, they blow inland in blow-outs. This can cause serious management problems. Look at this website Sand movement on a dune