8.2. Depositional Landforms Flashcards
Depositional Landforms
- produced by build up of sediment
- occurs where inputs > outputs (where rate of accumulation of sand and shingle exceeds the rate of its removal)
Beaches
- most typical of all coastal landforms
- usually made of sand and shingle
- usually form between HWM (high water mark) and the LWM (low water mark) - in inter-tidal zone
- sediment is moved longshore drift
- a dynamic landform like a conveyer belt
Beach zones
1) backshore zone
2) foreshore zone
3) nearshore zone
4) offshore zone
Backshore Zone
- part of the beach that is only underwater when there are storm waves or during the highest tides, so this zone is usually not under the influence of waves
- lower energy than the foreshore and therefore there is less potential to remove sand
- as a result, a greater amount of sand accumulates in the backshore environment
Foreshore Zone
- part of coastal zone that is exposed at low tide and underwater at high tide
- includes majority of wave action
- higher energy removes sand
- berm is the highest point on the beah and forms the foreshore-backshore boundary which changes seasonally
- foreshore gives some protection to backshore against the effects of wave and tidal energy
Nearshore Zone
- Seaward of the foreshore
- permanently covered with water, except at extremely low tides
- where waves break
Offshore Zone
- further out to sea
- beyond the influence of the waves
- sediment can build-up in bars
- if the bar grows enough to pierce the surface of the water, it can become a barrier island and a barrier beach
Role of Tides in the Formation of the beach profile
- ebb and flow of the tides make the beach a transitional zone between land and sea
- width and nature of foreshore and nearshore zones are determined by the tidal range as well as by the amount and type of beach sediment
- spring tides shape the berms at the top of the beach
- on beaches with a high tidal range, each falling tide reveals a new beach environment, washing clean of any sign of human activity
Role of waves in the Formation of the beach profile
- waves have energy to change the beach
- constructive waves move material up the beach, contributing to the berms and increasing gradient of the beach
- destructive waves comb material down the beach, contributing the longshore bars and reducing the gradient of the beach
- type of waves that break on any beach will change from day to day and season to season, depending on changing weather and climatic conditions like wind intensity and direction
The role of sediment in the formation of the beach profile
- beach sediment is in constant motion and arrives on the beach from a range of sources: cliff collapse, rivers and sea bed
- the larger the particle size, the steeper the beach
- shingle is more permeable than sand so the backwash drains into it, rather than across it
- shingle often makes up the upper part of the beach - thrown up by strong waves
- sand is less permeable than single so there is more backwash
- sand is smoother than shingle and causes less friction with the waves but the beach is gentler, so wider. This means the wave energy is dissipated over a wide area, not concentrated into a small area
Berm
terrace of a beach that has formed in the backshore, above the water level at high tide
Beach Cusps
semi-circular, scalloped depressions cut into the lower edge of the storm beach
- origins are controversial
Draw a Beach Diagram
- dunes
- backshore
- foreshore
- nearshore
- offshore
Swash-aligned beaches
- beaches where wave crests are parallel to the beach
- waves come into the shore at 90 degrees, usually due to wave refraction. The waves wash up the beach and then swash back down the beach
- there is therefore no longshore drift
- located in bays
- waves are parallel to the beach so the beach is swash aligned
- rare
Drift-aligned beaches
- formed by longshore drift (zig-zag migration of sediment along the beach)
- wave crests approach the beach at an angle
- hence, the beach will migrate laterally with the tide and the beaches are referred to as drift-aligned
- beach may well continue parallel to the drift and detach from the coastline to form spits