Role of deposition in formation of landforms

Role of deposition in formation of landforms

1. Deposition

   • Occurs when water not enough energy so drops particles
   • In areas where swash brings material up beach but backwash not enough energy to bring down again

2. Rowen spit

   • Spit doesn't form from change in direction of coast
   • Instead, LSD deflected by tidal flow and into deeper water
     • Waves don't break -> no swash/backwash
       • Material is deposited
   • Spit increased in size by LSD, waves and wind move sediment along coast and big swash, small backwash mean sediment is deposited, increasing the length
   • Curved end
     • SW winds and wave refraction
       • destructive energy dissipates as waves refract into bay allowing constructive waves to build up curved end
   • Biggest particles deposited out of 3 landforms as subjected to highest wave energy, so pebbles on spit as well as sand

3. Sand dunes

   • Situated on Rowen spit and Barmouth beach
   • Develop where strong offshore winds and a low gradient near shore slope
     • Sand is deposited in this environment and starts to build up
   • Marram grass establishes and roots hold sand in place, allowing consolidation and growth
     • Forms a more effective wind break starting a positive feedback loop: sand is deposited so dunes get higher, which means more sand is trapped and dunes get higher and so on
   • Human processes can encourage deposition and so, growth of sand dunes
     • 1972 causeway built across Bar Bach channel
       • Caused a large amount of deposition behind the causeway, facilitating the development of sand dunes
   • Sand dunes are effective form of natural coastal management, during high spring tide they act as natural wall, protecting what is behind them

4. Salt marsh

   • Area behind Rowen spit sheltered, only open to tidal flow and some refracted waves
     • They carry very fine grained sediments from river estuary, clay & silt
       • Particles flocculate in salt water to form larger particles, which are mainly deposited around high tide, as the flow velocities are low
   • The rate of deposition is highest on mud flats, where flow velocities are lowest and the low energy means the material is more likely to be deposited
     • From high tide mark, deposition extends seawards, which expands the mud flats
   • Salt marsh builds up higher, frequency of tidal flooding falls
     • Allowing vegetation to colonise. The plants (eg spartina) trap sediment, like marram grass, as energy from water is reduced - and so more sediment is deposited
       • Most vegetation is found in the lowest energy environment. (Areas only flooded in a high spring tide)
       • Plants which can withstand saline conditions, allow more deposition as they can withstand tide change

5. Conclusion

   • These landforms controlled by the energy the wind and waves have and so, governed by deposition
   • Enhanced greenhouse effect (312 ppm 1960s to over 400ppm today)
     • Results in rising global average temps which means eustatic sea level rise, due to melting of land based ice and thermal expansion of sea water
       • Deposition will change with rising sea levels
         • Dramatic sea level rise could mean spit submerged, destroyed sand dunes and inundated salt marsh
         • Gentle sea level rise, effects less severe, salt marsh and sand dunes could grow and rise with sea level