CASE STUDY - Rhone Delta (low energy coastal environment)

Question 1

Brief overview

Answer 1

• Has formed over the last 7000 years
• Total length of the coastline is 90km, covers an area of 1740km^2
• Lobate shaped delta - dominated by waves rather than the tide or river

Question 2

Physical factors that influence the formation of landforms

Answer 2

• Dominant wind direction (NW) typically has low wind speeds, however onshore winds from SSE and SE speeds can exceed 100km/hr
• Waves tend to be low in height and energy, heights of 0.5m to 1m in most cases, most frequent wave direction is SW (30% of total) - also due to the enclosed shape of the Mediterranean Sea which causes fetch distances to be relatively short in all directions (longest 900km from the SW)
• High levels of sediment accumulation from river deposition has created a gently sloping landscape - waves break early on the shore and their energy is dissipated by the beach and delta sediment
• High accumulation of sediment due to small tidal range (0.3m) and high deposition (due to flocculation of clay particles), Mediterranean is very saline, when slat water mixes with fresh, any clay particles present become adhesive and stick together forming larger, heavier clumps that can no longer be held in suspension, deposition was estimated to be 17 million m^3 per year

Question 3

Landforms within the landscape system

Answer 3

Pattern of sediment transport can be split into four cells in the delta
- Cell 1 dominated by SW-NE LSD which has formed the Gracieuse spit across the entrance to Port St-Louis at Marseilles
- Cell 3 has converging LSD currents, which has resulted in the formation of onshore bars - all along the coast, areas of lagoons can be seen where LSD has moved sand to create lagoons trapped behind the onshore bars and spits
- Dunes: The delta is very gently sloping, coastal beaches are narrow with low dunes behind them, dunes form when the winds blow from the sea and dry sand is moved up the beach by saltation, sand becomes trapped by obstacles and gradually the sand deposits will build up and become colonised by plants (eg. marram grass) which are adapted to salty conditions and have long roots to help them survive on shifting sands, the grasses trap more sand, forming an embryo dune which grow to form a ridge - over time dunes stabilise and the plants colonising them change as soils become established
- Delta covered by areas of brackish saltwater marshes behind sand dunes and around the edges of lagoons

Question 4

How and why the landscape system changes over time

Answer 4

• Climate change and increased sea levels has led to a rise in sea level of 2mm/yr since 1950 (Med), plus increased storm activity have affected the delta, leading to increased rates of erosion of beaches and dunes
• Although tidal range is small, winds from SE can rust in large wave surges - this + reduced sediment input from rivers = change in relationship between sediment inputs and outputs in the system resulting in rates of coastal retreat between 2-8m, notably at Stes-Maries
• Authorities have spent £15 million on coastal protection - including groynes, rip rap and sea walls to try to restore equilibrium in the system