Coast casestudy: Rhone delta

Question 1

Where is the River Rhone located and where does it flow into?

Answer 1

• Located: West of Marseilles in Southern France
• Flows into: Mediterranean Sea

Question 2

What two major distributaries does the delta lie between>

Answer 2

• The Grande Rhone
• The Petit Rhone

Question 3

How did the Rhone delta form?

Answer 3

• It has gradually formed over the last 7000 years as a result of sea level rise at the end of the last ice age.
• The current status of the shoreline was formed in the early 18th century (1700s) when a flood changed the course of one of the channels.
• As a result of the flood in the 1700s, material at the mouth of the abandoned channel moved to form the Beauduc Spit.

Question 4

How did human management influence the Rhone delta?

Answer 4

• Human management changed the position of the mouth of Grand Rhone, reducing it from 3 channels to one.

Question 5

What shape is the Rhone Delta?

Answer 5

Lobate shaped

Question 6

Why is the Rhone Delta lobate shaped?

Answer 6

• The delta is dominated by waves (rather than tide or river), this means waves reshape the delta by moving sediment at the edge by longshore currents.

Question 7

Why is the Rhone Delta a low energy environment?

Answer 7

• It enters the Mediterranean, which is enclosed —-> fetch distances are short in all directions, even with the longest fetch of 900km being disrupted by the Balearic Islands
• The dominant NW wind direction typically has low wind speeds —-> this produces waves that are low in energy and height
• There are high levels of sediment accumulation from river deposition so waves break early on the shore and their energy is dissipated by beach and delta sediment

Question 8

What is the reason for high deposition rate in the Mediterranean?

Answer 8

• Flocculation of clay particles
• The Mediterranean is very saline, so when salt water mixes with fresh water clay particles become adhesive and stick together.
• Clay forms larger, heavier clumps that leave suspension
• The Rhone has a small tidal range of 0.3m, creating few currents to carry material away.

Question 9

What is the typical discharge of the Rhone Delta between typical discharge and then in high flow conditions?

Answer 9

• Typical: 1500m³/second
• High flow: 6000m³/second

Question 10

What are the physical factors that influence the Rhone Delta?

Question 11

List the landforms associated with the Rhone delta?

Answer 11

• Onshore bars
• Lagoons
• Dunes

Question 12

How have waves influenced the Rhone delta and landforms?

Answer 12

• Waves come in 2 main directions - the most frequent: SW, which are low in height and energy. SSE and ESE waves are less frequent that are higher energy waves.
• Cell 1 is dominated by SW-NE longshore drift, wave action has formed the Gracieuse Spit across the entrance to Port St. Louis at Marseilles.
• Cell 3 has converging longshore drift currents - resulting in the formation of onshore bars.
• Generally, waves have created lagoons where longshore drift has moved sand to trap lagoons behind onshore bars and spits.

Question 13

How do dunes form in the Rhone?

Answer 13

• Dunes form when wind blows from the sea, and dry sand is moved up the beach by saltation.
• As material moves via saltation, the sand becomes trapped by obstacles on the berm (or the point of the highest spring tides)
• Gradually, sand deposits build up and become colonized by plants, such as marram grass that adapted to saline conditions and developed long roots to help survival on shifting sands.
• Vegetation consolidates the dunes, trapping more sand which forms an embryo dune - which then grows to form a ridge.

Question 14

What are the recent changes in the Rhone Delta?

Answer 14

• Climate change, accompanied by rising temperatures have impacted the Mediterranean.
• Sea level has risen 2mm/year since 1950, which is perpetuated by increased storm activity. This has led to increased rates of erosion of beaches and dunes in the Delta.
• Despite low tidal range, SE winds can result in large wave surges. The interaction between output and input has changed —> There is reduced sediment input from rivers and large wave surges. This has resulted in rates of coastal retreat between 2 and 8m.
  Authorities have spent 15 million Euros on protection such as groynes, riprap and sea walls to restore equilibrium.