Rhone Delta : low energy

Question 1

Where is the Rhône delta?

Answer 1

The river rhone runs into the mediterannean sea just to the west of Marseilles in southern France. Delta lies between the two major tributaries of the river Rhone, the grand rhone and the petite rhone, which diverge 4km north of the town of arles. In the 30km between this spit and the sea, the rhone splits into many channels to form a delta. The east branch, the grand Rhône, is the largest of the two branches and carries 85% of the river’s water into the Mediterranean sea

Question 2

Why is the Rhône delta a low energy environment?

Answer 2

As the Rhône enters the sea, the velocity/energy of the river falls. Due to the small fetch of the Mediterranean and climate of the area, there are low energy waves most of the year so more sediment is added than removed (sediment surplus). The longshore currents create bars and spits which shelter areas further from the energy of waves encouraging further deposition.

Question 3

What is the wave height?

Answer 3

0.5-1m in 80% of cases

Question 4

Sediment cell information

Answer 4

The Rhône delta has formed over the last 7000 years since the sea level rise of the last ice age ended. Cell 3 has a zone of converging longshore drift currents and a spit has formed at beauduc due to this

Question 5

What direction is the sediment moving in?

Answer 5

Wave direction is mostly SW (30%) which only produces low wave heights of 0.5-1m

Question 6

Prevailing wind direction

Answer 6

SW

Question 7

Importance of the sediment load?

Answer 7

River collects a large amount of sediment from the young sedimentary rocks of the french alps. Deposition in 1900 was estimated to be 17 million m3 per year (or 50 tonnes every minute)

Question 8

What is the tidal range and its importance?

Answer 8

Small tidal range of 30 cm reduces the amount of land exposed to coastal erosion which creates very few currents to carry material away

Question 9

Salinity and importance of this

Answer 9

High salinity of the Mediterranean Sea causes a lot of flocculation, which increases deposits

Question 10

Length of the Rhône?

Answer 10

90km

Question 11

Average discharge of the Rhône?

Answer 11

17 million m3 per year

Question 12

Source of the river Rhône?

Answer 12

Rhône glacier

Question 13

Source of sediment in the Rhône?

Answer 13

Debris deposited by valley glaciers that extend from the alps

Question 14

What is the Rhône valley’s land use?

Answer 14

Tourism

Question 15

Human impacts on the Rhône delta?

Answer 15

• Management of the river for hydroelectricity and to prevent flooding have affected the sediment load leading to a reduction of 17 million m3/year in 1900 1.5 million m3/year in 2000
• Reduced sediment load has caused coastal erosion e.g 8m eroded at sainte mariés de la mer since 1994
• Coastal management has been put in place to counteract this- this has led to 85% of the coastline now being managed and has prevented further erosion
• In the 19th century, the mouth of the Grand Rhône changed position due to human management, reducing it from 3 channels to one

Question 16

Short term natural changes

Answer 16

• The delta changes seasonally as river flow in the Rhône is seasonal from 1500m3 per second in the summer to 6000m3/second in spring and winter

Question 17

Long term natural changes

Answer 17

-delta has grown since sea level stopped rising 6-7000 years ago. The river deposited material in the deepened channel eroded by the river during the last ice age
-2mm sea level rise/yr since 1950 and increased storm activity has led to a reduction in beach size and dune erosion and this will continue with further climate change
-this threatens coastal towns on the delta such as saintes Mariés de la mer from coastal erosion

Question 18

How are the landforms along the Rhône delta interrelated?

Answer 18

-The delta is very gently sloping. Coastal beaches are narrow with dunes behind them. Behind the dunes, there are lakes and lagoons. In the nearshore area, there are longshore bars.

-dunes form when winds blow from the sea, and dry sand is moved up the beach by saltation. Obstacles on the berm of the point of the highest spring tides trap the sand. Gradually these build up and are colonised by plants such as marram grass which are adapted to the salty conditions

-area is covered by brackish salt water marshes and coastal salt meadows. Salt water lagoons, near the sea are formed where sand bars and spits have separated the sea.

-the formation of bars and spits further reduces wave energy so more deposition takes place.

Question 19

Change in rates of deposition between 1900 and 2000?

Answer 19

17 million m3 in 1900 to 1.5 million m3 in 2000

Question 20

What is the impact of the reduced sediment load?

Answer 20

8m eroded at sainte mariés de la mer since 1994