Coast casestudy: Rhone delta Flashcards
Low energy coastal environment
Where is the River Rhone located and where does it flow into?
- Located: West of Marseilles in Southern France
- Flows into: Mediterranean Sea
What two major distributaries does the delta lie between>
- The Grande Rhone
- The Petit Rhone
How did the Rhone delta form?
- 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.
How did human management influence the Rhone delta?
- Human management changed the position of the mouth of Grand Rhone, reducing it from 3 channels to one.
What shape is the Rhone Delta?
Lobate shaped
Why is the Rhone Delta lobate shaped?
- 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.
Why is the Rhone Delta a low energy environment?
- 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
What is the reason for high deposition rate in the Mediterranean?
- 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.
What is the typical discharge of the Rhone Delta between typical discharge and then in high flow conditions?
- Typical: 1500m³/second
- High flow: 6000m³/second
What are the physical factors that influence the landforms in the Rhone Delta?
- Wave energy
- Tidal range
- Wind energy
- Fluvial presence
List the landforms associated with the Rhone delta?
- Onshore bars
- Lagoons
- Dunes
How have waves influenced the Rhone delta and landforms?
- 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.
How do dunes form in the Rhone?
- 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.
What are the recent changes in the Rhone Delta?
- 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.
Why is the Rhone delta considered a low-energy coastal environment?
The delta is formed where the Rhone River meets the Mediterranean sea.
It is low energy due to low tidal ranges, weak wave action and significant fluvial sediment deposition from the Rhone River.
How much fluvial sediment does the Rhone carry per year?
35-40 million tonnes of sediment
How does the physical factor of wave energy effect landforms in the Rhone delta?
- The Mediterranean sea has low wave energy due to being sheltered.
- Low wave energy means waves are not strong enough to cause significant erosion, allowing sediment to accumulate.
- As accumulation exceeds removal, depositional landforms are formed.
- Longshore drift is limited by low wave energy, so material stays within the delta system rather than being transported away
How does the physical factor of tidal range affect landforms in the Rhone delta?
- The Rhone delta experiences microtidal range (less than 0.4m), so there is little vertical movement of water between high and low tide.
- As a result, tidal currents are weak which limits erosion and enhances formations of lagoons, for example.
How does the physical factor of wind affect landforms in the Rhone delta?
- Mistral wind forms DUNES as dry sand is moved up the beach by saltation.
How are dunes formed in the Rhone delta?
- Wind blows from the sea which moves dry sand up the beach.
- The sand becomes trapped by obstacles on the berm or the point of high spring tides.
- Gradually these sand deposits build up and are colonized by plants. These plants, such as marram grass, are halophytes and consolidate the dunes with their roots.
- Halophytes trap more sand, forming an embryo dune which grows to form a ridge.
- Overtime dunes stabilize.
How does the physical factor of fluvial presence impact landforms of the Rhone delta?
- The Rhone river carried 35-40 million tonnes of sediment annually.
- As the river enters the Mediterranean sea, velocity decreases, causing sediment to be deposited which forms distributaries.
- This continuous depositions expands seaward, creating new landforms.
