rhone - low energy case study

location of the Rhone

• southern france

• river flows into Mediterranean

• made up of grand and petit rhone (85% water in grand rhone)

• diverge 4km north of Arles

• within 30km it splits into many distributaries, forming a delta

what type of delta is the rhone

• arcuate delta

• curved and fan-shaped smooth edge

• wave dominated

how was rhone formed

• over last 7000 years due to sea level rise from end of last ice age

• flood moved course of rhone to present day position in 18th century

• material at mouth of abandoned channel was left, and transported via LSD forming Beaduc Spit

• mouth of grand moved in 19th century, 3 channels to 1 due to human management

why is the rhone a low energy coastal environment

• short fetch (longest is 900km)

• low wind speed mostly from NW

• low wave height and energy

• gently sloping coast so high levels of sediment accumulation from river deposition, waves break early on shore

rates and causes of deposition

• 1900 it was 17 million m³/yr

• river flow and deposition varies seasonally

• high deposition due to flocculation

• tidal range 0.3m creates few currents to carry sediment away

why did amount of sediment being carried decrease

• 20th century river was managed to prevent flooding and for HEP

• in 2000 estimated that river only carried 1.5 million tons a year

how do sediment cells affect the rhone

• cell 1 - movement of material from SW to NE port st louis

• cell 2 - material moved towards east

• cell 3 - 3 converging currents from NW and SE providing sediment

• cell 4 - net movement is toward NW

how is a bar formed and how does it affect the rhone

• LSD transported sand and deposited it as numerous offshore bars

• delta is sloping, beaches narrow with low dunes

• nearshore area has longshore bars - brackish water removes sand and deposits it

• between bars are longshore troughs

• behind bars and dunes are lagoons

how are spits formed and how do they affect the rhone

• 60 km of coastline are eroded, supplies sediment to beauduc spit and espiguette spit

• sandy input from rhone makes the gracieuse spit to east

• LSD in cell 1 deposits material forming gracieuse spit across entrance to port St Louis

• flooding in 18th century meant material at the abandoned channel has been transported by LSD to NW cell 2, forming smaller beaduc spit

• deposition of sediment in cell 4 formed espiguette spit

how is a lagoon formed and how does it affect the rhone

• areas of lagoon have formed behind many onshore bars and spits

• behind dunes so sheltered

• areas of stationary brackish water, flocculation occurs

• salt marsh formed

how is a dune formed and how does it affect the rhone

• aeolian processes from sea move dry sand up beach via saltation

• sand trapped behind obstacles on berm/point of highest spring tides

• sand deposits build up and become colonised by plants

• grasses trap more sand forming an embryo dune which grows into a ridge

• dune stabilises over time and plants colonising them change

• dunes protect salt marshes

long term changes in the rhone

• coastal retreat due to climate change

• gracieuse spit formation via LSD

• retreat of petit camargue coast due to poor sediment supply

how does climate change cause long term changes in rhone

• rising temperature causes thermal expansion and land ice melting contributes to sea level (rise of 2mm/year)

• increased storm activity means more erosion, wind from SE can cause large wave surges

• reduced sediment input from rivers and increased erosion rates - rates are between 2-8m at Stes-Maries

• authorities spent 15 million euros on coastal protection

how does LSD cause long term changes in rhone

• formed gracieuse spit

• retreat of old mouth and erosion of old channel left sediment

• sand derived from erosion was taken by LSD and made a spit

• elongation provides protection against storms for port of marseilles

• central spit is retreating - endangered sustainability of structure

• if spit fails, no salt marshes

how does poor sediment supply cause long term change in rhone

• causes retreat of petit carmague coast

• poorly supplied by petit rhone, which has a dam catching most of sediment

• retreating for decades

• reduction in width of beach

• retreat zone contributes to the advance of espiguette spit

what are the short term changes to the rhone

• destruction of small dunes - human trampling

• cliffs collapsing due to erosion

how does human trampling cause short term change in rhone

• destroy small dunes on lower foreshore

• considerable traffic of vehicles on beaches destabilises dunes

• on piemanson beach especially as its particularly protected from storms and is mostly stable

what isn’t a significant factor in low energy coastlines

geology

instead the effect of waves, wind and humans are more significant

socio economic aspects of rhone

• low population density (5 inhabitants/km²)

• salt production at petite carmague (10,000 ha) and salin de giraud (12,000ha), salt extraction basins protected by embankments/seawalls

• ore and tanker terminal, oil complexes and the solace iron and steel works

• tourism and recreation - 40,000 tourists/year, heritage

• fisheries and agriculture - local economic importance