Louisiana low-energy case study

• Louisiana coastline is a 650 km long coastline along the Gulf of Mexico making it 12x longer than the NYC
• This low energy coastline is dominated by depositional landforms due to the enclosed nature of the Gulf of Mexico which is sheltered from larger, more energetic, Atlantic Ocean swell waves
• This coastline consists of large amounts of fluvial sediment material inputs from the Mississippi River
• The micro-tidal range of 0.2m created by the enclosed Gulf of Mexico allows fine sediment to be deposited along the coastline
• The coastline sits in the Northern hemisphere subtropical high-pressure belt, experiencing high pressure and light winds throughout the year which produced small constructive waves which builds depositional landforms
• 1. Birds foot delta is the 7th largest delta on earth covering 31,000 square kilometres
• The BFD is formed by huge amounts of fluvial sediment transported along the River Mississippi
• Sediment released by the BFD is transported via longshore drift along the coastline which feeds the Chandeleuer islands and Grand Isle Barrier Reef
• The BFD took 7000 years to form with 6 differsnt lobes created every 1000-2000 years when the Mississippi switches (Metastable equilibrium) to a new path
• Unintentional human activity has reduced the sediment inputs by 60% since 1950 which has created a negative sediment budget and a net vertical subsidence of the delta at a rate of 12cm per year
• 2. Grand Isle Barrier Island is a barrier island 70km west of the BFD
• Longshore drift transports sediment from the BFD along the coast; onshore waves (constructive) waves push the sediment onshore forming barrier beaches
• Within the barrier island, a beach forms on the seaward side, followed by a sand dune which is held together by marram grass, followed by a salt-marsh with salt-tolerant plants which trap fine sediment, and the followed by mudflats and a low energy lagoon
• These landforms on the barrier island may rise due to dynamic equilibrium induced by rising sea levels
• 3. Isle Dernieres are a set of barrier islands East of Achafalaya Bay
• The isles were dramatically changed in the last island hurricane of 1856 when the islands were completely submerged by a 4m high storm surge, eroding the islands in hours and splitting the main island into 2 smaller islands
• 4. The Atchafalaya bay deltas are where 2 new deltas were formed around 50 years ago due to the flow of the Mississippi River changing
• The two deltas were formed due to human factors as well as hurricanes which redistributed sediment
• A human structure built by the US army in the 1960s diverted the flow of the river by 30% which caused a positive sediment budget forming these 2 new deltas in 1973 via a net vertical accretion
• These deltas have grown rapidly at 2.8 square kilometres per year
• The two deltas added a combined 34 square kilometres of land between 1989 - 1995 but lost 2 square kilometres between 1999 - 2004; this coincided with a 3 year period where hurricanes battered the deltas and there were no major floods to replenish the loss of sediment
• 5. The Chandeleuer islands are a 31km-long chain of inhabited islands
• These islands are constantly changing depositional landforms with fluctuating energy and material inputs - they take decades and centuries to form but can be destroyed within hours
• 85% of the lands surface area was lost during hurricane katrina in 2005