Low Energy Coastlines GG3025 Lecture 2/3

Low Energy Coastlines Salt marshes Mangroves copyright www.amarulasail.com/images/oct2003

Low Energy Coastlines Characteristics Estuarine Saline Temperature (temperate and tropical) Shelter Shallowness Ebb and Flow Currents Tidal range Maritime Salt marsh and Intertidal Flats

Salt marshes Typified by: Vegetated sward Drainage creeks and pans Wadden (intertidal flats) Seasonal algae (Enteromorpha sp.) Sea grasses Vegetation succession (salinity/submersion) Sometimes a vertical front/cliff to the saltmarsh separating from the intertidal flats

Salt marshes Rich ecology, highly productive (land to sea transition) Conservation interest – flora and fauna Subject to external influences such as: Drainage Pollution Land reclamation Changing land-sea relationships

Salt marshes Accumulation of fine sediments often in estuarine or margins of a bay or lagoon environments: Settling lag Flocculation Biological aggregation Colonised by vegetation Sediment settles through vegetation trapping, low current velocities Sediment recycling (ebb and flow channels)

Salt marshes Sediment sources: Rivers Sea Variable composition: silt, sand, clay, and organic content Windblown content in some cases

Salt marshes Studies have focused on: Models for salt marsh development Marsh Creek and Pan formation Growing/Decaying landform Salt marshes are studied in terms of hundreds rather than thousands of years (rocky coasts)

Salt marshes Upper level of the intertidal zone Clay and silts/silty clays often deposited on coarser sediments e.g. sands/gravels Distinguished from mud flats by vegetation Intertidal flats (wadden) comprise sand, silt, clay and mud (cohesion and plasticity) Mud originates from the land/estuary Sand/Silt from the sea But very generalised description

Salt marshes Sediment distribution Sediment grain size making up the wadden tends to decrease further inland Decreasing energy of wind and tide Complicated by localised tidal circulation Suspended material settles out of the water column (lag in settlement)

Salt marshes Scour lag during the ebb because of sediment cohesion Vegetation traps sediment Water covers wadden and the marsh via channels Settling of sediment Flocculation and biogenic trapping

Salt marshes Sediment trapped by vegetation due to wave dampening and impeding tidal flow Chemical micro-environment leading to clay flocculation Algal mats Colonies of intertidal animals Invertebrates trapping sediment Clay aggregation leads to increase in settling velocity – flocculation (especially where different salinities)

Salt marshes Sediment supply to salt marshes may be seasonal due to variations in: Physical Processes (reduced river flow in summer) Biological Processes (reduced algal activity in winter/plant growth in summer)

Salt marshes Sediment Recycling: continued minor erosion and deposition in channels through channel mobility, undercutting of creek banks, water circulation through the marsh, slumping Pan and Creek Formation: proto-pans are areas of standing water, proto-creeks the intertidal ebb and flood channels

Salt marshes Characteristics (research and field observation): Pans often non-vegetated (salinity) whilst partially colonised pans Pan enlargement is often active Channel pans have been found… possibly due to siltation/blockage of primary creeks, lateral spread of vegetation to create roofed and unroofed creek Plant debris stranded on marsh can cause marsh plant death and a secondary pan due to tidal scour Erosion of marsh fronts causing toppling of vegetated blocks to create fringing secondary marsh and secondary pans

Salt marshes Problems: No evidence of pan shallowing as a result of pan extension Water spilling from one pan to another (leading to pan extension) not supported as pans fill from bottom during rising tide Channel pans possibly created through silting creeks could be reversed through sub-surface water movement and this may provide new explanations for pan and creek development – including new pans on a mature marsh