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