Section 2: Beaches, estuaries Flashcards
What is a beach
Collection of non-cohesive material at the interface between dry land and the sea, actively ‘worked’ by waves, tides and currents
What is a barrier
A sand or single bar above high tide, parallel to the coastline and separated from it
Why are beaches and barrier important
Coastal protection and resilience
Creation of unique natural environments
Recreation
What is depth of closure
The depth at which waves start to touch the bed. Wave shoaling occurs, no longer circular waves. Not a fixed position, relative to wave condition
Lots of sediment moves onshore, very little offshore
What is a bar
Found within region of breaking wave, normally below low tide mark
Indicated where sediment collects
Summer: flatter offshore profile and berm
Winter: bar type profile
Material comes from both directs, where they meet forms a bar
Onshore movement due to interaction with shoaling wave
Offshore movement due to waves breaking and undertow
Can be multiple bars, with troughs and channels on bigger and wider beaches, waves have time to reform after the bar
If can see waves breaking out at sea, a white line, a bar
Rip channels can form
What is the swash zone
Upper part of the beach which is alterately wet/dry on timescales of breaking waves
Uprush (onshore) and backwash (offshore) flows
Large sediment movement
What is the beachface
Planar, relatively steep upper-part of beach profile subjected to swash
Swash zone can be considered
Steeper at the top
Tidal range determines size of beachface
What is a berm
Nearly plantar section landward of beach beach
Accretionary feature separated from beachface by berm crest
faces landward
Summer: Forms during long wave periods
What is the shoreline
Plantar, relatively steep upper part of beach profile subject to swash
Actual transition part between land and water
Dynamic
Summer (swell) profile
Calm conditions (low wave steepness)
Onshore transport
Creation of berm features
Longer period waves: short wave height compared to length
Sediment pushed up the beach, onshore transport
Grain size intersection between swell and storm profile will change
Bigger grain size more likely to see storm profile
Winter (storm) profile
Energetic conditions (high wave steepness)
Beach erosion and offshore transport
Creation of bar features (s)
Seasonal control, not much sediment lost from the system, dynamic equilibrium
In reality, big storms may take sand outside the depth of closure
Chronic loss of material
Macrotidal characteristic
> 4m
Sweeping zone of tidal concentration
Wave energy only focuses on area for small period of time
Flat, largely featureless
Big intertidal zones
Microtidal characteristics
<2m
Small zone of tidal concetration
Wave energy focused on an area for extended period of time
Steep with prominent berm
Small intertidal
Reflective beach
Tidal range is relatively low in relation to breaking wave height, big grain size, gravel beach
Microtidal characteristics
<2m
Small zone of tidal concertation
Wave energy focused on an area for extended period of time
Steep with prominent berm
Small intertidal
Dissipative beach
wide, flat small grain size, large tidal range
What is the height of a wave that breaks in 1m of water
0.78m (can vary significantly)
Longshore currents in the UK
Wave that break at an angle to the beach contours
Tidal variation
Variation in breaking wave heights along a beach
What is bed return flow
Offshore movement of water at the bed, part of vertically segregated circulation pattern
What is swash aligned
Parrall to wave crests, sediment is a closed system
Drift aligned
Oblique to wave crest. formed by LSC, open sediment cell
Brunn rule
SHoreline retreat derived by assuming erosion in the upper part of profile is balanced by deposition in the lower part
Define estuary
A partly enclosed coastal body of brackish water with one or more rivers flowing into it, with a free connection to an open sea
Why is river sediment retained in estuaries
Circulation patterns: retention of load
Aggregation: biological or flocculation (charge between clay particles)
Coastal plain estuaries
Rising sea level causes ocean to invade existing river valleys. ‘Drowned river valleys’. Common in US, Europe – Southampton Water.
Fjord estuaries
Glaciated valley. U shaped with steep walls. Common in Norway, Canada, Alaska,
New Zealand and Scotland.
Bar built estuaries
shallow and is separated from the open ocean by sandbar deposited parallel to coast by wave action. Common along US East coast and in India.
Tectonic estuaries
Produced by faulting or folding of rocks. Down-dropped area into which river flows. San Francisco Bay
Highly stratified estuary
Salinity contours (isohalines) horizontal and close together
Common: deep estuary, micro-tidal where there is sufficient river discharge to develop a fresh surface water mass, but discharge rate is insufficient to completely expel lower saline water
Large river input, small tidal input
Predominantly fresh down stream flow
Weak upstream flow of salt water
Highly stratified salt wedge estuary
Salinity (contours (isohalines) horizontal and close together
Common: where a river discharges to into a virtually tide-less sea. Position of salt wedge is not stationary, but depends on river flow
Slightly stratified, partially mixed estuary
Salinity contours dip steeply near surface and near bed
Common – tidal energy is sufficient (meso-tidal) to cause shear along the halocline. Turbulence caused by shearing – mixing. Salinity increases from head to mouth, but two basic water lays can be identified separated by zone of mixing.
Weak down stream flow, dominant upstream flow
Well-mixed estuary
Salinity contours nearly vertical
Common –tidal range is large to water depth (shallow estuaries) and turbulence produced by vertical shear on bottom large enough to mix estuary completely. Tidal flow much larger than river flow - macro-tidal coasts.
Very weak downstream flow
Wide well-mixed estuary
Salinity contours very close together
Laterally inhomogeneous – Estuary is sufficiently wide for coriolis force to be effective, river and sea water may become horizontally segregated
Inverse estuary eg Shark Bay Australia
Hypersaline, double the salinity of normal seawater
Inverse estuaries occur in dry, hot and arid climates where evaporation greatly exceeds the inflow of freshwater.
What is residence time
The period of time required for a water parcel to leave the domain implying the definition deals with moving individual pieces for a spatially varying situation
What is flushing time
The average amount of time freshwater spend in a system iethe rate whch fresh water is flushes out of an estuary
Low mean salinity in an estuary
suggests a long residence time, difficult for freshwater to get out, is mixed
High mean salinity in an estuary
suggests freshwater gets out quickly before it can be mixed with salt water
Residence time calculation
Time required to replace the total volume of freshwater with the estuary with ‘new’ freshwater from the river
Tres = Vf / R
Vf (little f) is total amount of river water accumulated in estuary
R is river flow (m^3/s)
Residence time calculation with fresh water fraction
f = 1 - Sm/Ss
f is freshwater fraction
Sm is mean salinity
Ss is salinty of the sea
Tres = fVe / R
