Medium Energy Environments

Lecture 4/5

GG3025

Beaches

• Medium Energy environments

• Deposition and Erosion

• Unconsolidated deposits of sand and gravel (pebbles) on a shore

• 40% of World’s coastline

• Many different ‘shapes’: straight, curved, pocket beaches in bays or coves

Beaches

Beaches

Beaches• Classification of beaches often based on nature of

sediment and composition

• Various sizes of sediment – coarse, fine, uniform, varied

• Most are ‘sand’ but some have pebbles scattered across the sand

• Sources: alongshore, onshore, beach shore, erosion, rivers and artificial nourishment. In the past from glacial sources e.g. Scotland.

Beaches

• Beaches can adapt their shape very rapidly to changes in energy inputs

• Beaches also have a capacity to dissipate energy

• Beaches can maintain themselves in dynamic equilibrium because its sediments are mobile

Beaches• The morphodynamic development of a beach

system has been identified as having 6 stages (Komar et al 1983)

• The dissipative state reflects the storm profile

• The reflective state equates to the swell profile

• “Intermediate states have complex morphologies and circulation patterns, because they contain both dissipative and reflective elements”

(Pinet, 1992, p265)

Beaches

copyrightPinet1992 p.265

Beaches

copyrightPinet1992 p.265

Beaches

Beaches

• Relict beaches – sediment sources are no longer available

– Natural or artificial diversion of a river– Dams– Anti-erosion works in a catchment

• Most (but not all) shingle beaches in the UK are relict

• Wave action leads to sorting – e.g. Chesil Beach, Dorset

Beaches

• Pebbles may be arranged in ‘patterns’ – cusps (patterns of coarser and finer sediments in the form of spurs and bays) or ridges running parallel to the shore

• Found on beaches exposed to ocean swell

• Can influence the pattern of swash as waves break

Beaches

Beaches

Beach Cusps copyright Pinet 1992 p266

Beaches

• Stable in both plan and profile

• Unstable – rapid changes during storms

• Active accretion / loss

• Relict – no ongoing sediment supply

• Plan and profile changes over time: hours, days or decades

Beach Profile

Copyright Pethick, 1989 p.93

Beaches

• In profile, beaches have three distinct sections: upper, middle and lower

– Upper is infrequently submerged– Middle is more steeply sloping– Lower has a shallower slope and may be

submerged even at low tides

Beaches• Beach Outlines in Plan: plan is affected by angle of waves, and trend of coastline (concave seaward), as well as patterns of sediment and distance from source of sediment (e.g. a river)

• Equilibrium Beach Plans: Cyclic (returns to original condition after disturbance) and Dynamic (changes whilst remaining in balance with ‘driving forces’)

• Beach Outlines in Profile: beach sediment and wave conditions – swash and backwash; tides; summer and winter

Beaches

• Beach materials may be moved and redistributed along a shoreline

• Back and forwards movement• Drift in one direction • Wind, Waves and currents• In bays beaches may accumulate at one

end or other• Beaches may occupy distinct pockets –

bounded by ‘reefs’ or headlands

Beaches• Coastal Sediment and Beach Budgets

• Volumes of sediment supplied to a certain sector by onshore and longshore drift and yields from the hinterland

and

• Volumes of sediment lost offshore, alongshore or landward

Over a time period

•Surveys, remote sensing and GIS

http://www.gisdevelopment.net/application/nrm/coastal/mnm/nrmmm007.htm

Beaches

Beaches• Tracing Beach Sediment Flow

– Longshore drift can be inferred by accretion patterns, deflection of river mouths

– Can be misleading if other movements e.g. from sea floor

– Pebbles of an unusual rock type can act as a natural tracer – from an outcrop or river mouth

– Patterns of beach sediment movement can be determined by introducing and following tracers

– Need to make sure tracer has same behaviour as the beach sediment

– Remote Sensing used to monitor and map sediments

Beaches

• Processes: waves, wind, tides and currents (see earlier lecture on these topics)

• Shapes and modifies beaches

• Erosion, deposition

• Treated separately as processes although acting together

Beaches

• Wave refraction, wave energy, and breaking waves will affect the beach

• Wave incidence and dimensions

• Height and steepness, grain size, sorting, and erosion

• Beach permeability

Beaches

Beaches

• Beaches that are ‘building’ receive more sediment from available sources than they lose onshore, alongshore or offshore

• Built up and outward (deposition)

• High and low tide lines advance seaward

• Where sediment losses exceed gains then erosion

Beaches

• Causes of beach erosion– Submergence and increased wave attack– Diminution of fluvial sediment supply– Reduction in sediment supply from cliffs– Reduction of sand supply from inland dunes– Diminution of sediment supply from sea floor– Extraction of sand and shingle from the beach– Increased wave energy– Interception of longshore drift– Change in the angle of incidence of waves

Beaches

• Causes of beach erosion

– Intensification of obliquely-incident wave attack

– Increased losses of beach sediment to backshore

– Increased storminess– Attrition of beach material

Beaches

• Causes of beach erosion

– Erosion due to beach weathering– Erosion due to increased scour by wave reflection– Erosion accompanying migration of beach lobes– Erosion due to rise in beach water table– Erosion due to removal of beach material by run-off– Erosion because of diminished tide range– Erosion by driftwood or following removal of a sea ice

fringe

Beaches

• Causes of beach erosion

– No single explanation– Contribution of one or other, on its own or

together will differ from place to place– Good example is that of the Lido di Jesolo,

fronting the lagoon of Venice on the Adriatic Coast

Beaches

• Beach erosion suggests that beaches are not naturally stable

• Transient features

• Evidence from geological column

• Widspread progradation however in Holocene times as a result of major marine transgression

Beaches

• Most beaches are ‘geologically’ of recent origin – late Quaternary transgression / Holocene

http://www.quaternary.stratigraphy.org.uk/about/history.html

Beaches•Beach Gradient and Sediment Size: gravels tend to assume steeper profiles than sand because more permeable, and beach face slope increases with pebble size

•Example: Chesil Beach – 5 degrees on fine shingle to 20 degrees on large cobbles

•Effect of swash and backwash

•Storm waves steepen profiles of shingle beaches and flatten those of sandy beaches

Beaches

Chesil BeachDorset

Beaches• Lateral Grading

– Fine to coarse sediment distribution along the shore

– Characteristic of some beaches

– Longshore drift may move finer particles further

– Reverse found on some beaches

– On Norfolk coast the downdrift coarsening of sand to shingle beaches is due to the preferential removal of sand to offshore bars as longshore drift progresses

– May also be due to wearing and attrition of sediment along the coast

Beaches• Beach Compartments:

– Compartments or cells occupied by beaches

– Breakwaters, headlands etc..

– Movement of sediments to and fro along the shore

– Material may move out along the coast by longshore wave and currents

– River mouths or tidal entrances impede longshore drift (similar to headlands or breakwaters)

– Accretion on updrift side, erosion on downdrift

Beaches

Beaches

Beach Berms

• Flat berms or terraces produced by swash deposition at high tide level

•Swash Berms are ridges parallel to coastline along the length of the beach

•Where swash action is prolonged (high and low tide) then two berms develop

Beaches

• Beach sediments once accumulated experience rounding and attrition

• Grains become smoothed and highly polished• Pebbles tend to be come slightly flattened and

thinner at right angles to the longest axis• Occasionally fluctuating water table (tides) within

a beach leads to precipitation of carbonates that ‘cement’ the beach sand into hard sandstone layers – ‘beach rock’

Beaches

• Effects of Artificial Features on beaches

– Sea walls, boulder ramparts (rip rap), concrete tetrapods may cause backwash on seaward side (less reflective though than solid seawalls

– Erosion

Beaches• Beach Nourishment:

– Artificial supply of sand to beach by engineers– Needs understanding of coastal

geomorphology– But experimental approach better than

theoretical models– Many sources: inland, alongshore, offshore– Needs to be durable sediment– Subject to same processes as ‘natural’

sediment– Monitoring guides further coastal

management

Beaches

Beach Nourishment:

• http://www.nap.edu/books/0309052904/html/index.html

• http://www3.csc.noaa.gov/beachnourishment/

• http://www.conshelf.com/services/beach-nourishment/beach-nourishment.html

• http://www.coastal.ca.gov/pgd/pgd-mon.html

Beaches

Beaches

Beaches

Sea Bright Before Nourishment Sea Bright After Nourishment