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Beaches Medium Energy environments Deposition and Erosion

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Presentation on theme: "Beaches Medium Energy environments Deposition and Erosion"— Presentation transcript:


2 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

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5 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.

6 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

7 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)

8 Beaches copyrightPinet1992 p.265

9 Beaches copyrightPinet1992 p.265

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11 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

12 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

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14 Beaches Beach Cusps copyright Pinet 1992 p266

15 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

16 Beach Profile Copyright Pethick, 1989 p.93

17 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

18 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

19 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

20 Beaches Coastal Sediment and Beach Budgets and
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

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22 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

23 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

24 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

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26 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

27 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

28 Beaches Causes of beach erosion
Intensification of obliquely-incident wave attack Increased losses of beach sediment to backshore Increased storminess Attrition of beach material

29 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

30 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

31 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

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

33 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

34 Beaches Chesil Beach Dorset

35 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

36 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

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38 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

39 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’

40 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

41 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

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Beach Nourishment:

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45 Beaches Sea Bright Before Nourishment Sea Bright After Nourishment

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