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

3. Beaches

4. Beaches

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

10. Beaches

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

13. Beaches

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

21. Beaches

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

25. Beaches

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

37. Beaches

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

42. Beaches http://www.nap.edu/books/0309052904/html/index.html
Beach Nourishment:

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44. Beaches

45. Beaches
Sea Bright Before Nourishment
Sea Bright After Nourishment