SHORELINE AND BEACH PROCESSES: PART 2 Implications for Coastal Engineering
Objectives of the lecture: Part 2 Show examples of coastal engineering Discuss the practical difficulties of ocean engineering at the coast Explain interactions between beach processes and structures Introduce beach processes and shoreline evolution Introduce beach management methods
Shoreline processes - an introduction Outline of presentation Practical importance of shoreline changes Geological evolution of the coastline Cliffs, beaches and the nearshore seabed Cliff recession; causes, consequences and counter-measures Beach changes; causes, consequences and counter-measures Longshore sediment transport - identification and control Cross-shore shore sediment transport - identification and control
Importance of shoreline changes Flooding of hinterland Erosion of coastal cliffs/ slopes Damage to structures/ property Siltation (killeşmek) of deepened areas Accretion of beach sediment
Coastal flooding - over-washing
Coastal flooding - over-washing
Coastal flooding - breaching
Coastal erosion
Happisburgh - July 2002
Happisburgh - August 2002
Happisburgh - October 2002
Happisburgh - December 2003
Damage to property - by erosion
Damage to property - by erosion
Damage to property - by overtopping
Harbour siltation
Geological origins/ inheritance Today s s shorelines are largely a result of geological processes, i.e. Tectonic processes (uplift, folding, continental drift); Production of sediments by glacial action; Sedimentation in lakes/ shallow seas; Long-term climate changes (glacial/ inter-glacial periods), leading to changes in sea level; Re-working of these sediments by changes in sea level.
Shoreline elements Cliffs and coastal slopes Beaches (and dunes) Nearshore seabed (to 5-10m 5 below lowest tide)
Cliffs and coastal slopes Cliffs top edges retreat (by definition!) Continually trying to adjust to a stable slope Land-sliding largely due to water flows from landwards Slips triggered by wave action Slips can be small & regular or large & rare (depending on geology) Debris ( talus( talus ) ) deposited after slip protects cliff face (temporarily) Hydrodynamic processes sort debris to provide (some) beach sediments
Cliffs and coastal slopes Cliff management methods Drainage (surface or internal) to reduce groundwater levels Planting vegetation to fix surface and de-water ground Direct face protection, e.g. grading / netting./ revetments Toe protection e.g. sills or walls, or higher beach levels Reduction of wave heights, e.g.by offshore breakwaters
Cliff slip - Overstrand,, Norfolk
Cliff toe protection - rock sill
Cliff protection - locking sediment
Cliff management - Robin Hoods Bay
Beach sediment transport Causes Waves (good at mobilising and stirring sediments) Currents (tide or wind-induced induced - good at carrying sediments) Winds - can transport dry sand in large quantities Human interventions - mining,, dumping waste materials Transport routes Parallel to beach contours - longshore transport Onshore-offshore offshore or cross-shore shore transport Vertical movements suspension or wind-borne
Longshore transport on beaches Causes Waves breaking obliquely to contours - causes currents Breaker height gradients - transport towards smaller waves Tidal currents - if strong at high tide Winds on dry sand beaches Clues to longshore transport (drift) direction Spits and diversion of river mouths Accumulation updrift against groins/ harbor arms etc. Erosion downdrift ofof breakwaters, seawalls, etc
Longshore transport of sand by wind
Massive beach spit, UAE
Effects of groins on longshore drift
Groin n on shingle beach
Long groin and spit, Dorset
Longshore drift - north Norfolk
Erosion downdrift from a seawall
Pocket beach - very little drift
Tombolo in Scotland - zero drift
Cross-shore shore transport on beaches Causes Waves travelling perpendicular to contours - onshore drift Breakers cause undertow current and suspend sand Beach porosity sucks down sediment particles in swash zone Winds on dry beaches blow sand onto dunes Tidal currents can carry sediments onshore or offshore Steep seabed slopes into deep water - offshore losses
Dunes - a sand reservoir
Beach management methods Sediment budgets First step is to understand sources, sinks and transfers along a section of coast; Sources include rivers, eroding cliffs, shell fragments Sinks include deep seabed, dredged channels, dunes Transfers longshore (and cross-shore) shore) transport Greater transfers in than out deposition (accretion) Greater transfers out than in - erosion
Beach management methods Advantages Relatively cheap and simple to improve / create beaches; Failure is gradual and obvious, with some self- healing; Environmental impacts are acceptable; Adds amenity value to coastline; Work well in areas where drift rates are low; Unlikely to lead to erosion of down-drift drift beaches; Maintenance methods are simple.
Beach management methods Disadvantages Rarely a permanent solution (half-life life 5-105 years at best); May allow some overtopping, and hence flooding; May cause siltation or wind wind-blow problems; Transfers longshore (and cross-shore) shore) transport; Often need regular maintenance or control structures ; May not be suitable for local problems on beaches with high longshore transport rates
Beach management methods Techniques Beach recharge,, i.e.adding extra sediment (from offshore); Re-cycling beach sediment (as at Seaford) to counter drift; By-passing sediment around obstructions to drift, e.g. harbours; Controlling drift rates by installing groynes,, breakwaters etc. Dune management and reducing reflections from seawalls; Reducing tidal currents close to the shoreline;
Beach recharge in progress
Hybrid groin / breakwater
Recharged beach with low groins
Beach management - guidelines 1 Establish long-term rates of change (expect erosion); 2 Identify sources, sinks and sediment budget; 3 Calculate drift rates 4 Predict future beach changes and validate against past changes; 5 Consider beach recharge/ recycling/ bypassing; Sources of extra sediment? Rates of loss, and maintenance commitments? 6 Assess/ model changes in beach and adjacent coastline 7 Consider control structures such as groynes,, breakwaters etc. 8 Re-model scheme and refine; 9 Carry out sensitivity tests, eg.. changes in wave climate.