Human activity intentionally causing change in a landscape system

Coastal system at Christchurch Bay

• Drift aligned coastline, exposed to dominant waves from the SW

• micro tidal range of C. 1m

• centre of bay is exposed to strong SW prevailing winds with a fetch across the Atlantic ocean 3000 miles/4800km - some protection is offered by Hengistbury Head on the western side of the bay

• unprotected areas of bay are retreating (erosion + MM) at a rate of 1-3m/year depending on geology, sheltering from hengistbury head, and spatial variation in management strategy

Sediment budget in Christchurch Bay

Inputs:

• cliff erosion - East Barton on Sea and Milford on Sea 58,000m³/yr

  • Chewton Bunny and Barton on sea = 50,00 m³/yr

• onshore transport

• beach nourishment - 50,000m³/yr

Outputs:

• transport from beach to sediment sinks in offshore zone i.e. shingles bank

• permanent output from Christchurch bay circulation system i.e. lost to sea

• total outputs = 505,000 m³/yr

Stores:

• beaches - 1.5 mill m³

• Hurst spit - 5 mill m³

• Shingles bank offshore bar - 42 mill m³

Human activity causing change intentionally within coasal landscape systems

• Groyne - intercepts transportation by LSD to another coastline

• Sea wall - dissipates KE by reducing erosion

• Sediment redistribution

• beach nourishment - using shingle reduces wave friction and dissipates KE and water volume lost due to percolation, so deposits sediment

• rock armour - dissipates KE

• gabion - dissipate KE

• managed retreat - natural erosion causing slumping of cliffs so natural input of sediment

Reasons for implementation of management strategy

• Hengistbury head Long Groyne 1938, built by Bournemouth council, preventing sediment redistribution and starving sediment in Christchurch Bay

• Hurst castle - intrinsic heritage value , special scientific interest (SSSI)

• Wetland Key haven - salt marsh, Biodiversity (SSSI), carbon store, UK signed up to Ramsar convention

• High value property that needs protecting across the whole bay

• tourism + leisure activities → jobs and economic value

Long Groyne at Hengistbury Head

Impact on processes and flows:

• limits LSD moving sediment from Poole Bay into Christchurch Bay, restricting movement of sediment but also absorbs wave energy and reduces rate of erosion

Effect of impacts on landforms:

• Mudeford spit to the East of the Long Groyne has depleted in sediment so further management has been required here e.g. beach nourishment and groynes

• over whole of CCB, there has been unintentional impact due to sediment input from Poole Bay being restricted. Therefore, coastline has retreated up to 60m, but now there is intentional management attempting to limit further erosion and encourage the accumulation of sediment

Managed retreat at Naish farm/Chewton Bunny

Impact on processes and flows:

• enables sediment input into the budget due to cliff collapse

• added sediment dissipates KE, reducing energy in the waves and enhancing deposition - this is a positive feedback mechanism and helps to re-establish equilibrium

Effect of these impacts:

• the cliffs at Naish Farm and Chewton Bunny are eroded at the base and destabilised due to the WCN, so slumping occurs leading to cliff retreat

• sediment is input into the beach store which is replenished naturally

• but coastline continues to retreat in certain areas where added sediment is not available due to groynes limiting LSD

Complex cliff stabilisation methods at Barton-on-sea

Impact on processes and flows:

• stabilisation technique intends to limit GPE and prevent MM via slumping

• achieved via drainage removing water and prevents a slip plane from developing due to lubrication

• steel sheets are added half way up the cliff to limit the downward movement of material

Effect of impacts:

• cliff is stabilised and therefore retreat is minimised - the beach is starved of an input of sediment (unintentional)

Rock armour and groynes at Milford on Sea, sea wall in front of historic White house

Impacts on processes and flows:

• rock armour dissipates KE, reducing erosion of cliff and sediment

• groynes trap sediment and limit LSD, beach profile becomes taller and beach becomes wider with sediment as sediment is trapped behind groynes → dissipates KE and encourages further deposition of sediment

• sea wall reflects KE and prevents erosion of the cliff

Effect of impacts:

• cliffs are protected

• sediment accumulates behind groynes

• intention is to re-establish equilibrium in this location

Beach nourishment at Hurst Spit

Impact on processes and flows:

• sediment budget added artificially

• KE is dissipated from the waves, encouraging more deposition - positive feedback

• enhancing sediment here protects SM from erosion as an intentional consequence

Effect of impact:

• spit is protected from KE

• SM is also protected

Sand mining in Mangawhai-Pakiri coastline of NZ

• sand is used for glass, concrete, filling roads, reclamations, building sites, and for renourishing beaches

• on average - 200kg of sand is used per person per year

• much of the sand is used to replenish the tourist beaches at Auckland - sand is taken from non-renewable resources

• when sand is removed from the system, a new equilibrium will be established

• sediment cell balance has been severely disrupted 1994-2004, 165,000m³/year of sediment was extracted

• Removal ceased from Mangawai, but Pakiri Beach removal continues - extraction exceeds inputs by 5 times

• there are few sizeable rivers in the area for input of sediment - rivers can contribute up to 80% of sediment in sediment cells

• due to nature of holocene sand, it is a non-renewable resource as it was brought in at the end of the last ice ace in a marine transgression = sediment deficit

• source sand = relic sand → removal of sand is permanent = extraction is unsustainable

Unintentional impact of Mangawhai-Pakiri sand extraction on processes and flows

Flows of sediment:

• transportation and transfers of sediment between the sediment stores is much reduced e.g. between the sand dunes, beaches, offshore zone and spit at Mangawhai

Processes and energy:

• due to reduction in sediment in the system, beaches which have become wider and flatter are less effective at dissipation KE, therefore rates of erosion have increased, making the dunes and spits more vulnerable to erosion and transportation

Effect of impacts of extraction from Mangawhai-Pakiri on changing coastal landforms

• Beaches - wider and flatter

• Dunes and spits eroded as less protection from wider, flatter beach. more energy hits dunes and spits

• Foredune ridges - undercut by wave action creating steep, seaward-facing scarps

• loss of vegetation in the dunes making them more vulnerable to aeolian erosion as the veg helps to stabilise the sediment

• two breaches of Mangawhai spit have occurred since 1978 i.e. the spits have been split into two - tidal currents were then altered leading to the sedimentation of Mangawhai harbour

The consequence of the changes caused by extraction in Mangawhai-Pakiri on the coastal landscape

• extraction rates at Pakiri beach exceed inputs by 5X = total sand supply in the landscape is being depleted

• stores of sediment in the landscape i.e. dunes, beaches and the sea bed, are diminishing

• transfers of sediment between the store has been reduced

• groynes have been installed to help restore equilibrium on the spit at Mangawhai

• has made the landscape more susceptible to the impacts of CC - SL rise, greater energy in the coastal system due to storm activity

• long term retreat by the end of the century is estimated at 35m