ECE student George Cottell reports on the Hengistbury Head/Christchurch Harbour fieldtrip

By ECE 2015-16 student George Cottell reports on the fieldtrip to Hengistbury Head and Christchurch Harbour:

This field trip saw groups from both Coastal Sediment Dynamics and Coastal Morphodynamics modules come together for the day to visit Hengistbury Head; part of a beautiful section of coastline on the South of England. The aim of the day was to investigate a number of coastal environments, be it natural or man-made.

The day started off with a chilly depart from the National Oceanography Centre, Southampton at 9:30am, arriving at a section of coast near Highcliffe. We started by exploring the geological setting, and investigating what effect the installation of rock groynes was having on the beach to the west of Highcliffe. There is a large littoral drift of sediment running from West to East along the coast.
The groynes used to be made out of timber, however, as they were approaching the end of their useful life, they were replaced by alternating long and short rock armour groynes. The short groynes were built with the intention of protecting the upper shingle beach and the long groynes as wave diffraction devices. The groynes seemed to have a positive effect as there was noticeable sediment accretion before each groyne.

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Slumping of the cliffs
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Groynes capturing littoral drift of sediment, and stabilising the beach. and cliffs behind

Next on the agenda was visiting the slumping cliffs to the east. The top most layer of the cliffs is a sandy-gravel layer. This is an important feature of the cliff as it permits the saturation of the cliff sediment with water. There is also a Barton clay layer below the gravel. The clay layer is impermeable to water which is important as it can act as a lubricating layer and encourage rotational cliff slumping.
Efforts to stabilise the cliff have been attempted in the past with plants being introduced to the cliff face in an attempt to consolidate the sediment and firm it up a bit. Also visible are large beams of metal which were attempts to reinforce the cliff face, rather like steel reinforced concrete.

After the cliffs, we got back on the coach and wet to eat lunch at a café near Hengistbury Head which was home to a flock birds which I’m sure wouldn’t hesitate to eat you given a fair chance…

 

 

 

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Groynes at Highcliffe showing the accumulation of sediment on the right hand side of each Groyne

 

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One of the birds getting over-confident during lunch

After eating lunch, we walked over to Hengistbury Head. At the tip of Hengistbury Head is beakwater dubbed ‘the long groyne’. This breakwater creates a separate coastal cell between the West of the headland and the East. This breakwater was constructed in the 1930’s in an attempt to counteract the erosion which was thought to be an unwanted by-product of the previously discussed groynes.

With more sediment being trapped by the groynes, there was less sediment to replenish the beach around the headland which caused substantial erosion to the beach and consequently, the cliff face.

Next we looked at the mudflat Inside the harbour itself from a
vantage point. The mudflat is a result of some interesting tidal processes within the harbour. At high tide, the water velocity is at a minimum within the bay. This means that any mud or silt can settle out of settle out of suspension when the flats are flooded at high tide. Any sand and heavier particles will have settled out before the water can reach high tide and so are accreted earlier.

After discussing the mudlflats, we started walking back to the coach and headed back home on the coaches.

The tidal inlet and barrier beach at Muderford, leading into Christchurch Harbour
The tidal inlet and barrier beach at Muderford, leading into Christchurch Harbour
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