The Crag formation is overlain by a succession of shallow-water deposits known as the Cromer Forest Bed Series (or Kesgrave formation). The Forest Bed series contains various layers of clays and sands deposited under both freshwater and marine conditions. The deposit represents a complicated series of Pleistocene climatic events that terminate at the beginning of the Anglian Glacial stage. The Forest Bed series is particularly well known for its fossils. When, during Glacial stages, ice sheets and glaciers spread over the ground they erode, transport and deposit huge quantities of material. However, once the ice melts and retreats this material is left as continuous sheets or mounds of heterogeneous and generally structure less material known as Till or Boulder Clay. Till can be a very variable deposit but it does have certain general properties. There will be a large range of grain sizes, with hard rock fragments up to boulder size surrounded by softer matrix material consisting of sands and clays. The rock fragments themselves may come from distant locations because glaciers have carried them; indeed the North Norfolk cliffs contain rocks from Scandinavia for precisely this reason. Generally, till has no internal structure or layers; however, in some instances the till is sorted by the huge volumes of water produced by the melting ice, and it may then form layers of sand and gravel or deposits of silts and clays. The
North Norfolk cliffs, because of their glacial origins, have two characteristics that are important when considering coastal erosion. Firstly, they are made of soft heterogeneous (made of many different deposits) material with a low shear strength, which means that they are very vulnerable to wave erosion and failure caused by undercutting. Secondly, the sand and gravel components of the glacial deposits are permeable, while the clays are impermeable. The impermeable clay structure can intercept water percolating through cliffs, causing it to pond above this layer and discharge from the cliff face. This leads to an increase in pore water pressure in the cliff, reduces the shear strength of the cliff, and may result in landsliding. Consequently the principle causes of erosion on the North Norfolk coast are wave action and the presence of groundwater. The severity of wave induced erosion will depend on beach width, beach gradient, the presence of defences and tidal range. A wide, shallow beach will absorb more incoming wave energy than a narrow, steep beach and, therefore, wave action becomes a dominant erosional process on narrow, steep beaches without any defences. Tidal range (tides are discussed in more detail later on) is an important factor as it determines the vertical height over which this erosional process can operate. The mean tidal range is about 4m at Weybourne and decreases to the east, dropping to about 3m at Happisburgh. Also because there are no rivers or other way to get material from inland this causes the waves only to pick up cliff material which means the cliffs are retreating. Also South East England is sinking and North West Scotland is rising because Scotland is bouncing after the weight of ice form the ice age was removed (isostacy).
This graph shows the metres per year of cliff material being swept away by the waves. As you can see Sheringham has one of the lowest, this may be because of the coastal protections at Sheringham. However the fastest retreat of the cliffs is between Trimingham and Overstrand reaching a high of just under 2.5m per year on average. While between Bacton and Walcott there is very little or even no erosion, all the low losses cold be aided by the use of coastal protection. Recently there had been as large mass movement at Overstrand.
These pictures show the devastating effects of weathering, isostacy and global warming.
What Sheringham Has done to Stop Erosion
The first line of defence at Sheringham is a natural sand beach and shingle bank, which are backed up by a mix of man made defences of groynes, promenades and sea walls which act as a last line of defence for extreme conditions. In the late 1980's there was great concern about the integrity of the coastal defences at Sheringham. Waves and tides had gradually removed the once-healthy beach from this exposed stretch of the North Norfolk coast allowing storm waves to attack the man-made defences inflicting considerable damage.
The sea walls, groynes and promenades (first built during the last century) have fixed the alignment of the frontage at Sheringham. Consequently, whilst the cliffs to the east and west of Sheringham have continued to erode naturally, the promenades now jut out by up to 70m seaward of the natural coast line. The exposed position of the beaches and defences means they are subject to an ever-increasing wave attack. Which means that the beaches at Sheringham are eroding and getting shorter and steeper thus leaving the sea walls to be undermined. Natural processes gradually carry shingle away from the beaches at Sheringham eastwards towards Cromer. The rate of drift of shingle gradually increases towards the east. Just west of Sheringham the drift of material tends to be in the opposite direction towards Blakeney. So whilst at the western end of the frontage the beach has not suffered quite so badly, the beaches in front of the Central and East Promenades have been stripped of shingle. This means that the sea walls are in drastic need of repair. The latest scheme to protect Sheringham is called Coast Protection Scheme 902 and consists of two stages and monitoring.
The first stage was the repair and renovation of 900 metres of the existing sea walls and the placing of large rock armour stone in front of them to act as buttresses and to absorb the energy of breaking waves. A weak bridge across the fishermen's slipway on the west beach was also renewed. The second stage was the work on the groyne system. All five of the old timber groynes at the west end from the Lifeboat Station to the Admiralty Slip were renewed or replace with new timber groynes and an extra groyne was built. The five remaining timber and steel groynes from the Admiralty Slip to the east end were removed and replaced with four new rock armour groynes. Additionally, 100 metres of the toe of the seawall on the east fisherman's beach was re-enforced and 90 metres of rock armour placed to the west of the Admiralty slip towards the Upcher groyne. As access was very poor and because large volumes of rock were required, it was planned to have the rock delivered by sea directly to the beaches.
The cost of stage 2 was approximately £1.85 million and work was completed in 1999. An integral part of the second stage was to monitor very carefully, over a number of years and within a predetermined monitoring programme, just how the existing beach and the sediment moving through the system responded to the new groyne design. It is also necessary to record how the rock armour and the refurbished sea-walls withstand storm surge conditions.
A traditional sea wall.
Groynes at Sheringham.
An overhead view of Sheringham.
In order to assess if the defences have been successful we aim to prove that
- Groynes have allowed beach material to build up
- The further away from the sea, the larger the pebbles
- Wav energy and type effects the beach profiles
To prove these I had to do different things for each aim. To prove that the groynes in Sheringham have allowed beach material to build up we did two beach profiles, one on either side of the groyne. To do a beach profile you have to get two ranging poles and one protractor. Then you plant one pole at the base of the beach and the other at a change in steepness then measure the angle between the pole at the base of the beach and the one at the change in steepness. Then you put the first pole where the second pole was and carry the fecund pole to the next change in steepness and repeat. To prove that the further away from the sea you are the smaller the pebbles, you have to measure every two metres up the beach and take five pebbles and measure their size then find the average and repeat. I also found out how many waves there were every minute by finding a fixed point at sea and counting how many wave crests passed each minute for ten minutes, also I timed the difference between each wave.
I chose to do this investigation to try to see if the three hypothesises are true. Such as if the groynes have allowed the beach material to build up. I faced some problems, like the protractor was quite hard to read and the pebbles were quite hard to measure, I overcame these by using my best estimate.
These are my analyses of wave energy at Sheringham and East Runton.
East Runton-
Wavelength= 3.13 X 6.1 X 1
= 19.093
Wave Energy= 740 X (0.3 X 0.3) X 19.093
= 1271.5938
The waves at East Runton are destructive because the mean number of waves breaking is 11.
Sheringham-
Wavelength= 3.13 X 7.1 X 1
= 22.223
Wave Energy= 740 X (0.3 X 0.3) X 22.223
= 1480.0518
The waves at Sheringham are very nearly destructive as the mean number of waves breaking per minute is around 8.15 which is nearer to 10.1 than 5.9.
In conclusion I think that Sheringham needs protecting for the people who live there because there homes would fall of the cliffs if the cliffs are not protected and also because Sheringham is needed for the local areas source of shops and services, which if they were gone could cause many problems. Sheringham is protected by the use of sea walls and groynes, which is quite effective considering that Sheringham loses very little of its land per year compared to places like Overstrand which have no protection. However I don’t think that the defences are sustainable because they are being undermined by the material that the earth is made of in north Norfolk, and the local council will not be able to constantly repair and renew the sea walls and groynes. East Runton’s plan of managed retreat is good for where it is put in to use because there are no towns or villages, very few houses close enough to collapse and it is free, managed retreat is also good for East Runton because it forms a natural protection for the inland area with sand and shingle beaches and sometimes a march.
My evaluation of the project is that we did find out what our aims asked, but there could have been improvements. For instance for finding out about pebble size we should of measured about 100 pebbles per two metres to get a fair result. Also we should have counted the amounts of waves every minute for one hour and we should have also counted the time between waves for ten minutes at a time for an hour.
Beach Profile for East Runton
Pebble Sizes in East Runton
Time between Waves breaking at East Runton
Average 6.1 seconds
Direction North-North East
Number of waves per minute at East Runton
Average 11
Direction North-North West
Sum of Difference= 2920 Correlation= 0.4
Sheringham Updrift Beach Profile
Sheringham Pebble Sizes
Sheringham Downdrift Beach Profile
Time between Waves Breaking at Sheringham
Average 7.1 seconds
Direction- North
Waves per Minute at Sheringham
Average 9.2 Direction North-North West
Sum of Difference= 10768
Correlation= -9