2B.8C - Temporal Variations in Coastal Recession
Rates of recession are not constant and are influenced by different factors, both short- and longer-term (wind direction/fetch, tides, seasons, weather systems and occurrence of storms.
Wind Direction and Fetch
Tides
Storms
Seasons
Weather Systems
Wind Direction and Fetch
- Winds change direction daily as different weather systems pass across or near the UK
- Rates of recession will be higher when the wind is blowing onshore
- Offshore winds produce calm conditions
- Dominant wind is the direction of the strongest wind
- Prevailing wind is the direction of the most common wind
- When wind blows from prevailing wind direction, strong winds produce large destructive waves and rapid recession
- Where wind blows from a direction where there is a large fetch, even moderate strength winds can build up large destructive waves and produce rapid recession
- Dominant wind direction often coincides with direction of largest fetch, as a large expanse of unobstructed open water allows wind to gain strength.
- In the North Norfolk coastline the rare dominant wind from the north with a 1,600 km fetch across the Norwegian and North Seas can produce recession rates of up to 8 m p.a.
Tides
- Rates of recession more rapid during high tide when deeper water in the foreshore zone allows waves to maintain a higher energy when they reach the backshore. (And they reach the backshore in the first place)
- Greater energy of impact on backshore increases erosion and rates of recession
- High tides occur twice each day, about 12 hours apart
- Twice each lunar month when the gravitational pull of the Sun and the Moon is aligned, particularly high Spring Tides occur
- Higher sea level produces deeper water near the backshore and faster recession
- IPCC estimates a 1 cm rise in sea level produces an average of 1 m horizontal erosion
- Global warming predicted to raise sea levels by 18-59 cm by 2100
Storms
- Storm events are deep (very low pressure) depressions
- They produce large, high energy destructive waves and fast rates of recession
- Global warming is predicted to increase intensity of atmospheric circulation - more frequent storm events - higher intensity of storm events - increased rates of recession
Seasons
- Storm events are more likely to occur in the winter when the contrast in temperature and pressure between the tropical and polar air masses is the greatest
- Consequently in the UK the rates of recession are faster in winter than summer
- In Holderness, in winter 2-6 m of erosion is common when storms, combined with spring tides, increase erosion rates (although the average erosion rate is 1.25 m - but this varies from 0-6 m per year)
- Number and intensity of storms affecting the UK each winter varies:
- Weaker storms during El Niño events every 2-7 years and during phases of low solar output in the 11 year cycle
Weather Systems
- UK located at the polar front between the boundary of warm tropical air of the Ferrel cell and the cold polar air of the Polar cell
- Interaction between warm and cold produces high air pressure anticyclone and low air pressure depression weather systems
- Anticyclones produce gentle winds and small waves - low rates of recession
- Depressions - areas of rising air (low surface air pressure)
- These produce strong winds - rapid rates of recession
- Depressions form in the North Atlantic and take several days to pass across the UK from the SW to NE. As they pass the spiralling inflow air produces changes in wind direction