The relative effect of the UK's stormy winter 2013-2014 on dune erosion: Case study of the Sefton Coast, Northwest England

摘要

The stormy United Kingdom (UK) "winter" of 2013-2014 caused significant erosion and flooding on many parts of the UK coast, including the Sefton coast in northwest England. The most destructive storms occurred on 5 December 2013 and 03 January 2014, but the extended "winter" period October 2013 to March 2014 was characterized by the occurrence of an unusually large number of storms which had a significant cumulative impact. On the Sefton coast the most serious dune erosion occurred along the northwest-facing part of Formby Point, where exposure to westerly and northwesterly waves is greatest and where the upper beach width above Mean High Water Spring (MHWS) tide level was less than 25 m at the onset of the 2013 -2014 winter. An analysis of LiDAR digital elevation models showed that the net frontal dune sediment loss on this part of the frontage in 2013-2014 represented 30% to 46% of the total sand volume loss between March 1999 and May 2014. Frontal dune volume loss in 2013-2014 was 243 × 10~3m~3, compared with a net gain of 789 × 10~3 m~3 between 1999 and 2013. The mid and upper beach above 0 m ODN lost 120 × 10~3 m~3 in 2013-14, compared with a loss of 743 × 10~3m~3 between 1999 and 2013. The sediment volume lost from the beach 1999-2013 was approximately matched by sediment accumulation within the frontal dunes, demonstrating the importance of aeolian transport process in contributing to net medium term dune toe recession. However, compared with longer term rates of change in the frontal dune toe position indicated by analysis of dune toe survey data, aerial photographs, and historical maps, the changes experienced in 2013-2014 were relatively small. The maximum retreat distance of the dune toe was 12.1m, and along most of the frontage ＜5 m, compared with maximum retreat distances of 380 m (6.3 m/yr) 1906-1966 and 173 m (5.3 m/yr) 1966-1999. Although the impacts of major storms are visually dramatic in the short term, their relatively infrequent nature and relatively short duration means the impact on longer term dune erosion rates is limited. Lower magnitude but higher frequency storm events, and aeolian transport of sand to the dunes, have a greater cumulative impact. There is no evidence that an increase in mean high water levels of 1.9 mm/yr since the early 1960s has resulted in accelerated dune erosion on the Sefton Coast. Average erosion rates, have in fact, declined over the past 100 years as the coastal planform has straightened and shown a anticlockwise rotation. A change from accretion to erosion on the central Sefton coast in the early 20th century appears to have been a response to changes in the offshore and nearshore bathymetry, caused partly by engineering works in the outer Mersey and Ribble estuaries. The evidence suggests that the coastal zone may now be approaching a new equilibrium following this disturbance. This study has confirmed the value of airborne LiDAR surveys in monitoring beach and dune erosion over both short and longer time periods. To be of greatest value, surveys should be undertaken before and after each winter period. The surveys should cover at least the most seaward 100 m of dunes and the beach down to mean sea level. Monitoring of changes in frontal dune and upper beach sand volumes provides a more comprehensive picture of dune erosion/accretion patterns than monitoring only of dune toe position, dune width, and dune crest height. Maximum information is obtained using 1D, 2D, and 3D morphometric measures in combination.