17. Can you explain in detail why the erosion rates differ at East Beach in Sidmouth?
Assess erosion with two methods
- Historic maps between 1890-1991 = 0.19m/yr cliff top and 0.15m/yr cliff toe
- Aerial photograph 1946-2015 = 0.19-0.27m/yr cliff top and 0.25m /yr cliff top and 0.25m /yr cliff toe
Future prediction ~ 20.9m in 100 years or 30.9m with a nominal further 10m buffer
Initially the OBC adopted BMP values however a further study was requested to check on sensitivity of previous rates
RHDHV reassessed the Halcrow calculations by repeating their calculation and looking at the variation of these results over a short time or longer time and by section. It was found that the erosion was quicker in recent past/short term and slower over a longer-term past. The analysis also showed transect 31 erosion was higher than the others. Therefore, it was suggested to be cautious and the worst transect should be used with the first 20 years based on the higher rate and the latter years on the longer-term average rates.
From transect 31 the rates for 2006-2018 are 2.1m/yr and 1946-2018 are 0.6m/yr. The future loss is estimated at 92.5m over 100 years.
Alternate source rates
These elaborated on the method used in the BMP using areas rather than a simple section. This is not dissimilar to the updated values adopted above.
DCC rates (2020) are undertaken with reference to apparent crest and toe lines on aerial surveys. These are then transacted at 3 sections to identity change in position of the area around the transect. The aerial photos were dated 2012, 2015, 2017, 2020. These are spilt into the rates at the top and bottom of the cliff. The top of the cliff is likely to be a reasonably well defined line bar any overhanging vegetation, however the position of the toe will also be a reflection of the height of the beach at the bottom as much as its position, so that if the beach is higher it would appear the toe of the cliff has receded whereas this may not be the same contour.
These have been used to come up with a high and low estimate line, where the high is and arbitrary 10m inland of the low line.
It should be noted that SWEEP was to inform possible CCMA’s and is therefore precautionary as not to leave out any properties potentially at risk or affected by changes in the future. This method is based on the digitisation of the cliff top form Lidar data (since 1998 and therefore resent short term data) and then an algorithm used to extrapolate this in the future including a nominal 0.5 speed up factor to account for climate change. These would appear to provide a significantly earlier loss of cliff compared to the methods above.