Coastal Change at Pakefield

The highest tides of the year occurred in early October 2019, coincident with a surge height of +0.5m and large, long period swell waves. Consequently the SWL exceeded ~1.7m (ODN) so the cliff was undermined and mass movement occurred in the form of topples, slides and slumps.

As the cliff face attempts to restore a stable angle of repose the erosion has been exacerbate by the sea meeting the cliff toe several more times in the last 2 months of wet windy weather & higher tides.

• Jan 2019 saw a significant tidal surge (SWL +2.8m ODN) and this brought cliff erosion which effected an area of the cliffs in front of PCP.

• There are very few brick built property on the cliff top- just within the policy unit LOW4.2.
• Recent increase in landowners / residents querying CPE's plan for management here.

• CPE undertook basic data analysis and produced a report. Aim - To assess whether average rates of erosion have changed since SMP adoption (2012) and whether an SMP policy review is justified.

DATA USED: Topographic profiles (since 1992), Lidar data and Aerial photos from CCO portal were the primary types of data analysed (+Aerofilm +Google Earth + Mike Page's aerials).

Method: The chainage at Cliff top, Cliff toe, MHW, MSL, & MLW were extracted from profiles S004 S005, S006, S007 & S008, for each year of monitoring data available (1999-2019 with some gaps).

Transects S005 (North Pakefield) & S008 (Kessingland) illustrate a positive trend; the short term rates are nearly 4x greater than the long term rates here.

At Transect S008 (Kessingland), the chainage starts to increase from year 2003 onwards. This is the time after which the Ness offers protection to the cliffs by increasing beach width >20m.

In the last decade, the beach at Transect S007 (Pontins end) has eroded more rapidly (~2m/yr) than S006 (PCP end) due to S007 being closer to the leading edge of Benacre Ness.

Empirical observations lead to the assumption that a period of accelerated erosion precedes the arrival of the Ness.

The general trend at S006 has been one of consistent erosion and shoreline transgression over the period of monitoring.

The MHW and MLW mark have eroded at an equivalent rate which maintains the characteristic gradient of the beach at 1:10.

Total landward movement of the MHW mark has been 35m over the last 27 years.

The MWL has retreated at a rate of 1.1m/yr over long term & 1.8m/yr over the short term.

The cliff top at S006 shows a total retreat of ~1m (within error of sampling), showing stability (until now). This was due to the beach still being wide and high enough to be able to naturally absorb wave action and defend the cliffs form all but the most extreme water levels.

The cross-shore beach width (between MHW and base of the cliff) was ~40m in early 2019*. This was twice the 'critical width' (20m); below which the cliffs are assumed to become vulnerable to marine processes.

*The beach has recently narrowed and exhibits a beach cliff >1m shown in photo above

Summary:

• As Benacre Ness migrates northwards - so does the extent of erosion that precedes it's arrival. (The rate of Ness migration may well be linked to the rate of shoreline retreat)
• Short-term (last 10 yr) average rates of coastal change are more rapid than the long-term average rates, signifying an acceleration of coastal change within the last decade at Pakefield.

Method: The most recent surface elevation data (2017) was subtracted from the earliest elevation data (2011) to produce a colour-scaled difference model. The areas of greatest surface elevation change over time are darkest. All elevation differences are within +5m to -5m. The most significant gain in elevation is at the leading edge of the Ness.

Summary:

• The leading edge of the Ness has migrated north ~0.5 km between 2011 & 2017. The rate of northward migration was estimated as 0.5km / 6yrs = ~80 m/ yr.
• Other studies have estimated Ness migration over longer term to be ~60 m/yr.

Method: This was not an exact science due to the mixed source of aerial photos used. The idea was to track the progressive lateral movement of active erosion along Pakefield cliffs (south to North) that is visible on aerial photos. The rate of movement was calculated using the distances between the northernmost point of active cliff erosion, and a known location - transect S006.

This calculation has been performed for years 1991-2019. The Figures conclude that the long term (28 year) rate of erosion advance varies between 5 and 25 m /yr. The median rate is 10 m/yr.

The epochs of greatest advance were between 1991 and 1996, and 2013-2014, presumably linked to extreme and sustained weather and water levels. In the latter event the leading edge of erosion pressure moved 115m north in one year.

The Southern end of Pakefield Caravan Park became threatened by cliff erosion in 2017; the same year as the seaward wall of the MOD rifle range fell collapsed on to the beach. In 2017 the width of the beach (cliff toe to MSL) at S007 was ~17m; which is under the critical width deemed necessary for cliff protection (20m).

Summary:

• Erosion pressure is ostensibly advancing northwards, ahead of the leading edge of Benacre Ness.
• The rate by which the erosion pressure has translated north is 15 m /year ± 10m.
• It is postulated that this increase in erosion pressure, which is preceded by a decrease in beach width, is a forerunner to the arrival of the Ness- the transfer of sediment from cliff budget to beach budget ahead of the Ness' arrival could be a very necessary part of the migration process.

Method: The extent of beach and/or cliff erosion incurred before the (assumed) arrival of the Ness was explored using four scenarios:

i) Continuation of the 10-year trend: Assumes ~80m / year Ness northward movement with 2m / year shoreline and cliff retreat rate.

ii) Continuation of the 25-year trend: Assumes ~70m / year Ness northward movement with 1m / year shoreline and cliff retreat rate.

iii) Ness migration slows + high erosion pressure: Assumes slow ~60m / year Ness northward movement that delays arrival of protection, combined with a rapid, 2.5m / year, shoreline and cliff retreat rate.

iv) Ness migration slows + low erosion pressure: Assumes ~40m / year Ness northward movement with 1m / year shoreline and cliff retreat rate.

Anticipating how the Pakefield frontage may evolve has helped CPE to consider the challenges that Pakefield faces & what actions may be required in the near future.

Pakefield North (S005) : Evidenced based and speculative scenarios finds the beach width is probably sufficient to prevent significant cliff erosion, before the arrival of Ness protection in future. However the beach width is predicted to reduce at a rate of between 1- 2m /yr for the next 15-30 years. Under current HtL policy intervention may be required to manage beach narrowing but probably not to resist cliff erosion.

Pakefield Central (S006): Under evidence-based scenarios i) & ii) there is a risk of cliff retreat after 2040 and beach narrowing at a rate of between 1m and 2m / year to precede that. Under Speculative scenarios more significant erosion is possible by 2035. Intervention over this part may be required to manage both beach narrowing and to resist cliff erosion.

Pakefield South (S007): Under evidence-based scenarios there is high risk of significant cliff retreat from present to 2040 and this could affect permanent properties. Under Speculative scenarios more significant erosion begins from ~2025 and extends to landward until ~2045. Intervention over this part is required soon to implement the current HtL policy. OR the policy could be reviewed.

SMP forecast of coastal change was reasonably based upon information available at the time of preparation but, given the elapsed time and modern rate of coastal change over Pakefield South, there are now appropriate grounds to consider a policy review- if community aspiration exists (e.g. NAI to MR)

The SMP erosion risk maps do not consider the future arrival of the Ness at Pakefield and show that erosion is predicted to continue along the whole frontage throughout epoch 2 & 3. This is where the SMP, and the erosion forecasts from CPE's basic data analysis differ.

• Want to make use of data being collected to support other SMP reviews (Slaughden, Bawdsey)
• Annual/ biannual/ triannual monitoring report on local coastal change would be helpful- with foresight into whether a departure from SMP predicted erosion rates is becoming evident at certain sites.
• Quick & simple construction of beach profiles from raw data.
• Volumetric change analysis (requires 3D processing licence)
• Easier access to the most contemporary collected data.
• More preparedness to ask for data analysis support/products from Geomatics.
• Innovative but accurate ways of processing & presenting data.