2 Context to erosion projections used in the ESC

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1 Halcrow Group Limited Lyndon House, 62 Hagley Road, Edgbaston, Birmingham B16 8PE tel fax halcrow.com Technical note Project LLWR ESC Technical Queries Date 11 December 2012 Subject Significance of the dune system at Drigg Ref ESC-TQ-SUE-022 Author Paul Fish 1 Introduction As part of their review of the 2012 ESC documentation for the LLWR, the Environment Agency has raised a series of Issues, Observations and Technical Queries. This Technical Note responds to the Agency s Issue Resolution Form (IRF) ESC-TQ-SUE-022. This IRF relates to the role of the Drigg dunes, which are discussed in the Level 3 ESC document Fish et al. (2010). In their review, the EA state We do not consider that the role and significance of the dune system in the evolution of the coast line has been clearly described in the ESC. We would like to see the Environmental Safety Case address and communicate in more detail the role (or lack of role) and significance of the dunes in the erosion of the repository. The specific information requested is as follows: Confirmation that the erosional and depositional behaviour of the sand based lithologies has been clearly defined and assessed. Clarification of the role and significance of the dune system in the coastal recession assessment. Description of the role and behaviour of the Drigg dune systems in response to the changing coastal conditions occurring during the erosion sequence. Assess the possibility for avulsion and its impact on the development of the local coast and impact on the LLWR site and disposal area. Confirmation of the current sand budget status of the Drigg dune system, are the dunes eroding or accreting? A supplementary query was also received in relation to the Bruun model for projecting coastal erosion and the relationship between rate of sea-level rise and rate of erosion projected by models in the ESC. 2 Context to erosion projections used in the ESC Before considering the specifics of the Drigg dune system, it is beneficial to revisit the methods used and conclusions of the coastal change assessment presented in the ESC. The Level 3 document that underpins the assessment is Fish et al. (2010). That report concluded that Based on the range of sea-level rise Prepared by Paul Fish Date 12 December 2012 Checked by Roger Moore Date 19 December 2012 Approved by Roger Moore Date 19 December 2012 Halcrow Group Limited is a CH2M HILL company

2 Technical note 11 December 2012 Page 2 of 10 projections and the available evidence concerning natural coastal change processes on the west Cumbrian coast, the LLWR is liable to be disrupted by cliff instability and erosion within a time frame of a few hundreds to thousands of years. This conclusion was reached by review of the combined outputs of a series of different numerical modelling approaches that were underpinned by knowledge of the current behaviour of the coastal system and understanding of how the coastline has responded to changes in sealevel and sediment supply over geological time. The approaches used to project future change were: simple extrapolation of historical rates of change (not considered reliable given the short length of reliable historical records and its representativeness of future conditions (i.e. historical records cover a period of static or falling sea-level, while all future scenarios involve rising sea-level) a simple Cliff Recession Model (CRM) that uses empirical relationships between beach volume, sea-level rise and cliff recession rate, and input data on future sediment supply from interpretation of geophysics and rates of sea-level rise from BIOCLIM projections. This approach is only appropriate for modelling the erosion rate of soft, cohesive cliffs such as those forming the Drigg frontage. the process-based 2-D SCAPE model, which used hydrodynamic data to model the development of the coastline over geological time periods and then extrapolate change into the future using projected increases in sea-level. This approach is only appropriate for modelling the erosion rate of soft, cohesive cliffs such as those forming the Drigg frontage. In addition to these cliff recession modelling approaches, a model for estuary and spit evolution was also presented to investigate the possibility of the Irt estuary expanding to erode the LLWR and the spit/dune systems at Drigg and Eskmeals. The results of this modelling were not significant to the LLWR as the time-periods to impact from estuary erosion occurred after the site had been breached by cliff recession. All projections are underpinned by a conceptual understanding of the coastline that has the following key points: the study area is a closed sediment cell that extends from St Bees Head to Ravenglass. Sediment is derived solely from cliff recession and temporary stores (such as beaches and dunes) within the cell. No significant volume of sediment enters the cell from adjacent coastlines. There is no net drift of sediment and material eroded from cliffs is liable to move to the north or the south, depending on prevailing wave direction. Sediment released from the cliffs is sorted and reworked by wave action and nearshore currents. All material is liable to move up- or down-drift in storms, with gravel being the least mobile. Gravel remains on the storm beach, sand remains on the lower beach and fines (silt and clay) are transported offshore to below MLW or into the estuaries. 3 Behaviour of the dunes in coastal erosion modelling 3.1 Introduction This topic has a series of sub-queries: 1. How the coastal recession model handles areas where dunes are present (such as CRM line profiles E and G) 2. Response of the model where sand is subjected to direct wave attack

3 Technical note 11 December 2012 Page 3 of Potential for avulsion of the Irt channel if all the dune sand is eroded, and the impact on the LLWR The cliff recession model considers the hinterland geology in terms of its gross particle size distribution in order to determine the future supply of beach-building sediment (i.e. gravel) that will be released by cliff recession. As stated above, this is because it is recognised that a central control on cliff erosion rate is the health of the beaches, which act as a buffer to wave energy, and therefore the supply of gravel is an important factor controlling cliff recession rate. The geophysical survey report identified a series of lithostratigraphic units (Figure 1) that can be compared to borehole records to determine characteristic particle size distributions (Figure 2). Geophysical lines were surveyed along a series of shore normal sections, so by calculating the percentage of gravel, sand and fines in each lithostratigraphic unit that are found along the section line above ordnance datum, the future supply of gravel can be estimated (Figure 3). Figure 1. Section of geophysical data from Line E, showing borehole calibration and interpreted lithostratigraphy Figure 2. Lithostratigraphic units that have characteristic particle size distributions Figure 3. Gross sediment supply calculated by aggregating particle size data for each lithostratigraphic unit that crops out above ordance datum along Line E. Note small increase in gravel moving inland

4 Technical note 11 December 2012 Page 4 of How the model handles dunes The coastal recession model is not applied to any coastal frontages that are formed exclusively of dunes. All modelled sections have a cap of dune sands of variable thickness, but the bulk of the material above ordnance datum that will be subjected to direct wave attack is till. For example, on Line G, which lies mid-way along the length of the Drigg dunes complex, the geophysics investigation shows c. 15m of glacial sediment above ordnance datum, with a covering of dune sand under 5m thick. Therefore, the coastal recession models account for the presence of dunes by simply considering the volume and size of sediment they will supply. As the sediment supplied is sand-sized it will not contribute to the storm beach. Therefore the model indicates more rapid erosion than would occur if cliff were comprised entirely of gravel. 3.3 Response of the model where sand is subjected to direct wave attack As stated above, none of the cliff recession modelling approaches are designed to consider the response of dunes to direct wave attack. Due to the presence of a till ridge that underlies the northern part of the Drigg promontory (at least as far south as Line G), the materials subjected to direct wave erosion will be tills and/or glacial outwash sands and gravels. According to the geophysical data and subsequent BGS borehole data, the southernmost section of the Drigg promontory is a gravel spit with a capping of dune sand (Figure 4). The gravel component of the spit can be seen in the base of dune blowouts at the southern tip of the promontory. The response of spits to rapid sea-level rise is determined by the supply of sediment. If the sediment supply is high, then the feature can expand in height and width in response to rising sea-levels. If sediment supply is insufficient, as is expected to be the case at Drigg, then the feature will be unable to grow at the same rate as sea-level rise and will be eroded or over-topped. Given the distance of the LLWR from the Drigg spit, even if the feature were eroded the implications to the site are insignificant in comparison to the projections of coastal erosion affecting the sites frontage. 3.4 Potential for avulsion of the Irt channel and impact on the LLWR Avulsions of river channels involve significant changes in their planform position. Avulsions are triggered by significant events that make fundamental changes to the shape of the river valley and channel. In the context of the estuary of the Irt, an avulsion may theoretically occur following erosion of part of the Drigg promontory/spit by the action of coastal erosion during a sustained storm, or fluvial erosion during a flood, allowing the river to reach the coastline via a shorter route. The evidence for a former channel of the river Irt passing through the spit is far from certain and an equivocal position is taken in Fish et al. (2010), the geophysical report and the subsequent BGS geological investigation. This reflects the fact these reports were compiled in rapid succession with limited opportunity for full integration of all new data with pre-existing data from BGS (2010) and Durham University investigations in 2005, 2006 and 2007 (reported in Halcrow 2005, 2006 and 2008). The principal evidence for a former route of the Irt through the Drigg promontory is: estuarine sediments below ordnance datum interpreted from geophysical data, interpreted as the landward extent of the former channel (line A) (see dashed red line on Figure 4) peat deposits below ordnance datum on the sea-ward side of the Drigg promontory at a similar northing to the estuary sediments that are interpreted to form the seaward expression of the channel (circled in red on Figure 4) historical maps from the 18 th century that indicate a different route of the River Irt. However, the estuarine sediments shown in the geophysics could simply represent a former meander associated with a channel in a similar position today (see blue line on Figure 4). The peat deposits located

5 Technical note 11 December 2012 Page 5 of 10 seaward of the till ridge that underlie the northern part of the Drigg promontory cannot easily be linked to the route of a presumed former channel. These sediments can instead be interpreted as kettle hole or dune slack deposits, formed at a time when the coastline and dune system extended further seawards, ground water levels were higher and the vegetation types were different. Historical mapping earlier than the first series OS data of the 19 th century is very unreliable, and different maps from the 17 th and 18 th centuries exist which show a range of orientations of the rivers Esk, Irt, Mite and the Drigg spit. Maps that have been used to support the presence of the channel also indicate that the coastline fronting the LLWR was c. 1km inland of its current position, which highlights the considerable uncertainty associated with these data. Furthermore, shallow cores and radiocarbon dated peats sampled by Durham University (reported in Halcrow 2005) in the vicinity of the presumed channel do not support the presence of a channel at this location, and instead indicate a continuous sand sheet was deposited over the last 6,000 years, with a cover of peaty soil formed in the last few hundred years. Possible margin of former meander, accounting for geophysical data Till promontory with dune cap Area where estuarine sediments are indicated to be present below OD. Gravel spit with dune cap Figure 4. Drigg promontory, showing sample locations and geophysics lines (from Fish et al. 2010)

6 Technical note 11 December 2012 Page 6 of 10 In summary, there is no unequivocal evidence that a river channel has ever penetrated through the Drigg spit/promontory. However, given the distance of the LLWR from the Drigg spit, even if a channel were to exist through the Drigg spit in the future, the implications to the site are insignificant in comparison to the projections of coastal erosion affecting the sites frontage. 4 Clarification of the role and significance of the dune system in the coastal recession assessment This aspect of the technical query is largely covered above. The contribution of dune sand that caps till cliffs is incorporated in the cliff recession model. No sections of pure dune/spit have been considered. The southern part of the Drigg promontory is a gravel spit with a capping of sand dunes and therefore the cliff recession model is not an appropriate tool. This feature is distant from the LLWR and if it were to be lost there are no direct implications to the site. No scenarios are considered credible for dune expansion occurring to such an extent that there is a positive benefit to the site. This is because there is no significant supply of sand in the system that would be needed to feed a growing dune system. The Drigg dunes (and other dune systems on the NE coast) developed over the past c. 5,000 years at time of very high availability of sediment from erosion of glacial sediments and falling sea-levels. These conditions are not thought likely to occur in the future. The soft sediments are largely eroded and exhausted now and along much of the St Bees to Ravenglass coastline the pre-glacial cliffs cut in hard Permo-Triassic sandstone are close to the shoreline. All projections used in the coastal assessment are for rising sea-levels. 5 Description of the role and behaviour of the Drigg dunes The present day configuration of the coastal landscape is shown in Figure 5. As has been explained in previous sections the dunes fronting the site are located on top of till cliffs (Figure 5, area A), while the dunes forming the spit proper sit on top of a former gravel spit (Figure 5, area B). Marine waters are able to pass around the end of the spit into the estuary complex and therefore up to the River Irt, which is at approximately the location where the A595 bridge crosses the river. In the future a number of possible responses of the dunes are conceivable. Defra (2007) outlines five simple scenarios for changes in cross sectional form (Figure 7). The present day coastal system surrounding the LLWR site is a closed system. This implies there will be no significant input of sediment to the system in the future and that coastal evolution will be characterised by reworking of existing sediment stores (in the dunes and till cliffs). This could lead to the dunes being moved landwards whilst the overall sediment volume stays the same or decreases. A hypothetical arrangement of the coast in the future is shown in Figure 6 (source: Fish et al. 2010). In this configuration it is possible that the spit configuration could play a role in determining water levels in the estuary. Although this could influence flood risk to the southern part of the LLWR site, such changes are likely to be of significantly less importance than the overall rise in sea level and associated cliff recession. In front of the LLWR site, it is possible that the dunes could have a limited influence on wave overtopping to parts of the site, although coastal recession is likely to have impacted site by this time. In order for the dunes to have a more significant influence in reducing flooding and erosion risk to the Drigg site over the long term, there would have to be a northerly movement of sediment located in the present day dune and spit complex to result in shoreline accretion in front of the site. This would need to be driven by a shift in the predominant wind/wave climate. The expected future forcing that will influence the development of the LLWR were summarised in Fish et al. 2010), which concludes a change in the wind/wave climate is unlikely over the next few hundred to a thousand years. In addition, there is considerable uncertainty over the response of the dunes to the very high projected rates of sea-level rise indicated by some projections.

7 Technical note 11 December 2012 Page 7 of 10 Figure 5. Present day configuration of the coastal landscape around Drigg. Figure 6. Hypothetical arrangement of coastal landscape around Drigg in the future. Figure 7. Schematic coastal dune development scenarios under rising sea level conditions (from Defra, 2007) 6 Confirmation of the current sand budget status of the Drigg dune system, are the dunes eroding or accreting? Information on the status of the Drigg dunes is available from the assessment of historical maps and aerial photos presented in Halcrow 2003 and These data indicate that the dunes have been in a stable equilibrium form since the mid 19 th century. The historical maps and photos show a spatially and temporally complex pattern of erosion and accretion of the dune front, with very limited net change in the shoreline position over the long term (when errors in the spatial data are accounted for). When the

8 Technical note 11 December 2012 Page 8 of 10 two most recent high quality aerial photos from 2002 and 2009 are compared, a consistent pattern of accretion is indicated, with localised embayments on the dune front being in-filled with sandy sediment that had become partially vegetated. Until future monitoring data are collected, it will remain unclear if this reflects a change in dune behaviour, however it is more likely to represent a short term effect caused by relatively calm sea conditions in the period before the 2009 image was captured. The site investigations undertaken by Durham University provide useful data on the longer time scales development of the dunes. Work undertaken by Durham in 2005 to 2007 (reported in Halcrow 2005 and 2008) provides a series of radiocarbon dates from peats above and below a continuous sand sheet that forms the low marsh land between the Drigg dunes and the channel of the River Irt. Organic sediments (peat) were sampled from four cores from this area providing an estimate of dune initiation of c years before present from 3 sites and c years before present from the deepest core. These results prove that dune initiation began by the mid-holocene, showing the data based on historical maps, which suggests the spit/dune system is far younger, is inaccurate. This sand sheet is covered by a layer of peaty soil which has been dated to c. 200 years before present, giving a guide to the date of dune stabilisation. The cessation in blown sand activity at this time may relate to changes climate, perhaps associated with the end of the stormy Little Ice Age period, or a change in land management that allowed vegetation to develop and stabilise the dunes. 7 Additional query on Bruun rule Additional questions were raised by the Environment Agency: does the shoreline recession rate modelled and assessed in the ESC increase if the rate of sea level change increases? does the SCAPE type model or the Bruun rule provide the most accurate recession rate? Both of the numerical modelling approaches (the Cliff Recession and SCAPE models) that formed part of the ESC coastal projection model have rate of sea-level rise as input parameters (Section 2). In both cases, there is considerable uncertainty over model behaviour in scenarios that have very high rates of sea-level rise (i.e. the high sea-level scenario of Fish et al. (2010), where rates of sea-level rise an order of magnitude greater than those seen in the historical record. This situation reflects the significant uncertainty in the science of long-term projections of coastal change, which is a result of the limited empirical datasets relating sea-level rise to coastal erosion, and uncertainty over the coastal response to the high rates of forecast sea-level rise that have no historical precedent. The Brunn model was reviewed by Halcrow in 2001, along with all available tools and models for coastal projection available at that time. This review did not include SCAPE, which was not developed until 2005 (Walkden and Hall, 2005). The Bruun model is largely based on the concept of an equilibrium beach profile and assumes that sediment eroded from cliffs, dunes and beaches is transferred seaward and deposited on the nearshore. This means the nearshore rises in line with sea-level and the sediment required to achieve this is balanced from erosion of the adjacent cliffs. In the 2001 report, it was concluded that while the method has been widely applied to short-term coastal changes with favourable results, there were concerns over its application to longer term changes (see Bray and Hooke (1997) for further discussion). The model has not been considered in subsequent reports and is not one of the approaches that feed the expert judgement work presented in the ESC (Fish et al. 2010). The principal limitations of the Bruun approach are:

9 Technical note 11 December 2012 Page 9 of 10 underlying geology does not influence the profile shape. The sandstone bedrock and the variability of the glacial sediments plays an important role in coastal morphology in west Cumbria; sediment grain size is the only variable determining shoreface profile variability; the use of a constant equilibrium profile, which implies that all sea floor profiles are in equilibrium with current and future sea level conditions. The concept of an equilibrium profile has been widely criticised; an instantaneous response to sea level rise. In reality, the response of the system is likely to lag behind due to the redistribution of eroded sediment; hydrodynamic processes are poorly accounted for; a simplistic mathematical relationship for equating the areas of erosion and deposition with landward migration of the coastline, meaning there is a net balance of sediment into or out of the profile from adjacent sediment sources. This may not be possible if sediment supply is limited, as is the case at west Cumbria. 8 Summary This note summarises the treatment of the sand based lithologies and the significance of dunes in the coastal evolution modelling underpinning the 2011 ESC. Responses to the five Technical Query Actions (ESC-TQ-SUE-022-A1 to A5) are as follows: A1. The dunes that front the LLWR and which form the northern part of the Drigg promontory rest on top of till deposits that constrain the rate of coastal recession. The dunes of the southern part of the Drigg promontory rest on gravels that may erode, but this would have little impact on future recession. The contribution of sand-sized material to the beach sediment budget is accounted for in the coastal erosion modelling. A2. The dunes have a limited significance in the evolution of the coastline over the timescale of a few hundred to a thousand years covered by the ESC. Under future conditions of rising sea level and limited supply of sand-sized sediment, there is little opportunity for expansion of the existing dune system or formation of new dune systems. A3. The entire frontage of the LLWR and the northern part of the Drigg promontory are underlain by a till ridge. Therefore the erosion assessments are based on cliff recession modelling that do not consider the response of dunes to direct wave attack. Under future conditions of potentially rapid rising sea level and limited supply of sand-sized sediment, existing sand dunes will be eroded. A4. The evidence for past avulsion of the course River Irt is equivocal, but even if an avulsion were to occur in the future, it would not affect timing of erosion of the disposal area, which is determined by erosion of the coastal frontage. A5. Under current climate and land management conditions, the dunes are stable. Geological evidence suggests dune accretion occurred over the past c. 6,000 years, but had ceased c. 200 years ago. 9 References Balson, P. S. (2010) Drigg Spit: Interpretative Report. British Geological Survey Commissioned Report, CR/10/ pp.

10 Technical note 11 December 2012 Page 10 of 10 Bray, M.J. and Hooke, J.M. (1997) Prediction of soft-cliff retreat with accelerating sea-level rise. Journal of Coastal Research, 13 (2), Defra (2007) Sand dune processes and management for flood and coastal defence Part 2: Sand dune processes and morphology. R&D Technical Report FD1302/TR. Fish P., Thorne M., Moore R. et al. (2010) Forecasting the Development of the Cumbria Coastline in the vicinity of the LLWR site. Quintessa document QRS-1443X-1. ESC Level 3 Report. Halcrow (2001) Review of historical and future potential coastal change at Drigg, Cumbria. British Nuclear Fuels plc May 2001 Halcrow Group Limited (2005) Coastal change at Drigg and Sellafield. Stage 1 coastal change projection model. Report to BNFL, June Halcrow Group Limited (2006) Coastal change at Drigg and Sellafield. Stage 2 coastal change projection model. Final report August Nexia Solutions Report Halcrow Group Limited (2008) Cumbria coastal studies. Stage 3 conceptual model. Final report March Nexia Solutions Report 8563 Walkden, M.J.A and Hall, J.W. (2005) A predictive mesoscale model of the erosion and profile development of soft rock shores. Coastal Engineering, 52(6):