Lyme Bay and South Devon Shoreline Management Plan: comments of West Dorset Coastal Research Group.

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1 Lyme Bay and South Devon Shoreline Management Plan: comments of West Dorset Coastal Research Group. The following represents the WDCRG consultation response to the draft SMP. Its scope is restricted to comments directed towards Chapter 6 and Appendix C of the draft SMP concerned with recommendations for future monitoring and research. The aim is to review the recommendations made, to identify possible gaps in knowledge and to indicate the opportunities for additional research that should be of benefit to the process of shoreline management and the development of scheme strategies. A draft document was produced initially by Dr M. Bray and then circulated amongst group members. Comments were received from the following and incorporated within this final document: Professor Denys Brunsden, Chideock. Dr Alan Carr, Wellington. Dr Jim Chandler, Loughborough University Mark Lee, University of Newcastle-upon-Tyne Attention is focused primarily upon the West Dorset coast, although many comments relate to general principles that should be applicable elsewhere along the south Devon SMP coastline. Some of the suggestions involve quite detailed work that it probably would not be feasible to fund in direct support of the SMP, or related Coastal Defence Strategy Plans. However, it is anticipated that the case for support from other directions (e.g. NERC, MAFF, EU etc.) should be strengthened considerably if relevant and valuable items can be identified and included within the SMP recommendations. Issues are tackled in the same order as presented in Chapter 6 of the SMP and comments are direct responses to that material. 6.2 Areas of Uncertainty Coastal Processes The draft plan states that a good understanding has been achieved of the coastal processes. The opinion of the group is that this statement is incorrect and should be withdrawn or altered in the SMP. There are many uncertainties concerning the following process related issues: 1.Long term wind climate. 2.Long term wave climate, especially swell waves. 3.Long term littoral sediment transport on the beaches. 4.Attrition losses of beach pebbles. 5.Exchanges of shingle between beaches and the nearshore bed. 6.Sediment transport in the offshore zone. 7.Approximate sediment budgets of the many pocket beaches within the SMP. 8.Breaching of barriers such as Chesil and Slapton. Furthermore, strategic management as advocated by an SMP also requires good knowledge of the behaviour of the landforms that result from the operation of coastal processes. It is the coastal landform e.g. beaches, cliffs, spits, dunes or saltmarshes, that provides habitats and 1

2 natural coastal defence and which may require management within an SMP. The following aspects of landform behaviour are important to identify: 1. The materials of which the landforms are composed, their origin and modes of delivery. 2. The interdependence of landforms and possible linkages between management actions and landform responses. 3. The potential scales of landform behaviour involving knowledge of the nature, magnitudes and frequencies of the changes that might occur. 4. The sensitivity of different landforms to external factors such as extreme events, management actions or more gradual climate changes e.g. shingle beach response to sequences of storm events of varying size? impacts of climate change on coastal landslide activity? beach response to variations in wave direction? 5. The consequences for geomorphological development, coastal defence and the natural environment of those changes considered likely or possible e.g. the threat to coastal defences posed by continued landslide activity in areas such as Lyme Regis, Seatown & West Bay. Much of this landform information can be developed using palaeo-environmental reconstruction, historical analysis and interpretation of landforms. In the case of the larger beaches (Chesil & Slapton) and spits (Dawlish Warren), late-holocene inheritance (up to 7,000 years before present) is important. The sediment budget (100 year timescale) is important for all beaches. In the case of the coastal landslides both ancient failures and the occurrence or recurrence of failures over the past years are important. In our opinion, such knowledge is not a luxury, but a necessity in a complex and dynamic environment where present day processes may not be a reliable guide to the full range of changes and responses that are possible. Indeed, many of the requirements for further research derive from the need to better define the relationship between contemporary processes and responses (reasonably well covered by the SMP and various engineering studies) and the longer term (50-100yrs.) trends towards which the landforms are evolving (not well covered to date). Monitoring will assist in the resolution of some of these uncertainties, but only slowly as the length of record increases. In the meantime, analysis of historical and archival sources should provide a valuable retrospective source of data. Studies involving the Lyme Regis Environmental Improvements and the assessment of Coastal Defence Improvements at West Bay are excellent examples of the types of holistic landform oriented approach that we feel the SMP should promote more strongly. In another example, all coastal cliffs on the Isle of Wight coast were classified into behaviour units for that SMP using methods developed within the recent MAFF funded Soft Cliffs research project Land Use, Human and Built Environment Two areas of uncertainty are identified: (i) detailed knowledge of ground levels in low-lying areas - this is essential to identify areas at risk of flooding and also to indicate the likely floodwater depths occurring under different scenarios for economic analysis and (ii) knowledge of the extent of properties situated upon ancient landslides susceptible to reactivation following marine erosion - this is being addressed at Lyme Regis, but may be relevant elsewhere e.g. Charmouth, West Bay. 2

3 6.2.3 Natural Environment Three areas of uncertainty are identified: (i) knowledge of ground levels in low-lying areas - to identify habitats at risk of saline flooding and (ii) the sensitivity or tolerance of important habitats and rare species to changes in environment - to estimate the losses or changes occurring with any change in strategic coastal defence options or with 'natural' changes occurring under a do nothing or managed retreat option (iii) the opportunities available for habitat creation (possibly required under EU Habitats directive if SMP actions lead to damage of EU designated habitats) Coastal Defences Reliable information relating to condition, standard of protection and residual life is much more difficult to provide in the case of soft defences where the beach is the main defence. Beaches vary significantly in the short term making assessments sensitive to conditions preceding the survey. Furthermore, generic assumptions of adequacy of soft defences are poorly developed and difficult to apply as they are less predictable in their performance than hard defences and the same design of soft defence may perform differently on a site by site basis. A solution could involve development of effective links between the programmes of beach and coastal defence structure monitoring within rolling programmes of beach management. Even then, some work would be needed to develop a reliable set of indicators permit monitoring of the actual standard of defence afforded by soft defences. 6.3 Recommendations for Future Monitoring Coastal Processes Appendix C provides much sound general advice on monitoring, but a significant number of uncertainties can be identified. We have sought to indicate how future monitoring might be improved and suggest that you add the following to the SMP: 1. The vertical aerial photography recommended should be very valuable. However, details are needed of methods of analysis and the purposes that might be served e.g. photogrammetric measurement to determine cliff retreat & beach volume changes - for each pocket beach? If so, an allowance is needed for setting up a network of stable precisely surveyed photogrammetric ground control markers. A reliable standardised analysis routine could then be devised to process results very efficiently. Additional flights (e.g. post storm) could easily be commissioned by individual authorities to record exceptional events occurring in more limited areas - processing of results would be standard and relatively cheap. A frequency in excess of once every 5 years would be preferable for areas where potential problems are identified. The choice of 1:10,000 scale photography is a little small for beach monitoring giving maximum attainable vertical accuracy of measurement of ±0.2m. 1:5,000 scale photography with 60% stereo overlap is preferable as used by the EA Southern Region Beach Monitoring Programme since 1973 between Bournemouth and the Thames. A larger number of photos would be needed to cover the coast at this scale such that the indicative cost could increase. 2. Consideration is needed to identify the some of the morphological features that might be monitored as indicators e.g. cliff-top, cliff-toe, edges of terraces within undercliffs, beach crest, shingle beach toe (where exposed) etc. 3. Having identified indicators it is important to devise a means of differentiating significant trends of concern to coastal defence from short term fluctuations or "noise." 3

4 Variation of indicators in excess of that attributable to normal variability might provide valuable early warning of major changes and future problems. For this reason it would be useful to establish normal variability based on historical analysis and to allow provision for periodic analysis and review of monitoring results. Consideration is needed of the possible actions that might be triggered when monitoring results exceed pre-determined thresholds. Such a programme would alter the management philosophy from being reactive to proactive. 4. Details are needed of the purposes to be served by the oblique photography. Its main use should be for visual representation and qualitative comparison. Given some care in camera and lens selection, flight paths and perspective etc., some quantitative measurements may be possible from such photography. However, vertical photography is always preferable where ease of analysis and precision of results are important. 5. The effects of any improvement or extension of the gabion matresses at Chesilton (MU 2) would need to be monitored especially carefully. Morphological changes resulting from high magnitude low frequency wave events would be the most important as impacts upon the coastal defences, natural environment and amenity values depend upon how the beach would interact with the new structural control under such conditions. Indeed, it could be argued that the present gabion matresses have yet to be tested fully by major swell wave events for which they were designed - hence they could still be viewed as experimental such that the results of extending them are uncertain and potentially problematic. 6. Greater allowance is necessary for monitoring of the effects of extreme events upon Chesil Beach. This should involve both the responses to typical cyclonic storms approaching from the SW as well as potentially more destructive swell and long period wave events. The coastal defence function of Chesil is dependent upon the integrity of the whole beach so monitoring should consider the whole structure to identify potential "weak points" that could form foci for breaching and eventual breakdown of the beach. This is a formidable task and would be assisted greatly by thorough historical analysis to establish natural variability and confirm the prevalence of any especially volatile sections as revealed by the Babtie (1997) study. Breaching and breakdown of the beach (either gradually or catastrophically) over the forthcoming years are felt to be genuine risks which were not addressed adequately by the Babtie (1997) study. A dual level approach is recommended comprising: (i) analyses of historical records of beach morphology (especially aerial photos) together with historical data concerning extreme events and (ii) improved monitoring of the contemporary beach (higher frequency than hitherto to help separate short term fluctuations from long term trends). The objective should be to develop a morphological model of beach behaviour which might improve confidence that events indicating possible breakdown could be detected early. Costs might be justified by the high scientific value of the results as well as by the severe consequences of an unpredictable beach breakdown. Costs of profiling might be reduced by application of new technologies to beach monitoring e.g. GPS field survey, airborne laser scanning (LIDAR), digital photogrammetry. The practicality, reliability and actual costs of these alternatives as well as their benefits of speed and resolution would need to be established by a feasibility study. Collectively, this work could form the basis of a scientific research project funded at national or European level - inclusion of this recommendation within the SMP may strengthen the argument for such work. 7. Reliable automatic weather stations could be established at key coastal sites. Wind data would be valuable for hindcasting of local wave climates. Precipitation data would be valuable in relation to study of landslide activity - especially if there are climate changes. 4

5 8. Consider monitoring of the levels of the nearshore sea bed in key areas where exchanges of shingle are suspected as at West Bay. Periodic sonar survey and sediment sampling along fixed shore normal profiles should suffice. Side-scan surveys could be applied to identify geomorphological features if necessary. It is recommended that opportunities are sought for development of monitoring proposals within the context of a regional initiative. This might provide a means of establishing common methods and standards for measurement, archiving and data analysis and be co-ordinated by a body such as SCOPAC with an overview of the results and future requirements of several adjoining SMPs. Costs of necessary feasibility studies could be shared. The monitoring most amenable to regional co-ordination would include wave and wind recording and beach monitoring. Tidal recording is already co-ordinated nationally by Proudman Oceanographic Laboratory with their network of 'A' Class gauges, although some other local authorities along the south coast have established their own gauges to provide improved resolution in flood hazard areas. A plan for regular updating of key analyses is needed to derive maximum benefit from the monitoring data e.g. beach volumes and trend analyses, sea-level rise, wave climate and extreme wave statistics, extreme sea-levels and joint probability studies etc. 6.4 Recommendations for Future Research A general opinion of the group is that the draft plan recommendations focus solely upon present day processes when significant uncertainties still remain in respect of the palaeoenvironmental evolution and historical behaviour. The latter should reveal key insights into the likely or possible long term behaviour of large and complex landforms such as Chesil Beach and the S.E. Devon and W. Dorset landslides. By contrast, several decades or more of monitoring data would need to be collected before similar understanding could be derived from contemporary process measurements. It is considered that conceptual evolutionary models need to be developed to indicate the possible longer term implications of changes detected by monitoring. Research should attempt to tackle requirements for estimates of future coastal evolution over scales of years as well as seeking to provide better predictions over 2-20 year periods. Both scales are important to shoreline management - the former to provide strategic vision and the latter to provide the knowledge to underpin specific policies and practices developed in working towards the long term goals Coastal Processes In research terms Chesil needs to be considered as a whole i.e. as a process unit rather than as two or three management units as indicated by the draft plan. It is felt that the study proposed to analyse Chesil monitoring results should be accompanied by work to develop a conceptual geomorphological model of the beach based the latest information available from palaeoenvironmental, historical and sediment budget research. It is regarded as a serious weakness that the long term sediment dynamics of the beach, its morphological changes and its internal structure remain poorly understood. Such a model should include the development of realistic scenarios involving the modes of shingle transport and breakdown of the beach - an event that should not be ignored by the SMP. Further academic research should be encouraged to provide vital further evidence of past evolution and behaviour of the beach. Research is needed to classify the behaviour of the coastal cliffs & landslides throughout the SMP coastline. The stability of all the cliffs - especially those of the Permian between Teignmouth & Torbay and the Devonian of Torbay - should be assessed. There are many severe landslide situations on this coast which may be very vulnerable to sea-level rise and climate change, but they have received little attention. It is recommended that the method of 5

6 identifying coastal behaviour units developed within the recently completed MAFF Soft Cliffs project (Rendel Geotechnics) is applied. This is primarily a reconnaissance level study and has been applied previously to the 100km of varied cliff coast around the Isle of Wight in support of its SMP. Management Plans need to be prepared for each pocket beach within the SMP which has a coastal defence function. They should address issues such as the long term and short term behaviour, sediment budgets, sensitivity to changes in environmental controls. This information should provide a framework for predicting the future behaviour over the next 50 years - to help identify potential conflicts between the proposed SMP policies and the longer term management objectives for the coastline (e.g. it may be possible that a particular beach will not be able to provide sufficient protection within the next 50 years - what should be done?). The plans, to a large extent, would be syntheses and interpretation of existing information, but could also include primary data collection (e.g. seabed sediments in the nearshore zone, within 1000m). Validation of the Meteorological Office Wave Model is an excellent idea, though it should be acknowledged that this model cannot easily account for far traveled swell waves or other long period waves so there may still be a requirement for long term wave recording, especially in the vicinity of Chesil. Note that recent research suggests that this coast is exposed to tsunami which may explain several cases reported of large waves appearing out of a calm sea Natural Environment A breach study of the type proposed for Slapton should also be considered for Chesil where the wave exposure and consequences of breaching are significantly greater. Relevant Research Options for Inclusion within SMP Recommendations: The following options should fill gaps in the information contained within the SMP and should be seen as additional to the research recommendations contained within the May 98 consultation document. The group believe that the results of the work suggested here should assist the implementation of SMP options and in time contribute strongly to future revisions of the SMP, thereby setting a firm basis for long term sustainable management of this coast. We therefore recommend that the following items be added to the SMP research recommendations: 1. Study of the origin and development of Chesil making use of all available palaeoenvironmental historical and sediment budget information. Linking of past development with contemporary monitoring and coastal process data. Development of a conceptual model of beach behaviour including possible breakdown of the beach (this type of approach is likely also to be effective for other process units within the SMP where better understanding of long term changes are needed). 2. Effects of extreme events upon Chesil Beach. Two approaches: (i) careful collection and evaluation of historical data and (ii) monitoring of contemporary beach responses to storms. Consideration be given to the application of new technologies for beach monitoring to help tackle problems caused by the size of Chesil. Items 1 & 2 should be addressed by a national or European funded scientific research project - inclusion of these recommendations within the SMP would strengthen the argument for such work. 6

7 3. Evaluation of consequences of results of items 2 & 3 for scientific interest, natural environment, built environment, coastal defence and amenity. 4. Encourage continuation of palaeo-environmental studies of Chesil and the Fleet Lagoon. 5. Encourage geomorphological investigation of the inshore seabed in support of studies of long term coastal evolution (relevant throughout Lyme Bay - especially between Chesil and Lyme Regis). 6. Precision measurements of ground levels in low-lying areas to identify lands, properties and key habitats at potentially at risk (photogrammetry or LIDAR). Especially valuable for study of consequences of Chesil Beach breakdown. 7. Continued wave monitoring to validate locally the Met. Office wave model. Give consideration to development of a means of defining a long term swell wave climate for Chesil. Consider installation of additional automatic coastal weather stations to record local winds and precipitation. 8. Classification of all cliffs into Cliff Behaviour Units (CBUs). These should provide an excellent first estimate of the nature and extent of ground movements possible should marine erosion accelerate or climate change intensify. Selected landslides where likely reactivation or possible first time failures would present a significant risk to properties may be identified for further investigation e.g. Isle of Portland, Torbay-Teignmouth. 9. Monitor scientific, educational and amenity interests. A means of quantifying the unique value of the SMP coast in these terms would be of great value in appraising scheme strategies. A definitive method has yet to be agreed, but visitors and other key indicators should be monitored in anticipation of practical methods becoming available in the future. 10. Seek regional co-ordination of monitoring through a body such as SCOPAC to identify best practice and standardise measurements, data archiving and analysis. 11. Promote preparation of Management Plans for each pocket beach within the SMP which has a coastal defence function. Any correspondence relating to this document should be addressed to the following: Dr Malcolm Bray Department of Geography University of Portsmouth PORTSMOUTH Hants. PO1 3HE malcolm.bray@port.ac.uk 7