Beach Modelling: Lessons Learnt from Past Scheme Performance Project: SC110004/S Guidance on approach to beach modelling

Guidance - Background  Environment Agency commissioned study  The project examined the behaviour of beaches on eleven sites  Compared actual behaviour with the expected performance from the original modelling and design.  A range of findings have been captured and used to develop guidance for future modelling of schemes  Published 2 nd May  Documents are on the Defra/EA R&D website and GOV.UK Defra/EA R&D website GOV.UK

Guidance - Audience  Guidance is aimed at coastal practitioners and those in commissioning organisations who may not have detailed technical knowledge of beach modelling.  Specifically it identifies points for them to be critically aware of when beach modelling is being contemplated or carried out.  It should also help the reader gain a better understanding of the approach to modelling, decisions that need to be taken along the way and the outputs from it.

Guidance - Contents  Final guidance provided in Beach modelling; lessons learnt from past scheme performance  Contents: –Section 1: Introduction –Section 2: Summary findings –Section 3: Comparative analysis sites –Section 4: Approach to beach modelling –Sections 5 to 9: Specific model tools and techniques detail –Section 10: Waves for beach modelling –Appendix – Comparative analysis summaries

Guidance – How to use it Understand the wider coastal process system and the data available (Section 4.1.2) Identify the Objective(s) of beach modelling in the context of overall beach management objectives (Section 4.1.1) Define ‘the problem’ (Section 4.2.1) Decide the modelling approach (Section 4.2.2, Section and Section 4.2.4) Set up and calibrate the beach model (Section 4.3) Undertake modelling (Sections 5 to 10) Analyse and interpret outputs to inform design (Section 4.4.1, Section 4.4.2, Section and Section 4.5.1) Further use of modelling to support ongoing beach management (Section 4.4.4, Section and Section 4.5.2)

Taking the Right Approach

Taking the right approach What is the objective?  Begin by establishing the ultimate outcome being sought (not just the modelling outcome) and the questions that the modelling needs to try to answer  Commissioning organisations are likely to get more benefit from stating what questions they want answered, rather than specifying what modelling they expect is required.

Taking the right approach What factors will determine the approach?  Important to know the driving factors along that coast, and the key aspects and variables for data inputs, by establishing: 1.the domain in which the problem belongs (long-shore / cross-shore); 2.the time scale and spatial scale in which the problem occurs; 3.which physical processes are important; and, 4.the combination and significance of variables.  Quality and amount of information available is also critical to making the choice.

Taking the right approach Understand potential limitations of different approaches  Uncertainties associated with using models and techniques to predict beach behaviour are inevitable. –Some are quantifiable, some unquantifiable.  Models are sometimes run outside of their range of validity –Need to considering the implications of doing so (degree of certainty in the outputs).  In some circumstances models are just not good enough to represent reality due to limitations or simplifications in the underlying processes represented. –Might still use, but apply a simplified representation only and recognise limitations on outputs

Taking the right approach Choosing the right models?  a range of approaches and tools are available to use in beach design and beach maintenance planning  Different approaches are applicable for different situations – a solution will often involve the use of more than one approach and a healthy dose of expertise in interpretation.  REMEMBER, modelling always produces an approximation of beach responses. Best value may therefore be obtained from relative assessments of alternative scenarios and different schemes rather than absolute outputs.

Undertaking Beach Modelling

Undertaking beach modelling Setting up Beach Models  Input data quality – models generally very sensitive to parameters used.  Important to define input variables at the outset: –Beach material grain size to be modelled –Beach contour(s) and/or profile(s) –Waves and water levels –Structure geometry  Boundary conditions –need to understand any influences from, and impacts on, neighbouring frontages and offshore.

Undertaking beach modelling Assumptions on beach sediment grading  Recognise that the beach model will have theoretical limits on its applicability relating the range of sediment sizes. Models are likely to: –assume that the beach sediment is uniform (e.g. a single D50 value); –assume uniformity in beach sediment cross shore, along shore and at depth; –assume that the threshold of motion is defined by the uniform particle size used; –assume full availability of sediment for transport and derive potential transport, erosion and accretion rather than actual.  Using outside these limits can be necessary and indeed still prove useful, but understand those limitations and use the outputs appropriately

Undertaking beach modelling Model calibration  Undertaken to demonstrate model represents behaviour of existing beach when results compared to actual data.  Problematic when changing the fundamental characteristics of a beach - as many schemes do! –Altering beach material size –Introducing or modifying control structures  Availability of data for calibration used to be an issue but situation improved considerably with monitoring programmes in place.

Sensitivity modelling  Should be used pro-actively to assess potential variations in outcomes to limitations/variability in input parameters  Use to inform selection of the preferred scheme  Sensitivity to certain parameters can produce not just a difference in rate of change, but alter the overall behaviour of that beach. Undertaking beach modelling

Accommodating uncertainty and changes  Adjusting the Design – to account for differences in the site or input conditions between when modelling originally done and implementation actually takes place.  Revisiting Modelling – particularly to assess how to respond to changes in behaviour rather than a trial and error adjustment to management regime.  Scenario Assessment – model different scenarios, e.g. changes in management regime or implementation details, to inform likely response to any such changes.

Understanding Beach Modelling Outputs

Beach modelling outputs – interpretation  Models are only one part of the design toolkit  Suitable expertise should be applied to interpret the results and identify any potential anomalies or spurious results  Certainty is often demanded (and is sometimes offered) – need to be realistic and recognise any limitations in the outputs, which need to be carried through to decisions based upon them.

 Document the approach taken to modelling –ensure others in future can use the outputs appropriately  Highlight limitations in the modelling outputs at the time of design development –convey uncertainties Beach modelling outputs - documenting the approach

Beach modelling outputs – management of the beach  Also document what management has occurred post- scheme to aid understanding of performance compared to pre-scheme predictions.  Information that should be recorded in association with beach management includes: –Date, volumes and locations of material placement. –Dates locations and details of structure modification. –Locations of borrow sites and volumes extracted for recycling.

Summary

 It is the appropriate application and interpretation of beach models that is key to successful scheme design. –Define the problem and the questions that need to be answered by modelling. Engage expertise early to establish how to address these questions. –Don’t overcomplicate. Recognise limitations and capabilities of models - model accordingly and deal with uncertainty appropriately –Maintain an auditable trail from model to design to management –Recognise that models are just part of the toolkit and they do not provide the definitive picture ….. but the outputs from these models are invaluable for the design and management of beach schemes Summary on Approach to Beach Modelling

Beach Modelling: Lessons Learnt from Past Scheme Performance Project: SC110004/S Guidance on approach to beach modelling