1. Beach Modelling: Lessons Learnt from Past Scheme Performance Project: SC110004/S Specific Modelling Tools & Techniques

2. Summary of Findings – Specific Tools and Techniques  In addition to common themes, a range of specific tool and techniques lessons are identified, covering: –One-line numerical beach plan shape models –Empirical beach plan shape methods –Cross-shore beach profile models –Physical models –Beach monitoring data for beach design and maintenance  Lessons highlighting the importance of defining the wave climate for beach modelling tools and techniques is also provided.

3. Summary of Findings – Specific Tools and Techniques

4. One-line numerical beach plan shape models 1.May be used to predict medium to long term shoreline changes. 2.Originally developed for straight sand beaches, but some have been extended (by varying model coefficients) to model shingle beaches. 3.Differences between modelled and actual transport rates can often be attributed to the use of sediment size coefficients’. 4.Where strong curvature of the shoreline or complex beach processes exist, the model may not be accurate or appropriate. 5.Drift rates are sensitive to wave height and direction. 6.Understanding gross drift rates as well as net drift is vitally important where drift reversals are encountered. Failure to do this can lead to misinterpretation of results.

5. Summary of Findings – Specific Tools and Techniques Empirical beach plan shape methods 1.Empirical plan shape methods are useful, quick and easy to apply, but do have limitations. 2.Stable (equilibrium) bay shape methods have proved to be exceptionally reliable for predicting beach shape where there are strongly unidirectional waves. 3.Although empirical rules can be effective tools, they are also very simplistic and issues that can affect their reliability include assumptions on availability of material supply and sediment transport processes. 4.With empirical beach plan shape methods, structures are generally presumed to be solid barriers, i.e. material cannot pass over or through them.

6. Summary of Findings – Specific Tools and Techniques Cross-shore beach profile models 1.These approaches are often relatively quick and easy to use. 2.The prediction of short term response for single wave and water level conditions is useful for informing on extreme aspects of beach response, e.g. during storms. 3.They are less useful for predicting longer term beach evolution. 4.No models or empirical tests are presently available to accurately model mixed beach profile response. 5.None of the presently available cross-shore profile models replicate beach response to bi-modal wave conditions.

7. Summary of Findings – Specific Tools and Techniques Physical models 1.A constraint of physical models for beach profile response is the scaling of smaller sized materials. Generally sand sized material cannot be accurately scaled. 2.Although 3-dimensional physical models can represent wave processes better than numerical models, they are not normally expected to provide precise results for sediment transport on sand beaches. 3.Physical models enable the likely actual behaviour of a beach to be observed directly, but are usually more costly to set up and run than numerical or empirical methods. 4.There is limited scope to return to a model to re-examine other scenarios at a later date, as the model may have been decommissioned.

8. Summary of Findings – Specific Tools and Techniques Beach monitoring data for beach design and maintenance 1.Monitoring provides an actual record of beach response, which models are attempting to produce, so a high degree of confidence can be placed in these. 2.However, this actual record is just a snapshot in time and may not be representative of all states of the beach between surveys. 3.A limitation of this method is that is cannot predict future beach response if future management practices are likely to differ from those current or previously.

9. Summary of Findings – Specific Tools and Techniques Importance of defining the wave climate (1 of 2) 1.The choice of wave modelling approach needs to be appropriate to deliver the transformations required for the type of beach modelling being undertaken. 2.Insufficient record length raises potential for biased/unrepresentative conditions and thus beach response predictions. 3.Sensitivity tests should be undertaken when only short record lengths are available. 4.Calibration of synthetic wave data with measured wave data can make significant improvements to the accuracy of sediment transport modelling. 5.Beach plan shape models are strongly dependent on accurate representation of significant wave height and direction. Good definition of bathymetry is therefore also critically important.

10. Summary of Findings – Specific Tools and Techniques Importance of defining the wave climate (2 of 2) 6.Cross shore processes such as run-up and breaching are linked closely with wave period. Wave models investigated within this study have shown the accuracy of wave period data to be inadequate. 7.Wave conditions characterised by bi-modal wave periods may result in higher wave run-up and a greater possibility of breaching than conventional modelling methods can currently deal with.

11. Beach Modelling: Lessons Learnt from Past Scheme Performance Project: SC110004/S Specific Modelling Tools & Techniques