CONTEXT AND JUSTIFICATION

The Council has a duty to monitor watercourses and, where they are in a condition likely to cause flooding, to act to reduce the risk of flooding to existing and new development, particularly in residential areas. Advice on what the Council should consider in terms of the statutory planning process is set out in National Planning Policy Guideline 7: Planning and Flooding (NPPG 7).

This guidance is based on Annex B of the Council for Scottish Local Authorities/Scottish Environmental Protection Agency Protocol on Flooding Issues. It deals only with the commissioning and undertaking of flood risk assessment studies at particular sites or over particular areas. It does not relate to studies of potential flood risk alleviation measures.

Flood risk assessments may be of a relatively minor nature, evaluating a small development on a low-risk site with minimal secondary effects, or may comprise major basin-wide studies for significant infrastructure developments. Preliminary or scoping studies may be required prior to a fuller assessment being undertaken.

The detail and technical complexity of a flood risk report will reflect the scale and potential significance of the study but, in all cases, whenever a flood risk assessment is undertaken for any location, the resulting report should address, as a minimum, the following requirements:

1. A location plan at an appropriate scale that includes geographical features, street names and identifies all water courses or other bodies of water in the vicinity. This should include drainage outfalls and, if necessary, cross-refer to their operational arrangements in the body of the report.

2. A plan of the site showing levels related to Ordnance Datum, both prior to and following development.

3. A more detailed indication, if appropriate, of flood alleviation measures already in place and of their state of maintenance and performance.

4. An assessment of the source of potential flooding - riverine, tidal or a combination of the two. This will determine whether combined probabilities need to be considered.

5. A plan of the site with existing information on the extent and depth of flood events or on flood predictions. Information may be anecdotal, photographic, survey results or model estimates. The events should be identified with date/time, source of the data and supporting information provided on any of the following:

• rainfall and/or return period;
• probability of occurrence of the flood or storm surge event; or
• a combination of all of these.

Recorded data are particularly valuable and, if available, should be highlighted along with evidence of any observed trends in flood occurrence. Any changes that have taken place since the last event should be identified.

6. A plan and description of any structures that may influence local hydraulics. This will include bridges and pipes/ducts crossing the watercourses together with culverts, screens, embankments or walls, overgrown or collapsing channels and their likelihood of choking with debris.

7. An assessment of the return periods or probabilities and any observed trends and the extent and depth of floods for the location and, if appropriate, routes and speed of water-flow. At this stage, best estimates, based on the most up-to-date findings, should also be made of climate change impacts on probabilities. Depending on the nature of the proposed development, it may be necessary to extend the assessment to the 0.2% probability, or 500-year flood, inclusive of climate change predictions.

8. A cross-section of the site showing finished floor levels or road levels, or other relevant levels, relative to the source of flooding and to anticipated water levels and associated probabilities.

9. An assessment of the likely rate or speed at which flooding might occur, the order in which various parts of the location or site might flood and the likely duration of flood events.

10. An assessment of the hydraulics of any drains or sewers, existing or proposed, on the site during flood events. The methodology for assessment must be clearly stated.

11. An estimate of the volume of water that would be displaced from the site for various flood levels following development of the site.

12. An assessment of the likely impact of any displaced water on neighbouring or other locations that might be affected subsequent to development. This should address the potential for change in the flooding regime both upstream and downstream of the site due to ground raising or flood embankments.

13. An assessment of the potential impact of any development on fluvial or coastal morphology and on the likely longer-term stability and sustainability.

GENERAL REQUIREMENTS FOR UNDERTAKING NUMERICAL MODELLING

It is likely that many flood risk assessments will require hydrological and/or hydraulic modelling. The following generic requirements represent good practice in undertaking numerical modelling. The sophistication, cost and safety implications of any development proposal should be reflected in the complexity, scope and precision of the models applied, the range of scenarios studied and the amount and range of input data collected. For flood risk modelling, it is particularly important to justify the type of model used (e.g. dynamic or steady-state) and to describe and list the input data. On occasions, data may have to be estimated by reference to neighbouring, or hydrologically similar, catchments and the methods applied must be clearly stated and their limitations emphasised.

The report of any modelling study should address all the following requirements at an appropriate level of detail.

1. Statement of objective - to explain clearly the situation being modelled and the objectives of the modelling study, including details of the output required from the model.

2. Justification of the model - to demonstrate that it is suitable for this study, including examples of previous applications in similar circumstances.

3. Technical description of model - history of the model, development history, published articles, details of the conversion of the model into a software package. Details of the experience and training of the model users.

4. Data - a model is only as good as the source data. The data required for the model must be clearly defined.

5. Data collection - all relevant data collection and measurement techniques should be quoted, including expected errors and relevant quality assurance. The raw data should be available to the client if required, as should details of the instrumentation and their calibrations.

6. Model calibration - it is important that the model is calibrated against a full data set representative of the range of conditions to be modelled. The model coefficients to be calibrated and the procedures to be used to optimise the calibration must be stated clearly. The choice of boundaries must be justified.

7. Model validation - data sets independent of those used for calibration must be employed for validation tests. Every effort should be made to validate the model across the range of conditions for which it will be run. Validation tests and analysis of model errors must be undertaken for the key variables required from the modelling study.

8. Sensitivity analysis - this analysis must be presented to demonstrate the effect on the key output parameters resulting from variation of input data and controlling assumptions.

9. Quality assurance - to demonstrate that the model has been subject to an evaluation procedure establishing its suitability for the relevant tasks.

10. Auditability - to ensure that there is a clear account of the modelling exercise for inspection by any appropriate auditors.

11. Reporting - clear description of the model including the underlying principles and implicit or explicit assumptions. Also a clear summary of the numerical output, the likely errors, bias, sensitivity and their implications for the objectives of the study and the conclusions.