Before commissioning a study, it is important to understand flooding mechanisms at the study location. A good understanding of the links between the sources and impacts of flooding can help identify the most appropriate modelling approach or whether modelling is the correct approach. A summary of the flooding mechanisms should be provided in the SoR to enable contractors to propose an appropriate methodology for the study.
To help understand the interaction of different actions across catchments and coastlines, the Responsible Authority should use the source–pathway-receptor–impact approach to build a conceptual model of the key processes which need to be considered in the study (Figure 3-1). This approach is a well-established framework in flood risk management. It provides a basis for understanding the causal links between the source of flooding, the route by which it is transmitted and the receptor, which suffers some impact: Sources are the weather events or conditions that result in flooding (e.g.
heavy rainfall, rising sea level, waves etc.); Pathways are routes between the source of flood waters and the receptor. These include surface and subsurface flow across the landscape, urban drainage systems, wave overtopping; Receptors are the people, industries and built and natural environments that can be impacted upon by flooding; Impacts are the effects on exposed receptors. The severity of any impact will vary depending on the vulnerability of the receptor. For any area there may be multiple sources, pathways and receptors which interact with each other. Source Pathway Receptor Impacts •e.g. rainfall, wind, waves •e.g. overtopping, overflow •e.g.
property, people, environment •e.g los of life, stress, material damage, environmental degradation Figure 3-1: The Source-pathway-receptor-impacts model, based on the DEFRA/Environment Agency R&D Technical Report FD2302/TR1 (Sayers, Gouldby, Simm, Meadowcroft, & Hall, 2003). In developing the conceptual model historic flood information for the area, including any anecdotal evidence, should be examined. A catchment walk-over in conjunction with a desk top review of Ordnance Survey maps and aerial photography should be used to identify physical features which may affect flood pathways and possible receptors.
Previous studies and SEPA’s national flood hazard maps http://map.sepa.org.uk/floodmap/map.htm can also be used to identify possible flooding mechanisms and whether they are adequately captured by the previous modelling approach. For coastal areas the National Coastal Change Assessment http://www.dynamiccoast.com/ should be used to identify areas where coastal change may be a factor influencing flooding. The historic flood datasets which may be available to help with developing a conceptual model are discussed in Section 5.4. Available data should be assessed during development of the conceptual model in order to determine the need for any additional data collection.
Responsible Authorities may consider assessing available data for studies later in a Flood Risk Management cycle so that data gaps can be identified and filled prior to commencement of the study. Information on relevant data is given in Chapter 5. A list of available data, with a brief description, should Flood Modelling Guidance for Responsible Authorities v1.1 11