be set out in the SoR, together with any survey requirements etc. SEPA can assist in determining the availability of hydrometric and other data for inclusion in the SoR. Knowledge of the catchment should be set out briefly in the SoR, including any key areas and known flooding mechanisms which need to be considered. These key features should also be marked on a location plan to be included with the statement of requirements. A catchment walkover with an appointed contractor can help develop a shared understanding of the study area.
Contractors should develop the conceptual model of the catchment, as understanding of the study area increases throughout the study, and describe the conceptual model in the modelling report. 3.4 Spatial Extent Key points for Responsible Authorities Use the conceptual model in setting the study extent. A single study covering a larger area is likely to be significantly cheaper than several smaller studies and may also be more accurate and robust Studies for inclusion in SEPA’s hazard maps should have a study area consistent with the strategic nature of the maps. The conceptual model described in section 3.2 should be used in setting the study extent.
Consideration should also be given to the area of interest, availability of calibration data, future use of the model and cost and time for a study The study extent should be sufficient to represent the assumed flooding mechanism (i.e. it should cover the flood source, flood pathway and any receptors). Boundaries of the study area should be sufficiently far away from the area of interest, considering flow controls, to have no impact on the results. Good places to set study boundaries are areas where the flood extents are relatively constrained for large events or where there is a hydraulic control such as a weir or tidal boundary.
SEPA’s flood hazard maps http://map.sepa.org.uk/floodmap/map.htm can be used with large scale mapping to identify constrained sections, which should be reviewed at a site visit. Data available for calibration should be considered in setting the study extent, as extending the study area to cover calibration data may significantly improve confidence in the study output. For fluvial studies covering gauged rivers it is strongly recommended that the study extent covers at least one and preferably two or more gauges to assist in calibration. This is discussed further in section 5.8 and Chapter 8.
The availability of topographic data should not be used to constrain the study area where other considerations suggest that a larger area would be more appropriate. The preference should be for additional data collection rather than a reduced study extent. Flood Modelling Guidance for Responsible Authorities v1.1 12