Table 3-1: Overview of different levels of assessment. Flood Modelling Guidance for Responsible Authorities v1.1 18 National (Strategic) Catchment (Feasibility) Local (Design) Example Objectives Producing national hazard maps to meet requirements of EU Floods Directive. Options appraisal. NFM studies. Screening maps for LUP in more complex areas. Detailed design of flood defences or NFM measures. Site specific FRAs for developments. General description Broad scale assessment; national overview; based on national datasets; strategic direction setting; e.g.
FRM planning -> national comparison; awareness raising; improved understanding Catchment scale assessment; improved understanding of the catchment; inform flood risk management option feasibility; flood warning area development; emergency planning local focus; consider interactions of sources; detailed representation of local features Scale National, large or multi- catchment, or long reaches of coastline. Catchment scale, major firths or long reaches of coastline with similar orientation. Small catchment or section of larger catchment, short lengths of coastline e.g. town frontage. Approaches Fluvial Use of historic flood extents, 1D or 2D hydraulic modelling.
Flows from national datasets e.g. CEH flow grid. 1D, 2D or 1D-2D hydraulic modelling. Hydrology using FEH methods and distributed inflows. Range of scenarios. Detailed hydraulic modelling (1D or 1D-2D); range of modelling scenarios; consideration of climate change. Local-scale assessment of design flows; explicit use of local information. Pluvial Use of historic flood extents, rapid flood spreading, 2D hydraulic modelling. FEH DDF model. Limited number of durations 2D hydraulic modelling. FEH DDF model. Range of scenarios. Coastal Use of historic flood extents, Level projection, rapid flood spreading, 2D hydrodynamic modelling. Hydrodynamic and wave transformation modelling.
2D hydrodynamic model for flood inundation. Range of scenarios. Consideration of climate change Detailed hydrodynamic and wave transformation modelling, overtopping modelling. Consideration of climate change. Consideration of coastal processes. 2D hydrodynamic model for flood inundation. Structures Fluvial Typically blockages caused by structures removed from DTM. Buildings likely to be removed from the DTM. Definition of key structures using basic information to represent height, length, etc. Structures such as foot bridges may be omitted. Explicit and refined definition of key structures and channel features. Pluvial Coastal Raised defences added to DTM. Overtopping analysis for defences.
Overtopping analysis for defences. Calibration/ Sensitivity Testing Check of selected locations against historic records. Reality checks Check against historic records; sensitivity testing. Calibration at gauges. Calibration at gauges for multiple events. Installation of additional gauging. Comparison with historic records. Sensitivity testing. Limitations Not suitable for site specific information. Large spatial range -> relatively lower spatial accuracy Assumptions on Hydrology and limited assessment of local influences Can be time consuming and expensive.
Data Requirements Fluvial DTM DTM Channel cross section survey DTM Channel cross section survey Pluvial DTM DTM Drainage network information Coastal DTM Bathymetry DTM Bathymetry Defence and beach profiles Quality Control Review of input data. Review of method. Consistency checks. Screening for physically unrealistic results. Review of input data. Review of models and hydrology by independent internal reviewer. Review of input data. Detailed review of models and hydrology by independent internal reviewer or external reviewer.