Flood Estimation Handbook (FEH)
The industry standard method for assessing flood flows in watercourses is the Flood Estimation Handbook (FEH) software produced by Wallingford HydroSolutions Ltd and NERC (CEH).
The assessment of flows in watercourses is important since other engineering processes use the results. For instance, the flows used in hydraulic river modelling are often derived by a FEH analysis and the results of the river modelling, may in turn, form an important part of a flood risk assessment (FRA). FRA's are a significant part of the planning process since they help to ensure that developments are not sited in areas prone to flooding.
The FEH, published in 1999, is a development of the Flood Studies Report methodology of 1975 and includes two methods for flood estimation, the Statistical procedure and the Revitalised Flood Hydrograph (ReFH) rainfall-runoff method. The statistical procedure can provide a flood peak of a given return period at both gauged and ungauged sites. The ReFH method can also be used to estimate peak flows but in addition can also generate design flood hydrographs and be used to analyse observed storms.
Statistical and ReFH Methods
Since the 1950's, flows in rivers have been systematically recorded and the analysis of this data is the basis of the statistical method. The statistical method itself incorporates several possible methodologies but in any event is a two stage process. The first stage is to estimate the QMED flow, that is the 1 in 2 year flood. The second stage is then to derive a growth curve for the site, this determines the relationship between the QMED flow and other return period flows. Depending on the data available for the study site this might be done solely on the data for the study site or might be derived from similar sites with suitable data by use of a 'pooled analysis'.
The ReFH method uses a mathematical model which estimates the losses to rain falling on the catchment, which includes infiltration into the ground, and routes the resulting runoff through the catchment. The model also estimates the initial base flow in the watercourse together with the increase in base flow caused by the infiltration of the rainfall into the ground causing groundwater levels to rise. The flow in the watercourse is the total of the runoff and the base flow.
The choice of approach to a particular study will usually be influenced by the aims of the study and the available information.
When carrying out assessments of the likely peak flows in rivers and streams, it should always be born in mind that, to quote the EA, 'flood estimation is an uncertain business'. To help to minimise the uncertainty we are aware that there is a need to think about the choices we make and not simply put data into the software and accept the results at face value.
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