• 15 Aug 2022
    • 10 Minutes to read
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    FRQSIM Method

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    Article summary


    The unit derives an inflow hydrograph from a catchment or subcatchment using FRQSIM (an abbreviation of 'flood FReQuency SIMulation'), a bespoke, fully distributed rainfall-runoff method. In Flood Modeller the generated hydrograph becomes a boundary condition equivalent to a QTBDY.

    This unit will either generate flow hydrographs for design return period events, derived from the UK Flood Estimation Handbook (FEH) or FRQSIM specific storm profiles, or will simulate runoff during historic events using recorded rainfall and other input data.

    The Import button can be used to populate most of the data fields with values transferred from the FEH CD - export the data from the FEH CD using the 'Export as CSV' feature.


    FRQSIM (an abbreviation of 'flood FReQuency SIMulation') is a bespoke, fully distributed rainfall runoff model that was originally developed by the Greater London Council (with later developments by the National Rivers Authority and the Environment Agency). It was initially developed specifically to provide design flows for flood alleviation schemes in the highly urbanised catchments of the Thames tributaries in LondonHowever, use of the model is not restricted to these areas.

    The catchment to be modelled is separated into 'node areas', which are equivalent to sub-catchments used in FEH methods. These are based not only on topographic information, but also on drainage networks including surface water drains, where this information is available. Each node area is then divided further into 'sub-areas' defined as either paved or open, depending upon the amount of open space within each sub area.

    The model differs from the FEH rainfall method as it uses a time area method to produce synthetic unit hydrographs (SUH). A separate SUH is produced for paved and open areas. A third SUH is produced, representing gardens and verges within urban areas. Time of travel estimates are based on estimations of the time it takes for water to travel from the nearest and furthest points of each sub-area to the node area outlet.

    Other notable differences from the FEH rainfall runoff method include the shape of the design storm profiles used. 10 storm profiles are available, based on 250 flood producing storms observed from over 30 years of rainfall records from across the London area. FRQSIM also includes a loss model which determines the effective rainfall for open areas. One other useful feature is the recognition of the finite capacity in the surface water drainage network. FRQSIM assumes that capacity of the network is the 5 year storm and that any rainfall above this will be stored in the model and released over subsequent time steps until all of the runoff has gone through the network.

    FRQSIM Operation

    A flow hydrograph can be generated from FRQSIM using a combination of inputs. These are catchment parameters taken from the FEH CD Rom, real or synthetic storm profile data, i.e. rainfall and some additional FRQSIM specific parameters. The sequence of FRQSIM modules that must be run to produce the hydrograph data is shown in Figure 1. The various input and output files involved with this sequence are also shown. The final hydrograph data is taken from the EO4 file produced by the EVENT program.


    Figure 1 Procedures required to generate hydrograph from FRQSIM software

    Each box in the above figure represents a separate sub-process within FRQSIM. These processes are defined as follows:

    • FEH CD ROM - not part of the FRQSIM suite of tools. Used to obtain catchment specific parameters.
    • FEHFRQ - converts FEH parameter data into FRQSIM compatible inputs.
    • UNIHYD - generates unit hydrographs for 3 different ground surface types; paved, open (natural ground) and garden.
    • EVENT - combines unit hydrograph data with specific storm profile to generate Flood Modeller hydrograph input.

    The top level of this FRQSIM folder contains all tools and associated data files. Subfolders within the FRQSIM folder will be used to hold all model specific files. Whenever FRQSIM is called from the Flood Modeller interface, Flood Modeller will write the current model specification into a subfolder created by the interface and named after the currently selected FRQSIM Flood Modeller node. If the folder already exists then existing model details will be overwritten. Some files within this subfolder will remain unchanged for different models. These will be automatically copied into the subfolder when it is created. This section of the document describes those files that will require editing in order to specify current model details.

    The user is not required to monitor these separate processes when using

    Data Input Form Format

    Catchment Details


    • Comment - Optional comment of up to 50 characters
    • Catchment area - In square kilometres.
    • Urban extent - Extent of urban/suburban land cover (range 0 - 1.0 although high values fall outside the applicability of the FEH methodology).
    • Easting - Easting coordinate of catchment outflow (not used in FEH calculations).
    • Northing - Northing coordinate of catchment outflow (not used in FEH calculations).
    • SAAR - Standard annual average rainfall (mm).
    • Reference Time, T0 - The assumed reference time (minutes) of outfall of the downstream point of the catchment. Default value is 10 minutes.
    • Open Area Fraction, gfac - Fraction of open area that contributes within the first 1/3 of the time of concentration of the catchment. Default value is 0.35.
    • Pfac, % Paved Factor - Percentage of urban catchment area that is paved
    • Power, % Paved Factor - Urbanised area = pfac * exp(urban extent * Power)
    • Channel Velocity Factor - Adjustment multiplying factor for velocity along channel to correct error in time to peak based on calibration. Default value is 1.0.

    Import button enables user to browse to an FEH catchment descriptor file (.csv). Flood Modeller will then read the required parameters from this file and populate the appropriate fields in the form.

    Event Data


    • Runoff - Percent runoff can either be entered directly (use Observed(PR)) or calculated from the Standard Percent Runoff (use FEH(SPR)) - in both cases enter the value as a percent (e.g. 40). The percent runoff can be allowed to vary during an event using the method suggested in the Flood Studies Report (set Percent Runoff to Variable).
    • Average rain intensity over 8 hours, .this is used for working out depth of flow for computing the velocity of flow in the open area during an average flood producing event. Default is 8.0mm/hr.
    • SPRHOST adjustment factor - Adjustment factor for standard percent runoff values, cspr. Default is 1.0.
    • Storm duration - Storm duration in hours. Must be greater than zero. For design events, an initial estimate of the critical storm duration, D, can be determined from the equation D=(1+SAAR/1000)Tp. The storm duration divided by the data interval should be an odd integer.
    • Storm area - Storm area in square kilometres. A zero signifies that the storm area is equal to the catchment area.
    • Data interval - Data and internal calculation time interval (hours). Suggested value is Tp/5 rounded to convenient number of whole or part hours such that the storm duration is an odd integer multiple of the data interval
    • Snow melt rate - Snow melt rate in mm/day. This is converted to the appropriate units and simply added to the runoff hydrograph.
    • Catchment wetness index - The catchment wetness index can either be calculated (select FEH) or can be directly entered (select Observed and enter the value in units of mm).

    The 'Load EVENT / UNIHYD file' button creates the required input data files for the FRQSIM EVENT and UNIHYD programs. Note, this will occur automatically if the user clicks on the 'Calculated Hydrograph' tab.

    Event Rainfall Data


    • Event rainfall - The event rainfall depth can either be calculated (select FEH) or directly input by the user (select Observed and enter a rainfall depth in mm).
    • ARF - The Areal Reduction Factor can be entered directly (0 < ARF < 1) or if set to zero then it is calculated.
    • Rainfall return period - Rainfall return period (in years). If zero then it will be calculated from the flood return period.
    • Flood return period - Flood return period (in years).
    • DDF Model Parameters - c, d1, d2, d3, e and f are the parameters of the FEH rainfall Depth-Duration-Frequency model. The parameters are obtained from the FEH CD.
    • Storm Profile - The storm (rainfall) profile can either be set to the predefined 50% Summer or 75% Winter profiles, a standard FRQSIM profile or a profile can be entered directly (select Observed and enter amounts of rainfall at each Data Interval for the full Storm Duration - the sum of these rainfall amounts must be equal to the 'Observed' rainfall depth (if used)). See FEH Vol 4 for criteria for selection. A suite of standard FRQSIM profiles are available in the drop down list for the user to select from.
    • Use equal return periods - Deselect this option if you want to set the rainfall and flood return periods to be different. The FEH advises that when using Summer design events (usually applies to more urban catchments) the return periods are equal.
    • The 'Calculate Depth' button is enabled when the FEH option is selected. This calculates a rainfall depth from FEH parameters. When suitable FEH parameters have been entered, clicking the button calculates a rainfall depth and writes the value to the rainfall depth textbox on the tab.

    Unit Hydrograph and Baseflow


    • Unit hydrograph data - The shape of the unit hydrograph (UH) can be either the FRQSIM calculated UH (select FRQSIM and click 'Calculate UH') or a user input shape (select Observed and enter ordinates of UH (in user-selectable units) at the Data Interval). Note that the default units of the UH are non-standard.
    • Area Type - FRQSIM requires 3 unit hydrographs for urban, open and garden surface types. This drop down switches the tabular display between the data from each area type.
    • Travel time specification - Either an observed value or calculated using FRQSIM method, which utilises the parameters DPLBAR and DPSBAR as used in the FEH and Longest Drainage Path (LDP). To directly input a value select Observed and enter value in units of hours.
    • DPLBAR - Mean drainage path length (km) from FEH CD.
    • DPSBAR - Mean drainage path slope (m/km) from FEH CD.
    • Longest Drainage Path - Parameter read from FEH CD Rom, but editable by user. Units = km.
    • Baseflow - Baseflow can either be calculated (select FEH) or user input (select Observed and enter the baseflow in m3/s).



    • Simulation type - Select full hydrograph, peak flow or baseflow only. Normal choice is full hydrograph. Baseflow only will remove the quick response runoff and impose only the baseflow. Similarly, peak flow will impose the peak of the hydrograph as a constant flow throughout the simulation.
    • Minimum flow - Forces the resultant hydrograph to be not less than the defined minimum flow (entered in units of m3/s). The minimum value may be required for program stability.
    • Time delay - Optional delay time (hours), e.g. if set to 2 hours then the hydrograph (actually the rainfall event) will start 2hrs after the start time of the simulation (this option does not affect the hydrograph displayed on the Calculated Hydrograph tab).
    • Hydrograph scaling method - The hydrograph calculated by this unit can be scaled either by a constant factor (eg 1.0 for no adjustment) or to achieve a defined peak flow (m3/s). This option is particularly useful when a hydrograph shape is required to be provided but the FEH statistical methods are being used to define the peak flow for a certain return period.

    Calculated Hydrograph

    The Calculated Hydrograph Tab provides details of the Flood Modeller FRQSIM rainfall-runoff method calculation and results. Appropriate data must be entered before clicking on this Tab and where data are not sufficient for the calculation then error messages will be generated; information on the error will be contained in the diagnostic file created.

    Clicking on this tab runs the complete sequence of FRQSIM tools to produce a hydrograph for the specified catchment and rainfall properties. This sequence must be run to generate the required flow/time data for the FRQSIM QTBDY node within the model before running a model simulation. If the flow/time data for the node are not generated then a model simulation can still be run, however the FRQSIM node will make no contribution to inflow into the model (it will appear as a null QTBDY in the model dat file).

    The areal rainfall, net (effective) rainfall, unit hydrograph and full hydrograph are tabulated and can be plotted (click on the Plot button). Text file data and results summaries can be viewed by clicking on the Data and Hydrograph buttons.


    FRQSIM Hydrological Model- Technical Manual, Flood Modelling Group, Environment Agency (Thames Region)

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