ReFH2 Method (ReFH2 Rainfall Runoff Method)

Required Data Entry

On opening a ReFH2 boundary unit within the Flood Modeller interface, you are presented with four tabs: Catchment, Rainfall, Options and Results. The first two tabs specify the input data for ReFH2; the Options tab allows for some model adjustment and the Results tab displays the values for the generated hydrograph and its intermediate constituent components (you can only view the Results tab if you have a licensed installation of the ReFH2 software).

Catchment tab

The Catchment tab required data entry detailed as follows:

Rainfall tab

The Rainfall tab requires data entry detailed as follows:

Note: The following variables are only relevant if ReFHVer ≥ 2.3

The rainfall data table is only editable if the tickbox options to use data from an xml descriptor file (on the rainfall and options tabs) are both unticked. In addition, tabular rainfall data can only be used with the FEH 2013 design rainfall option. The table allows for individual cells to be edited. The right-click menu on the table also provides options to cut, copy or paste individual or blocks of cells. Pasting can be to other parts of the table or to 3rd party software, e.g. MS Excel, for further editing (you can copy data back form Excel into the table as well).

V2.3 options tab

The data settings on the v2.3 options tab are only applied if the ReFH2 engine version on the Catchment tab is set to v2.3. Furthermore, in v2.3, all settings on this tab are optional and only need values specifying if they are required for your particular analysis. The options provided on this tab are as follows:

Note: These settings will still all be written to your network file (dat file) even if they are unticked to not be used. In this case they will have a “False” flag against them, which will instruct the ReFH2 software to ignore them.

Options tab

The Options tab required data entry detailed as follows:

The following variables have changed behaviour/constraints for Flood Modeller v5.1 (incorporating ReFH2.3):

Note: the following defaults are implied (i.e. the ReFH2 engine overrides whatever is set in the unit data, under the given circumstances)

• Plotscale is forced to be true if using point descriptors
• Rainmodel is forced to be 2013 if using point descriptors
• Rainmodel is forced to be 2013 if the ReFH2 engine version used is v2.3 (or later)
• If catchment area is specified explicitly within the unit data (CAREA) and a pre-defined descriptor file is being used, then this area is only used if the descriptor file is a point descriptor file; otherwise area is read from the catchment descriptor file

Generating Results

Prior to viewing the Results tab or running a simulation, the minimum input requirements are:

• The Catchment Descriptors (Catchment Tab)
• Storm Duration
• Data Interval
• Return Period (Rainfall Tab)

Although a recommended timestep range and storm duration are provided on the Results Tab, an estimate of these must still be provided initially.

Catchment descriptors may be imported from an xml file obtained from the FEH Web Service (note; this must be purchased), a csv file produced by the FEH CD-ROM (using the Import Catchment Descriptors button), or may be input manually. Note that only those descriptors used in the calculations are imported, in addition to the Easting and Northing grid reference, which are provided for information only (although may be used by post processing tools, e.g. TabularCSV, to extract coordinates).

The season (Rainfall Tab) has a default storm season setting of 'WINTER', but this can be changed to ‘SUMMER’. An appropriate selection can be influenced by the degree of urbanisation in the catchment (for URBEXT > 0.125 a summer storm is recommended).

By clicking on the Results tab, the user is effectively launching a boundary mode simulation of Flood Modeller of a model containing the single ReFH2 unit. This produces the output hydrograph and its constituents in tabulated form, and many of the intermediate calculated parameters. Recommended values for design storm duration and timestep are given, which can be subsequently checked against those originally entered. N.B. if subsequently adjusting the storm duration or time interval, these should be rounded so that the duration divided by the interval is an odd integer. The hydrographs and hyetographs may also be viewed graphically via the Plot button. The hydrograph (.zzh) and summary data (.zzb) output files are also produced and are able to be viewed via the appropriate button on the tab. Occasionally, the simulation may fail, e.g. due to bad input data or an inappropriate licence, and like any Flood Modeller simulation, will generate a run error and produce diagnostic output in such instances. The user is advised to check this output (N.B. The error dialogue box may appear partially hidden behind the "Please Wait..." dialogue box).

If the ReFH2 executable fails to run (E1797), there will be an accompanying message in the diagnostics (*.zzd) file of the format: “Error running ReFH2 executable; return code: n ”, where n is usually the return code specified in the ReFH2 documentation. In the first instance, the user is referred to this. However, there are other codes, not directly returned from the ReFH2 executable. If this is the case, usually these are error codes determined by the Windows system, e.g. refer to https://msdn.microsoft.com/en-us/library/windows/desktop/ms681381(v=vs.85).aspx for details. Specifically, the following two may be encountered:

• n=258 – ReFH2 executable timed out. By default, 30s is allowed for the ReFH2 executable to run and process the data. This should be much longer than ReFH2 software takes to run, and is introduced as an additional safety measure to Flood Modeller from hanging should an unexpected interruption occur in the ReFH2 software.
• n=9999 – [Non-Windows system code]. Unspecified return code. The ReFH2 executable process could not be launched for an undetermined reason.

Theory and Guidance

The ReFH2 boundary unit utilises the ReFH2 (Revitalised Flood Hydrograph 2) software to derive an inflow hydrograph for a catchment or sub-catchment. The ReFH2 method can be applied to both rural and urbanised catchments and can utilise either the FEH 1999 or the FEH 2013 rainfall models. ReFH2 is an updated version of the original ReFH method and was released by Wallingford HydroSolutions (www.hydrosolutions.co.uk) in 2015.

The hydrograph produced by the method is utilised in Flood Modeller as a boundary condition, equivalent to a Flow Time Boundary. It can therefore form inputs to a hydrodynamic or routing model. This inflow can be applied at the upstream end of a reach or may be distributed over a reach in conjunction with the Lateral Inflow unit. When run in boundary mode, the resultant hydrograph may be viewed using the Graphical or Tabular postprocessors; furthermore, summary hydrograph (.zzh) and data input (.zzb) files may be viewed prior to running a full simulation; this preview functionality is also available within the Flood Modeller interface.

Alternatively, the rainfall component of the ReFH2 Boundary can be used as a direct rainfall boundary (equivalent to an REBDY), when used in conjunction with a Lateral Inflow unit, by selecting the Hyetograph Boundary type option.

The ReFH2 Boundary is a rainfall-runoff model using procedures developed by UKCEH to update the original ReFH Method and the FSR/FEH Rainfall Runoff Method. The key changes from ReFH1 are:

• Revision of equations for estimating model parameters (T_p, C_MAX, BL, BR) from catchment descriptors.
• Improved representation of urban processes (implementation of Kjeldsen 2013 urbanisation)
• The catchment descriptor (regression) equations used to derive T_p, C_MAX, BL and BR at ungauged locations have been re-estimated and updated to include URBEXT2000 (ReFH1 used URBEXT1990).
• Special consideration to Scottish data:
  • Use of more Scottish gauged data (overlapping rain and flow data) for model calibration
  • Development of Scotland specific equations for the estimation of initial model conditions
• Improved handling of “alpha” (a) parameter:
  • Calibration of the method up to and beyond the 200-year design event (ReFH “alpha” parameter only calibrated to T=150 years)
  • Alpha performance related to SAAR and return period (in ReFH alpha is related only to return period)
  • Option to make estimates of initial model conditions (C_INI) independent of pooling-group bias, i.e. alpha not invoked (applied by default for Scotland)
• Applicable on permeable catchment, ReFH is not recommended for use if BFIHOST > 0.65
• ReFH2 can utilise FEH99 and FEH13 depth duration and frequency rainfall models
• Scaling of parameters for use on small catchments (plot scale). Plot scale is assumed to be <0.5km², i.e. below the limit of extracting catchment descriptors from the FEH Web Service.

Enhancements within the ReFH2.3 model (as per the website), and its incorporation within Flood Modeller include:

• The option to apply climate change factors to design rainfall.
• The option to specify several parameters directly within Flood Modeller, including ARF and SCF
• Closing the water balance within the ReFH rural model.
• Closing the water balance within the ReFH urban model, including the introduction of depression storage.
• Incorporation of the new BFIHOST19 descriptor from the FEH Web Service.
• Modelling of FEH13 rainfall is no longer limited to a maximum of 1:1000 year event.
• ReFH2.3 includes an option to use the legacy ReFH2.2 model, allowing you to recreate results created in older version of the software or revisit existing projects.

More information on the theory adopted within the ReFH2 method can be found on the Wallingford Hydrosolutions website (www.hydrosolutions.co.uk ), e.g. the ReFH2 Technical Guidance Document , or in the user guide that accompanies the ReFH2 software. It is strongly recommended that users of the ReFH2 Boundary read the Technical Guidance Document , produced by Wallingford Hydrosolutions. You should also be conversant with the methods described in the Flood Estimation Handbook and must be aware that design flow hydrographs produced by the ReFH2 method can still be highly inaccurate (as the model parameters are based solely on catchment descriptors).

Users should note that you cannot use observed rainfall data with the ReFH2 Method (you would need to revert to ReFH1 to incorporate observed rainfall into your boundary calculations).

When using the ReFH2 Method you should also note that separate methods have been derived specifically for Scottish catchments. These are invoked by selecting Scotland for the country setting. Currently equations are the same for all other UK countries (so selecting any other country will produce the same output).

Datafile Format

The following describes the format of a ReFH2 unit within a 1D river network file (.dat file). You can view this file in a text editor, e.g. Notepad.

Line 1 - Keyword 'ReFH2BDY' (case insensitive)

Line 2 - Label

Line 3 - Minrev, ReFHVersion

Line 4 - z, easting, northing, ngr

Line 5 - T, D, ∆t, SEASON, URBANISED, PLOTSCALE, override, DescType, CCUplift

Line 6 - Xmlflag

Line 7 - Xmlfile

Line 8 - Reportflag

Line 9 - Reportfolder

Line 10 - CAREA, COUNTRY, SAAR, URBEXT2000

Line 11 - DDFMETHOD, c, d1, d2, d3, e, f

Line 12 - DPLBAR, DPSBAR, PROPWET, BFIHOST, BFIHOST19

Line 13 - tdelay, bfonly, SCFLAG, scfact, hymode, SCALING, minflow

Line 14 - altbar, aspbar, aspvar, farl, fpext, fpdbar, fploc, ldp

Line 15 - rmed_1h, rmed_1d, rmed_2d, saar4170, sprhost, urbconc2000, urbloc2000

Line 16 - urbext1990, urbconc1990, urbloc1990

Line 17 - C_1km, d1_1km ,d2_1km ,d3_1km ,e_1km ,f_1km

The following lines are only valid for Minrev ≥ 5.1

Line 18 - usescf, usearf, usesc, useeda, useurbanarea, useirf, useif, usetpscaling, useds

Line 19 - scf, arf, sc, eda, urbanarea, irf, if, tpscaling, ds

Line 20 - mDDF2013D (referred to below as m), nDDF2013T (referred to below as n)

The following lines are only valid if m and n > 0

Line 21 - <blank>, T1, T2, …, Tn

Line 22 - D1, P1,1, P1,2, … P1,n
…
Line 21+i - Di, Pi,1, Pi,2, … Pi,n
…
Line 21+m - Dm, Pm,1, Pm,2, … Pm,n

Some of these parameters are entered as default values automatically by the Flood Modeller interface. These are defined as follows (in case you wish to manually edit values in a 1D river network (.dat) file):