A range of boundary options, including industry standard and regionally specific methods, provide the flexible to accurately represent water entering and leaving your models

Within each boundary category, Flood Modeller offers an extensive range of options enabling you to tailor your model to your precise requirements and available data. Rainfall data can be converted directly to inflow (optionally with loss through evaporation and/or infiltration), or flow data (e.g. a discharge hydrograph, having performed your own hydrological analysis), or hydrological boundaries providing full rainfall-runoff methodologies. 

Flood modellers can account for how water enters and leaves your model using flow or water level boundaries:

  • Flow boundaries define the variation of flow with time – most commonly utilised to provide inflows, e.g. at the upstream ends of a model

  • Water level boundaries define the variation of water level with time – most commonly utilised to describe how water leaves the model network

The range of boundary options to represent flows entering and leaving your models include:

Rainfall/Evaporation Boundary

The Rainfall/Evaporation Boundary provides a rainfall and/or evaporation boundary inflow into a model network. Rainfall data, evaporation data and infiltration data can all be specified separately and included as part of the inflow. 

Flow Time Boundary

The Flow Time Boundary models a discharge hydrograph (specifying flow versus time) as a boundary condition. This boundary unit is ideal for entering flow data after having used your own analysis methods. 

Abstraction Unit

The Abstraction Boundary unit allows you to specify flow entering your system based on specified logical rules, for example when a weir is open or when the water level has reached a specified height. It represents a discharge hydrograph without the time aspect needing to be specified.  

FEH Boundary

The FEH Boundary is a rainfall-runoff model based on procedures described in the Flood Estimation Handbook (1999).

FRQSIM Boundary

The FRQSIM Boundary derives an inflow hydrograph using FRQSIM (an abbreviation of 'flood FReQuency SIMulation'), a bespoke, fully distributed rainfall-runoff method. 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 FSSR16 Boundary is a rainfall-runoff model based on procedures set out in the Flood Studies Report (1975) and includes revisions contained in subsequent supplementary reports.

ReFH Boundary

The ReFH Boundary is a rainfall-runoff model using procedures developed by CEH to update the FSR/FEH Rainfall Runoff Method. This unit also provides ReFH Urban; an enhancement in order to better estimate design flows in heavily or very heavily urbanised catchments. This alternative method, which is based on the study published by Kjeldsen (2009, revised 2013), can be applied when there is a difference between the boundaries of the topographic and sewer/drainage catchments.

ReFH2 Boundary

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 in 2015. 

Generic Rainfall-Runoff (including Green-Ampt & SCS)

The Generic Rainfall-Runoff Boundary provides a range of globally applicable hydrological model components. Loss through infiltration can be applied using the Green-Ampt, or SCS (US Soil Conservation Service, now the Natural Resources Conservation Service) curve number methods. Rainfall-runoff models can be selected from the SCS unit hydrograph or Clark Unit hydrograph, and multiple options are provided for ascertaining time of concentration/lag.

The range of boundary options to describe the variation of water levels in your model (commonly used to describe how water leaves the modelled area) include:

Flow Head Boundary

The Flow-Head Boundary represents a flow against stage rating relationship. This boundary condition is also often referred to as a rating curve.

Normal/Critical Depth Boundary

The Normal/Critical Depth Boundary is a downstream boundary which automatically generates a flow-head relationship based on section data. The options available are to apply normal depth (from Manning's equation), or critical depth. This is a simpler alternative to the Flow Head Boundary in which the rating table is explicitly supplied.

Head Time Boundary

The Head Time Boundary allows the input of a stage hydrograph (specifying water level versus time) as a boundary condition. 

Tidal Harmonics Boundary

This boundary generates water level data utilising numerical methods based upon Admiralty Tide Tables and the constituent harmonics of the tide for a given location and date. Flood Modeller also provides tools that allow you to specify a varying mean sea level rate rise, to enable long term simulations enabling you to assess sea level rise scenarios in coastal and tidally influenced areas. 

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