1D River Boundaries

A boundary is required at the upstream end of a river network, to provide the inflow entering the system. A boundary is also required at the downstream end of a river network, allowing water to leave the system.

The routing equations are known to be “well-posed” if flow is provided upstream, and stage (or head) is provided downstream.

Flow-Time Boundaries

In a Flow-Time boundary, a user can manually provide the inflow hydrograph.

Flood Modeller provides functionality to Add a Flow-Time boundary to your river network. When Working with Flow-Time boundaries, alongside providing the flow data, it is necessary to ensure the label matches the connecting River Section node within the network.

Hydrological boundaries

As an alternative option for your upstream boundaries, Flood Modeller provides a selection of hydrological boundary nodes, each covering specific hydrological methodologies. These are added to a river network in the same way as Adding a Flow-Time boundary. For each hydrological boundary, parameters within the properties will need to be specified relevant to the methods employed.

The FEH rainfall-runoff boundary (FEHBDY) derives an inflow hydrograph from a catchment or sub-catchment. The FEH boundary is a rainfall-runoff model based on procedures described in the Flood Estimation Handbook (1999).

The Revitalised Flood Hydrograph a.k.a. ReFH rainfall-runoff boundary is a rainfall-runoff model using procedures developed by UKCEH to update the FSR/FEH Rainfall Runoff Method, in response to concerns that the FSR/FEH design model tended to overestimate design floods. ReFH improves the way that design events are modelled and has a number of advantages over the FSR/FEH unit hydrograph and losses model.

The ReFH2 rainfall-runoff boundary 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 Generic Event rainfall-runoff boundary provides a range of different hydrological model components to be integrated within the same boundary unit. The rainfall models implemented to date focus on the globally-applicable NRCS (Natural Resources Conservation Service), previously SCS (Soil Conservation Service) model, and an observed rainfall model. The loss models implemented include the SCS curve number and Green-Ampt models. A transformation model can be selected from SCS unit hydrograph or Clark unit hydrograph, and multiple methods are available for ascertaining time of concentration/lag. Simple constant and recession baseflow models can be defined.

The Flood Studies Update (FSU) methodology is research commissioned, managed and funded by the Office of Public Works (OPW) and is a substantial update of the Flood Studies Report (NERC, 1975), that provides methodologies for flood estimation in Ireland and the United Kingdom. Data obtained from the FSU Portal can be used directly within Flood Modeller.

Additional hydrology features

An upstream boundary is always required for a 1D river network, however Flood Modeller also provides additional options for making hydrology selections at simulation runtime.

The inflow parameters for a simulation can be adjusted - select an alternative return period in hydrological boundaries, scale Flow-Time series data, and more! Alternatively, inflow data can be provided via a specifically formatted .csv file, and this can also store multiple flow profiles for selection within your simulation.

In both cases, no adjustments are needed to the underlying network - the boundary nodes will Allow hydrology parameter overrides by default. Further information on utilising these methods within your simulations are provided in the River Simulations section.

Further tools which may be of interest if using hydrological boundaries in your model include the Optimal Storm Duration tool.

Downstream Boundaries

A river network also requires a downstream boundary to allow water to leave the system. These are added in the same way as upstream boundaries.

Using a Flow-Head boundary, a user can manually provide the relationship between the flow and stage at this downstream point.

By contrast, in a Head-Time boundary, the relationship provided is between the stage and time.

A Tidal Harmonics boundary is equivalent to a Head-Time boundary, using tidal data.

The Normal Depth boundary is suitable for use if the river conditions after the downstream boundary continue roughly as is (i.e. fairly uniform downstream). A user can select whether to provide normal depth or critical depth data in this boundary.

Alternatives

The alternative option for providing upstream and downstream boundaries is by linking your 1D river network to a 1D urban network or to 2D components. Details of this can be found in the Integrated Modelling section of this manual.