Lateral Inflow
  • 08 Aug 2022
  • 6 Minutes to read
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Lateral Inflow

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The Lateral Inflow unit acts as a distributor to apportion the inflow across multiple river reaches or reservoirs as opposed to applying a point inflow from a boundary to a single node. The total inflow from such a boundary can be divided according to reach length, surface area or user specified proportions (that must total 1.0).


Field in Data Entry Form


Name in Datafile

Node Label

label of connecting boundary unit

Label 1

Distribution Method

Lateral inflow distributed by reach lengths (wqhtstr='REACH'), surface areas (wqhtstr='AREA'), or user-defined weight factors (wqhtstr = 'USER')



Number of units receiving lateral inflow


Node Labels

Labels for receiving river sections and reservoirs (must be lateral labels of these)


Custom Weight Factor/Effective Area

Corresponding weight factors or user-defined area for each receiving unit - only present if user-defined or if the second column is ticked. Otherwise, (relative) reach lengths and surface areas will be calculated at runtime


Use Weight Factor /Effective Area

Flag signifying that calculated reach weight factor or area is to be overridden by the user defined value entered in the data file (wfact) (overflag='OVERRIDE').


Theory and Guidance

The Lateral Inflow unit acts as a distributor to apportion the inflow from a QT-type boundary (i.e. including hydrological boundaries) or rainfall/evaporation boundary (REBDY) across multiple river reaches or reservoirs as opposed to applying a point inflow from a boundary to a single node. The total inflow from such a boundary can be divided according to reach length, surface area or user specified proportions (that must total 1.0).

The lateral inflow unit always operates in conjunction with a single boundary unit. Therefore, the lateral inflow will share the same label as the connecting inflow boundary. This is the single node label that appears on the Network Properties chart, and also at the top of the property form or when selecting 'Edit Node Labels'.

The unit will also hold the labels of the 'dummy' distribution nodes that provide the connectivity to the river sections and reservoirs that it passes flow into - these can be inserted/viewed in the grid of the Lateral Inflow property form. The receiving river sections or reservoirs will also hold the labels of the lateral inflow dummy nodes within the data file. Each river or reservoir unit can hold up to four distribution nodes (i.e. from four different lateral inflow units) - these are named Lateral [Inflow] Labels within the interface.

It is advisable to add the distribution nodes to the receiving unit before doing so to the Lateral Inflow unit - this will then add the node labels to the pull down list which can subsequently be accessed from the Lateral Inflow form.

There will be three methods available for the lateral inflow unit to divide up a boundary inflow; reach length, surface area or user-defined proportions (or weight factors). The latter would apply to distributed lateral inflows and the sum of the weight factors would be 1.0 otherwise a warning message will be displayed. For direct rainfall/evaporation inputs, the weight factors are not required to sum to 1.0. They represent the surface area over which the input is applied and the connected REBDY unit would actually have units of rainfall/evaporation intensity per unit area. Therefore the equivalent inflow is calculated from the product of these two values.

It is possible to integrate user defined weight factors with either area or reach length distributed factors. User defined reach factors must be between zero and one and the combination of all user entered factors for a lateral inflow must sum to less than or equal to one. The value of one minus this total is then proportioned by reach length to calculate the remaining undefined factors.

For area-distributed inflows, an effective, constant surface area can be entered to override the calculation of actual surface area at runtime.

For river units, the mass balance component of the St Venant equations is



where the lateral inflow is given by




w is the weight factor

dx is the reach length

Lateral3 is the average flow at the lateral inflow node over a time step, i.e. the flow from the connected boundary inflow

The structure of this equation shows that a lateral inflow cannot be applied to a river section with zero reach length. Attempting to do this will lead to an error message being displayed.
Lateral inflow units require have a 'dummy' equation for stage to be defined. It should be noted that this will not influence results within the main model network in any way and will only act to ensure consistency within the Flood Modeller model. Results at each dummy node can be viewed as normal, although flow is the only meaningful variable produced.
The influence of the lateral inflow on the momentum equation would be neglected.
For a reservoir unit the mass equation will incorporate lateral inflows as follows:




wLateral3(1)is incorporated into the net flow into the reservoir

Each connecting node of the lateral inflow unit will have the following mass equation defined if the lateral inflow is connected to a QT-type boundary unit:



If the lateral inflow is connected to a rainfall/evaporation (REBDY) unit the equation becomes:




 Lateral8is obtained from the REBDY unit (note units have to be converted from the default mm/hr)

This equation directly links to the mass equation defined for the REBDY unit:



It should be noted that the lateral inflow connectivity is a significant departure from normal Flood Modeller connectivity rules. To illustrate this, the following is a typical partial schematisation showing how Flood Modeller incorporates lateral inflows with the appropriate node labelling. N.B. Normal Flood Modeller connectivity rules apply to continue to/from nodes S1 and S4.

Unit type

Unit Node label

Lateral Inflow (Distribution) node label(s)

Boundary (e.g. QTBDY)



Lateral Inflow

















If the lateral inflow is to be proportioned by reach length then the individual weighting factors will be constant throughout the run and hence the weight factors are calculated during the read phase. This is achieved by dividing the individual reach lengths (dx) by the total reach length of all connected river sections. Similarly if the lateral inflow is proportioned by user-defined weight factors then these will be read from the data file and stored during the read phase. However, if the lateral inflow is proportioned by surface area then the weighting factors will constantly be changing and, thus, will be recalculated at each time step based on the surface areas from the previous time step. As mentioned previously, this process can be overridden by a constant, effective area, which is entered in the weight factor location of the data file.


The following errors may be generated:





Version number is not supported in this version of Flood Modeller

The unit was generated using a more recent version of Flood Modeller. Contact your supplier for an upgrade


Error reading Lateral Inflow data

Generic data file read error


User defined weight factors do not sum to 1.0

Occurs at read time. User must change one or more weight factors


Boundary inflow to Lateral unit not recognised or not entered

Must be a QT-type boundary unit or rainfall/evaporation input unit


Lateral Inflow receiving nodes not recognised or not entered

Distribution nodes must be river, reservoir or Muskingum lateral labels


Reach length factors cannot be used in conjunction with receiving Reservoir units

Lateral inflows which are connected to Reservoir units must have user-defined or surface area weight factors


River or Reservoir unit already has maximum number of lateral inflows

No more than 4 lateral inflows per River or Reservoir unit


Lateral Inflow applied to River or Muskingum Section of zero length

Impossible to calculate inflow per unit length


Lateral inflow weight factor type not recognised

Must be 'USER', 'REACH' or 'AREA'


Node connecting to receiving unit not recognised as lateral inflow

Lateral inflow distribution nodes must be present in lateral inflow unit


This Muskingum type will not receive lateral inflows

Must be VPMC or VPMC section Muskingum type


Lateral inflow cannot be applied at end of reach

Reach length must be greater than zero

Datafile Format

Line 1 - keyword "LATERAL", version number "#REVISION#1", [comment]

Line 2 - Label 1

Line 3 - Keyword for weight-factor "REACH", "AREA" or "USER"

Line 4 - Integer nunit

Line 5 to 4+nunit - Label(i), [wfact(i)],[overflag(i)]

Data File example


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