 08 Aug 2022
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Lateral Inflow
 Updated on 08 Aug 2022
 6 Minutes to read

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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).
Data
Field in Data Entry Form  Description  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 userdefined weight factors (wqhtstr = 'USER')  wqhtstr 
n/a  Number of units receiving lateral inflow  nunit 
Node Labels  Labels for receiving river sections and reservoirs (must be lateral labels of these)  Label(i) 
Custom Weight Factor/Effective Area  Corresponding weight factors or userdefined area for each receiving unit  only present if userdefined or if the second column is ticked. Otherwise, (relative) reach lengths and surface areas will be calculated at runtime  wfact(i) 
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').  overflag(i) 
Theory and Guidance
The Lateral Inflow unit acts as a distributor to apportion the inflow from a QTtype 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 userdefined 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 areadistributed 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
where w is the weight factor dx is the reach length 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:

where wis 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 QTtype boundary unit:


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

where is 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)  IF1   
Lateral Inflow  IF1  LAT1, LAT2, LAT3 
RIVER  S1  LAT1 
RIVER  S2  LAT2 
RIVER  S3  LAT3 
RIVER  S4   
General
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 userdefined 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.
Errors
The following errors may be generated:
Code  Message  Comments 

E1620  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 
E1621  Error reading Lateral Inflow data  Generic data file read error 
E1622  User defined weight factors do not sum to 1.0  Occurs at read time. User must change one or more weight factors 
E1623  Boundary inflow to Lateral unit not recognised or not entered  Must be a QTtype boundary unit or rainfall/evaporation input unit 
E1624  Lateral Inflow receiving nodes not recognised or not entered  Distribution nodes must be river, reservoir or Muskingum lateral labels 
E1625  Reach length factors cannot be used in conjunction with receiving Reservoir units  Lateral inflows which are connected to Reservoir units must have userdefined or surface area weight factors 
E1626  River or Reservoir unit already has maximum number of lateral inflows  No more than 4 lateral inflows per River or Reservoir unit 
E1627  Lateral Inflow applied to River or Muskingum Section of zero length  Impossible to calculate inflow per unit length 
E1628  Lateral inflow weight factor type not recognised  Must be 'USER', 'REACH' or 'AREA' 
E1629  Node connecting to receiving unit not recognised as lateral inflow  Lateral inflow distribution nodes must be present in lateral inflow unit 
E1630  This Muskingum type will not receive lateral inflows  Must be VPMC or VPMC section Muskingum type 
E1631  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 weightfactor "REACH", "AREA" or "USER"
Line 4  Integer nunit
Line 5 to 4+nunit  Label(i), [wfact(i)],[overflag(i)]