Lateral Inflow

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

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

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:

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:

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.

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 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.

Errors

The following errors may be generated:

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