Abstraction

This unit models a time dependent abstraction from an open channel system. Abstractions are also used to enable Flood Modeller simulations to model channel bifurcations.

Data

Theory and Guidance

This unit models a time dependent or rules-based abstraction from an open channel system. Abstractions are also used to enable Flood Modeller simulations to model channel bifurcations. Abstractions can be set by giving a time history of values or by using 'logical rules'.

This unit is used to represent abstractions from an open channel system. Abstraction amounts can be either time-dependent or operated using a LOGICAL switch type, where the Rules sub-block that sets the abstractions is inserted within the unit.

This unit can be used as both a boundary condition, where the unit functions in the same way as a Flow Time Boundary unit, or as a two-noded mid-reach unit. If you want to use the abstraction unit as a boundary condition only one hydraulic label should be defined; otherwise two labels should be defined in the same way as, for example, a WEIR unit.

For the LOGICAL switch type, the unit has two modes of operation, controlled from the time switch data: AUTOMATIC, where the amount to be abstracted is set from the control rules; and MANUAL where the amount abstracted is fixed and derived from the abstraction amount defined in the appropriate part of the time switch data. For time switch operation, the abstraction quantities are set directly within the data set, and intermediate abstractions are interpolated according to model time.

If the unit is operated with LOGICAL switching, and the associated output is undefined during AUTOMATIC operation, the abstraction flow will be set to the most recent abstraction amount. This will occur, for example, when no operating rules currently apply for a driving Rules unit. When this occurs a warning message will be displayed.

Abstraction amounts should be chosen to follow the sign convention of flows in Flood Modeller models. For example if one hydraulic label is defined then flows out of the system will be negative, whereas if two labels are present then positive flow means flow from label 1 to label 2.

It should be noted that the flow at this unit will always be set equal to the abstraction flow at all times during the run. This may mean, for example, that unrealistic flows occur elsewhere in the network. It is up to you to ensure that the set abstractions (or rules and conditions governing the output of the connected control unit) guarantees realistic flow conditions throughout the system.

Equations

The equation used is simply:

The abstraction flow is either set directly from the data (time switching or MANUAL operation for LOGICAL switching) or is equal to the output of the control rules (AUTO operation for LOGICAL switching).

General

If the user needs to simulate complicated control rules to drive the abstraction amount for this unit, the connected control unit should be a Rules unit.

Datafile Format

Line 1 - keyword `ABSTRACTION' [comment]

Line 2 - Label1, [Label2]

Line 4 - swtype, [CLabel]

Line 5 - ndat, tlag, tm, rptflg

If swtype=`TIME':

Line 6 to Line 5+ndat - time, abstra

If swtype=`LOGICAL':

Line 6 to Line 5+ndat - time, opmode, [abstra]

Note If LOGICAL switching is used, the RULES data sub-block should appear immediately after this unit in the datafile.

where:

Label1 = Label of upstream node of abstraction unit

Label2 = Label of downstream node of abstraction unit (only required if unit is two-noded)

Clabel = Label of connected control unit (up to eight characters) - only required for AUTOMATIC operation with CONTROL switching - not used in Flood Modeller Routing

swtype = Switch type - either `TIME' for abstraction amount controlled according to model time, or `LOGICAL' for abstractions set from included logical RULES sub-block. A third option, 'CONTROL', is not used in Flood Modeller Routing.

ndat = Number of time/operating mode/abstraction data sets

t_lag = Optional time-datum adjustment. This value can be used to reduce all time values by the given value. For example, if the Time-Datum adjustment is set to 2.2 and the units of time are hours then all time values will be reduced by 2.2 hours (the hydrograph is moved 2.2 hours earlier). Default value is 0.

t_m = optional keyword or value for units of time in the following data set. Can be any numerical multiplier or one of the following: seconds (the default), minutes, hours, days, weeks, fortnight, lunar (month), months (of 30 days), quarter, years or decades.

time = Model time (s)

opmode = Operating mode (keyword `AUTO' or `MANUAL')

abstra = Abstraction amount (m³/s) (only required for LOGICAL switching if corresponding operating mode is `MANUAL')