Boundary Data

The unit sets the concentrations of transported variables at an inflowing model boundary.

Description

Water flowing into the model will carry dissolved and suspended material within it. It is necessary to define the concentrations of the material at the inflowing boundaries in order for the model to operate. The boundary conditions are defined using the module, BOUNDARY. A separate module is required for each inflowing boundary node.

Boundary conditions need only to be set at boundary nodes where water enters the model. The concentration of variables at outflowing boundaries is determined from the upstream conditions. Therefore the downstream end of a river model may not need boundary conditions, so long as the flow does not reverse. However, a tidal boundary where water enters the model during a flood tide would need a boundary condition set. 1D Water Quality Solver will fail during its simulation if it encounters a boundary with inflowing water, for which no concentration boundary condition has been set.

At a tidal boundary, the concentration of the outflowing water is used to determine the concentration at the boundary during the ebb tide. The specified boundary condition only comes into effect during the flood tide. At the turn of the tide it is possible that artificial transients will be induced in the model by sharp changes in the concentration at the boundary. In order to smooth out this effect a ramp function is used. The boundary concentrations are linearly interpolated between those at the end of the ebb tide and those specified at the boundary. The length of the ramp function may be varied using the variable 'ntramp' which is set in the general data section of the Water Quality datafile. 'ntramp' is set as the number of time steps over which the smoothing should take place. In some cases, the ramp function may need to be applied for lengthy periods, say more than 1 hour. In such cases it may be advisable to extend the model seaward so that the return of dissolved and suspended substances on the early part of the flood tide is simulated.

The boundary conditions are set as concentrations of the dissolved or suspended variable, except in the case of dissolved oxygen where the user may elect to specify the percentage saturation, instead of the concentration. The concentration may be allowed to vary with time (use CTBDY) or with discharge (use CQBDY). Time varying boundary conditions may be set using dates, seconds, minutes, hours, days, weeks, lunar months, months, years, decades or a user defined time unit. Discharge varying boundary conditions are set using units of m/s.

Boundary conditions must be set for all the transported variables (listed at the end of each water quality process module) that are modelled by the 1D Water Quality simulation. The following variable names, with appropriate units, may be used at boundary nodes:

• Coliforms (user defined units)

• Decaying pollutant (user defined units)

• Conservative pollutant (user defined units)

• Salt (ppt)

• Temperature (C)

• Suspended sediment (mg/l)

• Dissolved oxygen (mg/l) or Percentage saturation DO (% sat)

• Fast BOD (mg/l)

• Slow BOD (mg/l)

• Fast nitrogen (mg/l)

• Slow nitrogen (mg/l)

• Ammoniacal nitrogen (mg/l)

• Nitrite-n (mg/l)

• Nitrate-n (mg/l)

• Suspended particulate fast BOD (mg/l)

• Suspended particulate slow BOD (mg/l)

• Phytoplankton (mg/l)

• Detrital carbon (mg/l)

• Phosphate (mg/l)

• Suspended benthic algae (mg/l)

• Detrital nitrogen (mg/l)

• Detrital phosphorus (mg/l)

• Adsorbed phosphorus (mg/l)

• Silicate (mg/l)

• Detrital silicon (mg/l)

In some cases it is possible to combine the different variable names together. The following combined variable names may be used:

Datafile format

Line 1                     BOUNDARY
Line 2                     label
Line 3                     ntbdy
Line 4                     keyword 'CTBDY' or 'CQBDY' If line 4 is the keyword 'CTBDY', the following line is read:
Line 4a                     Format. If line 4a is the keyword 'MULTIPLIER', the following line is read:
Line 4b                     fmult.
Line 5                     Sorpol (a). If line 5 is a combined variable name (see above) then the following line is read:
Line 5a                     ratio, ratio1
Line 6                     ndat
If line 4a is the keyword 'DATE', the following line is read:
Line 7                     ihour, imin, iday, imonth, iyear, conc
Or if line 4a is not the keyword 'DATE', the following line is read:
Line 7                     conc, time
Or if line 4 is keyword 'CQBDY', the following line is read:
Line 7                     conc, flow
Line 7 is repeated 'ndat' times.
Lines 4 to 7 are then repeated 'ntbdy' times.
Last line                     keyword 'BOUNDARY' or 'END'

Where:

If the keyword 'BOUNDARY', the boundary conditions for a new boundary node are specified. The keyword 'END' is used to specify the end of the boundary conditions section of the 1D Water Quality datafile.

General

The BOUNDARY module may only be used once at each model boundary nodes. Boundary conditions must be specified for all the water quality variables which are being simulated by 1D Water Quality Solver. If the user specifies boundary conditions for water quality variables which are not being simulated, then 1D Water Quality Solver will ignore this redundant information.

Overview

Datafile Layout

Related Topics

Preparing a Water Quality Datafile

Running a Water Quality Simulation

Boundary/Initial Conditions