13. HEGADAS

User Guide MAIN MENU

General introduction

HEGADAS is the HGSYSTEM model for heavy gas dispersion from area sources. It is one of the oldest and most complex HGSYSTEM models and has many features and options.

HEGADAS has a steady-state version, called HEGADAS-S, and also a time-dependent (transient) version, called HEGADAS-T. Parameters of both versions are discussed in this chapter. HEGADAS-S can also be used for finite-duration releases.

HEGADAS can use either the standard HGSYSTEM multi-compound, two-phase aerosol thermodynamical model or the full hydrogen fluoride (HF) chemistry and thermodynamics. Both descriptions are discussed in separate chapters of the Technical Reference Manual.

HEGADAS can be used for three scenarios:

        (1) evaporating pool scenario
        (2) post-HEGABOX run
        (3) post-AEROPLUME/HFPLUME run

In case (1), source term data generated from LPOOL (and HFFLASH if applicable) can be used. LPOOL (and HFFLASH) will generate link files with this information.
In case (2), HEGABOX will generate a link file plus an observer data file which will be used by HEGADAS-T.
Case (3) occurs when the near-field jet model (AEROPLUME or HFPLUME) makes a transition to HEGADAS at the end of the jet simulation. Relevant data is again communicated through a link file.

HEGADAS uses the concept of 'breakpoints'. Using a breakpoint, HEGADAS calculations can be interrupted or initiated at a specified downwind location. At a 'partial' breakpoint some variables may be changed (e.g. surface roughness). At a 'full' breakpoint a complete HEGADAS run is initiated, typically after running a jet model, this happens in scenario (3) above. More details on 'breakpoints' can be found in the HGSYSTEM 3.0 Technical Reference Manual section 7.A.3.4.1., 7.A.4. and 7.C.3.1.

POSTHS and POSTHT are the HGSYSTEM version 3.0 post-processors to plot HEGADAS results. They are discussed in Chapter 15.

Range of applications and limitations

HEGADAS is intended for heavy gas dispersion simulations. It should not be used for buoyant clouds or clouds that become buoyant. It should also not be used for clouds which have considerable momentum of their own.

Guidance for use

HEGADAS is a complicated model. To appreciate its full capabilities the user should read some of the technical background information in the Technical Reference Manual.

Use of defaults for many parameters is strongly advised (see discussion below).

The HEGADAS-T model is quite complex to use and interpret. For every new case the user wants to analyse, it is usually necessary to run the model several times to get a feeling for the results. Run times will be long (several minutes), especially for HF runs.
Concentration and cloud information will only be given for certain output times. The choice of the output times is important in interpreting results. Using the option to generate output times automatically is recommended.

For finite-duration releases where the travel time to a distance of concern is greater than the release duration, HEGADAS-T should be used instead of HEGADAS-S.

The HEGADAS code is quite robust: numerical problems seldom occur. If numerical problems do occur (especially for HF), slightly changing the input parameters might be helpful.

The report file use the following nomenclature:
DISTANCE m downwind distance fron source (x coordinate)
CONC - ground level centre-line concentration; % molar fraction of pollutant (all HF assumed to be monomer)
SZ m vertical dispersion coefficient Sz
SY m vertical dispersion coefficient Sy
MIDP m half-width b of the middle part of the cross-wind concentrtion profile (see Figure 7.A.1)
YCU, YCL m cross-wind distance from cloud centre-line (y coordinate) at which CONC is equal to CU or CL respectively. See CLOUD input block for CU and CL.
ZCU, ZCL m height at which CONC is equal to CU or CL respectively (m).
RIB - bulk Richardson number Ri*
TMP °C ground level centre-line temperature
CA kg/m3 ground level centreline concentration

HEGADAS INPUT PARAMETERS

A description of all the input parameters that can occur in a HEGADAS input file will be given. Some parameters are specific for either the transient or steady-state model. Unless specified, parameters apply to both versions.

The HEGADAS-S input file has the DOS filename 'casename.HSI' where 'casename' is the user-supplied name of the problem. For HEGADAS-T the file name is 'casename.HTI'.

In the following, actual keywords are given in capitals and in bold. The descriptions of less important parameters or parameters that need not normally be set by the user, are given in a smaller font.

All parameters, except TITLE, occur in blocks preceded by a specific block keyword.
For HEGADAS-S these block keywords are: CONTROL, AMBIENT, GASDATA, DISP, CLOUD, TRANSIT and POOL.
For HEGADAS-T these block keywords are: CONTROL, AMBIENT, DISP, CLOUD, GASDATA, CALC, TRANSIT, TIMEDATA, AUTOTIM, and MMESOPT.


The TITLE keyword does not occur in a parameter block.

TITLE The title of the current problem to be run with AEROPLUME.
  At most 50 alphanumeric characters.
  Optional, no default.

The CONTROL block contains the parameters which controlling the type of calculations performed in the HEGADAS run.

ISURF Indicator for surface heat effects and water vapour effects (-).
  ISURF = 2, 3 or 4.
  Optional, default is 3.
  ISURF = 2: no heat transfer or water-vapour effects. Only for isothermal problems without HF.
  ISURF = 3: heat transfer from ground to cloud is taken into account, but not water vapour effect.
  ISURF = 4: in addition to heat effect, also water vapour transfer from ground to cloud is accounted for. Use this value for dispersion over water (no HF).
  For a run involving HF, ISURF must always be 3.
ICNT Concentration data print indicator (-)
  ICNT = 0 (inactive) or 1 (active).
  Optional, default is 0.
  When ICNT = 1, concentration data is written to a file 'casename.HS1' or 'casename.HT1'. The following is written at every reporting downwind distance:
  - y-co-ordinates (ycu,ycl) where ground-level concentration equals CU or CL,
  - heights (zcu,zcl) where ground-level concentration equals CU or CL,
  - cumulative volumes (Vcu,Vcl) (m3) where concentrations exceed CU or CL for the part of the cloud up to present downwind position,
  - cumulative vapour contents (Mcu,Mcl) (kg) where concentrations exceed CU or CL for the part of the cloud up to present downwind position.
IMTYPE Type of HEGADAS run (-).
  IMTYPE = 1, 2 or 3.
  Optional, always set by HEGADAS program itself.
  IMTYPE = 1 for a HEGADAS-S run, 2 for a HEGADAS-T run and 3 for a post-HEGABOX run.
PRTOBSD Print observer data indicator (-)
  PRTOBSD = 0 (inactive) or 1 (active)
  Optional, default is 0.
  HEGADAS-T only.
  When PRTOBSD = 1, observer dispersion data is written to a file 'casename.HT2'.
ICSCOR Cloud shape correction indicator (-)
  ICSCOR = 0 (inactive) or 1 (active)
  Optional, default is 1, should normally not be changed.
  HEGADAS-T only
OBSEPS Convergence tolerance for observer concentrations (-)
  0.005 <=OBSEPS <=0.2.
  Optional, default is 0.05.
  HEGADAS-T only.
  Only change if observer setting fails. Program will clearly indicate this.

The AMBIENT block contains parameters describing the conditions of the ambient atmosphere. In HEGADAS ambient pressure is always taken to be 1.0 atm.

Z0 Reference height for ambient wind speed U0 (m).
  0.1 <=Z0 <=50.
  Mandatory.
U0 Ambient wind velocity at height Z0 (m/s).
  1.5 <=U0 <=20.
  Mandatory.
AIRTEMP Ambient air temperature at height ZAIRTEMP (°C).
  -50 <=AIRTEMP <=50.
  Mandatory.
ZAIRTEMP Height at which ambient air temperature is given (m).
  0 <=ZAIRTEMP <=50.
  Optional, default is 0.
TGROUND Ground (substrate) temperature (°C).
  -50 <=TGROUND <=50.
  Optional, default is ambient air temperature at ground level (z = 0).
RHPERC Relative air humidity (%).
  0.0 <=RHPERC <=100.
  Optional, default is 0.

DISP block contains parameters describing the dispersion characteristics.

ZR Land surface roughness (m).
  10-5 <= ZR <=1.
  Mandatory.
PQSTAB Pasquill/Gifford stability class.
  PQSTAB = A, B, C, D, E or F (character).
  Optional, default is D.
AVTIMC Concentration averaging time (s).
  0 <=AVTIMC <=3600.
  Optional, default is 600.
  AVTIMC should not be specified by the user if the CROSSW parameter DELTA (d) is specified below.
  Specifying AVTIMC will only affect the calculation of the cross wind diffusion in HEGADAS. For HEGADAS-T, if the user wants to apply time-averaging (or time-smoothing) of calculated concentrations c(t) to obtain (average) concentrations cav(t), where , then the post-processor POSTHT should be used. For HEGADAS-S AVTIMC is only used if the user requests a finite duration correction in the postprocessor POSTHS (input block ERRORFUN
MONIN Monin-Obukhov length (m).
  -500 <=MONIN <=109.
  Optional, normally HEGADAS itself calculates the Monin-Obukhov length based on the value of PQSTAB.
CROSSW Cross-wind dispersion coefficient.
  Following this keyword, three parameters can be specified: MODSY, DELTA and BETA. They specify the formula used for the cross-wind diffusion coefficient.
  MODSY Power law (1) or Briggs law (2) indicator (-)
  MODSY = 1 or 2.
  Optional, default is 2.
  DELTA First coefficient in formula (m1-BETA or -).
  0.02 <=DELTA <=0.6.
  Optional, default calculated by HEGADAS.
  BETA Second coefficient in formula (- or 1/m).
  10-6 <=BETA <=1.0.
  Optional, default calculated by HEGADAS.
  Normally the user only has to specify MODSY. Choose a value of 2 if the HEGADAS analysis involves distances larger than 1 km, otherwise set MODSY to 1. The values of DELTA and BETA should normally not be user-specified.
CE Gravity spreading law constant (-).
  1.0 <=CE <=1.3.
  Optional, default is 1.15 and should normally not be changed.
CD Gravity spreading law constant (-).
  0 <=CD <=106.
  Optional, default is 5 and should normally not be changed.
SIGX Downwind dispersion coefficient formula.
  Following this keyword, three parameters can be specified which specify the formula used to calculate downwind (x-direction) diffusion.
  HEGADAS-T only.
MODSX Formula indicator (-).
  MODSX = 1, 2 or 3.
  MODSX = 1 uses the original HEGADAS-4 formulation.
  MODSX = 2 uses the Ermak/Wilson's formulation.
  MODSX = 3 uses the adapted Chatwin/Wilson's formulation.
  Optional, default is 2.
ASIGX Bulk Richardson number (-).
  0 <=ASIGX <=1000.
  Optional, default calculated by HEGADAS.
BSIGX Second coefficient b in formula (-).
  10-4 <=BSIGX <=1.2.
  Optional, default calculated by HEGADAS.
  Normally the user should not set any of these parameters.

GASDATA block contains the source emission data, pollutant composition and thermodynamic data. The reservoir or initial stack mixture consists of 100 % wet pollutant by definition. Wet pollutant is defined to consist of the dry pollutant plus any water present in pollutant (as specified by WATERPOL).

THERMOD Thermodynamics model indicator (-).
  THERMOD = 1 (no HF) or 2 (HF).
  Optional, default is 1.
  THERMOD = 1 means that the normal ideal gas or aerosol thermodynamics will be used. THERMOD = 2 indicates that the full HF chemistry and thermodynamics will be used.
GASFLOW Dry pollutant source emission rate (kg/s).
  10-7 <=GASFLOW <=105.
  Must be specified if FLUX is not given.
  Either GASFLOW or FLUX must be specified.
FLUX Dry pollutant source emission rate (kg/s/m2).
  10-7 <=FLUX <=10.
  Can be specified instead of GASFLOW if POOL block is given.
  Either GASFLOW or FLUX must be specified.
WATERPOL Mole fraction water (liquid plus vapour) in wet pollutant (-).
  0 <=WATERPOL <=1.0.
  Optional, default is 0.0.
WPICKUP Mole fraction of initial pick-up of liquid water by pollutant (-).
  0 <=WPICKUP <=1.0.
  Optional, default is 0.0.
TEMPGAS Pollutant temperature (°C).
  -270 <=TEMPGAS <=150.
  Either TEMPGAS or ENTPOL must be user-specified.
ENTPOL Pollutant enthalpy (J/mole).
  -109 <=ENTPOL <=109.
  Either TEMPGAS or ENTPOL must be user-specified.
HFLIQFR Liquid mass fraction in initial HF release (-).
  0 <=HFLIQFR <=1.
  Optional, default is 0.
  Only relevant when using HF thermodynamics (THERMOD = 2). TEMPGAS must also be specified using this parameter.
  For a pool of liquid HF, HFLIQFR should be set to 1.0 and TEMPGAS to the pool temperature.
  For a pressurised releases with HF flashing off, HFLIQFR is the post-flash liquid in the original HF and TEMPGAS the post-flash temperature. The modules HFPLUME or HFFLASH can be used to find these parameters.
CPGAS Specific heat at constant pressure of the dry pollutant (J/(mole K)).
  5 <=CPGAS <=300.
  Mandatory. Using HGSYSTEM module DATAPROP to find CPGAS is strongly recommended.
MWGAS Molar mass of dry pollutant (kg/kmole).
  2 <=MWGAS <=200.
  Mandatory. Again, use of DATAPROP to calculate MWGAS is recommended.
HEATGR Natural convection heat transfer group
  5 <=HEATGR <=100.
  Optional, default is 24.
  Use of DATAPROP to calculate HEATGR is strongly recommended.
SPECIES Pollutant compound properties. Using this keyword at least once implies the use of the full two-phase (aerosol) model or a mixture consisting of at least one compound (excluding water). If the SPECIES keyword is not specified, ideal gas thermodynamics is used with gas properties given by CPGAS and MWGAS. Condensation or freezing of water is still taken into account.
  Only relevant if THERMOD = 1.
  The SPECIES keyword plus parameters must be specified for every compound in the mixture, except water. The sum of the molar fractions used in the SPECIES keywords (see parameter #2 below) must equal 1.0 - WATERPOL.
  The use of DATAPROP to generate the input parameters when the SPECIES keyword is being used, is strongly recommended.
  Currently a maximum of 8 species can be specified (excluding water). Please note that DATAPROP allows for more species to be specified. DATAPROP also splits dry air (if specified) up into nitrogen and oxygen, thus generating two compounds instead of one. Thus the HEGADAS link file generated by DATAPROP could contain the SPECIES keyword more than 8 times. The user should combine or remove compounds if this occurs.
  Please note that HEGADAS has restrictions concerning the aerosol structure: either a single two-compound aerosol (two compounds with the same aerosol class) or a number of separate one-compound aerosols (a series of compounds with all a different aerosol class) are allowed (not regarding water and dry air), other combinations are not supported. Sometimes this restriction can cause a fatal HEGADAS error message during a post-AEROPLUME run, because AEROPLUME has no such restrictions and allows any combination of at most eight compounds.
  For example: propane and butane can be combined (two-compound aerosol, both compounds have same aerosol class). Propane, butane and oxygen can not be combined because propane and butane have the same aerosol class and for more than two compounds all aerosol classes must be different.
  Following the SPECIES keyword of a certain compound a block of 12 parameters (#1 to #12) must be specified:
#1 compound name (maximum of 12 characters).
#2 mole fraction in pollutant mixture (-).
0<= #2<= 1.
#3 aerosol class (-).
-1<= #3 <= 50.
#4 specific heat of vapour (J/(mole K)).
5<= #4 <=300.
#5 specific heat of liquid (J/(mole K)).
0<= #5 <=10 3 .
#6 heat of vaporisation (J/mole)
0<= #6 <=10 5
#7 critical temperature (K).
0<= #7 <=10 4
#8 critical pressure (atm).
0<= #8 <=10 3
#9 vapour pressure function coefficient B1.
-10 8 <= #9 <=10 8 .
#10 vapour pressure function coefficient B2.
-10 8 <= #10 <= 10 8 .
#11 vapour pressure function coefficient B3.
-10 8 <= #11<= 10 8 .
#12 vapour pressure function coefficient B4.
-10 8 <= #12 <= 10 8 .
Note: the saturated vapour pressure of the compound is described by the Wagner function:

P v (T) = P c x exp { [ B 1 x Q + B 2 x Q 1.5 + B 3 x Q 3 + B 4 x Q 6 ] / T r }
where T is the vapour temperature, P c the critical pressure, T c the critical temperature, T r = T/T c and Q = 1 - T r .

The optional CLOUD block controls the output of gas cloud information.

NSOURCE Number of report interval along vapour blanket (secondary source) (-).
  1 <=NSOURCE <=20.
  Optional, default is 4.
  HEGADAS-S only.
NFIX Number of fixed-sized report intervals with length DXFIX (-).
  1 <=NFIX <=105.
  Optional, default is calculated by HEGADAS.
  HEGADAS-S only.
DXFIX Length of the NFIX fixed-sized report intervals (m).
  10-4 <=DXFIX <=105.
  Optional, default is L/5, where L is downwind length, calculated by HEGADAS.
  HEGADAS-S only.
XGEOM Geometrical factor to increase reporting interval (-).
  1.0 <=XGEOM <=2.0.
  Optional, default is 4.
  HEGADAS-S only.
  Applies to downwind distances after NFIX·DXFIX meter.
  Reporting interval is NFIX·DXFIX + (XGEOM)i·DXFIX, where i = 1, 2, ...
XEND Last downwind distance for HEGADAS-S calculations (-).
  0.01 <=XEND <=106.
  Optional, default is 106.
  HEGADAS-S only.
XSTEP Reporting interval in downwind direction (HEGADAS-T) (m).
  0.01 <=XSTEP <=104.
  Optional, default is 25.
  HEGADAS-T only.
XSFACT Multiplication factor in variable XSTEP algorithm (-).
  0.01 <=XSFACT <=100.
  Optional, default is 1.0 (algorithm not used).
  HEGADAS-T only.
  IF XSFACT is 1.0, then variable XSTEP algorithm is invoked, otherwise XSTEP is a constant for this HEGADAS-T run.
XSEPS Tolerance in variable XSTEP algorithm (-).
  10-5 <=XSEPS <=100.
  Optional, default is 0.05.
  HEGADAS-T only.
  If XSFACT is 1.0, then variable XSTEP algorithm is invoked and XSEPS will be used, otherwise XSTEP is a constant for this HEGADAS-T run.
CAMIN Termination ground level concentration (kg/m3).
  10-7 <=CAMIN <=5.0.
  Optional, default is that HEGADAS will use COMIN.
  Calculations do not go further downwind when concentration CAMIN is reached. Calculations will never go beyond XEND (HEGADAS-S).
CU Upper concentration limit for output (kg/m3).
  10-7 <=CU <=5.0.
  Optional, default is that HEGADAS will use CUV.
  For iso-concentration output and cumulative cloud data. See ICNT keyword.
CL Lower concentration limit for output (kg/m3).
  10-7 <=CU <=5.0.
  Optional, default is that HEGADAS will use CLV.
  For iso-concentration output and cumulative cloud data. See ICNT keyword.
COMIN Volumetric equivalent of CAMIN (vol-%).
  10-5 <=COMIN <=100.0.
  Optional, default is 0.1 %.
  Calculations do not go further downwind when concentration COMIN is reached. Calculations will never go beyond XEND (HEGADAS-S).
CUV Volumetric equivalent of CU (vol-%).
  10-5 <=CUV <=100.0.
  Optional, default is 2.0 %.
  For iso-concentration output and cumulative cloud data. See ICNT keyword.
CLV Volumetric equivalent of CL (vol-%).
  10-5 <=CLV <=100.0.
  Optional, default is 0.1 %.
  For iso-concentration output and cumulative cloud data. See ICNT keyword.

The CALC block specifies the output times. It is applicable for HEGADAS-T only. To specify output times in HEGADAS-T the user must use either the CALC block or the AUTOTIM block, but not both.

TSTAR Output time for which cloud data is calculated and printed (s).
  1.0 <=TSTAR <=106.
  Mandatory if AUTOTIM block not used.
  This keyword must be repeated for each output time in increasing order. Up to 20 output times can be specified.

AUTOTIM block applies to HEGADAS-T only. It instructs HEGADAS-T to calculate an 'optimal' set of output times between two user-specified limits. To specify output times in HEGADAS-T the user must use either the CALC block or the AUTOTIM block, but not both.

TFIRST First output time in automatic output time algorithm (s).
  1.0 <=TFIRST <=106.
  Mandatory if CALC block not used.
TLAST Last output time in automatic output time algorithm (s).
  1.0 <=TLAST <=106.
  Mandatory if CALC block not used.
DTMIN Initial (minimum) step between output times (s).
  10-5 <=DTMIN <=103.
  Optional, default is 20.
  The HEGADAS-T output time algorithm will increase DTMIN by repeatedly multiplying with OTFACT, whenever the algorithm decides that the step size can be increased.
OTEPS Tolerance in automatic output time algorithm (-).
  10-5 <=OTEPS <=100.
  Optional, default is 0.05.
OTFACT Multiplication factor for DTMIN in output time algorithm (-).
  0.01 <=OTFACT <=100.
  Optional, default is 1.5.
  Every time the algorithm decides that the step between output times can be increased, it multiplies the current step with OTFACT.

The POOL block applies only to HEGADAS-S with gas-blanket calculations (evaporating pool scenario). It specifies the pool dimensions.

PLL Pool length (m).
  10-3 <=PLL <=106.
  Mandatory if POOL block is used.
PLHW Pool half-width (m). 10-3 <=PLHW <=106. Mandatory if POOL block is used.
  N.B. The pool is also called the 'primary source' in HEGADAS documentation. If more gas evaporates than can be taken up into the atmosphere, a 'secondary source' or vapour blanket will form. See HEGADAS description in Technical Reference Manual

The TIMEDATA block applies to HEGADAS-T only. This block specifies either the time-dependent source data (evaporating pool scenario, SOURCE records) or time-dependent data at a breakpoint (post-AEROPLUME/HFPLUME scenario, BRKDATA records). Not relevant for a post-HEGABOX run.

The block keyword TIMEDATA itself has two parameters: INCRT and ITYPBR:
INCRT Every (INCRT + 1)th record is read (-).
  0 <=PLL <=99.
  Optional, default is 0 (read all records). Do not normally change this parameter.
ITYPBR Type of data. (-).
  0 <=ITYPBR <=3.
  Optional, default is 0.
  ITYPBR = 0 for evaporating pool scenario with gas-blanket calculation. SOURCE records need to be specified.
  ITYPBR > 0 for full breakpoint at specified downwind location. Value of ITYPBR indicates type of data in BRKDATA records.

TSTPOOL Start time at which source (pool or breakpoint) becomes active (s).
  -104 <=TSTPOOL <=105
  Optional, default is 0.
  Zero data (no source) assumed for times less than TSTPOOL.
TSTEPR Size of time step and bund size. This keyword has two parameters: TSTEP and RBUND.
  TSTEP Size of time step between records (s).
  0 <=TSTEP <=103
  Mandatory.
  RBUND Radius of bund/dike (m).
  0 <=RBUND <=104.
  Optional, no default.
SOURCE Source data for evaporating pool scenario (ITYPBR = 0). The SOURCE keyword has three parameters: pool radius R, evaporation rate E and (optional, for bunds only) vapour height H.
  R radius of pool at current time (m).
  0 <=R <=104
  Mandatory.
  E (evaporation) release rate of dry gas (kg/s).
  0 <=E <=106
  Mandatory.
  H height of vapour (m).
  0 <=H <=106
  Optional, for bunds only.
  The SOURCE records may occur up to 100 times. The i-th occurrence corresponds to time i·TSETP. The pool is assumed to be cylindrical. SOURCE data records will be generated by LPOOL when it creates a link file for HEGADAS-T.
BRKDATA Source data for post AEROPLUME/HFPLUME (full breakpoint) scenario (ITYPBR > 0).
The BRKDATA keyword has three mandatory parameters: cloud half width BEFF, DATA2 and DATA3, where the type of the latter two depends on the value of ITYPBR
  BEFF cloud half-width (m).
  0 <=BEFF <=106
  Mandatory.
  DATA2 ground level wet-gas molar fraction (-) (ITYPBR=1,2) or cloud height (m) (ITYPBR=3).
  0 <=DATA2 <=106
  Mandatory.
  DATA3 cloud height (m) (ITYPBR=1) or dry-gas flow (kg/s) (ITYPBR=2,3).
  0 <=DATA3 <=106
  Mandatory.
  The BRKDATA records may occur up to 100 times. The i-th occurrence corresponds to time i·TSETP. BRKDATA will be generated by the jet models (AEROPLUME or HFPLUME) when they create a link file to HEGADAS-T.
Either the SOURCE keyword (ITYPBR = 0) or the BRKDATA keyword (ITYPBR > 0) must be specified, but not both.

The optional TRANSIT block is used to specify a (full or partial) breakpoint. Apart from DISTS, the only keyword that applies to HEGADAS-T is ZRS. All keywords can be used for HEGADAS-S.

DISTS Downwind (x) location of breakpoint (m).
  0 <=DISTS <=105
  Mandatory.
WS Effective cloud half-width at breakpoint (m).
  -105 <=WS <=105
  Optional.
  HEGADAS-S only.
  A negative value indicates that only calculations downwind of the breakpoint should be carried out.
HS Effective cloud height at breakpoint (m).
  0 <=HS <=105
  Optional.
  HEGADAS-S only.
CONCS Ground level mole fraction wet-gas at breakpoint (m).
  0 <=CONCS <=1.0
  Optional.
  HEGADAS-S only.
ZRS Surface roughness downwind of breakpoint (m).
  10-5 <=ZRS <=1.0
  Optional.
TMPS Cloud temperature at breakpoint (°C).
  -270 <=TMPS <=50
  Optional.
  HEGADAS-S only.
MSFLOW Total flow (pollutant plus entrained air) at breakpoint (kg/s).
  10-6 <=MSFLOW <=105
  Optional.
  HEGADAS-S only.
AVCONC Cloud-averaged concentration at breakpoint (kg/m3).
  10-7 <=AVCONC <=5.0
  Optional.
  HEGADAS-S only.

For HEGADAS-S, at least one of the keywords CONCS, HS, MSFLOW, AVCONC or ZRS should be given. This data will normally be generated by AEROPLUME or HFPLUME.
For HEGADAS-T only a full breakpoint can be specified or a partial surface roughness breakpoint. For a full breakpoint only the DISTS parameter needs to be specified, all other data must be given in a TIMEDATA block. For a partial surface roughness breakpoint DISTS and ZRS must be given. The data for a full breakpoint will normally be generated by AEROPLUME or HFPLUME.

The TRANSIT block can occur more than once (multiple partial breakpoints). They must be given in increasing order of DISTS. This applies to both HEGADAS-S and HEGADAS-T.

The MMESOPT block contains the 'switches' which indicate the use of the extra options developed by Earth Technology and sponsored by Martin Marietta Energy Systems. An indicator value of 1 means that the corresponding option is used, a value of 0 means that the option is inactive.

IMETP Indicator for use of meteorological pre-processor (-).
  IMETP = 0 or 1.
  Optional, default is 0.
  For IMETP = 1, an additional file 'casename.MET' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
IDEP Indicator for use of wet and dry deposition model (-).
  IDEP = 0 or 1.
  Optional, default is 0.
  HEGADAS-S only.
  For IDEP = 1, an additional file 'casename.DEP' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
ICANY Indicator for calculation of canyon effects (-).
  ICANY = 0 or 1.
  Optional, default is 0.
  HEGADAS-S only.
  For ICANY = 1, an additional file 'casename.CNY' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
IFLUC Indicator for calculation of centre-line concentration fluctuations (-).
  IFLUC = 0 or 1.
  Optional, default is 0.
  HEGADAS-S only.
  For IFLUC = 1, an additional file 'casename.PTL' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
ILIFT Indicator for use of plume lift-off description (-).
  ILIFT = 0 or 1.
  Optional, default is 0.
  HEGADAS-S only.
  No additional data needed.
  When ILIFT = 0, HEGADAS will stop program execution when plume lift-off is detected. When ILIFT = 1, lift-off is taken into account. See Chapter 9 in the HGSYSTEM 3.0 Technical Reference Manual for details.
IAVG Indicator for calculation of variation of concentration with averaging time (-).
  IAVG = 0 or 1.
  Optional, default is 0.
  HEGADAS-S only.
  No additional data is needed.
  For IAVG = 1, an additional file 'casename.PTL' must be supplied. See Chapter 18 for a description of the data which must be given in this file.

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