Project Technical Documents

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Models and Data Requirements

HITERM uses a number of models for describing technological risk and emergency situations. These primarily include models for :

• chemical spills or runaway/unwanted reactions, leading to:
  • atmospheric dispersion of a toxic gas or evaporating liquid; (a short description of a Lagrangian model prepared by the GMD)
  • fires and explosions of a gas, liquid, or evaporating liquid;
  • transport and dispersion in soil, groundwater and surface water systems;
• pre-processor codes such as:
  • 3D diagnostic wind field model;
  • source models such as spill/pool or tank evaporation models;
• routing algorithms for transportation risk analysis
• impact models.

Examples for third-party models that are being studied for potential inclusion in HITERM include:

A number of simple, usually analytical models describing phenomena such as

• outflow from containers (liquids, vapor, gas)
• turbulent free jets
• spray releases (instantaneous and continuous)
• evaporation
• heat radiation
• dispersion
• vapor cloud explosion
• ruptures of vessels

can be found in the famous Yellow Books, Methods for the calculation of the physical effects of the escape of dangerous material (liquid and gases), Part I and II, Report for the Committee for the Prevention of Disasters, Published by the Dutch Directorate-General of Labour, Ministry of Social Affairs, Voorburg, the Netherlands (first edition: 1979).

Model data requirements

A typical model candidate for integration is SLAB:

The SLAB model has been developed to simulate the atmospheric dispersion of denser-than-air releases over flat terrain. The model treats continuous, finite duration, and instantaneous releases from four types of sources: and evaporating pool, an elevated horizontal jet, a stack or vertical jet, and an instantaneous volume source. While the model is designed to treat denser-than-air releases, it will also simulate cloud dispersion of neutrally-buoyant releases. Consequently, a typical SLAB simulation covers both the near-field dense gas phase and the far-field passive gas phase.

See: "User's Manual for SLAB: An Atmospheric Dispersion Model for Denser-Than-Air Releases" by Donald L. Ermak, available through the National Technical Information Services (NTIS), order number DE91-008443.

The SLAB data requirements are listed below:

c   Simple Prediction Heavy Gas Dispersion Model
c     Developed 1990 by DL Ermak
c
c   Meteorological Variables
c
c     - za    ambient measurement height (m)
c     - ta    ambient temperature ('k)
c     - ua    ambient wind speed (m/s)
c     - z0    surface roughness height (m)
c     - rh    relative humidity (percent)
c     - wmae  molecular weight of ambient air (kg)
c     - cpaa  heat capacity of ambient air (kg)
c     - wma   molecular weight of dry air (kg)
c     - cpa   heat capacity of dry air at const. p. (j/kg-'k)
c     - wmw   molecular weight of water (kg)
c     - cpwv  heat capacity of water vapor (j/kg-'k)
c     - cpwl  heat capacity of liquid water (j/kg-'k)
c     - rhoa  density of ambient air (kg/m3)
c     - pa    ambient atmospheric pressure (pa=n/m2=j/m3)
c     - hmx   mixing layer height (m)
c     - uastr ambient friction velocity (m/s)
c     - stab  stability class values
c
c    class   value      description
c      a      1.0      very unstable
c      b      2.0        unstable
c      c      3.0    slightly unstable
c      d      4.0         neutral
c      e      5.0     slightly stable
c      f      6.0         stable
c
c   default   0.0    input 'ala' for stability
c
c     - ala   inverse monin-obukhov length (1/m)
c             (ala is an input parameter only when stab=0.0)
c
c   source variables
c
c     - idspl spill source type
c           1 - evaporating pool release
c           2 - horizontal jet release
c           3 - vertical stack/jet release
c           4 - instantaneous or short duration evaporating pool release
c     - wms   molecular weight of source gas (kg)
c     - cps   heat capacity at const. p. (j/kg-'k)
c     - ts    temperature of source material ('k)
c     - rhos  density of source gas (kg/m3)
c     - qs    mass source rate (kg/s)
c     - as    source area (m2)
c     - ws    vapor evaporation rate or vertical jet source velocity (m/s)
c     - bs    source half width; bs=.5*sqrt(as) (m)
c     - tsd   continuous source duration (s)
c     - qtcs  continuous source mass (kg)
c     - qtis  instantaneous source mass (kg)
c     - hs    source height (m)
c     - us    horizontal jet source velocity (m/s)
c     - tbp   boiling point temperature ('k)
c     - cmed0  initial liquid mass fraction
c     - dhe   heat of vaporization (j/kg)
c     - cpsl  liquid heat capacity (j/kg-'k)
c     - rhosl liquid density of source material (kg/m3)
c     - spb   saturation pressure constant  (default: spb=-1.)
c     - spc   saturation pressure constant  (default: spc=0.)
c
c   additional variable  definitions
c
c     - tav   concentration averaging time (s)
c     - zp(i) heights of concentration calculation; i=1,4 (m)
c     - xffm  far field length (m)
c     - ncalc sub-step multiplier (input parameter)
c     - nssm  number of calculation sub-steps (nssm=3*ncalc)
c     - grav  acceleration of gravity (m/s2)
c     - rr    gas constant (j/mol -'k)

Another example would be the evaporation models SOURCE and EVAP, that generate a dynamic source terms for a spill of an evaporating liquid:

C     SOURCE.f
C     This subroutine computes the evaporation rate from a pool
c     of a (volatile) liquid.
c
c     INPUT:
c
c     kmax ..... maximum number of time steps (seconds)
c     dt ....... time step [s]
c     vtot ..... total amount spilled [m3]
c     tspill ... duration of spill [s]
c     widthp ... maximal width of pool [m]
c     rpar() ... vector of input parameters (see evapor.f);
c                partially used and/or updated in this subroutine:
c     rho = rpar(15)  [g/m3]
c     rho ...... density of liquid [g/m3]
c ----------------------------------------------------------------------
C     EVAPOR.f
c     This subroutine computes the evaporation rate per unit area
c     at time t after the spill of a (volatile) liquid dependent on
c     geographical, climatological and meteorological data of the
c     environment as well as the physico-chemical properties of the
c     liquid, the surrounding air and the underlying ground.
c     Ref.: P.I. Kawamura and D. Mackay,
c           J. Hazardous Materials, 15 (1987) 343-364.
c
c     INPUT:
c
c     t ...... time elapsed since the spill [s]
c     sa ....... solar altitude [rad]
c     cloud .... cloud cover [%]
c     ubar10 ... wind speed measured at 10 m [m/s]
c     tempa .... ambient air temperature [K]
c     diamp .... pool diameter or downwind length of pool [m]
c     depthp ... initial depth of the pool [m]
c     wtmol .... molecular weight of the liquid [g/mol]
c     dab ...... diffusivity of the liquid in air [m2/s]
c     conliq ... thermal conductivity of the liquid [J/msK]
c     boilp .... boiling point of the liquid [oC]
c     vapht .... heat of vaporization (=latent heat of evaporation) [J/mol]
c     a ........ } coefficients for the vapor pressure equation of the
c     b ........ } form     press = exp(a-b/(T+c)) [Pa]
c     c ........ } where    T ... temperature [K]
c ------------------------------------------------------------------------


Basic parameters of the accident:

• substance,
• mass spilled,
• duration of the spill

 

have to be supplied by the operator either directly, or indirectly
by identifying a contained or vehicle of known content; 


Other parameters (location, time) are either known automatically through the
systems real-time clock and the identification of a storage vessel or a
location determined by GPS  for a vehicle;


alternatively, they can be supplied by automatic link-up to monitoring 
stations for data such as the meteorological variables.