The Release Models

The model system for the computation of the source strength consists of different individual submodules with regard to the release type. The HITERM system explicitly includes:

jet release of gas (Ermak 1991)
evaporating pool release.

If the source strength is known, the release models are no longer necessary or performs very simple. However, in case of an accident very often only some geometric quantities (e.g. of a spill) are known.

Most of the parameters needed for the source strength estimation are very uncertain. To take these uncertainties into account, a Monte Carlo simulation is used to determine a source strength distribution over time instead of a single number for the source strength.

Key parameters of these time dependent distributions are used as an input for the dispersion model. Such key parameters can be, for example, the emission rate at the mean and at the 95th percentile of the probability functions.

Although explosion or burning may also be important release types, their explicit handling in the system is impossible for practical reasons (no data, expensive computational burden). With a high level of probability, for explosions an immediate total release of the substance can be assumed. For release types, due to burning the determination of the source strength and the properties of the released substances may be very uncertain. However, this is a basic problem, and not merely a problem of the modelling tools.

For fire type release, the applied methodology is similar to buoyant jet release, taking into account the especially broad range of uncertainty for the model input parameter.

The input for the release models is formed by a set of substance specific parameters and a set of release specific parameters such as release type, release and meteorological conditions. An example of the substance specific data follows:

MOLECULAR WEIGHT [KG]   0.00070906
HEAT CAPACITY OF GAS [J/(KG*K)]     498.1
HEAT CAPACITY OF LIQUID [J/(KG*K)]          498.1
SOURCE TEMPERATURE [K]  239.1
HEAT OF VAPORIZATION [J/K]  287840.
BOILING POINT [K]   50.
GAS DENSITY OF SOURCE [KG/M3]   1.9
LIQUID DENSITY OF SOURCE [KG/M3]    1574.
SATURATION PRESSURE CONST. A    1978.
SATURATION PRESSURE CONST. B    -27.
DIFFUSIVITY [M**2/S]    20.
THERMAL COND. OF LIQUID [J/MSK]     0.23

where

MOLECULAR WEIGHT: molecular weight of source gas
HEAT CAPACITY OF GAS: heat capacity of the gas at constant pressure
HEAT CAPACITY OF LIQUID: heat capacity of the liquid
SOURCE TEMPERATURE: temperature of source material at release time and location
HEAT OF VAPORIZATION: heat of vaporization of the liquid MLI> BOILING POINT: boiling point temperature of the liquid
GAS DENSITY OF SOURCE: density of source gas
LIQUID DENSITY OF SOURCE: density of a liquid source
SATURATION PRESSURE CONST. A / B: coefficients for the vapour pressure equation
DIFFUSIVITY: diffusivity in air
THERMAL COND. OF LIQUID: thermal conductivity of the liquid.

Not all of the data listed above may be necessary. Depending on the release type, some data can be skipped. For example, for pure jet release of gas, only the parameters for the gas phase are used. An example for the release and meteorological parameters can resemble the following:

RELEASE TYPE    1
GAMMA [K/M] -0.038
STABILITY CLASS         3
MIXING HEIGHT [M]   -2000.
WIND SPEED [M/S]    5.
WIND DIRECTION          30.
MEASUREMENT HEIGHT [M/S]    100.
ROUGHNESS LENGTH [M]    0.5
SURFACE TEMPERATURE [K] 279.
PRESSURE [HPA]      1013.
SOURCE HEIGHT [M]   50.
REL. HUMIDITY [%]   50.
CLOUD [%]       20.
LONGITUDE               15.3
LATITUDE                52.1
DAY [D] 8
MONTH [M]   8
TIME [HOURS]    15.4
SOURCE AREA [M**2]  2       0.5
DEPTH [M]                   2.5
MASS SOURCE RATE [KG/S] 5.2
RELEASE DURATION [S]    1500.
VERTICAL RELEASE VELOCITY [M/S] 20.4
HORIZONTAL RELEASE VELOCITY [M/S]   0.
SOURCE TEMPERATURE [K]  350.

where:

RELEASE TYPE: spill source type
- 1 - evaporating pool release
- 2 - horizontal jet release
- 3 - vertical stack/jet release
- 4 - instantaneous or short duration evaporating pool release
GAMMA: vertical temperature lapse rate (refer to Table 1)
STABILITY CLASS: stability class (refer to Table 1)
MIXING HEIGHT: mixing height above surface (refer to Table 1)
WIND SPEED [M/S]: ambient wind speed at MEASUREMENT HEIGHT
MEASUREMENT HEIGHT: height of the meteorological sensors above ground
ROUGHNESS LENGTH: typical aerodynamic roughness length in the model domain (see Table 2)
SURFACE TEMPERATURE: ambient surface temperature at release location
SOURCE HEIGHT: release height above ground
REL. HUMIDITY: relative humidity of the ambient air
PRESSURE: ambient atmospheric pressure
CLOUD: cloud cover (100% refers to totally clouded sky)
LONGITUDE: longitude of the release point
LATITUDE: latitude of the release point
DAY: day of the month
MONTH: month of the year
TIME: local standard time
SOURCE AREA: source area
DEPTH: depth of the spill
RELEASE DURATION: total release time
VERTICAL RELEASE VELOCITY: vertical jet release velocity
HORIZONTAL RELEASE VELOCITY: horizontal jet release velocity
MASS SOURCE RATE: mass source rate (release rate)
SOURCE TEMPERATURE: temperature of the source material.