RiskWare  On-line Reference Manual

Release Level 1.9
Release Date 2000 06

Revision Level 1.0

SLAB: An Atmospheric Dispersion Model for Denser-Than-Air Releases


SLAB is a computer model that simulates the atmospheric dispersion of denser-than-air releases. The types of releases treated by the model include continuous, finite duration, and instantaneous releases from a ground-level evaporating pool, an elevated horizontal jet, a stack or elevated vertical jet, and an instantaneous volume source. Except for the evaporating pool source which is assumed to be all vapor, all of the remaining sources may be either pure vapor or a mixture of vapor and liquid droplets.

Atmospheric dispersion of the release is calculated by solving the conservation equations of mass, momentum, energy and species. The conservation equations are spatially averaged so as to treat the cloud as either a steady state plume, a transient puff, or a combination of the two depending on the duration of the release. A continuous release (very long source duration) is treated as a steady state plume. In the case of a finite duration release, cloud dispersion is initially described using the steady state plume mode and remains in the plume mode as long as the source is active. Once the source is shut off, the cloud is treated as a puff and subsequent dispersion is calculated using the transient puff mode. For an instantaneous release, the transient puff dispersion mode is used for the entire calculation.

The mathematical description of the physics of heavy gas dispersion (gravity spread, reduced turbulent mixing, etc.), as well as the description of the normal atmospheric advection and turbulent diffusion processes, are inherently included in the conservation equations. The thermodynamics of liquid droplet formation and evaporation is treated by assuming local thermodynamic equilibrium. Transport of the vapor-droplet mixture is treated as a single fluid and neglects gravitational settling and ground deposition of the droplets. The thermodynamic effect of ground heating when the cloud is cooler than the ground surface is also included.

The time-averaged concentration predicted by SLAB depends on not only the various physical phenomena associated with the dispersion equations, but also on the specified concentration averaging time. This is due to cloud meander. As the concentration averaging time is increased, more cloud meander can occur resulting in an effectively wider cloud. In the limiting case of a trace gas release (where all dense-gas effects are negligible) over grassy terrain (Z0 = 0.02 m) and a specified 15-minute concentration averaging time, SLAB yields a cloud concentration and width that corresponds to the values estimated by the standard Gaussian-plume dispersion curves for a rural environment.


The user's guide, entitled "User's Manual for SLAB: An Atmospheric Dispersion Model for Denser-Than-Air Releases" by Donald L. Ermak, is available through the National Technical Information Services (NTIS), order number DE91-008443. The NTIS Sales Desk can be reached at (703)-487-4650, or orders by mail can be sent to NTIS, 5285 Port Royal Road, Springfield VA 22161.

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