Laboratory investigations were conducted in a stably stratified towing tank to investigate the effects of stability and terrain on plume dispersion under extreme atmospheric conditions. A three dimensional idealized model hill (Gaussian shaped saddle) at a scale of 1:2500 was towed through the tank to simulate flow over complex terrain. With reference to plume dispersion over flat terrain, the complex terrain effects revealed from visualization results are (1) the plume rise and spread increases as a result of the reduction of local flow speed-the upstream blocking phenomenon; (2) the plume tends to flow around rather than over the hill, which greatly enhances the lateral spread; (3) pollutants accumulate in the semi-stagnant region immediately upstream and below the ridge line; and (4) downstream of the ridge, pollutants carried over and around the model remain aloft for small Froude number but are carried downward along the lee surface as the Froude number increases. Some quantitative measurements were made as well. A feasibility study of numerical simulation of plume dispersion in stably stratified flow over complex terrain was also conducted. It is shown that a small Froude number expansion technique may be useful for modeling single source diffusion problems when the source lies below the mountain tops. Several methods are discussed for the imposition of radiation boundary conditions in numerical calculations.

EPA project officer: William H. Snyder. Prepared by Flow Research, Inc. "November 1974." Includes bibliographical references (p. 19-20) "EPA-650/4-74-044."