Abstract
Several overwater plume dispersion models under low level release height condition are designed to contribute to impact-prediction of air pollution caused by exhaust gass emitted from artificial establishment overwater in coastal or offshore region. These models are verified by three separate coastal or offshore diffusion experiments in USA.
Gaussian plume equation is used in the overwater disperision model. Diffusion widths (σy, σz) on Pasquill-Gifford-Turner's chart (PGT chart) are applied to the overwater dispersion in devising the method of use.
It is noticeable that turbulence intensities over sea surface are about a half of these over a flat short grass plain corresponding to the field of PGT chart, because roughness length (z0) of sea surface is smaller than the above-mentioned grass plain by about two orders. This is clarified by both of observation and similarity theory in surface boundary layer. This valuable relationship is extended to one of diffusion width over sea surface and such grass plain. Moreover, the gradients of az over sea surface are assumed to be smaller than that over land surface because the thickness of surface boundary layer over sea surface are smaller than that over land surface mainly due to the roughness length. They are the original devices. The following two overwater plume dispersion models are recommended,
(1) The relations of Ls and atmospheric stability categories (A-F) in case of z0=3cm on Golder (1972)'s chart and L observed by bulk method are combined in this model. Categories ofσy, σz on PGT chart are respectively shifted by two ranks to more stable ranks except the category F to be shifted by one rank, namely A→C, B→D, C→E, D→F, E→G, F→G. Theσy, σz of category G are respectively extrapolated on PGT chart;σy (G)= (2/3)·σy (F), σz (G)= (3/5)·σz (F).
(2) In case of no observation of atmospheric stabilities, σy'σz of category G are always used. It is applicable to the case in which safety tolerance is able to establish especially highly.
