Re: Gaussian Plumes - how far out can you be?

["Ebrahim Esmaili" <Ebrahim_Esmaili@fernald.gov>]

     Dave,
     
     One sigma out of central ordinate (on each side) accounts for about 
     70% of area under Gaussian curve, so extending out to few sigma (3-4) 
     should be OK.  For a quick check on your impact area, you may use a 
     simple Gaussian plume equation (excluding the cross wind term) and 
     calculate the centerline concentration.  Using following equation,
     
     tolerance limit concentration/centerline concentration = 
     exp[-1/2(Y/sigmaY)**2]
     
     and using sigmaY curves (for stability B and downwind distance of 
     interest), then calculate the crosswind distance Y for your impact 
     area. If you have access to USEPA SCREEN or ISC models, you may do a 
     more rigorous calculation to generate isopleths and footprint of the 
     plume.

Ebrahim Esmaili, Ph.D.
Parsons Infrastructure & Technology Group Inc.
ebrahim_esmaili@parsons.com        
______________________________ Reply Separator _________________________________
Subject: Gaussian Plumes - how far out can you be?
Author:  radsafe@romulus.ehs.uiuc.edu at FE-INTERNET
Date:    3/10/98 8:33 PM


     
Radsafers:
     
Some coworkers and I are doing some analysis of accident consequences at 
our site.  One aspect we were looking at is the width of area 
contaminated at or above a given level after the plume has passed over.
     
Has anyone done some analysis (or found a good reference) that indicates 
how far out from centerline the Gaussian plume model is valid?  I 
realize it is only an approximation, and it seems to me that I remember 
one of the problems being "edge effects" - the Gaussian plume never goes 
to 0 but the real plume does.  Is it OK to be several sigma out  - or is 
there some limit.
     
To illustrate the problem - we wanted to identify the area contaminated 
above 1E4 Bq/m^2.  At 500m from the stack in Class B weather, the area 
was 800m wide.  This seems wrong.
     
If anyone has useful info or insight - please advise. 
Thanks
Dave Tucker
     
_____________
     
Dave Tucker, CHP
Health Physicist
Radiation Protection Branch
AECL - Chalk River Labs
Chalk River, Ontario
Canada
KOJ 1JO
Ph (613) 584-8811 X4505
tuckerd@aecl.ca
Tucker