July 1, 1993


MEMORANDUM

SUBJECT:  Stack-Structure Relationships--Further clarification of our
          memoranda dated May 11, 1988 and June 28, 1989

FROM:     Russell F. Lee, Meteorologist
          Techniques Evaluation Section, SRAB (MD-14)

To:       Richard Daye, Regional Meteorologist
          Region VII


     It has come to our attention that, if the suggestions provided in the
Model Clearinghouse memoranda to you of May 11, 1988 and June 28, 1989, are
followed mechanically, they can sometimes result in downwash calculations
being made for a Good Engineering Practice (GEP) stack.  This, of course,
is in conflict with the definition of GEP stack height.  It could also
result in unnecessarily restrictive emission limits being required of the
source.  

     It was never our intention to suggest modeling a GEP stack as though
downwash were occurring.  The purpose of this memorandum is to suggest a
procedure to avoid this problem in future cases where the approach
described in the Clearinghouse memoranda, or a similar approach, is being
used.  The problem may have occurred in the case addressed in our original
memoranda.  However, we do not believe that there is sufficient likelihood
of significant error to justify re-evaluating the modeling for that case. 

     The problem can occur only for a tall building (i.e., a building whose
height is greater than its projected width for at least some directions). 
The GEP stack height is a function of the projected building height and the
projected building width, which varies by wind direction.  The maximum GEP
height is determined only from those circumstances when the wind crosses
the building blowing to or from the stack.  For circumstances when the wind
does not cross the building, but the stack is close to the building (as
described in my Clearinghouse memoranda), the projected building dimensions
provided to Industrial Source Complex (ISC2) should be restricted to be no
greater than that used in defining the maximum GEP height.  Otherwise, ISC2
may calculate downwash from a stack that meets GEP, as if it did not meet
GEP criteria.

     It is our intention to include this approach in the development of a
building orientation preprocessor that is currently being prepared by this
Branch.  The Regional Modeling Contacts will be given an opportunity to
review that preprocessor when it is completed. 

cc:  D. Atkinson
     P. Eckhoff
     J. Irwin
     D. Wilson