September 20, 1989

MEMORANDUM

SUBJECT:  Stack Located Within 5L of Dam

FROM:     James L. Dicke, Chief
          Techniques Evaluation Section (MD-14)

TO:       Lewis Nagler, Regional Meteorologist
          Region IV

     In response to your request to Dean Wilson, the Model
Clearinghouse has reviewed the situation you presented where a
power plant stack is located within 5L (where L is the lesser
dimension, height or projected width, of nearby structures) of
the Saluda Dam in South Carolina.  The Clearinghouse position is
that the large lateral extent of the lake and dam preclude
consideration of the structure as a building and thus should be
treated as "rolling" terrain in ISC.

     Guidance contained in Section 3.6 of the "Guideline for
Determination of Good Engineering Practice Stack Height..." is
applicable if the source wishes to justify a stack height for
modeling greater than that given by Equation 1
due to nearby terrain, i.e., not further than 0.8 km from the
stack.  It appears from the information you presented that the
GEP stack height given by Equation 1 is 202.5 feet.  From your
diagram, the height of the dam with a road on top is greater than
0.4 HG (81 feet) and is closer than 0.8 km to the stack.  This
case appears similar to Figure 13(b) in the stack height
guideline.  The extent of the dam and lake appears to be greater
than 2 miles or 10 HT, thus meeting the fluid modeling criteria
for nearby terrain.

     Thus, the source has the option to perform fluid modeling to
ascertain whether a GEP stack height greater than 202.5 feet can
be justified.  In the absence of such a justification the source
should be modeled at 202.5 feet, or at actual stack height if the
stack can be grandfathered; in either case, downwash is assumed
not to be occurring and building dimensions should not be input
to ISC.

     Incidently, we were unable to reconcile the 162 feet height
of the dam in your figure with the handwritten figures on the
terrain map.  These figures indicate that the dam top is 377 feet
MSL while the stack base is 201.5 feet MSL.  The simple
difference of these two numbers suggests that the dam height
should be 175.5 feet.

     If you have any further questions, please contact me.

cc:  D. Grano, AQMD (MD-15)
     D. Wilson, TSD (MD-14)