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[Fwd [Fwd NC 43 connector ICI]]
Subject: [Fwd [Fwd NC 43 connector ICI]]
From: Colin Mellor <cmellor@dot state nc us>
Date: Wed, 03 May 2006 12 55 06 -0400
To: brian wrenn@ncmail net
Brian,
here are Todd Kennedy's responses to your questions regarding the NC43 connector
ICI He has provided comment on question #5 which was the one I couldnt answer
Let me know if this covers your concerns
Colin
"Kennedy, Todd" wrote
Colin, I decided to go ahead and provide the comments for all 6 since I had
already prepared them prior to your email about # 5 1- Values are total load in
tonnes over the seven year period 2- We are not aware of an economic
development model for the project Development procedures for the land use
scenarios are discussed in 4 2 2 and 4 2 3 The change in land use between
scenarios is likely not a reliable economic indicator 3- One of the objectives of the
study was to represent average hydrologic or climate conditions On this point, the
model simulation period is appropriate As discussed in 4 3 1, the mean rainfall for
the simulation was within 4 percent of the long term average at the COOP station
(>25yrs) Most modeling studies of pollutant loading use a 10 year or less
simulation period More often than not, the simulation period is 5 years or less
(e g , most North Carolina TMDL modeling, Haith and Shoemaker 1987, Donigian
2002, CH2M Hill 2003) The Jordan Lake GWLF (Tetra Tech 2003) that I was
involved with used a 10 year simulation period Note that these references are
contained within the report 4- Section 4 5 5 states "rates for residential land
uses with values increasing with imperviousness" The statement of trend is only
for residential land uses Values are from Kuo et al (1988) Accumulation for
commercial land uses (typically higher in imperviousness) from this reference fall
in between the various densities of residential land use There does not appear to
be a linear relationship between imperviousness and accumulation rates across all
land uses Kuo et al (1988) did not contain accumulation rates for roadways
Therefore, available export rates from FHWA and EPA were collected As
discussed in the report, these export coeffiencts are not the same as accumulation
rate, hence the iterative process used Note that the larger issue here is the lack
of locally collected data This is a common constraint in modeling Values from the
literature must be adapted in an appropriate and reasonable manner in these
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instances 5- The comment in 4 7 1 states "most BMPs convert little runoff to
infiltration" For most of the widely used, traditional BMPs, this statement holds
true Infiltration BMPs are used less often (though are gaining in use) and typically
treat less area on a treatment area per BMP basis The issue about the impact of
BMPs on long term runoff rates or volumes would probably hold true even for
infiltration BMPs, assuming that the lion's share of shift in volume is from surface
to subsurface runoff and little goes into deep seepage The important point of the
comment was to address the peak flow requirement This is addressed in number
6 below 6- Since the model does not explicitly consider the peak flow
requirement, it might be considered conservative That is, control of peak flow via
the existing regulation may be expected to provide additional control of runoff in
addition to those considered directly in the study More importantly, however, is
that the peak flow control would be a requirement regardless of the scenario
Therefore the change in the peak of the hydrograph or difference in the scenarios
should be insignificant, particularly considering that nearly all of the new
development would require the control Note that the peak flow requirement is for
greater than or equal to 15%, imperviousness levels according to the Neuse Model
Plan document (1999) Low density development (<15%) does not change
between scenarios Todd J. Todd Kennedy
Environmental Management
Stantec Consulting
Ph [phone]
Fx [phone]
tkennedyCaD_stantec com
www stantec com
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