Low Impact Development and
the CT General Stormwater
Preparing for the NEXT BIG
Willimantic, CT
Copyright Trinkaus Engineering, LLC
Steven Trinkaus
 Invited presenter on LID in Taiwan, South
Korea and China,
Consultant to Pusan National University,
Korean Water and Land & Housing Institute in
South Korea for LID projects
Licensed Professional Engineer (CT)
CPESC and CPSWQ certifications
Over 31 years in the Land Development Field
with 14 years applying LID
What is Low Impact Development?
LID is an ecologically friendly approach to
site development and storm water
management that aims to mitigate
development impacts to land, water, and
air. The approach emphasizes the
integration of site design and planning
techniques that conserve natural
systems and hydrologic functions on a
All development will have some impact on
our environment. The implementation of
LID design concepts and treatment
systems can minimize the environmental
impacts associated with development
and will lead to development approaches
which are sustainable in the future.
The Old Way
Source: University of Arkansas Community Design
Source: University of Arkansas Community Design Center
The New Way
The General Stormwater Permit
 Certification that development plans are
in compliance with following Guidance
 2002 Erosion Guidelines,
 2004 Storm Water Quality Manual
The General Stormwater Permit
 Permit is not meant to be or implied to
be a design document,
 It is simply a requirement to ensure
that development plans meet minimum
erosion control and stormwater
management standards (one size fits all
Applicability of General Stormwater
Development projects with site
disturbance greater than 5 acre with
local permitting,
Requires preparation of Stormwater
Pollution Control Plan (SWPCP),
Not correct terminology: we do not want
to control pollution, we want to PREVENT
pollution (Stormwater Pollution Prevention
Plan) [SWPPP – Industry Standard)
2002 Erosion & Sediment Control
Guidance document to be referenced in
local land use regulations to require
preparation of plans conforming to this
Municipal agencies need to ensure
erosion control plans are in compliance
with the standards and specifications
found in this document
2004 Storm Water Quality Manual
Purpose is to provide guidance and is
intended to augment, rather than
replace professional judgement in the
design of stormwater management
systems, focusing on addressing water
LID & General Permit
Neither 2002 Guidelines, 2004 Manual
or Appendices of 2011 contain design
processes and methodologies for LID,
The Appendices discuss in general terms
land use approaches for LID, but they
are not requirements,
The requirements to consider LID
conflict with many local land use
regulations (home rule issue?)
LID & General Permit
Very few municipalities refer to 2004
Storm Water Quality Manual in their
Result is that applicants do not provide
information on water quality,
groundwater recharge & channel
protection flow as part of their project
LID & General Permit
Several Technical definitions are not
Effective Impervious Area
Not defined by Rational method runoff
Runoff Reduction Practices
Must be defined by a storm size and soil
conditions, not a one size fits all approach
Section 5 (b) of Permit
Estimate of average runoff coefficient
has nothing to do with GP,
Calculations supporting the design of
sediment and floatable removal controls
(no equation for this requirement)
Section 5 (b) (v) of Permit
(v) Runoff Reduction:
“quantified as total annual postdevelopment runoff volume reduced
through canopy interception, soil
amendments, evaporation, rainfall
harvesting, engineering infiltration,
extended infiltration or evapotranspiration” Very few hydrologic models
can determine and evaluate the losses
associated with these items
Section 5 (b) (v) of Permit
(v) Runoff Reduction and LID
Appears to be a requirement to provide
runoff reduction (but this is not a standard
found in the 2004 manual),
This is in conflict with Appendix B of GP,
(v)(d) Does not define how infiltration tests
are to be done, yet states you must use
50% of the field-measured infiltration rate
Section 5 (b) of Permit
(h) Calculation illustrating retention of
WQV or 50% WQV (2004 manual does
not mention 50% standard),
(i) The size of the storm for runoff
reduction is not defined,
(C) stormwater measures designed &
implemented in accord with 2004
Manual & DOT qualified list
Section 5 (b)(C) of Permit
Due to age of 2004 Manual (12+ yrs),
design standard for Bioretention is not
No specification for soil media,
 No sizing criteria,
 Stated 3’separation to SHGW prevents use
except in deep well drained soils,
 If manual does not provide design
specifications, people will use the web
(google it), not a good solution
Section 5 (b)(C) of Permit
(a) requirement to retain 50% of WQV
for redevelopment projects is a “one
size fits all approach” which is not
feasible on many sites,
The GP is telling designers how to
design their project (not intent of
What is “Maximum Extent Achievable”
Not defined in GP
Section 5 (b)(C) of Permit
(b) Only applicable to site with less than
40% effective impervious cover,
Is arbitrary standard,
 One blanket standard with minimal
 (ii)(a) Requires Runoff Reduction & LID to
meet volume reduction requirements, yet
Manual does not have volume reduction
Section 5 (b)(C) of Permit
(ii)(b) Goal is 80% removal of TSS,
How do you assess the annual load for postdevelopment conditions?
 How to you evaluate the effectiveness of
your treatment system(s) to meet or
exceed this goal?
 Only focusing on TSS does not mean there
will not be adverse impacts on water quality
Issues with LID in Permit
Appendix B
Encouragement to use LID measures to
reduce impacts of development and
address stormwater quality issues,
Encouragement of LID rarely results in
the successful implementation of LID
The result when LID is encouraged
1. Bioretention are
infiltration systems – do
the soils next to a
wetland infiltrate?
2. Bottom of system is 6”
above observed seasonal
high groundwater level
3. Bottom of system is 2’
below ex. grade in
4. Treating parking lot
runoff – require 3’
vertical separation to
groundwater – not
The result when LID is encouraged
1. Ponding more
than 3 days
rainfall event
2. Very few
3. Site was not
prior to
installation of
4. No sizing
S. Trinkaus Photo
The result when LID is encouraged
1. Used outdated
detail for
2. Inappropriate soil
media (too much
3. Use of filter fabric
(causes clogging,
reduced or no
Detail used for system on prior slide
Bioretention moving to Wetland
Bioretention moving to Wetland
The result when LID is encouraged
Rainfall Event on 9/2/12
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The result when LID is
Potential Problems:
-Poor Natural Soils
-Design Calculation in Error
-Incorrect Soil Mixture
1 week later – 9/9/12
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The result when LID is encouraged
1. Overflow grate set
flush to soil
surface, NO
2. Notch on left side
has no function,
parking pitches
away from facility
3. 24” of soil media on
top of Structural
fill with no
underdrains (Where
would the water go
if it could
Target/Lowes – Southington, CT S. Trinkaus Photo
The result when LID is encouraged
Design of bioretention systems were an ‘after
thought’ in the design process – system is
located on ‘graded ridge line in parking area’ –
what water will enter system
Target/Lowes – Southington, CT, S.Trinkaus photo
The result when LID is encouraged
Ponding because
system has no
underdrain & soil
media was placed on
top of compacted
structural fill
The result when LID is encouraged
1. At low point is flush catch basin
grate directly connected to
hydrodynamic separator
2. No available storage for runoff
3. Balance of island is raised, not
depressed, good plants are only
Staples – Branford, CT
CT DEP Photo
1. Runoff can only
enter near low end
of sloping facility
2. Runoff must make
90 degree turn into
3. Minimal storage
around overflow
Evergreen Walk – Windsor,
The result when LID is encouraged
Runoff cannot enter facility with accumulated
sediment blocking the way
Runoff will not make a 90 degree
turn when flowing along a curb line
Evergreen Walk – Windsor, CT S. Trinkaus Photo
The result when LID is encouraged
Water will ALWAYS take the path of least resistance!!!
Evergreen Walk – Windsor, CT S. Trinkaus Photo
Classical Magnet School – Hartford
Since water is
ponding on the
soil surface,
this would not
be a good
location for a
rain garden.
It appears
that plants
were just
added to this
area with no
use of an
media that
Photo: M. Rickel Pelletier
Classical Magnet School – Hartford
More ponded
water at far
end, does not
bode well for
infiltration of
Photo: CT DEP
Issues with LID in Permit
Appendix B
Incorrect terminology (LID is not
intended to maintain pre-development
hydrology (what size storm, not stated),
A goal of LID is to mimic predevelopment hydrology for small
frequent rainfall events)
Issues with LID in Permit
Appendix B
Refers to 2004 Manual and Appendix to
provide guidance on implementing LID,
Neither document is a LID design
States roof runoff is clean and can be
infiltrated via dry wells, roof runoff is
not clean (40% of nitrogen &
phosphorous loads come from
atmospheric deposition)
Issues with LID in Permit
Appendix B
Direct discharge to groundwater of
stormwater without pre-treatment is
considered a Class V Injection Well
which requires permit from EPA,
States that infiltration could be
considered in historically industrialized
areas. (In the LID community,
infiltration is prohibited for this type
of land use)
LID in Areas with High Seasonal
Water Table or Hardpan Layer
Language in this section implies a “one size
fits all approach” to dealing with the soil
LID is a performance based approach to
stormwater management and can be applied
on all types of soils, but the design of the
system must be adjusted to reflect soil
LID in Areas with High Seasonal
Water Table or Hardpan Layer
A longer travel time through the soil does
not equate to improved water quality
treatment (this is a valid statement for onsite sewage disposal systems, but not for
stormwater management systems),
More than infiltration tests are necessary
for the design of LID systems
LID in Areas with High Seasonal
Water Table or Hardpan Layer
“bioretention systems should be planted
with water tolerant/wetland plants” if
located in areas with a high seasonal
groundwater table”, if you do this, the
bioretention system will fail as the language
in the permit is telling you that water will
pond for extended periods of time and thus
only wetland plants will be able to grow
The incorporation of LID information
and requirements in the CT GP are not
They will not lead to the proper
implementation of LID,
Can place applicants in a Catch 22
between local stormwater requirements
and the GP
The Construction General Permit is not
the correct format to have LID
strategies implemented in CT,
LID is more than stormwater
management and by focusing in on
stormwater alone, does not lead to the
creation of sustainable projects
Contact Information
Steve Trinkaus, PE, CPESC, CPSWQ
Trinkaus Engineering, LLC
114 Hunters Ridge Road
Southbury, CT 06488
203-264-4558 (phone & fax)
THE END – Questions?
Copyright Trinkaus Engineering, LLC