B. Impervious Surface Issue PaperMEMORANDUM
TO: Todd Gerhardt, City Manager
CITY OF -
FROM: Terry Jeffery, Water Resources Coordinator
CHANHASSEN
7700 Market Boulevard DATE: May 10, 2010 e�
PO Box 147
Chanhassen, MN 55317 SUBJ: Impervious Surface Issue Paper
Administration
BACKGROUND
Phone: 952.227.1100
been processed for this hardcover limit. Over this time frame, two additional
Fax: 952.227.1110
On August 22, 1994, the City adopted Ordinance 297 — Stormwater Management.
Building Inspections
This was in direct response to Minnesota Rules Chapter 6120.2500 through
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6120.3900: Shoreland Management which became law in 1989.
Fax: 952.227.1190
Staff has prepared this issue paper to inform council about the feasibility of
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allowing alternative best management practices as a mitigating factor in
Engineering
exceeding hardcover allowances on residential properties.
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shoreland rules, which stipulates no more than 25% impervious cover in those
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On May 24, 2004, the City adopted Ordinance 377 which limited the maximum
Finance
lot coverage for all structures and paved surfaces to 25% in areas zoned single
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family residential. This was in response to drainage and flooding issues
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throughout the City.
Park & Recreation
Impervious surface is defined in the Chanhassen City Code as:
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been processed for this hardcover limit. Over this time frame, two additional
Fax: 952.227.1110
Impervious surface means any material that substantially
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Fax 952.227.1110
reduces or prevents the infiltration of stormwater. It shall
Recreation Center
Point area of Lake Minnewashta and the Carver Beach area of Lotus Lake. Both
2310 Coulter Boulevard
include, but not be limited to, gravel driveways, parking
Phone: 952.227.1400
area, buildings and structures. (20)
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Ordinance 377, these areas would still require a variance under the DNR
Chanhassen is a Community for Life - Providing for Today and Planning for Tomorrow
From May of 2004 through the current date, thirteen (13) variance requests have
Planning &
been processed for this hardcover limit. Over this time frame, two additional
Natural Resources
requests were made and withdrawn; in both instances, a way to comply with City
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Fax 952.227.1110
Code was found. The majority of the requests have come from the Red Cedar
Point area of Lake Minnewashta and the Carver Beach area of Lotus Lake. Both
Public Works
of these areas were fully developed prior to the implementation of most, if not all,
1591 Park Road
water quality rules at the state and local level. Even if the City amended
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Ordinance 377, these areas would still require a variance under the DNR
Fax: 952.227.1310
shoreland rules, which stipulates no more than 25% impervious cover in those
Senior Center
areas within 1,000 feet of lakes and 300 feet of streams and rivers. This was
Phone: 952.227.1125
codified in the City Code with Ordinance 297. Of the thirteen variance requests
Fax: 952.227.1110
and the two that were withdrawn, only six would fall under City Ordinance 377.
The remaining nine would still be required to meet the 25% impervious
Web Site
requirement under the Shoreland Rules set forth in Minnesota Rules Chapter 6120
wwadchanhassennn,us
and City Ordinance 297.
Chanhassen is a Community for Life - Providing for Today and Planning for Tomorrow
Todd Gerhardt
May 10, 2010
Page 2 of 5
YEAR
HARDCOVER VARIANCE
REQUESTS (UNDER 377)
OTHER VARIANCE REQUESTS
(EG. SETBACKS, ETC)
2004
2 (0)
18
2005
4 (2)
19
2006
1 (0)
16
2007
3 (3)
13
2008
1 (0)
11
2009
2(l)
11
2010
0 NA
2
Table 1. Number of hardcover variance requests per year versus number of other variance requests.
These requests have prompted discussion regarding the possibility of allowing the use of
alternative paving materials, collectively referred to as pervious paver systems, to mitigate some
overage of the hardcover limit.
The DNR has offered some guidance on the use of permeable pavement systems. They
discourage them in general on sites where the 25% is already exceeded: "On degraded sites
where 25% impervious is already exceeded, the retrofitting of permeable pavement systems may
be of lesser value." 1
ANALYSIS
The intent of the hardcover limit is to minimize, if not prevent, damage as a result of excessive
runoff. It is known that impervious surfaces, such as asphalt and concrete, result in dramatic
increases in runoff volumes and that this increase can result in erosion problems, localized
flooding and basement damage as well as damage to other infrastructure. In March of 2003 the
Center for Watershed Protection issued a report summarizing the result of 225 studies relating
stream indicators to impervious surface coverage. The results showed that stressed conditions
resulted for streams having between 10% and 25% hardcover and that beyond 25 %, severe
degradation takes place. While the review looked primarily at stream indicators, it did show that
phosphorous export to water bodies increases proportionate to the amount of hardcover. It has
been shown that reservoirs with over 25% impervious cover cannot sustain safe drinking water
supplies.
In addition, by increasing the amount of runoff, the amount of infiltration is decreased, thereby
diminishing the amount of precipitation available for groundwater recharge. Another benefit is
the preservation of green space which has been positively correlated to property values.
It was staff's intent to bring a discussion of alternative best management practices that could be
used to allow for a nominal increase in hardcover on residential lots. However, the more staff
began to consider the issue, the less practical an alternative approach seemed. The following is a
discussion of the inherent limits and restrictions to increasing hardcover allotments.
1 Permeable Pavement Systems in Shoreland Areas, A Guidance to Local Government Units, DNR Waters, St. Paul,
Minnesota September 10, 2003
2 Schueler, Tom. 2003. Impacts of Impervious Cover on Aquatic Systems. Center for Watershed Protection.
Todd Gerhardt
May 10, 2010
Page 3 of 5
MS4 PERMIT AND NONDEGRADATION PLAN
As part of the City's National Pollution Discharge Elimination System: Municipal Separate
Storm Sewer System (MS4) Permit, we performed a Non - Degradation Assessment of the Major
Watersheds within the City. Based upon the modeling performed for the City, it is estimated that
under current conditions, the volume of runoff will increase by 50% between 1990 and 2020
with the majority occurring between 2000 and 2020. Of the 12 major watersheds delineated for
Chanhassen, all but two significantly exceed the Nondegradation Volume Management
requirements.
WATERSHED
1990 CONDITIONS
2000 CONDITIONS
2020 CONDITIONS
ac. ft. / year
ac. ft. / year
ac. ft. / year
Chaska Creek
99
118
155
Lake Ann
36
41
64
Lake Lucy
285
347
460
Lake Mitchell
116
193
193
Lake Riley
279
318
483
Lake Susan
713
908
1074
Lotus Lake
668
682
682
Lower MN River
75
76
103
Minnehaha Creek
368
386
429
Rice Marsh Lake
517
567
622
Bluff Creek
1,364
1,731
2,977
TOTALS
4,520
5,367
7,242
Table 2. Surface runoff by target year per watershed measured in acre feet per year.
In the above table, the 1990 condition represents the flow volume that the City is to achieve
through the implementation of best management practices and planning tools. In all cases, a
reduction in volume is required. In the case of Lake Riley, Lake Lucy, Lake Susan and Bluff
Creek, this target reduction is significant.
EXISTING DRAINAGE AND FLOODING ISSUES
Over the last thirty (30) months, we have been made aware of
more than 70 nuisance areas by residents pertaining to
drainage, erosion or flooding. All of these issues are
intrinsically related to the amount of runoff. Any activities
which promote additional runoff will only serve to exacerbate
these issues and will result in the creation of new issues.
These complaints do not consider gully erosion tributary to
area waters such as Riley Creek, Bluff Creek and Lotus Lake.
These complaints do not consider emergency repairs or capital
improvement projects in response to volume issues.
When a pond and a storm sewer conveyance system is designed, certain assumptions are made.
One major assumption pertains to the density of development; the amount of impervious surface
relative to pervious surface. While developers are required to design to a certain threshold, we
try to minimize the extent to which developable land is used for stormwater treatment and do not
have them over - design a pond (i.e. there is not extra capacity within these ponds).
Todd Gerhardt
May 10, 2010
Page 4of5
LIMITATIONS AS A RESULT OF SOIL AND GEOLOGIC CONDITIONS
Soils in this area were formed from glacial till and have high
clay content. This tends to result in more poorly drained soils
than would be typical of soils with a higher sand content.
These soils, which tend to be in the soil hydrologic groups C
and D (these are poorly drained soils) and/or have a shallow
water table do not lend readily to infiltration features such as
pervious paver systems. It is possible to install them but it
would likely require a considerable amount of data acquisition,
engineering and soil corrections which would necessitate
placing the burden of proof on individual property owners at a
considerable expense.
OWNERSHIP AND MAINTENANCE RESPONSIBILITIES
Even if they were to undertake the upfront due diligence, there is little assurance that: (1) the
landowner would take steps to assure that the feature is working per design; (2) the landowner
would take steps to correct the feature if it were not functioning properly; (3) would undertake
long term maintenance for the features; and (4) that the feature would stay in place into the
future. (i.e. that a subsequent landowner would not replace the pervious system with an
impervious system more to their liking.)
This in turn generates numerous questions. How does the City ensure that these features remain
in place and functioning per design? Many of the more standard tools — easements and escrows
— do not seem appropriate for this application. Does the City require a Conservation Easement
or Drainage and Utility Easement over a driveway or patio on a private residence? Does this
mean that the City would become responsible for the maintenance of infiltration features on
private property? Does the City require an escrow that can be drawn upon to make repairs
should the resident not be willing or able to? These questions are not unique to Chanhassen.
Other city staff are evaluating these same questions and, as of yet, I have not heard of a city that
has developed a method to address these issues.
Staff had also considered the possibility of providing banking credits. Upon further discussion it
seemed clear that the administration and management of this program may be daunting. This
program also would not be cost effective. In order for this program to have any net benefit on
the overall hardcover, it would need to be developable land. This would likely mean purchasing
a lot within a subdivision and then selling portions of this lot as needed to owners within a
development. Taking an average 15,000 S.F. lot and removing the right -of -way and setbacks a
net credit of approximately 8,000 S.F. would be available. This would result in a square foot
cost of about $12.5 /S.F. assuming a lot cost of $100,000. Additionally, this would remove this
lot from the tax revenue for the City. The other option would be to use undevelopable land
which would do nothing for the overall percentage of impervious cover.
Todd Gerhardt
May 10, 2010
Page 5 of 5
CONCLUSION
It was the intent of this issue paper to discuss the ramifications of allowing alternative hardcover
materials to allow for the 25% maximum hardcover to be exceeded. Staff has concluded that the
current conditions do not warrant additional dispensation for the placement of alternate
hardcover. The volume of runoff already exceeds what is allowed under the Clean Water Act as
described in the City MS4 Permit.
Being that on average, fewer than two hard cover variances are requested annually, the majority
of land owners are able to abide by this limitation. Further, the majority of these requests came
from landowners in those areas fully developed prior to the 1970's on smaller lots. These are the
situations which the variance process was intended for and these are the situations for which it
has worked well. The use of pervious pavement systems might be used as a condition of
approval for a variance request but not as a practice to negate the need for a variance.
ATTACHMENTS
Figure 1. Resident Concerns — Surface Water
GAENGUerryTosition Papers\Hardcover\Hardcover Issue Paper.docx
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