A. Impervious Surface DiscussionAdministration
/I
This issue paper will discuss the reasons for limiting the amount of impervious
MEMORANDUM
CITY OF
TO:
Todd Gerhardt, City Manager
CHMNSEN
exceeded without need for a variance. Before going into greater detail, the findings
Terry Jeffery, Water Resources Coordinator
7700 Market Boulevard
DATE:
November 28, 2011 .
PO Box 147
and an increased likelihood of localized and regional flooding. As the volume of
Chanhassen, MN 55317
Finance
water that would otherwise be conveyed has nowhere to go. This results in
Phone: 952.227.1140
SUBJ:
Impervious Surface Issue Paper
Administration
Phone: 952.227.1100
This issue paper will discuss the reasons for limiting the amount of impervious
Fax: 952.227.1110
surface at 25% of the lot area. It will also look at the ramifications of allowing
Building Inspections
alternative hardcover materials so that the 25% maximum hardcover could be
Phone: 952.227.1180
exceeded without need for a variance. Before going into greater detail, the findings
Fax: 952.227.1190
can be summarized as follows:
Engineering
1. An increase in hard cover leads to an increase in stormwater infrastructure costs
Phone: 952.227.1160
Fax: 952.227.1170
and an increased likelihood of localized and regional flooding. As the volume of
runoff exceeds the capacity of the existing system, surcharge conditions result as
Finance
water that would otherwise be conveyed has nowhere to go. This results in
Phone: 952.227.1140
localized flooding. To remedy this condition, existing infrastructure must be
Fax: 952.227.1110
upsized and additional infrastructure must be added at a substantial cost to all
community members.
Park &Recreation
Phone: 952.227.1120
Fax: 952.227.1110
2. An increase in hard cover will result in an increase in overall maintenance cost of
the system. Increased sediment delivery reduces the effective life cycle of
Recreation Center
detention ponds, sump manhole structures and other structural BMPs. This
2310 Coulter Boulevard
requires additional expense to maintain these BMPs.
Phone: 952.227.1400
Fax: 952.227.1404
3. An increase in hardcover leads to an increase in runoff rates and volume. This
Planning &
can contribute to gully erosion and mass soil loss through escarpments. It can
Natural Resources
also cause other city infrastructure, such as sanitary sewer, to be at risk.
Phone: 952.227.1130
Fax: 952.227.1110 4. An increase in hardcover will lead to a decrease in water quality. As hardcover
increases so does delivery of TSS, nutrients, metals and other pollutants to the
Public Works water resources. An increase in hardcover also results in an increase in base flow
7901 Park Place
Phone: 952.227.1300 water temperature. All of these changes are detrimental to fish habitat,
Fax: 952.227.1310 recreational benefit and general aesthetics of lakes, streams, wetlands and ponds.
Senior Center 5. As there is no way to guarantee that alternative surfaces would be maintained into
Phone: 952.227.1125 the future, there is no way to be certain that a net increase in hardcover will not
Fax: 952.227.1110 result.
Web Site 6. On average, there are less than two (2) hardcover variances requested per year.
www.ci.chanhassen.mn.us
The majority of these requests is from property owners within the shoreland
district for Chanhassen area lakes and streams and would still, therefore, fall
under the purview of Chapter 6120 of the Minnesota Rules.
Chanhassen is a Community for Life - Providing for Today and Planning for Tomorrow
Todd Gerhardt
November 28, 2011
Page 2 of 9
BACKGROUND
For over 25 years, the maximum lot coverage for all structures and paved surfaces has been
limited to 25% in areas zoned single - family residential. This was in response to drainage and
flooding issues throughout the City.
Impervious surface is defined in the Chanhassen City Code as:
Impervious surface means any material that substantially reduces or
prevents the infiltration of stormwater. It shall include, but not be limited
to, gravel driveways, parking area, buildings and structures. (20)
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Figure 1: Variances by Type: 1990 -2010
From 1990 to 2010, there have been 342 variance requests submitted to the City for
consideration. Of those 342 requests, 32 have been for relief from the 25% maximum hardcover
allowance.
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. While many of these systems have been installed and are
functioning per design, these are most commonly located on property owned and maintained by
local government units or commercial properties where a management company is responsible
for maintenance.
Todd Gerhardt
November 28, 2011
Page 3 of 9
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
25% Shallows + 25 *. Deep
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W Impenions
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Figure 2: Effect of Increase Impervious Cover on Surface Hydrology
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 rationale behind limiting hardcover and 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
November 28, 2011
Page 4 of 9
EFFECT OF HARDCOVER ON WATER QUALITY
Numerous studies show that the urbanization of a watershed has a pronounced negative impact
on water quality. The 1983 US EPA National Urban Runoff Program (NURP) study indicated
that while there is not a dramatic amount of variability between different urban land uses, there is
a significant difference in pollutant concentrations from urban sources to that produced from
undeveloped lands 3 . These pollutants are commonly divided into the following categories:
sediments and floatables; pesticides and herbicides; organic materials; metals; hydrocarbons;
bacteria and viruses; and nutrients. The concentration of these pollutants is directly proportional
to the degree of urbanization - the more intense the urban development, the greater the pollutant
concentration within the storm water runoff from the developed area.
Of the 1,774 listed impairments to Minnesota waters in 2010, 428 or 24% were listed as impaired
for eutrophication. This is second only to bioaccumulative toxins such as mercury which is
largely a result of atmospheric deposition rather than surface runoff. Lake Susan, Lake Riley
and Lotus Lake are all impaired for eutrophication. Nutrient concentrations, specifically
nitrogen and phosphorus, are largely predictive of a water body's trophic status and the amount
of algal biomass produced.
This indirectly affects the biological oxygen demand and available oxygen within a water body.
This algal biomass production may be filamentous algae or it may be cyanobacteria — commonly
referred to as blue -green algae. Contact with blue -green algae can result in skin irritation,
irritation of the eyes and ears, fever, nausea, and upper respiratory ailments in humans and
animals. There is a very strong positive correlation between the eutrophication of a water body
and the urbanization of the watershed.
Walker reported "cause- effect relationships 00,000
linking urban development to lake and
reservoir eutrophication are well 10,000
established" Walker goes on to say "Urban
watersheds typically export 5 to 20 times as
much phosphorus per unit per year as • 000
compared to undeveloped watersheds in a o
region." 6
100
Suspended solids are comprised of
phytoplankton and fine soils particulates
such as clay, silt and sand. These
particulates adversely impact benthic
habitats and can negatively impact fish
HOURS DAYS Y.ED MONTHS
it ".1E *►
Figure 3. Relational Trends of Freshwater Fish Activity to Turbidity and Time
3 US EPA. 1983. Results of the Nationwide Urban Runoff Program: Volume I - Final Report.
Water Planning Division. Washington, DC. NTIS Publication No. 83- 185552.
4 United States Environmental Protection Agency (EPA). 1999. Preliminary Data Summary of Urban Storm Water
Best Management Practices. EPA - 821 -R -99 -012 Washington D.C.
5 Minnesota Department of Health (MDH). 2011. Blue -Green Algal Bloom and Microcystin. Site Assessment and
Consultation Unit
6 Walker, W.W. 1987. Phosphorus Removal by Urban Runoff Detention Basins. Lake and
Reservoir Management: Volume III.
Todd Gerhardt
November 28, 2011
Page 5 of 9
reproduction. The fines can smother newly hatched larvae and fill in void spaces that would be
used as habitat by organic organisms. The suspended solids also limit light penetration which
adversely impacts emergent and submergent vegetation growth. This diminished light
penetration also limits photosynthesis resulting in diminished input of oxygen into the water
column.
This input of sediment also acts as a delivery method for other pollutants that bind to these
particles and increases maintenance costs for ponds and other stormwater infrastructure. In order
for the wet detention ponds to function appropriately they need the dead storage volume
(permanent pool) as modeled in the original design. Increases in hardcover lead to an increase in
sediment delivery and a decrease in the life cycle of the pond. This increase in sediment delivery
also results in an increased frequency at which the sump catch basin manholes must be
maintained.
The last major effect urbanization has on water quality to be discussed in this issue paper is its
effect on temperatures of surface water features. Water temperature has an impact on several
important measures of water quality. Most importantly it affects the solubility of dissolved
gasses. Specifically, an increase in temperature results in a decrease in the amount of oxygen
and carbon dioxide available for respiration. This issue is further compounded by the fact that
metabolism, respiration and dissolved oxygen demand are all directly proportional to water
temperature. Thus oxygen demand is actually increasing as the available oxygen is decreasing.
Not only will this affect the typical flora and fauna of area lakes and streams, it could have very
deleterious effects on such cold water regimes as designated trout stream, Assumption Creek and
Bluff Creek which may behave more similar to a cold water assemblage of fish.
MS4 PERMIT, NONDEGRA DA TION PLAN and VOLUME
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.
7 Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for
quantitative assessment of risk and impact. North American Journal of Fisheries Management. 16: 693 -727
8 Malina, J. F. 1996. "Chapter 8: Water Quality." Water Resources Handbook. L. W. Mays, ed.
McGraw -Hill. New York, NY.
9 California State Water Resources Control Board (SWRCB). 1963. Water Quality Criteria.
Second Edition. Publication No. 3 -A., pp. 284 -285.
Todd Gerhardt
November 28, 2011
Page 6 of 9
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 1. 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 ad ' ' y serve to
exacerbate these issues and will result in the creation of n Exposed Sanitary Sewer laints 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 impr- )vement projects in response
to volume issues.
When a pond and a storm sewer conveyance system
are designed, certain assumptions are made. One
major assumption pertains to the density of J
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). Eventually capacity may be exceeded in a
system and the city will need to repair or replace the
system. One such project was done in 2008 within the
West Central Lotus Lake Watershed specifically to
address capacity issues at Kerber Boulevard. This
included installing several new stormsewer structures,
upsizing a half mile of pipe, creating a new berm and a
series of ditch checks and restoring a wetland. This was
just the first phase in a proposed three- phased project.
Todd Gerhardt
November 28, 2011
Page 7 of 9
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 a property owner were to undertake the upfront due diligence (as shown in the attached
plans), it could be problematic to assure 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 adequately 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 would 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 tributary to the conveyance system
and receiving water. This land would be prohibitively expensive and would not generate
revenue for the City.
The City has gone to considerable lengths to ease any burden that may result from this hardcover
requirement. The city does not count pools or retaining walls against the impervious surface
calculations. Further, wetlands and pond areas do not count against hard surface calculations
even though these features have 100 percent runoff.
CONCLUSION
It was the intent of this issue paper to discuss the ramifications of allowing alternative hardcover
materials so that the 25% maximum hardcover could be exceeded without need for a variance.
Based upon numerous reasons, staff has concluded that the current conditions do not warrant
Todd Gerhardt
November 28, 2011
Page 8 of 9
additional dispensation for the placement of alternate hardcover. Once again, these reasons can
be summarized as follows:
1. An increase in hard cover leads to an increase in stormwater infrastructure costs and an
increased likelihood of localized and regional flooding. As the volume of runoff exceeds
the capacity of the existing system, surcharge conditions result as water that would
otherwise be conveyed has nowhere to go. This results in localized flooding. To remedy
this condition, existing infrastructure must be upsized and additional infrastructure must
be added at a substantial cost to all community members.
2. An increase in hard cover will result in an increase in overall maintenance cost of the
system. Increased sediment delivery reduces the effective life cycle of detention ponds,
sump manhole structures and other structural BMPs. This requires additional expense to
maintain these BMPs.
3. An increase in hardcover leads to an increase in runoff rates and volume. This can
contribute to gully erosion and mass soil loss through escarpments. It can also cause
other city infrastructure, such as sanitary sewer, to be at risk.
4. An increase in hardcover will lead to a decrease in water quality. As hardcover increases
so does delivery of TSS, nutrients, metals and other pollutants to the water resources. An
increase in hardcover also results in an increase in base flow water temperature. All of
these changes are detrimental to fish habitat, recreational benefit and general aesthetics of
lakes, streams, wetlands and ponds.
5. As there is no way to guarantee that alternative surfaces would be maintained into the
future, there is no way to be certain that a net increase in hardcover will not result.
6. On average, there are less than two (2) hardcover variances requested per year. The
majority of these requests is from property owners within the shoreland district for
Chanhassen area lakes and streams and would still, therefore, fall under the purview of
Chapter 6120 of the Minnesota Rules.
Most of these requests came from landowners in those areas fully developed prior to the 1980'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 to mitigate for the increase, but not as a
practice to negate the need for a variance. Even in these instances, the systems should be
designed and installed by a qualified professional and a long -term operations and maintenance
program should be developed and implemented. An example of what a typical plan set submittal
might look like is included in the attachments. These systems do provide an overall benefit to
surface water management in many applications. The use of these systems below the 25%
threshold should be encouraged to provide for water quality benefits to area lakes, streams,
wetlands and ponds. The use of these systems should also be encouraged below the 25%
threshold to reduce the burden on existing infrastructure, thereby reducing operations and
maintenance costs and to minimize future system requirements.
Todd Gerhardt
November 28, 2011
Page 9of9
ATTACHMENTS
1. Exhibit 1 — Impervious Surface Coverage.
2. Example Mitigation Plan (3).
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PERMEABLE PAVER CALCULATIONS
Area of Drainage:
Infiltration Rate of Native Soils:
Designed Drawdown Time:
Permeable Paver Storage Available:
Rain Event Rate:
Gallons of Water Generated:
Gallons of Water Infiltrated:
Gallons of Water Sent Offsite:
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