The Benefits of Better Site DesignFeature article from Watershed Protection Techniques. 3(2): 647-656
The Benefits of Better Site Design
in Commercial Development
odern commercial development is domi-
nated by the parking lot. Indeed, as much as
half of the entire surface area of a typical
office park or shopping center is devoted to parking. No
one has ever stepped up to claim that they invented the
parking lot, and their reluctance is understandable: the
parking lot is a prime habitat for the car and not much
else.
From an environmental standpoint, parking lots
rank among the most harmful land uses in any water-
shed. Parking lots not only collect pollutants that are
deposited from the atmosphere, but also accumulate
pollutants that leak, drip or wear off cars. Researchers
have found that parking lot runoff can have extremely
high concentrations of nutrients, trace metals and hy-
drocarbons. Parking lots also influence the local air and
stream temperatures. In the summer months, pavement
temperatures can exceed 120 degrees Fahrenheit, which
in turn increases local air temperatures five to 10 degrees
compared to a shaded forest. Parking lots can also
exacerbate smog problems, as parked cars emit greater
levels of smog precursors under extreme heat island
conditions (Scott et al., 1999).
Perhaps the greatest environmental impact of park-
ing lots is hydrological in nature. Simply put, there is no
otherkind of surface in awatershed that produces more
runoff and delivers it faster than a parking lot. When this
runoff is discharged into a headwater stream, its great
erosive power steadily degrades the quality of down-
stream habitats, unless exceptionally sophisticated
stormwater practices are installed.
Is it possible to design a better parking lot? At first
glance, there seems to be little opportunity to incorpo-
rate better site design into parking lots. However, the
better site design techniques described earlier in this
issue suggest a key design strategy: work to incremen-
tally shrink the surface area of the parking lots and
then use the space saved to integrate.functional land-
scaping and better storniwater treatment within the
parking lot. Through a series of relatively minor design
adjustments, it is possible to reduce the surface area of
parking lots by five to 20%. These design adjustments
include curbing excess parking, incrementally reducing
parking demand ratios, providing credits for mass tran-
sit, shrinking stall sizes, narrowing drive aisles, and
using grid pavers for spillover parking areas.
In this article, we examine some of the benefits of
employing better site design as they apply to commer-
cial development.. As with the residential redesign, this
analysis also uses the Simplified Urban Nutrient Output
Model (SUNOM) to compare actual commercial devel-
opment sites constructed in the 1990s with the same
sites redesigned utilizing better site design techniques.
The two commercial developments analyzed include a
retail shopping center and a commercial office park.
Our fairly conservative approach to parking lot
redesign is intended to reflect realistic opportunities in
a suburban setting. For example, we did not utilize
shared parking, porous pavement, or structured park-
ing in any of the redesigns, although each of these
techniques is very effective. Nor did we reduce the basic
footprint or size of the buildings in either scenario,
although smaller "boxes" may well have been more
appropriate for the zoning. Instead, our basic approach
was to make a series of relatively modest changes in
parking lot design to shrink parking lot area, and then
implement better landscaping and stormwater treat-
ment measures within the saved space.
This article reports on the potential benefits of
parking lot redesign in terms of reduced runoff, pollut-
ant export and development costs. It also reviews the
initial experience of communities that are experimenting
with new and innovative parking lot designs, and con-
cludes with some implications for both the engineer and
watershed manager.
25
Redesign of the Old Farm Shopping Center
The undeveloped Old Farm shopping center, lo-
cated in the City of Frederick, Maryland, was primarily
meadow, with some shrubby forest and a few farm build-
ings. Bordered by two major arterial roads and served by
existing public water and sewer, the site was a prime
candidate forcommercial development (Figure 1).
Construction of the shopping center site parcel
commenced in 1992. The 9.3 acre site is atypical suburban
"strip" shopping center with two large retail stores, other
retail space, a gas station and a drive-in bank (Figure 2).
In terms of surface cover, the shopping center devoted
50% of its total area for parking, as compared to 16% for
the actual footprint of the retail buildings. Another 24%
of the surface area was devoted to landscaping or storm -
water treatment. Less than 10% natural cover was re-
tained on the site, and part of the project encroached on
the 100-yearfloodplain and the stream buffer. The entire
site was mass graded during construction. The basic
layout was designed to accommodate the car, with gen-
erous parking located in front of the stores. The parking
lot design provided 5.2 full-size stalls per 1,000 square
feet (sf) of retail space, which exceeded the already
generous local parking requirement of five spaces per
1,000 sf. According to the most recent national parking
research, only 4.0 to 4.5 spaces are needed to serve
shopping centers (ULI,1999).
The stormwater treatment system at Old Farm con-
sisted of an infiltration basin located near the rear of the
shopping center that captured runoff from about a third
of the site, and three oil grit separators that provide
some treatment for the remaining two-thirds of the site.
After discharging from the oil/grit separators, runoff
traveled through a series of storm drains that extended
along the road and eventually discharged to the stream
(albeit without detention of any kind). It should be
noted that recent performance monitoring has shown
that oil grit separators have little or no pollutant removal
capability (see articles 119 and 120).
The Redesigned Old Farm Shopping Center
The Old Farm shopping center was redesigned
using a "U-shaped" layout that maintained the same
amount of gross floor area, but sharply reduced the site
area devoted to parking (Figure 3). The new design
reduced walking distances, encouraged pedestrian use,
and created a more intimate shopping experience. Park-
ing dropped from 50% of the total site area to 38%,
primarily because the parking .demand ratio was re-
duced from 5.2 spaces to 4.4 spaces per 1,000 sf of retail
area.
The rationale for the lower parking demand was
justified in two ways. First, no extra parking spaces were
allowed beyond those required by the locality. Second,
,4
27
77
Pre- Conventional Parking Innovative Parking Lot
Developed Lot
Runoff no practice
-
24.5
20.6
(incheslyr) practices
2.6
18.1
15.1
Infiltration no practice
2.7
3.4
11.8
9.1
8.9
(incheslyr) practices
the existing parking demand ratio was reduced by about
15 % to reflect actual parking demand more accurately.
As a result, the total number of parking spaces dropped
from 343 to 291. In addition, 17% of the parking stalls
were designed for compact cars, which require slightly
smaller stalls than standard full-sized spaces. Taken
together, these changes eliminated slightly more than
one acre of parking area, which provided enough space
to design a more effective landscaping and stormwater
treatment system.
Several parking lot islands were increased in size
and converted into bioretention areas to treat stormwa-
ter. Other elements of the stormwater treatment system
included a sand filter, an infiltration trench, and a filter
strip. Furthermore, 25 % of the entire parking area was
designated for "spillover parking," and grid pavers
were used rather than normal paving materials. The grid
pavers helped store the first few tenths of an inch of
rainfall that would have otherwise run off the parking lot
(ICPI, 2000). Lastly, the redesign enabled reforestation
and greater protection of the buffer along the stream
that runs along the edge of the property. Asa result, the
proportion of natural cover at the site climbed from 7%
to 19% as a result of the parking lot redesign.
Comparative Hydrology at the Old Farm Shopping
Center
As expected, the construction of the original shop-
ping center dramatically changed the hydrology of the
site (Table 1). The increase in impervious cover from 1 %
to more than 70% increased annual runoff volume by a
factor of nine. The infiltration basin used in the original
design helped put some runoff back into the ground, but
even so, annual runoff was seven times greater than the
pre -development condition. The redesigned parking
lot, by virtue of its lower impervious cover and improved
stormwater practices, produced about 20% less runoff
than the original design. Nevertheless, the stormwater
practices at the redesigned parking lot were not able to
match the pre -development hydrology.
Comparative Nutrient Output from the Old Farm Shop-
ping Center
The conversion of the meadow into a shopping
center greatly increased nutrient export from the site;
the SUNOM model indicated that annual phosphorus
and nitrogen export would increase tenfold as a result
of the development ( see Figure 4 ). Nutrient export from
the shopping center was dominated by stormwater
runoff, as the model indicated that stormwater runoff
contributed about 95% of the annual nutrient export
from the site. Nutrient loads were not greatly reduced by
the infiltration basin or oil/grit separators that were
installed at the conventional parking lot. Nutrientexport
was still projected to be eight to 10 times higher than pre -
development conditions, even after these stormwater
treatment practices were installed.
In contrast, the redesigned parking lot sharply
reduced nutrient export (Figure 4). In fact, the rede-
signed parking lot without stormwater practices pro-
duced about the same nutrient load as the conventional
parking lot with stormwater practices. This reduction
was a direct result of the lower impervious cover asso-
ciated with the redesigned parking lot. When the rede-
signed parking lot was combined with more sophisti-
cated stormwater practices (i.e., bioretention, sand fil-
ter, infiltration trench and filter strip), the total nutrient
export was half that of the conventional parking lot with
stormwater practices. It is interesting to note, however,
that this load was still about five times higher than that
produced by the meadow prior to development.
Comparative Cost to Develop the Old Farm
Shopping Center
The cost to develop the redesigned parking lot was
marginally lower than the cost for the conventional
parking lot — about 5%. Considerable cost savings
wererealized due to less paving, shorter sidewalks, and
fewer curbs and gutters, but these savings were largely
offset by added costs for improved stormwater prac-
tices, landscaping and grid pavers. Overall, the esti-
mated cost to build the conventional parking lot was
$782,500, compared to $746,270 for the redesigned park-
ing lot. The extent of potential cost savings depends
heavily on the level of sophistication of the original
stormwater treatment system. In this case, the unsophis-
ticated stormwater practices used in the conventional
i
50
parking design were fairly inexpensive, but were also not
40
effective in removing nutrients.
30
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Summary
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2 - Conventional Design (no practices)
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Redesigning the 270 Corporate Office Park
The second case study involved the redesign of a
typical suburban office park. The 12.8 acre parcel is
located in Germantown, Maryland in the mildly sloping
terrain of the Piedmont (Figure 6). The existing cover at
the site was almost entirely meadow, except for a few
trees and an old farm pond that bisected the property
boundary. No wetlands or other sensitive natural fea-
tures were evident on the site. The site was zoned for
office development, and existing infrastructure made it
an attractive candidate for development. An existing
network of public water and sewer, electric, gas, and
other utilities ran along the frontage of a large arterial
road.
The layout of the conventional suburban office
park design is depicted in Figure 7. The project included
a pair of five -story office buildings, surrounded by a sea
of parking. Over half (52%) of the surface cover at the
office park was devoted to parking, as compared to only
11% for actual footprint of the office building. Most of
the remainder of the site was utilized for landscaping,
stormwater treatment or turf. Only 2% of the natural
cover was retained on the site, and nearly all of the parcel
was mass graded during construction.
As with many suburban office parks, the location
of the building and parking were primarily oriented
toward the car. The parking lot was sized using a parking
demand ratio of 3.1 spaces per 1,000 sf of building, which
slightly exceeded the minimum parking requirements of
the locality. As aresult, the parking lot created room for
745 standard stalls, along with 331arger stalls for vans
and disabled access. The parking bays also featured
roomy aisles between the stalls (24 feet wide). The
design was intended to provide some amenities for the
office workers, including a short path system between
buildings, an ornamental stormwater pond, and some
landscaping in required setbacks and parking islands.
The conventional design featured the classic "pipe
and pond" approach to stormwater management. Park-
ing lot runoff was initially collected by a curb and gutter
system that sent runoff into underground storm drain
pipes that, in turn, discharged into two very small wet
ponds. Each pond served roughly half of the site and
was expected to have a reasonably good capability to
remove nutrients.
�A
The Redesigned 270 Corporate Office Park
The redesigned site employed a number of tech-
niques to minimize impervious cover and improve storm -
water treatment (Figure 8). The office park featured the
same amount of office space, but the two office towers
were situated closer to the road to shorten utility exten-
sions, and pedestrian access to a bus stop was provided
to encourage the use of public transportation.
The key strategy employed in the redesign was to
incrementally reduce the size of the parking lot, and this
was achieved in five ways. First, no excess parking spaces
were allowed over those required by the local parking
demand ratio. Second, the local parking demand ratio was
reduced by 8% to reflect actual parking demand. Third,
the parking demand ratio was reduced by another 10% to
reflect the proximity to the bus stop. Fourth, the size of
approximately 20% of allparking stalls was downsized to
accommodate compact cars. Lastly, drive aisles in many
parkingbays werereduced from24 feet in width to 20 feet.
Combined, these measures reduced the total parking lot
area by nearly 30%, or about two acres. Once again, the
savings in paving gave the designer more room to inte-
grate landscaping with more effective stormwater treat-
ment.
For example, larger landscaping islands were in-
stalled in the parking lot to plant shade trees, and some
of these areas were also converted into bioretention areas
to treat stormwater. A dry swale was used to treat storm -
water within a landscaped setback area in another part of
the site. About 15% of the lot was designated for spillover
parking, and grid pavers were used to attenuate runoff in
this area. The basic stormwater management goal was to
attenuate, treat, or recharge as much runoff from smaller
storms as possible in the parking lot itself. Runoff from
larger storms was treated in a wet detention pond near the
outlet of the property.
As a result of the redesign, roughly 14% of the office
park was eitherretained in natural land cover orreforested
(compared to 2% under the conventional design). This
green space, combined with the water features and a
walking path, created a more tranquil environment for
office workers. Overall, the total impervious area associ-
ated with the redesigned office park dropped from 68% to
53%.
Comparative Hydrology, for the-2 70 Corporate Center
Office Park
The hydrological story was much the same for the
270 Corporate Center as for the shopping center. Con-
struction of the conventional design sharply increased
annual runoff volumes and decreased infiltration (Table
2). Runoff did not increase as much in the redesigned
parking lot, primarily because its impervious cover was
much lower. Annual runoff volumes were 21 % lower in
the redesigned parking lot compared to the conven-
tional design, and infiltration volumes were42%higher.
Despite these improvements, the redesigned parking
lot was unable to mimic the hydrologic conditions prior
to development.
Nutrient Output at the 270 Corporate Center Office
Park
As expected, the conversion of the meadow into
an office park greatly increased nutrient export. Annual
phosphorus and nitrogen export increased roughly ten-
fold, according to the SUNOM model (Figure 9). Aswith
the shopping center, stormwater runoff was found to
generate about 95 % of the annual nutrient export from
the site. The two wet ponds were reasonably effective
in removing nutrients at the conventional office park,
but still resulted in nutrient export that was seven to
eight times higher than pre -development conditions. In
contrast, the redesigned parking lot sharply reduced
nutrient export (Figure 9). The combination of lower
impervious cover and more effective stormwater prac-
tices reduced nutrient export by about 40 to 50%, when
compared to the conventional parking lot design with
stormwater practices.
Hydrologic Factor Pre- Conventional Redesigned Parking Lot
Developed Parking Lot
Runoff (incheslyr) 2.7 23.9 18.9
Infiltration (incheslyr) 11.8 2.6 3.7
Note: no change In the annual volume of runoff or infiltration was calculated as a result of the stormwater
practices installed at either the conventional or redesigned parking lot.
five or ten percent. For many communities, however,
this modest step may seem like a terrifying leap, possi-
bly off a cliff.
Developers, bankers, retailers and drivers all have
a shared interest in abundant and convenient parking,
and it is hard to convince them that any attempt to
downsize parking lots, however modest, will not work
against this goal. This kind of thinking is quite under-
standable. Most people can easily recall the rare situ-
ation where parking was hard to find, but the more
common situation where parking is plentiful generally
escapes our everyday notice.
Small wonder, then, that so many communities
are prone to inertia when it comes to changing parking
codes. Perhaps the only way watershed advocates can
overcome this inertia is to document the existence of
excess parking capacity in each community. Indeed, it
is a rather simple step for volunteers to count cars and
photograph empty stalls during peak times at similar
commercial land uses to demonstrate how generous
local parking requirements actually are.
A small but growing list of communities are now
experimenting with their parking standards and parking
lot designs, including cities like Scarborough, Ontario;
Oakland, CA; Olympia, WA; Sacramento, CA; Bellevue,
WA; Davis, CA and Prince George's County, MD. Each
community has worked in different ways to redesign
their parking lots, and many of their successful experi-
ences are recounted in Better Site Design: A Handbook
for Changing Development Rules in Your Community
(CWP,1998a).
Given the prevalence of parking lots in our urban
landscape and the environmental harm they cause, we
need to fundamentally change the way that parking lots
are sized and designed. The modest ideas presented in
this article are merely an initial step in this direction. A
wide range of professions collectively influence the
form and function of parking lots, including engineers,
hydrologists, landscape architects, urban foresters,
soil scientists, developers, leasing agents, plan review-
ers, transportation researchers and many, many others.
Working together, these groups can move us closer
toward the goal of a truly sustainable parking lot, i.e.,
onethat not only provides carhabitat, but also prevents
damage to other habitats, as well. - JAZ
References
Center for Watershed Protection. 1998a. Better Site
Design: A Handbook_ fir Changing Development
Rules in Your Community. Site Planning Round-
table. Ellicott City, MD.176 pp.
Center for Watershed Protection.1998b. Nutrientload-
ing From Conventional and Innovative Site De-
velopment. Chesapeake Research Consortium,
Ellicott City, MD. 56 pp.
Interlocking Concrete Pavement Institute. 2000. Perme-
able Interlocking Concrete Pavements: Selec-
tion, Design, Construction and Maintenance.
Washington, DC.
Scott, Klaus I., James R. Simpson, and E. Gregory
McPherson.1999. "Effects of Tree Cover on Park-
ing Lot Microclimate and Vehicle Emissions."
Journal q fArboriculture 25(3):129-142.
Urban Land Institute. 1999. Parking Requirements_ fir
Shopping Centers: Summary Recommendations
and Research Study Reports. 2nd Edition. Wash-
ington, DC. 81 pp.