URN:NBN:fi:tsv-oa7794
DOI: 10.11143/7794
How does imperviousness develop and affect runoff generation in an
urbanizing watershed?
GERALD KREBS, ULLA-MAIJA RIMPILÄINEN AND OUTI SALMINEN
Krebs, Gerald, Ulla-Maija Rimpiläinen & Outi Salminen (2013). How does im-
perviousness develop and affect runoff generation in an urbanizing watershed?
Fennia 191: 2, pp. 143−159. ISSN 1798-5617.
Imperviousness associated with urbanization remains one of the biggest chal-
lenges in sustainable urban design. The replacement of forests, marshlands, buff-
ers, and wetlands with impervious surfaces, strongly influences hydrological
processes in urbanizing areas. This study analyzes the contribution of four con-
structed surfaces types – roofs, yards, roads, and an international airport – to
surface runoff within a 21 km2 watershed, and presents the development over
five decades (1977−2030). The land-cover model, used to assess watershed im-
perviousness in 2030, utilizes coefficients between impervious areas generating
surface runoff and the floor area, developed during the study. The conducted
imperviousness analysis allows the evaluation of land-use development impacts
on the stream network, and the identification of hydrologically active areas for
urban planning and stormwater management. Research reveals the importance
of yard imperviousness related to suburban residential housing for stormwater
runoff generation, and the impacts of transport-related imperviousness on storm-
water runoff.
Keywords: urban planning, land-use, imperviousness, stormwater runoff, imper-
viousness-components
Gerald Krebs, Aalto University School of Engineering, P.O.Box 15300, FI-00076
Aalto, Finland. E-mail: gerald.krebs@aalto.fi
Ulla-Maija Rimpiläinen, City of Vantaa, Kielotie 28, 01300 Vantaa, Finland and
Aalto University School of Arts, P.O.Box 11300, FI-00076 Aalto, Finland. E-mail:
ulla-maija.rimpilainen@vantaa.fi
Outi Salminen, University of Helsinki, Department of Forest Sciences, P.O.Box 27,
FI-00014 University of Helsinki, Finland. E-mail: outi.m.salminen@helsinki.fi
Introduction
In the year 2000, 70.8% of the European popula-
tion lived in urban conditions and the percentage
will increase to 77.4% in 2030. Development in
Finland follows the pattern of Western Europe;
82.2% of the population lived in urban areas in the
year 2000 and about 86.2% will in 2030 (United
Nations 2012). Urban development is associated
with the construction of building facilities and infra-
structure to inhabit enlarging future population,
leading to higher levels of impervious land cover.
The fraction of impervious cover within the gross
watershed area is defined as the degree of impervi-
ousness. Grimm et al. (2008) note that impervious-
ness is among the most important land cover modi-
fications affecting stream condition in urban areas.
Land cover modifications alter the water-
shed hydrology and affect both, surface and
subsurface water bodies. These modifications
are seen in a decrease of forested stream buffer
and wetland areas that provide capacity for re-
tention and purification (Brabec et al. 2002).
The impacts on water bodies include increased
stress on stream hydrology, channel morphol-
ogy, water quality, and stream ecology im-
posed by changes in stormwater quality and
quantity (Brabec 2009), and changes in the
eco-hydrological diversity of watersheds
(Booth & Jackson 1997). Further impacts in-
clude reduced infiltration, increased surface
runoff, higher peak discharge in streams,
shorter travel time, and more severe pollutant
loads influencing both surface water quantity
144 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
and quality (Lee & Heaney 2003; Beighley et
al. 2009). Stream water quality is deteriorating
as direct urban runoff transports pollutants
from contaminated surfaces (such as roads,
parking lots, roofs, urban parks, and recrea-
tional areas) into aquatic systems, without ad-
equate filtration (Bannerman et al. 1993; Ruth
2003). Besides surface waters, imperviousness
also affects groundwater bodies. Impacts in-
clude problems in water supply and reduced
low flow volumes in streams, caused by a re-
duction of the groundwater recharge rate (Ar-
nold & Gibbons 1996; Haase 2009).
The impacts of land cover modifications on
water bodies can be assessed using the degree
of imperviousness. It is an important indicator
of stream health (Schueler 1994), and can be
used as an index to address complex urban en-
vironmental issues (Arnold & Gibbons 1996).
Even though threshold values are always con-
troversial (Brabec 2009), Schueler (1994) con-
cludes in a review of several stream degrada-
tion studies that they produce a fairly consist-
ent result – stream degradation already occurs
at levels of low imperviousness (10%−20%),
and becomes unavoidable at 20%−30% im-
pervious land cover (Arnold & Gibbons 1996).
Haase (2009) reported in a long-term water
balance study for the city of Leipzig that direct
runoff doubles at a degree of imperviousness
around 20% compared to unsurfaced land.
Furthermore, evapotranspiration rates clearly
declined and groundwater recharge slightly re-
duced at the same level of imperviousness.
Imperviousness can be subdivided into a
natural and an artificial component. Natural
imperviousness considers areas with elevated
surface runoff due to varying soil types, sur-
face condition and vegetation, whereas artifi-
cial or manmade imperviousness refers to are-
as where changes in stormwater runoff quality
and quantity are caused by human activities.
This manmade imperviousness can be subdi-
vided into three components: 1) roof surfaces,
2) road surfaces, and 3) impervious surfaces
on yards including terraces, parking areas, and
driveways from different land-use types exist-
ing in the watershed. Additionally, in this
study, the development of the imperviousness
generated by the Helsinki-Vantaa airport was
assessed separately as a fourth component due
to the uniqueness of this land-use type. Com-
mon land-use assessments relate the percent-
age of imperviousness with a land-use type.
However, there are no standardized methods
for deriving these relationships; therefore,
there is typically a high variation in the frac-
tion of impervious area within the same land-
use type (Canters et al. 2006). Direct methods
of land-use assessment, including ground sur-
veys, interpretation of large scale aerial photo-
graphs or satellite imagery (Beighley et al.
2009) were developed to overcome these in-
accuracies. In practice, the direct methods are
time consuming and costly, and can usually
only be applied in small watersheds (Canters
et al. 2006).
The objective of this research was to analyze
the contribution of different impervious sur-
face components and their properties, both, to
imperviousness and stormwater runoff genera-
tion within the watershed of the Kylmäoja
stream in Southern Finland. This analysis con-
sists of an assessment of the past land-use de-
velopment between 1977 and 2007 and an
imperviousness prediction for 2030. The fore-
cast of imperviousness is difficult and implies
numerous assumptions, such as future build-
ing and population density, building heights,
and the distribution of residential and com-
mercial areas. In this study, the land-use and
population data from 1977−2007 was ana-
lysed to assess the watershed imperviousness
development. The same data was used to de-
velop coefficients between floor-, roof-, and
yard areas. These coefficients were then used
to create a connection between future build-
ing densities, floor areas, and impervious sur-
faces in 2030. The hydrological impacts of the
imperviousness development were evaluated
adopting a categorization scheme from Schuel-
er (1994) for ultimate imperviousness, and
runoff calculation using the rational method.
Stormwater runoff in this study refers to runoff
generated on constructed impervious surfaces
in urban areas. The stormwater runoff does not
include runoff generated in natural impervious
areas outside the constructed areas. The ana-
lyzed components included the common ele-
ments of urban development (Schueler 2000),
such as rooftops, yard areas, and roads from
various land-use types (residential, commer-
cial, and industrial areas), as well as the Hel-
sinki-Vantaa airport. Three mitigation scenari-
os are presented and their efficiency evaluated
for the study catchment.
FENNIA 191: 2 (2013) 145How does imperviousness develop and affect runoff ...
Methods and materials
Site description
The research site was the Kylmäoja watershed
(Fig. 1) located north of Helsinki in Finland,
and shared between the city of Vantaa and the
municipality of Tuusula. Both, Vantaa and Tuu-
sula are part of the Greater Helsinki Metropoli-
tan Region with about one million inhabitants
(Population Register Centre 2011). The water-
shed area is 20.83 km², with about 75% or
15.71 km² located in the city of Vantaa, and
25% or 5.12 km² in the municipality of Tuusula
(Krebs 2009). The watershed is dominated by
suburban residential land-use with scattered
small industrial areas. Furthermore, parts of the
Helsinki-Vantaa airport are located within the
watershed (Fig. 1). The urban areas (Tikkurila
and Ruskeasanta) located in the southern part
of the watershed, and the surfaces of the Hel-
sinki-Vantaa airport (opened for traffic in 1952,
FINAVIA 2012) in the northeast, were mostly
constructed before the first year (1977) of the
land-use assessment conducted in this study.
The northern areas in Vantaa (residential areas
such as Ilola in the beginning of the 1990s) and
Tuusula (focus on industrial development since
the 1990s) were developed more recently, and
are still subject to major construction activities
(Vantaan kaupunki 2008; Tuusulan kunta
2009). The selected watershed represents a typ-
ical sub-urban watershed in Southern Finland,
with a strong development of residential hous-
ing on the one hand and fast industrial devel-
opment due to good traffic connections on the
other hand.
The stream network consists of four branches
– eastern, central and western headwaters join-
Fig. 1. Aerial image of the Kylmäoja watershed, located in Vantaa and Tuusula, Southern Finland. The map shows the most
urbanized areas in the southern watershed area (Tikkurila and Ruskeasanta), residential areas like Ilola in the centre of the
watershed area, the Helsinki-Vantaa airport in the west, and important future developments like Leinelä (Vantaa) and the
Business Park FOCUS (Tuusula). The stream network of Kylmäoja and the border between Vantaa and Tuusula are also shown
(Modified, original Vantaan kaupunki).
146 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
ing the main branch. The entire watershed was
divided into eleven subcatchments according
to the stream network, with three subcatch-
ments being transboundary, shared by the city
of Vantaa and the municipality of Tuusula (Fig.
2). The division into subcatchments can guide
city planners in Vantaa and Tuusula to set fo-
cuses and objectives for urban planning and
stormwater management by defining specific
areas of importance within the catchment area.
Land-use analysis
The watershed and stream network were delineat-
ed with the software Geoinformatica (Jolma 2007)
using the D8 method (Fairfield & Leymarie 1991)
on a 25 m-raster digital elevation model (DEM).
The actual stream network location was deter-
mined with aerial images produced in the year
2007; differences between the actual and mod-
elled stream network were corrected accordingly.
The catchment and subcatchment borders were
modified to correspond with the existing stormwa-
ter sewer network. The conducted delineation was
based on data from the year 2007 and thus repre-
sents the catchment borders and stream network at
this time point. The results of the delineation were
used for the entire time period assessed in the
analysis.
Spatial analysis focused on the distribution of
impervious surfaces of different land-use types
(such as residential, commercial, and industrial ar-
eas) within the watershed to assess the develop-
ment of ultimate imperviousness (UI) within the
Kylmäoja watershed between 1977 and 2030;
therefore, the development of the roof area, the
road surface area, the yard imperviousness, and
the area of the Helsinki-Vantaa airport was investi-
gated over the study period. Both, the size and the
quality of these surfaces were evaluated based on
Fig. 2. The eleven subcatchments defined during this work. The subcatchments 1, 2, and 6 are transboundary subcatchments
shared by Tuusula and Vantaa. The effects of the conventional sewer pipe network on the catchment are also shown. Grey
areas belong to the topographical catchment but are drained into nearby streams; black areas are not part of the topograph-
ical watershed but are piped into the Kylmäoja stream. The spatial properties of the watershed and the subcatchments are
presented in the table on the right hand side (size, area located in Vantaa and Tuusula, and fraction of the entire watershed).
FENNIA 191: 2 (2013) 147How does imperviousness develop and affect runoff ...
housing and road datasets in Vantaa, aerial pho-
tography, and historical maps.
The land-use assessment was conducted for a
period of five decades from 1977 to 2007, and fur-
ther on to 2030. The analysis from 1977 until 2007
is based on existing construction. Roof and imper-
vious yard areas were derived from aerial photog-
raphy; and floor areas and plot-ratios (maximum
permitted floor area divided by the plot area) were
compiled from building permit archives. The fore-
cast for 2030 is based on zoning plans, providing
information about plot-ratios to define building
density in the future.
Forecast of imperviousness requires a link be-
tween impervious surface area development and
parameters of urban growth, such as future build-
ing or population density. Few methodologies are
available to predict imperviousness (creating a link
between imperviousness and population density
or land-use, respectively) and generally have
shortcomings concerning small areas (Reilly et al.
2004). Therefore, in this study, coefficients were
chosen to link the floor area of a building (used in
urban planning) with the building roof- and yard
area (relevant for runoff) to predict imperviousness
development in small urban scale. This approach
allows for consideration of local peculiarities in
suburban dwellings, as the coefficients were de-
veloped for the specific study catchment.
Coefficients between the stormwater relevant
variables (rooftops and yard imperviousness) and
zoning variables (such as the floor and living area)
were developed for the period 1977−2007 (Eq.
1−3). The coefficients were developed for each
subcatchment, each observed year, and each
building type defined by the city of Vantaa. Con-
sideration of different areas, time periods and
building types accounted for dwelling develop-
ments, and changes in architectural style within
the study area.
The developed coefficients in Vantaa differed
from the coefficients in Tuusula, due to the varia-
tion in available data. The coefficients developed
in this work are the relationship between roof and
floor areas for Vantaa (used also for Tuusula)
(1)
the relationship between impervious yard and
floor areas for Vantaa
(2)
and the relationship between impervious yard and
roof areas for Tuusula
(3)
where AR [m
2] is the roof area, AF [m
2] is the floor
area, and AY [m
2] is the yard area.
The above coefficients were derived using spa-
tial data for the time period before 2007. As there
was no data on floor area for existing buildings in
Tuusula for 1977−2007 available, the coefficient
derived for subcatchments with similar structure in
Vantaa (CR Vantaa) was used also for Tuusula sub-
catchments. Furthermore, for the same reason, the
impervious yard area was put in relation to the
roof area, obtained directly from aerial images, in-
stead of the floor area (CY Tuusula). While the selected
approach hinders the direct comparison of devel-
oped coefficients between the cities, it does not
affect the prediction of impervious yard areas in
Tuusula for 2030 and the comparison of impervi-
ousness development between the cities.
These coefficients were implemented to derive
impervious surfaces from the expected floor area
in 2030. The future development of floor area
(based on plot-ratios) was adopted from the pro-
jections made by the city of Vantaa, as the building
density within the watershed in 2030 is expected
to be less than the maximum allowed density.
Three different site types exist in the current devel-
opment plans: undeveloped areas, fill-in sites, and
sites where the plot-ratio utilization can be opti-
mized. Undeveloped areas have defined densities
in the plans but the plot-ratios are not yet defined.
Thus, this type of development involves the largest
uncertainties in future floor area forecasts. On the
other hand, the time of realization in the currently
undeveloped areas is rather clearly defined. Fill-in
sites are empty plots within already developed ar-
eas, for which the plot-ratio and hence maximum
floor area defined. Estimates for the future floor
area are rather certain, as due to the high real es-
tate prices in the capital area of Finland (Suomen
virallinen tilasto 2010), these sites can be assumed
to be used to the maximum plot-ratio defined.
Sites for plot-ratio optimization are sites where the
existing buildings have a smaller floor area than
would be allowed by the authorities. Whereas it
can be assumed that the maximum plot-ratio will
be utilized at some future point (driven by real es-
tate prices), the actual time of realization involves
uncertainties, as it is often driven by ownership
changes.
148 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
Ultimate imperviousness
Ultimate imperviousness (UI) is the sum of the im-
perviousness generated by the categories assessed
in the land-use analysis. Those were the road and
roof areas, impervious yard areas, and the Helsin-
ki-Vantaa airport. UI considers constructed imper-
viousness and does not include naturally impervi-
ous surfaces such as rock formations. The effects of
land-use changes on the Kylmäoja stream were
evaluated using the determined ultimate impervi-
ousness (UI) as an indicator of stream health
(Schueler 1994). UI is used both as an indicator
and as a guideline criterion for stormwater man-
agement practices and objectives. UI categoriza-
tion follows the threshold values suggested by
Schueler (1994), defined as sensitive streams (wa-
tershed imperviousness 0−10%), impacted streams
(watershed imperviousness 11−25%), and non-
supporting streams (watershed imperviousness
>25%). No such classification exists for river ba-
sins in Finland, whereas the Schueler classification
is based on a large number (18) of catchment-
stream relationship studies carried out across the
USA for various catchment types and scales.
Stream condition in Schueler’s review (1994) con-
sidered stream parameters such as channel stabil-
ity, water quality, and stream diversity as well as
catchment parameters, such as land-use controls,
development rights, and the condition of buffer
zones.
Runoff generation analysis
Runoff generation calculation considered different
surface materials and adopted the values for runoff
coefficients from German (ATV-DVWK-REGEL-
WERK 2000) and Finnish (RIL 2004) manuals.
Runoff generation used the rational method and
runoff coefficients characterizing different surface
types (Eq. 4).
(4)
where Q [l s-1] is the discharge, Ai [ha] is the area
of contributing surface types, q [l s-1 ha-1] is the
rainfall intensity, ci [-] is the runoff coefficient of
contributing surface types, and i [-] is the number
of different surface types within the watershed.
The runoff generation analysis focused on
changes in the peak of a design runoff event fol-
lowing modifications in catchment impervious-
ness. The design peak runoff is calculated using
Eq. (4), and it is induced by a 15 min design rain-
fall intensity that occurs once in 50 years. The de-
sign rainfall intensity is adopted from Kilpeläinen
(2006), who summarized precipitation statistics
from Helsinki-Kaisaniemi between 1951 and
2000, and reported that a 15 min rain event with a
return period of 50 years generates a precipitation
of 255 [l s-1 ha-1]. The selected rainfall intensity was
used for all simulations conducted in this study.
Roof areas were assigned with a runoff coeffi-
cient of 0.9 (Kibler 1982; ATV-DVWK-REGEL-
WERK 2000; RIL 2004). Road surfaces made of
asphalt (almost 98% of the roads within the Ky-
lmäoja watershed were asphalt-paved in 2007)
were assigned with a runoff coefficient of 0.95 and
a value of 0.60 characterized the minor share of
public gravel covered roads (ATV-DVWK-REGEL-
WERK 2000). Yard surfaces are more variable and
differ depending on the building type. Aerial im-
ages and in-situ study showed that in low density
residential areas around half of the yard surfaces
were covered with asphalt and half with gravel,
described by a runoff coefficient of 0.63. In medi-
um dense residential development areas the as-
phalt-gravel ratio was found to be 80−20%, de-
scribed by a runoff coefficient of 0.82. Yard areas
associated with apartment blocks, industrial build-
ings, and other service facilities had mainly as-
phalt covered yard areas with an assigned runoff
coefficient of 0.95 (ibid.). Asphalt areas associated
with the Helsinki-Vantaa airport were assigned the
same runoff coefficient (0.95) as the bituminous
road surfaces.
Stormwater runoff was simulated for the ana-
lyzed land-use in the Kylmäoja watershed for the
years 1977, 1992, and 2007. Furthermore, the
stormwater runoff was estimated for the year 2030
in four scenarios to evaluate impacts of current
plans as well as the potential of three mitigation
approaches. All four scenarios for 2030 consid-
ered construction development within the water-
shed between 2008 and 2030. For scenario 1, roof
and impervious yard area of new buildings were
calculated using the developed relationships (CR
Vantaa, CY Vantaa, and CY Tuusula).
The U.S. Environmental Protection Agency
(2006) concluded in a report that higher-density
development creates less runoff, and impacts the
watershed less than low-density development at
all scales for a given development. Generated run-
off from a building depends on the contributing
roof area; therefore, a higher structure – with sev-
eral floor levels – has a smaller footprint than a
∑
FENNIA 191: 2 (2013) 149How does imperviousness develop and affect runoff ...
building with less floor levels, and thus, generates
less runoff due to the smaller roof area. Thus, for
scenario 2, an increase in building height – the
floor area being unaffected – was suggested and
therefore used a lower coefficient CR Vantaa.
Scenario 3 evaluated the effects of more pervi-
ous surface material use for yard areas, simulated
by a runoff coefficient variation. Depending on the
land-use type, the proportion of impervious mate-
rials used on yards was reduced by 20% (instead
of asphalt, the utilisation of gravel, wide tiles, al-
ternative pavers or vegetated areas was supported)
to evaluate the potential reduction in runoff for
2030. While for scenario 1, the yard runoff coeffi-
cient for detached houses and low-rise housing
areas was set to 0.63 the reduction of impervious
materials by 20% (80% of the area pervious mate-
rials) results in a runoff coefficient of 0.43. For row
houses the share of asphalt was reduced to 50%,
with a runoff coefficient of 0.63 (0.82 in scenario
1). The share of asphalt in yard areas for offices
and apartment blocks was reduced to 80% (from
100% in scenario 1), resulting in a runoff coeffi-
cient of 0.82.
Scenario 4 assumed that 10% of buildings in
the Kylmäoja watershed would have an extensive
green roof installed by the year 2030. While sce-
narios 1−3 were calculated for the study water-
shed, scenario 4 uses estimates based on a green
roof runoff reduction study conducted in Brussels
(Mentens et al. 2006). They used regression equa-
tions and concluded a potential runoff reduction
of 2.7% for the city of Brussels for the assumption
of 10% of the roofs receiving an extensive green
roof.
Results and discussion
Relationships of area types
The coefficients between the constructed roof area
and the floor area (CR Vantaa), derived for buildings
constructed between 1992 and 2007 are shown in
Table 1. The coefficients are presented for four cat-
egories of building types: 1) Low-rise residential
(detached and row housing), 2) high-rise residen-
tial area (apartment blocks), 3) service areas
(meant for public and private facilities such as
schools, daycare but also road administration and
water supply), and 4) workplace areas (including
office buildings as well as industrial production).
CR Vantaa for low-rise residential areas varied from
0.69 to 0.92 (i.e., a floor area of 100 m2 is associ-
ated with roof area of 69−92 m2). The correspond-
ing coefficients in high-rise residential areas found
in five subcatchments are lower and range from
0.37 to 0.52. The coefficients for service areas
were found to be equal to the ratios for high-rise
residential areas. The coefficients (0.80−0.81) for
workplace areas show little variation, except for
office areas found in subcatchment 2 (0.62) (Table
1).
Both coefficients concerning yard impervious-
ness (CY Vantaa (Eq. 2) and CY Tuusula (Eq. 3)) show far
greater variation within and between the catego-
ries. In low-rise residential areas in Vantaa the co-
efficients found range from 0.33 to 0.95, i.e., an
impervious yard area of 33−95 m2 was related to
100 m2 of floor area. Apartment block coefficients
in Vantaa range from 0.52 to 0.88 and service ar-
eas have an impervious yard area of 52−69 m2
connected to 100 m2 of floor area. The largest var-
iation in coefficients was found for industrial pro-
duction and maintenance areas, where values in
Vantaa reach from 0.62 to 2.52. Large yard imper-
viousness connected to low floor area was found
for retail parks, logistics and transportation com-
panies, all three having maximum two-storey-
buildings and hence little floor area compared to
roof area and large asphalt areas for parking, ma-
neuvering, and exhibition (Table 1). While the
yard imperviousness coefficients for Tuusula had
to be related to the roof area rather than the floor
area (as in Vantaa) they follow a similar pattern.
The coefficient found for low-density residential
areas was with 0.78 significantly lower than found
for industrial areas in Tuusula (1.72−2.15). Both
apartment blocks and service areas do not exist in
the Tuusula areas of the watershed.
Watershed Imperviousness
UI was used to assess the impacts of land-use
changes on the Kylmäoja stream. UI is presented
as a percentage for each of the eleven subcatch-
ments as well as for the entire Kylmäoja watershed
for the years 1977, 1992, 2007, and 2030 (Fig. 3).
The classification thresholds are based on the
study review conducted by Schueler (1994). This
classification scheme was applied in the Kylmäoja
watershed, because there are no water quality data
available from the stream to directly compare im-
perviousness analysis results to actual stream
health. Even though the effects of imperviousness
150 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
on stream health are affected by site and climate
conditions, Schueler’s classification (1994) is
based on a wide range of studies (18) conducted
under varying climate conditions and thus allows
conclusions on stream condition and urban devel-
opment impacts on streams. Furthermore, it pro-
vides guidelines for urban planning concerning
the future development. At the beginning year of
analysis in 1977, the highest degree of impervi-
ousness was found for the subcatchments in south-
ern Vantaa and around the Helsinki-Vantaa air-
port. According to Schueler’s (1994) categoriza-
tion, the identified imperviousness implies a re-
duction in channel stability, water quality, and
stream diversity. The resource objective for this
stream condition is shifted from protection to
maintenance to achieve a better ecological condi-
tion. The six remaining subcatchments had imper-
viousness rates below the threshold value of 10%,
defining these sections of Kylmäoja as healthy with
stable channels and beds, good water quality, and
stream diversity. The airport subcatchment showed
little change with an increase of 2% from 1977 to
1992 whereas residential development effects can
be seen in the imperviousness of the subcatch-
ments 7 and 9. Two subcatchments remained be-
low the threshold of 10% in 1992, while the two
most urban subcatchments 10 and 11 exceeded
the threshold of 25%. This degree of impervious-
ness results in poor channel stability, water quality,
and biodiversity, and moves the resource objec-
tives from maintenance of the affected section to
minimization of pollutant loads reaching down-
stream waters. In 2007 five subcatchments ex-
ceeded the threshold of 25% located within the
southern areas of the Kylmäoja watershed. Clear
Table 1. CR Vantaa, CY Vantaa, and CY Tuusula for the eleven subcatchments defined for the construction period 1992−2007.
A0
Detached house,
row house, low-
rise residential
area
Ak
Apartment
blocks, high-rise
residential area
P
Area for services
T
Industrial
buildings, office
buildings, work
place area
CR CY CR CY CR CY CR CY
1 (Vantaa) 0.82 0.78 0.52 0.88 0.80 0.87
1 (Tuusula) 0.82 0.78 0.80 2.15
2 (Vantaa) 0.92 0.92 0.62 0.62
2 (Tuusula) 0.82 0.78 0.80 1.72
3 (Vantaa) 0.78 0.71 0.81 2.52
4 (Vantaa) 0.78 0.71 0.80 0.87
5 (Vantaa) 0.72 0.68 0.39 0.67 0.80 0.87
6 (Vantaa) 0.91 0.32 0.52 0.88 0.80 0.87
6 (Tuusula) 0.82 0.78 0.80 2.14
7 (Vantaa) 0.84 0.79 0.80 0.87
8 (Vantaa) 0.69 0.33 0.80 0.87
9 (Vantaa) 0.79 0.95
10(Vantaa)
()()(Vantaa
)
0.77 0.72 0.37 0.69 0.37 0.69 0.80 0.87
11(Vantaa)
(Vantaa)
0.72 0.68 0.37 0.52 0.37 0.52 0.80 0.87
FENNIA 191: 2 (2013) 151How does imperviousness develop and affect runoff ...
increase is also seen in the earlier rather undevel-
oped subcatchments 1 and 6, which drain into the
central and eastern headwaters of Kylmäoja. The
forecast for 2030 expects seven subcatchments to
exceed the threshold value for a non-supporting
stream, and five are classified to be impacted. No
subcatchment within the Kylmäoja catchment will
be sensitive anymore, in other words all sections
and the entire stream will be exposed to degrada-
tion at some level.
UI for the entire Kylmäoja watershed shows a
permanent growth due to land-cover change from
9% in 1977 to 19% in 2007. In 2030 UI is ex-
pected to reach 26%. While the link between fu-
ture building density and future impervious sur-
faces was developed based on catchment data
from 1977 to 2007, the forecast is based on future
projections and involves uncertainties concerning
the actual building density in the watershed by
2030. These uncertainties are induced by factors
such as the political and economic environment
both within the communities sharing parts of the
Kylmäoja watershed as also neighbouring cities
and municipalities. Besides the level of watershed
imperviousness, another important indicator for
the effects of land-use on the quality of streams is
Fig. 3. Imperviousness in the watershed 1977−2030. Whereas in 1977 six stream sections (and the whole stream) were
sensitive and five sections impacted, in 2007 only one section was sensitive, five impacted, and five non-supporting. In 2030
four sections will be impacted and seven non-supporting (and the whole stream). No part of Kylmäoja will be sensitive any-
more. Imperviousness of the entire watershed is shown above the observation year.
152 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
the distance of impervious areas from the aquatic
system (Brabec 2009). This criterion was not inves-
tigated during this work, but would be an impor-
tant indicator of stream health for further studies
on this watershed.
The EU Water Framework Directive (European
Parliament and Council 2000) defines the good
ecological status for water bodies as the target ob-
jective, and demands water resources manage-
ment at river basin scale. As watershed borders are
determined by topographical conditions, the wa-
tershed scale approach requires cooperation of
authorities (reaching from regional to international
scale) in transboundary basins. UI was therefore
evaluated separately for the three transboundary
subcatchments of the Kylmäoja watershed. While
the subcatchment areas in Vantaa are mainly dom-
inated by suburban residential developments, are-
as in Tuusula are in focus of industrial develop-
ment, also due to residential building restraints
north of the Helsinki-Vantaa airport.
Besides the Vantaa part of subcatchment 2, due
to the presence of the Helsinki-Vantaa airport
since 1952 (FINAVIA 2012), the transboundary
subcatchments had little degree of imperviousness
in 1977, ranging between 0−5%. In subcatchment
2, the degree of imperviousness will be slightly
higher within the Tuusula areas (25%) than within
the Vantaa areas (23%) in 2030. In subcatchment
6, imperviousness in Tuusula will yield at 21%,
whereas in Vantaa it will be 12%. The largest dif-
ference in imperviousness development was found
for subcatchment 1. Already in 2007 the impervi-
ousness in Tuusula was 30% compared to 14%
within the residential areas on the Vantaa side of
the border. This difference will enlarge further in
2030, with 43% of imperviousness in Tuusula and
22% of imperviousness in Vantaa by then (Fig. 4).
The high degree of imperviousness within the
Tuusula part of the watershed is especially influen-
tial on stream health, as the areas drain into the
headwaters of Kylmäoja. Consequently, all physi-
cal, biological and chemical impacts are effective
for the entire stream length. Furthermore in sub-
catchment 6, the city of Vantaa has established a
nature reserve of 11.3 ha (Kylmäojan korpi) in
Fig. 4. The maps show the de-
velopment of imperviousness
for the three transboundary
subcatchments 1977−2030.
FENNIA 191: 2 (2013) 153How does imperviousness develop and affect runoff ...
2002 (Rantalainen 2004) south of the municipal
border, receiving the increasing stormwater runoff
from industrial areas in Kulomäki located just
north of the border. The results also show that co-
operation between municipalities concerning
stormwater management is not sufficient yet and
has to be improved to meet the requirements of the
EU Water Framework Directive.
Both the network of the Kylmäoja stream and
the catchment and subcatchments were delineat-
ed based solely on data from the year 2007. Com-
parisons of the results with aerial images taken
prior the year 2007 showed that the stream net-
work and the total catchment boundaries were not
affected by the development of new areas within
the Kylmäoja watershed. However, the possible
impact of new developments 1977−2007 on sub-
catchment boundaries prior 2007 possibly affects
the calculated degree of imperviousness due to
extensions of the stormwater sewer network.
Stormwater runoff generation on impervious
areas – Scenario 1
The stormwater runoff was calculated using the ra-
tional formula (Eq. 4) with source areas derived
during the spatial analysis, runoff coefficients and
a 15 min rain with an intensity of 255 [l s-1 ha-1].
Between 1977 and 2007, the lowest increase in
generated stormwater runoff was found in sub-
catchment 2, which includes the main parts of the
Helsinki-Vantaa airport, and in subcatchment 4,
which is located south of the airport. As stated ear-
lier Helsinki-Vantaa airport opened for traffic in
1952, before the first year of the spatial analysis
and has little expanded within the Kylmäoja catch-
ment since then. The presence of the airport leads
to a high stormwater runoff generation already in
1977, but construction restraints around the air-
port have limited increase of imperviousness dur-
ing the study period. The largest increase in runoff
generation between 1977 and 2007 occurred in
subcatchment 6 where the value increased more
than tenfold from 1977 to 2007. The subcatch-
ment located in northeast of Kylmäoja was mostly
undeveloped until 1992, when residential con-
struction started in the Vantaa part of the subcatch-
ment and industrial development increased in
Tuusula (Fig. 5).
Based on current urban development plans in
Tuusula and Vantaa, stormwater runoff generation
increase is likely to reach the largest value within
subcatchment 6, mainly due to the business park
FOCUS (Tuusulan kunta 2009) and the residential
development plans in Leinelä in Vantaa (Vantaan
kaupunki 2008). The reported generated runoff
uses a rainfall intensity of 255 [l s-1 ha-1] suggested
by Kilpeläinen (2006). The possible changes in
rainfall intensity due to climate change and the ef-
fect on the generated runoff have not been investi-
gated in this study. However, the conducted de-
tailed analysis of impervious surfaces in the water-
shed allows the application of different rainfall in-
tensities or frequency storms for future studies.
Cheng and Wang (2002) reported an increase
of 27% in peak discharge associated with an im-
perviousness growth from 4.78% to 11.03% for
a 204 km2 watershed in Taiwan. Kim et al. (2011)
assessed the impacts of urbanization on runoff
using SWAT-SWMM simulations on the white
rock creek watershed in Texas, US, and conclud-
ed an increase in surface runoff by 12% when
doubling the watershed imperviousness. Yu et al.
(2003) used a distributed rainfall-runoff model
for a 35 km2 watershed in Taiwan. They found
that the water balance fraction of surface runoff
increased from 10% to 17% associated with a
watershed imperviousness increase from 20% to
64%. In our study, the predicted increase in sur-
face runoff caused by an imperviousness growth
from 19% to 26% was found to be 41%. The
stronger increase in runoff for the Kylmäoja
catchment might be explained by the larger area
of directly connected impervious surfaces pre-
sent at a higher degree of imperviousness. While
the selected approach using the rational method
allows the assessment of the runoff generation
from impervious surfaces, an assessment of the
water balance and watershed surface runoff re-
quires the inclusion of pervious catchment are-
as. This approach could be conducted in a fu-
ture study based on the conducted land-use
analysis. Furthermore, similar hydrologic assess-
ments have been conducted in earlier studies
(Jang et al. 2007; Haase 2009; Spencer et al.
2009).
The role of four different runoff source areas
was investigated: roofs, roads, yards and the
Helsinki-Vantaa airport. For the years 1977,
1992, 2007 and 2030 the generated stormwater
runoff was calculated by source area to identify
their influence on runoff and to show the poten-
tial of the source area for applying corrective
measures in urban planning (Fig. 6). Runoff gen-
erated on natural impervious areas within the
154 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
watershed (such as open rock formations) was
not considered in the conducted runoff analysis.
While these areas do contribute to the overall
runoff in the watershed, their contribution does
not affect the development of runoff generated
due to urban construction, as this construction
usually replaces pervious areas (such as fields or
forests) rather than open rock formations. Thus,
in this study, the contribution of natural impervi-
ousness to watershed runoff was assumed to re-
main constant over time and discarded in the
presented contribution of analysed impervious-
ness components.
In 1977, more than half of the stormwater
runoff in the Kylmäoja watershed was gener-
ated on the bituminous runways and associ-
ated areas of the Helsinki-Vantaa airport. The
contribution of roof, yard and road surfaces to
stormwater runoff in 1977 ranged between
15% and 18%. The total traffic-related area
(TTA) contribution, considering stormwater
runoff from the airport, yard areas, and road
network, accounted for about 84% of storm-
water runoff within the Kylmäoja watershed in
1977. As the Helsinki-Vantaa airport is a
unique source of stormwater runoff, the traffic
related runoff excluding the airport areas was
also investigated, to allow for comparison
with other suburban watersheds. This subur-
ban traffic-related area (STA) describes the
contribution of traffic-related areas, if only
roofs, yards, and roads are taken into account
for runoff calculation, and thus represent
100% of the stormwater runoff in the water-
shed. In 1977, 77% of the stormwater runoff
within the watershed was generated on STA.
Fig. 5. Stormwater runoff
development and increase
for 1977−2007 (A) and
2007−2030 (B) for the 11
subcatchments and the Ky-
lmäoja watershed in Sce-
nario 1.
FENNIA 191: 2 (2013) 155How does imperviousness develop and affect runoff ...
From 1977−1992 a decreasing influence of
the Helsinki-Vantaa airport on stormwater
runoff and an increasing impact of roof- and
yard surfaces can be seen. Consequently, also
the influence of TTA and STA decrease to 77%
and 65%, respectively. Observation in 2007
indicates further reduction of the relative in-
fluence of the Helsinki-Vantaa airport gener-
ating 28% of stormwater runoff in the Kylmäo-
ja watershed. Stormwater runoff from yard ar-
eas equals the impact of the airport, whereas
roofs generate slightly less runoff.
The forecast for 2030 indicates that the
most important source of stormwater runoff
generation will be the yard areas accumulat-
ing 34%. Roofs will account for 30%, the Hel-
sinki-Vantaa airport for 19% and road surfac-
es will have the smallest share (17%). TTA will
generate 70% and STA 63% of the stormwater
runoff within the Kylmäoja watershed in 2030.
In Tuusula, the extent and ongoing growth
of impervious yard surface, caused by the
high degree of industrialisation, is especially
influential. The impervious yard area in Tuu-
sula alone will account for 14% of runoff gen-
eration within the watershed in 2030, twice as
much as roofs (7%) and even almost four
times more than roads (3%). Besides the ex-
tension of these areas, the industrial focus in
Tuusula also implies that practically 100% of
these yard areas are asphalt covered and thus
highly impervious. Detached houses, as an
example, were found to have typically 50% of
their driveways covered with asphalt and 50%
with gravel surface.
Traffic related areas (STA) in a medium den-
sity suburban area ranges from 63% to even
70% (City of Olympia 1994), depending on
the layout of parking facilities and streets. STA
within the Kylmäoja catchment in 1977 was
higher (77%) than measured in Olympia in
1994. Over time the value decreased to 65%
in 2007 and 63% in 2030, within the range of
the City of Olympia (1994) study. The reduc-
tion is explained by residential areas, which
were more densely built after their first devel-
opment and which had the roof areas increas-
ing faster than the road areas.
Approaches for improvements – Scenario
2 higher structures reduce the roof area
The lowest coefficient between roof area and
floor area for apartment blocks and office
buildings was found to be 0.37 and 0.40, re-
spectively. Both values derive from buildings
constructed between 1993 and 2007. The val-
ue of 0.4 describes a building with three to
four floors. For scenario 2, the coefficient was
reduced to 0.20, thus every apartment- and
office building to be constructed 2008−2030
has six floors (instead of three). The reduction
potential (Scenario 1 vs. Scenario 2) naturally
depends on development plans within the wa-
tershed as only apartment blocks and office
buildings are influenced. Furthermore, the po-
tential is the largest in areas, where the ex-
pected increase in stormwater runoff is the
largest. For the subcatchments 6 and 8, a re-
duction potential of 16% and 12% was found.
The reduction equals 6 [l s-1 ha-1] for both sub-
catchments. The reduction for the whole Ky-
lmäoja watershed was found to be 4 [l s-1 ha-1]
or 7% (Fig. 7). These results confirm conclu-
sions drawn by the US Environmental Protec-
tion Agency (2006) suggesting that high-den-
sity development generates less runoff.
Even though the attempt to support high and
therefore slim structures for these building
types offers a potential for reduction of storm-
water runoff, it is uncertain how large the ef-
fect is in reality. The question, how the yard
areas belonging to structures are designed, is
crucial. The reduction of roofage is only then
a reasonable approach, if the reduced imper-
vious area is not compensated by construction
of parking lots, clearing the positive effect.
Fig. 6. Contribution of the four analyzed components to
stormwater runoff in the Kylmäoja watershed 1977−2030.
156 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
Approaches for improvements – Scenario 3
perforation of yards
As stated earlier the yard areas contribute sig-
nificantly to the total stormwater runoff. This
influence suggests that the perforation of yard
areas through the use of less impervious mate-
rials could be a feasible approach for storm-
water runoff mitigation. Depending on the
land-use type, the proportion of impervious
materials used on yards was reduced by 20%
to evaluate the potential reduction in runoff
for 2030. All changes are, as conducted for
the higher structure approach, applied for
buildings to be constructed 2008−2030. The
reduction potentials (Scenario 1 vs. Scenario
3) for the subcatchments 6 and 1 were found
to be 8% and 6%, respectively. The reduction
equals 3 [l s-1 ha-1] for both subcatchments.
The reduction for the whole Kylmäoja water-
shed was found to be 2 [l s-1 ha-1] or 4% (Fig.
8).
In contrast to the attempt of supporting
higher structures, the reduction of impervious
yard area is also applicable to existing yards,
even though the changes in yard surface types
were only applied for buildings to be con-
structed, to allow for direct comparison. Fur-
thermore, the variation of yard area impervi-
ousness is effective for all building types in
the catchment – reaching from a one-family
house to an apartment block – and does not
Fig. 7. The peak runoff gener-
ated from impervious areas
in the Kylmäoja watershed
calculated for the 11 sub-
catchments and the Kylmäo-
ja watershed in 2030 (Sce-
nario 1 and 2) and the runoff
reduction potential of Sce-
nario 2.
Fig. 8. The peak runoff gen-
erated from impervious ar-
eas in the Kylmäoja water-
shed calculated for the 11
subcatchments and the Ky-
lmäoja watershed in 2030
(Scenario 1 and 3) and the
runoff reduction potential
of Scenario 3.
FENNIA 191: 2 (2013) 157How does imperviousness develop and affect runoff ...
depend on building height. Stone (2004) ar-
gues that a reduction of impervious cover
through driveway perforation would have
‘substantial benefits for the region’ when ap-
plied in Madison, USA. Even though his sug-
gested increase of infiltration (from zero to
80%) for driveways is far more progressive
than applied in this study (plus 20% infiltra-
tion), the importance of yards for zoning prac-
tices is evident.
Approaches for improvements – Scenario 4
green roofs
The increase of green areas in cities does not
only have a positive eco-hydrological effect,
but enhances life quality in urban areas. Un-
fortunately, due to limited space and high
land prices, the recreation and preservation
of green areas in urban circumstances are of-
ten regarded to be very expensive if not im-
possible (Mentens et al. 2006). Roofs will ac-
count for almost one third of stormwater run-
off in the Kylmäoja watershed in 2030; there-
fore, the mitigation of these areas is worth
evaluating. Mentens et al. (2006) conducted
an intensive literature review to investigate
the actual effect of green roofs on yearly run-
off in Brussels and found that extensive roof
greening – a substrate layer between 30 and
140 mm in depth – on 10% of the roofs would
reduce the annual runoff in the urban area by
2.7%. To assess the effect of green roofs on
stormwater runoff generation in Kylmäoja,
we follow this suggestion and assume that
10% of the buildings in the watershed receive
an extensive green roof by 2030, reducing
the stormwater roof runoff by approximately
38%. The investigation showed that 30% or
almost one third of the generated runoff in
2030 will derive from roofs. The reduction of
stormwater runoff by 38%, applied for 10%
of the rooftops in the watershed, results in a
total reduction of 3.8% for the runoff gener-
ated from roofage. The stormwater runoff in
the whole Kylmäoja watershed can be re-
duced by about 1.2%. As mentioned, neither
seasonal changes nor climate differences be-
tween the area in Belgium (Mentens et al.
2006) and Finland were considered, but the
estimation shows the potential for runoff re-
duction using green roofs.
Conclusions
This study showed that the land-use changes with-
in the Kylmäoja watershed have been rapid until
the present day and will be also until the target
year of this study, 2030. Based on the reported re-
lationships between land-use and stormwater run-
off, approaches for reduction of runoff peaks were
evaluated. The attempt to support construction of
higher buildings, thus having less roofage, was es-
timated to clearly reduce stormwater runoff. Re-
duction potential for the Kylmäoja watershed was
found to be 7% for the Kylmäoja catchment and
up to 16% for individual subcatchments. Howev-
er, this tool is not effective without implementation
of best stormwater management practices in the
saved yard areas. If the reduction of roof area is
compensated by increased parking places, the
positive effect is diminished. The impact of yard
areas on stormwater generation and hence the
stream health became obvious. The contribution
of impervious yard areas to stormwater runoff gen-
eration is higher than the contribution of the actual
associated building roofs. Therefore it is important
to closely investigate the runoff processes and po-
tential mitigation measures in yard areas. The at-
tempt to reduce asphalt surfaces on yard areas
seems to be the easiest mitigation approach ap-
plicable in the Kylmäoja type of an urban area. In
the proposed mitigation scheme the yard areas of
detached houses contained no asphalt, maximum
50% of the row house yard areas were asphalt
covered, and apartment blocks, office buildings
and industrial facilities had less than 80% asphalt
covered yard areas. These changes were only ap-
plied for newly constructed sites, and as such the
estimate proved to be less effective than the reduc-
tion of roof area in newly constructed buildings
with a reduction potential of 4% for the Kylmäoja
watershed and up to 8% for the subcatchments.
Nevertheless, the reduction of impervious yard
surfaces is, compared to the reduction of roof area,
applicable for both new and existing buildings and
related with low costs. Recognizing that 34% of
the accumulated stormwater runoff in Kylmäoja
will derive from impervious yard areas, the runoff
mitigation potential of those areas is of major im-
portance to reduce the degrading effects of devel-
oped land on the Kylmäoja stream. The approach
of introduction and support of green roofs in the
watershed showed an estimated reduction poten-
tial of stormwater runoff of about 1.2%, when 10%
of the roofs in the catchment area were assumed to
158 FENNIA 191: 2 (2013)Krebs, Gerald, Ulla-Maija Rimpiläinen and Outi Salminen
have an extensive green roof. Even though the use
of extensive green roofs results in less stormwater
runoff reduction than the reduction of roof area
and yard perforation, the potential of the approach
with 30% of stormwater runoff deriving from roof
areas is existing.
ACKNOWLEDGEMENTS
The authors want to thank Prof. Harri Koivusalo for
his comments on the manuscript. Furthermore, we
wish to thank Prof. Ari Jolma, Prof. Riku Vahala,
and Prof. Pertti Vakkilainen for their support and
valuable advices during the conducted research.
The personnel of the City of Vantaa and the mu-
nicipality of Tuusula were very helpful in providing
data and giving an insight to processes in urban
planning and stormwater management. Funding
for this research was provided by the City of Van-
taa, the STORMWATER project (grant A30116),
the HYDROSYS project (grant 224416, DG INF-
SO), and Maa- ja vesitekniikan tuki Foundation.
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