Water and Environmental EngineeringDepartment of Chemical EngineeringMaster Thesis 2013
Daniel Kibirige & Xing Tan
Evaluation of Open Stormwater Solutions
in Augustenborg, Sweden
Postal address Visiting address Telephone P.O. Box 124 Getingevägen 60 +46 46-222 82 85
SE-221 00 Lund, Sweden +46 46-222 00 00
Web address Telefax
www.vateknik.lth.se +46 46-222 45 26
Evaluation of Open Stormwater Solutions
in Augustenborg, Sweden
By
Daniel Kibirige & Xing Tan
Water and Environmental Engineering
Department of Chemical Engineering
Lund University
August 2013-05
Supervisor: Professor Jes la Cour Jansen
Co-supervisor: Erik Mårtensson
Examiner: Associate Professor Karin Jönsson
Picture on front page: Snaps of Open stormwater solutions in Augustenborg.
Photo by Xing Tan
i
Acknowledgements
We would like to thank all people involved in the completion of this master thesis degree
project. Firstly, we are grateful to God for good health and wisdom during our studies.
Secondly of particular mention we would like to recognise our supervisors Prof. Jes la Cour
Jansen (VA-teknik) and Erik Mårtensson (DHI) for their patience and guidance throughout
the compilation of this thesis. We would also like to acknowledge Peter Ahlström (Malmö
municipality) and Tomas Wolf (VA SYD) for the elevation and temperature data that they
provided us with. Daniel would like to express gratitude the Erasmus Mundus for financial
support during the entire study in Sweden. Last but not least we both would like to be grateful
to our friends and family in Sweden for the care and support we received during these past
two years. Finally we would like to dedicate this thesis to our parents who guided us through
childhood until now and who have patiently awaited our return after the completion of our
studies.
i
iii
Summary
The stormwater system has gone through major changes in past decades. The study area
chosen was Augustenborg Eco-city located in the Southern County of Sweden, Skåne.
Currently, its area covers approximately 32 hectares and has a total population of about 3000
people. Within this area other than a residential area, there are a number of features to the
eco-city. These features include: a school, music theme playground, green roofs & areas, and
open stormwater systems.
Augustenborg eco-city is one of Sweden´s largest urban sustainability projects and is
renowned for its effective open stormwater systems. In this study, a hydrological model was
built to evaluate the hydraulic performance of these systems. The aim was to investigate the
efficiency of the current open stormwater system on the rate of flow during extreme climatic
events. The objectives of the study were: firstly, to evaluate the efficiency of the open
stormwater system in Augustenborg eco-city for improvement, and secondly, to analyse
predictability of potential flood under extremely high precipitation conditions and suggest
measures to deal with this risk.
The methodology used, involved; the design of a hydrological model by MIKE SHE, as well
as a number of visits to Augustenborg eco-city to check the precise location the open
stormwater systems. Different types of data (from different sources; DHI, Geological Survey
of Sweden) were collected namely: meteorological data, elevation data, and geological data.
The hydrological model consisted of a seven different components in the model. These
components include: model domain and grid, topography, climate, land use, overland flow,
unsaturated flow and saturated zone. Of the seven much emphasis will be placed on
components that relate to water movement that are specified in the model which include;
overland flow (OL), evapotranspiration (ET). In this study there was no observed data which
meant that there it was no calibration and validation to compare the results of the model.
The results for the simulation consisted of three different scenarios; 1 year prediction of
hydrological balance, 10 year extreme rainfall event and 100 year extreme rainfall event. The
main focus of the results in all three scenarios is based on water balances (evapotranspiration)
and overland flow (runoff).
In scenario 1, the mean daily rainfall was chosen for the period 2006-2008. Initially a 10 year
period was chosen however after analysis of rainfall events, the period 2006-2008 had large
amounts of high rainfall events which would gave more credit to our study as we were
looking into the efficiency of the system under high rainfall events. In this scenario the water
balance was reasonable with a ratio of precipitation to evapotranspiration of 75% and depth of
overland flow of 0.65 m.
In scenario 2, a total of two weeks were simulated; one week before and after the 31st of July
in order to see how the flooding propagates during the event. The extreme rainfall event was
calculated by taking the averages between two time periods at which rainfall was recorded
e.g. 12h00-14h00, 14h00-14h15 etc. A figure of 62 mm/day (showing a total accumulation of
rainfall) was calculated and inserted into the last day of July. Generally over the entire area
the depth of overland flow was in the range of 0.15 m with only swales above 0.60 m
indicating that rather small amounts of water were flowing on the surface but most of the
water was able to be infiltrated into the system.
iii
iv
Finally in scenario 3, similarly a two week simulation (one week before and after the
abovementioned date) was made. The same procedure was used as in scenario 2 to calculate
the precipitation value that was inserted on the last day of July. An estimated amount of 128
mm/day precipitation rate was obtained according to accumulation like in the example of the
10 year extreme rainfall event. In this scenario, not only are the swales at risk of not being
able to hold the water which in turn will cause flooding with a maximum water depth of
above 1 m in some places. In addition, some residential areas are also under the risk flooding,
which puts the system under stress in handling the 100 years extreme rainfall because most of
soil is saturated under such conditions.
All the simulation results showed that most of the open stormwater solutions performed their
specific function effectively, especially swales, therefore it can be concluded that the open
stormwater system in Augustenborg is well suited to handle current climatic conditions and a
10 year extreme event. The 100 year extreme event posed the most risk to the area and
flooding was event. In summary if the current open stormwater solutions are well maintained,
Augustenborg will continue to be an example to other eco-cities around the world.
iv
Table of Contents
1 Introduction 1
1.1 Background 1
1.2 Objectives 1
1.3 Aim 1
1.4 Limitations 2
2 Presentation of fundamental components in the study 3
2.1 Drainage systems 3
2.2 MIKE SHE 5
2.3 Open stormwater solutions 6
2.3.1 Open channel (ditches) 7
2.3.2 Green Roofs 8
2.3.3 Swales 9
2.3.4 Storage ponds 9
3 Description Augustenborg Eco-city 11
3.1 Description of study area 11
3.2 Description of open stormwater systems in Augustenborg 14
3.2.1 Overview of the open storm water system in Augustenborg 14
3.2.2 Open channel 16
3.2.3 Green roof 16
3.2.4 Swales 17
3.2.5 Storage ponds 17
4 Materials and Methods 19
4.1 Data collection of study area 19
4.2 Field investigations 19
4.3 Detailed description of MIKE SHE Model 19
4.3.1 Introduction 19
4.3.2 Model domain and Grid 20
4.3.3 Topography 20
4.3.4 Climate 21
4.3.5 Land Use 23
4.3.6 Vegetation 23
4.3.7 Overland flow 25
4.3.8 Unsaturated Flow 26
4.3.9 Soil profile definition 27
4.3.10 Saturated Zone 28
4.3.11 Calibration data 29
4.4 Description of open stormwater solutions in MIKE SHE model 29
5 Results 31
5.1 Scenario 1 31
5.2 Scenario 2 32
5.3 Scenario 3 35
6 Discussion 39
6.1 Scenario 1 39
6.2 Scenario 2 40
6.3 Scenario 3 40
7 Conclusion 41
8 Suggestion and recommendations 43
References 45
Appendix
List of Figures
Figure 2.1 Processes in MIKE SHE model Source: DHI ........................................................... 6 Figure 2.2 Green roof Structural Model (Peck, et al., 2009) ...................................................... 8 Figure 3.1 Aerial Orthophoto showing Augustenborg location in Sweden. Source: Google
Earth.......................................................................................................................................... 11 Figure 3.2 Aerial Orthophoto of Augustenborg. Source: Google Earth ................................... 11 Figure 3.3 Playground area. Source: Daniel Kibirige .............................................................. 12 Figure 3.4 Open grass banks. Source: Xing Tan ...................................................................... 13 Figure 3.5 Storage ponds. Source: Daniel Kibirige .................................................................. 13 Figure 3.6 Drainage channel. Source: Daniel Kibirige ............................................................ 14 Figure 3.7 Overview of the stormwater system in Augustenborg (VA SYD, n.d) .................. 15 Figure 3.8 Shallow channel. Source: Daniel Kibirige .............................................................. 16 Figure 3.9 Green roof installation at Augustenborg eco-city. Source: Daniel Kibirige ........... 16
Figure 3.10 Storage ponds in Augustenborg. Source: Xing Tan .............................................. 17 Figure 4.1 Schematic layout of MIKE SHE model .................................................................. 19 Figure 4.2 Design of boundary in Augustenborg, in MIKE SHE Model ................................. 20 Figure 4.3 Model area of Augustenborg ................................................................................... 21 Figure 4.4 Daily mean air temperature (2007) ......................................................................... 22 Figure 4.5 Monthly potential evapotranspiration ..................................................................... 22 Figure 4.6 The key vegetation properties are Leaf Area Index (LAI as shown in blue line) and
Root Depth (RD as shown in red line) ..................................................................................... 24 Figure 4.7 Impervious area shown in blue in Augustenborg .................................................... 26 Figure 4.8 Soil profile definitions ............................................................................................. 27 Figure 4.9 Swales area defined in red ....................................................................................... 30 Figure 4.10 Long open ditch system (blue line) is defined around the buildings .................... 30 Figure 5.1 Mean Precipitation (2000-2011) ............................................................................. 31 Figure 5.2 Water Balance (2007) ............................................................................................. 32
Figure 5.3 Depth of Overland Flow (2007) .............................................................................. 32 Figure 5.4 Water balance of 10 years extreme events for 2 weeks simulation ........................ 34 Figure 5.5 Depth of Overland flow after precipitation for 10 years extreme event (Left to
Right; 30 minutes, 3 hours and at bottom 3 days) .................................................................... 35 Figure 5.6 Water balance of 100 years extreme events for 2 weeks simulation ...................... 37 Figure 5.7 Overland flow depth after precipitation for 100 years extreme event (Left to Right;
30 minutes, 3 hours and at bottom 3 days) ............................................................................... 38
List of Tables
Table 2.1 Different Manning values of roughness coefficient (M) according to material type.
Adapted from French, 2007 ........................................................................................................ 8 Table 3.1 Illustration of different solutions mentioned in Figure 3.7 ....................................... 15 Table 4.1 Soil Profile ................................................................................................................ 27 Table 4.2 Swale definitions ...................................................................................................... 28 Table 4.3 Geological layers ...................................................................................................... 28 Table 4.4 Computational layers ................................................................................................ 29 Table 5.1 Precipitation data on 31st July, 2007 for 10 years extreme event from DHI ........... 33 Table 5.2 Precipitation data on 31st July, 2007 for 100 years extreme event from DHI ......... 36
1
1 Introduction
1.1 Background
The activities of man in many ways affected water cycle processes today. This is mainly
through the abstraction of water supply as well as creating an obstruction in the natural
rainfall cycle through the construction of impervious surfaces (Butler & Davies, 2004). For
example when rain falls, water should either evaporate or infiltrate into the soil however in
urban areas this is not the case. In urban areas there is barely any infiltration due to large
areas of the cities being paved (Villarreal & Bengtsson, 2005). It is this regard that Butler &
Davies (2004) contend that the above mentioned ways have led to the development of
different drainage systems (wastewater and stormwater) in urban areas. Urban drainage
systems, in particular, stormwater systems direct the flow of runoff and are expected to
prevent the flooding.
The stormwater system has gone through major changes in past decades (Malmö Stad, n.d.).
As a result of urbanization, open stormwater systems in urban areas are becoming more and
more popular and have been widely used in many cities all over the world. According to
Kleidorfer, et al. (2009), urbanization has had a huge impact on water resources which has
dramatically increased surface runoff. It was found that as a result of urbanisation there is a 20%
increase in rain intensity (Kleidorfer, et al., 2009). It is in this regard that open stormwater
systems are of paramount importance in this fast developing environment.
Augustenborg eco-city; one of Sweden´s largest urban sustainability projects, is renowned for
its sustainable urban planning, waste management as well as the effective open stormwater
systems (Malmö Stad, n.d.). Generally, the function of an open stormwater system can be
divided into four categories: infiltration, storage, detention and slow transport of runoff. In an
open stormwater system, rainfall runoff is handled by different types of open stormwater
system solutions e.g. green roofs, vegetated swales, open ditches and storage ponds.
Therefore, in order to improve and enhance the entire open stormwater system in
Augustenborg, a hydrological model was designed to evaluate the hydraulic performance of
these solutions.
1.2 Objectives
The objectives of the study are:
to evaluate the efficiency of the open stormwater system in Augustenborg eco-city for
improvement
to analyse predictability of potential flood under extremely high precipitation
conditions and suggest measures to deal with this risk
1.3 Aim
The aim of the study is to investigate the efficiency of the current open stormwater system on
the rate of flow during extreme climatic events.
1
2
1.4 Limitations
1. Uncertainties
Not all the data needed in the MIKE SHE model were able to be collected; therefore, several
assumptions were made e.g. the value of horizontal and vertical conductivity in both
saturated and unsaturated zone.
2
3
2 Presentation of fundamental components in the
study
2.1 Drainage systems
A group of researchers believed that the historical background of drainage systems date back
to many centuries ago (Cone, 2005). In the 18th
century in some urban areas; human waste
was harvested (used for fertiliser) and stormwater was reused for household activities. The
former method was abandoned due to the outbreak of disease whereas the latter was
developed into a more structured system (Chocat, et al., 2001). As time moved forward the
design of the conventional sewer system was designed and resulted in a modified version of
the water transport system we have today. Furthermore, in order to enhance the development
of drainage systems, the use of computer models became more prominent in the last century
(Butts, Payne & Overgaard, 2004).
On the contrary, Chocat et al. (2001) states, “…the latest developments in drainage
calculations are due not to technology but to philosophical realignments based on four
overriding concepts: a) introduction of sustainable development; b) acceptance of the
ecosystem approach to water resource management; c) impacts of drainage on receiving
waters; and d) recognition that complexity of the urban environment requires an integrated
approach.” It is in this regard that this thesis contends that combination of technological
advancements as well philosophical assumptions from the past hold ground for the better
management of drainage systems now as well as in the future.
Drainage systems are categorized into natural and conventional systems. Natural drainage
systems relate to drainage through infiltration and storage properties of semi natural features
(Ahmed, 2010). These features include; swales, ponds, detention basins etc. Conventional
drainage systems are the usual drainage of water through a piped network. There are two
types of conventional systems namely; combined and separate. Combined systems transport
wastewater and stormwater in the same pipe whereas separate systems have two different
pipes transporting the respective flows (wastewater and stormwater). In European countries,
the most prevalent system is the combined system whereas in Sweden only 15% of sewer
systems are combined with the rest separate (Berggren, 2007). In addition, the separate system
can be in a number of forms; piped, open, or a combination of these two. An open system usually
contains a combination of the following: ponds, channels, wetlands, green roofs, green infiltration
surfaces and porous paving (Ahmed, 2010). In a combined system; the pipe transporting water
is normally only 10% full and the rest is only used during the rainy season, meaning that is
almost never used to full capacity; this being its main disadvantage. Then, the advantage of
the separate system is that stormwater and wastewater do not mix, but due to two pipes, the
cost of construction is much higher.
The combination of drainage methods used brings up another interesting topic, in field of
water management, that no one facet can be considered as the best solution to proper drainage
but rather a combination of facets. So in order to achieve better drainage systems, future
research needs to focus also on specialised solutions (open stormwater systems) rather
general solutions (conventional system) or a combination of the two (Cone, 2005). On the
contrary, at times specialised solutions can be expensive and may not be able to be used in all
parts of the world e.g. developing countries. Thus, in developing countries, there is a need for
3
4
link between specialised solutions as well as generalized solutions to find a common ground
that is suitable to ensure sustainability.
Literature of the past few years shows a trend, showing that sustainability is taking priority in
terms of the development of drainage systems (Villarreal, Semadeni-Davies & Bengtsson,
2004). However, in its true sense much is left to be desired when it comes to standard of these
sustainable drainage systems. The major problem of the past of integrating design techniques
with ecological issues still prevails. With urbanization increasing at a very fast rate this
integration becomes paramount for the development of drainage systems. Steiner (2002)
believes that integrating human needs, design parameters as well as ecological factors need to
followed for better sustainability. He further goes on to argue that conventional methods need
to be upgraded to Best Management Practices (BMP’s). BMP’s are structural methods used
in the construction of stormwater systems. The synonym for BMP’s in Europe is Sustainable
Urban Drainage Systems (SUDS). One of the main goals of stormwater management systems
is to reduce the impacts of urban development on the movement of water in urban areas
(Scholes, Ellis & Revitt, 2007). In order to have full efficiency of these systems, stormwater
management systems can be designed in a series. This means that a series will comprise of a
number of components that will have its specific function. Some components will control
flooding, others water quality as wells as storage of runoff. Also, they will have value such
as: aesthetic and recreational benefits (Villarreal, Semadeni-Davies & Bengtsson, 2004).
More recently, stormwater management systems have been used to mimic real life natural
processes. This has been done with aids of sophisticated computer models such as the MIKE
SHE and MIKE URBAN models. These models will be discussed further later in this chapter.
An example of series components in a stormwater management system are described by a
“suburban retrofit” in Sweden; of which the exact location is not mentioned (Villarreal,
Semadeni-Davies & Bengtsson, 2004). Villarreal (2004) further suggests that an unnamed
report says that the system comprised of open channel which received water from
surrounding areas and a detention pond for storage of water. He further iterated that
stormwater management systems should perform at least more than one function to ensure
sustainability in all seasons. For example, aesthetics can be of huge benefit during dry periods
where human uses may conflict with open channel use. Like in the example in Sweden, the
major function of the stormwater system was only conveyance of water, which caused
residents to dislike the system. This resulted in litter collecting in the system; however that
was not the only reason for the litter. The litter was made visible also due to the displacement
of vegetation but overall the attitudes of residents was not good due to the fact the system had
only one purpose. Furthermore, (Mehler & Ostrowski, 1998) agrees with Villarreal,
Semadeni-Davies & Bengtsson (2004), that conventional stormwater systems are not able to
solve many complex issues relating to stormwater and water quality issues, thus better
stormwater systems such as BMP’s should be used.
Other BMP’s that have been deemed to be a solution to stormwater management are also
under the spotlight. Green roofs are a popular stormwater management system that is being
scrutinized carefully to assess their true effect on the retention of stormwater. In the past, roof
runoff was regarded as a source of clean drinking water, irrigation and other household uses.
However, according to past studies in Northern Europe much is left to be desired about the
quality of the runoff water (Good, 1993).There was an increase in the levels of zinc (from
galvanized roofs surfaces) in runoff water. Although the levels of zinc where low to harm
4
5
human health, it was harmful to aquatic life. This leaves room for research and investigations
into the storage possibilities of stormwater in areas where galvanized zinc roofs are still used.
On the other side of the coin, currently, the use of galvanized roofs across the world has been
reduced and concrete roofs are used, which negate the principle of reduction of runoff. The
response to this problem was then solved with the introduction of a green roof or also known
as a vegetated roof. Obviously, vegetated roofs which are slightly more expensive to
construct do perform the desired function of reducing the flow rate of runoff.
As has been discussed in this chapter, in order for better movement of water, the design of
stormwater management facilities need to be revised. Literature shows that stormwater
designs are improving but at a slow rate (Butler & Davies, 2004). Not only does the
advancement of design principles need to change but also the professionals involved in the
construction is vital in the implementation of better stormwater management systems.
Mihelcic, et al. (2003) states, “…together the mature fields of the physical sciences,
engineering, economics, and human behavioural studies to address the critical issues of
sustainability”, can go a long way in achieving sustainable stormwater solutions. It is in the
regard that the use of computer aided modelling software has become vital for future
development.
2.2 MIKE SHE
Hydrological modelling relates back to the late 60’s where proposed a blueprint for
modelling the hydrological cycle began (Freeze & Harlan, 1969). In this blueprint, flow
processes where described using derived partial differential equations. This made the model
very mathematical and complicated. Then in 1977, a consortium was formed between three
European organisations; Système Hydrologique Européen (SHE). These organisations used
Freeze & Harlan’s (1969) equations as a base to improve hydrological modelling. From this
was the emergence of several MIKE software.
MIKE SHE is an advanced integrated hydrological modelling software that is part of the
MIKE group of software owned and designed by the Danish Hydrological Institute (DHI)
(Refsgaard & Storm, 1995). It is based on deterministic and physically based modelling
systems, making its application in research very purposive (Abbott et al., 1986). The models
main function relates to the movement of water in relation to the hydrological cycle. So,
basically the model describes a number of hydrological processes e.g. interception,
evapotranspiration, overland and channel flow, flow in the unsaturated zone and saturated
zone, snow melt as well as the exchange of water between aquifers and rivers (Figure 2.1).
According to Butts, Payne & Overgaard (2004) the abovementioned processes can be
represented with different degree of emphasis depending ones goals of the study, the
availability of field data and the modeller’s choices.
5
6
Figure 2.1 Processes in MIKE SHE model Source: DHI
In order to describe these processes, MIKE SHE uses a number of equations to calculate
flow. For example partial equations are used in a number of processes namely: channel flow
(one dimensional Saint-Venant equation), overland (two-dimensional Saint-Venant equation),
unsaturated (one-dimensional Richard Equation) and saturated subsurface flows (three-
dimensional Boussinesq equation). Analytical methods are used in the model to describe
evaporation; interception and snow melt (DHI, 2004).
In addition, potential evapotranspiration, precipitation, elevation, soil type and cover, are
represented using network grids. Within each of the grids, vertical and horizontal layers are
described at a number of depths. This is seen as lateral flow between the grids which occurs
as overland flow or saturated flow. One assumption that is made when Richard Equation is
used is that horizontal flow is negligible in the unsaturated zone compared to the vertical
flow.
2.3 Open stormwater solutions
Decentralized open stormwater systems are becoming more and more popular in recent years
and are starting to be widely applied in small and rural communities where they can be most
cost-effective (National Small Flow Clearinghouse, 2000)
In Augustenborg, stormwater from roofs and other impervious surfaces is collected in gutters
and channelled on through canals, ditches, ponds before finally draining into a traditional
combined sewer system (VA SYD, n.d.).
6
7
In this section background on four main open stormwater solutions will be introduced and the
function of each will be discussed, focusing on the relevant aspects with regard to the
subsequent modelling steps. The four main open stormwater solutions includes: open channel
(ditches), green roofs, swales and storage ponds.
2.3.1 Open channel (ditches)
Open channels are a very important component in an open stormwater system and can be
categorized into two different types: natural and artificial channels. Artificial channels
include; irrigation canals, navigation canals, spillways, sewers, culverts and drainage ditches.
Open channels are usually constructed in a regular cross-section shape and are used for
diverting the flow stormwater, which in turn results in the decreasing the rainfall runoff peak
flow (Chow, 2009).
In order to know how much flow can be transported through an open channel or ditch, the
manning equation is usually used because it is simple and precise.
The Manning equation in the SI units system is defined as follows:
⁄
⁄ Equation 2.1
Whereby:
Q = flow rate (m3/s)
A = flow area (m2)
R = hydraulic radius (m)
M = Manning roughness coefficient
Sf = Friction Slope
According to equation 2.1, the flow rate is a function of flow area, hydraulic radius and
Manning resistance coefficient, however, Manning roughness, is very difficult to measure
directly in practice. The estimation of the roughness coefficient (Table 2.1) of the channel is
the most challenging task because different materials are used for lining the channel.
Generally, the value of Manning coefficient is estimated as the same value measured in
previous successful designs.
7
8
Table 2.1 Different Manning values of roughness coefficient (M) according to material type.
Adapted from French, 2007
Material Manning coefficient
Concrete (rough) 68
Concrete (smooth) 85
Plastic 80
Grass (lawn) 20
Cement mortar (neat) 90
Masonry 40
Rubble 30
2.3.2 Green Roofs
Different source control techniques have been applied in an open stormwater systems in order
to reduce the risk of flooding (Stahre, 2008). The primary goal is to decrease rainfall runoff
entering the combined sewer system which can be easily overloaded during high precipitation
seasons.
A green roof/vegetated roof (Figure 2.2), is a living surface of plants growing in a soil layer
on top of the roof (Stahre & Geldof, 2003). It is one of the most efficient solutions in open
stormwater systems aiming at reducing peak rainfall runoff flow (Stahre, 2008). Generally
there are 3 different types of green roofs; extensive, semi-intensive and intensive green roofs
depending on the depth of planting medium and the amount of maintenance they need
(Family Business Institute, n.d.). Extensive green roofs traditionally support 5-10 kilograms
of vegetation (drought resistant species) per square metre while intensive roofs support 35-70
kilograms of vegetation (trees and shrubs) per square metre and semi-intensive green roofs
support ranges between 10 to 35 kilograms of vegetation (herbs and meadow vegetation) per
square metre (Family Business Institute, n.d.).
Figure 2.2 Green roof Structural Model (Peck, et al., 2009)
8
9
2.3.3 Swales
A swale is a vegetated open channel, planted with a combination of grasses and other
herbaceous plants (City of Indianapolis, n.d.). A swale can slow down the rapid flow of
stormwater runoff by ponding water between its loping sides. Ponding not only slows the rate
of flow but also separates pollutants from storm water. Once the swale becomes full, the
excess water will overflow over berms and flow into the main long ditches.
Swales have been used for runoff control from rural highways and residential streets for
many years, besides; they also provide infiltration and water quality treatment. As an efficient
open system solution, swales are becoming more and more popular because they can look
more aesthetically than a rock-lined drainage system and are generally simple to construct
and maintain (City of Indianapolis, n.d.).
2.3.4 Storage ponds
Storage ponds are usually used to slow the rainfall runoff by ponding a certain volume of
water performing as a detention container (Akan, 1993). The advantages of storage ponds are
mainly the following aspects:
reduce the peak flow downstream
reduce the erosion led to sewage system through sedimentation process
help to reduce the pollutant through setting particles
The routing equation is generally used for calculating the outflow for a specific reservoir,
especially for a pond (Akan, 1993).
Equation 2.2
Whereby: I is the inflow rate
O is the outflow rate
S is the water volume in storage pond
t is the time
9
11
3 Description Augustenborg Eco-city
3.1 Description of study area
The study area is Ekostaden Augustenborg which was formally known as Augustenborg in
the early 90’s. When translated from Swedish, Ekostaden Augustenborg means Augustenborg
Eco-city. Augustenborg Eco-city is located in the third biggest city of Sweden; Malmö, in the
Southern county of Skåne (Figure 3.1 & Figure 3.2).
Figure 3.1 Aerial Orthophoto showing Augustenborg location in Sweden. Source: Google
Earth
Figure 3.2 Aerial Orthophoto of Augustenborg. Source: Google Earth
Augustenborg is an area that dates back as far as the 1950s (Shukri, 2010). At that time it was
the first public housing area in Malmö that had its own coal heating system and laundry. By
the early 1980’s, due to lack of employment and the then diminishing industrial industry in
Sweden, less people wanted to stay within the area (European Sustainable Urban
Development Projects, n.d.). Then, in 1997, Peter Lindhqvist of the Department for Internal
11
12
Services, Bertil Nilsson; former headmaster at the school in Augustenborg, and Christer
Sandgren of MKB (Malmö´s public housing company), came together and formed a coalition
suggesting the development of an eco-friendly industrial park (Ekostaden Augustenborg,
n.d.). Among many other reasons; deterioration of the buildings as well as basement flooding
was the major factors that led to suggestions of renovation and creation of the eco-friendly
industrial park. It is 1997 that Augustenborg received its current name of Ekostaden
Augustenborg and development of the Eco-city started in 1998 and was completed in 2002.
Currently, its area covers approximately 32 hectares and has a total population of about 3000
people (Shukri, 2010). Within this area other than a residential area, there are a number of
features to the eco-city. These features include: a school, music theme playground (Figure
3.3), green roofs & areas, and open stormwater systems, (Ekostaden Augustenborg, n.d.).
Figure 3.3 Playground area. Source: Daniel Kibirige
The most significant modification in Augustenborg which relates most to this study was the
change in the handling of stormwater. Open stormwater systems were designed in place of
the conventional sewer system. The intention of this system was to help reduce flooding
which was a major problem. There are number of open stormwater systems namely: green
roofs, open ditches, swales, and retention ponds (Stahre, 2008). Stahre (2008) further goes on
to group these different systems into three categories; local infiltration, storage and flow
detention and slow transport. The facilities that belong to each in the abovementioned
categories will be briefly discussed below and will be thoroughly explained in the next
section.
Local infiltration consisted of green roofs and grass turf. Green roofs or vegetated roofs were
created in the form of a Botanical Roof Garden. This Botanical Roof Garden was the first of
its kind created in world (Stahre, 2008). It is situated on top of workshops in the
Augustenborg area and consists of a number of sections. These sections include; a public
viewing area and a research section for pilot studies. Visitors are allowed to come and view
the roof garden as an attraction. The research section of the roof is handled by the
Scandinavian green roof institute (SGRI), which aims at promoting and developing better
ideas in the field of green roofs in Scandinavia and the world at large. SGRI conducts many
activities such as, international workshops, facilitation of research activities and Scandinavian
Green Roof Award. Then, Grass turfs (Figure 3.4) are found in the form of open grass banks
in Augustenborg. Their main function would be infiltration but also delay the runoff.
12
13
Figure 3.4 Open grass banks. Source: Xing Tan
Storage and flow detention in Augustenborg are in the form ponds and temporary storage
facilities. Storage ponds range in size and are located all around the study area (Figure 3.5).
,
Figure 3.5 Storage ponds. Source: Daniel Kibirige
Slow transport facilities are found in different “drainage corridors”. A drainage corridor can
be seen in Figure 3.6 and is from the southeast to the southwest corner of the study area
(Stahre, 2008). They are in the form of concrete drainage canals, concrete cube canals,
vegetated cannels.
13
14
Figure 3.6 Drainage channel. Source: Daniel Kibirige
3.2 Description of open stormwater systems in Augustenborg
3.2.1 Overview of the open storm water system in Augustenborg
In Augustenborg, there is one inlet and two outlet ponds. The stormwater is firstly collected
and stored in the storage area (see No.2 in Table 3.1), after that, the water is pumped
underground and transported into a ditch system through the inlet pond. The long ditch
system is indicated as blue line in Figure 3.7 and thereafter stormwater travels to the outlet
ponds and finally it is led into a piped sewage system.
In the open system at Augustenborg, stormwater is generally conveyed through concrete
canals, open ditches or pumped between two ponds underground. The water is then
channelled out on the lawn or wetland in the event of an extreme rainfall. The “onion gutters”
(Figure 3.8) and the “cube canal”, can be found all around the eco-city and they are designed
for creating more resistance when water move. The ponds mainly contribute as a detention
volume while the green roofs reduce the peak flow. During heavy rainfall, the excess water in
the pond will be led to the sewage system in Lönngatan. As shown in Figure 3.7, different
numbers represent different open system solutions (Table 3.1).
14
15
Figure 3.7 Overview of the stormwater system in Augustenborg (VA SYD, n.d)
Table 3.1 Illustration of different solutions mentioned in Figure 3.7
Number Implication Number Implication
1 The Augustenborg botanical
roof garden area
9 Basketball court, storm water
collected here travel to 10
2 Storm water storage pumped
underground from storage area
10 Ditch through the park
3 Concrete canal 11 Outlet pond Nr 1
4 Wetland 12 Storage pond
5 Onion gutters 13 Macadam-bottomed ditch
6 Storage pond 14 Storage pond
7 Block flat with green roof 15 Constructed tone canal
8 Cube canal 16 Outlet pond Nr 2
15
16
3.2.2 Open channel
According to a report by International Green Roof Institute, an open drainage system, in the
form of a shallow ditch, was constructed across the park area in Augustenborg in order to
reduce the rainfall runoff speed (Figure 3.8), (Stahre & Geldof, 2003).
Figure 3.8 Shallow channel. Source: Daniel Kibirige
The swale is mostly filled up during wet weather conditions and during dry weather
conditions the drainage ditch and banks is dry. It is obvious that an open channel can be
easily integrated in the park environment and the other advantage is that the construction cost
is low (Stahre & Geldof, 2003).
3.2.3 Green roof
In Augustenborg residential area, a vegetation cover of sedum grass on top of 9,500 m2 of the
roofs is applied in order to reduce the stormwater runoff, and in Augustenborg Botanical
Roof Garden, the green roofs reduce about 50% of the annual runoff (Stahre, 2008). A
section of the green roof installation at Augustenborg eco-city is shown in Figure 3.9.
Figure 3.9 Green roof installation at Augustenborg eco-city. Source: Daniel Kibirige
The green roofs can be applied in those housing estates where most of the buildings had a flat
roof, and those applied green roofs managed to reduce the stormwater runoff from the roofs
efficiently (Figure 3.9). They also played a role as an effective insulation for the buildings.
16
17
The green roofs also contribute as saving energy for heating system and protect the roof
construction.
3.2.4 Swales
In Augustenborg, artificial swales can be found all around and are designed to manage
rainfall runoff, filter pollutants and increase rainfall infiltration. The constructed swales are
generally low tract of land connected to an open channel or constructed canal. When it rains,
the rainfall runoff will firstly reach the swale, at the point where stormwater begins to
infiltrate into the ground. If the soil is saturated as a result of heavy rain, the excess runoff
will flow downwards along the slope of the swale and finally into the open channel. As
mentioned in open channel section, the canals are designed to accommodate wet weather
conditions whereas during dry weather conditions they are totally dried out. Conversely, the
swale system plays an important role especially during dry weather conditions, most of
stormwater infiltrates downward from the swale land instead of being transported to the open
channel system.
3.2.5 Storage ponds
In order to be able to handle stormwater from the residential area as well as garden parts of
Augustenborg, large detention volumes are required. The best solution was to build several
storage ponds for the excess stormwater during heavy rainfall season. In Augustenborg, the
main function of storage ponds is to delay the peak flow by storing a certain volume of
stormwater in the ponds, thus reducing the peak flow. They can also help reduce pollutant
from stormwater through sedimentation (Akan, 1993); as a result, the sewer system and
treatment plant will receive less pollutant than usual.
There are several storage ponds that exist in Augustenborg and all the ponds constitute a
certain detention volume (Figure 3.10). However, the ponds vary in appearance. The ones in
courtyards in the neighbourhood all look somewhat different with different “stamp” on each
(VA SYD, n.d.). The purpose is to make the local resident realise that each of their courtyard
should be kept clean since they own it.
Figure 3.10 Storage ponds in Augustenborg. Source: Xing Tan
Different vegetation was planted at the inlet of each pond. Before stormwater travels into the
storage ponds, vegetation will absorb nutrients from the stormwater acting as a filter. It is
very important to keep the water clean in the pond; the water in different ponds is pumped
between each other to ensure the water does not become stagnant.
17
19
4 Materials and Methods
This study involved the design of a hydrological model. A MIKE SHE model was created to
simulate the flow of water during extreme climatic events.
4.1 Data collection of study area
Data about Augustenborg eco-city was collected from a number of sources namely: Malmö
municipality, VA SYD, Sveriges Geologiska Undersökning (SGU); known as Geological
Survey of Sweden, internet and literature. The data collected included: location data,
topography data, land use map, soil map, meteorological data (rainfall and temperature).
4.2 Field investigations
A number of visits to Augustenborg eco-city were done in person to check the precise
location of open stormwater solutions.
4.3 Detailed description of MIKE SHE Model
4.3.1 Introduction
The principles behind the creation of the MIKE SHE model for Augustenborg used in this
study will be discussed in this chapter. Particular reference will be made to parts relating to
the modelling of the open stormwater solutions. There are seven different components in the
model. These components include: model domain and grid, topography, climate, land use,
overland flow, unsaturated flow and saturated zone (Figure 4.1). Of the seven much emphasis
will be placed on components that relate to water movement that are specified in the model
which include; overland flow (OL), evapotranspiration (ET).
Figure 4.1 Schematic layout of MIKE SHE model
Model Domain & Grid
• GIS to MIKE Grid
Topography • Digital Elevation
Model
Climate
• Temperature
• Precipitation
• Reference Evapotranspiration
Landuse •
Vegetation
Overland Flow
• Manning number
• Detention storage
• Initial water depth
• Subsurface leakage coefficient
Unsaturated Zone
• Soil profile definition
Saturated Zone
• Geological layers
• Computational layers
•Drainage
20
4.3.2 Model domain and Grid
Augustenborg is located in the southeast part of Malmö Municipality with the coordinates of
55° 34′ 47″ N, 13° 1′ 29″ E. The eco-city is surrounded by roads to its north, west and south
and a railway to its east. Therefore, the roads and railway constituted as fixed head
boundaries which prevented inflow from surrounding areas. These boundaries were then used
for both the surface water and groundwater components in the model.
Before setting up a hydrological model for a specific area, it is important to describe the
boundary. Therefore the first step was to define the model area. The boundaries of the model
were roughly chosen due to constraints in data. It was assumed that the boundaries of surface
water were defined not to allow the inflow of water from surrounding areas. As for the
groundwater ideally a larger area would have been used as the surface and groundwater do
not coincide due to lack of data covering a larger area; the same boundary conditions as in
surface water were used.
Initially, in this case, a xyz ASCII file containing local elevation data in Augustenborg was
interpolated to the a “MIKE grid” by using MIKE SHE TOOLBOX and the catchment is
defined by creating a Dfs2 file from a shape file. As shown in Figure 4.2 the blue area shows
the defined geometric area in the model and boundary of the whole Augustenborg eco-city as
well as its absolute location.
Figure 4.2 Design of boundary in Augustenborg, in MIKE SHE model
4.3.3 Topography
In MIKE SHE application, topography was generated by using elevation data (shown as
black dots in Figure 4.3) obtained from Malmö stad and based on the topography data the
upper boundary of the model was defined. Figure 4.3 shows the Digital Elevation Model
(DEM) for the area with the elevation corrected for all buildings. In the model, the elevation
of all buildings was raised to 1 m higher than ground surface to indicate to the model that
water should flow around the building and not infiltrate into them. Elevation is highest in the
Southeast area and lowest in Northwest and in the Eastern area it is over 20.8 m (shown in
red).
20
21
Figure 4.3 Model area of Augustenborg
Topography (Figure 4.3) is usually defined through a DEM either as a point theme shape file,
or an ASCII file (DHI, 2012). In this case, an ASCII file containing the elevation data of
Augustenborg was converted to a dfs2 file by using the MIKE ZERO TOOLBOX.
4.3.4 Climate
The climate in Malmö can be classified as mild and oceanic regardless of its northern
location compared to other coastal cities in Europe. The maximum temperature during
summer ranges from 18 to 21C and minimum temperature varies between 10 and 12C; with
the rare occasion of +25C (Figure 4.4). Winter temperatures vary from season to season but
on average the temperatures are between -3 and -4C with the odd day of winter reaching
temperatures of -10C (Climate Zone, 2004). Also, in the winter season; snow falls during the
months of December to March. Rainfall in this region is moderate throughout the year and an
average of 600 mm of rainfall per year is recorded. The driest months of the year are from
January till June which usually record less than 50 mm per month. The second half of the
year has more rainfall; at times in access of 100 mm per month.
21
22
Figure 4.4 Daily mean air temperature (2007)
In MIKE SHE, climate section is made up of four different parts including: precipitation,
reference evapotranspiration, air temperature and snow melt.
The precipitation rate, reference evapotranspiration and air temperature were all specified by
dfs0 files created for each respective section. Precipitation data from 2000 to 2011 was used
for creating the precipitation rate curve however emphasis was placed on the years 2006-
2008. Average monthly evapotranspiration (Figure 4.5) data from January to December was
used and repeated for the years 2001 to 2011.
Figure 4.5 Monthly potential evapotranspiration
In MIKE SHE model, the evapotranspiration processes were separated and modelled in
different processes (DHI, 2012): Firstly, some of the rainfall runoff is intercepted by the
vegetation canopy, from which part of the storm water evaporates. The remaining water
travels to the soil surface, and contributes as either surface runoff or percolates into the
-10
-5
0
5
10
15
20
25
1/1
/20
07
2/1
/20
07
3/1
/20
07
4/1
/20
07
5/1
/20
07
6/1
/20
07
7/1
/20
07
8/1
/20
07
9/1
/20
07
10
/1/2
00
7
11
/1/2
00
7
12
/1/2
00
7
Tem
pe
ratu
re (
°C)
Month
0
20
40
60
80
100
120
140
1 2 3 4 5 6 7 8 9 10 11 12
Po
ten
tial
Eva
po
tran
spir
atio
n
(mm
/mo
nth
)
Month of the year
22
23
unsaturated zone. Some of the infiltrating water will be evaporated from the higher parts of
the roots zone or will be transpired by the vegetation roots. The remainder of the infiltrating
water will be recharged to the groundwater and stored in the saturated zone.
MIKE SHE uses the Crop Reference Evapotranspiration rate for all calculations of
Evapotranspiration and was calculated as follows:
Equation 4.1
Where
ETref is the Reference Evapotranspiration
Kc is the Crop Coefficient specified in the Vegetation Development Table that adjusts the
Reference ET rate for different vegetation types.
The maximum amount of ET that can be removed within one time step is
Equation 4.2
Recent 15 years temperature data was collected from SMHI and latest 11 years data was
selected for setting up a df0 file in MIKE SHE. The mean temperature value was then used as
the daily temperature values and inserted in MIKE SHE model. Precipitation data was
collected from VA SYD while evapotranspiration data was provided by DHI.
When referring to the snow melt section, the value of degree-day coefficient (amount of
water that melts per day) was set to 2 mm/C/day as a constant distribution while the values
of other parts e.g. melting temperature, min snow storage, max wet snow fraction, initial total
snow storage and initial wet snow fraction are all set to 0.
4.3.5 Land Use
The land use in Augustenborg comprises of residential buildings, a school, musical
playground, trees, open lawn, wetlands and open stormwater solutions (green roofs, open
ditches, swales etc.). The newly built school building with natural materials, ground source
heat pump and solar thermal panels is located in the northern part of the eco-city.
4.3.6 Vegetation
Vegetation affects the hydrological cycle mainly through evapotranspiration, which is the
sum of evaporation and plant transpiration from the Earth's land surface to atmosphere.
Evaporation accounts for the movement of water to the air from soil and Transpiration
accounts for the movement of water within a plant and the subsequent loss of water as vapour
through stomata in its leaves. Evapotranspiration is an important part of the water cycle
therefore the vegetation factor should be defined properly in MIKE SHE.
In the MIKE SHE model, vegetation properties were set same over the whole area.
Vegetation properties were used to calculate the actual evapotranspiration from crop
reference evapotranspiration defined under climate section which was discussed previously
(DHI, 2012).
23
24
The key vegetation properties are Leaf Area Index (LAI) and Root Depth (RD). Generally the
LAI and Root Depth can be specified directly as a time series. Otherwise, they could be
defined as a crop cycle in the vegetation properties editor.
Necessary information on LAI and RD can be found from the agronomy department at local
university or relevant government sector (Yan, et al., 2012). In this case, the data type of
vegetation was selected as vegetation property file from which almost all the vegetation types
could be found. Figure 4.6 shows the vegetation properties of a mix of grass, bushes and trees
in Augustenborg.
Figure 4.6 The key vegetation properties are Leaf Area Index (LAI as shown in blue line) and
Root Depth (RD as shown in red line)
For each crop stage, three vegetation parameters needed to be specified (DHI, 2012):
1. LAI - The Leaf Area Index was defined as the area of leaves per area of ground surface.
This can vary between 0 and 7 depending of the vegetation type and the values of LAI are
widely available in the literature for most main plant types.
2. RD - The Root Depth of the crop was defined as the depth below ground in millimetres to
which roots extend (Allen, 1998). It usually changes from season to season. The soil type was
taken into consideration since some crops might develop different root distribution upon the
characteristics of soil.
3. Crop coefficient (Kc) - The crop coefficient adjusts the Reference ET rate relative to the
actual evapotranspiration for different vegetation types (DHI, 2012). Since most crops may
differ one has to consider two situations namely quoted in DHI (2012):
“In the early crop stages, where LAI of the farm crop is lower than the LAI of the
reference grass crop, the evapotranspiration of the farm crop is less then the calculated
reference evapotranspiration. This is accounted for in the Kristensen & Jensen ET
calculation, since a crop LAI is used as input. Therefore, for most field crops it is
therefore not necessary to specify Kc values below 1 in the early crop stages.”
24
25
“In the crop mid-season the opposite situation may occur where crop potential
evapotranspiration is larger than the calculated reference evapotranspiration of the
reference grass crop. This is not handled in the ET calculations, and Kc values above
1 may therefore be relevant for some crops in the mid-season during the period where
crop leaf area index is at its maximum.”
A value of 1 was chosen in this simulation. This meant that the assumption was made that the
maximum evapotranspiration rate will equal the reference evapotranspiration rate.
4.3.7 Overland flow Overland flow over Augustenborg area varies depending on the season as well as the
occurrence of an extreme climatic event. Generally the movement of water is channel through
different stormwater systems. Infiltrations occurs on permeable areas such as swales, green
roofs and open banks whereas asphalt and concrete surfaces allow the flow of water in a
controlled and safe manner.
The finite difference method was selected to simulate overland flow. It can be seen there are
several items required for calculation processes and these items are included in the main
dialogue of the model for overland flow. There items include:
1. Manning number, is equivalent to the Stickler roughness coefficient,
2. Detention Storage, is used to limit the amount of water that can flow over the ground
surface,
3. Initial Water Depth on the ground surface, is used for the overland flow calculations and
the initial water depth is usually set to 0,
4. Surface-Sub surface leakage coefficient.
Surface-subsurface leakage coefficient reduces the infiltration rate at the ground surface by
reducing both the infiltration rate and the seepage outflow rate across the ground surface
(DHI, 2012). Figure 4.7 show that all the buildings represented an impervious surface (in
blue), which meant the vertical hydraulic conductivity was very limited (close to 0).
25
26
Figure 4.7 Impervious area shown in blue in Augustenborg
The value of the leakage coefficient was set to zero for all buildings as water cannot infiltrate
through the buildings and the rest of the area was set to have full contact to the UZ/SZ zones.
4.3.8 Unsaturated Flow
Unsaturated zone is the area where some of the pores are filled with water. It is directly under
the top surface that accounts for percolation of water that has been infiltrated through the top
layer of soil. The flow of water in this zone is vertical as most of it is flowing downwards into
the saturated zone. The depth of the unsaturated zone is dependent on the level of the water
table. The water table is the border line between the unsaturated zone and the saturated zone.
In MIKE SHE there are three methods that are used to calculate unsaturated flow. These
methods include Richards’s equation, the gravity flow and the two-layer water balance. In
this instance Richards equation was used which was specified in the simulation specification
dialog. Richards equation is quoted by DHI (2012) as the, “driving force for transport of
water in the unsaturated zone is the gradient of the hydraulic head, h, which includes a
gravitational component, z, and a pressure component, . This equation is shown below:
( ) (
) Equation 4.3
Where,
K; is the hydraulic conductivity
is the pressure head
z; is the elevation
26
27
θ is the water content
t; is time
Furthermore, Pachepsky (2003) contends the Richards’ equation is the most often used in
modelling of water transport. Its application has been easy and widely used in the calculation
of unsaturated flow conditions, hence its selection for use in our model.
In this simulation the method of classification that was chosen from the model dialogue was
“Calculated in all Grid points”. This option was chosen as it is recommended for smaller
scale studies, or studies where the classification system becomes intractable, which in our
case, it’s a smaller scale study.
4.3.9 Soil profile definition
In this model the unsaturated flow is represented by the definition of the soil profile. The
dominant soil covering layer in Augustenborg is till (Figure 4.). The till was divided in to
vertical layers, the upper more permeable than the lower. A topsoil (Matjord) layer was added
to the geological description (Table 4.1 and Table 4.2). Table 4.2, displays the geological
description of swales in the model.
Figure 4.8 Soil profile definitions
Table 4.1 Soil Profile
From depth To depth Soil type
1 0 m 0.3 m Matjord 0
2 0.3 m 2 m Coarse Till (0-0.5 m)
3 2 m 5 m Coarse Till (0.5-2 m)
Note: The Matjord, was inserted as a top layer of soil.
1
0
Undefined Value
119664 120340
[meter]
6161400
6161500
6161600
6161700
6161800
6161900
6162000
6162100
6162200
6162300
[meter]
27
28
Table 4.2 Swale definitions
From depth To depth Soil type
1 0 m 0.3 m Matjord 0
2 0.3 m 2 m Gravel
3 2 m 5 m Coarse Till (0,5-2 m)
4.3.10 Saturated Zone
The saturated zone is the area where all the pores are completely filled with water. When
water enters this zone it is classified as groundwater. The top of this zone is called the water
table and is the boundary between it and the unsaturated zone.
The creation of the saturated zone in the MIKE SHE is based on 3D Finite Difference
Method which involves defining the geological model, vertical numerical discretisation,
initial conditions and boundary conditions.
The MIKE SHE Graphical User interface shows that the initial conditions are defined as a
property of numerical layer whereas the geological models as well as the vertical
discretisation are independent. The boundary conditions such as wells, drains and rivers are
also defined independently.
4.3.10.1 Geological layers
Geological layers were created using the layers that were discussed in the unsaturated zone.
In each layer, lower level, horizontal and vertical hydraulic conductivity, specific yield and
storage where chosen. This is shown in Table 4.3:
Table 4.3 Geological layers
The lower level values define the bottom of geological layers relative to the ground hence
their negative values. The hydraulic conductivities are a function of soil texture and are
related to the ease with which water flows through the soil (DHI, 2004). This mean that
compacted soil have a lower conductivity unlike loose, coarse soils (DHI, 2004) and (Fetter,
2000).
Soil Layers
Lower level Kh Kv Specific Yield Storage
Matjord -0.3 1e-006 1e-006 0.2 0.0001
Course Till 0-0.5 -2 2e-005 5e-006 0.2 0.0001
Course Till 0.5-2 -5 1e-006 1e-007 0.2 0.0001
28
29
4.3.10.2 Drainage
Drainage in the MIKE model was set based on permeability. Most areas in the study area are
permeable due to the open stormwater solutions. The areas that are not permeable are
concrete ditches, parking places and buildings. In MIKE SHE, the areas where buildings are
were set to -1.7 m relative to the ground taking into account the geological layering of soil.
Other areas where set to zero indicating that water would be able to infiltrate into those areas.
4.3.10.3 Computational layers
Of the three geological layers, it was decided to make two computational layers. The first two
layers (Matjord and Course Till 0-0.5 m) were combined into one layer and the bottom layer
(Course Till 0.5-2 m) was left as a layer on its own (Table 4.4) in order to represent the
separate soil layers.
Table 4.4 Computational layers
Computational Layers Lower level Initial Potential Head
L1 -2 -1.5
L2 -5 -1.5
4.3.11 Calibration data
In this study there was no observed data which meant that there it was no calibration and
validation to compare the results of the model.
4.4 Description of open stormwater solutions in MIKE SHE model
In MIKE SHE, all the open solutions including storage ponds (Figure 4.3), swales and open
channel are properly defined as shown in, Figure 4.9 and Figure 4.10. Green roofs were not
defined in MIKE SHE because the large amount of rainfall cannot be handled by green roofs
in other words; the capacity of green roofs for handling stormwater is limited thus they were
ignored in MIKE SHE model.
29
30
Figure 4.9 Swales area defined in red
Figure 4.10 Long open ditch system (blue line) is defined around the buildings
30
31
5 Results
This chapter presents efficiency of the open stormwater systems in Augustenborg eco-city
and the predictability flooding under extremely high precipitation conditions. The results are
presented in three simulation scenarios namely: 1year prediction of hydrological balance, 10
year extreme rainfall event and 100 year extreme rainfall event. The main focus of the results
in all three scenarios will be based on water balances i.e. a reasonable amount of
evapotranspiration and overland flow (runoff) since there was no calibration data to compare
the model results.
5.1 Scenario 1
In this scenario, the current open stormwater system was assessed for its efficiency in
response to rainfall under the current weather conditions which was represented by measured
rainfall events. The mean daily rainfall was chosen for the period 2006-2008 (Figure 5.1
Mean Precipitation (2000-2011)). Initially a 10 year period was chosen however after
analysis of rainfall events, the period 2006-2008 had large amounts of high rainfall events
which would gave more credit to our study as we were looking into the efficiency of the
system under high rainfall events. To further justify the above mentioned point closer
emphasis was placed on the year 2007 as one of the highest mean precipitations of 178 mm
per month was recorded. In 2007, the water balance showed a ratio between rainfall and
evapotranspiration of 75% (Figure 5.2). In 2007 overland flow was only seen in small regions
and there was notable minimal flooding in the system on the South western part of Figure 5.3.
Then in the month whereby precipitation was the highest in July (111 mm); overland water
depth is 0.65 m (Figure 5.3) which is also minimal showing that there is little amount of
flooding in the area during the season where there is continuous rainfall.
Figure 5.1 Mean Precipitation (2000-2011)
0
50
100
150
200
250
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
Rai
nfa
ll (m
m/m
on
th)
Year
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
31
32
Figure 5.2 Water Balance (2007)
Figure 5.3 Depth of Overland Flow (2007)
5.2 Scenario 2
In order to evaluate any water system it is essential to assess the risk of flooding in the area.
In this scenario the system in Augustenborg was put under a 10 year extreme rainfall event
which takes place on the 31st of July, 2007. A total of two weeks were simulated; one week
32
33
before and after the 31st of July in order to see how the flooding propagates during the event.
The extreme rainfall event was calculated by taking the averages between two time periods at
which rainfall was recorded (Table 5.1) e.g. 12h00-14h00, 14h00-14h15 etc. A figure of 62
mm/day (showing an accumulation of rainfall) was calculated and inserted into the last day of
July. All other parameters were kept the same except the storing of results. In this scenario
the overland flow was stored every 12 minutes to see the effect of flooding more clearly.
Table 5.1 Precipitation data on 31st July, 2007 for 10 years extreme event from DHI
Time Precipitation rate (mm/h)
2007/7/31 12:00 0.00
2007/7/31 14:00 2.76
2007/7/31 14:15 5.00
2007/7/31 14:30 6.60
2007/7/31 14:40 9.30
2007/7/31 14:45 12.72
2007/7/31 14:50 17.40
2007/7/31 14:52 24.24
2007/7/31 14:55 33.84
2007/7/31 14:57 57.36
2007/7/31 15:02 103.80
2007/7/31 15:05 57.36
2007/7/31 15:07 33.84
2007/7/31 15:10 24.24
2007/7/31 15:15 17.40
2007/7/31 15:20 12.72
2007/7/31 15:30 9.30
2007/7/31 15:45 6.60
2007/7/31 16:00 5.00
2007/7/31 18:00 2.76
2007/7/31 0:00 1.42
2007/8/01 12:00 0.84
33
34
In Figure 5.4, the water balance shows precipitation is much higher compared to
evapotranspiration (103 mm compared to 33 mm) of which 33% of the water leaves the
system as evapotranspiration. Water directly infiltrates to the saturated zone (63 mm) and it
can be inferred that the upper soil layer is not saturated yet because there is very little outflow
(1 mm) observed in the unsaturated zone. Therefore the open stormwater system is capable
for handling a 10 years extreme event.
Figure 5.4 Water balance of 10 years extreme events for 2 weeks simulation
Overland flow is presented in Figure 5.5 showing how the extreme event propagates from 30
minutes to 3 hours and finally how the system is after 3 days. It can be inferred that after the
stormwater infiltrates into the soil (shown by blue colour in Figure 5.5) and only a few areas
will be flooded e.g. the swale area on the western part, where the most prevalent height of
flood is about 0.6 m and at the highest height above 0.8 m. These values are low enough for
the system to recover after a number of days as it rarely rains continuously in South Sweden
unlike tropical climates.
34
35
Figure 5.5 Depth of Overland flow after precipitation for 10 years extreme event (Left to
Right; 30 minutes, 3 hours and at bottom 3 days)
5.3 Scenario 3
In this scenario a 100 year extreme event was simulated. Once again the event took place on
the 31st of July, 2007. Similarly a two week simulation (one week before and after the
abovementioned date) was made. The same procedure was used (with values from Table 5.2)
to calculate the precipitation value that was inserted on the last day of July. An estimated
amount of 128 mm/day precipitation rate was obtained using the same method as in the 10
year extreme rainfall event.
35
36
Table 5.2 Precipitation data on 31st July, 2007 for 100 years extreme event from DHI
Time Precipitation rate (mm/h)
2007/7/31 12:00 0.00
2007/7/31 14:00 4.47
2007/7/31 14:15 8.64
2007/7/31 14:30 11.88
2007/7/31 14:40 17.64
2007/7/31 14:45 25.20
2007/7/31 14:50 36.36
2007/7/31 14:52 53.28
2007/7/31 14:55 77.76
2007/7/31 14:57 136.80
2007/7/31 15:02 205.92
2007/7/31 15:05 136.80
2007/7/31 15:07 77.76
2007/7/31 15:10 53.28
2007/7/31 15:15 36.36
2007/7/31 15:20 25.20
2007/7/31 15:30 17.64
2007/7/31 15:45 11.88
2007/7/31 16:00 8.64
2007/7/31 18:00 4.47
2007/7/31 0:00 2.15
2007/8/01 12:00 1.22
From the water balance (Figure 5.6) it can be seen that the precipitation was even much
higher compared to 10 years extreme event (169 mm compared to 10 3mm), of which 119
mm of water infiltrates to the saturated zone while 33 mm evaporated. The evapotranspiration
is still 33 mm which is the same as in the 10 year extreme event simulation results because no
change has been made on the vegetation or soil. However, 5 mm of water at the boundary
flow was observed in unsaturated zone compared to no flow of water out of the boundary in
36
37
the UZ in the 10 year extreme event simulation. This could mean that in the 100 year event
some areas within the area would be at risk of flooding as there is complete saturation in the
unsaturated and saturated zones causing outflow of water at the boundary.
Overland flow was recorded in this scenario as in scenario 2 (30 minutes, 3 hours & 3 days).
By checking of the overland flow depth (Figure 5.7), most of the swales are flooded with an
average depth of water of about 0.8 m with a few areas above 1.4 m. Some residential areas
are also under risk of flooding, which puts the system under stress in handling the 100 years
extreme rainfall because most of soils are already saturated under such a condition. The top
layer of soil is predicted to be more or less saturated and this could lead to more areas being
flooded.
Figure 5.6 Water balance of 100 years extreme events for 2 weeks simulation
37
38
Figure 5.7 Overland flow depth after precipitation for 100 years extreme event (Left to Right;
30 minutes, 3 hours and at bottom 3 days)
38
39
6 Discussion
In this chapter a detailed explanation into each step of modelling as well as the results of each
scenario will be explained and interpreted in line with our aims and objectives.
Generally the input of data into the model will be briefly explained and justified as to why
certain parameters where chosen. Firstly looking at the quality of input data, it can be said
that the data was of good and acceptable quality as the results of the simulations look
reasonable with a few exceptions here and there. Geographical data of Augustenborg could
have been better to show more elevation points to ensure a better representation of the
topography. This would have helped in the knowing different slopes more precisely. In
addition, more elevation points should be added to have a larger area around the
Augustenborg area to ascertain that there is no flow entering Augustenborg.
With regard to the geological layers, more information regarding the type of soils present
across the entire study area would have made the results even more credible. As only two
types of soil (Matjord and Upper & lower Course Till) were used, this could have been one of
the reasons for the extremely high evapotranspiration in the water balance. Also if vegetation
had been described in the model possibly transpiration could have been better represented
and this could have resulted in a lower evapotranspiration value.
Green roofs are an integral part of Augustenborg however as mentioned in the results section
their function would be limited in this study which looked into major flooding as they do not
have the capacity to hold a lot of water and the little water they do hold would not be
significant in a large rainfall event. However, if green roofs can be described in the model it
could have been some sort of canopy interceptor but due to the design of the model domain it
would have been difficult to describe green roofs clearly as the buildings were set as
impermeable.
Finally with regard to the authenticity of the results it would have been better if we were able
to calibrate and validate the model but due to a lack of observed data this made our results
more of a stepping stone to future studies. However, temperature and rainfall data did add
credibility to the results indicating that during summer months there was a large increase in
precipitation which is common in the South of Sweden during the summer months. Also the
mean temperature range was relatively close to what literature states it should be. In addition
the hydraulic conductivity used in the geological layers was within the standards of the
Geological Survey of Sweden.
MIKE SHE hydrological model was created and three main scenarios investigated. These
scenarios revealed the following:
6.1 Scenario 1
This scenario showed how the current system acts in response to normal amount of rainfall
during a three year period (2006-2008) with particular interest in the year 2007. In this year
(2007), there was a large amount of rainfall recorded which drove the researchers to choose
this year as the major sample in determining whether or not the system is capable of handling
normal rainfall with slight variations.
39
40
From the results the system was able to handle the amount of rainfall over the year 2007 as
all the parameters on the water balance was reasonable. This period did however show the
best representative scenario over a ten year period (2000-2011). From precipitation data it can
be seen that these years (2006-2008) clearly were of best for the analysis to the system in the
event of large daily as well as monthly events of rainfall. In July 2007 the highest amount of
overland flow recorded was above 0.8 m but due to the swales within the area, the water was
able to be infiltrated and stored in the unsaturated and saturated zones.
6.2 Scenario 2
With every system that is designed it has to be tested under the most extreme conditions no
matter how impossible those conditions seem. In this scenario a 10 year rain event was tested
to see how the system would respond in relation a high rain event over a period of time. A 12
hour period was chosen and simulated. The results showed that the system was capable of
handling a 10 year event over a period of 12 hours with specific time interval. Generally over
the entire area the depth of overland flow was in the range of 0.1 5m with only the swale
above 0.6 m indicating that rather small amounts of water were flowing on the surface but
most of the water was able to be infiltrated into the system so that there was no flow of water
through the boundary and no water was flooding the residential area.
6.3 Scenario 3
A similar method was used as in scenario 2 but in this case it was aimed at looking at a longer
period of 100 years. The 100 year period yielded different results and for the first time the
unsaturated zone reached its threshold and there was overland flow on the ground surface.
There were a number of areas of values above 1m which indicated that there is severe a risk
of flooding during this extreme event. Also 5 mm of outflow was recorded from the water
balance as the water flowing out of the system. This is an indication that in the unlikely event
of an extreme rain for more than 24 hours, there will definitely be an accumulation of water
that will cause a high risk of flooding. This range is marginal in a lot of areas around the
system and possibly the system would be under high alert to possible flooding and measures
need to be in place to ensure the probable damage caused by flooding is reduced. However,
with global trends including climate change it can be said that this event puts the system
under a lot of pressure to handle so much water. Thus, it can be concluded that during this
scenario the system is under a dangerous risk of flooding and there needs to some measures
put in place to ensure that if large amount of rainfall falls like in this scenario it will be able
to handle it.
As stated in one of the objectives of this study, certain measures of dealing with risk of
flooding need to be implemented due to flooding e.g. 100 year extreme event. One particular
measure that could be enhanced further is channelling of more water out of the open
stormwater solutions in Augustenborg into the municipal sewer to reduce the amount of
water collecting in the storage ponds that could cause overflow.
40
41
7 Conclusion
The overall efficiency of the system during current conditions (2007) and a 10 year extreme
rainfall event are able to handle large amounts of water over a short period of time. The 100
year extreme event was not able to handle the flow well enough and there was major flooding
in the South Western part of Augustenborg and in the North here and there. This flooding
would be a problem for the residents.
Most of the open stormwater solutions performed their specific function with a few
exceptions. It can be seen the areas where ponds are located, there is no flooding meaning
that they worked as a storage medium even in the model which is its function (storing water).
Open ditches are also performing their function well because the 100 year simulation result
shows very few areas are under serious flooding. Swales are the most efficient and are
considered as the most useful solution in this system, most stormwater is transported and
stored around the swales which are reasonable distance from the residential areas where the
risk of flooding is greatest.
The open stormwater system in Augustenborg is well suited to handle extreme rainfall events
over a 10 year period provided the current conditions are well maintained, whereas it is not
well suited enough to handle a rain event of a 100 years.
41
43
8 Suggestion and recommendations
The first suggestion is, since, a pipe network of the stormwater system was not taken into
consideration, and possibly it should be considered by the using MIKE URBAN (urban water
modelling software). This could have shown more analysis of how water is transported after
being channelled through the specific open stormwater solutions. The current MIKE SHE
model can be coupled with MIKE URBAN. Coupling of models is becoming a common
trend in the field of hydrology and it is believed that this can improve the results of this study
even more. MIKE SHE will look at the hydrological cycle of the entire system whereas
MIKE URBAN will focus on the actual piping system and description of the open system in
the area.
Secondly as one of objectives was to suggest ways of minimising the risk of flooding the
researchers suggest that describing more open stormwater solutions will improve the model.
Green roofs need to be described in the model to kind of act as an interceptor for falling
rainfall to see what effect this will have on the flow of water in the system.
43
45
References
Ahmed, N. (2010). Runoff water quality from a green roof and in an open storm water
system. Lund, Sweden: Division of Water Resources Engineering, Lund University.
Akan, A. (1993). Urban stormwater hydrology : a guide to engineering calculations.
Lancaster Pa: Technomic Pub. Co.
Allen, R. G. (1998). Crop evapotranspiration - Guidelines for computing crop water
requirements. FAO.
Berggren, K. (2007). Urban drainage and climate change - impact assessment. Sweden:
Licentiate thesis, Lulea university of technology.
Butler, D. & Davies, J. W. (2004). Urban drainage (2nd ed.). London: Spon Press.
Butts , M. B., Payne, J. T. & Overgaard, J. (2004). Improving streamflow simulations and
flood forecasts with multimodel ensembles. In Liong, Phoon & Babovic (Ed.). (pp.
1189-1196 ). Singapore: World Scientific Publishing Company.
Chocat, B., Krebs, B., Marsalek, J., Rauch, W. & Schilling, W. (2001). Urban drainage
redefined: from stormwater removal to integrated management. Water Science &
Technology, 43(5), 61–68.
Chow, V. T. (2009). Open Channel Hydraulics. Leeds University.
Christensen, F. D. (2004). Coupling between the river basin mangement model (MIKE
BASIN) and the 3D Hydrological model (MIKE SHE) with the use of the OPENMI
system. 6th International Conference on Hydro informatics.
City of Indianapolis. (n.d.). Stormwater Design and Specification Manual. Retrieved May 12,
2013, from http://www.uwrwa.org/bmpTool/factSheets/4_7_Swales.pdf
Climate Zone. (2004). Climate zone. Retrieved May 16, 2013, from http://www.climate-
zone.com/climate/sweden/celsius/malmo.htm
Cone, W. C. (2005). Stormwater Management Trends: A review of tools, techniques and
methods for design and development of the land with implications for sustainable
design. Landscape Architecture & Regional Planning Masters Projects.
Daniel, E. B., Camp, J. V., LeBouf, E. J., Penrod, J. R., Dobbins, J. P. & Abokowitz, M. D.
(2011). Watershed Modeling and its Applications: A State-of-the-Art Review. The
Open Hydrology Journal, 5, 26-50.
DHI. (2004). MIKE SHE User Manual. Horsholm, Denmark.
45
46
DHI. (2011). MIKE 11 - river modelling unlimited. Retrieved May 01, 2013, from
http://www.dhisoftware.com/Products/WaterResources/MIKE11.aspx
DHI. (2011). MIKE URBAN - modelling water in the city. Retrieved May 12, 2013, from
http://www.dhisoftware.com/Products/Cities/MIKEURBAN.aspx
DHI. (2012). Help guide. Denmark.
Ekostaden Augustenborg. (n.d.). Retrieved March 29, 2013, from
http://www.rolfsdotter.se/pdf/Ecocity_Aug.pdf
European Sustainable Urban Development Projects. (n.d.). Retrieved March 29, 2013, from
http://www.secureproject.org/download/18.360a0d56117c51a2d30800078401/121075
0905824/Ekostaden_Malm%C3%B6_Sweden.pdf
Family Business Institute. (n.d.). Green Roofs, Living Architecture. Retrieved March 29,
2013, from http://www.family-business-experts.com/green-roofs.html
Fetter, C. W. (2000). Applied Hydrogeology (4th ed.).
Freeze, R. A. & Harlan, R. L. (1969). Blueprint of a physically-based, digitially simulated
hydrologic response model. Journal of Hydrology, 9, 237-258.
French, R. (2007). Open Channel Hydraulics. Highlands Ranch Colorado: Water Resources
Publications LLC.
Good, J. C. (1993). Roof runoff as a diffuse source of metals and aquatic toxicity in
stormwater. Water Science and Technology, 28, 317-321.
Kleidorfer, M., Moderl, M., Sitzenfrei, R., Urich, C. & Rauch, W. (2009). Water Science
Technology, 60, 1555-1564.
Malmö Stad. (n.d.). Retrieved April 2, 2013, from http://www.malmo.se/English/Sustainable-
City-Development/Augustenborg-Eco-City/The-Green-City.htm
Mehler, R. & Ostrowski, M. (1998). Comparison of the efficiency of best stormwater
management practices in urban drainage systems. London,UK.
Mihelcic, J. R., Crittenden, J. C., Small, M. J., Shonnard , D. R., Hokanson, R. D., Zhang, Q.,
Chen, H., Sorby, S A., James, V. U., Sutherland, J. W., Schnoor, J L. (2003).
Sustainability Science and Engineering: The Emergence of a New Metadiscipline.
Environmental Science & Technology, 37(23), 5314-5324.
National Small Flow Clearinghouse. (2000). Pipeline,decentralized wastewater treatment
system. Pipeline.
Pachepsky, Y., Timlin, D. & Rawls, W. (2003). Generalized Richards’ equation to simulate
water transport in unsaturated soils. Journal of Hydrology, 272, 3-13.
46
47
Peck, S. W., Cameron, R. D. & Liptan, T. (2009). Green Roofs: Beautiful and Innovative
Solutions to Stormwater Pollution.
Scholes, L., Ellis, B. & Revitt, M. (2007). Drives for future urban storm water management.
University of Birmingham, U.K.: First SWITCH scientific meeting.
Shukri, A. (2010). Hydraulic Modeling of Open Stormwater System in Augustenborg,
Sweden. Lund: Lund University.
Stahre, P. (2008). Blue- Green Fingerprints in the City of Malmö, Sweden. Malmö: Malmö
stad, VA-SYD.
Stahre, P. & Geldof, G. D. (2003). New Approach to Sustainable Stormwater Planning.
Malmö: Augustenborg´s Botanical Roof Garden.
Steiner, F. (2002). Human Ecology: Following Nature's Lead. USA: Island Press.
Thompson, J. R., Sorenson, H. R., Gavin, H. & Refsgaard, A. (2004). Application of the
coupled MIKE SHE/MIKE 11 modelling system to a lowland wet grassland in
southeast England. Journal of Hydrology, 293, 151–179.
USGS. (2010). Unsaturated Flow Basics. Retrieved April 25 , 2013, from
http://wwwrcamnl.wr.usgs.gov/uzf/unsatflow/unsatflow.html
VA SYD. (n.d.). Eco-city Augustenborg. Malmö.
Villarreal, E. L. & Bengtsson, L. (2005). Response of a sedum green-roof to individual rain
events. Ecological Engineering, 25, 1-7.
Villarreal, E. L., Semadeni-Davies, A. & Bengtsson, L. (2004). Inner city stormwater control
using a combination of best management practices. Ecology Engineering, 22(279-
298).
Yan, H., Wang , S. Q., Billesbach , D., Oechel , W., Zhang, J. H., Meyers , T., Martin, T A.,
Matamala, R., Baldocchi , D., Bohrer, G., Dragoni, D. & Scott, R. (2012). Global
estimation of evapotranspiration using a leaf area index-based surface energy. Remote
Sensing of Environment, 124, 581-591.
47
Appendix.
EVALUATION OF OPEN STORMWATER SYSTEMS DURING EXTREME
EVENTS: A CASE STUDY OF AUGUSTENBORG
Daniel Kibirige and Xing Tan
Water and Environmental Engineering, Department of Chemical Engineering, Lund Univer-
sity, Sweden
Abstract
The stormwater system has gone through major changes in past decades. A hydrological
model using DHI’s MIKE SHE was built for the area of Augustenborg Eco-city in Sweden,
in order to evaluate potential flooding under 10 year and 100 year extreme precipitation con-
ditions. The result showed that during the 10 year extreme event, the depth of overland flow
was in the range of 0.15 m with only a few areas above 0.60 m indicating that rather minor
overland flow. In the 100 year event not only the swales are at risk of flooding with a maxi-
mum depth of water of above 1 m in some places but also some residential areas. Most of
the open stormwater solutions performed their specific function effectively, especially
swales, and the open stormwater system in Augustenborg is well suited to handle a 10 year
extreme event but not 100 year extreme event.
Introduction
The activities of man have in many ways
affected water cycle processes today
(Butler & Davies, 2004). For example
when rain falls, water should either evapo-
rate or infiltrate into the soil however in
urban areas this is not the case due to ur-
ban development. There is more paving in
urban areas than green areas causing the
high flow of water. This puts urban drain-
age systems under a lot of pressure in han-
dling the increasing amount of water fol-
lowing through them.
The stormwater system has gone through
major changes in past decades (Malmö
Stad, n.d.). It was found that as a result of
urbanisation there is a 20% increase in rain
intensity (Kleidorfer, et al. 2009). This
article discusses the effect of two extreme
rainfall events on an open stormwater sys-
tem using a MIKE SHE model.
Study area
Augustenborg eco-city located in the
Southern county of Skåne, Sweden, was
the area of study. It covers an approximate
area of 32 hectares and has a total popula-
tion of about 3000 people (Shukri, 2010).
Augustenborg is Sweden´s largest urban
sustainability project and is renowned for
its sustainable urban planning, waste man-
agement as well as the effective open
stormwater system (Malmö Stad, n.d.).
The different solutions found in Augusten-
borg include: green roofs, vegetated swales
open ditches and storage ponds.
Figure 1 Aerial View of Augustenborg
MIKE SHE model
A MIKE SHE model was built to mimic
the open stormwater systems in Augusten-
borg in order to see the flow of water over
the area. In the model there are seven dif-
ferent components. These components are:
model domain and grid, topography, cli-
mate, land use, overland flow, unsaturated
flow and saturated zone (Figure 1). Of the
seven much emphasis was placed on com-
ponents that relate to water movement
which include; overland flow (OL).
Figure 2 Illustration of MIKE SHE Model
Figure 2 shows the steps that were under-
taken in creating the model. The first step
set a model domain and grid showing the
exact area. Then the second step was to
define the model area using the MIKE
SHE Toolbox and elevation data to show
the boundary of the area. Thereafter a Dig-
ital elevation model (DEM) was created
using ArcGIS to show how the topography
varied.
The third input into the model was climate
data. Climate data consisted of, tempera-
ture (Figure 3), evapotranspiration (Figure
4) and precipitation (Figure 5) which were
collected from DHI.
Figure 3 Temperature Data
Model Domain & Grid
Topography
Climate
Landuse
Overland Flow
Unsaturated Zone
Saturated Zone
-10
0
10
20
30
1/1
/20
07
3/1
/20
07
5/1
/20
07
7/1
/20
07
9/1
/20
07
11
/1/2
00
7
Tem
pe
ratu
re (
°C)
Months
Figure 4 Evapotranspiration data
Figure 5 Rainfall Data
The fourth step is to describe land use by
illustrating the open ditches and swales.
The green roofs were ignored because their
impact during extreme rainfall conditions
would be minimal. In the MIKE SHE
model, vegetation properties were set same
over the whole area. Vegetation properties
were used to calculate the actual evapo-
transpiration from crop reference evapo-
transpiration (DHI, 2012). The key vegeta-
tion properties designed in the model were
are Leaf Area Index (LAI) and Root Depth
(RD), (Yan, o.a., 2012). Generally the LAI
and RD can be specified directly as a time
series. Otherwise, they could be defined as
a crop cycle in the vegetation properties
editor.
The fifth component that was described in
the model was overland flow. The finite
difference method was selected to simulate
overland flow. There were several items
required for calculation processes in the
main dialogue of the model for overland
flow. There items include:
1. Manning number; was equivalent to the
Stickler roughness coefficient
2. Detention Storage; was used to limit the
amount of water that can flow over the
ground surface
3. Initial Water Depth on the ground sur-
face; was used for the overland flow calcu-
lations and the initial water depth is usual-
ly set to 0
4. Surface-Sub surface leakage coefficient;
the value of the leakage coefficient was set
to zero for all buildings as water cannot
infiltrate through the buildings and the rest
of the area was set to have full contact to
the UZ/SZ zones.
The sixth step in the design of the model
was defining the unsaturated zone. In
MIKE SHE there are three methods that
can be used to calculate unsaturated flow.
These methods include Richards’s equa-
tion, the gravity flow and the two-layer
water balance. In this instance Richards
equation was used which was specified in
the simulation specification dialog. Rich-
ards equation is quoted by (DHI, 2012) as
the, “driving force for transport of water in
the unsaturated zone is the gradient of the
hydraulic head, h, which includes a gravi-
tational component, z, and a pressure com-
ponent, ψ.” Its equation is:
( ) (
)
Where,
K; is the hydraulic conductivity
ψ is the pressure head
z; is the elevation
0
20
40
60
80
100
120
140
1 3 5 7 9 11
Po
ten
tial
Eva
po
tran
spir
atio
n
(mm
/mo
nth
)
Months of the year
0
50
100
150
200
250
Rai
nfa
ll (m
m/m
on
th)
Year
Jan
Feb
Mar
Apr
May
Jun
θ; is the water content
t; is timed
Soil profile definitions were defined using
Tables 1-4, listed below:
Soil profile definition
Table 1 Soil Profile
From
depth
To
depth
Soil type
1 0m 0.3m Matjord 0
2 0.3m 2m Coarse Till (0-0.5m)
3 2m 5m Coarse Till (0.5-2m)
Note: The Matjord was inserted as a top
layer of soil.
Table 2 Geological layers
Soil layers
Lower
level
Kh Kv Specific
Yield
Storage
Matjord -0.3 1e-
006
1e-
006
0.2 0.0001
Course
Till 0-0.5
-2 2e-
005
5e-
006
0.2 0.0001
Course
Till 0.5-2
-5 1e-
006
1e-
007
0.2 0.0001
Table 3 Swale definitions
From
depth
To depth Soil type
1 0m 0.3m Matjord 0
2 0.3m 2m Gravel
3 2m 5m Coarse Till (0.5-2m)
The seventh step in design of the model
was the saturated zone. The creation of the
saturated zone in the MIKE SHE is based
on 3D Finite Difference Method which
involves defining the geological model,
vertical numerical discretisation, initial
conditions and boundary conditions.
The MIKE SHE graphical user interface
shows that the initial conditions are de-
fined as a property of numerical layer
whereas the geological models and the
vertical discretisation are independent.
Drainage
Drainage in the MIKE model was set
based on permeability. Most areas in the
study area are permeable due to the open
stormwater solutions. The areas that are
not permeable are concrete ditches, park-
ing places and buildings. In MIKE SHE,
the areas where buildings are located were
set to -1.7 m (an estimated figure indicat-
ing no infiltration in the top soil; matjord
& upper course till layers) relative to the
ground taking into account the geological
layering of soil. Other areas where set to
zero indicating that water would be able to
infiltrate into those areas.
Computational layers
Of the three geological layers two compu-
tational layers were formed. The first two
layers (Matjord and Course Till 0-0.5)
were combined into one layer and the bot-
tom layer (Course Till 0.5-2) was left as a
layer on its own (Table 4).
Table 4Computational layers
Computational
Layers
Lower
level
Initial Potential
Head
L1 -2 -1.5
L2 -5 -1.5
Calibration data
In this study there was no observed data
which meant that there was no calibration
and validation to compare the results of the
model.
Description of open stormwater solu-
tions in MIKE SHE model
In this study some of the open solutions
including storage ponds, swales and open
channels are defined (Figure 6 and Figure
7).
Figure 6 Swales area defined in red
Figure 7 Long open ditch system (blue line) is
defined around the buildings
Results
The system in Augustenborg was put un-
der a 10 year extreme rainfall event which
took place on the 31st July, 2007. A total of
two weeks were simulated; one week be-
fore and after the 31st July in order to see
how flooding propagates during the event.
Using data obtained from DHI (for 31st
July 2007), the extreme rainfall event was
calculated by taking the averages between
two time periods at which rainfall was
recorded e.g. 12h00-14h00, 14h00-14h15
etc. (Table 5). A figure of 62 mm/day
(showing to accumulation of rainfall) was
calculated and inserted into the last day of
July.
Table 5 Precipitation data on 31st July, 2007
for 10 years extreme event from DHI
Time Precipitation rate(mm/h)
2007/7/31 12:00 0.00
2007/7/31 14:00 2.76
2007/7/31 14:15 5.00
2007/7/31 14:30 6.60
2007/7/31 14:40 9.30
2007/7/31 14:45 12.72
2007/7/31 14:50 17.40
2007/7/31 14:52 24.24
2007/7/31 14:55 33.84
2007/7/31 14:57 57.36
2007/7/31 15:02 103.80
2007/7/31 15:05 57.36
2007/7/31 15:07 33.84
2007/7/31 15:10 24.24
2007/7/31 15:15 17.40
2007/7/31 15:20 12.72
2007/7/31 15:30 9.30
2007/7/31 15:45 6.60
2007/7/31 16:00 5.00
2007/7/31 18:00 2.76
2007/7/31 0:00 1.42
2007/8/01 12:00 0.84
All other parameters were kept the same
except the storing of results. In this case
the overland flow was stored every 12
minutes to see the effect of flooding clear-
ly.
In Figure 8 the water balance shows pre-
cipitation is much higher compared to
evapotranspiration (103 mm compared to
33 mm) of which 33% of the water leaves
the system as evapotranspiration. Water
directly infiltrates to the saturated zone (63
mm) and it can be inferred that the upper
soil layer is not saturated yet because there
is very little outflow (1 mm) observed in
the unsaturated zone. Therefore the open
stormwater system is capable for handling
a 10 years extreme event based on the hy-
drological cycle.
Figure 8 Water balance of 10 years extreme
events for 2 weeks simulation
Overland flow is presented in Figure
showing how the extreme event propagates
from 30 minutes to 3 hours and finally
after 3 days. It can be inferred that after
the stormwater infiltrates into the soil
(shown by blue colour in Figure 9) only a
few areas will be flooded (green-to-yellow
areas in Figure 9), where the most preva-
lent height of flood is about 0.6 m and at
the highest height above 0.8 m. These val-
ues are low enough for the system to re-
cover after a number of days as it rarely
rains continuously in South Sweden for
extreme flooding to occur. Thus the Figure
8 is in agreement with Figure 9 indicating
that the system can indeed handle a 10
year extreme rainfall event.
Figure 9 Overland Flow 10 year event
Similarly in the 100 year extreme event the
process was simulated using the same pro-
cedure as the 10 year event. Also, the same
procedure was used (as in the 10 year
event) to calculate the extreme precipita-
tion value that was inserted on the last day
of July (Table 6). An amount of 128
mm/day was obtained according to accu-
mulation of rainfall for each time period
where it rained.
Table 6 Precipitation data on 31st July, 2007
for 100 years extreme event from DHI
Time Precipitation rate(mm/h)
2007/7/31 12:00 0.00
2007/7/31 14:00 4.47
2007/7/31 14:15 8.64
2007/7/31 14:30 11.88
2007/7/31 14:40 17.64
2007/7/31 14:45 25.20
2007/7/31 14:50 36.36
2007/7/31 14:52 53.28
2007/7/31 14:55 77.76
2007/7/31 14:57 136.80
2007/7/31 15:02 205.92
2007/7/31 15:05 136.80
2007/7/31 15:07 77.76
2007/7/31 15:10 53.28
2007/7/31 15:15 36.36
2007/7/31 15:20 25.20
2007/7/31 15:30 17.64
2007/7/31 15:45 11.88
2007/7/31 16:00 8.64
2007/7/31 18:00 4.47
2007/7/31 0:00 2.15
2007/8/01 12:00 1.22
From the water balance (Figure 10), pre-
cipitation is much higher compared to 10
years extreme event (169 mm compared to
103 mm), of which 119 mm of water infil-
trates to the saturated zone while 33 mm
evaporated. The evapotranspiration is still
33 mm which is the same as in the 10 year
extreme event simulation results because
no change was made to the vegetation or
soil layers. However, 5 mm of water at the
boundary flow was observed in unsaturat-
ed zone compared to no flow of water out
of the boundary in the UZ in the 10 year
extreme event simulation. This could
mean that in the 100 year event some areas
within the area would be at risk of flooding
as there is complete saturation in the un-
saturated and saturated zones causing out-
flow of water at the boundary.
By checking of the overland flow depth in
(Figure 11), most of the area where swales
are located flooded with an average depth
of water of about 0.8 m with a few areas
above 1.4 m. Some residential areas are
also under risking flooding, which puts the
system under stress in handling the 100
years extreme rainfall because most of soil
is saturated.
Figure 10 Water balance of 100 years extreme
events for 2 weeks simulation
Figure 11 Overland Flow 100 year event
Discussion
The results showed that the system was
capable of handling a 10 year event over a
period of 12 hours. Generally over the en-
tire area the depth of overland flow was in
the range of 0.15 m with only the region
were swale are located above 0.6 m indi-
cating that rather small amounts of water
flowing. It can be inferred that most of the
water infiltrated into the system and there
was no flow of water through the boundary
and flooding in the residential area.
The 100 year period yielded different re-
sults and for the first time the unsaturated
zone reached its threshold and there was
overland flow on the ground surface.
There were a number of areas of with val-
ues above 1m which indicated that there is
severe a risk of flooding during this ex-
treme event. Also 5mm of outflow was
recorded on the water balance indicating
water flowing out of the system. This is an
indication that in the unlikely event of an
extreme rain for more than 24 hours, there
will definitely be an accumulation of water
that will cause a high risk of flooding.
In addition, with global trends including
climate change it can be said that this
event puts the system under a lot of pres-
sure to handle so much water. Thus, it can
be concluded that during this scenario the
system is under a dangerous risk of flood-
ing and there needs to some measures put
in place to ensure that if large amount of
rainfall falls like in this scenario it will be
able to handle it. On the contrary, it is also
rare that a 100 year extreme event can oc-
cur in the South of Sweden but we should
not neglect anything hence the investiga-
tion.
Most of the open stormwater solutions
performed their specific function with a
few exceptions. It can be seen the areas
where ponds are located, there is no flood-
ing meaning that they worked as a storage
medium even in the model which is their
function (storing water). Open ditches also
performed their function well because
from the 100 year simulation result, very
few areas are under serious flooding even
if the soil is saturated meaning the open
channels transport water to the outlet
ponds efficiently. Swales are the most effi-
cient and are considered as the most useful
solution in this system, most stormwater is
transported and stored around the swales
which are reasonable distance from the
residential areas where the risk of flooding
is greatest
Conclusion
The overall efficiency of the open storm-
water systems during a 10 year extreme
rainfall event shows that the system is able
to handle large amounts of water over a
short period of time. The 100 year extreme
event was not able to handle the flow of
water well enough and there was major
flooding in the South Western part of Au-
gustenborg and in the North here and
there.
References
Butler, D., & Davies, J. W. (2004). Urban
drainage (2nd ed.). London: Spon Press.
Climate Zone. (2004). Climate zone.
Retrieved May 16, 2013, from
http://www.climatezone.com/climate/swed
en/celsius/malmo.htm
DHI. (2012). Help guide. Denmark.
Kleidorfer, M., Moderl, M., Sitzenfrei, R.,
Urich, C., & Rauch, W. (2009). Water
Science Technology, 60, 1555-1564.
Shukri, A. (2010). Hydraulic Modeling of
Open Stormwater System in Augustenborg,
Sweden. Lund: Lund University.
Villarreal, E. L., & Bengtsson, L. (2005).
Response of a sedum green-roof to
individual rain events. Ecological
Engineering, 25, 1-7.
Yan, H., Wang , S. Q., Billesbach , D.,
Oechel , W., Zhang, J. H., Meyers , T.,
Martin, T A., Matamala, R., Baldocchi ,
D., Bohrer, G., Dragoni, D & Scott, R
(2012). Global estimation of
evapotranspiration using a leaf area index-
based surface energy. Remote Sensing of
Environment, 124, 581-591.