Bluegreengrey (BGG) solution for future climate and flood-resilient urban drainage network by enhancing the natural hydrological cycle

Abstract

A loop of urbanization, increasing impervious surface in urban areas, increasing temperature,
rainfall, surface runoff and flooding, and lack of new spaces have been adding challenges to urban
drainage systems for decades. The climate change impacts are exposing the existing vulnerable
conventional urban drainage systems to more challenges in the future. A new idea of sustainable
drainage system named Bluegreengrey (BGG) which was introduced by EDGEs brings a solution to
retrofit the existing landscape and drainage system in a more efficient way to detain stormwater in
open subbase layer of existing roads and use it for more healthy green lives in urban areas while
solving the basement flooding problem.
In southern Sweden, downpouring will be frequent due to climate change, which threatens existing
drainage systems in urban areas and may lead to more basement flooding. In this study, a district in
Trelleborg municipality in the south of Skåne, the southernmost county of Sweden, experiencing
repeated basement flooding for a long time was investigated with BGG structures to test their
performance to mitigate basement flooding in this area. Incorporating BGG with proper settings of
existing LID tools in the model software Mike+ was one of the outcomes of this study. Mike+ is a
modeling software introduced by DHI.
The hydraulic and physical parameters were chosen as per guidelines for usage of Mike+ and
practicing properties of BGG suggested by EDGEs. The porosity of substrate and storage layers and
the infiltration capacity of the top surface layers are the dominant factors to define the capacity of
BGG structures. The size and distribution of BGG structures, the formation and size of catchments,
and the size of rainfall events cause variations in the surface runoff reduction capacity of BGG
structures. By storing stormwater and providing it for trees and plants BGG enhances the natural
hydrological cycle in urban areas which was concluded from the stormwater distribution results with
BGG deployed in the model. The model was run with a 10-year synthetic rainfall event provided by
DHI with a climate factor of 1.25 and then BGG structures were deployed along several roads until
they can make the basements of all the households claimed flooding, become free from flood. A real
rainfall event of 2021 was provided by the Trelleborg municipality. This real rainfall event recorded
in 2021 was found to be equivalent to a 50-year rainfall event. The same model with deployed BGG
structures was then tested with this real rainfall event to see their performance with a 50-year
rainfall event. The results showed the characteristics of BGG structures to enhance the natural
hydrologic cycle and at the same time, flood-claimed households remain out of flood risk.
In this study, BGG structures reduced the surface runoff produced from impervious surfaces in the
case area by up to 96% for individual catchments for 10-year rainfall events. The average surface
runoff reduction capacity of BGG structures was 48% for the 10-year rainfall. On average the BGG
structures infiltrated 0.08m3
/sec, stored 0.37m3
/sec, and produced 0.11m3
/sec runoff when the
inflow in the BGG structures was 0.56m3
/sec. Replacement of 14% of the existing impervious
surface with BGG in a catchment can reduce the surface runoff by 50% for 10-year rainfall. The
deployed BGG structures showed the same performance for 50-year rainfall and reduced the runoff
by 48%. As the surface runoff capacity is subjected to the size of BGG structures, catchments, and
rainfall it cannot be summarized with any specific number in case of reduction capacity. BGG
structures were found in this study to reduce the discharge rate and flow velocity through the pipes.
The discharge rate and flow velocity through the outflow pipe indicate a smoother flow discharge
which is good for the downstream recipients and biodiversity as well.
iv
The impact of source control or disconnecting impervious surface areas of private lands from the
drainage network was also tested in this study. As obvious the runoff volume reduced with reduced
impervious surface in the catchments.

Popular Abstract

Bluegreegrey system for holistic stormwater and drainage pipe network management
Reducing basement floods and overall urban flooding risk with climate change impacts become a
challenge for many cities and countries due to the lack of sufficient spaces. Bluegreengrey (BGG)
system reduces flood risk, mimics natural hydrology, and lessens the demand for new spaces.
Climate change impact, rapid urbanization, increasing property value, and care for biodiversity and the
overall environment are needed to be considered for a sustainable urban drainage system. On the
other hand, accommodating trees and plants and managing stormwater needs extra space. The BGG
system uses the subbase layer of existing pedestrian, bike, and flexible lanes of traffic to retain and
store stormwater. While they accommodate tree root zones and reuse the naturally treated
stormwater for trees and green lives in urban areas. These are done by using materials with higher
porosity in the subbase layer while ensuring enough bearing capacity for medium traffic load.
In this study, a district of the Trelleborg community was chosen as a case study and the calibrated
model was used Mike+ to analyze and assess the efficiency of the BGG system. The case study area has
a record of repeated claims of basement flooding for a long time. A separate stormwater network was
also built in the area, but many claims of basement flooding were still recorded. The model was tested
with a 10-year design rainfall with a climate factor of 1.25 and the risk of basement flooding and other
drainage parameters was studied. Then BGG system was deployed in the model to analyze their
performance and to make the pipe network functional with 10-year rainfall. A real rainfall which is
equivalent to a 50-year rainfall was also tested with the same deployed BGG system. A test was
performed to assess the ability of source separation to reduce basement flood risk in the study area.
The model results revealed the capacity of the BGG system to reduce the runoff and make the existing
drainage pipe network functional with 10 years rainfall with climate factor. The first approach of the
study found the minimal deployment and spatial distribution of the BGG system to ensure the pipe
network out of basement flood risk. The model also showed the same deployment of the BGG made
the pipe network functional for an intense 50-year rainfall also. When the slope of the catchment and
the link between two adjacent nodes are within 5%, replacing 14% of existing impervious surfaces in
the catchment with the BGG system can reduce the runoff by 50% for 10-year design rainfall. The
runoff was reduced to almost one-tenth of that of the existing system with the BGG system. In the BGG
system stormwater passes through two layers and nutrients like Nitrogen and Phosphorus are used by
the trees and vegetation. Hence it is expected to do 70-80% treatment of stormwater by its process.
At the same time ensuring water and nutrients supply for trees and plants ensures reuse of
stormwater, reducing the heat island effects, and working as a carbon sink for urban areas. The flow
velocity and discharge rate for outflow were reduced significantly with the BGG system. It indicates
fewer hazards and risks of flooding in urban areas in terms of downstream environmental impacts. It
indicates safety for biodiversity downstream as well.
The BGG system mimics natural hydrology distributing stormwater in different phases and thus
reducing the runoff and load in the pipe network system. The BGG system can be adapted to the
existing landscape and proper distribution and deployment of the BGG system can make the existing
pipe networks functional for 10-year rainfall. This system can be a solution for basement flooding even
under more intense rainfall with longer return periods because of climate change'simpact as well. The
study uncovered the BGG as a holistic solution to overall urban flooding.

• author: Asif Rifat, Adnan
• supervisor: Shifteh Mobini
• organization: Division of Water Resources Engineering
• course: VVRM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Bluegreengrey, Sustainable stormwater, Basement flooding, Urban flooding, Pipe Network, BGG, Climate change, Hydrology, Mike+.
• publication/series: TVVR 5000
• report number: TVVR 23/5004
• ISSN: 1101-9824
• language: English
• additional info: Examiner: Rolf Larsson
• id: 9139285
• date added to LUP: 2023-10-09 16:15:47
• date last changed: 2023-10-09 16:15:47