Modelling the impact of implementing
Water Sensitive Urban Design at a
catchment scale
Luca Locatelli
Phd Student
DTU - Environment
P.S. Mikkelsen (DTU), O. Mark (DHI), K. Arnbjerg-Nielsen (DTU), S. Gabriel
(ORBICON), B. Bockhorn (KU), H. Taylor (ORBICON), H. Larsen
(ORBICON), A.S. Blicher (ORBICON), M.J. Kjølby (DHI), P. J. Binning
(DTU).
Aim of the project
To model the impact of retention-detention basins (’skybrudfaskiner’)
• Annual water balance
• Estimate initial conditions for single design events
• Peak flow reduction on design events
What are retention-detention basins
(’skybrudfaskine’)?
Why retention-detention basins?
Detention storage handles design storm events
(e.g. return period >2 year)
Detention volume << retention volume
Retention storage handles everyday events
(e.g. return period <2 year)
Retention storage reduces runoff volume
Literature on detention-retention systems
• Roldin, M.; O. Fryd; J. Jeppesen; O. Mark; P. J. Binning; P. S. Mikkelsen and M. B.
Jensen. 2012. "Modelling the Impact of Soakaway Retrofits on Combined Sewage
Overflows in a 3 Km(2) Urban Catchment in Copenhagen, Denmark." Journal of
Hydrology, 452, pp. 64-75.
• Roldin, Maria; Ole Mark; George Kuczera; Peter S. Mikkelsen and Philip J.
Binning. 2012. "Representing Soakaways in a Physically Distributed Urban Drainage
Model – Upscaling Individual Allotments to an Aggregated Scale." Journal of Hydrology,
414-415, pp. 530-38
• Sønderup, H.; J. Jørgen Linde; S. Gabriel and P.S. Mikkelsen. 2002. "Rainwater
Tanks Are Less Efficient Than Detention Basins for Cso Abatement," Global solutions for
urban drainage : 9ICUD, Portland, Oregon, 8-13 September 2002
• Jeppesen, Jan. 2010. "Quantitative Hydrological Effects of Urbanization and Stormwater
Infiltration in Copenhagen, Denmark," PhD Thesis. Department of Earth Sciences, Aarhus
University, Denmark
Case study area
0.6 hectares of impervious area, k= 10-6 m/s
Model (1)
TOOL: MIKE URBAN (DHI) + Soakaway model (Roldin et al. 2012)
METHOD: Single soakaways can be modelled as a single
aggregated unit (Roldin et al. 2012)
Model (2)
THE SOAKAWAY MODEL (Roldin et al. 2012)
Calculates the infiltration rates from a ’basin’ in Mike Urban
Qin
Qin
QQ
of of
h
Qf
w
l
Method
1. Model Baseline scenario and identify the area to be disconnected
Single event with 10 year return period
2. Model the annual water balance
Soakaway model, continuous simulation using 22 years rainfall time series
3. Estimate initial conditions (water content)
Soakaway model, continuous simulation using 22 year rainfall time series
4. Design the detention-retention basin and model it in Mike Urban
Regional CDS Ver_3.2.xls
+
The retention-detention basin is modelled as a ’basin’ node in Mike Urban
with an overflow pipe controlled by a maximum flow rate valve.
Results
Baseline scenario
CDS rain with T=10 years and a safety factor of 1.1
Black nodes = water above terrain
Depth below
surface (m)
Results (1)
Can we reduce sewage overflow by disconnecting areas?
88% disconnected!
Results (2)
Annual water balance (1)
Results (3)
Annual water balance (2)
Results (4)
Initial conditions
Results (5)
Design retention-detention basins
Detention-retention volume = 3.4 m3 per 100m2 of impervious area
(impervious area = 0.6ha)
Results (6)
With combined detention-retention
CDS rain with T=10 years and a safety factor of 1.1
Conclusions
• Detention-retention basins can help avoid sewer surcharge
• Detention-retention basins infiltrate a large percentage of the annual stormwater
runoff  smaller load to sewage treatment plant
A soakaway design return period T= 0.1 years infiltrates approx. 80% of the annual stormwater runoff
• Increasing soakaway volumes leads to diminishing returns in performance: the larger
the design return period of soakaways the smaller the stormwater volume infiltrated
per unit volume of soakaway
• Soakaways are on average between 20% and 60% full at the beginning of the
simulation. The higher the design return period the more likely the soakaway is empty
at the beginning of the simulation.
Thank you for your attention
Luca Locatelli
lulo@env.dtu.dk
Results (3)
Annual water balance (2)