Impacts of combined sewer overflows on a large urban river - Understanding the effect of different management strategies.
To support decision makers in planning effective combined sewer overflow (CSO) management strategies an integrated modelling and impact assessment approach has been developed and applied for a large urban area in Berlin, Germany. It consists of an urban drainage model, a river water quality model and a tool for the quantification of adverse dissolved oxygen (DO) conditions in the river, one of the main stressors for urban lowland rivers. The coupled model was calibrated successfully with average Nash-Sutcliffe-efficiencies for DO in the river of 0.61 and 0.70 for two validation years. Moreover, the whole range of observed DO concentrations after CSO down to 0 mg L-1is simulated by the model. A local sensitivity analysis revealed that in the absence of CSO dissolved oxygen principally depends on phytoplankton dynamics. Regarding CSO impacts, it was shown that 97% of the observed DO deficit can be explained by the three processes (i) mixing of river water with CSO spill water poor in DO, (ii) reduced phytoplankton activity due to CSO-induced turbidity and (iii) degradation of organic matter by heterotrophic bacteria. As expected, process (iii) turned out to be the most important one. However depending on the time lag after CSO the other processes can become dominant. Given the different involved processes, we found that different mitigation schemes tested in a scenario analysis can reduce the occurrence of critical DO deficits in the river by 30-70%. Overall, the study demonstrates that integrated sewer-river-models can be set up to represent CSO impacts under complex urban conditions. However, a significant effort in monitoring and modelling is a requisite for achieving reliable results.
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany. Electronic address: mathias.riechel@kompetenz-wasser.de.
Mathias
M
Riechel
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany.
Andreas
A
Matzinger
Berliner Wasserbetriebe, Neue Jüdenstrasse 1, 10179 Berlin, Germany.
Erika
E
Pawlowsky-Reusing
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany.
Hauke
H
Sonnenberg
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany.
Mathias
M
Uldack
Berliner Wasserbetriebe, Neue Jüdenstrasse 1, 10179 Berlin, Germany.
Bernd
B
Heinzmann
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany.
Nicolas
N
Caradot
Senate Department for Urban Development and the Environment, Württembergische Strasse 6, 10707 Berlin, Germany.
Dörthe
D
von Seggern
Kompetenzzentrum Wasser Berlin, Cicerostrasse 24, 10709 Berlin, Germany.
Pascale
P
Rouault
D004784:Environmental Monitoring; D005618:Fresh Water; D008962:Models, Theoretical; D045483:Rivers / Q000382:microbiology*; D060753:Water Quality
CSO; Dissolved oxygen; Impact assessment; Integrated modelling; Mitigation measures; River Spree