Keywords
combined sewer overflows, dissolved oxygen, impact assessment, model validation, sensitivity analysis
Start Date
1-7-2012 12:00 AM
Abstract
In the city of Berlin combined sewer overflows (CSO) can lead to severedepressions in dissolved oxygen (DO) of receiving urban rivers and hence to acutestress for the local fish fauna. To quantify CSO impacts and optimize sewermanagement strategies, a model-based planning instrument has been developed.It couples the urban drainage model InfoWorks CS which simulates hydraulics andpollutant transport in the sewer with the river water quality model QSim whichsimulates hydraulics, mass transport and various biogeochemical processes in thereceiving water body. To identify simulated CSO impacts, concentration-durationfrequency-thresholds for DO are applied to river model results via an impactassessment tool. Two kinds of impacts are distinguished: i) suboptimal conditionsand ii) critical conditions for which acute fish kills are possible. In the case of Berlin,suboptimal conditions are observed on up to 92 days per year, predominantlyduring periods of low discharge and high temperatures whereas critical conditionsonly occur after CSO. For model calibration and validation, continuousmeasurements in both river and sewer are used. First simulations show goodaccordance between simulated and measured DO concentration in the river withNash-Sutcliffe efficiencies between 0.70 and 0.79 for an eight-month time period atthree different river monitoring points. However, to assure satisfactory modelperformance for adverse DO conditions in particular, impact assessment results formeasured and simulated data are compared. Regarding suboptimal DO conditionssimulated and measured data show good agreement. Nevertheless modelrepresentation for critical conditions is poor for some river sections and requiresfurther improvement for CSO conditions. The results underline the importance ofcombining different validation approaches when dealing with complex systems.
Validation and Sensitivity of a Coupled Model Tool for CSO Impact Assessment in Berlin, Germany
In the city of Berlin combined sewer overflows (CSO) can lead to severedepressions in dissolved oxygen (DO) of receiving urban rivers and hence to acutestress for the local fish fauna. To quantify CSO impacts and optimize sewermanagement strategies, a model-based planning instrument has been developed.It couples the urban drainage model InfoWorks CS which simulates hydraulics andpollutant transport in the sewer with the river water quality model QSim whichsimulates hydraulics, mass transport and various biogeochemical processes in thereceiving water body. To identify simulated CSO impacts, concentration-durationfrequency-thresholds for DO are applied to river model results via an impactassessment tool. Two kinds of impacts are distinguished: i) suboptimal conditionsand ii) critical conditions for which acute fish kills are possible. In the case of Berlin,suboptimal conditions are observed on up to 92 days per year, predominantlyduring periods of low discharge and high temperatures whereas critical conditionsonly occur after CSO. For model calibration and validation, continuousmeasurements in both river and sewer are used. First simulations show goodaccordance between simulated and measured DO concentration in the river withNash-Sutcliffe efficiencies between 0.70 and 0.79 for an eight-month time period atthree different river monitoring points. However, to assure satisfactory modelperformance for adverse DO conditions in particular, impact assessment results formeasured and simulated data are compared. Regarding suboptimal DO conditionssimulated and measured data show good agreement. Nevertheless modelrepresentation for critical conditions is poor for some river sections and requiresfurther improvement for CSO conditions. The results underline the importance ofcombining different validation approaches when dealing with complex systems.
