The semi-sewer river: hydraulic backwater effects and combined sewer overflow reverse flows in Central Brussels reduce deoxygenation impact further downstream.

In 2011 and 2012 the dissolved oxygen content in the low-discharge river Zenne was monitored continuously, every 5 minutes, downstream of Brussels city centre, making it possible to document the complex mechanisms by which combined sewer overflow (CSO) spills affect both the hydraulics and the oxygen balance of the hydrosystem. In addition to oxygen demand impacts, proportions of water volumes are such that the oxygen-devoid sewage water discharged from CSOs contributes significantly to the oxygen deficit observed in the river further downstream. It is shown that ensuing unexpected hydraulic behaviour, such as a full river-flow reversal, can explain the dual nature of oxygen sag following major CSO events. At times the semi-sewer river plays the role of an in-stream stormwater tank, effectively attenuating the environmental impacts of Brussels CSOs.