Hydraulic functioning of bioswales under polder conditions

A field-survey in Rotterdam

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Bioswales contribute to climate resilience, as they positively impact the urban water infrastructure by improving the water balance and water quality. However, current bioretention design ignores the facts that different designs in different soils and climatic locations produce different performance results.
The effect of polder conditions on the hydraulic performance, as well as the effect of different storm-types and initial conditions will be researched in this report. To this end, 5 bioswales located in Rotterdam, The Netherlands are monitored on their hydraulic behaviour. The discharge of the drain, groundwater levels in and at the edge of the bioswale and the water level in the bioswale are monitored for 4 of the bioswales. One bioswale only has groundwater and surface water measurements. The polder conditions resulted in a low volume reduction and faster and stronger reacting drain. However, the peak reduction and peak delay were still quite good due to the low permeability of the top-soil. For smaller storms, even the goal for volume reduction could be met. It was found that the structural porosity governed the infiltration, but large plants can increase the permeability too much. In addition, the textural porosity should start high enough to allow for vegetation development.