Evaluating the Performance of Bioswales using a Hydrological Groundwater Model

Abstract

The expected effects of climate change on increased and more frequent rainfall events ask for more innovative solutions to manage urban stormwater. Sustainable Urban Drainage Systems (SuDS) offer an eco-friendly method to disconnect stormwater from the sewer system. The Municipality of Rotterdam, the Netherlands, has integrated multiple SuDS into its drainage network, including bioswales. Bioswales are vegetated areas that slow down, collect, and filter (storm) runoff. However, uncertainty exists regarding their performance under different conditions. This thesis aims to answer the following research question: How do bioswales perform under various conditions, as evaluated by a hydrological groundwater model?

The bioswale groundwater model used in this thesis, developed by Deltares, utilizes the Unsaturated Zone Flow (UZF) package of MODFLOW to simulate the hydrological response of bioswales. The model was calibrated and validated using existing monitoring data, and a one-at-a-time sensitivity analysis was performed to identify the most influential factors affecting bioswale performance. The case-study bioswale was tested under design storms reflecting current and 2050 summer and winter conditions, as well as prolonged wet winter rainfall. Two design scenarios were proposed to improve bioswale performance.

The case-study calibration results showed that the model could realistically simulate water levels and discharges. However, the existence of preferential flow in the unsaturated zone, not accounted for by the UZF package, led to a time-lag in modelled drain discharge. The sensitivity analysis indicated that infiltration parameters strongly influence emptying time, peak discharge, and time-lag in the model. The performance assessment showed that the case-study bioswale met the emptying time criterion, but the peak discharge limit was exceeded during summer events. Simulating prolonged wet winter rainfall showed that consecutive rainfall events could be more critical regarding winter bioswale performance, compared to a single winter design storm. The bioswale design improvements demonstrated that relocating the drain from the centre to the side of the bioswale, thereby increasing the distance water needs to travel, significantly reduced peak discharge, though at the cost of longer emptying times. Widening the bioswale increased storage volume; therefore, the connected paved surface area could be increased, but the effect of adding additional drains on bioswale performance was limited.

To increase the understanding of bioswale performance, further empirical research on vegetation, macropores, and preferential flow is recommended, along with improvements to the modelling of these processes. In terms of model application, the bioswale groundwater model, with some adjustments, can be applied to other SuDS types that might be less sensitive to the natural influences of vegetation change and macropores. Combining the modelling of individual bioswales and SuDS, as done in this study, with urban-scale modelling could significantly improve Rotterdam’s climate resilience.