Assessing spatial and temporal variability of grass swale infiltration in shallow groundwater conditions

Implementing stormwater green infrastructure in shallow groundwater areas presents major challenges that could restrict widespread adoption of swales in such areas. These limitations are driven by concerns about reduced swale infiltration capacity, which negatively impacts the effectiveness of green measures in managing runoff volumes. This study evaluates the spatial and temporal distribution of infiltration rates in a 30-m grass swale section using a Modified Philip-Dune infiltrometer and full-scale infiltration testing. Groundwater levels were continuously monitored by three piezometers adjacent to the grass swale to assess the impact of unsaturated zone depth on the swale infiltration capacity. Results showed that infiltration rates varied widely from 13 mm/h at the swale bottom to 98 mm/h on the right slope and highlighted the potential overestimation of swale capacity when relying only on point measurements of infiltration. Results from a full-scale infiltration test revealed an overall swale infiltration rate of only 4 mm/h, which is below the values recommended in the literature for swale applicability. A 52 % decrease in infiltration rates was observed between 2022 and 2024. Experimental results indicated that the grass swale had the capacity to recover its storage and managed a subsequent rainfall event within 15 h of the full draw-down. While the findings did not show a strong correlation between swale infiltration rates and the depth of the unsaturated zone, the results underscore the need to balance the soil permeability and groundwater protection for effective stormwater management.