Long-term performance assessment and future proofing of a raingarden for stormwater management

Abstract

Raingardens are a type of Sustainable Drainage Systems (SUDS) that provides a nature-based solution to stormwater control. Research into the performance of raingardens is relatively scarce in the UK and effort is needed to provide data and improved scientific understanding to deliver a strong evidence-base. The overarching goal of this study is to evaluate the long-term performance of a raingarden, designed over a low-conductivity soil. To this effect, an experimental raingarden with two amended soil types (Sandy loam with 24% fines content; Loamy sand with 13% fines) was installed within the Royal Botanic Garden Edinburgh (RBGE) in Scotland, and monitored between Summer 2019 and Winter 2022-23. This study documents the qualitative and quantitative methods in evaluating a field scale raingarden. Antecedent soil moisture between storm events, saturated hydraulic conductivity, and climate season were found to be key factors that controlled surface ponding in the sandy loam media. The time to drain ponded water in this media in Winter was on average twice longer than that of Summer due to 71% lesser median percolation in the cold season. In comparison, loamy sand media exhibited a lesser influence of temperature on infiltration, with average values in Winter being 40% lower than those in summer. Computer model of the raingarden developed in this study predicts that infiltration in the sandy loam soil must be at a minimum of 64 mm/h to prevent overflow from the raingarden under a reasonable worst- case scenario projected for 2050. The efficiency of evapotranspiration (ET) removal on rainfall was on average of 43% in summer highlighting the significance of the role of vegetation in the hydrological cycle of the raingarden. Species Festuca altissima, Primula posioni and Ligularia fischeri, Rodgersia pinnata and Gunnera manicata were found not suitable in a raingarden setting due to either stringent habitat requirements or prone to wilting during prolonged dry periods. Organic wood mulch is preferred over gravel mulch as the later potentially increases the solar radiation exacerbating heat stress in times of drought. Overall, the results of this work indicate that raingardens installed over low-conductivity soils can still be an effective technology for stormwater management, when designed suitably. The findings of this work provide clear and transferable insights that can be applied to the design, monitoring and maintenance of raingardens in similar climatic and soil conditions, both in the UK and around the world