Runoff modelling and assessment of treatment wetland performance using a triangular form based multiple flow algorithm

Abstract: Eutrophication is one of the biggest threats to aquatic ecosystems. As nutrient load increases in marine environments in the Öresund and Baltic Sea and dead zones grow, Swedish municipalities further their efforts in limiting the amount of nutrients delivered into those environments. Since the majority of nutrients enter aquatic systems through surface flow as a result of nutrient leakage, a reliable model predicting flow accumulation and pattern is needed. Such a model is suggested in this thesis. It utilizes a dynamic triangular form based multiple flow algorithm in combination with several physical parameters such as the surface roughness and infiltration rate. The model is shown to deliver more detailed results than a static single flow based algorithm, utilized by many municipalities today. It additionally indicates the location and amount of water accumulated during a precipitation event in the wetland in the study area. Thus, it can be utilized to analyse the capacity of treatments wetlands which are often used to filter nutrients through sedimentation before they enter water bodies. A brief analysis of the wetland’s effectiveness was made for a rainfall event with a 10 year return period, showing that bigger particles (sand) were mainly deposited in the wetland, while particles of smaller sizes (silt and clay) were transported through the wetland without deposition and thus entered the downstream aquatic systems.