Eutrophication degrades the quality of freshwaters and hinders their utilisation. Much of the nutrients causing eutrophication are introduced to water bodies in runoff from agricultural and residential areas. The degradation of freshwaters causes notable costs for societies. Pond–wetland treatment systems are one of the means used in water protection to purify runoff from these areas. In the Finnish climate their retention performance relies heavily on sedimentation of total suspended solids (TSS) and attached nutrients.

The goal of this thesis is to improve knowledge on how geometry and flow conditions of ponds and wetlands affect their ability to retain TSS load. The advantage of this thesis com-pared to previous studies is the utilisation of accurate, measurement-based modelling of pond and wetland topography. This study compares two pond–wetland treatment systems in SW Finland.

Dry topography was mapped with terrestrial laser scanning and bathymetry of the treatment systems was mapped with the use of integrated sonar of an ADCP platform and VRS-RTK-GNSS. Combined DEMs were used as input data in 2D and quasi-3D flow modelling that simu-lated flow during seasonal peak discharges. Flow in and out of the treatment systems was measured with water pressure sensors from April to December in 2017. Water quality data was derived from continuous turbidity sensor data that was calibrated and converted to TSS content with water samples analysed in laboratory. The combination of different data was utilised to analyse connections between water quality and discharge, the changes in the treatment systems’ water area and volume and finally the effects of flow patterns on TSS sedimentation and resuspension.

Results show that single factors thought to influence the treatment process do not ade-quately indicate the way a treatment system functions. Water residence time seems to be an important factor for water purification but good hydraulic efficiency does not necessarily lead to retention of suspended solids. Flow patterns should be taken into account in planning. In pond–wetland treatment systems flow characteristics and geometry of the downstream wetland in particular seem to be of importance to retention performance. Operation of ponds and wetlands should be monitored throughout the year with high frequency data on discharge together with nutrient and TSS loads.