Urban Runoff and Snowmelt Quantity and Quality Processes in Snow Deposits and Hydrologic Abstractions

University dissertation from Luleå

Abstract: Urban stormwater hydrology of cold regions, including northern Sweden, is characterized by freezing temperatures, accumulation of precipitation and pollution in seasonal snowpacks, release of such accumulations during short melt periods, major contributions of snowmelt to runoff, rainwater storage in snowpacks, and increased runoff from frozen grounds. Thus, the hydrology of cold regions differs significantly from that of the regions without frost and snow, and such differences need to be accounted for in stormwater management. Additional uncertainties are introduced into these considerations by climate change impacts on urban drainage, which in northern Sweden include additional hydraulic loading on urban drainage systems (UDS) and stormwater control measures (SCMs). Thus, the main objective of this thesis is to advance the understanding of urban drainage operation during the winter/spring transition in cold regions, by examining the processes affecting stormwater quantity and quality during this period, and the resulting implications for stormwater management and adaptation to climate change. To advance the understanding of runoff generation processes with respect to seasonal changes, experimental and modelling approaches were used. Infiltration into frozen soils was investigated in laboratory experiments with two engineered (sandy) soils holding two different initial water contents, 5 and 10%. The infiltration was significantly impaired by a complete blockage of soil pores in the initially wetter soil (i.e. 10%). Higher soil moisture and finer soil gradation slowed down the process of thawing and restoration of the infiltration capacity. In the modelling approach, the factors affecting the runoff generation were systematically investigated for a set of scenarios developed for two case studies in the Lindsdal (Kalmar) and Kiruna catchments. Winter scenarios were characterized by reduced infiltration, compared to the summer conditions. The simulation results showed a significant runoff volume increase, by 275%, in Lindsdal. Simulations of rain-on-snow events revealed that the snowpack depth strongly affected surface runoff by providing potential storage capacity. Consequently, even though the volume of runoff from rain-on-snow events in Kiruna increased by 180-500% for a range of snowpack depths, the smaller increases applied to the greater snowpack depths. Seasonal changes and rain-on-snow events greatly impacted the UDSs performance by producing more flooded nodes and surcharged sewer pipes. The impact of climate change was studied for future climate scenarios in two ways: applying upscaled precipitation in the Lindsdal catchment, and applying uplifting factors to both precipitation and temperature on the basis of Representative Concentration Pathways (RCPs), for low and intermediate emission scenarios, RCP2.6 and 4.5. The results showed adverse impacts of climate change on the UDS performance. Accumulations of pollutants in snowpacks were investigated by compiling concentrations of TSS and trace metals (Cd, Cr, Cu. Ni, W, and Zn) in snow samples from 11 sites, with various traffic densities, in Trondheim (Norway) and Luleå (Sweden). Data analysis for individual sites confirmed the assumption of a linear trend in accumulation of constituents with time (0.65