Littoral wetlands and lake inflow dynamics

Abstract

Wetlands are increasingly recognized as important water treatment systems, which efficiently remove nutrients, suspended sediments, metals and anthropogenic chemicals through sediment settling and various chemical and biological processes. This thesis tackles three interconnected aspects of wetland physics. The first is wetland circulation, which is one of the most important design parameters when constructing wetlands for water quality improvement because it regulates the residence time distribution, and thus the removal efficiency of the system. Field work demonstrates that wetland circulation changes from laterally well mixed during low flows to short-circuiting during storms, which in combination with a reduced nominal residence time undermines the wetland treatment performance. The second important physical mechanism is thermal mediation, i.e. the temperature modification of the water that flows through the wetland. This change in water temperature is specifically important in littoral wetlands, where it can alter the intrusion depth in the downstream lake. Numerical analysis in conjunction with field observations shows that littoral wetlands located in small or forested watersheds can raise the water temperature of the lake inflow during summer enough to create surface inflows when a plunging inflow would otherwise exist. Consequently, more land borne nutrients and chemicals enter the epilimnion where they can enhance eutrophication and the risk of human exposure. The third and last physical mechanism considered in this thesis is the exchange flows generated between littoral wetlands and lakes. Field experiments show that during summer and fall, when river flows are low, buoyancy- and wind-driven exchange flows dominate the wetland circulation and flushing dynamics. More importantly, they can enhance the flushing by as high as a factor of ten, thus dramatically impairing the wetland potential for removal and thermal mediation.