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The influence of multiscale hyporheic flow on solute transport

Implications for stream restoration enhancing nitrogen removal

Time: Mon 2022-09-05 10.00

Location: F3, Lindstedtsvägen 26 & 28, Stockholm

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Language: English

Subject area: Civil and Architectural Engineering, Hydraulic and Hydrologic Engineering

Doctoral student: Ida Morén , Resurser, energi och infrastruktur

Opponent: Professor Roy Haggerty, Louisiana State University, USA

Supervisor: Professor Anders Wörman, Resurser, energi och infrastruktur; Associate Professor Joakim Riml, Resurser, energi och infrastruktur

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QC 20220815


Stream water that flows into and out of streambeds is called hyporheic exchange flow (HEF).It continuously interacts with groundwater and thereby affect the water quality of local streamreaches as well as downstream recipients by providing an environment where solutes andenergy can be retained and degraded. Because of anthropogenic activities, many streams andrivers have been physically, chemically and biologically degraded during the last centuries andnatural functions, such as HEF, have to some extent been lost. The general aim of this thesiswas to advance the understanding of the physical controls of HEF in small streams and toinvestigate how HEF influences solute reactive transport in streambeds and surface waternetworks before and after stream restoration. To reach the aim, the consistency and deviationbetween HEF parameters evaluated with two common approaches were investigated in tendifferent alluvial streams with low discharge, shallow depth and moderate slope. The twoapproaches were: 1) developing and using a deductive hydro-mechanical model to assessed therelationship between the multiscale streambed geomorphology and the reach scale averageHEF parameters, and 2) evaluating HEF parameters from in-stream tracer tests using a 1Dlongitudinal transport model. The relatively high consistency between the approaches connectstheories that previously have been relatively fragmented and provides a tool for upscaling(parameterizing) of HEF in solute transport models over stream networks based onindependent observations of stream topography, streambed sediment properties and in-streamhydraulics. Applying the modelling framework at the network scale and supporting it withcomprehensive datasets provided information regarding physical mechanisms and spatialvariability of HEF as well as its influence on longitudinal solute transport. Specifically, thefractal properties of the water surface profile were shown to represent the average HEF velocitywell. Furthermore, hydraulic head variations over shorter wavelengths (0.1-5 m) were found todrive the main part of the HEF and the static hydraulic head variations dominated over dynamichydraulic head variations as drivers of HEF in all investigated streams. Moreover, this thesishighlights the importance of the hyporheic zone as a bio-chemical and mechanical filter forstream water. It shows that common engineered stream restorations can influence HEF andimprove the water quality in local stream reaches as well as downstream recipients. Specifically,the thesis presents exact solutions to the nitrogen transport, which shows that the mass removalof nitrogen in the hyporheic zone is either transport or reaction limited and that the maximalremoval rate corresponds to an optimal hyporheic residence time and a typical denitrificationDamköhler number. The results also show that potential exists to reduce the agriculturalnitrogen load to the Baltic Sea by stream restorations that optimize the hyporheic residencetimes. However, the large spatiotemporal variability in the potential between reaches stressesthe importance for further studies on which processes that are driving HEF under specifichydromorphologic conditions and careful design of stream restoration measures at each localstream reach.