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Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low., QC 20230629

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, hydrauli, Vattendrag utgör en volumetrisk liten men viktig del av den hydrologiska cykeln.Vattendrag fungerar som snabba transportleder för vatten, lösta ämnen och energioch ytvattnet interagerar även kontinuerligt med utströmmande grundvatten i den såkallade hyporheiska zonen, vilket är sedimenten närmast vattendragen. Flödet avytvatten in och ut ur den hyporheiska zonen kallas hyporheiskt utbyte och kanpåverka vattenkvalitén i det lokala vattendraget och i nedströms recipienter genomatt skapa förutsättningar för retention och nedbrytning. På grund av antropogenaaktiviteter har många vattendrag blivit fysikaliskt, kemiskt och biologiskt nedbrytna,och naturliga funktioner såsom hyporheiskt utbyte har gått förlorade. För att skyddavattendrag från fortsatt destruktion, och för att restaurera vattendrag och förbättravattenkvalitén i lokala vattendrag och nedströms recipienter krävs en ökad fysikaliskförståelse och tillförlitliga modeller för prognosering av hyporheiskt utbyte. Trots attden hyporheiska zonen studerats ingående under de senaste decennierna misslyckasofta befintliga modeller med att inkludera alla väsentliga temporala och spatiala skalor,och de är därför svårt att generalisera resultaten för större avrinningsområden. Detgenerella syftet med denna avhandling var att öka den fysikaliska förståelsen för hurflödet genom den hyporheiska zonen i små vattendrag drivs och att undersöka hurdet hyporheiska utbytet påverkar ämnestransporten i små enskilda vattendrag ochnätverk av vattendrag. Dessutom syftade avhandlingen till att undersöka betydelsenav restaurering av vattendrag för det hyporheiska utbytet och transporten av kväve ismå jordbruksdiken. Metoderna som användes i den här avhandlingen inkluderarbåde omfattande undersökningar i fält och semi-analytisk matematisk modellering.Ett ramverk av modeller etablerades, vilket inkluderade både en deduktiv,hydromekanisk modell som utvärderar korrelationen mellan den flerdimensionellabottentopografin och hydrologin i ett vatte, QC 20220815

Hyporheic exchange flow (HEF) can generally be quantified through two different approaches. The first approach, which is deductive, entails physically based models, supported with relevant observations. The second approach includes inductive assessments of stream tracer tests using solute transport models, which provide a useful mathematical framework that allows for upscaling of results, but included parameters often have a vague physical base, which limits the possibilities of generalizing results using independent hydromorphological observations. To better understand how the physical basis of HEF-quantifying parameters relates to stream hydromorphology at different spatial scales, we cross-validated the results from (a) tracer test assessments using a 1D solute transport model that accounts for HEF and (b) an independent hydromechanical model that represents HEF driven by multiscale pressure gradients along the streambed interface. To parameterize the models, topographical surveys, tracer tests, and streambed hydraulic conductivity measurements were performed in 10 stream reaches, differing in terms of geomorphology, slope, and discharge. The results show that the models were cross-validated in terms of the average exchange velocity, providing a plausible physical explanation for this parameter in small alluvial streams with low discharges, shallow depth, and moderate slopes. However, the hydromechanical model generally resulted in wider residence time distributions and occasionally higher average residence times compared to the tracer test assessments. From the cross-validated multiscale hydromechanical model, we learned that water surface profile variations were the main drivers of HEF in all investigated streams and that spatial scales between 20 cm and 5 m dominated the estimated HEF velocity., QC 20211221

Although hyporheic exchange has been shown to be of great importance for the overall water quality of streams, it is rarely considered quantitatively in stream remediation projects. A main driver of hyporheic exchange is the hydraulic head fluctuation along the streambed, which can be enhanced by modifications of the streambed topography. Here we present an analytical 2-D spectral subsurface flow model to estimate the hyporheic exchange associated with streambed topographies over a wide range of spatial scales; a model that was validated using tracer-test-results and measurements of hydraulic conductivity. Specifically, engineered steps in the stream were shown to induce a larger hyporheic exchange velocity and shorter hyporheic residence times compared to the observed topography in Tullstorps Brook, Sweden. Hyporheic properties were used to parameterize a longitudinal transport model that accounted for reactions in terms of first-order decay and instantaneous adsorption. Theoretical analyses of the mitigation effect for nitrate due to denitrification in the hyporheic zone show that there is a Damkohler number of the hyporheic zone, associated with several different stream geomorphologies, that optimizes nitrate mass removal on stream reach scale. This optimum can be limited by the available hydraulic head gradient given by the slope of the stream and the geological constraints of the streambed. The model illustrates the complex interactions between design strategies for nutrient mitigation, hyporheic flow patterns, and stream biogeochemistry and highlights the importance to diagnose a stream prior remediation, specifically to evaluate if remediation targets are transport or reaction controlled., QC 20180112

QC 20211116

QC 20220817