(1) School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

Transcription

1 The dynamics of hyporheic exchange flows during storm events in a strongly gaining urban river Mark O. Cuthbert 1, V. Durand 1,2, M.-F. Aller 1,3, R. B. Greswell 1, M. O. Rivett 1 and R. Mackay 1 (1) School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK (2) Present address: UMR 8148 IDES, Bât 504, Faculté des sciences, Université Paris Sud 11, ORSAY CEDEX, France (3) Present address: Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, UK m.cuthbert@bham.ac.uk

2 Research Gaps Very few studies addressing the existence of an exchange zone in the presence of strong groundwater discharge Relatively little research regarding the HZ in the urban setting especially with regard to possible contaminant attenuation potential High resolution temporal variability in exchange flows not well understood The wider project is targeted at these areas and has generated a large data set but this talk just aims to highlight a few novel findings/concepts that have arisen from the research.

3 Study site Study Site

4 Study Site Context

5 River Head (mad) Differential Head (cm, positive upwards) Electrical Conductivity (ms/cm) Field Observations: Hydraulics Strong changes in EC in upper part of 1.1 river bed in response Cost to storm effective events self-build system for measuring and logging differential pressure 1.05 Gaining condition under low flows Reversal of hydraulic gradients during 0.8 storm events Storm events: Greswell elevated et al (2009), river stage Journal Of Hydrology, 373: Apr Apr Apr Apr-09

6 Depth (m Field Observations: Chloride Profiles We seem to have persistent SW-GW mixing in the top 30 cm in part of the reach Cl (mg/l) River range 24/10/ /06/ /06/ /07/ /10/2008 Borehole range Any suggestions welcome WHY? Topographically induced exchange? Hydrodynamically induced exchange? Sampling method? Flow reversals plus dual porosity? Maybe chloride isn t conservative in this case? Enhanced diffusion through vertical momentum transfer? Ellis et al 2007, Journal Of Contaminant Hydrology, 91: 58-80

7 Field Observations: Gas Gas collected up to 0.8 m below river bed Composition: nitrous and some methane not yet well constrained Hydrochemical data consistent with biogenic production of gas e.g. through denitrification Volumetrically up to 14% by volume not well constrained 0.1 m

8 Hypothesised Effects of Gas Hydraulic: enhanced depth of flow reversal during storm events Increased specific storage: Reduced effective S s g( n m m ) 0.5 n /( n 1) K s K 1 1 hydraulic conductivity: sats s Reduced effective porosity: v q n n p H ( n 1) / n 2 Thermal: enhanced thermal diffusivity Non-homogeneous diffusion: a ( 1 n) C. I n

9 Modelling Results: Hydraulics 102 River stage variation 101 Bank Sy = 30% Particles tracked over 2 days Bank Sy = 3% Flow spiralling due to flow reversal during storm events This increases at depth with available bank storage 30% increase in exchange flow volume due to gas >2 x depth of flow reversal in channel centre due to gas

10 Temperature fluctuation (deg C Modelling Results: Thermal Observed diurnal temperature fluctuations much larger than predicted for saturated sediments (e.g. around 6 times larger at 0.5 m depth) Range expected from literature values for saturated sediments River: daily sinusoidal boundary condition Analytical forward model for a 1-D infinite medium output at 0.25 m below river bed Time (hr) Range of field observational results Annual temperature fluctuations enhanced by 4 to 30%

11 Conclusions Persistent mixing observed in a strongly gaining reach: hypotheses still need testing Bed/bank storage controls flow reversal in gaining rivers: implications for river restoration Flow spiralling may lead to enhanced dispersion in river bank/bed: implications for contaminant attenuation Large gas accumulations may significantly alter flowpaths, depth/volume of flow reversals and thermal regime: implications for biological functioning and biogeochemical processes Beware temperature tracer methods in the presence of accumulated river bed gas?

12 Any questions or suggestions? Freeze coring, River Tame, 2009 Forthcoming paper: Impacts of river-bed gas on the hydraulic and thermal dynamics of the hyporheic zone. by M. O. Cuthbert, V. Durand, M.-F. Aller, R. B. Greswell, M. O. Rivett and R. Mackay In Review for Advances in Water Resources Special Issue (Guest Editors: Fleckenstein, Krause, Hannah, Boano)