the dynamics of hyporheic exchange flows during storm ... · the dynamics of hyporheic exchange...
TRANSCRIPT
The dynamics of hyporheic
exchange flows during storm
events in a strongly gaining
urban riverMark O. Cuthbert 1, V. Durand1,2, M.-F. Aller1,3,
R. B. Greswell1, M. O. Rivett1 and R. Mackay1
(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, 91405 ORSAY CEDEX,
France
(3) Present address: Lancaster Environment Centre, Lancaster
University, Bailrigg, Lancaster, LA1 4YQ, UK
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.
Study Site
Study site
Study Site Context
Field Observations: Hydraulics
-4
-2
0
2
4
6
8
10
Diffe
ren
tia
l H
ea
d (
cm
, p
ositiv
e u
pw
ard
s)
0.8
0.85
0.9
0.95
1
1.05
1.1
Ele
ctr
ica
l C
on
du
ctivity (
mS
/cm
)
93
93.2
93.4
93.6
93.8
14-Apr-09 16-Apr-09 18-Apr-09 20-Apr-09
Riv
er
He
ad
(m
AD
)
Reversal of hydraulic gradients during
storm events
Storm events:
elevated river stage
Strong changes in EC in upper part of
river bed in response to storm events
‘Gaining’ condition
under low flows
Greswell et al (2009), Journal Of Hydrology,
373: 416-425.
Cost effective self-build system for measuring
and logging differential pressure
Field Observations: Chloride Profiles
We seem to have persistent SW-GW mixing in the top 30 cm in part of the reach
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.0 50.0 100.0 150.0 200.0
Cl (mg/l)
De
pth
(m
)
River range
24/10/2007
05/06/2008
27/06/2008
31/07/2008
17/10/2008
Borehole range
WHY?
Hydrodynamically
induced exchange?
Topographically
induced exchange?
Any suggestions welcome…
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
Sampling method?
Field Observations: Gas
0.1 m
• 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
Hypothesised Effects of Gas
• Hydraulic: enhanced depth of flow
reversal during storm events
Hp
nmmngS s
)( Increased specific storage:
Reduced effective
hydraulic conductivity: 2/)1()1/(5.0 11
nnnn
sat ssKsK
n
qv Reduced effective porosity:
• Thermal: enhanced thermal diffusivityn
I
n
Ca .)1(
Non-homogeneous diffusion:
Modelling Results: Hydraulics
90 91 92 93 94 95 96 97 98 99 100
97
98
99
100
101
102
90 91 92 93 94 95 96 97 98 99 100
97
98
99
100
101
102
• 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
Particles tracked over 2 days
River stage
variation
Bank Sy = 30%
Bank Sy = 3%
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)
• Annual temperature
fluctuations enhanced
by 4 to 30%-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 12 24 36 48
Time (hr)
Te
mp
era
ture
flu
ctu
atio
n (
de
g C
)
River: daily sinusoidal
boundary condition
Range of field
observational results
Range expected from
literature values for
saturated sediments
Analytical forward model for a
1-D infinite medium output at
0.25 m below river bed
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?
Any questions or
suggestions?
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)
Freeze coring, River Tame, 2009