Salt-tracer experiments to measure
hyporheic exchange in gravel-bed sediments
Marcel van der Perk
1, Ellen L. Petticrew
2, Philip N. Owens
2, Rineke Hulsman
1, and Linda Wubben
11
Department of Physical Geography, Utrecht University, the Netherlands;
2University of Northern British Columbia, Prince George BC, Canada m.vanderperk@geo.uu.nl / Fax: +31 30 2531145 / Phone: + 31 30 2535565
Introduction
Many gravel-bed rivers in the central interior of British Columbia (BC),
Canada, represent important salmon spawning habitats. The success of salmon spawning depends on the hyporheic flow conditions, which
control the transfer of oxygen and heat between the surface water layer and the interstitial water in the gravel bed. To understand and quantify
the depth distribution of hyporheic flow in gravel bed sediments, a series of tracer experiments were performed in large outdoor flumes at the
Quesnel River Research Centre, Likely, BC, Canada (Fig. 1).
Figure 1 Outdoor flume at the Quesnel River Research Centre
Figure 2 Experimental set-up
V ≈ 0.034 m/s
18.9 m
2 m 0.3 m
Loc1 = 3 m Loc 2 = 10.1 m Loc 3 = 15.9 m
V ≈ 0.034 m/s
18.9 m
2 m 0.3 m
Loc1 = 3 m Loc 2 = 10.1 m Loc 3 = 15.9 m
Methods
The flume was filled with a 30 cm thick layer of well-sorted gravel (Fig. 2).
The flumes were filled with aerated local groundwater. Subsequently, dissolved common salt was added to raise the electrical conductivity
(EC). At the start of each experiment local groundwater was discharged at the upper end of the flume. At three locations downstream from the
inlet, the decrease in EC was monitored in both the water layer and at a fixed depth of 0.05 m, 0.1 m, or 0.2 m in the gravel bed until the EC
remained constant. Each experiment was replicated three times. Table 1 lists the main parameters of the experiments. The normalised
breakthrough curves measured at different depths were used to calibrate a straightforward numerical model (Fig. 3).
z = 0.05 m
x = 0.05 m
water depth = 0.20 m
Horizontal flow velocity:
exponential depth distribution
z k h
h z V e
V ( ) (0)
Vertical exchange rate:
Proportional to horizontal flow velocity
) ( )
(z V z
Vv h
Figure 3 Schematic overview of the numerical model
Results and perspectives
The calibration results of the (Tables 2-3; Figs. 4-5) reveal that the
exchange rate at the sediment-water interface is as high as 13.5 l/m2/s, but rapidly decreases to 0.2 l/m2/s at 10 cm depth. The results of these experiments provide good perspectives to further study the penetration and deposition of fine sediments in gravel beds.
Parameter Value
Water flow velocity 0.034 m/s
Longitudinal dispersion coefficient 0.008 m2/s
Interstitial water flow velocity at z = 0 Gravel porosity Water flow velocity
Parameter Value exponent k
best fit 41 m-1 minimum 37 m-1 maximum 42 m-1 α
best fit 0.99 minimum 0.60
maximum 1.00 -30
-25 -20 -15 -10 -5 0
1E-05 0.0001 0.001 0.01 0.1 1 10 100
Vertical exchange rate (l/m2/s)
Depth (cm)
Best fit Mean Minimum Maximum
Table 2 Fixed model parameters
Table 3 Calibrated model parameters
Figure 4 Calibrated vertical exchange rate as function of depth
Figure 5 Measured and modelled breakthrough curves Table 1 Main parameters of the flume experiments
Flume dimensions 18.9 m 2 m
Depth water layer 20 cm
Thickness gravel layer 30 cm
D50 gravel 39.1 mm
gravel porosity 0.4
Longitudinal gradient gravel bed 0.05%
Initial Electrical Conductivity in flume 400-800 µS/cm
Electrical Conductivity local groundwater 150 µS/cm
Location 1 - Water
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC modelled
measured
Location 2 - Water
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC
Location 3 - Water
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC
Location 2 - Gravel -5 cm
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC
Location 1 - Gravel -10 cm
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC
Location 2 - Gravel -10 cm
0 0.2 0.4 0.6 0.8 1
0 500 1000 1500 2000 2500 3000
Time (s)
Normalised EC
