Sedimentary roles on hyporheic exchange in karst conduits at low Reynolds numbers by laboratory experiments Sedimentary roles on hyporheic exchange in karst conduits at low Reynolds numbers by laboratory experiments

The relative roles of the sediment grain
size/permeability, conduit flow rate and conduit geometry/
angle on the hyporheic exchange between a karst conduit
and its underlying sediments under low Reynolds numbers
(Re) were investigated by means of laboratory experiments
and numerical simulations. Two laboratory analogues
consisting of siphon structured glass tubes (with bend angles
of 15 and 45°) were used for the experimental studies. Tracer
experiments were performed in each analogue with sediments
of variable grain size (0.45 mm, 0.4–0.7 mm, 1 mm) to characterize
the transport properties of contaminants originating
from the sediments. Numerical simulations were used to probe
the exchange flow patterns and exchange flux magnitudes
between the conduit and sediment. Tracer experiments demonstrated
a zone of forward flow and a zone of reverse flow in
the sediments that were independent of grain size, which were
reproduced well by numerical simulations. The exchange flux
ranged from 0.02% for fine grains to 2% for coarse grains
under the experimental flow conditions. A linear relationship
between the exchange flux and the conduit Re value, which
was independent of the conduit geometry and sediment grain
size, was established with numerical simulations. This study demonstrated that sediment grain size/permeability has no influence
on the exchange flow patterns; however, relative to the
conduit flow rate and conduit geometry/angle, sediment permeability
has a much stronger influence on the exchange rate
of hyporheic flow.