ABSTRACT:

This is supporting data for the manuscript "Recovering head and flux distributions at the sediment-water interface for arbitrary, transient bedforms by inversion of photographic time series". The data set includes raw video of bed form and dye motion for three tracer tests conducted in a glass-sided flume which are discussed in the manuscript. Also contained are digital feature representations for those tests, including sediment-water interface (SWI) location, SWI head distribution, and dye front locations.

ABSTRACT:

Clogging of streambeds due to clay deposition influences the stream-subsurface exchange flux and thus directly modulates hyporheic ecological and biogeochemical processes. Clogging of sandy streambeds has previously been studied under losing and gaining flows and during streambed movement, but not when these two flow conditions coincided. We conducted flume experiments to quantify the combined effect of moving bedforms and losing or gaining flows on kaolinite deposition and streambed clogging. The experiments were conducted by adding pulses of kaolinite in a flume packed with sand under a streamwater velocity of 25 cm/s. We measured the deposition rates, dynamics of hyporheic exchange flux (HEF) and vertical hydraulic conductivity (Kv), and the vertical distribution of kaolinite at the end of the experiments under two losing and two gaining flows (Darcy velocity of 10 and 20 cm/day). Kaolinite deposition led to clogging and reduction in Kv and HEF under all flow conditions. Deposition occurred faster under losing flow conditions than under gaining flow conditions. However, the changes in Kv and HEF were similar under losing and gaining flow conditions for similar kaolinite concentrations in the bed. Our results indicate that the deposition patterns of kaolinite were more influenced by bedform movement than by losing or gaining flow conditions. This implies that bedform morphodynamics control local-scale clogging of sandy streambeds and should be accounted for when studying the hydrology of catchments at larger scales.

ABSTRACT:

Previous modeling studies of hyporheic exchange induced by moving bedforms have used a Lagrangian frame of reference, typically a simulation domain that moves with an individual bedform. However, that approach is not suitable for simulating the exchange and accumulation of fine particles at a specific location by the migration of a series of bedforms, which commonly occurs in sand-bed streams. Here we present a novel simulation framework that represents mobile bedforms with a moving-interface domain and determines the resulting hyporheic transport using particle tracking. Simulation results successfully reproduce observations of clay deposition in sand beds, and the resulting development of a low-conductivity layer near the scour zone. Increased bedform celerity and filtration both lead to shallower depth of clay deposition, and a more compact deposition layer. While increased filtration causes more clay to deposit, increased celerity reduces deposition by flattening hyporheic exchange flowpaths.

ABSTRACT:

Hyporheic exchange flux (HEF) plays an important role in the transport of nutrients and sediments in stream ecosystems. Deposition of fine suspended sediment particles can clog the streambed, reduce permeability, and lead to a reduction in HEF, resulting in impairment of various ecological processes. However, the dynamics of fine particle deposition and streambed clogging are still not well understood, especially when the bed is in motion. We conducted flume experiments to study the effects of coupled sand-kaolinite dynamics on HEF. Three experiments with a mobile sand bed and constant discharge were conducted in a laboratory flume through repeated kaolinite injection pulses at a fixed increment. HEF and participating porewater volume were assessed using salt and dye tracer tests. Kaolinite deposition rates were inferred from turbidity measurements while deposition patterns were measured using core samples. We found that fine sediment primarily accumulated within a layer below the bedform scour zone, and that this layer was thicker when kaolinite was added in larger pulses. This low-permeability layer led to an overall reduction in HEF, which declined linearly regardless of the pulse increment concentration. However, the rate of reduction in participating pore volume was higher for larger kaolinite addition increments, because faster deposition of kaolinite protected the deposits from scour and resuspension. These results indicate that clogging occurs not just during and after high-flow events, but also under constant flow conditions in which clay particles and hyporheic exchange lead to the formation of a low-conductivity layer in the bed.