We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%-142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of fl ow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform.However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The fi rst bend served as a fi lter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this fi ltering bend and was lost to the overbank surface.

[1] The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive a large zone of river-groundwater mixing, or hyporheic exchange. Flume experiments and numerical simulations show that river water downwells into the riverbed upstream of a channel-spanning log and upwells downstream. Exchange rates are greatest near the log and decay exponentially with distance upstream and downstream. We developed equations for bed pressure profiles and hyporheic exchange rates in the vicinity of a channel-spanning log that can be used to evaluate the impact of LWD removal or reintroduction on hyporheic mixing. The magnitude of pressure disturbance along the bed (and thus hyporheic exchange) increases with the fraction of channel depth blocked by the log and channel Froude number. Exchange rates are relatively insensitive to relative depth of the log (gap ratio). At natural densities, LWD in lowland streams drives reachaveraged hyporheic exchange rates similar to a ripple-covered bed. However, the length scales and residence times of hyporheic exchange due to LWD are greater. By removing LWD from streams, humans have altered patterns and rates of hyporheic exchange, which influence habitat distribution and quality for invertebrates and fish.Citation: Sawyer, A. H., M. Bayani Cardenas, and J. Buttles (2011), Hyporheic exchange due to channel-spanning logs, Water Resour.

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[1] The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive river-groundwater mixing, or hyporheic exchange, and heat transport within the hyporheic zone. We quantify hyporheic fluid and heat exchange induced by current interaction with channel-spanning logs using two approaches: laboratory flume experiments and numerical simulations that link turbulent open-channel fluid flow, porous fluid flow, and heat transport. Flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio (a fraction of the channel depth blocked by the log), channel Froude number, and sediment permeability. Upstream of a log, downwelling water transports the river's diel thermal signal deep into the sediment. Downstream, upwelling water from shallow flow paths has a thermal signal similar to the surface water, while upwelling water from deep flow paths forms a wedge of buffered (low-amplitude) temperatures. Since most hyporheic water emerges from shallow flow paths, upwelling water has limited potential to buffer surface water temperature. Because of historic channel clearing practices, modern rivers have unnaturally low densities of LWD. A key implication is that LWD removal has contributed to thermal homogenization and potential degradation of hyporheic habitats. LWD reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone. However, hyporheic exchange near LWD may not impact diel surface water temperatures at the reach scale.