Effects of suspended sediment characteristics and bed sediment transport on streambed clogging

Abstract

Abstract:Fine sediment deposition in streambeds can reduce pore water fluxes and the overall rate of hyporheic exchange, producing deleterious effects on benthic and hyporheic ecological communities. To increase understanding of the factors that control the reduction of hyporheic exchange by fine sediment deposition, we conducted experiments in a laboratory flume to observe changes in the rates of solute exchange and kaolinite clay deposition as substantial amounts of kaolinite accumulated in the streambed. Two long-term experiments were conducted, with durations of 14 days and 29 days. Use of a laboratory flume system allowed steady stream flow conditions to be maintained throughout both experiments, and alternating injections of known quantities of kaolinite and a sodium chloride tracer were used to assess the effect of clay accumulation on hyporheic exchange directly. In the first experiment, there was no bed sediment transport and kaolinite deposition formed a highly clogged near-surface layer that greatly reduced hyporheic exchange. Application of a fundamental model for advective hyporheic exchange indicated that the effective permeability and porosity of the streambed decreased substantially during the course of the experiment. In the second experiment, the kaolinite was prepared with different surface properties to be more mobile, and the experiment was conducted with a small degree of bed sediment transport. As a result, no distinct clogged layer developed, and the rate of hyporheic exchange was found to remain approximately constant throughout the experiment (29 days). These results indicate that increasing fine sediment loads, e.g. those that occur from changes in land use, can have substantially different impacts on hyporheic exchange and associated ecological processes depending on the stream flow conditions, the rate and frequency of bed sediment transport, and the extent of interaction of the introduced fines with bed sediments.

The effect of coarse gravel on cohesive sediment entrapment in an annular flume

“…The clogging layer cuts off the advective pumping process and correspondingly reduces additional infiltration of suspended fine grain sediments into the streambed. Rehg et al (2005) further reported that under shear stress conditions that cause a sand bed to be mobilized a clogging layer was prevented by the mobile bed, which results in continual pumping of fine sediment into the bed. Schalchli (1992) conducted a series of flume experiments using bed sediments collected from the Langeten River in Switzerland.…”

“…Because of the need to understand the impacts of cohesive sediment and associated contaminants on aquatic biota (Ankers et al, 2003;Droppo and Stone, 1994) and the physical loss of habitat through infilling of interstitial spaces with cohesive sediment in stream beds (Cobb et al, 1992), an increasing number of recent experimental and field studies have advanced knowledge of cohesive sediment transport and storage mechanisms in aquatic systems (Droppo and Amos, 2001;Packman et al, 2000;Rehg et al, 2005;Stone and Krishnappan, 2003;Krishnappan and Engel, 2006;Krishnappan, 2007). Deposition of cohesive sediment in gravel bed streams can influence pore water chemistry and nutrient cycling (Grimm and Fisher, 1984;Nagorski and Moore, 1999;Worman et al, 2002) as well as alter the porosity and conductivity of stream beds (Packman and MacKay, 2003).…”

“…Rehg et al (2005) demonstrated that the mobilization of a sand bed with bed-forms (dunes) increased the amount of suspended sediments entrapped in the bed. They attributed increased sediment entrapment to bed mobilization that prevented the formation of a clogging layer in the upper surface of the streambed.…”

“…They attributed increased sediment entrapment to bed mobilization that prevented the formation of a clogging layer in the upper surface of the streambed. Clogging layers formed under static bed conditions through the infilling of the bed pore voids when clay particles are pumped advectively into the bed (Rehg et al, 2005). The clogging layer cuts off the advective pumping process and correspondingly reduces additional infiltration of suspended fine grain sediments into the streambed.…”

“…Several studies highlight the importance of bed form and bed mobility on the entrapment of cohesive sediments (Schalchli, 1992;Krishnappan and Engel, 2006;Rehg et al, 2005). Packman et al (2000) developed a bed-form induced advective pumping model to describe the infiltration potential of fines into a streambed comprised of sand.…”