Changes in bedload transport rate associated with episodic sediment supply in a Japanese headwater channel

Abstract

2We conducted field monitoring of bedload transport rate associated with experimental sediment 3 release in a natural channel to clarify behavior of the supplied sediment on mixed size bed. 4Observation of bedload rate at two sites along the 30 m channel reach revealed that downstream 5 migration of finer particles delay compared with coarser particles. Ratio of the bedload sediment that 6 deposited during the migration was higher for finer sediments. These behaviors of the mixed size 7 particles were clear during passage of the sediment wave without changes in water discharge. Flashing 8 peak of discharge that caused artificially by opening of the dam gate did not destroy channel bed 9 structure including steps and pools formed by coarser sediments, and only small amount of bedload 10 was mobilized. Both reach scale channel features including steps, pools, and riffles as well as fine 11 scale features (i.e., armor coat) likely increased critical shear stress of particles and decreased bedload 12 rate during our experiment. Extreme sediment supply induced two types of sediment deposition; (1) 13 filling the pools in reach sales and (2) the intrusion of fine particles into the coarser sediment that 14 formed an armour layer. The all grain size fractions can deposit as type (1) when shear stress of stream 15 water is not enough to entrain bedload particles, while deposition type (2) occurs when finer sediment 16 pass over channel bed on which armor coat is well-developed. Deposition of finer sediment into 17 coarser sediment that forms armor coat is affected by grain size distribution of bed surface sediment. 18Thus, impact of the sediment supply on downstream channel depends on both bedforms and grain size 19 distribution of bed surface sediment over which the supplied sediment pass. 20 21

Short-term responses of macroinvertebrate drift following experimental sediment flushing in a Japanese headwater channel

“…6; also see Imaizumi et al 2009). The ratio of deposition was greatest for the 1-2 mm size class (80%) followed by the 2-4 mm size class.…”

“…We assumed that the bed load sediment consisted of particles larger than sand ([1 mm in diameter), because particles smaller than 1 mm are likely to be carried in suspension during rough and turbulent flow conditions in steep channels (Gomi and Sidle 2003). The bed load transport rate of the HelleySmith samplers was calculated from the dry weight of the sampled sediment by dividing by the duration of the sampling event (30 s on average) (Imaizumi et al 2009). Bed load transport values were multiplied by the ratio between the channel cross-sectional width (which changed over time) and the width of the sampler (7.8 cm) to estimate the amount of bed load across the entire channel width.…”

“…The gate was closed at 11:55 to reserve water for the next sediment flushing. Water height was measured within the UR using a capacitive water level probe (Trutrack, WT-HR) with a measurement range of 0.5 m (accurate to within 5 mm) at intervals of 30 s. Detailed information for discharge estimation from the stage gauge was provided by Imaizumi et al (2009). After opening the gate, we began collecting bed load sediment ([1 mm in diameter), suspended sediment (\1 mm in diameter), drift macroinvertebrates, and transported organic matter.…”

“…Sediment that accumulates in the pond is released once a year to maintain the sediment storage capacity of the dam. By opening the gate of the dam, both water and sediment are simultaneously flushed out downstream (Imaizumi et al 2009). The first flooding pulse appears at a given point downstream, followed by the pulse of suspended sediment.…”

“…Mass movements and bank failures are natural episodic sediment events, and such sediments directly enter stream channels (Gomi et al 2002;Imaizumi and Sidle 2007). When sediment supply overwhelms transport capacity, sediment particles tend to be deposited on the channel bed (Montgomery and Buffington 1997;Imaizumi et al 2009). Excess sediment supply can fill in pools in pool-riffle and step-pool channels (Lisle 1982;Lisle and Hilton 1992).…”