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The Effect of Sand Content on Sediment Transport, Deposition, and Bar Morphology

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The Effect of Sand Content on Sediment Transport, Deposition, and Bar Morphology The Effect of Sand Content on Sediment Transport, Deposition, and Bar Morphology By John Kemper Advisor: Dr. Karen Prestegaard April 26th, 2012 GEOL394H 1 Abstract Studies of stream adjustment to urbanization have focused primarily on the geomorphic consequences of increases in discharge, such as channel widening (e.g. Hammer, 1972, Pizutto et al., 2000). Urbanization can also affect sediment supply, grain size, or sediment transport mechanics, which can alter channel stability and morphology. Sediment deposition can initiate feedbacks among geomorphic and hydraulic variables leading to significant alterations of stream morphology. This has been documented in lower Little Paint Branch Creek by Blanchet [2009], although the underlying mechanics for these changes were not fully investigated. In summer 2012, simple alternate bars in the channel at Cherry Hill Road were replaced by large diagonal bars with very sharp bar fronts. These elevations of these bars changed were reduced during floods associated with Hurricane Sandy. These changes occurred over short timescales (< 6 months) and coincide with an increase in upstream sand supply associated with major road construction (of the Inter-County Connector or ICC) in the watershed (Blanchet, 2009). This research is designed to test the hypothesis that changes in bar morphology were caused by varying amounts of sand in the bedload, which reduces the critical dimensionless shear stress of the gravel, thus increasing sediment transport and deposition rates. (Wilcock and Crowe, 2003). Test of this hypothesis were based on field data collection and modeling of the sediment transport conditions before and after the Hurricane Sandy flood. Data collection included field measurements of channel morphology, water surface gradient, and grain size analysis of surface and subsurface material. Field data show that dimensionless shear stress for Hurricane Sandy was well below the * τ crit for homogenous gravel, indicating that sand content is responsible for lowering the critical dimensionless shear stress, leading to the substantial sediment transport observed. Using time series of important flow parameters a model for sediment transport in lower Little Paint Branch Creek was created. Suspended sediment profiles indicate that 500 µm and larger sand is not suspended at significant heights above the bed. Rouse number calculations indicate that sand is accumulated during small events and winnowed during large events. The model shows that a substantial number a small events occurred in the months prior to Hurricane Sandy, leading to the lowering of the critical dimensionless shear stress. This suggests that sand content, and the small flow events that lead to the accumulation of sand, have a significant impact on bar morphology and channel morphology for systems experiencing a sudden influx of sand sized sediment. 2 Introduction Stream channel morphology results from the deposition and erosion of sediment in alluvial channels. Although streams indicate a wide range of morphology, the location of sediment deposition generates three main planform patterns: straight, braided, and meandering (Leopold and Wolman, 1957). Even straight channels indicate patterns of sediment deposition either as riffle-pool sequences, which are vertical accumulations of gravel spaced along the channel bed, or as alternate bars that are deposited along alternating banks. The spacing of riffle- pool sequences and alternate bars are about 5-7 times the width of the channel (Leopold and Wolman, 1957). The alternate bars accompany a wandering thalweg, or line of greatest depth, which alternates from bank to bank as well. There is a continuum in channel form between channels with alternate bars and those with meander bends. The wavelength of the meander is 10-14 times the width of the channel; a meander wavelength contains 2 alternate bar sequences (Leopold and Wolman, 1957). In braided streams, sediment is initially deposited near the channel center, which is usually the location of highest bedload transport. Diversion of flow around the central bar forms two or more anastomosing channels and a resulting channel morphology that is wider and shallower than straight and meandering streams for equivalent discharges. Braided channels occur on steeper slopes than meandering channels with similar discharge. This makes sense because braided channel streams are generally mostly gravel and, for the same bankfull discharge, it requires a greater shear stress to transport gravel than sand. (Leopold and Wolman, 1957). In addition, sand is more easily suspended and thus swept to the channel edges by secondary currents (Dietrich and Smith, 1979). Channel morphology develops due to the interactions between flow and sediment transport in stream channels. Pioneering researchers identified systematic downstream changes in average channel morphology (termed hydraulic geometry) in most river systems (Leopold and Maddock, 1953). Average downstream hydraulic geometry relationships indicate that bankfull width is proportional to the square root of the discharge (w Q0.5). In a given stretch of river, Q is mostly constant, but width exhibits systematic changes between riffles and pools (Andrews, 1980) or between reaches with and without channel bars (Ferguson et al., 1993). To explain downstream hydraulic geometry, Parker (1979) hypothesized that most channels were “threshold channels” that moved sediment when the critical dimensionless shear stress was exceeded and that this threshold was achieved at the bankfull stage. Therefore, for a given energy gradient, the channel adjusts width until the channel depth generates a bankfull shear stress (~gdS, with = fluid density, g = gravitational constant, d = depth, S = water surface gradient) that is near the threshold of motion for the bed material size. Parker [1979] explained threshold channels by comparing the distributions of bankfull shear stress ( = gdS) and channel depth within a river cross section. If average bankfull shear stress is near critical, the distribution of shear stress indicates higher bankfull shear stress values ~ 20% above critical in the center of the channel, whereas shear stresses on the channel banks are below critical. In other words, is variable across the channel, and is above crit (shear stress required for incipient motion) for gravel in the center of the channel. This allows channels to transport sediment while maintaining stable channel banks. Therefore, for a threshold gravel bed stream (with a given gradient), grain size determines the threshold (bankfull) depth which, in turn, influences velocity. The threshold channel maintains itself through varying discharge events through positive and negative feedbacks and the importance of the adjacent floodplain in 3 carrying discharge through the width of the floodplain, which limits the rate at which depth increases in the channel. The original theory for threshold channel morphology assumed a constant critical dimensionless shear stress for a channel. Dimensionless shear stress is the ratio between fluid shear stresses and grain resisting forces (Shields, 1938): ∗ 1 where is shear stress, s is the ratio of sediment density s to water density , g is gravity, and * D50 is median grain size (measured as intermediate axis of a particle). is therefore the ratio of * fluid shear stress to grain resisting forces and crit is the dimensionless shear stress required for incipient motion (critical dimensionless shear stress). The original work on critical dimensionless shear stress was conducted on homogeneous sediment in flumes (Shields, 1936). Since that time, investigators have determined that the size distribution of surface particles (Parker and Klingeman, 1982; Wilcock, 1986) and the influences of sand on gravel transport (Ferguson et al, 1992; Wilcock and Crowe 2003) both significantly lower the critical dimensionless shear stress for gravel transport. The increase in sand content lowers the friction angle of the gravel, causing a decrease in dimensionless shear stress required for incipient motion. The increase in sand content causes the gravel sized particles (2-256 mm) to roll across the smaller sand particles ( mm to 2 mm) or, in other words, ride on a carpet of sand, thus lowering critical dimensionless shear stress. Thus, in gravel bed streams, changes in the supply of sand or the sorting of bed material can result in changes in sediment transport mechanics and thus sediment transport rates. These changes in transport can result in changes in sediment deposition and channel morphology. * Due to the dependence of crit (for gravel) on sand content, it introduces another parameter into the mechanics of gravel bar formation that has been largely ignored by previous studies. By combining the observations of Leopold and Maddock [1953], Leopold and Wolman [1957], Parker [1978], and Wilcock and Crowe [2003], we find that sand content could be an important parameter for gravel bar formation that has not been previously investigated. Partitioning of Shear Stress It is also important, when studying the effects of shear stress on sediment transport and gravel bar formation, to separate the shear stress into two components: the mean boundary shear stress exerted on bed surface particles and mean boundary shear stress exerted on form roughness of bars which, when summed together, equal the total mean boundary shear stress (Lisle et al., 1993). Surface particle shear stress can be calculated by where is fluid density,
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