Bed Material Grain Research Articles

Experiments on Gravel‐Sand Transitions: Behavior of the Grain Size Gap Material

AbstractRiver bed sediments often lack fine gravel between 1 and 5 mm, a phenomenon referred to as the “grain size gap.” The gap corresponds to the rapid reduction in grain size associated with the gravel‐sand transition (GST), where median bed material grain size reduces from ∼10 mm gravel to ∼1 mm sand. Fine gravel grain sizes are often present in hillslope sediment, so it is not clear why they are absent on riverbed surfaces. We present a phenomenological laboratory experiment examining changes in sediment dynamics across a GST to examine the behavior of grain size gap material when included in the bed and feed grain size distributions. Our observations indicate that where sand falls out of washload, forming persistent surficial deposits at the GST, grain size gap material experiences enhanced mobility. This is due to hydraulic smoothing by sand that occurs because of a geometric effect, where medium sand bridges interstitial pockets in fine gravel bed surfaces. Our experiments show that gap gravel flux is enhanced by sand deposition, making gravel beds mobile at the threshold of motion. We are unable to maintain an immobile grain size gap gravel bed when sand is fed which explains why gravel beds composed of 1–5 mm particles are so rare on Earth. We hypothesize that in natural systems, grain size gap particles are either buried in the diffuse extension of GSTs or transported into coastal and marine environments where they are more commonly observed.

Achieving Equilibrium as a Semi‐Alluvial Channel: Anthropogenic, Bedrock, and Colluvial Controls on the White Clay Creek, PA, USA

AbstractBased on well‐developed hydraulic geometry relations for width and depth, classic studies initially interpreted the Mid‐Atlantic White Clay Creek (WCC) as a quasi‐equilibrium, alluvial channel. Subsequent studies document the legacy of colonial‐age watershed disturbances and urban development, confounding earlier classifications. To investigate this matter, we contribute new data from reach‐scale geomorphic mapping, and observations and modeling of bed material transport. WCC's longitudinal profile reflects a history of bedrock incision, while hydraulic geometry equations for width and depth indicate quasi‐equilibrium cross‐sectional adjustment. Alluvial landforms such as pools and riffles, bars, and actively forming floodplains occur at all 12 study sites, but exposures of bedrock and colluvium are also common. The ratio of bankfull to threshold Shields stress averages 1.41 (range: 0.41–2.63), suggesting that WCC is an alluvial, threshold, gravel‐bed river. However, a numerical model of WCC bed material transport and grain size, calibrated to bedload tracer data, demonstrates that 22% (range: 8%–73%) of bed material is composed of immobile, locally sourced cobbles and boulders, while the remaining bed material represents mobile, sand to cobble‐sized alluvium; this leads us to classify WCC as a semi‐alluvial river. Additional computations suggest that channel morphology is insensitive to bed material supply. Field observations imply that bankfull Shields stresses do not represent channel adjustments to achieve stable banks; rather, width adjustment likely reflects cohesive bank processes. Despite the numerous and contradictory labels applied to WCC (i.e., quasi‐equilibrium, Anthropocene, bedrock, semi‐alluvial, and gravel‐bed), each term contributes insight that any single conceptual model would be unable to provide alone.

Image-Based Bed Material Mapping of a Large River

The composition or bed material plays a crucial role in the physical hydromorphological processes of fluvial systems. However, conventional bed material sampling methods provide only pointwise information, which can be inadequate when investigating large rivers of inhomogeneous bed material characteristics. In this study, novel, image-based approaches are implemented to gain areal information of the bed surface composition using two different techniques: monocular and stereo computer vision. Using underwater videos, captured in shorter reaches of the Hungarian Danube River, a comparison of the bed material grain size distributions from conventional physical samplings and the ones reconstructed from the images is carried out. Moreover, an attempt is made to quantify bed surface roughness, using the so-called Structure from Motion image analysis method. Practical aspects of the applicability of image-based bed material mapping are discussed and future improvements towards an automatized mapping methodology are outlined.

Decadal-scale morphological adjustment of a lowland tropical river

Compared with temperate regions, much less is known about the dynamics of tropical river systems. Tropical rivers are typically characterised by pronounced seasonal changes in precipitation, large sediment loads and high rates of lateral channel migration across often very low-gradient and densely populated floodplains. Understanding the controls on channel migration or change is integral to our ability to fully predict and build resilience against flood risk and wider river-related hazards. Here, we analyse channel and confluence migration over the last ~40years along a ~85km reach of the Cagayan River and one of its tributaries, the Pinacanauan de Ilagan (Luzon, Philippines) using optical satellite imagery captured during this period. Combining this with spatial variations in channel pattern, valley width and new bed material grain size data, we demonstrate that sediment transport and deposition are key drivers of the observed tropical channel morphodynamics in this region. The high sediment supply generated in the catchment headwaters (by mass-wasting of hillslopes triggered especially in typhoons) results in high aggradation rates and channel widening on the lower gradient alluvial plain. We suggest that this aggradation enhances local confluence and lateral channel migration rates, which can reach >300m per decade, and that lateral migration rates of tropical rivers are typically greater than those of temperate rivers. Channel morphodynamics have implications for how to best manage these types of tropical river systems, where hard bank protection structures may result in a complex geomorphic response and flood risk mapping may need to include assessment of sensitivity to varying channel position and topography.

Experimental investigation of the equilibrium scour depth below submerged pipes both in live-bed and clear-water regimes under the wave effect

Pipelines are the key structures to carry fluids such as oil, petroleum, gas and water; any inadequate design induces bottom stability problems causing unintended consequences. In the present study, the equilibrium scour depth below submerged pipes due to regular waves was investigated experimentally. During the experiments various pipe diameters, bed material grain sizes and wave characteristics were used. The novelty of the study is that the experiments were performed both in live-bed and clear-water regime conditions. The analysis of the experimental data showed that the equilibrium scour was correlated with Shields parameter as well as Keulegan-Carpenter number, especially for live-bed scour condition. However, in the case of clear-water scour, the Keulegan-Carpenter number was found as a unique non-dimensional parameter that dominates the scour phenomenon. New empirical relations were also proposed for clear-water and live-bed scour separately.

Can beaver impact promote river renaturalization? The example of the Raba River, southern Poland

The European beaver (Castor fiber) was reintroduced in the Polish Carpathians in the 1980s after a few centuries of absence. It gradually colonized suitable habitats in the Raba River valley and elsewhere. The question arises as to whether beaver activity can play a role in the local improvement of hydromorphological conditions and spontaneous renaturalization of the Raba River channel. Field surveys were performed in morphodynamically and structurally homogeneous reaches of the river. Traces of beaver activity were identified and used to estimate the studied beaver population. Local beaver impact on the studied river channel was also determined. The Raba channel is trained along about 80% of its length and considerably incised. Traces of beavers activity were found in 16 out of 31 river reaches, mainly in the upper and lower river course. The study showed that relatively flat channel gradient, small maximum bed-material grain size, and high channel sinuosity favour beaver presence. The largest number of beaver habitats was identified in river reaches strongly altered by man and characterized by a uniform channel structure. Beaver impact on channel structure varies depending on differences of the river channel features in upper and lower reaches of the Raba River channel. In upper reaches, the impact of beaver activity (mostly dams) is reflected in increased lateral erosion, while slower water current reduces the tendency for bed degradation. In lower reaches, beaver impact is mostly limited to bank fragmentation (slides and burrows). Lateral erosion, accumulation of material at the toe of riverbanks, and wood debris accumulation all produce a local impact on river channel width. These beaver-initiated processes mostly alter artificially homogenized river reaches. Beavers may actually play a substantial role in future renaturalization of both upper and lower reaches of the Raba River.

Ecological state of a mountain river before and after a large flood: Implications for river status assessment

Assessment of the ecological status of rivers is key to monitoring the achievement of the environmental goal of the EU Water Framework Directive and the success of restoration projects. In summer of 2009 and 2010, repeated assessments of physical habitat conditions and of fish and benthic invertebrate communities were performed at low-flow conditions in 10 unmanaged and 10 channelized cross-sections of the Biała River, Polish Carpathians. Between the two surveys, an 80-year flood occurred, significantly affecting habitat characteristics and river communities. In unmanaged cross-sections, active channel width increased, whereas the degree of cross-sectional variation of flow velocity decreased. In channelized cross-sections, the increase in active channel width and the cross-sectional variation of flow velocity was accompanied by a decrease in bed-material grain size. Before the flood, the unmanaged cross-sections hosted 2.3 times more benthic invertebrate taxa than the channelized ones, whereas after the flood, the number of taxa they supported was so reduced that the taxonomic richness of benthic invertebrate assemblages in both cross-section types became similar. In comparison to pre-flood conditions, the abundance of fish juveniles (YOY) in unmanaged cross-sections was reduced nearly by half; before the flood they hosted 5 times more juvenile individuals than channelized cross-sections and only twice as many after the flood. Finally, a differing assessment of flood impact on the ecological river quality was obtained with the invertebrate-based BMWP-PL index and the European Fish Index, with the former indicating a significant reduction of the quality in unmanaged cross-sections and the latter pointing to no such change. The results indicate that assessments performed before or after a major flood may yield significantly different results for the quality of abiotic and biotic elements of the river ecosystem. Final assessment should thus be based on repeated surveys to balance the effect of extreme hydrological events.

Excess River Sedimentation at Bridges in the Raya Graben (Northern Ethiopia)

AbstractThis study investigates the temporal variability and effects of bio‐climatic factors and bridges on local hydro‐geomorphic conditions that cause excess sedimentation and flood hazard in ephemeral rivers of the Raya graben (northern Ethiopia). Normalized difference vegetation index (NDVI) was analyzed using Landsat imageries of 1972, 1986, 2000, 2005, 2010, and 2014. Daily rainfall data of three meteorological stations were considered to analyze the temporal trends and return time intervals of rainfall intensity in 24 h. Stream‐bed gradient and bed material grain size were measured in 20 river reaches. Results show that the maximum NDVI values were recorded in the time interval from 2000 to 2014. Nevertheless, during this decade, the study bridges experienced the largest excess sedimentation, in some bridges up to 1·6 m. Different factors to account for it are discussed as no evidence of rainfall pattern, accountable for an increase in sediment delivery from the headwaters, is found. The field data show that channel narrowing at bridges is the main cause of thick upstream sedimentation and of increasing frequency of overbank flows during the last 15 years. A simple hydraulic approach is presented to explain the variability of sediment deposition at bridges and change in bed material grain size between the main stem and the reach right upstream of the bridge. Copyright © 2016 John Wiley & Sons, Ltd.

Grain sorting in the morphological active layer of a braided river physical model

Abstract. A physical scale model of a gravel-bed braided river was used to measure vertical grain size sorting in the morphological active layer aggregated over the width of the river. This vertical sorting is important for analyzing braided river sedimentology, for numerical modeling of braided river morphodynamics, and for measuring and predicting bedload transport rate. We define the morphological active layer as the bed material between the maximum and minimum bed elevations at a point over extended time periods sufficient for braiding processes to rework the river bed. The vertical extent of the active layer was measured using 40 hourly high-resolution DEMs (digital elevation models) of the model river bed. An image texture algorithm was used to map bed material grain size of each DEM. Analysis of the 40 DEMs and texture maps provides data on the geometry of the morphological active layer and variation in grain size in three dimensions. By normalizing active layer thickness and dividing into 10 sublayers, we show that all grain sizes occur with almost equal frequency in all sublayers. Occurrence of patches and strings of coarser (or finer) material relates to preservation of particular morpho-textural features within the active layer. For numerical modeling and bedload prediction, a morphological active layer that is fully mixed with respect to grain size is a reliable approximation.

Variable Shields number model for river bankfull geometry: bankfull shear velocity is viscosity-dependent but grain size-independent

ABSTRACTThe bankfull geometry of alluvial rivers is thought to be controlled by water and sediment supply, and characteristic sediment size. Here we demonstrate a novel finding: when bankfull shear velocity and bankfull depth are correlated against bed material grain size and bed slope, they are to first order independent of grain size and dependent on water viscosity. We demonstrate this using a similarity collapse for bankfull Shields number as a function of slope and grain size, obtained with data for 230 river reaches ranging from silt-bed to cobble-bed. Our analysis shows that bankfull Shields number increases with slope to about the half power. We show that the new relation for bankfull Shields number provides more realistic predictions for the downstream variation of bankfull characteristics of rivers than a previously used assumption of constant bankfull Shields number.

Morphology and controls on the position of a gravel‐sand transition: Fraser River, British Columbia

AbstractAlluvial river channels often exhibit a relatively abrupt transition from gravel‐ to sand‐bedded conditions. The phenomenon is well documented, but few prior studies have analyzed the spatial variability through reaches where transitions occur. The downstream fining pattern observed in the Fraser River is cited as a classic example of an abrupt gravel‐sand transition in a large alluvial channel. However, important questions regarding the exact location of the transition, its sedimentology and morphology, and what controls its location remain unanswered. Here we present observations of the downstream change in bed material grain size, river bed topography, and channel hydraulics through the reach within which the transition occurs. These observations indicate that the gravel‐sand transition is characterized by a terminating gravel wedge, but there are patches of gravel downstream of the wedge forming a diffuse extension. We show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility. We argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed. We also find that sand in suspension declines rapidly at the downstream end of the wedge, suggesting that sand is delivered to the bed, completing the sedimentary conditions for a gravel‐sand transition. We propose that the break in river slope associated with the transition is a consequential feature of the transition.

Simulation Method of Particle Size Scale for Fine Sand Model

In sediment movable bed model, the basic similar conditions include the flow movement similar and the sediment movement similar. In water flow movement similar conditions, the velocity scale of the Freund's similarity and resistance similarity is inconsistency, and the flow resistance is the function of bed material grain size and flow intensity. In sediment movement similar, the particle size scale forms are not consistent too, and they are also related to the flow conditions, so the sediment partial size scale is difficult to determine. According to the research, the similar conditions can be focused on primary and secondary one, and put the primary one as the design basis. In view of the waterway regulation, the regulation flow discharge should be the design basis, others allowed a certain deviation; In the straight reach, resistance similarity should be the design basis, Freund's similarity allowed a certain deviation; In the bend reach, the Freund's similarity and resistance similarity should be simultaneously satisfied; For the bed load model, the incipient motion should meet similarity condition; The suspended sediment model should meet the sediment suspension similarity; For the non-uniform sediment, median particle size should be the design basis. According to the above principles, the sediment particle size scale can be determined.

Bedload Equation Analysis Using Bed Load-Material Grain Size

Abstract Twelve predictive bedload sediment transport equations are rated against 14 sets of gravel-bed river field data collected by handheld bedload sampler in Narmab River, northeastern Iran. To evaluate these formulas two types of grain size namely bedload and bed material were used. The results show that the equations of Engelund and Hansen, Van Rijn and Einstein perform well with bed material grain size, while Shocklitsch, Meyer-Peter and Mueller, and Frijlink yield the best results using the bedload grain size.

Flow and sediment dynamics in channel confluences

Confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large, low‐gradient main channel commonly occur in nature, but they have not yet been investigated. Measurements of the three‐dimensional velocity field, turbulence, sediment transport, bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the Upper Rhone River, Switzerland. The difference between the low‐flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone. Due to this bed discordance, the tributary flow penetrates into the main channel mainly in the upper part of the water column, whereas the main‐channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two‐layer flow structure in the confluence zone. In confluences with high supply of coarse sediment from the tributary, the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near‐bed flow originating from the main channel. The sediment transport capacity is further increased by the three‐dimensionality of the flow, which is characterized by maximum velocities occurring near the bed, and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. A conceptual model is proposed for the hydro‐morpho‐sedimentary processes, and compared to existing conceptual models for confluences with different characteristics.

Analysis of the Sediment Characteristics of the Caofeidian Sea Area

Caofeidian harbor is a large size deep-water harbor currently under construction in the eastern coast of north China. The construction began in 2003. Based on the measurements and analysis of the sediment samples collected from 2008 to 2010, the characteristics of suspended sediment concentration, suspended sediment and bed material grain diameter of the Caofeidian sea area with the progress of the engineering are presented. The measurements and analysis reveal that i) the suspended sediment concentration is low and decreases with the engineering progress; ii) the suspended sediment is very fine and becomes coarse in the deep-water channel; iii) the bed sediment in the east of Caofeidian foreland is much coarser than that in the west part.

Effect of sediment pulse grain size on sediment transport rates and bed mobility in gravel bed rivers

Sediment supply to gravel bed river channels often takes the form of episodic sediment pulses, and there is considerable interest in introducing sediment pulses in stream restorations to alter bed surface grain size distributions and bed mobility. A series of laboratory experiments was conducted in order to examine how sediment pulse grain size and volume affects the mobility of bed material in gravel bed channels. Pulses used in the experiments were composed of either the fine tail or the median of the subsurface bed material grain size distribution. Bed material refers to sediment in the channel prior to the pulse introduction exclusively. Both types of pulse were finer than the bed material surface median. Two pulse sizes were used, which were either equivalent to the volume of sediment required to cover the entire bed one median subsurface bed material grain diameter deep (full unit) or 1/4 of this volume (1/4 unit). The latter was designed to produce a transitory pulse. With the exception of the 1/4 unit coarse pulse, introduction of the sediment pulses to the channel caused dramatic increases in the bed load flux. The coarse sediment pulses fine the bed surface and coarsen the bed load. Finer pulses also fine the surface, but the bed load fines while the bed material load (that excludes pulse material) coarsens. The greatest effects on the fractional transport occurred during the full unit fine pulse where the pulse covered the greatest bed surface and effectively smoothed the bed, increasing near bed velocity and mobilizing the coarse particles. Overall, the coarse pulses were not very effective at mobilizing bed material. The large fine pulse mobilized ∼35% of the bed material surface (∼35% of its input weight) and was most effective at mobilizing the surface. However, the small fine pulse mobilized 50% of its input weight as it passed through the channel, making it the most efficient at mobilizing the bed material.

Transient response in longitudinal grain size to reduced gravel supply in a large river

The first extensive dataset on subaqueous bed material grain size in a large river subject to reduced sediment supply is investigated alongside bathymetry, modeled flow, and sediment flux. Results suggest that following sediment supply decline and a shift to a finer sediment supply, the gravel‐sand transition (GST) in fluvial systems extends and subsequently migrates upstream. The non‐abrupt (∼125 km) GST in the Sacramento River corresponds with a hump in the long profile, indicating recent downstream redistribution of sediment that impacts grain sizes. The hump is composed of sediments winnowed from upstream gravel beds that accumulate downstream where slope declines. This increases local sorting values and coarse sediment flux rates in the GST, leading to further gravel loss by burial and net efflux. Thus, in a transient response to sediment supply changes, whether anthropogenic or natural, the GST extends upstream as a longitudinally patchy bed modulated by bedload sheet transport that favors the loss of gravel.

Downstream patterns of bed material grain size in a large, lowland alluvial river subject to low sediment supply

A new data set of bimodal subaqueous channel bed sediments was analyzed for longitudinal patterns in grain size. It yielded two interesting observations: (1) separate fining trends in d50 exist for gravel and fines that overlap for ∼175 river kilometers, and (2) this overlap in fining trends results in a protracted (nonabrupt) gravel to sand transition. These suggest bed patchiness that is interpreted in the context of inherent grain size bimodality, localized hydraulic sorting due to spatially heterogeneous (and low) sediment supply, and local deviations in channel gradient. The data also show that tributaries have a minor impact on main stem downstream fining due to basin shape and tectonic history. In addition, subaqueous channel sediments were observed to be finer than nearby sediments collected from exposed bars, which may indicate a narrow active zone of transport where bars are disconnected from sediment movement. This research has implications for bed material collection and theories of downstream fining.

Influence of bed material size heterogeneity on bedload transport uncertainty

The bed material grain size distribution of gravel bed streams is often spatially heterogeneous. The heterogeneity is actually a random parameter, even for a “well‐mixed mixture,” which potentially causes the transport rate for a given bed material to become an uncertain variable. The cause of bed material heterogeneity is the nonuniformity of the bed material, which is analyzed in this paper using examples from field observations and experimental data. The Monte Carlo simulation method is applied to study the uncertainty of the bedload transport rate using Wilcock's experimental data (P. R. Wilcock et al., 2001; P. R. Wilcock and J. C. Crowe, 2003). Each realization of the Monte Carlo simulation employed a randomly generated grain size distribution field for the entire simulation domain. With sufficient realizations the simulation results were adequate to show that the transport rates were a random variable and the mean transport rates did not fall on a single‐valued curve when the local heterogeneity was taken into account. The results indicate that the bedload transport rate of nonuniform sediment has an intrinsic uncertainty that can result solely from the bed material. The results also partially account for the scatter of the fractional transport rate within Wilcock's experimental data. This study presents an important concept in understanding the uncertainty associated with estimates of sediment transport.

The rate and pattern of bed incision and bank adjustment on the Colorado River in Glen Canyon downstream from Glen Canyon Dam, 1956-2000

Closure of Glen Canyon Dam in 1963 transformed the Colorado River by reducing the magnitude and duration of spring floods, increasing the magnitude of base flows, and trapping fine sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the first cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffles decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized. Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sand-bars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage-discharge relations. In the downstream part of the study area where riffles controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species. Models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average flows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2) the potential importance of exceptional flows resulting from management decisions.