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Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate, a series of flume experiments were performed where the flow was reduced rapidly from equilibrium conditions. Sediment transport rates and bed elevation data across the flume and over a 15-meter-long test section were collected during the experiments to allow detailed examination of evolving bedform dynamics. It was found that relict bedforms stopped moving completely after flow reductions, and the mode of sediment transport was shifted to small bedforms that arose rapidly over dune stoss sections throughout the test section. The changes in sediment transport with time as the sand bed adjusted to the new flow rate was found to agree with predictions based on the relations proposed in Wren et al. (2020). Wavelet analysis is used to visualize changes in length and amplitude scales during the bed transition process.

Experiments were conducted in a laboratory flume in which gravel and total bed-load rates were measured continuously using independent methods that allowed evaluation of the effect of four ...

Floating breakwaters are commonly used for shoreline protection in coastal areas and may offer a relatively cost-effective method for embankment protection in irrigation reservoirs. This s...

Passive acoustic technology has potential as a surrogate method for monitoring coarse bedload transport in rivers and streams. This has been demonstrated by both laboratory experiments and field studies where it was found that acoustic energy generated by particle collisions was proportional to the amount of bedload in transport. One factor that has inhibited further development of the technique is that comparison of acoustic data is complicated by the wide variety of hardware used. A second factor is the need for each research group to assemble hardware, which is often expensive and designed for laboratory use. In addition, the hardware used in the field is rarely calibrated. In order to demonstrate how an entire acoustic data acquisition system should be calibrated for coarse bedload data collection, this manuscript presents the assembly and calibration of an inexpensive acoustic system comprised of readily available equipment. The following are presented here: assembly of the system, key operating parameters and suggested specifications, system calibration, and experiences from multiple deployments of the example system.

Recent sedimentation rates are useful for quantifying how changes in a watershed affect soil erosion; however, typical geochronological methods for dating sediments are limited in temporal resolution, particularly for newly deposited sediments. We used sediment traps to measure short-term sediment accumulation rates in a natural oxbow lake whose watershed has a mix of agricultural and forested land. Precipitation data from a local Soil Climate Analysis Network (SCAN) site and water quality measurements from Beasley Lake were used to explain intra-annual patterns in sediment deposition. We found that sediment accumulation in the traps was highest in July and August, while the highest rate of runoff, indicated by increased precipitation and total suspended solids, occurred in late winter and early spring (February-April). Our results indicate that the trapping efficiency of Beasley Lake may be lower than expected due to timing of particle settling predominantly in the summer months. The rate and timing of sediment accumulation in Beasley lake was highly seasonal and occurred when water temperature and pH increased, potentially through interactions with algal blooms. Our findings contribute new understanding of the interactions between suspended sediments, algal biomass, and water chemistry in a natural oxbow lake and provide support for using sediment traps to measure intra-annual variability in sedimentation rates in oxbow lakes.

Irrigation reservoirs are widely used in parts of the Mississippi River alluvial floodplain to supplement limited groundwater resources. Constructed using local soils that are often low in ...

It can be difficult to predict sediment transport in sand-bedded ephemeral streams because of rapidly changing conditions that may not be sustained long enough to reach equilibrium. This si...

The structure of a mobile bed in a laboratory channel composed of sand and gravel (D50=8 mm) was characterized over a series of experiments with steady flows from a low flow, where only th...

Ephemeral streams with mobile beds represent a challenging environment for sediment transport prediction. Bed topography from relatively infrequent high flow events may persist after hydrog...

Short-term studies of wetlands show that wetlands scavenge pollutants, thereby mitigating the pollution of downstream water bodies. However, there are few long-term studies that demonstrate this effect. This study provides a wealth of data demonstrating that there is variable long-term mobility of metals and arsenic in wetland-lake systems. More than 900 samples were analyzed from 14 soil and sediment cores collected from six wetland-lake systems in the same floodplain. Cores were analyzed for a suite of trace elements (As, Co, Cu, Ni, Pb, Zn are reported in this paper), organic matter content, clay content, and water content. Sediment age was estimated using 210Pb and 137Cs data, facilitating comparison of element concentrations in contemporaneous wetland and lake sediments. In all six systems, sediment-laden water passes through the wetland before settling into the lake. In three of the systems, element concentrations were consistently higher in lake sediments, indicating a history of remobilization of wetland elements and subsequent deposition in lake sediments. Results in the other three lakes were variable, documenting unique long-term histories of element mobility and sequestration in otherwise similar systems.

On-farm irrigation reservoirs, together with tailwater recovery systems, are commonly used to store water in the Mississippi River Valley alluvial floodplain to supplement limited groundwater resources. The inner slopes of the earthen embankments that enclose these reservoirs are subjected to continuous erosion due to wind-generated waves. Various methods have been used to protect the embankments, but none of them have been shown to be both sustainable and cost-effective. In an effort to assess levee protection strategies, a range of treatment techniques, including levee configurations and surface treatments, were applied on the embankments of the University of Arkansas Pine Bluff (UAPB) Lonoke Demonstration Reservoir in 2007. The treatment techniques included 17 different slope configurations along the east and west embankments and five chemical and mechanical treatments along the north and south embankments. Embankment erosion was measured by comparing new surveys conducted in 2015 and 2017 with as-built information provided by the USDA Natural Resources Conservation Service (NRCS). The change in levee profile between the surveys and the as-built profiles was used to quantify the effectiveness of the treatment methods for protection against wave action. An artificial neural network (ANN) prediction model was developed to establish relationships between erosion parameters (eroded area and maximum retreat) and four independent variables (existence of berm, face slope, maximum effective fetch, and probability of wind speed). Data from 27 sections were used for training, and data from seven sections were used for testing. Almost all of the slope treatments were significantly eroded over the eight to ten years that elapsed between construction and the surveys. Although having a berm did not significantly reduce the total eroded volume, in most cases it delayed the bank retreat. For any two identical slope configurations, the one with the longest fetch typically had at least slightly higher erosion and retreat. In general, milder slopes performed better than steeper slopes. The survey results showed that geo-synthetic fabric, fly ash, and soil cement were more effective at reducing wave erosion than the other treatment methods. Keywords: Embankment erosion, Irrigation reservoir, Levee erosion, Wave erosion.

The use of surface water resources for irrigation in the state of Arkansas increased over the years following 2000 due to groundwater depletion. In order to reduce dependence on groundwater, irrigation reservoirs and tailwater recovery systems are used to capture and store water for irrigation. Irrigation reservoir levees are typically constructed from local soils that are often low in cohesion, and the use of rock armoring and geotextiles is limited due to high costs, resulting in levees that are susceptible to erosion. Wind-driven waves erode the exposed inner slopes of the levees, resulting in the need for frequent repairs, an added expense for agricultural producers. Evaluation of the status of irrigation storage reservoirs and identification of design factors that contribute to erosion are needed to address the problem. A survey was conducted in 2013 to 2015 to assess the current state of levee erosion and attempt to identify associated factors. It was found that 79% of the 584 homogeneous levee segments contained within 148 surveyed reservoirs had block failures and near-vertical slopes. Despite regional winds with preferential southerly and northerly directions, levees of all orientations were damaged by waves. For the surveyed irrigation reservoir levees, soil type, vegetation, inner slope, and berm presence were found to be poor predictors of the state of impairment. The most important variable associated with levee failure was maximum effective fetch length, such that longer fetches were more likely to have block failures and greater loss of top width.

Quantifying the effectiveness of erosion control practices in watersheds remains a difficult problem. Determination of recent sediment accumulation rates for lake sediments in agricultural watersheds using radioisotopes, such as lead-210 (210Pb) and cesium-137 (137Cs), is potentially a valuable means of assessing the effectiveness of soil conservation practices at the watershed scale. The predominance of sediment arriving in runoff from the watershed, variable sedimentation rates, and mechanical mixing of soil in nearby fields all present challenges in the conversion of radioisotope data to sedimentation rates. Four sediment cores from Beasley Lake, Mississippi, were used to demonstrate the application of the Constant Initial Concentration (CIC) model for calculating sediment age from the distribution of 210Pb bottom sediments. The activity of 137Cs was used to supplement the 210Pb data by providing a benchmark date within the core to calibrate the CIC model. Three of the four cores showed reductions in sediment accumulation rate within the 30 years prior to core collection in 2008 and 2011 by at least 50% relative to rates from before the adoption of soil conservation measures in the watershed. The most recently resolved rates were approximately 0.5 cm y−1 (0.2 in yr−1). The study demonstrates the application of the CIC model for developing sediment accumulation chronologies in agricultural catchments.

In river hydraulics, there is a continuing need for characterizing bed elevations to arrive at quantitative roughness measures that can be used in flow depth calculations and for improved p...

This study was conducted to improve prediction of the depth of erosion of sand (D50 = 0.3–0.9 mm) from immobile gravel (D50 = 36.1 mm) under steady uniform flows with bed shear stresses from 0.1 to 0.9 of that required to entrain the gravel. This situation, often encountered downstream of dams, has important implications for habitat restoration. Steady uniform flows were imposed on a flume channel containing a mixture of sand and gravel until sediment concentrations in the flow exiting the channel became small. The elevation of sand relative to gravel was measured after each experiment and compared poorly to calculated depths from published relationships whose predictions were based in part on the equivalent grain roughness of the bed. An improved predictive relationship was developed by using the cumulative distribution function of the surface gravel elevations to scale the shear velocity available for transporting sand from the gravel substrate.

Laboratory experiments were carried out in a wave tank to investigate wave induced beach profile evolution and scarp recession on cohesive and non-cohesive model embankments. The cohesive embankment was constructed using a natural soil mixture, and the noncohesive embankment was constructed using cylindrical shaped polyethylene terephthalate (PET) pellets of uniform size. A standardized packing procedure was used for both cohesive and non-cohesive embankments. The onshore face slope of the embankment was approximately 30 degrees. The morphological evolution of the embankment was monitored using water level sensors and a time-lapse camera. Waves quickly eroded the bank face near the free surface and formed a scarp. While an equilibrium profile was established relatively quickly and at a much steeper slope for the non-cohesive embankment, the cohesive embankment continued to erode for a much longer period. The scarp of the cohesive embankment retreated by a series of wave undercutting and slump failures. An empirical relationship between incident wave height and embankment erosion was established for the tested conditions. The measured beach profiles were compared with the developed analytical solution.

Continuous monitoring of gravel transport in rivers is necessary for understanding the effect of dam removal on river systems. Impact plates, such as those deployed on the Elwha River in th...

Successful sediment management of streams and rivers downstream of dams requires a method for predicting sand transport across a range of conditions that can include sand transported below ...

The use of ultrasonic acoustic technology to measure the concentration of fine suspended sediments has the potential to greatly increase the temporal and spatial resolution of sediment measurements while reducing the need for personnel to be present at gauging stations during storm events. The conversion of high-frequency attenuation and backscatter amplitudes to suspended silt and clay concentration has received relatively little attention in the literature. In order to improve the state of knowledge, a laboratory investigation was undertaken by the National Center for Physical Acoustics in cooperation with the USDA-ARS National Sedimentation Laboratory. In these experiments, two immersion transducers were used to measure attenuation and backscatter from 20 MHz acoustic signals propagated through suspended clay (smectite and kaolinite) and silt particles. The resulting data includes attenuation values for a wide range of concentrations (0.3–14 g/L) and particle sizes (0.03–14 μm diameter). Attenuation curves for each particle were compared to the theoretical attenuation curves developed by Urick (1948) and Sheng and Hay (1988) for scattering as presented by Landers (2010) [5,11,12]. In addition, it was found that the backscatter signal could be used to discriminate between suspensions dominated by clay or silt.

Bed load transport rates are difficult to predict in channels with bed material composed of sand and gravel mixtures. The transport of bed load was measured on Goodwin Creek, and in a laboratory flume channel with a similar bed material size distribution. The range of bed load transport rates measured in the laboratory channel were similar to those measured in the channel of Goodwin Creek; however, the shear stresses calculated on Goodwin Creek were three times greater than in the laboratory channel for similar bed load transport rates. Much of this difference in shear stress was removed by applying the drag partitioning technique of Einstein, although significant differences between the two sets of data remain. Predictions of bed load transport rates using three previously published transport relations were good for most flows for the laboratory data. For the Goodwin Creek bed load data, predicted transport rates were close to measured ones for low flows but diverged by an order of magnitude or more for high shear stresses. Improved methods of shear stress partitioning are needed to improve the performance of bed load transport relations on streams of this type.

Mobile-bed laboratory measurements allow observation of interactions between turbulent sediment-laden flows and irregular, non-stationary bed topography. Water in mobile-bed experiments is typically clouded by suspended particles, complicating the use of optical and laser based measurement techniques. In such conditions, acoustic techniques can be used to measure turbulence, suspended sediment transport, and bed topography; however, these procedures also have unique challenges. Here, the effect of suspended particles on acoustic Doppler turbulence measurement, the effect of particle size on acoustic measurement of suspended-sediments, and a method for statistically describing bed topography (measured acoustically or otherwise) are addressed. It is found that suspended particles have a small effect on Reynolds stresses measured with an acoustic Doppler velocimeter and that those effects could be minimized by excluding data collected with the highest and lowest acoustic amplitudes. The effect of particle size on acoustic backscatter amplitude is most strongly pronounced for particles smaller than 0.3 mm. Semivariogram analysis is shown to be a viable method for analyzing complex bed topography produced by mobile-bed experiments.

Experiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges at approximately the same flow depth of 0.2 m for 11 different elevations of sand in the gravel bed. Sand transport was measured using both physical samples and a density cell. For a given flow rate, increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32–44% and increases in sand transport by three orders of magnitude. For the highest two discharges, the sand merged into a small number of long and low bed forms that translated through and over the gravel bed. A collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated...

At base-flow conditions, ephemeral streams with mobile beds may inherit bed topography caused by previous higher flow events, resulting in bed topography that is not in equilibrium with flow conditions. Major flow events can generate large bedforms that result in hydrodynamics and sediment transport that are not in equilibrium with subsequent lower flow conditions. To simulate a large storm runoff–driven stream flow followed by lower base-flow conditions, changes in sand bed topography and sand transport caused by a simultaneous reduction in discharge and flow depth in a laboratory flume with an equilibrium sand bed and steady, approximately uniform, flow were investigated. Bed topography was evaluated using second-order structure functions, semivariograms, and statistical analyses. Sediment transport responded rapidly to a near-instantaneous negative step in mean flow depth and discharge. The relative contributions of reduced bedform height and celerity to the decreased transport rate are quantif...

Bed load is part of the total sediment load of a stream, comprising the coarsest fraction of mobile material that is frequently in contact with the stream bed. Sediment conveyed downstream may fill reservoirs and channels, impeding navigation, increasing the likelihood of flooding, and degrading water quality and aquatic habitats. Local erosion and deposition of bed material may also cause instability of channel banks. Any long-term program of channel stabilization or improvement must account for bed-load transport in order to avoid channel aggradation or degradation. The determination of bed load rates has been concentrated in four general methods (Hubbell 1964): (1) direct measurement by collecting physical samples, (2) using bed-load transport relations, (3) measuring the erosion or deposition of bed material sediment in a confined area, or (4) measuring bed-load transport indirectly using surrogate technologies. None of these techniques is suitable for a wide range of uses. No single bed-load transport relation has been shown to have general applicability (e.g., Gomez and Church 1989; Vanoni 2006, p. 221–222; Scheer et al. 2002). Many streams do not have a convenient area to carry out erosion or deposition measurements. Accurate determination of bed load using direct measurement methods is difficult because the rate and size of sediment in transport varies dramatically in time at a point and spatially at a given time over a channel cross section of a channel even when the flow is steady (Ehrenberger 1932; Hubbell et al. 1985; Whiting et al. 1988; Kuhnle and Southard 1988; Kuhnle et al. 1989; Gray et al. 1991; Bunte and Abt 2005). Matters are further complicated by unsteady flows. Designing a device that will sample with an equal efficiency over widely varying transport rates is itself a significant challenge. An additional challenge is to collect enough samples at a point and in a cross section to adequately define the mean rate of bed-load transport for a given flow strength. Recent improvements in technology have allowed the development of devices that do not collect physical samples of the sediment but collect indirect or surrogate information that is directly related to the transport of bed-load sediment. These surrogate methods have been divided into active and passive types by Gray et al. (2010), depending on whether the sensors emit signals and record properties of the reflected sound or passively record naturally generated sound emissions. Examples of active sensors consist of acoustic Doppler current profilers that track motion of the bed sediment (Rennie et al. 2002) and active tracking of tagged sediment particles (Emmett et al. 1996; Nichols 2004). Examples of passive sensors include impact pipes (Papanicolaou et al. 2009; Mizuyama et al. 2010), impact plates (Krein et al. 2008; Moen et al. 2010; Rickenmann et al. 2014), geophones (Tsakiris et al. 2014), and devices capable of sensing sediment-generated noise (SGN) associated with the transport of gravel and coarser sediment (Thorne 1986a; Barton et al. 2010). The following paragraphs will focus on passive acoustic techniques for measuring SGN using hydrophones deployed in the transporting flow. Bed load made up of gravel (diameter >2 mm) and larger particles has been shown in previous studies to generate sound through particle collisions and bed collisions, the amplitude of which is related to the particle transport rate (e.g., Thorne 1985, 1986a; Barton 2006). Passive acoustic instrumentation has several characteristics that make it attractive for use in measuring bed load. It is well suited for remote deployment because the hardware needed to sense and record sound is relatively inexpensive. Passive acoustic methods, which record sound that is generated at some distance from a hydrophone, is nonintrusive so the phenomenon being measured is minimally affected by the measurement process. Passive acoustic instrumentation integrates sound from a finite area, decreasing bias caused by spatial heterogeneity of bed-load transport. Continuous monitoring of bed-load transport in a minimally invasive manner is readily accomplished during infrequent high-magnitude events when coarse gravel and cobble sediments are in motion. These events are infrequent but have been shown to be responsible for transporting the majority of bed-load sediment in some streams (Kuhnle 1992). Challenges of using passive acoustics to measure bed load include the generally unknown size of the measurement volume and the acoustics of fluvial environments. This paper reviews the current state of knowledge supporting the use of passive acoustic technology for bed load monitoring using hydrophones deployed in the transporting flow to directly record interparticle collisions. It is shown that while prior SGN work has addressed the generation and characteristics of SGN signals and qualitative/empirical approaches to quantifying bed load using SGN in field settings, a lack of work to date on the propagation of SGN signals in the riverine environment limits SGN as a broadly applicable methodology. In the following section, the concept of using SGN to measure bed load is introduced, along with the development of the method to date. The acoustic characteristics of the riverine setting are then presented, including propagation, transmission losses, and boundary effects. Since sediment transport may not be the most dominant source of underwater sound in a river, the

Successful sediment management of streams and rivers downstream of dams requires a method for predicting sand transport across a range of conditions that can include sand transported below ...

Cleaning of fine sediment out of gravel streambeds has become an important method to restore affected stream habitats. Introducing the increased flows needed to entrain fine sediments without eroding the coarser fractions of the bed and potentially destroying its usefulness as a habitat requires careful selection of flow strength. Toward this end, a series of flume experiments was conducted on the entrainment of sand from immobile gravel beds to determine the depth to which the sand could be eroded without the gravel being mobilized. A series of steady flows was imposed on three gravel beds filled with sand with median sizes of 0.2, 0.3, and 0.9 mm. The shear stress of the series of flows ranged from just above the critical shear stress for initiation of motion of the sand up to 0.9 times the critical stress of the 36.1-mm gravel. The experiments continued until the rate of fine sediment exiting the channel was very small. It was found that the cleanout depth of the sand was reasonably predicted b...

The transport of bed material and fluid turbulence are affected by many factors, including the fine sediment load being carried in a channel. Current research has focused on sand-sized particles introduced to gravel beds, while the effect of silt load on sand transport has received less attention. Experiments on the effects of silt load, in concentrations 0-26,900 mg l −1 , on sand transport were performed with a recirculating laboratory flume using three different sand bed configurations: ripples ( Fr =0.24), dunes ( Fr =0.34), and dunes ( Fr =0.48). Three Acoustic Doppler Velocimeters were arranged to measure flow and turbulence quantities simultaneously in one vertical. Sand transport did not change in a consistent manner with increasing silt load, increasing up to 4,000 mg l −1 for dunes ( Fr =0.48) and up to 2,000 mg l −1 for dunes ( Fr =0.34) and then declining to near the clear water case with increasing silt concentrations. Silt addition for the ripple case caused a relatively small change in sand transport, decreasing with added silt up to approximately 2,000 mg l −1 and then increasing as silt went up to approximately 10,000 mg l −1 . Dunes ( Fr =0.48) decreased in length and height as silt increased, while dunes ( Fr =0.34) did not show a consistent trend. A clear trend of decreasing Reynolds stress with increasing silt concentration was observed in the ripple case, with a 33% reduction in near-bed Reynolds stress caused by an 8,900 mg l −1 concentration of silt.

Floating breakwaters are typically used to protect small marinas or for shoreline erosion control on limited-fetch water bodies in which wavelengths are relatively short, such as lakes, reservoirs, and bays. In spite of the large amount of literature on floating breakwaters, there is still a lack of knowledge on the performance of cylindrical breakwaters, which, as a result of their simplicity, can minimize construction and material costs. In this paper, we describe a laboratory investigation of the hydrodynamic interaction of cylindrical breakwaters with monochromatic waves in deep and transitional water depths such as are found in small reservoirs. The tested breakwater models feature single- and multiple-cylinder cross sections combined with bottom-moored, arm-restrained (connected to shoreline), and pile-restrained configurations. When the breakwater models were fixed, the reflection was higher than in the partially restrained models, and the efficiency was strongly dependent on draft ratio (zd/d, whe...

The effectiveness of erosion control methods is difficult to measure, hampering the development of management practices and preventing accurate assessment of the value of erosion control structures over time. Surface erosion can vary widely over an area, particularly if gully erosion is present, and the use of sediments transported in streams for quantifying erosion is hindered by the highly variable nature of fluvial sediment loads. When a watershed drains into a lake, accumulated sediments have the potential to yield information about historic rates of sedimentation that can be used to evaluate the effectiveness of previous erosion control measures. In the present study, sediments from five natural oxbow cutoff lakes in the Mississippi River alluvial floodplain were dated using lead-210 decay rates and bomb-pulse derived cesium-137 with the goal of relating trends in sedimentation rate to reductions in erosion due to management practices. It was found that the radioisotope dating methods were best used in concert with known dates for implementation of management practices. Changes in sedimentation rate over time frames as short as 12 years were detectable. Larger lakes showed smaller changes in sedimentation rate relative to smaller lakes.

The earthen levees commonly used to form irrigation reservoirs are subjected to significant embankment erosion due to wind-generated waves. The design of bank protection measures relies on adequate prediction of wave characteristics based on wind conditions and fetch length. Current formulations are based primarily on winds and waves in large water bodies and do not provide an optimal fit to waves in small water bodies such as irrigation ponds. Based on wind and wave data collected in an irrigation reservoir near Carlisle, Arkansas, the coefficients in a commonly used equation for wind wave prediction were improved for use in irrigation reservoirs. Details of the development of the new coefficients as well as data collection procedures are presented here. With the new empirical coefficients, the RMS error is 0.01 m for energy-based significant wave height and 0.06 s for peak wave period.

The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency (recurrence frequency) at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation (σ) of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L

In the absence of identifiable macrofossils in lacustrine sediments, radiocarbon dating must rely on pollen or bulk sediment fractions. Bulk sediment fractions are not generally preferred because they contain an unknown mixture of organic material of variable age, they may contain dead carbon such as lignite that is difficult to eliminate, and material of aquatic origin may be subject to reservoir effects. If the various processes that contribute carbon to the system are relatively constant over time, however, changes in 14C activity with depth may be used to accurately estimate sediment accumulation rates even if the absolute ages are erroneous. In this study, fine-grained fractions (250–710 μm organic material, humic acids extracted from 14C activities of the bulk fractions were highly linear with depth and produced consistent calculated sediment accumulation rates similar to, and perhaps more reliable than, rates determined using twigs or charcoal.

A series of laboratory flume experiments were conducted to characterize lateral variations of suspended sediment over dunes. Forty experimental runs were made using flow depths of 0.328 and 0.127 m in a 1.2-m-wide channel. The Froude number was 0.47 and the median diameter of the bed material was 0.52 mm. Point samples of suspended sediment and depth-integrated values calculated using acoustic backscatter data were collected simultaneously at two lateral positions at spacings of 0.41, 0.20, 0.10, and 0.05 m. Root mean square differences between the paired samples increased from 30 to 94% and from 20 to 45% of the mean concentration for point samples and depth-integrated samples, respectively, as the lateral spacing between samples was increased. Lateral variations in the concentration of suspended sediment should be considered when designing sediment sampling strategies over dune beds.

Many sampling projects have been initiated or modified in recent years to quantify the effects of water quality protection and enhancement programs. Although comprehensive references on the theory and procedures related to discharge data collection have been published, similar guides to water quality sampling are not available. Several sources provide general guidance on sampling project design and on manual sampling procedures, but only recently has detailed information on automated storm water quality sampling been developed. As a result, a compilation of available information on the design of water quality sampling projects is needed to support sound decision-making regarding data collection resources and procedural alternatives. Thus, the objective of this article is to compile and present practical guidance for collection of discharge and water quality constituent data at the field and small watershed scale. The guidelines included are meant to increase the likelihood of project success, specifically accurate characterization of water quality within project resource constraints. Although many considerations are involved in establishing a successful sampling project, the following recommendations are generally applicable to field and small watershed studies: (1) consider wet-weather access, travel time, equipment costs, and sample collection method in the selection of sampling site numbers and locations; (2) commit adequate resources for equipment maintenance and repair; (3) assemble a well-trained, on-call field staff able to make frequent site visits; (4) establish reliable stage-discharge relationships for accurate discharge measurement; (5) use periodic manual grab sample collection with adequate frequency to characterize baseflow water quality; (6) use flow-interval or time-interval storm sampling with adequate frequency to characterize storm water quality; and (7) use composite sampling to manage sample numbers without substantial increases in uncertainty.

Lateral and longitudinal variability in suspended-sediment concentration was investigated over dune and upper-regime plane beds under steady, uniform flow in a laboratory flume. A much greater sampling time was required to reliably predict mean concentration in a dune bed than for an upper-regime plane bed. Laterally separated, simultaneous samples over both dune and upper-regime plane beds were poorly correlated.

Understanding the interactions between sediment transport and turbulence in a supercritical flow is useful in the study of river hydraulics and fluvial systems and the modeling of such flows in nature. Toward this end, 11 profiles of suspended-sediment concentration and fluid velocity were collected in supercritical conditions over low-relief antidunes in a recirculating laboratory flume. It was found that velocity profiles agreed well with the law of the wall, and that turbulence intensities were similar to those in clear-water flows. The classic Rouse equation was found to under-predict concentration values in the upper 60-80% of the flow depth and a slightly modified version was proposed and successfully tested against experimental data from the present study and others.

Wetland vegetation can attenuate storm surge and waves and thus play an important role in coastal protection. In this study, laboratory experiments were used to quantify wave attenuation as a function of vegetation type, density, and height, as well as wave conditions. Wave attenuation characteristics were investigated under regular and irregular waves for rigid and flexible model vegetation in addition to live Spartina alterniflora and Juncus roemerianus, two common coastal species. Vegetation densities were 156–623 stems/m2 for model vegetation, 405–545 stems/m2 for S. alterniflora, and 2,857 stems/m2 for J. roemerianus. Bulk drag coefficients (CD) of the vegetation species were calibrated based on wave gauge data and video images, and regression equations were derived for the drag coefficient as a function of Keulegan-Carpenter number (KC). CD did not depend significantly on the relative vegetation height with respect to water depth for rigid vegetation models in the emergent and submerged cond...

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent f...