Reducing Sediment Transport Research Articles

Afforestation of degraded grasslands reduces sediment transport and may contribute to streamflow regulation in small catchments in the short-run

Land use change in the Southern Grasslands biome with the introduction of exotic, fast-growing forest species is a controversial topic, because of the potential effect on water and soil resources. The aim of this study was to assess the effects of afforesting degraded grassland on streamflow and sedimentation in subtropical headwater catchments. Two small, headwater catchments were studied, one planted with Eucalyptus saligna (EC: 0.83 km2) and another with grasslands and extensive livestock (GC: 1.10 km2). Rainfall, runoff, and sediment discharge were monitored from September 2013 to March 2017. The results show two-fold greater surface runoff and sediment yield occurred in GC than in EC. Maximum and mean runoff coefficients were, respectively, 45.5 and 10.2% in GC, and 12.4 and 2.2% in EC; suspended sediment yield was, respectively, 67.9 and 22.4 Mg km−2 in GC and EC; and bed load sediment yield was only 0.053 and 0.006 Mg km−2. El Niño year with high rainfall produced the greatest annual sediment yield, with values 4.2 times greater in GC (167.7 Mg km−2) than in EC (39.9 Mg km−2). Hydrographs/sedimentographs synchronization and hysteresis patterns suggest faster sediment delivery in GC than in EC, but further studies are needed to conciliate sediment sources results with sediment fingerprinting. Staggered forest harvest (21% of the planted eucalyptus) led to sediment yield comparable to the observed during the pre-harvest period, and lower than in grassland catchment. In conclusion, well‐managed forest plantations, including staggered forest harvesting and preservation of riparian forest, are less prone to sedimentation than degraded grassland under intensive grazing. Well-managed afforestation of degraded lands contributes to soil conservation in the studied region, whereas low ecological performance of degraded grassland calls for soil and pasture management practices to increase forage offer to grazing animals, while conserving soil and water resources in the catchments.

Further Development of Small Hydropower Facilities Will Significantly Reduce Sediment Transport to the Pantanal Wetland of Brazil

Small hydropower (SHP) facilities, are commonly being built around the world. SHPs are viewed as less environmentally harmful than larger dams, although there has been little research to support that assertion. Numerous SHPs have been built, and many more are in development or proposed, in rivers that drain into the Pantanal, a world-renowned floodplain wetland. Three river systems with the largest contributions of sediments to the Pantanal—the Cuiaba, upper Taquari, and Coxim rivers—remain largely undammed. The upland tributaries transport sediments and nutrients into the Pantanal, thereby affecting geomorphological dynamics and biological productivity of downstream floodplains. This study presents measurements from upstream and downstream of current hydropower facilities, most of which are SHPs, throughout the upland watersheds of the Upper Paraguay River basin to reveal how these facilities may affect the transport of suspended sediments and of bedload sediments. In addition, a predictive model using artificial neural networks estimates the impact of building 80 future SHPs on sediment transport based on observations at current facilities as well as the spatial distribution of future facilities. More than half of current facilities retained suspended sediments: 14 of the 29 facilities showed >20% net retention of suspended sediments, two others retained between 10-20%, seven were within 10%, and six showed >10% net release. Bedload sediment transport was a small component of total sediment transport in rivers with high total sediment loads. Multiyear series of satellite images confirm sediment accumulation in several cases. Model predictions of the impacts of future hydropower facilities on suspended sediment concentrations and transport show retention of a large fraction (often much >20%) of sediment inputs. Summing riverine transport rates for inflows into the Pantanal indicates that currently envisioned future hydropower development would reduce the suspended sediment transport by ~62% from the current rate. This study shows that if SHPs are built on sediment-rich rivers, this may prove problematic for the facilities as well as for downstream ecosystems. These results support recommendations that several river systems presently lacking dams in their lower reaches should be excluded from future hydropower development to maintain the sediment supply to the Pantanal.

The effectiveness of a multi-row Tamarix windbreak in reducing aeolian erosion and sediment flux, Niatak area, Iran

Windbreaks are barriers that are widely used to reduce wind speed and aeolian erosion. Most windbreak studies have been done in wind tunnels and generally used rigid objects rather than live plants and most of these studies report on the modification of the flow field and not on the effectiveness at reducing sediment transport. A series of experiments were conducted to monitor the effectiveness of a fourteen-rows Tamarix windbreak in the field and in particular to measure the reduction in sediment erosion and transport. Over the course of six dust storm events, with mean wind speed ranging from 9.4 to 18.2 ms−1, sediment flux and wind speed were measured at seven heights (z/h = 0.05, 0.1, 0.2, 0.4, 0.8, 1.4 and 1.8 where h is the mean height of the windbreak) at five locations upwind (x=−100 m), within (x = 100 and 256 m) and downwind (x = 448 and 560 m) of the windbreak. Largest reduction of sediment flux (a reduction of 50%) was observed at the end of the windbreak (x = 256 m). The decrease in silt and clay component is quite uniform but the relative concentration of sand (>100 μ) generally decreases within the windbreak and remained lower downwind of the windbreak compared to the upwind value in two of the three events for which sediment size data are available. The windbreak is effective in aeolian erosion control under a variety of dusty conditions and, in general, the potential of the windbreak is optimum due to its optimal porosity (39%) and structure (multiple-row design). Because Tamarix withstand harsh conditions, it is optimal to use it as a windbreak for wind and blown-sediment control in arid regions.

The Effectiveness of Adaptive Beach Protection Methods under Wind Application

A physical model test was carried out to evaluate a measure of reducing sediment transport in a condition of erosive wave incidence. The erosion trend was analyzed in a beach profile consisting of 0.1 mm sand, and a scenario in which 1 mm and 5 mm materials were applied to the erosion section was conducted. The effects of beach nourishment profiles with different sand diameters were verified by comparing the results when the submerged breakwater was installed. In addition, because high waves are usually accompanied with strong wind, to determine the wind effect, morphological change was examined under waves only and the coexistence of waves and wind together. The experimental results showed that sediment transport around the shoreline decreased in a condition of nourishment with 1 mm grains, and the total amount of morphological change was similar to the case in which the submerged breakwater was installed. The results illustrated that a change in wind velocity increased the wave energy density, as well as the range of morphological change.

Chute cutoff-driven abandonment and sedimentation of meander bends along a fine-grained, non-vegetated, ephemeral river on the Bolivian Altiplano

The origin and development of meandering river planforms has long been a focus of research in the geosciences. Most attention has focused on perennial meandering rivers with well developed riparian vegetation assemblages, and while increasing interest is turning towards the dynamics and sedimentology of ephemeral meandering rivers with sparse to no vegetation, additional field data are required. As a contribution, this study presents a remote-sensing and field-based analysis of chute cutoff-driven abandonment and sedimentation of meander bends along the fine-grained, non-vegetated, ephemeral Río Colorado on the Bolivia Altiplano. Along the 25km long sinuous study reach, located between 25 and 50km upstream of the present-day margins of Salar de Uyuni, quasi-regular flood events (typically at least one per year) drive bend cutoffs and wider channel-floodplain dynamics, despite low specific stream power (<10W/m2) and cohesive (dominantly silt and clay) bed and bank sediments. Along the reach, twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. Three chute cutoffs (CC1, CC2, CC3) occurred between 1996 and 2016; for one bend (CC3), high-resolution (<0.65m) satellite imagery and field investigations reveal the details of pre-, mid- and post-chute cutoff processes and sedimentary products. Together, the findings suggest that for any given bend, an increase in bend amplitude (mean ratio of meander bend length to chute channel length ∼4) combines with a typically high diversion angle between the channel-belt axis and the upstream limb of the meander bend (mean ∼98°) to result in declining flow efficiencies. This enhances overbank flooding and promotes deposition along the upstream limb of the bend. Overbank flooding promotes the development of chute channels, which commonly initiate as shallow (typically <1m) headward eroding channels, while sediment derived from overbank flow and headward erosion tends to be deposited in the downstream limb of the bend. By obstructing flow, such deposition reduces sediment transport capacity within the meander bend as a whole, thereby further inducing deposition within the upstream limb. During subsequent floods, deepening and widening of a dominant chute channel and sedimentation and shallowing of the meander bend continues. Along the reach, episodic La Niña-driven flood events drive phases of more rapid bend migration and clusters of chute cutoffs. Our findings contribute to a more comprehensive understanding of meandering river dynamics across the full range of Earth’s conditions, and also may help to improve interpretations of Earth’s pre-vegetation rivers and meandering fluvial forms on other planetary bodies.

Frequency analyses of peak discharge and suspended sediment concentration in the United States

Compound extreme events with concurrent peak discharges and suspended sediment concentrations have substantial impacts such as floods, infrastructure failure, pollution, increased water treatment costs, and threats to aquatic life. It is thus essential to understand the spatiotemporal dynamics of water and sediment transport during such extreme flood events. Copulas are used to conduct multivariate flood frequency analyses and provide relationships between design variables and recurrence intervals that are useful in hydrology and water resource management. We used hydrological data from 20 gauging stations on important rivers across the United States with daily records spanning at least the last 30 years. We extracted annual maximum discharges (Qmax) and corresponding suspended sediment concentrations (SSCmax) at these 20 stations. We analyzed the temporal trends of Qmax and SSCmax by using the Mann–Kendall non-parametric statistical test. Then, we investigated the interdependency of Qmax and SSCmax through various correlation coefficients and used the Multivariate Copula Analysis Toolbox (MvCAT) for flood frequency analyses. We found Qmax decreased significantly for one river, whereas SSCmax decreased significantly for 10 of the 20 rivers over the analysis period. Five of rivers selected as statistically suitable for copula analysis were of the smallest size and with the fewest dams. Seven of rivers showed significantly decreasing SSCmax trends were of the largest size and with the most dams. The remaining eight rivers were of moderate size. In larger watersheds, the number of dams more strongly affected sediment transport dynamics. Soil condition and forest cover exerted stronger influences on SSCmax in smaller than in medium-sized watersheds, and soil condition was better correlated with the spatial SSCmax distribution than forest cover. Using MvCAT, we computed the optimal continuous marginal distribution functions for the five suitable rivers, and the optimal copula was different in each case. Our results indicate reduced sediment transport during extreme flood events. Copula functions are very useful in such flood frequency analyses, and measures should be applied according to local conditions to achieve optimal water resource management and planning.

Evolution of Suspended Sediment Budget in the Deltas of Lake Baikal Tributaries

The monitoring, field, and satellite datasets are used to estimate changes in suspended sediment budget for the Selenga and Upper Angara river deltas. Sediment deficit during recent decades dominates delta evolution due to decrease in the sediment delivery to the delta since the middle of the 1970s by 51% for the Selenga River and by 70% for the Upper Angara River. The observed decline in the sediment runoff and peak water flows associated with the sediment deposition within the inundated flood-plain of the deltas leads to the reduction of the sediment aggradation rates over both deltas. Under low water flows we observed longitudinal increase of the sediment transport rates along Selenga River delta; the Upper Angara constantly reduces sediment transport rates under various hydrological conditions.

Sediments of Matsushima Bay, Northeastern Japan: Insights Gained From 5 Years of Sedimentological Analysis Following the 2011 Tohoku Earthquake‐Tsunami

AbstractMatsushima Bay was less affected by the 2011 Tohoku Earthquake Tsunami than other Pacific coastal areas of northeast Japan because of the sheltering effects of islands in the bay mouth. To understand the recovery from the 2011 tsunami of benthic environments in Matsushima Bay, we conducted surveys of the geochemical properties of surface sediments over 5 years following the earthquake and compared them to sedimentary data before the earthquake. Before the tsunami, the mud fraction (&lt;63‐μm size) proportion of the surface sediments varied over a wide range, whereas after the tsunami, the mud content range was relatively narrow. During 2012–2015, the mud content was linearly correlated with the total organic carbon (TOC) content, but the slope of the relationship differed from that before the tsunami. The tightly coupled mud‐TOC relationship and the almost constant C/N ratios in the surface sediments suggest that the bay sediments were resuspended and transported by the tsunami. In addition, loss of some organic matter sources in the bay may partly account for the uniform C/N ratios. By 2016, the slope of the mud‐TOC relationship was almost the same as the pretsunami value, but the C/N ratios remained constant. These results suggest that ecosystems and biogeochemical cycles in the bay had not yet fully recovered their pretsunami state. Trace element compositions of core samples indicated that sediment sources were little changed by the tsunami, probably because islands in the mouth of the bay reduced sediment transport into the bay from distant sources.

River temperature and the thermal-dynamic transport of sediment

Spatial and temporal variability of river temperature, and its impact on sediment transport, is explored. Global river temperatures, computed daily using 1980–2010 climate, reflect latitudinal and seasonal variations in solar radiation, wind patterns, humidity, and water sources, and can be cooler or warmer than air temperatures. Subpolar, Polar and High Plateau rivers are cold (TMEAN: <8 °C), and of intermediate variability (Tσ: 6° to 10 °C), are highly sensitive to temperature variations, which together favor a river's ability to transport sediment. Temperate, Subtropical and Alpine rivers are of intermediate temperatures (TMEAN: 5° to 26 °C) with a maximum in variability (Tσ: >8°); their transport responsiveness is dependent on their minimum temperature. Tropical rivers are warm (TMEAN: >25 °C), less sensitive to temperature swings, and possess a reduced sediment transport capacity; their narrow temperature variability (Tσ: 1° to 4 °C) favors finer-grained sediment transport. Fluid density of river water ranges by 3.5%, and more rarely increases by 170% and thereby increases sediment transport 2.8-fold. Kinematic viscosity has a global range of 72.5%, and works to reduce sediment transport as the river warms. A 25 °C increase in water temperature, due to some combination of river elevation descent, or flow to warmer latitudes, or seasonal increases, can increase grain settling velocities by 7.3%, 42%, 85%, 90% for a 1000-μm, 250-μm, 62.5-μm, or a 10-μm grain respectively, and reduce sediment transport by 90% for a grain size population centered at 62.5 μm, or 300% if centered at 10 μm. The common practice of using fluid density and temperature constants (ρf = 1000 kg/m3, T = 20 °C) in transport models, can greatly under-predict subpolar and alpine river transport and over-predict sediment transport in a tropical river. Temperature variations impact rivers that transport a preponderance of fine sediment, a characteristic of most large rivers. These trends in river temperature and sediment transport have important ecological and anthropogenic implications. The effects on sediment transport will be especially important to quantify under a warming climate and where human influences, such as urbanization, may change the riverine thermal regime.

Reduced sediment transport in the Chinese Loess Plateau due to climate change and human activities

The sediment load on the Chinese Loess Plateau has sharply decreased in recent years. However, the contribution of terrace construction and vegetation restoration projects to sediment discharge reduction remains uncertain. In this paper, eight catchments located in the Loess Plateau were chosen to explore the effects of different driving factors on sediment discharge changes during the period from the 1960s to 2012. Attribution approaches were applied to evaluate the effects of climate, terrace, and vegetation coverage changes on sediment discharge. The results showed that the annual sediment discharge decreased significantly in all catchments ranging from −0.007 to −0.039 Gt·yr−1. Sediment discharge in most tributaries has shown abrupt changes since 1996, and the total sediment discharge was reduced by 60.1% during 1997–2012. We determined that increasing vegetation coverage was the primary factor driving the reductions in sediment loads since 1996 and accounted for 47.7% of the total reduction. Climate variability and terrace construction accounted for 9.1% and 18.6% of sediment discharge reductions, respectively.

Bedform development and its effect on bed stabilization and sediment transport based on a flume experiment with non-uniform sediment

Non-uniform sediment deposited in a confined, steep mountain channel can alter the bed surface composition. This study evaluates the contribution of geometric and resistance parameters to bed stabilization and the reduction in sediment transport. Flume experiments were done under various hydraulic conditions with non-uniform bed material and no sediment supply from upstream. Results indicate that flume channels respond in a sequence of coarsening and with the formation of bedform-roughness features such as rapids, cascades, and steps. A bedform development coefficient is introduced and is shown to increase (i.e. vertical sinuosity develops) in response to increasing shear stress during the organization process. The bedform development coefficient also is positively correlated with the critical Shields number and Manning׳s roughness coefficient, suggesting the evolution of flow resistance with increasing bedform development. The sediment transport rate decreases with increasing bed shear stress and bedform development, further illustrating the effect of bed stabilization. An empirical sediment transport model for an equilibrium condition is proposed that uses the bedform development coefficient, relative particle submergence (i.e. the ratio of mean water depth and maximum sediment diameter), modified bed slope, and discharge. The model suggests bedform development can play a primary role in reducing sediment transport (increasing bed stabilization). The model is an extension of Lane׳s (1955) relation specifically adapted for mountain streams. These results explain the significance of bedform development in heightening flow resistance, stabilizing the bed, and reducing sediment transport in coarse, steep channels.

Hydrodynamics of steep streams with planar coarse‐grained beds: Turbulence, flow resistance, and implications for sediment transport

AbstractThe hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows, energetic subsurface flow, and macro‐scale form drag from immobile boulders and channel and bed forms. Heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro‐scale form drag; however, little work has explored steep river hydrodynamics in the absence of complex bed geometries. Here we present theory for the vertical structure of flow velocity in steep streams with planar, rough beds that couples surface and subsurface flow. We test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes (0.4–30%). Experimental flows have a nearly logarithmic velocity profile far above the bed; flow velocity decreases less than logarithmically toward the bed and is nonzero at the bed surface. Velocity profiles match theory derived using a hybrid eddy viscosity model, in which the mixing length is a function of height above the bed and bed roughness. Subsurface flow velocities are large (&gt;1 m/s) and follow a modified Darcy‐Brinkman‐Forchheimer relation that accounts for channel slope and shear from overlying surface flow. Near‐bed turbulent fluctuations decrease for shallow, rough flows and scale with the depth‐averaged flow velocity rather than bed shear velocity. Flow resistance for rough, planar beds closely matches observations in natural steep streams despite the lack of bed forms or channel forms in the experiments, suggesting that macro‐scale form drag is smaller than commonly assumed in stress‐partitioning models for sediment transport.

Small Reservoirs as a Beneficial Management Practice for Nitrogen Removal.

There are few beneficial management practices (BMPs) with demonstrated efficacy in snowmelt-dominated regions. Small reservoirs are a BMP that can help mitigate flooding and reduce sediment transport, while reducing export of dissolved nutrients. To understand controls on nitrate removal and assess how this ecosystem service can be optimized, denitrification activity was measured in reservoirs and stream pools of the Tobacco Creek Model Watershed (Manitoba, Canada) via the chloramphenicol-amended acetylene block technique. Denitrification activity was positively correlated with nitrate and sediment organic carbon (SOC), and negatively correlated with sediment particle size and pH. Reservoirs exhibited higher denitrification activity than stream pools and were associated with higher levels of SOC, higher nitrate in early summer, and lower concentrations of dissolved oxygen. Nitrate was added to a set of samples to test for nitrate saturation, an indicator of poor ecological status, where nitrate concentrations exceed the denitrification capacity of microbes. Forty-nine percent of measurements demonstrated nitrate saturation, indicative of the need for additional remediation activity. Findings from this research suggest this BMP has higher capacity for nitrogen removal than stream pools because of higher denitrification rates and a higher apparent threshold for nitrate saturation, coupled with increased residence times. Results also inform the construction of additional reservoirs, which have been identified as a priority BMP in this region. Siting reservoirs in areas where conditions contribute to buildup of fine sediments and planting riparian vegetation to foster high organic C availability may help optimize denitrification, although tradeoffs in terms of other ecosystem services must be considered.

Reduced sediment transport in the Yellow River due to anthropogenic changes

The erosion, transport and redeposition of sediments shape the Earth's surface, and affect the structure and function of ecosystems and society(1,2). The Yellow River was once the world's largest carrier of fluvial sediment, but its sediment load has decreased by approximately 90% over the past 60 years(3). The decline in sediment load is due to changes in water discharge and sediment concentration, which are both influenced by regional climate change and human activities. Here we use an attribution approach to analyse 60 years of runoff and sediment load observations from the traverse of the Yellow River over China's Loess Plateau - the source of nearly 90% of its sediment load. We find that landscape engineering, terracing and the construction of check dams and reservoirs were the primary factors driving reduction in sediment load from the 1970s to 1990s, but large-scale vegetation restoration projects have also reduced soil erosion from the 1990s onwards. We suggest that, as the ability of existing dams and reservoirs to trap sediments declines in the future, erosion rates on the Loess Plateau will increasingly control the Yellow River's sediment load.

Identification and management of critical erosion watersheds for improving reservoir life using hydrological modeling

Sustainable management of water resources requires identification and management of critical erosion areas for reducing the reservoir sedimentation. A process-based distributed model SWAT (Soil and Water Assessment Tool) was used to identify critical erosion watersheds in Damodar catchment and tested soil and water management strategy to reduce sediment transport to reservoirs for improving their useful life. The model was calibrated and validated using measured runoff and sediment yield from two watersheds and two reservoir inflows. The validated model was also tested for its appropriateness by comparing the identified critical erosion area of the catchment with the erosion map prepared by Soil Conservation Department (SCD), Damodar Valley Corporation (DVC). The results show that the critical erosion area identified using modeling results matched spatially well with the DVC manually prepared area. Further, the validated model has been used to simulate the sedimentation in the reservoirs. The simulated sedimentation rate is 1.12 and 3.65 Mm3/year, respectively, for Konar and Panchet reservoirs for the studied period (1997–2001), which is reduced to 0.98 and 1.80 Mm3/year, respectively, when the critical watersheds are treated with conservation measures. As a result of model identified and implemented management strategy, Konar and Panchet reservoirs will have an additional useful life of 8 and 85 years, respectively. Results show a successful incorporation of distributed hydrological modeling for identifying critical watersheds, developing effective management strategy for controlling soil erosion, reducing reservoir sedimentation and improving their useful life.

Aeolian dynamics of beach scraped ridge and dyke structures

Where urban areas are situated close to a beach, sand dunes act as protection from flooding and erosion. When a dune has been removed or damaged by erosion, dune, ridge or dyke re-building using heavy machinery, a process known as beach scraping, is a common method of restoration. Following construction, natural accretion of sediment on the backshore is preferable as it facilitates sustained natural dune building, growth of vegetation, and habitat creation and reduces the need for further beach scraping.This study investigates the near surface flow and transport potential for three artificial structure designs: a single ridge, a double ridge and a dyke. The three shapes contained an identical volume of sand and were preceded by 50m of beach at an angle of 3°. A computational fluid dynamic model (CFD) was created for each scenario to calculate wind flow and shear velocity from 4 different wind directions at 22.5° intervals from 0° (onshore) to 67.5°. From this data sediment flux was predicted along a two dimensional transect for each of the scenarios.For all structures, shear velocity on the beach and stoss slope decreased as incident wind direction became more oblique; conversely shear velocity in the lee of the crest increased. A reduction in shear velocity at the foot of each structure also occurred and appears related to stoss slope, with the greatest reduction at the toe of the dyke structure (stoss slope 34°) and the least before the single ridge (stoss slope 17°).Specifically the results suggest that the double ridge structure is the most resilient to aeolian erosion. Shear velocity reduction on the back beach is comparable to the dyke and sediment flux from the stoss slope of the double ridge structure may become trapped in the swale between the two ridges encouraging sediment deposition, thus reducing sediment transport beyond the dunes and backshore. Although the dyke structure underwent the greatest reduction in shear velocity on the back beach it experienced substantial sediment flux at the crest and along the top of the structure, making it susceptible to erosion during a strong wind event. The highest sediment transport rate was calculated at the crest of the single ridge, and the single ridge structure also created the smallest reduction of shear velocity on the back beach, thus making it less desirable than the double ridge.

Unraveling bed slope from relative roughness in initial sediment motion

AbstractUnderstanding incipient sediment transport is crucial for predicting landscape evolution, mitigating flood hazards, and restoring riverine habitats. Observations show that the critical Shields stress increases with increasing channel bed slope, and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms, particle interlocking across the channel width, and large bed sediment relative to flow depth (relative roughness). Here we use scaled flume experiments with variable channel widths, bed slopes, and particle densities to separate these effects which otherwise covary in natural streams. The critical Shields stress increased with bed slope for both natural gravel (ρs = 2.65 g/cm3) and acrylic particles (ρs = 1.15 g/cm3), and adjusting channel width had no significant effect. However, the lighter acrylic particles required a threefold higher critical Shields stress for mobilization relative to the natural gravel at a fixed slope, which is unexpected because particle density is accounted for directly in the definition of Shields stress. A comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical Shields stress in our experiments. These changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion. Increased relative roughness can explain much of the observed heightened critical Shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore, pumice, or ice clasts on Titan.

Large-scale dam removal on the Elwha River, Washington, USA: Fluvial sediment load

The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21±3millionm3 of sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000±3,400,000tonnes of transported sediment, with 1,100,000 and 7,100,000t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000±80,000t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292m3/s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300mg/L and gauged bedload transport as large as 24,700t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000t in year 1 and 5,400,000t in year 2. Of the total 6,300,000±3,200,000t of suspended-sediment load, 3,400,000t consisted of silt and clay and 2,900,000t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000±360,000t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of <2-mm bedload was estimated to be on the order of 1.5Mt.

Wind Erosion Reduction by Scattered Woody Vegetation in Farmers' Fields in Northern Burkina Faso

AbstractWind erosion is an important soil degradation process on agricultural fields in the Sahel and is strongly affected by scattered woody vegetation. This paper analyses the effect of scattered vegetation on sediment transport in agricultural fields in northern Burkina Faso. A model was developed to simulate the changes in wind speed and sediment transport around shrubs and trees. The model was applied by using field measurements on wind speed, wind direction, and sediment transport, obtained from two farmers' fields during the rainy season of 2003. Vegetation characteristics and the density of vegetation elements differed per field. The model was used for scenario studies to test the effect of height, number, element type and spatial arrangement of vegetation elements on aeolian sediment transport. The local effects of vegetation elements on wind speed and sediment transport are small compared with the effects caused by the changes in the aerodynamic roughness length and changing wind speed at a larger scale. With relatively small changes in the characteristics of scattered woody vegetation, sediment transport can change considerably. An optimal arrangement of vegetation elements in an area in itself does not exist; it is an interrelation between the number of vegetation elements, the silhouette area and the type of vegetation elements present. This interrelation makes the use of scattered vegetation as a wind erosion control strategy attractive, as it fits in a variety of farming systems and can easily be adapted to specific needs of farmers. Therefore, scattered woody vegetation can be used to reduce sediment transport. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Impact of human activities and infrastructure works on hydro morphological characteristics of Alfeios River, Greece

&lt;p&gt;The Alfeios River is the greatest in length and flow-rate river in Peloponnisos and constitutes an important water resource and ecosystem of Western Greece. A number of infrastructure works and human activities have been constructed and are operating in Alfeios River Basin, while in the past extensive gravel extraction occurred. The impacts of infrastructure works and gravel extraction mainly in the Lower Alfeios Basin on the hydromorphological river characteristics are described and analyzed. Increased conductivity values were observed at the position close to the steam electric power plant due to the discharge of wastewater. The results show that gravel extraction and infrastructure works, in conjunction with the reduced sediment transport rates, cause long-term adverse effects on riverbed erosion as well as on the water level.&lt;/p&gt;