Sediment Erosion Rates Research Articles

Soil erosion and deposition characteristics of slope surfaces for two loess soils using indoor simulated rainfall experiment

Sediment deposition is an important part of the erosion process. Slope and soil type affect the spatial distribution of erosion–deposition; however, little is known about where erosion and deposition occur specifically and how they interact with influencing factors. To address this issue, four rainfall simulation experiments were conducted in a 1 m × 5 m plot with varying slopes (i.e., 5°, 10°, 15°, and 20°) and simulated rainfall for 1 h at a rate of 60 mm h−1. For the two soils, the runoff rate and sediment concentration were correlated with a similar temporal pattern. Runoff increased gradually and ultimately approached a steady state, and the sediment concentration decreased gradually and ultimately approached a steady state, which indicated that the erosion processes of the two soils were controlled by the detachment-limited condition. The sediment concentration and erosion rate were positively correlated with slope. However, the onset of runoff was delayed for AS soil, which also had a relatively high sediment concentration. The distribution range and area of deposition for the two soils were negatively correlated with the slope gradient, and the main deposition area occurred at the bottom of the plots. In contrast, the SD soil was more susceptible to deposition, and the deposition thickness was mainly concentrated within a 0–4 mm depth; this concentration was not observed for AS soil because of its high erosion intensity. Overall, this similarity also existed in the spatial variations of silt and fine sand contents in the soil surface. The silt and fine sand-sized particles were the primary eroded particle sizes; however, selective mechanisms of these two particle sizes were obviously different in the two soils. The results indicated that the erosion–deposition characteristics and the main eroded materials differed with varying erosion intensities and soil types. These findings are essential for a more comprehensive understanding of soil erosion mechanisms under conditions without rill formation.

Enhanced erodibility of deep-sea sediments by presence of calcium carbonate particles

Using an erosion chamber, erodibility experiments on two different deep-sea sediments (mainly clayey silt) acquired in the western Pacific Ocean have been conducted to estimate the erosion rate and its potential controlling factors. Overall, the erosion rates of both deep-sea sediments were in the range of 10−7–10−5 kg m−2 s−1 which was two or three orders of magnitude lower than that of sediments in shallow environments. Such erodibility, however, exhibited two contrasting patterns depending on the carbonate compensation depth (CCD), which determines carbonate contents in sediments. At one site (MC1), above the CCD, the sediments (carbonate content 18.32 ± 1.00 wt%) were easily resuspended by surface erosion, resulting in a high eroded mass (2.49 × 10−2 kg m−2 at shear stress of 0.6 Pa). At another site (MC4), below the CCD, the sediments (carbonate content 0.06 ± 0.04 wt%) were less resuspended by floc erosion, resulting in a low eroded mass (3.70 × 10−3 kg m−2 at shear stress of 0.6 Pa). Over the entire range of bed shear stress, the erosion rate above the CCD was about 1–6 times higher than that below the CCD. The difference in carbonate content between two sites caused different variations in the bed resistance with depth. The erodibility above the CCD was enhanced, probably due to the particle roughness and low bulk density. Once the bed shear stress exceeded about 0.39 Pa, the erosion type of MC1 shifted from depth-limited to transitional erosion. However, the erosion type of MC4 always remained depth-limited over the entire range of bed shear stress. These results underline the effect of calcium carbonate on resuspension and erosion behaviors of the deep-sea sediments.

Porewater nutrient enrichment alters benthic-pelagic coupling on intertidal sandflats

With the intensification of urban development, agriculture, and forestry, many coastal waters are becoming enriched with nitrogen (N). Long term, excess terrestrial N accumulates in the sediment porewaters impacting benthic fauna and microphytobenthic productivity, altering the complex bio-geophysical interactions that occur on the seafloor. These interactions may influence sediment erodibility and resuspension rates on the seafloor, which have wider ecosystem consequences through changes in water column turbidity, the fluxes of organic matter and sediment porewater nutrients into the water column, and the redistribution of deposited terrestrial materials (i.e. pollutants and sediment). We conducted a field experiment to explore how bio-geophysical interactions change in the context of porewater N enrichment. The experiment enriched plots (4 m2) on an intertidal sand flat in Tauranga Harbour, New Zealand, with three concentrations of N fertiliser (0, 150 and 600 g N m−2; n = 6). After 10 weeks, we collected intact sediments from the plots, and in the laboratory, subjected these sediments to sequential increases in water flow velocity (between 0.05 and 0.4 m s−1) using annular flumes (bed area = 0.17 m−2). We calculated sediment erosion rates and thresholds, measured fluxes of NH4+ and Chlorophyll a across the sediment-water interface and characterised the sediment properties and macrofaunal communities. We anticipated complex feedbacks to occur where porewater N enrichment would increase microphytobenthic biofilm growth, stabilising the sediments (and reducing benthic-pelagic coupling), but at the same time, N enrichment might also alter the macrofaunal community, which graze the microphytobenthos and physically mix the sediment, with a potential destabilising effect (and increase in benthic-pelagic coupling). Although our enriched treatments doubled the biomass of microphytobenthos, altered the macrofauna communities and enhanced the fluxes of Chlorophyll a and NH4+ across the sediment-water interface, sediment erosion thresholds and rates were unaffected. Our results highlight a potential mismatch between the micro- and macro-biotic processes and the larger scale physical forces on dynamic intertidal sandflats, and this mismatch has implications for our ability to scale up small scale processes to entire ecosystems.

Predicting sediment discharges and erosion rates in deep time—examples from the late Cretaceous North American continent

AbstractDepositional stratigraphy represents the only physical archive of palaeo‐sediment routing and this limits analysis of ancient source‐to‐sink systems in both space and time. Here, we use palaeo‐digital elevation models (palaeoDEMs; based on high‐resolution palaeogeographic reconstructions), HadCM3L general circulation model climate data and the BQART suspended sediment discharge model to demonstrate a predictive, forward approach to palaeo‐sediment routing system analysis. To exemplify our approach, we use palaeoDEMs and HadCM3L data to predict the configurations, geometries and climates of large continental catchments in the Cenomanian and Turonian North American continent. Then, we use BQART to estimate suspended sediment discharges and catchment‐averaged erosion rates and we map their spatial distributions. We validate our estimates with published geologic constraints from the Cenomanian Dunvegan Formation, Alberta, Canada, and the Turonian Ferron Sandstone, Utah, USA, and find that estimates are consistent or within a factor of two to three. We then evaluate the univariate and multivariate sensitivity of our estimates to a range of uncertainty margins on palaeogeographic and palaeoclimatic boundary conditions; large uncertainty margins (≤50%/±5°C) still recover estimates of suspended sediment discharge within an order of magnitude of published constraints. PalaeoDEMs are therefore suitable as a first‐order investigative tool in palaeo‐sediment routing system analysis and are particularly useful where stratigraphic records are incomplete. We highlight the potential of this approach to predict the global spatio‐temporal response of suspended sediment discharges and catchment‐averaged erosion rates to long‐period tectonic and climatic forcing in the geologic past.

Glacial sedimentation, fluxes and erosion rates associated with ice retreat in Petermann Fjord and Nares Strait, north-west Greenland

Abstract. Petermann Fjord is a deep (>1000 m) fjord that incises the coastline of north-west Greenland and was carved by an expanded Petermann Glacier, one of the six largest outlet glaciers draining the modern Greenland Ice Sheet (GrIS). Between 5 and 70 m of unconsolidated glacigenic material infills in the fjord and adjacent Nares Strait, deposited as the Petermann and Nares Strait ice streams retreated through the area after the Last Glacial Maximum. We have investigated the deglacial deposits using seismic stratigraphic techniques and have correlated our results with high-resolution bathymetric data and core lithofacies. We identify six seismo-acoustic facies in more than 3500 line kilometres of sub-bottom and seismic-reflection profiles throughout the fjord, Hall Basin and Kennedy Channel. Seismo-acoustic facies relate to bedrock or till surfaces (Facies I), subglacial deposition (Facies II), deposition from meltwater plumes and icebergs in quiescent glacimarine conditions (Facies III, IV), deposition at grounded ice margins during stillstands in retreat (grounding-zone wedges; Facies V) and the redeposition of material downslope (Facies IV). These sediment units represent the total volume of glacial sediment delivered to the mapped marine environment during retreat. We calculate a glacial sediment flux for the former Petermann ice stream as 1080–1420 m3 a−1 per metre of ice stream width and an average deglacial erosion rate for the basin of 0.29–0.34 mm a−1. Our deglacial erosion rates are consistent with results from Antarctic Peninsula fjord systems but are several times lower than values for other modern GrIS catchments. This difference is attributed to fact that large volumes of surface water do not access the bed in the Petermann system, and we conclude that glacial erosion is limited to areas overridden by streaming ice in this large outlet glacier setting. Erosion rates are also presented for two phases of ice retreat and confirm that there is significant variation in rates over a glacial–deglacial transition. Our new glacial sediment fluxes and erosion rates show that the Petermann ice stream was approximately as efficient as the palaeo-Jakobshavn Isbræ at eroding, transporting and delivering sediment to its margin during early deglaciation.

Breaching Flow Slides and the Associated Turbidity Current

This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.

Erosion by Wave Action of Consolidated Cohesive Bottom Containing Cohesionless Sediment

The erosion processes of consolidated cohesive sediment under irregular breaking waves were formulated to predict the profile evolution of a cohesive sediment beach with a layer of sand. The cohesive sediment is eroded by turbulence generated by wave energy dissipation caused by wave breaking and bottom friction. Sand released from the eroded sediment is transported onshore or offshore by wave action. The cohesive sediment erosion rate is increased by a thin mobile layer of sand and decreased by a thick sand layer. The complicated interactions of waves, sand and cohesive bottom were simplified and incorporated into an existing cross-shore numerical model. The model was compared with flume experiment data with measured till erosion rates on the order of 0.05 cm/h. The calibrated model was used to simulate the temporal change of the till erosion rate for a duration of 100 h. The scale effect was examined in hypothetical prototype tests of the same till with a length ratio of 1/4. The prototype 200-h simulations predicted much larger till erosion near the shoreline and sand deposition in the surf zone. The model will need to be compared with actual field data.

Decomposing macroalgae (Ulva spp.) impacts benthic macrofauna and surface sediment erosion

Patches of decomposing macroalgae can be beneficial to intertidal regions, providing habitat and food, or can create anoxic conditions inhospitable to benthic organisms. These opposing outcomes warrant further investigations into biotic-abiotic processes associated with macroalgae. Here, differences in surface sediment erosion measures (erosion threshold (Ʈc; N m−2), erosion rate (ER; g m−2 s−1), and sub-surface erosion rates (me; g N−1 s−1)) were examined at sites dominated by the suspension-feeding clam or deposit-feeding bivalve (Austrovenus stutchburyi and Macomona liliana, respectively) after 30 days of exposure to decomposing macroalgae (Ulva spp.). The Austrovenus site was chosen to represent a species-rich, functionally diverse macrofaunal community. The nearby Macomona site had similar sediment characteristics, yet had a less abundant and diverse faunal community. Despite the equal amounts of Ulva recovered from both sites (< 3% of the initial 3 kg wet weight m−2 added), differences in surface erosion were measured. One day after Ulva removal, an initial increase in surface erosion (−Ʈc and +ER) was measured at the Austrovenus site, but after 14 days there was no difference compared with control plots. At the Macomona site, Ulva addition stabilized sediments (+Ʈc and -ER), an effect that persisted for 14 days. These differences were reflected in the macrofaunal community. At the Austrovenus site, the macrofaunal community recovered within 14 days whereas differences between Ulva and controls persisted at the Macomona site. This work emphasizes how subtle differences in benthic community composition and recovery can result in spatial and temporal variations in sediment erosion potential on intertidal sandflats.

Sediment Availability and Erosion Rates on Off‐Highway Vehicle Trails in the Ouachita Mountains, USA

AbstractFactors influencing sediment availability are assessed and erosion rates are quantified for an off‐highway vehicle (OHV) trail system in the Ouachita Mountains of Arkansas. As of May 2012, the Wolf Pen Gap trail system included 77.0 km of "trails" which consist of county roads; open and closed Forest Service roads; and open and closed OHV trails. For a given trail length, the sediment volume available to be eroded is determined by bare trail width and sediment depth. Four condition types are defined that group trail sections based on statistically different trail widths or depths. Trail construction method appears to influence sediment availability differences more than erosion potential (as indexed by trail slope gradient and length). The range for annual trail erosion rates is estimated as 75 and 210 tonne/ha/yr. The high and low rates are obtained using two independent methods. The 210 tonne/ha/yr rate is computed from mean sediment capture at 30 sediment traps installed for 0.5–1.0 year. The 75 tonne/ha/yr rate is computed assuming all available trail sediment measured in a one‐time sampling is eroded over the next year. We argue in support of this assumption and suggest both rate values may be conservative. Trail erosion rates and sediment trap observations indicate frequent trap cleanout will be needed to continue sediment capture from All Terrain Vehicle trails.

The Relative Role of Mangroves on Wave Erosion Mitigation and Sediment Properties

Fringe mangroves face waves daily and are thought to protect against erosion in low wave energy sites and undergo erosion if exposed to high wave energy. We aimed to understand the effects of fringe mangroves on erosion and sediment dynamics and of wave exposure on seedling density at three sites of increasing wave energy. Sediment properties (mean grain size, sorting, and bulk density) were assessed within each site in unvegetated and mangrove-vegetated shores in wet and dry seasons. In addition, we estimated seasonal erosion/accretion rates for 2.4 years and seedling density in two zones from the forest edge with contrasting wave exposure. Regression analysis was carried out to explain sediment properties and erosion rate variance in response to the vegetation volume that opposes wave energy and to explain erosion rates in response to wave energy. Mangrove-vegetated shores reduced erosion rates from 3 to 15 times in the two sites with higher wave energy, while the vegetated site with the lowest wave energy experienced accretion compared to minor erosion along the unvegetated shore. Shores with greater Rhizophora mangle basal areas and vegetation volumes favored deposition of particles with low settling rates, different sediment classes, reduced erosion rates, and increased shoreline stability. Mangrove seedling density decreased between 2 and 43 times from the low wave exposure zone to the high wave exposure zone at the forest edge in studied sites. In order to increase vegetation volume, coastal adaptation based on mangroves must limit human disturbances and facilitate epiphytic relationships with oysters.

CALCULATION MODEL AND EXPERIMENTAL STUDY ON SEDIMENT EROSION OF SLOPE UNDER EXTERNAL-SOIL SPRAY SEEDING

Due to the current lack of theoretical study on rain erosion of the slope under external-soil spray seeding, the calculation model applicable to slope sediment erosion under external-soil spray seeding was derived from hydraulics theory and energy method. Designed slope scouring test on indoor model, conducted 3 separate scouring tests under 3 different slope rates (1:0.75, 1:1, 1:1.25) and 3 different rainfall intensities (120, 150, 180mm/h). The rationality of the calculation model was verified by the experimental data. Meanwhile, also analysed the developing regularities of slope erosion and relationship among sediment erosion, rainfall duration, rainfall intensity and slope rate. The results show that the relative error between the theoretical value and measured data is within 18%, and the calculation model can accurately predict the sediment erosion of the slope under external-soil spray seeding to some extent; the slope under external-soil spray seeding is lack of erosion resistant ability, and the slope erosion is mainly rill erosion and layered surface erosion; the amount of sediment erosion is positively correlated with rainfall intensity and slope rate. The relationship between growth rate and rainfall duration basically remained the same during the first 20 minutes, and then gradually decreased.

Comparative numerical analysis between two designs of Francis runner blades in sediment affected conditions

Sediment erosion of hydraulic turbine is the major problem from the perspective of operation and maintenance in power plants of Himalayan and Andes region. The effects of sediment erosion on turbine components need to be predicted in advance during the design phase so that the best design with better sediment handling can be installed in real power plants. In this paper, comparison of performance and erosion of two different designs of Francis turbine i.e. design I and design II are carried out. Full turbine steady state numerical simulations are carried out for 8 different guide vane openings using shear stress transport (SST) k-ω turbulence model. Sediment erosion analysis is carried out using Tabakoff erosion model for both the designs, numerical hill charts are constructed using Ned and Qed values for all the operating conditions. Comparison of pressure distribution along pressure and suction side of GV surface between design I and a similar measurement in a previous experiment is carried out, which shows a good agreement between numerical and experimental results. Hydraulic efficiency and the sediment erosion rate density on the runner blades of design I for all operating conditions is higher than that of design II. The difference in efficiency is less than 1.85%for all operating conditions while sediment erosion rate density is much less in design II, which shows that design II is a better option for this turbine.

Testing the Prediction Ability of LEM-Derived Sedimentary Budget in an Upland Catchment of the Southern Apennines, Italy: A Source to Sink Approach

Landscape evolution models (LEMs) represent one of the most promising approaches to evaluate sedimentary budget, although factors such as the high number of parameters or the difficulty evaluating the robustness of the results can represent a limitation in their application in natural landscapes. In this paper, the Caesar–Lisflood LEM has been applied in a small catchment (i.e., about 9 km2) of southern Italy draining an artificial reservoir in order to test its ability to predict sediment flux and erosion rate. Short-term (i.e., about 20 years) estimation of the sediment volumes accumulated in the reservoir has been reconstructed by a bathymetric survey and compared to the results coming from the coeval LEM simulations. Results indicate a good accordance between LEM-based erosion volume estimations and direct sedimentation assessment, thus testifying to the high potential of such models to solve issues of sedimentary budget and short-term landscape modification.

Predictions of the equilibrium depth and time scale of local scour below a partially buried pipeline under oblique currents and waves

This paper presents experimental results on temporal developments of local scour below a partially buried pipeline under oblique currents/waves. The effects of the flow incident angle (α ≤ 45°, α = 0° denotes that the currents/waves are perpendicular to the pipeline) and the embedment-to-diameter ratio (e/D ≤ 0.5, e/D = 0 denotes no embedment) on the equilibrium depth and time scale of scour were quantitatively investigated. The experimental results indicate that with the increase of the embedment-to-diameter ratio from 0 to 0.5, the equilibrium depth of scour decreases by up to 30 and the time scale increases by approximately 5 times. With the increase of the flow incident angle from 0° to 45°, the equilibrium depth decreases by approximately 25% and the time scale increases by approximately 0.7 times. A uniform empirical relationship between the equilibrium depth for arbitrary conditions (S, i.e., for α > 0° or e/D > 0) and that for a conventional case (S0, for α = 0° and e/D = 0) is newly proposed for both current and wave conditions based on theoretical derivation and curve fitting to the experimental data, where S0 can be easily predicted from existing empirical formulae. The correlation analysis indicates that the experimental results of the equilibrium depth can be well represented by the newly proposed formula. For predicting the time scale of scour process, a theoretical approach based on the erosion rate of sediment, which was originally proposed for a conventional case, is expanded to arbitrary conditions. It is indicated that the time scales of scour obtained from the theoretical approach are well coincident with the experimental values. Finally, an empirical relationship between the time scale for arbitrary conditions (T) and that for a conventional case (T0) was established based on the theoretical derivation. The newly proposed empirical formula is found to provide satisfactory predictions of the time scale compared with the present experimental results and those published.

The influence of permeability on the erosion rate of fine-grained marine sediments

Laboratory experiments have been carried out to investigate the erosion behaviour of a number of marine sediments which were reconstituted and sieved in various ways to ensure a large variation in soil properties such as (i) particle size distribution (including median grain size and fines content), (ii) bulk density, and (iii) hydraulic permeability. Based on the experimental results, it was found that for finer marine sediments the erosion rate at a given shear stress was sensitive to changes in these soil properties. No unique relationship was found between changes in the erosion rate and changes in the bulk density or fines content. More specifically, we find that the erosion rate at a given shear stress reduces with increases in density and fines content, but by an amount that is different for different sediments. In contrast, we show that there appears to be a unique relationship between permeability and erosion rate, such that permeability may be used to predict the erosion rate for the marine sediments at any density. By reinterpreting existing experimental results in the literature, we find that this same relationship between permeability and erosion rate is apparent for quartz sediments. We propose an empirical relationship, which fits well the erosion rate behaviour of finer sediments close to the threshold shear stress.

Check dams and sediment control: final results of a case study in the upper Corneja River (Central Spain)

The sediment yield and erosion rate in an area can be estimated by calculating the volume of sediment retained by check dams. However, results can show unrealistic and imprecise conclusions if the sediment volume is not measured accurately, as tolerable erosion rates are usually below 10 t ha−1 year−1. Our previous research developed a topographic method (the Sections Method) for measuring the sediment wedges created by check dams and the sediment yield with a high degree of precision to avoid the aforementioned problem. Until now, however, it had only been tested for a few check dams. Here, we present a final and complete analysis applying this method to 113 check dams in a granitic restored area in the upper basin of the Corneja River (Central Spain), under a continental Mediterranean climate, to estimate an accurate mean erosion rate over 50 years, as well as the downstream influence of the check dams. Our results show that the check dams trapped 5365.93 m3 sediment, which represents a sediment export of 0.096 t ha−1 year−1 and a total sediment yield of 5.6 t ha−1 year−1. These values are significantly higher (> 18%) than those obtained with other, simpler geometric methods currently in use, such as the Prism and Pyramid Methods, as we have already argued in previous papers. It is therefore important to take this issue into account. Moreover, we confirmed a number of morphological changes. As a result of the check dam effect, the slopes of the streambed are 11% lower than the original streambed slope, with a maximum reduction of up to 39%, leading to higher infiltration rates in the streambeds, lower energy waterflows, and flood lamination. Furthermore, the sediment wedges created a new land surface of over 5000 m2 permitting the development of agroforestry uses such as riparian woods, orchards, cropland, or pastureland. These final results show that check dams were highly effective and played a positive role in retaining and controlling sediment.

Effects of bioturbation on the erodibility of cohesive versus non-cohesive sediments along a current-velocity gradient: A case study on cockles

Soft-bottom bioturbators are ecosystem engineers in the sense that they can have considerable effects on sediment erodibility and resuspension. The common cockle Cerastoderma edule is a bioturbating filter feeder that is widespread along the European Atlantic coastline. Its presence and activity can decrease sediment erosion thresholds in cohesive sediments but little is known about its effect on non-cohesive sediments. Using controlled annular flume experiments, we investigated the relative effects of different cockle densities on sediment resuspension in cohesive vs. non-cohesive sediments by assessing the following: (i) the mud and sand burrowing behavior of cockles, (ii) critical erosion thresholds, (iii) the mass of eroded sediment and (iv) erosion rates. Our results show that cockles were more active in non-cohesive sediment compared with cohesive sediment. Despite their lower activity, the presence of cockles in cohesive sediment increased sediment erodibility by reducing the critical erosion threshold (Ucrit) and increasing both the mass of eroded sediment and erosion rate. In contrast, cockles had no effect on erodibility in non-cohesive sediment, especially on the eroded sediment mass and erosion rate. The mass eroded was not significantly different between cohesive and non-cohesive sediments when cockles were present. Our experiments show that the increased erodibility of cohesive sediment due to the bioturbation by cockles is density dependent: higher cockle density results in stronger effects on erodibility. Moreover, this increase in cohesive sediment erosion due to cockle bioturbation was positively correlated with current velocity. In contrast, the erosion of non-cohesive sediment only depended on the current stress and was unaffected by cockle density. Considering the high abundance of C. edule, its widespread distribution and its extensive activities, the results of this study could be widely applicable to intertidal mud flats around the world.

Numerical Model for Bank Erosion in the Brahmaputra River

A method is proposed to predict bank erosion and sand bar migration in river reaches where suspended sediment transport is dominant. The method focuses on the influence of the lateral bed slope on the erosion and deposition rate of suspended sediment, as well as on the profile of lateral bedload transport, assuming that geometric similarity holds in the bank region. In the proposed model, the erosion and deposition rate can be evaluated using either the bed shear stress at a reference location or the average bed shear stress in the bank region. In order to simulate bank erosion and associated bank shifting with a depth-integrated-base treatment, stretchable grids were added to the conventional coarse grid system near the bank. The proposed method, including the bank erosion model, is applied to the lower reach of the Brahmaputra River, which is ∼90 km long and ∼12.50 km wide. The computed results on bank shifting, sand bar migration, and sediment transport rates are compared with data obtained from field investigations and remote sensing. These results suggest that the proposed method is applicable for predicting sediment issues in river reaches dominated by suspended sediment.

Synthesis of Common Management Concerns Associated with Dam Removal

AbstractManagers make decisions regarding if and how to remove dams in spite of uncertainty surrounding physical and ecological responses, and stakeholders often raise concerns about certain negative effects, regardless of whether these concerns are warranted at a particular site. We used a dam‐removal science database supplemented with other information sources to explore seven frequently raised concerns, herein Common Management Concerns (CMCs). We investigate the occurrence of these concerns and the contributing biophysical controls. The CMCs addressed are the following: degree and rate of reservoir sediment erosion, excessive channel incision upstream of reservoirs, downstream sediment aggradation, elevated downstream turbidity, drawdown impacts on local water infrastructure, colonization of reservoir sediments by nonnative plants, and expansion of invasive fish. Biophysical controls emerged for some of the concerns, providing managers with information to assess whether a given concern is likely to occur at a site. To fully assess CMC risk, managers should concurrently evaluate site conditions and identify the ecosystem or human uses that will be negatively affected if the biophysical phenomenon producing the CMC occurs. We show how many CMCs have one or more controls in common, facilitating the identification of multiple risks at a site, and demonstrate why CMC risks should be considered in the context of other factors such as natural watershed variability and disturbance history.

Spatio-temporal variation of water flow and sediment discharge in the Mahanadi River, India

The transport of sediments by rivers to the oceans represents an important link between the terrestrial and marine ecosystem. Therefore, this work aims to study spatio-temporal variation of the sediment discharge and erosion rate in the Mahanadi river, one of the biggest rivers in India, over past three decades vis-à-vis their controlling factors. To understand the sediment load variation, the trend analysis in the time series data of rainfall, water and sediment discharge of the Mahanadi river were also attempted. The non-parametric Mann-Kendall and Sen's methods were used to determine whether there was a positive or negative trend in the time series data with their statistical significance. The occurrence of abrupt changes was detected using Pettitt test. The trend test result represents that sediment load delivered from the Mahanadi river to the global ocean has decreased sharply at the rate of 0.515×106tons/year between 1980 and 2010. Water discharge and rainfall in the basin showed no significant decreasing trend except at only one tributary. The decline in sediment discharge from the basin to the Bay of Bengal is mainly due to the increase in the number of dams, which has recorded the increase from 70 to 253 during the period of 1980 to 2010. Over the past 30years the Mahanadi river has discharged about 49.0±20.5km3 of water and 17.4±12.7×106tons of sediment annually to the Bay of Bengal whereas the mean erosional rate is 265±125tons/km2/year over the period of 30years in the basin. Based on the current data (2000–2001 to 2009–2010), sediment flux and water discharge to the ocean are 12±5×106tons/year and 49±16km3/year respectively; and ranking Mahanadi river second in terms of water discharge and sediment flux to the ocean among the peninsular rivers in India.