Peak Sediment Research Articles

Erosion characteristics of outburst floods on channel beds under the conditions of different natural dam downstream slope angles

To explore the influence of natural dam downstream slope angles on the erosion characteristics of outburst floods on channel beds, five natural dam failure tests with different dam downstream slopes were conducted. The results show that the increase in the dam downstream slope angle enlarges the breaching discharge and shortens the duration. The farther away from the dam, the weaker the erosion intensity is on the channel bed. The absolute value of the erosion rate at each section is smaller in the late stage of the failure process, and the erosion intensity gradually decreases. For the channel bed near the dam, the erosion rate is larger, and the erosion ability is stronger than those for the channel bed farther from the dam. With the increase in the dam downstream slope angle, the erosion rate increases in the early stage, which changes little in the late stage. The sediment concentration first increases and then decreases, and with an increase in the dam downstream slope angle, the peak sediment concentration first decreases and then increases. For natural dams with gentle dam downstream slopes, the peak sediment concentration is primarily affected by the dam volume; when the dam downstream slope gets steep, the breaching discharge is the dominant factor affecting the sediment concentration. The final topography of the channel bed after dam failure under different dam downstream slope angles is basically the same. When the failure process comes to an end, sandbars are formed on the channel bed, which are distributed on both sides of the channel, forming a braided river downstream. The channel bed surface represents an obvious “S” shape.

Impact of the construction of cascade reservoirs on suspended sediment peak transport variation during flood events in the Three Gorges Reservoir

Peak sediment transport during flood events is very important to the sustainability of the Three Gorges Reservoir (TGR), and thus, in-depth research on peak sediment transport must be performed for the refined regulation of this reservoir. The variation in peak sediment transport and its corresponding causes following the completion of the cascade reservoirs in the lower Jinsha River in the TGR are still unclear. In this study, based on daily discharge and suspended sediment concentration time series, hysteresis patterns and peak sediment transport times were used to determine the peak sediment transport variations. The temporal peak sediment transport variations and their origins were revealed based on wave theory and a one-dimensional hydraulic model. The results show that peak sediment hysteresis has become more obvious recently, and the greatest influencing factor is the construction of cascade reservoirs. Rising water levels in the TGR contributed to slowing SSC, doubling the proportion of anti-clockwise patterns, and increasing the peak sediment transport time by approximately 31% after 2003. The cascade reservoirs in the lower Jinsha River reduced the peak discharge by approximately 15%, which had a relatively large influence on the SSC peak, increasing the SSC peak transport time by approximately 22% and increasing the proportion of the anti-clockwise pattern at the inflow reach. These results can provide references for reservoir regulation during flood events; more significant sediment hysteresis means the regulation timing should be changed. The lag between the peak sediment and peak discharge is affected by the flow process and water level and can be used to help develop a reservoir regulation model.

Effects of hypolimnetic aeration on the quantity and quality of settling material in a eutrophied dimictic lake

Effects of hypolimnetic aeration (pumping of epilimnetic water into the hypolimnion) on the quantity of settling material in eutrophied Lake Vesijärvi, Finland were studied by comparing spatially comprehensive gross sedimentation rates as dry and organic matter prior to aeration activity and during two aerated years. Possible changes in the organic matter (as loss on ignition, LOI), carbon (C) and nitrogen (N) contents and changes in the C/N ratio of the settling material and surface sediment were quantified. Thermal stratification broke up earlier due to aeration and was followed by sedimentation peaks. The absolute amount of dry and organic matter as well as C and N settling to the lake bottom were significantly higher in the aerated years. Increased sedimentation rates were especially pronounced in the deep zones indicating enhanced sediment focusing. Increased sedimentation of C and N reflected higher primary production during the aerated years, which most likely was associated with increased temperature and turbulence and the subsequent regeneration and recycling of nutrients in the water body. Aeration seemed to slightly enhance degradation, but contrary to its ultimate aim, it failed to decrease the phosphorus content of the water column and deposits of organic material in the deep zones of the lake.

Character and sedimentation of “lingering” Macondo oil to the deep-sea after the Deepwater Horizon oil spill

During the active 87-day Deepwater Horizon spill in the northern Gulf of Mexico, a significant fraction of the spilled Macondo oil was transported to the seafloor via the sedimentation of marine snow. Here we present a detailed characterization of oil that arrived together with marine snow at a 1400 m deep sediment trap six weeks to 13 months after the spill had ended. These data give insight into the nature and evolution of the sedimentation of the marine snow and oil, the latter of which remained as droplets in the water column after the spill ended. Four pulses of oil flux were recognized; three of which were associated with peak sedimentation rates of diatoms. Detailed chemical analysis (TPH, alkylated PAH, and petroleum biomarker fingerprints) reveal the sinking oil's lack of evaporation and photo-oxidation, which indicated it was not derived from the sea surface but had “lingered” within the water column after the spill. Measurable amounts of the increasingly weathered (biodegraded and water-washed) Macondo oil was collected in the trap for ~1 year after the active spill ended, over which time the oil flux decreased overall. The results indicate that sinking diatom aggregates and other marine snow scavenged measurable amounts of weathered Macondo oil droplets remaining in the water, and carried them to the deep-seafloor for approximately 1-year after the spill ended.

Assessing the performance of a physically based hydrological model using a proxy‐catchment approach in an agricultural environment

AbstractPhysically based models are useful frameworks for testing intervention strategies designed to reduce elevated sediment loads in agricultural catchments. Evaluating the success of these strategies depends on model accuracy, generally established by a calibration and evaluation process. In this contribution, the physically based SHETRAN model was assessed in two similar U.K. agricultural catchments. The model was calibrated on the Blackwater catchment (18 km2) and evaluated in the adjacent Kit Brook catchment (22 km2) using 4 years of 15 min discharge and suspended sediment flux data. Model sensitivity to changes in single and multiple combinations of parameters and sensitivity to changes in digital elevation model resolution were assessed. Model flow performance was reasonably accurate with a Nash–Sutcliffe efficiency coefficient of 0.78 in Blackwater and 0.60 in Kit Brook. In terms of event prediction, the mean of the absolute percentage of difference (μAbsdiff) between measured and simulated flow volume (Qv), peak discharge (Qp), sediment yield (Sy), and peak sediment flux (Sp) showed larger values in Kit Brook (48% [Qv], 66% [Qp], 298% [Sy], and 438% [Sp]) compared with the Blackwater catchment (30% [Qv], 41% [Qp], 106% [Sy], and 86% [Sp]). Results indicate that SHETRAN can produce reasonable flow prediction but performs less well in estimation of sediment flux, despite reasonably similar hydrosedimentary behaviour between catchments. The sensitivity index showed flow volume sensitive to saturated hydraulic conductivity and peak discharge to the Strickler coefficient; sediment yield was sensitive to the overland flow erodibility coefficient and peak sediment flux to raindrop/leaf soil erodibility coefficient. The multiparameter sensitivity analysis showed that different combinations of parameters produced similar model responses. Model sensitivity to grid resolution presented similar flow volumes for different digital elevation model resolutions, whereas event peak and duration (for both flow and sediment flux) were highly sensitive to changes in grid size.

Main influencing factors of sediment peak transmission time in flood season of the Three Gorges Reservoir (TGR)

Based on the measured data of sediment peak transport after the application of the TGR, the relationship between sediment peak transmission time and different influencing factors in the TGR was studied. The result shows: Main influencing factors of sediment peak transmission time in flood season of the TGR are sediment peak concentration of Cuntan station and flood retention coefficient, and the relationship between them is the positive proportion, in which the influence of inflow sediment peak on the sediment peak transmission time is greater than flood retention coefficient. Research results can provide reference for Sediment regulation of the TGR during flood season.

A Numerical Model for Fluvial Transport of Subglacial Sediment

Sediment discharge from glaciers impacts downstream aquatic habitats, hydropower operations, and river infrastructure. Since discharge of subglacial sediment will evolve in response to glacier retreat, estimating future subglacial sediment dynamics is of great relevance. To develop tools and methods to better constrain the responsible processes, we present a till dynamics model that accounts for limited sediment access coupled to a subglacial hydrology model to describe the evolution of a subglacial till layer over a glacier's longitudinal profile in one dimension. Synthetic simulations examining the effects of changing hydrology highlight the importance of properly constraining both the erosion of underlying bedrock and the subglacial sediment connectivity. This is because changes in hydrology alter the timing of peak sediment discharge but only marginally affect total sediment discharge once the subglacial sediment reservoir is exhausted. Model simulations for real‐world glaciers yield insights into the distribution of sediment along the glacier bed, including locations where sediments are deposited or exhausted. Comparison between model results and field data shows that total and peak sediment discharge, as well as interannual variability, can be captured with acceptable skill for periods ranging from hours to decades. The results from this model show that modeling subglacial sediment transport on decadal to subdaily scales is possible but requires processes such as bedrock erosion and sediment connectivity to be considered.

Erodibility of synthetic water repellent granular materials: Adapting the ground to weather extremes

Granular materials with synthetic water repellent coatings have great potential to be used in ground interfaces (ground-atmosphere-vegetation and ground-structure) as infiltration barriers, due to their altered hydrological properties (suppressed infiltration and decreased sorptivity). However, very few studies have evaluated the impact of synthetic soil water repellency on soil erosion. This paper investigates the effect of water repellency on soil erosional behavior, including splash erosion and rill processes. Twenty-four flume tests were carried out on model slopes under artificial rainfall; soils with three wettability levels were tested, including wettable (contact angle, CA < 90°), subcritical water repellent (CA ~ 90°) and water repellent (CA > 90°). Various rainfall intensities (230 mm/h, 170 mm/h, 100 mm/h and 40 mm/h) and grain sizes (Fujian sand and sand/silt mixture) were adopted. Erosional variables, including splash erosion rate, average sediment concentration, peak sediment concentration and time to peak sediment were measured to quantitatively analyze the behavior. This study confirms the impact of water repellency on soil erosion and unveils the possibility to reduce infiltration at ground-atmosphere interface with controlled soil erosion. The results revealed that: (1) synthetic water repellency does not necessarily lead to increased soil erosion yield; its impact is dependent on grain size with the soil erosion loss increasing for Fujian sand, but decreasing for sand/silt mixtures; (2) splash erosion is positively correlated to soil water repellency and high rainfall intensity, regardless of grain size; (3) the erosion processes for sand/silt mixtures are particle size selective and not affected by soil water repellency, whereas this phenomenon is not observed with Fujian sand.

Extreme sediment fluxes in a dryland flash flood

A flash flood on 28th September, 2012, rose to a peak discharge of 2357 m3 s−1 from zero within one hour in the ephemeral Nogalte channel in SE Spain. Channel morphology and sediment sizes were measured at existing monitored sites before and after the flood and peak flow hydraulics calculated from surveyed floodmarks and cross-sections. Maximum peak sediment fluxes were calculated as ~600 kg s−1 m−1, exceeding maximum published, measured dryland channel values by 10 times and common perennial stream fluxes by 100 times. These high fluxes fit the established simple bedload flux - shear stress relations for dryland channels very well, but now extended over a much wider data range. The high sediment fluxes are corroborated by deposits at >1 m height in a channel-side tank, with 90 mm diameter sediment carried in suspension, by transport of large blocks and by massive net aggradation as extensive, structureless channel bars. Very high sediment supply and rapid hydrograph rise and recession produced the conditions for these exceptional sediment dynamics. The results demonstrate the extreme sediment loads that may occur in dryland flash floods and have major implications for catchment and channel management.

Accuracy of sedimentgraph modeling from topography map scale and DEM mesh size

The evaluation of scale effects on modeling performance of sedimentgraphs as the ultimate outputs of the hydrological simulation is vital for adaptive watershed management. The present study therefore analyzed effectability of simulated sedimentgraphs components in association with different topographic maps with various vector scales. The whole procedure was materialized to select the critical scale and cell size for the Galazchai Watershed, Iran. To this end, the stormwise sedimentgraphs were modeled for 23 recorded events using the Clark's Instantaneous Unit Hydrograph (IUH) model stemmed for developing Instantaneous Unit Sedimentgraphs (IUSGs) incorporated with dimensionless sediment concentration distribution (DSCD) based on the vector scales of 1:25000, 1:50000, 1:100000 and 1:250000 and cell sizes of 5, 10, 20 30, 50, 100 and 200 m. Some 644 direct sedimentgraphs (DSGs) were then evaluated based on Relative Errors (REs) for sediment volume, peak sediment, time to peak, base time and the Coefficient of Efficiency (CE). The results confirmed that REs for peak sediment, time to peak and CE were sensitive to cell size. The results further verified that the cell sizes of 5, 20, 30, 50 and 100 m were critical cell sizes in viewpoint of time to peak. In addition, the vector scales of 1:50000 with cell size of 50 m, and 1:100000 with cell sizes of 5 and 10 m were critical vector scales and cell sizes based on RMSE evaluation criterion. It is concluded from the current research that the accuracy of simulation of sedimentgraph was influenced by map scales and mesh sizes.

Anatomy of a late Quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI

The data set of the island of West Caicos consists of a combination of high-resolution lidar and digital imagery, radiometric data, and amino acid racemization (AAR) D/L values as well as extraordinary preservation of sedimentary and bio-constructed deposits on the island. Together with a well-established regional stratigraphic framework from the neighboring Bahamian island chain and detailed field mapping, the West Caicos data set provides an opportunity to advance our understanding of the link between carbonate stratigraphy and the controlling parameters of sea level, sediment supply, tectonics, and climate. The exceptionally well-preserved stratigraphic record of the mid–late Pleistocene from West Caicos allows recognition of marine isotope stages (MIS) 9/11, 5e, and Holocene. The west coast of the island provides a continuous 8.4 km exposure of the MIS 5e record that enhances our understanding of the intra-MIS 5e bipartite succession in this region and supports a sea-level lowering that occurred during the last interglacial (LIG).West Caicos accreted as a series of eolian, foreshore–shoreface, barrier-reef, and fringing-reef facies. Shoreline positions for all but the MIS 7 and MIS 5a/c deposits are now captured in the sub-meter geodetic-quality topobathy lidar survey. The Star Town unit of MIS 9/11 age forms the southern margin of the island as well as the central karst-modified spine, with uranium–thorium (UTh) coral ages of between 368 and 365 thousand years ago (ka) and a D/L Glu average of >0.60. Early MIS 5e strata include the Railroad Ridge unit in the center of the island, which was previously identified as one of the older units. The Railroad Ridge unit is a north–south linear dune ridge that has a D/L Glu average of 0.52 and indicates the onset of sediment sources from the east associated with a flooded central Caicos Platform. Approximately synchronously, the early MIS 5e South Reef unit (+3.75–4 m sea-level elevation) nucleated on the western coast of West Caicos, attached to the middle Pleistocene Star Town unit. The South Reef corals yield an average coral UTh age of 126.5 ka, with an average D/L Glu from skeletal matrix of 0.46. This transitional barrier/fringing reef is unconformably overlain by the late MIS 5e Boat Cove unit. Corals in the basal transgressive phase of this upper MIS 5e Boat Cove unit yield an average 120.6 ka UTh and average D/L Glu of 0.45. Shallow-marine, foreshore, and eolian facies of the late MIS 5e rest on a wave-cut terrace that cuts the early MIS 5e South Reef unit by as much as 3 m down to a +0.5 m above present sea-level elevation. The upper MIS 5e Boat Cove unit displays a maximum of +4.5 m elevation above present-day sea level, measured from the middle of the foreshore. This estimate may be a minimum value as the exposures on the west coast are part of a forced regressive foreshore sequence. Last stages of the upper MIS 5e deposition are seen in a series of regressive dune-beach ridge deposits referred to as the Northeast Ridges unit, with sea-level positions ranging from +4.5 m early in the central part of West Caicos, to +0.5 m on the far northeast coast, eventually building down below present-day sea level. This sea-level history fits well with previously documented records in the Bahamas and globally, and the extensive erosion supports a short-lived (between 126 and 120 ka) intra-MIS 5e relative-sea-level fall. Holocene deposition on West Caicos is restricted to northern and eastern margins of the island as a combination of strandplain and dune ridge mixed skeletal–oolitic units.Using the high-resolution digital elevation model (DEM) and linked digital geologic map makes it possible to compare areas of the different Pleistocene and Holocene units quantitatively and to constrain volumetric assessments and accumulation rates on the island for different units. The peak sediment volume production/accumulation rate—between 6.5 and 8 × 106 m3 or between 2.2 and 2.7 × 106 m3 per ka—is observed in the upper MIS 5e unit. This rate is 5 times greater than the 0.5 × 106 m3 per ka estimated for the Holocene ooid dune-ridge/strandplain complex, suggesting that the combination of higher sea level, greater wave and wind energy, and warmer ocean temperatures may have fostered the higher rate of chemically precipitated sediment supply.

Geomorphic Evolution of a Gravel‐Bed River Under Sediment‐Starved Versus Sediment‐Rich Conditions: River Response to the World's Largest Dam Removal

AbstractUnderstanding river response to sediment pulses is a fundamental problem in geomorphic process studies, with myriad implications for river management. However, because large sediment pulses are rare and usually unanticipated, they are seldom studied at field scale. We examine fluvial response to a massive (~20 Mt) sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States, in an 11‐year before‐after/control‐impact study of channel morphology and grain size. We test the hypothesis that for a given flow magnitude, greater geomorphic change occurs under sediment‐rich conditions than under sediment‐starved conditions. Channel response to flow forcing was significantly different during the sediment‐pulse peak, 1–2 years after dam removal began, than earlier or later. During peak sediment supply our hypothesis was supported; major geomorphic change occurred under low flows and unit stream power ≤60 W/m2. However, by 4–6 years after dam removal began, rates of geomorphic change and sensitivity to stream power had decreased substantially such that our hypothesis was no longer unequivocally supported. These findings are consistent with a two‐phase conceptual model of dam‐removal response, involving a transport‐limited state followed by a more supply‐limited state. From comparisons with other dam removals and natural sediment pulses, we infer that the longevity of sediment‐pulse signals in gravel‐bed rivers depends upon gradient, river discharge, valley morphology, and sediment grain size. Stream power associated with substantial geomorphic change varies with sediment supply, such that assigning a general threshold stream power to gravel‐bed rivers may be untenable.

Turbidity and dissolved organic matter as significant predictors of spatio‐temporal dynamics of phosphorus in a large river‐floodplain system

AbstractPhosphorus (P) inputs are increasing in river‐floodplain systems, but the factors which influence the dynamics of this nutrient are not clear. To assess P dynamics in this kind of river, the main channel of the Middle Paraná River, 3 anabranches, 9 secondary channels, and 20 lakes (7 permanently connected and 13 temporarily connected to the fluvial system) were sampled. Multiple linear regressions were applied to explain spatio‐temporal patterns of P through commonly measured limnological variables. Particulate P increased during the sediment peak (evaluated through turbidity). Soluble reactive P (SRP) was positively associated with dissolved organic matter (DOM, mainly the chromophoric fraction), which increased during high waters in the fluvial system but was highly variable in each kind of aquatic environment. In temporarily connected lakes, vegetated zones dominated by emergent macrophytes displayed the highest SRP and chromophoric DOM concentrations. The flood and sediment peak positively affected P load in the river due to the increase in dissolved and particulate fractions, respectively. In addition, particle‐bound alkaline phosphatase activity was positively associated with SRP concentration and load. These results suggest that the sediment peak incorporates particulate P in the system while the floodplain is a P source during floods through exportation of the dissolved fraction. Dissolved P could be largely exported associated with DOM, which stimulates phosphatase biosynthesis by decreasing P bioavailability. The effect of aquatic macrophytes on P dynamics seems to be influenced by DOM exudation. According to these considerations, DOM should be taken into account to analyse P dynamics in river‐floodplain systems.

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ13C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ13C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1 mg l−1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ13C levels (> − 40‰). In contrast, larvae were markedly 13C–depleted (down to − 60‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.

Impact of river interventions on alluvial channel morphology

Rivers play a significant role for sustainment and development of human race. The number of river interventions is increasing to meet water and energy demand. The present paper discusses impacts of Rengali dam on Brahmani river, India. The first-order impact assessment is carried out with hydrological data analysis between upstream and downstream gauging stations. The second-order impact study integrates seasonal stream power evaluation and temporal variation of Brice Index (BI) and cut-off length at a critical river reach. The flow duration and sediment duration curves at downstream gauging station revealed that, lean season discharge has increased (260–300%) and peak suspended sediment concentration has decreased for downstream reach. The seasonal steam power analysis quantified increasing trend of pre-monsoon stream power (296 W/m to 811 W/m) for the river. The planform alteration study showed initiation of braiding (BI of 0.14 in 1975 to 2.33 in 2016), formation of multiple cut-offs and mid-channel bars predominantly in post-dam period. Two stages of planform responses (1985–2009 and 2009–2016) have been observed for the river after dam closure.

Sediment control function of river notches

The purpose of this study is to investigate the control function and mechanisms of natural river notches. Physical and numerical experiments are analyzed in this study for two representative types of sediment events: high intensity and short duration Type A sediment disaster events, and low intensity and long duration Type B moderate non-disaster events. Two dimensionless parameters, sediment trapping rate and reduction rate of peak sediment transport, are defined to evaluate the sediment control function of river notches. Study results indicate that the contraction ratio of the notch has a significant influence on sediment control function, with high contraction ratios resulting in both high sediment-trapping and high reduction rates. River notches provide better sediment control during Type A events than Type B events. The sediment control mechanism of river notches is the result of multiple interactions among river flow, sediment transport, and riverbed variation. Analysis of these interactions supports the significant protection role of river notches on sediment control for disaster events

Understanding land use and cover change impacts on run‐off and sediment load at flood events on the Loess Plateau, China

AbstractThe Loess Plateau has been experiencing large‐scale land use and cover changes (LUCCs) over the past 50 years. It is well known about the significant decreasing trend of annual streamflow and sediment load in the catchments in this area. However, how surface run‐off and sediment load behaved in response to LUCC at flood events remained a research question. We investigated 371 flood events from 1963 to 2011 in a typical medium‐sized catchment within the Plateau in order to understand how LUCC affected the surface run‐off generation and sediment load and their behaviours based on the analysis of return periods. The results showed that the mean annual surface run‐off and sediment load from flood events accounted for 49.6% and 91.8% of their mean annual totals. The reduction of surface run‐off and associated sediment yield in floods explained about 85.0% and 89.2% of declines in the total annual streamflow and sediment load, respectively. The occurrences of flood events and peak sediment concentrations greater than 500 kg/m3showed a significantly downward trend, yet the counterclockwise loop events still dominated the flood event processes in the catchment. The results suggest that LUCC over the past 50 years resulted in significant changes in the water balance components and associated soil erosion and sediment transportation in the catchment. This was achieved mainly by reducing surface run‐off and sediment yield during floods with return period of less than 5 years. Run‐off–sediment load behaviour during the extreme events with greater than 10‐year return periods has not changed. Outcomes from this study are useful in understanding the eco‐hydrological processes and assisting the sustainable catchment management and land use planning on the Loess Plateau, and the methodologies are general and applicable to similar areas worldwide.

Hydrological and sedimentation conditions in a non-tidal lagoon during ice coverage – The example of Vistula Lagoon in the Baltic Sea

The main objective of this study was to compare conditions for sedimentation in a lagoon in the presence and absence of ice coverage. The winter conditions of Vistula Lagoon (hydrology, suspended solids in water, ice, and snow) were studied during the period 2011–2016. Three-dimensional numerical modelling was used to simulate the water level variations and currents under ice coverage. Satellite images were collected to reproduce the ice-coverage cycle.In general, the hydrological and sedimentation conditions in the winter period without ice coverage were highly similar to those in the warm period of the year. The suspended sediment concentration was 22–32 mg/l.Ice cover eliminates wind forcing, and water exchange with the sea and river discharge become the main forcing factors. Strong vertical salinity stratification is established under ice, and river water intrusion spreads much further under ice in the lagoon area than in the absence of ice.The water level variation in the inlet is the main dynamic factor influencing the water motion under ice throughout the lagoon. Water level variations with a period of less than 12–24 h are damped by the ice cover, and oscillations with a period of longer than 1 day easily spread under the ice.The currents in the lagoon are characterised by vertical stratification with a maximum in the middle layer due to friction at the bottom and at the ice layer, currents are sufficient to produce re-suspension only in the vicinity of the inlet. In the absence of ice, the currents have maximums at the surface and in the near-bottom layer.The suspended solid concentrations under the ice cover (5–10 mg/l) are less than those in the summer (20–35 mg/l), and vertical distribution is characterised by two maximums: near the bottom and at the surface. The sedimentation rate under ice was experimentally estimated as 2–4 mm/year.During the ice period, the rivers are the single sediment sources, and most sediments settle in the close vicinity of the river mouths and are redistributed along the lagoon area only after the ice melt.Air-born sediments stored in the ice and snow cover (on the ice) contain particles of anthropogenic origin (ashes and soot), inorganic mineral substances and organic substances. Because it appears that most ice melts within the lagoon, the sedimentation peak is expected to occur in spring and is caused by both the increase of river inflow and melting of ice.

Analysis on the Runoff and Sediment Yielding of “7.26” Rainstorm in 2017 in the Dali River Basin

Hydrological regimes of the “7.26” rainstorm and flood in 2017 in the Dali River basin are illustrated, the runoff and sediment of the flood is calculated. The relationship between peak discharge, runoff and sediment of history floods and the corresponding mean rainfall and combined rainfall factor are established respectively. It is found that the point of “7.26” flood locates at the upper left or in the middle of the history points group. The results show that after the implementation of water and soil conservation and ecological restoration policy for several decades, the underlying surface of the Dali River basin has changed to some degree, but encountered with the high intensity rainfall such as "7.26", it will still generate the flood of high sediment concentration.

A laboratory study of channel sidewall expansion in upland concentrated flows

Gully erosion contributes large amounts of sediment within watersheds around the world. Gully widening constitutes about 80% of total soil loss, especially in the presence of a plow pan which manifests a less or non-erodible soil layer. Current knowledge on sidewall toe scour (scour arcs) and tension crack processes in gully widening is limited. Thus, simulated channel sidewall expansion tests, where the channel bed was fixed to represent a non-erodible layer, were designed to investigate how inflow rate, slope gradient and initial channel width affect channel widening processes. Soil boxes (2.0 m-long, 0.3 m-wide and 0.5 m-deep) with two slope gradients (15° and 20°), four inflow rates (1.0, 2.0, 3.0 and 4.0 L min−1) and two initial channel widths (4 and 8 cm) were subjected to clear-water overland flow. Photogrammetry was used to detect tension crack and width variations of channels. The results show that sediment delivery and channel width increase with the increase of inflow rate, bed slope and the decrease of initial channel width. Exponential equations were used to predict the channel width time series. Time lag occurred between sediment peak and soil block failure. Toe scour, crack development, sidewall failure and block detachment and transport, in sequence, were the four main processes of channel widening. Basal scour arc length, tension crack length and width decreased with initial channel width and increased with time, flow discharge and bed slope. Basal scour arcs were divided into three patterns according to different shapes in comparison to the failure arcs. Sediment delivery equations based on the disaggregation of concentrated flow entrainment and mass failure were also fitted. This study provides new insight on improving gully erosion measurements and prediction technology.