Sediment Movement Research Articles

Response of the gravel–sand transition in the Yangtze River to hydrological and sediment regime changes after upstream damming

AbstractIn the Yangtze River, the Three Gorges Dam (TGD) has altered the flow‐sediment regimes, resulting in channel degradation and bed surface coarsening, which have been reported in recent years. However, there has been no systematic study on the impacts of the TGD on the downstream gravel–sand transition (GST). Based on detailed field observations and numerical modelling, this study fills this gap and reveals significant adjustments in the GST and the leading causes. The results show that after operation of the TGD began, the GST migrated 49.5 km downstream during 2003–2010 due to a reduction in the suspended‐load supply (grain size < 0.5 mm) and then remained stable from 2010 to 2015 under the control of a braided river morphology. In addition, as channel degradation continued, the steepening bed profile upstream of Chenjiawan and the lowering of the downstream water level caused some gravel within the GST to become more frequently mobile during low‐flow periods (discharge <15,000 m3/s). Combined with the effects of the TGD‐induced prolonged low‐flow season and reduced frequency of large floods (discharge ≥ 40,000 m3/s), the GST exhibited a poorly understood phenomenon in which gravel was mainly transported downstream during low‐flow periods. However, these processes affected only localized sediment movement, contributing little to overall GST migration in the Yangtze River. The results also show that when a low sediment supply persists long term, additional GST migration after transient downstream migration depends primarily on the river morphology. This article has implications for the channel regulation and evolutionary processes of GSTs below dams.

Erratum for “Sediment Movement over Type A Piano Key Weirs” by Binit Kumar, Subhojit Kadia, and Zulfequar Ahmad

Erratum for “Sediment Movement over Type A Piano Key Weirs” by Binit Kumar, Subhojit Kadia, and Zulfequar Ahmad

Eulerian–Eulerian multiphase models for simulating collapse of submarine sediment column with rheological characteristics in air–water flow

Submarine massive sediment movement, which may devastate submarine infrastructure and cause huge waves threatening the coastal areas, combined with the air–water interface is a significant issue of the multiphase flows in the areas of ocean engineering and geoscience. Currently, simulating rheology-based sediment flows is still a challenge because of the complex intergranular dynamics and its coupling with computational fluid dynamics. In particular, air–water–sediment multiphase models that can simulate such rheology-based flows while also taking into account the accurate capture of the air–water interface have not been developed yet. Coupling with the coupled level set method and volume of fluid method in the air–water two-phase flow, this study develops two new Eulerian–Eulerian multiphase models for simulating the rheology-based sediment flows. The difference between the two developed models is that how to model the drag force of the particle–fluid interactions. To demonstrate their capabilities, the two developed models are employed to simulate the gravitational collapse of submarine sediment column in air–water flow with loose-packing case and dense-packing case. The results of collapse process, induced air–water interface fluctuation, and contractancy/diltancy behavior are all agree well with those in previous experiments and simulations. In addition, comparing the results of two developed models, it can be concluded that different modeling methods of the drag force not only directly affect the water–sediment interactions, but also indirectly influence the air–water interface fluctuations and the intergranular stresses remarkably.

Morphodynamics and Evolution of Estuarine Sandspits along the Bight of Benin Coast, West Africa

It is well known that estuarine systems are significantly affected by hydrodynamic conditions such as river discharge, storm surges, waves and tidal conditions. In addition to this, human interferences through developmental projects have the capability of disrupting the natural morphological processes occurring at estuaries. In West Africa, the goal to improve standards of living through large-scale dam construction, offshore ports and coastal erosion countermeasures has triggered alarming changes in the morphodynamics of estuarine systems. The estuaries at the Volta River mouth (Ghana) and “Bouche du Roi” inlet (Benin), located along the Bight of Benin coast, West Africa, were selected as two case study sites to examine their long-term morphodynamics and sandspit evolution. In this study, we primarily analyzed estuarine morphology using remotely sensed images acquired from 1984 to 2020. We further estimated the longshore sediment transport for this region using results from the image analysis and the depth of active sediment motion. Our results reveal that the longshore sediment transport rates for this region are in the magnitude of 105–106 m3/year. Comparative analysis with other estuaries and sandy coasts suggests that the longshore sediment transport along this coast has one of the largest rates estimated in the world.

An Eulerian two-phase flow model investigation on scour onset and backfill of a 2D pipeline

The capability of an Eulerian two-phase model, SedFoam, in simulating the onset of scour underneath a pipeline and the backfill process is investigated. When a pipeline is slightly buried in the sediment bed, the scour onset can be caused by the piping process, which is due to seepage flow moving underneath the pipeline driven by the upstream–downstream pressure difference. To directly resolve piping as part of the scour simulation has been a challenge in the single-phase models. Alternatively, the two-phase models may be capable of simulating piping and backfill as it can resolve the interactions between the flow, structure, sediment transport, and seepage flow using a single set of governing equations and closures. To prove this point, SedFoam is validated by two laboratory experiments for the onset of scour underneath a pipeline. For piping driven by a prescribed upstream–downstream pressure difference, the model captures the temporal evolution of the pore-pressure gradient and the resulting fine-scale bathymetric change around the pipe consistent with the measured data. The model further provides insight into the seepage flow and the creeping movement of sediments during the onset of scour. When simulating piping driven by a unidirectional current, SedFoam successfully predicts piping driven by the upstream–downstream pressure gradient due to the incoming flow deceleration by the presence of pipeline and flow separation. As a proof-of-concept application, SedFoam is applied to simulate pipeline scour driven by an oscillatory flow. Although the boundary layer streaming effect is neglected, the model result shows realistic scour onset and the development of a scour hole. To demonstrate the model’s capability to simulate the backfill process, the pipeline is then lowered artificially into the scour hole as an idealized treatment of the complex pipeline sinking process during scour. The resulting burial depth due to backfill is similar to that predicted by the empirical formula.

Evaluation of Longshore Drift on the East Coast of Maengbang Beach (Gangwon, Korea) through Wave and Depth Changes

Kim, G.-S.; Ryu, H.-S., and Kim, S.-H., 2021. Evaluation of longshore drift on the east coast of Maengbang Beach (Gangwon, Korea) through wave and depth changes. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 499–503. Coconut Creek (Florida), ISSN 0749-0208. Numerical simulation of the sediment by the Delft3d model was conducted to examine the changes in the longshore drift transport caused by long-term wave changes at Maengbang Beach. Representative waves were identified with input reduction tools using NOAA NCEP wave data of about 40 years (i.e., January 1979 to May 2019). To verify the adequacy of the model, simulated wave and depth changes were compared with corresponding observed data for about 23-month period (from March 2017). Error analysis showed a bias of 0.05 and root mean square error of 0.23, indicating that the model results were satisfactory. Further, the observed and the simulated values of change in depth were similar. To examine the effects of long-term changes in wave characteristics, the NOAA wave data were classified into representative wave grades, and the annual trends of the representative waves were analyzed. After assessing the weight of each wave class considering the projected wave environment for year 2100, the amounts of sedimentation, deposition, and longshore drift transport were assessed for the same period. Sedimentation pattern would not change significantly compared to the current state, and the amount of the local longshore drift will likely decrease. While a local increase in the Longshore drift transport may occur, the overall drift and the amount of sediment movement are likely to remain at current levels. Regarding the coastal longshore drift under climate change, the study considered the average sea level rise as per scenarios RCP 2.6, RCP 6.0, and RCP 8.5 (IPCC, 2013). The net amount of sediment movement and its variability are projected to increase with the rise in sea level for most areas. Regarding the amount of sediment movement, local movement at the center of the coast will likely decrease compared to the present state, and, consequently, waves would have a greater impact on the edge due to sea level rise.

Cohesive forces between sediment particles and its impact on incipient motion of sediment: a review

The understanding of the incipient motion of cohesive sediment is a great challenge in the research of sediment incipient motion. It is recognized that cohesive forces are critical in determining the characteristics of cohesive sediment incipient motion. Based on the analysis of current research, cohesive forces are summarized in two main categories (containing six types) according to the formation mechanism, and the formation mechanism and influencing factors of the typical electric double layer force and EPS cohesive force are explored. The impact of cohesive forces on cohesive sediment incipient motion is examined in terms of two aspects, i.e. sediment stability and incipient motion modes. Based on the discussion above, it is concluded that current studies generally focus on one certain cohesive force and seldom take the interaction and comprehensive effect between cohesive forces into consideration, lacking systematicness. Moreover, most of the current research achievements based on measured data are empirical and short of quantitative relationship derived from mechanism level, suffering from a theoretical deficiency. It is suggested that the various cohesive forces should be considered comprehensively in line with the specific sedimentary environment. What's more, the formation mechanism research of cohesive forces should be concerned fundamentally, and the quantification research of cohesive forces needs more attention.

Geomorphological information mapping of debris-covered ice landforms using Google Earth: An example from the Pico de Posets, Spanish Pyrenees

Geomorphological mapping records ‘snapshots’, making it difficult to investigate change and sensitivity in changing environments. The high resolution now available in Google Earth (GE) permits re-investigation of landscape features/landforms previously recorded in the literature or mapped. Transects across geomorphologically interesting locations can be provided with decimal degree WGS 84 locations and bearings via GE. Landform elements are identified, located and coded onto transects that summarise geomorphological information. Transects crossing rock cliffs, scree slopes, snowpatches, protalus ramparts, ‘vanishing’ glaciers, moraines and rock glaciers are investigated using GE on the Pico de Posets in the Spanish Pyrenees. Observations of geometry/form, materials and processes involved in ice and sediment movement are linked to the Randolph Glacier Inventory and map records. The transect mapping shows that accumulated weathered rock debris insulated a remnant Little Ice Age glacier that formed a lateral moraine and a contiguous rock glacier as the glacier wasted down. Landforms previously considered to indicate permafrost are shown not to do so. Glacigenic formation of moraines, protalus rampart and rock glaciers provides the simplest explanation for their origin.

Three-dimensional direct numerical simulation of flow induced by an oscillating sphere close to a plane boundary

The dynamic interaction of submerged spherical structures with ocean waves alters the local flow field and is responsible for the movement of sediments over the seabed, which would have detrimental effects on the ecosystems. The present research aims to gain insight into the effect of a plane boundary on the vorticity dynamics and shear modifications on the plane boundary induced by the flow due to an oscillating sphere. The flow field around an oscillating sphere close to a plane boundary is investigated numerically by three-dimensional direct numerical simulation of the Navier–Stokes equations. The sphere oscillates perpendicular to the plane boundary modeling the seabed. The flow structure, resulting mean and fluctuating shear stress distribution on the plane boundary, and the Lagrangian particle transport are investigated as a function of the sphere oscillation amplitude, Reynolds number, and the distance of the sphere from the plane boundary. The systematic correlations for the variation of maximum shear stress on the plane boundary are estimated from scaling parameters. The time-mean results are then linked to the Lagrangian particle transport where it is observed that the Lagrangian path lines track the mean streamlines closely. Further, the particles at the stagnation zone cease drifting and oscillate about a constant mean position. The practical significance of this result is that a ring of zero mean shear on the seabed is found toward where particles congregate. This provides a guide for the optimum location for the mooring anchors, minimizing the risk of anchor scour or undermining.

Review on Coastline Change and Its Response Along the Cotonou Coast, Benin in the Gulf of Guinea, West Africa

The global surface temperature has risen critically over the past century and according to the IPCC Fifth Assessment Report 2014, existing risks in natural and human systems will worsen. Coastal erosion is mostly caused by climate change and among all the coastal areas at risk, Benin, which is part of the Gulf of Guinea, has been ranked very highly as a vulnerable region. Therefore, in this review, we focus on the evolution of coastline change in Cotonou of Benin, summarizing its resultant impacts and applied measures around the coast area by reviewing previous studies. Signs of coastal erosion in Cotonou appeared in 1963. After 39 years, the east shoreline of Cotonou has retreated by 885 m, resulting in the disappearance of more than 800 houses. To solve this problem, Benin authorities built seven groynes in 2013, and have increased the number of the structure as a way to interrupt water flow and limit the movement of sediment. Over the region, shorelines appeared preserved accordingly. In contrast, areas located further east, where groynes were not installed, have suffered from intensive erosion at a rate of 49 m/yr. In the future, as a next step, the effectiveness of groynes should be studied with local and broader perspectives.

Geo-ecological cues for mass nesting synchronization of Olive Ridley turtles along Rushikulya estuary in Odisha, east coast of India

In this study, we have analyzed how geo-ecological cues for endangered Olive Ridley turtles’ mass nesting behavior got modified by impact of four severe cyclones during 2010-2019 that made landfall in the vicinity of Rushikulya estuary, which is one of the largest mass nesting congregation (arribada) sites in the world. Analyzing last 10 years of shoreline dynamics, we show that even the slightest modification in beach morphology influenced their nesting behavior in Rushikulya rookery. Shoreline change analysis showed periodic phases of high/low erosion and the northward longshore sediment movement, which becomes impeded by the southern spit, the length of which increased by about 1800 m. During the analyzed period, the nesting behavior of Olive Ridley turtle was greatly influenced by changes in land use and land cover pattern around the Rushikulya rookery. Such reductions in tree cover and marshy land areas were majorly driven by anthropogenic activities and extreme weather events, such as cyclones. We also report increased mortality of turtles, no or false mass nesting events due to significant loss and/or erosion of the nesting sites due to cyclones. The results indicate that conservation of Olive Ridley turtles should be more holistic, or ecosystem centric, rather than species centric. It is important to maintain the ecological integrity of their habitat for highly synchronized mass nesting event and eventually their survival.

Release flux of heavy metals from river sediments at different flow rates

Abstract This paper simulates sediment motion under different hydrodynamic conditions, aiming to investigate the release flux of heavy metals in river sediments. During the lab experiments, carried out in a circular rectangular flume device, water velocity in the flume was altered by controlling the gate switch, and the flow rate was controlled from 0 to 1 m/s. Sediment from the Le'an River and chlorine-removed tap water were used as experimental sediment and water, respectively. Through analyses of Cu, Zn, Cd, and Pb concentration in water at different flow rates, the relationship between the release flux (y) of Cu, Zn, Cd, and Pb and the flow rate (x) was established with a fitting error of less than 15%. In order to judge the reliability of the conclusions, experimental results were verified outdoors. The results showed when the sediment particle size is between 0 and 250 μm, within 1 hour, a quadratic polynomial correlation between the release flux of Cu, Cd, and Pb from river sediments and water velocity when the water pH is 5–9 and the flow rate is 0–65 cm/s; when the water pH is 5–9, the flow rate is 0–35 cm/s, the release flux of Zn from river sediments was shown to have a quadratic polynomial relationship with water velocity. The error between the calculated and measured values of heavy metals released from sediment in the Le'an River were within 5–30%. Our results can provide a theoretical reference for the control and treatment of heavy metal pollution in rivers and further improve corresponding water quality models.

Against all odds: Harbor porpoises intensively use an anthropogenically modified estuary

AbstractHarbor porpoises (Cetacea) are present in the North Sea throughout the year but periodically enter adjacent estuaries, which due to human activities are among the planet's most threatened aquatic systems. However, the occurrence of harbor porpoises in estuaries has rarely been studied. In this work, harbor porpoise occurrence at two stations in the anthropogenically modified Ems Estuary (Germany, Netherlands) was modeled using a machine learning approach (Random Forest) that drew on 8 years of acoustic monitoring data with C‐PODs together with environmental data. Harbor porpoises were present year‐round at both stations. According to the models, their detection was mainly explained by season, tide, and noise level, with the highest detection probabilities in spring, at high tide, and at low noise levels. The seasonal and tide‐dependent occurrence of harbor porpoises coincided with prey availability. Presumed feeding activity was detected in 47% of all harbor‐porpoise‐positive 10 min blocks and indicated the importance of the estuary as a regular feeding area. The elevated noise levels detected at one station were attributed to tidal‐induced currents and sediment movements. The results of this study can help to improve estuarine management through measures that include conducting dredging and disposal activities when harbor porpoise occurrence is less likely.

Measurement of Turbulent Flows and Shear Stress on Open Channels

The phenomenon of turbulent flows becomes a virtual object in any changes in open channel flow hydraulics. Turbulent flow and shear stress have a role in the geometrical changes of bed channel and sediment movement. The dynamics of turbulent flow are consequences of hydraulic channel dynamics. Turbulent flow has excessive kinetic energy resulting in resistance force because the increase of friction effect and infraction in turbulent flow creates a complex phenomenon. Shear stress is in the eternal pressure of flow against the deformation of the primary basic form of channel. The research aims to analyze turbulent flow, shear stress, and bed scours’ phenomena and potential. Measurement of turbulent flows is by measuring the flow velocity in four segments at a distance of 100 cm each. The channel’s cross-section is divided into nine parts and five measurement points in the flow depth of inner and outer regions. There are three variations of channel discharge and slope, i.e., low discharge (Q1), medium discharge (Q2), large discharge (Q3), and downward slope (S1), medium slope (S2), and high pitch (S3). The parameter of turbulent flow analysis, shear stress includes flow velocity average (U), flow depth (h), channel slope (S), viscosity (φ), the mass density of the liquid (ρ), the characteristic length or hydraulic radius (L/R) by using an empirical equation approach. Turbulent flow analysis used dimensionless Reynolds’ number equation approach. The effect of hydrodynamic on turbulent flow causes the distribution of shear and scour stress, transport, and sediment deposition. The increase in the slope of the channel affects the increase in the values of shear stress.

On the use of IMU (inertial measurement unit) sensors in geomorphology

AbstractInertial measurement units (IMUs) are mobile sensors assemblies constructed using a combination of MEMS (micro electrical mechanical systems) accelerometers, gyroscopes and magnetometers. Both the technology and its applications to geomorphic problems are developing rapidly, since they demonstrate the prospect of monitoring individual sediment grains, of various sizes, during transport and at high frequency. This prospect has numerous implications which range from hazard identification and warning to complex theoretical derivations for sediment transport modelling. At the same time, the deployment of IMUs needs to be underpinned by a number of technical considerations regarding the limitations of the technology and the physics of the inertial measurements. IMU measurements should be reported in a manner that allows for clear understanding of the scope of the study, with sufficient detail for repeatability and clear error characterization. At a secondary level, IMU measurements should be linked clearly with the physics of sediment motion. Here the author highlights five technical issues which can lead to the misinterpretation of IMU measurements. His scope is to begin a dialogue towards a collective agreement on a presentation/reporting protocol for IMU measurements in geomorphic studies that will allow for the coherent contextualization of the technology and accelerate its scientific impact within geosciences.

The propagation of wave scour along the spanwise direction of submarine pipelines in case of clear-water regime

The local sediment motion around cylindrical structures laid on erodible seabed occurs with wave and/or current effects and causes significant stability problems. In this study, the development of the scour hole along the spanwise direction of the pipeline and the scour propagation velocity were investigated experimentally. The time dependent measurements beneath the test pipe were performed in a large wave channel. In the experiments, the measurements were recorded by using ultrasonic equipment. Since the coarse bed materials were used, all of the experiments were carried out in the clear-water scour regime condition, differently than live-bed data in literature. The experimental results reveal that the scour propagation along the spanwise direction of the submarine pipe is not relevant with Shields parameter under clear-water condition; however, it is inversely related to the Keulegan-Carpenter number. By analyzing the findings of the study, a new empirical relation was established to estimate the non-dimensional scour propagation velocity based on the Keulegan-Carpenter number along the spanwise direction of submarine pipe, in case of wave effect. The statistical significance of the equation was demonstrated by the correlation coefficient and scatter index.

Experimental Analysis of Incipient Motion for Uniform and Graded Sediments

So far, few studies have focused on the concept of critical flow velocity rather than bed shear stress for incipient sediment motion. Moreover, few studies have focused on sediment mixtures (graded sediment) and shape rather than uniform sediment for incipient motion condition. Different experiments were conducted at a hydraulic laboratory at the University of Guilan in 2015 to determine hydraulic parameters on the incipient motion condition. The aim of this study is to conduct a comparison between uniform and graded sediments, and a comparison between round and angular sediments. Experiments included rounded uniform bed sediments of 5.17, 10.35, 14 and 20.7 mm, angular uniform sediment of 10.35 mm, and graded sediment. Results demonstrated that angular sediment has a higher critical shear velocity than rounded sediment for incipient motion. Results also showed that for a given bed sediment, although critical shield stress and relative roughness increased with the bed slope, the particle Froude number (based on critical velocity) decreased. In terms of the sediment mixture, the critical shear stress (Vc*) was higher for the graded sediment than for the three finer uniform sediment sizes. The finer fractions of the mixture have a higher particle Froude number than their corresponding uniform sediment value, while the coarser fractions of the mixture showed a lower stability than their corresponding uniform sediment value. Results demonstrated that the reduction in the particle Froude number was more evident in lower relative roughness conditions. The current study provides a clearer insight into the interaction between initial sediment transport and flow characteristic, especially particle Froude number for incipient motion in natural rivers where stream beds have different gravel size distribution.

3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs

Deposits in dam areas of large reservoirs, which are commonly composed of fine-grained sediment, are important for reservoir operation. Since the impoundment of the Three Gorges Reservoir (TGR), the sedimentation pattern in the dam area has been unexpected. An integrated dynamic model for fine-grained sediment, which consists of both sediment transport with water flow and gravity-driven fluid mud at the bottom, was proposed. The incipient motion driven by gravity in the form of fluid mud was determined by the critical slope. Shallow flow equations were simplified to simulate the gravity-driven mass transport. The gravity-driven flow model was combined with a 3D Reynolds-averaged water flow and sediment transport model. Solution routines were developed for both models, which were then used to simulate the integral movement of the fine-grained sediment. The simulated sedimentation pattern agreed well with observations in the dam area of the TGR. Most of the deposits were found at the bottom of the main channel, whereas only a few deposits remained on the bank slopes. Due to the gravity-driven flow of fluid mud, the deposits that gathered in the deep channel formed a nearly horizontal surface. By considering the gravity-driven flow, the averaged error of deposition thickness along the thalweg decreased from −13.9 to 2.2 m. This study improved our understanding of the mechanisms of fine-grained sediment transport in large reservoirs and can be used to optimize dam operations.

The Beautiful and the Dammed: Defining Multi-Stressor Disturbance Regimes in an Atlantic River Floodplain Wetland

Natural hydrological fluctuations within river floodplains generate habitat diversity through variable connections between habitat patches and the main river channel. Human modification of floodplains can alter the magnitude and frequency of large floods and associated sediment movement by interrupting these floodplain connections. The lower Wolastoq | Saint John River and its associated floodplain wetlands are experiencing anthropogenic disturbances arising from climate change, increased urbanization in the watershed, changing upstream agricultural landscape practices, and, most notably, major road and dam construction. By comparing digitized aerial images, we identified key periods of change in wetland extent throughout an ecologically significant component of the floodplain, the Grand Lake Meadows and Portobello Creek wetland complex, with significant erosion evident in coves and backwater areas across the landscape following dam construction and significant accretion around the Jemseg River following highway construction. Connectivity and hydrological regime also influenced other habitat components, namely nutrients and metals retention, as well as the composition of the local macrophyte community. These findings address two key aspects of floodplain management: (1) understanding how hydrological alteration has historically influenced floodplain wetlands can inform us of how the ecosystem may respond under future conditions, such as climate change, and (2) the mechanisms by which habitat diversity and disturbance regimes filter biological communities, with the potential for patches to host a rich biodiversity continuously supporting critical ecosystem functions.

Probabilistic description of coarse particle motion above threshold by particle tracking velocimetry method in an experimental study

Sediment motion behavior plays an important role in the sediment and hydraulic engineering, though its physics is still not fully understood. Ignoring the stochastic nature of the sediment transport leads to various equations for bedload transport which are now being challenged due to their results. In this study, the non-suspended particle motion (bedload transport) in different hydraulic conditions was assessed by a particle tracking technique called the Particle Tracking Velocimetry (PTV). The results of the PTV were applied to describe the particle behavior throughout the probability distribution functions. Knowing the particle motion behavior would be a guidance to learn more about the parameter/s governing the particle transport in different sediment transport regimes. After calibrating and validating the frames (resulted from the PTV), the instantaneous particle velocity was measured. Different probability distribution functions were assessed with Kolmogorov-Smirnov criterion to find the best function which fits the collected data (i.e. the particle velocity). It was shown that the probability distribution function is Log-Normal for lower particle Reynolds number and on the other hand, in the higher particle Reynolds number, the Normal distribution is best describing the particle velocity. The results of this research also could be applied in similar hydraulic conditions in eco-hydraulic field, specifically macro-plastic movement as bedload in river courses.