River Bed Degradation Research Articles

Impacts of grade control structures on riverbed degradation

Sediment flux of many rivers has been significantly reduced due to human activities caused by economic development, leading to increasingly severe riverbed degradation. To prevent riverbed degradation, grade control structures (GCSs) have been widely applied in degrading channels. Existing studies have not provided a good understanding of the effects of GCSs on flow characteristics and bed morphology in degrading channels, limiting the management of degrading channels. A series of flume tests with no sediment supply are conducted to investigate the effects of GCSs on upstream water levels and riverbed morphology in degrading channels. The experimental results indicate that: (1) in the initial stage of degradation, the water surface slope in the backwater reach is linearly and negatively correlated with the GCS-height Froude number, based on the average flow velocity upstream of the backwater reach due to GCS and the height of GCS; (2) the effective protection bed length upstream of GCS is approximately equal to the length of the reach where the flow velocity is less than the critical velocity for sediment motion in the backwater zone; (3) for sequential GCSs, the effective protection bed length will decrease if GCS is located in the backwater reach of the downstream GCS. A semi-analytical calculation method of the effective protection length and equilibrium bed profile upstream of GCS in degrading channels is proposed based on the critical condition of sediment motion and weir flow formulas. The computed values by the proposed calculation method agree well with the experimental data of the present study.

River bed degradation in a compound channel after base level fall

The bed degradation of a compound channel induced by a base level fall can significantly impact the flood conveyance capability and stability of riverine structures. This study found that severe compound channel degradation is due to the coupled effects of flood discharge and base level fall. Laboratory experiments examined the impact of base level fall on the grain size distribution at the bed surface, flow discharge in the main channel and floodplains, and longitudinal profile of the bed elevation. Two riverbed degradation patterns of a compound channel after base level fall were clarified. Relative to the original bed slope, degradation pattern (I) had a higher bed slope, but degradation pattern (II) had a lower one. A calculation method was proposed to determine the critical conditions for identifying these two riverbed degradation types. Both experimental and published data confirmed that the proposed method is valid for identifying these two compound channel degradation patterns.

Dam‐induced riverbed degradation in the Saskatchewan River Delta

AbstractRiver deltas formed by the uninterrupted flow of water carrying sediments and nutrients provide many benefits to humans, including natural resources (e.g., water, aquatic, and terrestrial species) and fertile land. However, the sustainability of river deltas is threatened by large hydraulic infrastructure (e.g., upstream dams), other human activities, and climate change. For example, the Saskatchewan River Delta in Northern Canada is threatened because dams upstream interrupt the sediment transport, resulting in declining sediment levels. The lack of sediment affects the ecology and environment of the delta. This paper will address how dam operation can be adapted to decrease erosion and maintain hydrological, ecological, and environmental outcomes from human activities in the Saskatchewan River Basin, especially the South Saskatchewan River and the river delta. The cross‐sectional data along the Saskatchewan River below the E.B. Campbell Dam were considered for unsteady flow simulations via HEC‐RAS. Insufficient sediment reaches the Saskatchewan River Delta, resulting in ecological changes. The purpose is to give decision‐makers or related stakeholders insight into developing strategies for reducing erosion, replenishing moisture, and restoring sediment in the Saskatchewan River Delta to minimize dam‐induced issues.

Pengaruh Groundsill Pada Degradasi dan Agradasi Dasar Sungai Winongo (Studi Kasus Simulasi Dengan Sedimen D50)

One of the rivers in Yogyakarta with sand as its riverbed material is the Winongo River, which has a high potential for riverbed degradation or agradation. Upstream and downstream of the groundsill construction, respectively, can experience degradation and agradation of sediment transport. In this research, the effectiveness of the MPM, Engelund Hansen, and Laursen Copeland equations on HEC-RAS 6.3.1 is examined in terms of determining the level of overall degradation and agradation of the Winongo River bed. The discharge data used in modeling is in the form of secondary data taken from the 2021 DPUPESDM using two discharge conditions, namely wet discharge (Feb-Mar) and dry discharge (August-Sept). There are 796 cross sections along the river's 41.3 kilometer length, 9 of which are groundsill structures. For all equations, d50 serves as the grain diameter. Based on the results of the simulation, the upstream groundsill frequently agradation while the downstream groundsill tends to degradation. The MPM and Engelund Hansen equations are closer to actual field survey than the Laursen Copeland equation, according to the simulation using the three equations. The nine groundsills on the Winongo River still have the potential to harm the river bank by collapsing the downstream portion of the structure because the condition of degradation in the downstream groundsill is more prevalent than aggradation in the upstream groundsill. Due to the average d50 grain size, it is more likely that models used to predict changes in river bed elevation may degrade

Assessment of River Regime of Chenab River in Post-Chiniot Dam Project Scenario

Dams and reservoirs trap most sediments, and clear water can cause downstream riverbed degradation or aggradation. As a result, the river adjusts its dynamics and channel geometry to regain equilibrium between sediment supply and transport capacity. This study aimed to assess the river regime of the Chenab River in the post-Chiniot Dam Project scenario using a one-dimensional numerical model. After calibration and validation using historic flows and river surveys, simulations were carried out for 5, 10, and 30 years. The sediment model was validated with Brune’s curve, which showed a Nash–Sutcliffe efficiency value of 0.734. The results showed that the river experienced continuous degradation of sediments for the first 16 years and showed a maximum erosion of 8 m at 680 m downstream of the dam. The reach experienced aggradation at 15 km downstream of the dam for the first 10 years and then became stable and showed a maximum deposition of 0.9 m. The ratio of sediments passed through the dam to sediments transported out of reach varied from 0.833 to 0.921, showing that the river reach would continue to attain equilibrium even after 30 years of reservoir operation. The study would be helpful for the prediction of possible future changes in the Chenab River.

Effects of sediment supply on scour at submerged grade control structures

Grade control structures (GCSs) are common river training structures constructed to prevent riverbed degradation. Existing scour predictors for GCSs are based on flume experiments with either no sediment supply or equilibrium sediment transport conditions, the quantitative impacts of sediment supply on the scour at submerged GCSs are still unclear. The present study reports a series of flume experiments to investigate the effects of sediment supply on scour at submerged GCSs. The experimental results show that, compared with the equilibrium sediment transport condition, a decrease of upstream sediment supply rate can reduce the upstream scour depth (down to zero), increase the downstream scour depth (maximumly by 2.3 times) and change the scour process at a submerged GCS. If the sediment supply rate is close to the sediment transport capacity of the flow, the downstream scour depth first increases rapidly and then oscillates obviously around an equilibrium value (i.e. quasi-live-bed scour). If the sediment supply rate is close to zero, the downstream scour depth oscillates initially, then increases monotonically with time towards an equilibrium value (i.e. quasi-clear-water scour). Based on the experimental data and theoretical analysis, the quantitative impacts of sediment supply rate on the equilibrium scour depth at submerged GCSs are evaluated; and calculation methods considering the sediment supply are proposed for estimating the equilibrium scour depth at submerged GCSs.

Sand Harvesting, Environmental Degradation and Livelihoods in North Rift Kenya

In Kenya, thousands of households rely on sand harvesting as their main source of livelihood. Sand harvesting is common in Kenya’s arid and semi-arid areas, but left uncontrolled it depletes water catchment areas, and thus, the need to promote sustainability by striking a balance between it and environmental conservation. This paper illustrates how sand harvesting is affecting the environment in West Pokot County. Findings indicate that sand scooping reduces surface water quality and quantity 251(61%), leads to river bed degradation 311(87.4%), sand harvesting increases erosional valleys 308(86.6%), there is contamination of water and scarcity of water due to sand harvesting 289(81.1%), sand harvesting affects the flow of the river downstream 280(78.7%), reduces land for farming 145(40.7%), storage of sand causes destruction of vegetation cover 214(60%) and destruction of the forest cover 206(48%). The paper concludes that sand harvesting is accompanied by disastrous environmental effects, which raises questions on the cost-benefits and sustainability of the sand harvesting activities in the study area. Consequently, the paper recommends for measures to be put in place to surmount the hazardous environmental effects and enhance the multiplier effects of sand harvesting on livelihood security.

Scour at River-Crossing Cylindrical Structures in Degrading Channels

Channel degradation can expose river-crossing cylindrical structures (e.g., pipelines, canals, and tunnels) that are completely buried in the riverbed, altering their surrounding flow patterns to cause scour that threatens their structural safety. This paper reports a case study of scour at a river-crossing canal in a degrading river. Based on the historical topography and hydrological data, the cause, development process, and performance of countermeasures for scour at the river-crossing canal were analyzed, and recommendations for scour protection are proposed. The findings from 53 experiments of the scour at a river-crossing cylindrical structure in a degrading channel are presented. The experimental results indicate that there are three scour development modes at a river-crossing cylindrical structure in a degrading channel: (1) the exposure of the buried cylindrical structures induces downstream scour, with no tunnel erosion (i.e., scour below the bottom of the cylindrical structure) occurring at the cylindrical structure, (2) the riverbed degradation downstream of the cylindrical structure increases the water head over the cylindrical structure and seepage below the cylindrical structure, inducing tunnel erosion at the cylindrical structure, and (3) tunnel erosion is induced when a large-scale bedform trough approaches the cylindrical structure. Based on the experimental data, the critical conditions of each scour development mode were determined. The effects of the flow intensity, dimensionless water depth, and bed material roughness on the equilibrium scour depth were analyzed, and predictors for estimating the equilibrium scour depth at a river-crossing cylindrical structure in a degrading channel are proposed.

Influence of cohesion on California bearing ratio of clay–gravel mixtures

An understanding of the behavior of cohesive sediment is required to solve various engineering problems such as scour around bridge elements, mitigation of soil erosion, pavement design, river bed degradation, stable channel design. Pavement foundation designers principally use the California bearing ratio (CBR) to describe the subgrade and subbase materials and their strength. Several laboratory experiments were done to study the variation in the CBR of cohesive mixtures comprised of clay–gravel mixtures. Nine different clay–gravel mixtures were used in which the clay content varies from 10% to 50% by weight. The variation of the CBR with clay percentage, moisture content, and undrained shear strength parameters was studied. The CBR value reduces with the increase in the moisture content and clay fraction in the mixtures and increases with an increase in the dry density of the mixture under unsoaked conditions. The CBR also increases with the increase of the angle of internal friction of clay–gravel mixtures. A functional relation has been identified to estimate the CBR of clay–gravel mixtures. Using multiple linear regression analysis (MLRA), a relation is proposed to estimate the CBR of clay–gravel mixtures under unsoaked conditions. A statistical analysis was done to judge the behavior of the pertinent variables on the CBR. The proposed relation predicts the CBR of clay–gravel mixtures very well. Artificial neural network (ANN) analysis using R programming was also done to determine the effects of the pertinent variables on the CBR. ANN methodology was applied to predict the contribution of each variable. Three different methods: Garson algorithm, Olden algorithm, and Lek's profile model are used to assess the influence of variable parameters. The Olden algorithm and Lek's profile both show positive association of cohesion with CBR in an unsoaked condition. The role of moisture content was found to be marginally negative in the Olden algorithm and Lek's profile results. It is found that both the ANN and MLRA models are accurate in predicting the CBR of clay–gravel mixtures. It was further found that the MLRA and ANN models are reliable and rapid tools for correct assessment of the CBR of cohesive soil mixtures using the basic soil properties.

Rainfall Induced Soil Erosion and Sediment Yield Assessment in Upper Brahmaputra River Basin

Abstract Riverbank erosion, aggradation, and river bed degradation are significant threats to most Indian watersheds. A quantitative valuation of the spatial distribution of soil erosion at a watershed level is vital for sustainable watershed planning and management practice. Very few studies can be found that focus on soil loss with sediment yield patterns in the upper Brahmaputra river basin due to massive erosion-deposition processes. Therefore, it is intended to study the soil loss and sediment yield pattern in the upper Brahmaputra river basin at the selected outlet of Majuli Island, located in South Asia. The principal objective is to evaluate soil erosion and sediment yield transported downstream of Majuli Island using the revised universal soil loss equation (RUSLE) and sediment delivery ratio (SDR) models, respectively, for 1979-2014. The study is conducted based on data availability and efficient models like RUSLE and SDR to measure soil erosion and sediment yield. Also, sediment yield maps for 36 years are prepared for the whole catchment considering rainfall as a variable. The maximum area contributing to soil erosion is observed around the hilly region having steep slopes. However, the highest sediment deposition is found near Majuli Island. This study would help take preventive measures for the erosion process and watershed management to minimize soil erosion in the watershed.

Massive Riverbed Erosion Induced by Inappropriate Grade Control: A Case Study in a Large-scale Compound Channel

Sediment flux in rivers has dropped globally in the past decades due to the increasing anthropogenic stressors, leading to significant riverbed degradation that may endanger the instream structures, collapse the banks and affect the riverine environment. To stabilise riverbeds, grade control structures (GCSs) have been widely applied in degrading channels. However, further understanding of the impacts of GCS on large-scale compound channels (with a main channel and floodplains) is needed. This paper reports a failure case of grade control in a large-scale compound channel (Shi-ting River, China), including a comparison of the bed profile measured using the structure from motion (SfM) technique before and after the flood season. The impacts of two improperly designed GCS on the bed morphology of the studied river reach are analysed and the reasons for failure of the present case are discussed based on numerical flow modelling. The results of numerical flow modelling and the measured bed profile indicate that the skewed mainstream flow direction in a curved main channel and the local scour downstream of a high drop GCS (3 ∼ 8 m) led to massive erosion on the floodplain near the GCS, significantly increasing the width and cross-sectional area of the main channel near the GCS. Consequently, the flow bypassed the GCS through the eroded area of the floodplain, leading to the dysfunction of the GCS. Due to the GCS dysfunction, a 5-year return period flood caused over 1.3 million m3 of erosion in the studied river reach (length ≈ 3.1 km), inducing severe bed degradation (up to 10 m) and reducing the width/depth ratio of the main channel upstream of the GCS. Through comparing the present study with the documented cases from literature, it is found that the bed degradation induced by GCS failure in compound channels develops much faster than those induced by sediment mining, weir or dam construction and reafforestation. Based on the analysed failure reasons, recommendations are provided for GCS design in large-scale compound channels.

A new mixing equation for bed material composition in bed form dominant conditions

Vertical mixing beneath the bed surface and its effect on sediment composition has long been underestimated. This paper proposes a mixing equation to illustrate the temporal and spatial variations of bed material composition for bed form dominant rivers. A continuous mass conservation equation for elevation-specific nonuniform bed material composition is initially presented. A vertical mixing equation is finally derived as a diffusion-type partial differential equation with a source term characterized by a vertical diffusion coefficient and the geometric dimensions of bed forms. Coupled with boundary conditions, this equation is further developed in a vertical mixing model at the scale of bed forms, in which the variations of bed material composition are clearly illustrated. The equation accounts for vertical exchange fluxes driven by bed form migration and their effect on bed material composition, which helps elucidate the mixing mechanism. In application to riverbed degradation, the proposed equation is capable to describe armoring development in consideration of the interior mixing, which provides more satisfying predictions for bed surface materials. The new mixing equation helps gain insight into predicting the variations of bed material composition during morphological changes.

Environmental Impacts of Sand Mining in Some Coastal Communities in Port Harcourt Metropolis Nigeria

Sand mining activity is one of the serious environmental problems, as the rivers are widely exploited for river bed materials like sand resulting in land and river bed degradation as well as loss of riparian habitat. The aim of this research was to assess the environmental impacts of sand mining activities in some coastal communities of Port Harcourt metropolis. Three communities namely Choba, Abuloma and Chokocho were selected for the study because of good access to waterfront and regular outcry by communities affected due to the level of environmental degradation. Formal interviews with operators of sand mining activity and stakeholders as well as the administration of questionnaire was employed. Questionnaires were designed to collect information on the characteristics of the sand mining activities in these coastal communities. The study lasted for a period of 12 months. Results obtained show that there were major impacts of sand mining namely soil erosion, road destruction, loss of vegetation, noise pollution across the coastal communities engaged in this activity. Choba recorded soil erosion with 27% as a major problem, Abuloma had noise pollution with 33% while in Chokocho, soil erosion ranked 32% as major impact of the activity. The study therefore, revealed that inspite of its economic importance, sand mining activity is causing more harm than good, hence it is suggested that environmental legislations regarding sand mining activities must be put in place to check and ameliorate the impacts. Community stakeholders should seek constant interface between the sand miners to resolve issues arising from the activity and to proffer intervention measures to ensure sustainability.

Handling of Krueng Jantho Riverbed Degradation with Groundsill Planning

Krueng Jantho river has an important role in the utilization of water resources, watershed conservation, and control of water damage. Dredging of river material has resulted in damage to the ecological factors of the Krueng Jantho river, specifically the degradation of the riverbed which can trigger the collapse of river banks and bridge pillars. The location under review is located in Alue Gintung Village, Seulimuem District, Aceh Besar Regency. Data and maps collected are watershed maps, river discharge data from the Krueng Aceh water level measurement post (AWLR), and topographical and river cross-section data. The method of calculating the generation of discharge data is used the Markov Chain method, the analysis of flood discharge plans using the Pearson Log type III distribution method, and the groundsill building planning for river bottom management and security. The groundsill building for the 25 yearly discharge is planned to be 80.00 m wide, groundsill 1,00 m high and 4.00 m thick groundsill lighthouse.

Regional relationships for bankfull hydraulic geometry and discharge in the southern Andes of Ecuador: An application in the estimation of geomorphological runoff thresholds

The bankfull represents the boundary between the processes taking place in the canal and the floodplain. The geometry of the bankfull is highly correlated with the drainage area, making it possible to establish regional bankfull geometry relationships that have wide application in canal restoration, studies of river bed degradation, dimensioning of infrastructure, hydrological modeling, among others. In this study, the regional bankfull geometry relationships were determined for an Andean zone in southern Ecuador, getting coefficients comparable to those reported in literature determined in other mountainous areas of the continent. Correlation coefficients higher than 0.60 were obtained, considered statistically significant. The geomorphological runoff threshold, which represents the effective amount of rainfall needed to produce bankfull discharges, was determined using regional bankfull geometry relationships, geomorphological characteristics of the drainage basins in the study area, and the NRCS unit hydrograph. The geomorphological runoff threshold varies from 0.8 mm to 8.50 mm and is based on the concept of the unit hydrograph; therefore it presents the same application limitations and uncertainties associated with the determination of the characteristics of the basin, the bankfull and the scale of work. Future works are proposed considering more detailed scales, a greater number of observation sites and the inclusion of additional variables of the drainage area such as slope, aspect and precipitation, to improve the estimates.

Detection of Stage‐Discharge Rating Shifts Using Gaugings: A Recursive Segmentation Procedure Accounting for Observational and Model Uncertainties

AbstractThe stage‐discharge rating curve is subject at many hydrometric stations to sudden changes (shifts) typically caused by morphogenic floods. We propose an original method for estimating shift times using the stage‐discharge observations, also known as gaugings. This method is based on a recursive segmentation procedure that accounts for both gaugings and rating curve uncertainties through a Bayesian framework. It starts with the estimation of a baseline rating curve using all available gaugings. Then it computes the residuals between the gaugings and this rating curve with uncertainties. It proceeds with the segmentation of the time series of residuals through a multi‐change point Bayesian estimation accounting for residuals uncertainties. Once the first set of shift times is identified, the same procedure is recursively applied to each sub‐period through a “top‐down” approach searching for all effective shifts. The proposed method is illustrated using the Ardèche River at Meyras in France (a typical hydrometric site subject to river bed degradation) and evaluated using several synthetic data sets for which the true shift times are known. The applications confirm the added value of the recursive segmentation compared with a “single‐pass” approach and highlight the importance of properly accounting for uncertainties in the segmented data. The recursive procedure effectively disentangles rating changes from observational and rating curve uncertainties.

Experimental Study on Critical Shear Stress of Cohesive Soils and Soil Mixtures

HighlightsErosion tests were performed to study the critical shear stress of cohesive soils and soil mixtures.Linear relationships were observed between critical shear stress and cohesion of cohesive soils.Mixture critical shear stress relates to noncohesive particle size and cohesive soil erodibility.A formula for calculating the critical shear stress of soil mixtures is proposed and verified.Abstract. The incipient motion of soil is an important engineering property that impacts reservoir sedimentation, stable channel design, river bed degradation, and dam breach. Due to numerous factors influencing the erodibility parameters, the study of critical shear stress (tc) of cohesive soils and soil mixtures is still far from mature. In this study, erosion experiments were conducted to investigate the influence of soil properties on the tc of remolded cohesive soils and cohesive and noncohesive soil mixtures with mud contents varying from 0% to 100% using an erosion function apparatus (EFA). For cohesive soils, direct linear relationships were observed between tc and cohesion (c). The critical shear stress for soil mixture (tcm) erosion increased monotonically with an increase in mud content (pm). The median diameter of noncohesive soil (Ds), the void ratio (e), and the organic content of cohesive soil also influenced tcm. A formula for calculating tcm considering the effect of pm and the tc of noncohesive soil and pure mud was developed. The proposed formula was validated using experimental data from the present and previous research, and it can reproduce the variation of tcm for reconstituted soil mixtures. To use the proposed formula to predict the tcm for artificial engineering problems, experimental erosion tests should be performed. Future research should further test the proposed formula based on additional experimental data. Keywords: Cohesive and noncohesive soil mixture, Critical shear stress, Erodibility, Mud content, Soil property.

Long-term effects of dam operations for water supply to irrigation on downstream river reaches. The case of the Ribb River, Ethiopia

ABSTRACT This work investigates the applicability of an analytical method for quick assessments of the long-term morphological effects of different dam operations on downstream river reaches with the idea to apply the method in feasibility studies to identify the least morphologic-impacting operation scenario. The Ribb River (Ethiopia) is used as a study case. The analytical method estimates the idealized, new equilibrium of the river bed profile without considering the duration of the morphological evolution. We apply the analytical method distinguishing sand-bed from gravel-bed reaches. The outcome of the analytical method is compared to that of a calibrated one-dimensional river morphology computer model. The analytical method overestimated the morphological changes compared to the one-dimensional model. By establishing the upper limits of the impact, the analytical method identifies a theoretical maximum river bed degradation near the base of the dam. If all sediment is trapped in the reservoir, the method allows distinguishing the effects of different dam operation scenarios, but only for gravel-bed river reaches. However, the method can also be applicable for sand-bed reaches if there is sediment input from the upper reaches. Further research works should be done to validate both methods if they indeed allow to detect the least impacting scenario, considering that data showing the effects of long-term dam operations on the downstream river reaches are lacking.

The effect of river flow velocity distribution on indications of the occurrence of degradation of the Tambong River basin

Floods that have high flow velocity can be indicated that there is degradation in the riverbed. If the deterioration of the riverbed is allowed to continue, it can cause damage to the existing building infrastructure around the river. Tambong River, located in the Kabat District of Banyuwangi, has experienced river overflowing. Process of measuring flow velocity uses the current meter. To determine the shape and contour of the Tambong River flow velocity distribution using the Surfer Software. In the Tambong River basin, the middle river of the downstream has the highest river flow velocity distribution of 0.92 m/sec, with a discharge of 1.75 m3/sec. It was indicated that the Tambong River Basin in the middle of the downstream would experience river bed degradation by showing a Reynold of 426, 997.33 and a Froude value of 0.55 so that it includes turbulent flow types and flow types subcritical. Average flow velocity distribution in the middle of the Tambong River basin is 0.901 m/sec and was indicated to have degraded as thick as 2.10 m/year. It can be concluded that the middle part of the Tambong river is a flood-prone zone due to the considerable velocity of the river flow.

Scouring of Replenished Sediment through Reservoir Flood Discharge Affects Suspended Sediment Concentrations at Downstream River Water Intake

Dredging is a commonly used sedimentation management strategy to remove mechanically deposited sediment from reservoirs. However, dredged sediment disposal is costly. Dredged sediment can be considered a beneficial resource and used for riverbed replenishment to prevent downstream riverbed degradation and improve aquatic habitats. This study investigated the feasibility of using dredged deposits with cohesive sediment for replenishment at the Shihmen Reservoir. Using the criterion of critical scour velocity, we conducted hydraulic assessments and identified the feasible replenishment area as the experimental domain. A physical model was developed to mimic the scouring process in the replenishment area. By applying dynamic similarity for scouring fine replenished sediment, we derived the regression relationship between flow-critical velocity and sediment-dry density, and used it for model ratio scaling of the grain size, dry density, and concentration in the physical model. Scoured sediment concentrations were measured to study the scour ratio at various flood discharges. Experimental results indicated that the scour ratio was related to factors such as flood discharge, flood duration, and water content of the replenished sediment. The reduction ratio of the concentration of sediment scoured from the replenishment area to the concentration of sediment at the downstream water intake was approximately 90% in the present study.