Changes In Channel Morphology Research Articles

Modelling of Sediment Exchange between Suspended-Load and Bed Material in the Middle and Lower Yellow River, China

The focus of this paper is on studying novel approaches to estimate sediment exchange between suspended-load and bed material in an unsteady sediment-laden flow with fine-grained sand. The erosion-deposition characteristics of the channel have close relation with the variation of size compositions of both suspended-load and bed material. These aims are addressed by deducing the sediment exchange equations from the mass conservation perspective and establishing a river-sediment mathematical model based on the theory. The model is applied in the middle and lower Yellow River, China, and calibrated and verified under both deposition and erosion conditions using a generalized channel and a large quantity of measured data in the Yellow River basin. The results indicate that the grading curves of suspended-load and bed material calculated by the mathematical model are close to those of the measured data. The temporal and spatial variations in the mean sizes of suspended-load and bed material, flow rate, sediment concentration and erosion or deposition volume estimates during the entire flood process can be accurately predicted. The model performance is considered acceptable for determining the sediment exchange process and the change in channel morphology for unsteady sediment-laden flow.

Stream channel restoration increases climate resiliency in a thermally vulnerable Appalachian river

We quantified stream temperature response to in‐stream habitat restoration designed to improve thermal suitability and resiliency of a high‐elevation Appalachian stream known to support a temperature‐limited brook trout population. Our specific objectives were to determine if: (1) construction of deep pools created channel unit‐scale thermal refugia and (2) reach scale stream channel reconfiguration reduced peak water temperatures along a longitudinal continuum known to be highly susceptible to summer‐time warming. Contrary to expectations, constructed pools did not significantly decrease channel unit‐scale summer water temperatures relative to paired control sites. This suggests that constructed pools did not successfully intercept a cool groundwater source. However, we did find a significant effect of stream channel restoration on reach‐scale thermal regimes. Both mean and maximum daily stream temperatures experienced significantly reduced warming trends in restored sections relative to control sections. Furthermore, we found that restoration efforts had the greatest effect on stream temperatures downstream of large tributaries. Restoration appears to have significantly altered thermal regimes within upper Shavers Fork, largely in response to changes in channel morphology that facilitated water movement below major cold‐water inputs. Decreased longitudinal warming will likely increase the thermal resiliency of the Shavers Fork main‐stem, sustaining the ability of these key large river habitats to continue supporting critical metapopulation processes (e.g. supplemental foraging and dispersal among tributary populations) in the face of climate change.

Lateglacial changes in river morphologies of northwestern Europe: An example of a smooth response to climate forcing (Cher River, France)

Climatic oscillations occurring during the Pleniglacial and Lateglacial periods are generally associated with several changes in fluvial morphology and processes. This study investigated two reaches of the Cher River basin (Noyers-sur-Cher and Thénioux) regarding morphological adjustments during the Lateglacial period and the early Holocene. Transitions from the Pleniglacial to the Lateglacial and from the Lateglacial to the early Holocene (ca. 14,700–11,500 cal. BP) are considered as interesting study cases because it consists of rapid and abrupt climate oscillations generally clearly recorded in current floodplains. The sites studied are located in the Cher River floodplain, which is one of the main tributaries of the Loire River. Several investigations have been conducted in the main valley, but few of them specifically focused on geomorphological evolutions of medium-sized catchments such as tributaries of the Loire River. Therefore, we examined this specific period through investigations recently conducted in the middle valley of the Cher River. This study aimed to (1) provide a geomorphological approach to fluvial adjustments during the Lateglacial from cases located in lowland areas of a medium-sized basin and (2) analyse the spatio-temporal variability of fluvial responses. Assessment of geomorphological trajectories of the two reaches studied highlights that fluvial metamorphosis occurring during the Lateglacial and early Holocene seems to be characterised by lateral readjustments and a low signal of morphological instability (shallow incision and aggradation processes, slow and progressive change in channel morphology). This observation is enhanced by the comparison with known geomorphological evolution patterns of adjacent basins. The intrinsic components of the Cher basin such as slope and valley width were considered as explanatory factors for the lack of morphogenetic processes. In addition, the geographical location of the reaches studied may also have led to a moderated expression of climate forcing. Indeed, the elevation as well as the north– south and east–west gradients may contribute to the low level of geomorphological adjustments in response to these climatic oscillations. This gradient could also have support this the fluvial response because of the weakly developed loess cover, which may have limited the fine sediment supply and the aggradation of the valley bottom.

Seasonal morphodynamic evolution in a meandering channel of a macrotidal estuary

The spatio-temporal morphodynamics in relation to the fortnightly tidal variations and seasonal river discharge variations in a meandering estuarine channel were investigated. Intensive topographic surveys (longitudinal and transverse) every 3–4 months using a digital sonar system equipped with a differential global positioning system and continuous monitoring of salinity and turbidity with moored instruments were carried out during 2009–2012 in the upstream section of the highly turbid macrotidal Chikugo River estuary, Japan. The results revealed that the tidal flow dominated in the estuary for most of the year except for the rainy season in which river flow dominates. During the low flow season, tidally induced net upward sediment transport dominates and deposition takes place in the upstream section especially in the inner part of the meander. It contributes towards the strengthening of the existing estuarine turbidity maximum (ETM) in the upstream section of the estuary, and the channel capacity reduced gradually during this period. However, large flood events led to the breakdown of this ETM zone in the upstream during the rainy season and exported the sediments to the downstream tidal flat which resulted in the rapid increase of channel capacity. This further exhibited that the ETM zone was migrated to the downstream part of the estuary during the rainy season.There were significant differences in the seasonal trends of morphological evolution in the estuary which further greatly influenced the channel capacity. The drastic changes in channel morphology due to the sediment export by the high river flow during the flood season was approximately recovered through the sediment import by the fortnightly tidal cycle during the dry season over a seasonal basis. The study concludes that the morphological changes in the tide dominated estuaries are strongly influenced by the fortnightly tidal variations as well as the seasonal river discharge variations.

Multidecadal Geomorphic Evolution of a Profoundly Disturbed Gravel Bed River System—A Complex, Nonlinear Response and Its Impact on Sediment Delivery

AbstractA 2.5‐km3 debris avalanche during the 1980 eruption of Mount St. Helens buried upper North Fork Toutle River valley and reset the fluvial landscape. Since then, a new drainage network has evolved. Cross‐sectional surveys repeated over nearly 40 years at 16 locations along a 20‐km reach of river valley document channel evolution. We analyze spatial and temporal changes in channel morphology using two new metrics: (1) a shape index that defines the degree of U‐shaped or V‐shaped valley geometry and (2) an alluvial phase space diagram that relates bed degradation or aggradation to increases or decreases in cross‐sectional area. Unlike a simple, linear response model previously proposed, our analysis reveals channel development has been distinctly nonlinear and nonsequential. Rather than following a sequential trajectory of (1) channel initiation and incision, (2) aggradation and widening, and (3) episodic scour and fill with little change in bed elevation, long‐term channel evolution has been more complex with vertical and lateral adjustments intertwined throughout. Our analysis reveals channel evolution has followed a complex trajectory that has migrated nonsequentially through several phase space domains including degradation and aggradation with widening and narrowing, bed‐level fluctuations with little change in cross‐section area, and changes in cross‐sectional area with little change of bed elevation. Persistent channel widening and reworking of the channel bed are responsible for maintaining elevated sediment delivery from this basin. Elevated sediment delivery is likely to persist until valley floor widths greatly exceed that of the channel migration zone, and/or channel slopes and valley walls stabilize.

The Impacts of Water Abstraction in Tropical Rivers: A Case of South West Upper Tana Basin, Kenya.

The study to determine the impacts of water abstraction in South West Upper Tana Basin which is part of the larger Upper Tana basin was undertaken in four major rivers, Thika, Kimakia, Kiama and Chania. Data for this study was obtained from the Kenya Meteorological department (KMD), Water Resources Management (WRA) and questionnaire surveys. The questionnaires were administered in the period between January 2018 and December 2018 to determine the impacts of water abstraction in both dry and wet seasons. Regression and correlation analysis was used to determine the nature and strength of relationships between different variables. The results of the study indicate that the main impacts of water abstraction are hydrological, environmental and socio-economic. The hydrological impacts include reduced water levels, changes in stream morphology, decreased turbidity and reduced siltation. Socio-economic impacts include increased income from crop production and conflicts while environmental impacts were mainly changes in riparian vegetation. The study established that the best combination of impacts of water abstraction in the South West Upper Tana Basin were changes in water turbidity, decline in water levels, improved crop yields, changes in channel morphology and siltation with a coefficient correlation r of 0.57 and coefficient of determination R 2 of 0.52. To mitigate these impacts, there is a need for better enforcement of legislations on water abstraction and the monitoring of the river basins to curb water over-abstraction. Public awareness campaigns and involvement of stakeholders could also minimize these impacts. Keywords: Water abstraction, hydrological impacts, socio-economic impacts, environmental impacts, South West Upper Tana Basin (SWUT). DOI : 10.7176/JEES/9-5-08 Publication date :May 31 st 2019

Comparison of different spatial interpolation methods for historical hydrographic data of the lowermost Mississippi River

ABSTRACTThe lowermost Mississippi River (LMR) is important to the environment and economy of continental United States. Although bathymetric data have been collected over many decades at numerous cross-sectional sounding points, there has been no consensus on appropriate interpolator for generating bathymetry. Such interpolation is critical to reliable assessments of channel morphology and channel change, which serve for dredging, engineering projects, and mapping of navigation hazards. This study aimed to identify an optimal spatial interpolation for mapping the river bathymetry from cross-sectional sounding measurements. We evaluated a variety of spatial interpolation methods including Inverse Distance Weighting (IDW), Ordinary Kriging (OK), Radial Basis Function (RBF), and Local Polynomial Interpolation (LPI). In addition, we also considered the anisotropic form of IDW (Elliptical IDW as EIDW), that of OK (OKA), and Universal Kriging (UK). Two reaches in the LMR, located between approximately RM (River Miles) 170–140 and RM 60–35, were chosen as the study area. Those interpolators were compared in terms of root-mean-square error (RMSE), mean absolute error (MAE), bias, and coefficient of determination (r2). Our results demonstrate that both of RBF and OKA performed the best in mapping the bathymetry of study reaches. Furthermore, our results also indicate that the addition of anisotropy can significantly reduce RMSE by 5–20%, as compared to isotropic methods. The findings better inform other researchers on selecting a proper interpolation technique for mapping river bathymetry, particularly for other reaches of the Mississippi River.

Dataset of Long-term Monitoring on the Change in Hydrology, Channel Morphology, Landscape and Vegetation Along the Naeseong Stream (I)

Dataset of Long-term Monitoring on the Change in Hydrology, Channel Morphology, Landscape and Vegetation Along the Naeseong Stream (I)

Dataset of Long-term Investigation on Change in Hydrology, Channel Morphology, Landscape and Vegetation Along the Naeseong Stream (II)

Dataset of Long-term Investigation on Change in Hydrology, Channel Morphology, Landscape and Vegetation Along the Naeseong Stream (II)

Predicting spatiotemporal changes of channel morphology in the reach of Teesta River, Bangladesh using GIS and ARIMA modeling

The Teesta is one of the most dynamic river systems in Bangladesh, supporting the local livelihood patterns of millions of riparian people over a long time. However, there is an increasing concern regarding such a dynamic system, due to its channels being subjected to regular shifting and threatening engineering projects, which results in various socio-economic and environmental problems. Therefore, the objective of the study is to investigate the spatiotemporal changes of channel morphology and predict mid-line channel shifting in the reach of the Teesta River, Bangladesh during 1972–2031, using multi-temporal Landsat images data with GIS, spatial autocorrelation index, and an autoregressive integrated moving average (ARIMA) model. The results show that the temporal changes in channel shifting rates were mostly affected by high sedimentation in the river flow path, whereas the spatial changes along the channel reaches were primarily controlled by the differences of river banks, as demonstrated by the various bar area formations. The relationship between the bar area and the braided index exhibits a strong positive correlation (R2 = 0.96, p < 0.01), while a moderate correlation occurs (R2 = 0.50; p < 0.05) between the sinuosity index and time, suggesting a high rate of sedimentation and changing the flow of the river affected by the channel banks. The results of the ARIMA model indicate that the maximum mid-line channel shifting will occur in the rightward direction for the period between 2017 and 2024, and leftward mid-line channel shifting between 2024 and 2031. The spatial autocorrelation results suggest that the decreasing bar area increases the spatial pattern of the channel. Overall, the maximum lateral channel shifting occurs in the right bank side; thus, the channel reach is sensitive to lateral shifting and requires protection utilizing engineering structures, as a huge population and extensive agricultural lands are being displaced on its rightward floodplain.

Channel forms and vegetation adjustment to damming in a Mediterranean gravel‐bed river (Serpis River, Spain)

AbstractThis paper focuses on the analysis of changes observed in channel morphology in the Serpis River (Alicante, Spain), a gravel‐bed river dammed since 1958. The paper analyses flow series and several aerial images, prior and subsequent to dam construction, to analyse changes in channel morphology and vegetation colonisation using Geographical Information Systems (GIS) techniques. Results show a concatenation of morphological changes throughout an adjustment sequence (60 years), which started with the transformation from wandering to single thread channel pattern, was followed by a slow vegetation encroachment, and culminated with the stabilization of channel migration. The role of vegetation (particularly Salicaceae species) has been critical in controlling floods' effectiveness, reducing river mobility and shifting, and consolidating a channel planform model adapted to the post‐dam flow conditions.

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

AbstractThe morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Examining the utility of river restoration approaches for flood mitigation and channel stability enhancement: a recent review

Increased anthropogenic activities, especially within the river corridor, have progressively disrupted natural flow regimes and segmented channel–floodplain connectivity. Consequent alterations in flow dynamics have caused geomorphic and hydrological changes in channel morphology and behaviour, decreasing their natural replenishment capacity, thereby causing their degradation. The simultaneous disconnection of the channel from its natural floodplain due to intensive cultivation and surface paving has also reduced possible floodwater outlets or seepage zones, which would have otherwise functioned as storm run-off storage buffers that reduced the flood peak downstream. Such occurrences of flow disruption, especially in low-relief topographic regions buffeted by high intensity monsoonal storms, accentuate flood-like situations, since natural channel capacities to transport excess storm-water are diminished, with this further enhancing ongoing or inducing riverbank erosion. Such events have accounted for much loss of life, livelihood and property worldwide and to mitigate their intensity, river restoration techniques have been promoted. These place greater emphasis on ecological management measures that attempt to restore and rehabilitate the geomorphic and hydrological functionality of the stream to a prior, more pristine state to achieve channel stability and regulate flows. Thus, frameworks based on river corridor management, riparian buffer creation and the River Styles Framework instituted guidelines, aided by geospatial analysis techniques, have yielded significant management strategies for such flood- and erosion-affected degraded channel corridors. This paper presents a recent review of such ecological techniques of flood and channel management, especially in monsoonal environments, while highlighting the issues concerning their implementation in developing nations.

Downstream channel changes and the likely impacts of flow augmentation by a hydropower project in River Dikrong, India

ABSTRACTThe Dikrong River, a tributary of the Brahmaputra River Basin in India, has been receiving additional water from an adjacent river since 2002 under the operation of the 405 MW Ranganadi hydel project. Flow duration curves and indicators of hydrological alteration analyses reveal that the downstream river flow has been significantly altered post-2002. Examination of the channel pattern in a Geographic Information System environment using satellite images from 1973 to 2014 displays that the channel planform has also changed over time, wherein it has become wider, less meandering and more braided. Sinuosity decreased from 1.9 in 1987 to 1.66 in 2002 and has stabilized since then. Simultaneously, braiding increased from 1.55 (1987) to 1.81 (2014). The average bankfull channel width from 1973 to 2014 varied between 378 and 573 m. The river also displays erratic and unevenly distributed bank migration and erosion patterns. However, channel changes could be observed even prior to the hydel project’s influence through flow addition. Thus, this study analyses the temporal and spatial changes in downstream channel morphology of the Dikrong River and examines whether they have been influenced by the hydrological alterations caused by the hydel project.

Impacts of gravel-bed rivers transformation on fluvial ecosystems and human society: Examples from the Czech flysch Carpathians

In the last centuries, gravel-bed rivers in developed countries have undergone rapid changes in channel morphology. The most serious problems include channel transformation related to progressive channel narrowing, incision or bed sediment coarsening. The main reasons for transformations were connected to the human interventions, which affected water and sediment fluxes in the basins. This paper summarizes contemporary research activities focused on these negative effects of channel transformations in the Czech flysch Carpathian rivers (the Morávka, the Olše and the Ostravice draining the highest mountainous areas of the Beskydy Mts). As the result of channel transformations, progressive changes in fluvial ecosystem were observed. The initial phytosociological survey demonstrates a higher biodiversity in the floodplain along the preserved multi-thread river channel than along the deeply incised channel in the Morávka River basin. Our observations of aquatic communities demonstrated that the channel transformation connected with incision and coarsening of bed sediments negatively affected fish or lamprey populations in the studied rivers. Regulation, damming and incision of channels caused changes of hydrological regime linked with gradual drying of floodplain. Additionally, a large set of hydraulic structures, bridges or weirs were affected by undercutting and progressive destruction in the Ostravice, Olše and Morávka River basins, which is assigned to increased transport capacity of regulated channels together with decreased sediment supply from mountainous parts.

Riparian Vegetation and Digitized Channel Variable Changes After Stream Impoundment

Following stream impoundment, rivers respond via changes in sediment dynamics, channel morphology, and vegetation distribution. Such changes have occurred along the Provo River, Utah, located within the Intermountain West of the U.S. Jordanelle Dam was built on the Provo River in 1992 after the majority of dam construction in the United States and therefore allows for a large-scale GIS analysis using aerial photographs, available before and after construction. To evaluate the effects of the dam, this study examines reach scale channel changes with respect to vegetation distribution and species richness. Post-impoundment, the authors find that channels downstream of the dam have become more stable, allowing for vegetation colonization, as exhibited in land cover changes from bare soil to grass. This results in greater species richness owing to colonization of a more stable riparian zone, ultimately changing habitat conditions. Identifying and understanding the impacts of the Jordanelle Dam on vegetation is necessary for protection of this valued ecosystem as rapid development continues.

Using high-resolution bedload transport tracer measurements to investigate the characteristics of bedload transport over a large urban flood event

Channel morphological change is often evaluated by employing sediment transport models since field data during high magnitude low frequency events is rarely available. However, sediment transport rate estimates are heuristic at best to within 1 - 3 orders of magnitude. Mimico Creek is an urban gravel-bed channel in Southern Ontario, Canada that has undergone intensive event-based sediment transport sampling and inter-event bed material particle tracking over a three-year period. A HEC-RAS model was developed of the study reach and calibrated to a series of discharge events where in-situ bedload sampling occurred. Both step-wise discharge and unsteady flow simulations were evaluated to compare sediment transport rates for a range of transport models which included the Meyer-Peter Müller and the Wilcock-Crowe. Calibration curves were developed to estimate sediment discharge in Mimico Creek. The results of the calibrated model were used to calculate the mean travel distance of bed material using the expression for the volumetric rate of bed material transport. Results from the modelling exercise found mean travel distances were similar and in some cases larger than those observed from field measurements, considering both mobile and immobile particles.

Recent geomorphological evolution of a natural river channel in a Mediterranean Chilean basin

In this paper we analyse the recent geomorphological evolution of the River Ñuble, an unmodified wandering river in the Mediterranean region of Chile, highlighting the relation between the observed hydroclimatic trends in the region and the morphology of the river. The work documents the recent changes (2003–2016) in channel morphology (i.e., narrowing, simplification, vegetation encroachment) and, in particular, how the river has progressively reduced its geomorphic activity during the last decade. Changes have been detected using aerial imagery and quantified by means of a series of channel form indices that allow tracking the geomorphological evolution of an 8-km river segment. The catchment has experienced a clear decrease in the frequency and magnitude of flood events, although this fact is not fully supported by a generalized reduction in the analysed rainfall series. We relate the observed river relaxation and the associated channel changes with the reported period of low hydroclimatic activity, which follows a humid period that occurred in the Pacific region during the 1970s and 1980s, connected to the PDO (Pacific Decadal Oscillations) that particularly affected the northern and the central parts of Chile. Altogether, this resulted in generalized river stability and a simplification of the drainage pattern of the channel. In absence of river training practices and other major changes in the catchment, the reduction of formative discharges appears thus to be the main control of the recent evolution of this gravel-bed river.

Channel morphology changes and their relationship to valley bottom geology and human interventions; a case study from the Vistula Valley in Warsaw, Poland

The aim of the study was to determine the role of the geological setting of channel zone in the morphological variability of a river reach situated between engineered and seminatural channel sections. Investigations were carried out in the Vistula River channel in Warsaw. The study included the drilling of 60 geological boreholes in the river channel and the registering of sedimentary structures of modern river-channel sediments. The morphology of the channel was defined based on a series of bathymetric measurements of different river levels. To identify the reasons for the variation in the channel-bed morphology, numerical modelling of river flow was performed. Calculations were made using the FLUENT CFD (Computational Fluid Dynamics) model. The stabilization of bottom erosion along the engineered reach of the river was found to be associated with a shallow depth to the top of the basement of modern alluvial sediments, composed of erosion-resistant glacial deposits. The suballuvial basement in this part of the river channel also serves as a local base level in relation to the river reach upstream.

THE EVOLUTION OF CHANNEL SYSTEM IN THE NORTHEAST MALAY BASIN, TERENGGANU OFFSHORE

This study aims to evaluate the evolution of the channel system in the Pleistocene-Recent succession in the northeast Malay Basin based on channel morphologies. The variable types of channel morphologies were determined from seven seismic time slices of study area. The fluvial channel became wider and low-sinuous when the sea-level rise. Meanwhile, the fluvial channel became narrower and high-sinuous when sea levels decreased. The point bars are seen in the meandering curve of the high-sinuosity channel. This occurred because of the sea-level decrease and more sediment being deposited in the study area. The point bars morphology do not appear in the transgressive event. An oxbow lake appeared in the 525 ms seismic time slice and is labelled as the oldest channel system. The channel morphologies then changed, and the oxbow lake does not appear in the younger system channel (Recent). The channel morphology changes have been proven to be affected by global sea-level changes.