Excess Sediment Supply Research Articles

Morphological changes of a developing sandbar along the shoreline of Sonadia Island, Bangladesh between 1972 and 2006 using remote sensing

ABSTRACT In a dynamic coastal environment, the morphology of a sandbar is anticipated to change regularly, particularly when there is a large annual sediment supply. The study explored the nature and magnitude of a developing sandbar’s morphological changes between 1972 and 2006 in Sonadia Island, Bangladesh, using five different satellite images. The sandbar’s changes across the shoreline were calculated using field survey data and manual-drawing outlines of overlaid maps. The study revealed continuous and rapid morphological changes of the sandbar along the Island’s shoreline. The net annual rate of shoreline displacement ranges between 3.94 m and 7.79 m during the period. These might directly result from the Island being exposed to huge wave and tidal action and excessive sediment supply around the Island. Transect 3 represents the sandbar’s head and appears to be the most active zone with the most significant annual shoreline displacement of 22.00 m. The sandbar’s far end around Transect 7 is more stable with a yearly shoreline displacement of 0.50 m. Changes were more drastic between 1999 and 2006 than between 1972 and 1999. These rapid changes after 1999 could indicate that the sandbar is becoming increasingly vulnerable to anthropogenic changes and the adverse effects of sea-level rise.

Dynamic mechanism of three-dimensional mixed-size grain/bed collision on non-flat bed using discrete element method

The impact of particles on slope bed is not only one of the main processes of eolian saltation, but also lead to the sand surface damage and provide sufficient materials for the sediment supply process in desert rivers. In order to reveal the mechanism of excessive sediment supply in desert rivers, it is necessary to study the grain-bed impact process, which plays an important role on the wind erosion. In this study, based on the discrete element method (DEM) and previous researches, a three-dimensional numerical simulation is carried out to study the influence of the incident angle, incident angle, and lateral incident angle on grain-bed collision under different bed surfaces. Finally, by fitting the simulation data, we give the three-dimensional splash function of ejection velocity, ejection angle, and lateral ejection angle as well as lateral ejection velocity with the change of impact velocity, impact angle, and lateral impact angle on different bed surface. This study will enhance our understanding of the processes of eolian sand movement and will provide useful information about the mechanisms that drive sediment replenishment to desert rivers.

Formation of paleovalleys in the Central Himalaya during valley aggradation

The formation of paleovalleys in mountainous regions is considered to result from extreme events such as landslides and glacial or landslide lake outburst floods. According to IPCC (2012) the extreme events are rarest of the rare weather/climatic events when the climate/weather variable is significantly above or below the defined threshold value. The present study suggests that paleovalleys can also form during years long periods of valley aggradation. A series of paleovalleys thus formed runs parallel to the present river course. In this study, we suggest that paleovalleys in the Alaknanda valley of the Central Himalaya have formed in two ways: 1) major valley aggradation and 2) local events of landsliding and lake breaching. Most of the paleovalleys in the Alaknanda valley formed during a major valley aggradation phase (between 15 and 8ka). Paleovalleys formed due to local landsliding also formed around 8ka. Local landslides were triggered due to high rainfall in lower valley reaches during unstable climatic conditions. Therefore, the formation of paleovalleys both by regional and local mode within 15–8ka indicates that the valley was receiving excess sediment from upper catchment as well as from lower reaches during this period. This phase of excess sediment supply and valley aggradation coincides well with post glacial climatic amelioration. Therefore the study underlines the role of climate in the time scales of 103 years in shaping the landscape of an active mountain like the Himalaya. The role of other landscape changing agencies such as tectonics and erosion is not accounted in the present study.

A Case of Normal Regression with Sea Level Transgression: Example from the Ganurgarh Shale, Vindhyan Basin, Maihar Area, M.P., India

Abstract The Ganurgarh shale, a formation belonging to the Bhander Group of Vindhyan basin is investigated using field based detailed lithofacies and petrofacies analyses in order to interpret the depositional environment in a sequence stratigraphic context. Five major lithofacies have been recognized consisting of calcareous sandstones, laminated mudstones, rippled siltstones, red-grey shales and sandy limestones characterized by small to large-scale cross-bedding, ripple cross-lamination of wave and current origin, parallel lamination, low-angle horizontal bedding, flaser and lenticular bedding, mud-cracks, salt pseudomorphs, convolute bedding and load structures. The constituent lithofacies are recurring and grouped into three lithofacies associations where, the association A is composed of fining upwards and B with coarsening upwards cycles at the lower and middle levels of the succession respectively, are dominantly arenaceous whereas, the association C occurring at upper levels is fining upwards (FU) and becomes calcareous with meager representation of clastics. Petrographically, the section offers three main petrofacies viz., (a) sandstone- (b) siltstone- (c) sandy limestone-petrofacies. Lithofacies characters complimented with petrography show that deposition occurred within the shoreface (subtidal) to foreshore intertidal domain involving tidal flats with sub-environments ranging from intertidal to supratidal. However, lithofacies associations within the Ganurgarh shale of Maihar area represent a case of normal regression during sea level transgression. In the beginning, probably because of excessive sediment supply the sea level had a falling trend during an overall transgressive phase ultimately culminating into limestone sedimentation.

Application of Loosely Coupled Watershed Model and Channel Model in Yellow River, China

Insufficient flow and excessive sediment supply in the Middle Yellow River lead to persistent sedimentation and rising flood levels in the lower river channel. Better understanding of the relationship between sediment erosion, transportation, and deposition can help in the decision making of soil and water conservation in the Coarse Sediment Source Area (CSSA) and sediment reduction works in the lower channel. A linked simulation can help elucidate the interrelationship. The goal of the current research is to simulate the whole sediment process from the CSSA to the downstream channels of the Yellow River. To achieve this goal, we use the Digital Yellow River Model (DYRM), a watershed hydrology and sediment yield and transportation model plate, coupled with a one-dimensional (1D) unsteady model for the hyper-concentrated sediment-laden flow in complex sections, to study the sedimentation process in the entire watershed. The performance of the simulation of flood in year 1977 was evaluated using the percent bias, Nash-Sutcliffe statistics of the residuals, and percentage difference. Simulation of the sediment process in 1977 shows that linked simulation can describe the hydrographs of discharge and sediment concentrations, especially flood in the downstream main channels. Compared with the underestimated discharge and sediment load in the main channel, the discharge of most tributaries is overestimated, whereas most of the sediment load is underestimated. The inconsistency indicates that more efforts are required to conduct better simulation, including data input and data processing, and both DYRM and 1D models.

Combining allostratigraphic and lithostratigraphic perspectives to compile subregional records of fluvial responsiveness: The case of the sustainably entrenching Palancia River watershed (Mediterranean coast, NE Spain)

We use a combined allostratigraphic and morpholithostratigraphic approach to establish a relative stratigraphy of macroscale sediment-landform units in the Mediterranean Palancia River watershed (986-km2, NE Spain). Four alloformations signifying important changes in (sub)regional scale geomorphic valley-floor evolution were identified based on data from 1120 field sites and age determinations, and from analyzing high resolution geodata. The formation of the widespread and thick Pleistocene alloformation can be attributed to climatically-induced excessive sediment supply and flood activities during Pleistocene cold intervals — rather than representing time-lagging response to Plio/Pleistocene neotectonic uplift. Triggered by the turn to Holocene climatic conditions, three successively inset alloformations illustrate how stream grading and floodplain narrowing continuously have progressed over the Holocene. The overall degradational valley-floor evolution in the Holocene is interpreted as a response to the antecedent, overly valley-floor aggradation. Allostratigraphic and morphostratigraphic data suggest that the abandonment of the two earlier Holocene alloformations geomorphologically represents a pulsed turn toward intensified entrenchment rather than pulsed sedimentation. The most important benefit of amalgamating allostratigraphic and (morpho)lithostratigraphic concepts is that allostratigraphic ordering provides a formally conclusive approach to scale up (morpho)lithostratigraphic information from the reach scale to much larger scales of geographical extent. Consequently, applying allostratigraphic principles opens a perspective to moving forward toward analyzing the relationships between climate, neotectonics, sea level change, human impact, and fluvial response in coupled hinterland-coastal systems that require to evaluate sedimentary information at larger spatial scales.

Widespread aggradation in the mountainous catchment of the Alaknanda–Ganga River System: timescales and implications to Hinterland–foreland relationships

Cut-and-fill type fluvial terraces are ubiquitous in the Lesser Himalayan zone of the Alaknanda–Ganga (Ganges) rivers system, which flows perpendicular to Himalayan lithotectonic units and traverses a steep climatic gradient. Lithofacies analysis of the sedimentary sequences of cut-and-fill terraces indicated that the valley aggradation took place via (1) channel bar development and excess sediment supply, (2) debris flows composed of mixed rounded to sub-rounded lithoclasts, resulting from episodic high intensity rainfalls in the upper catchment or (3) debris flows or rockfalls generated by local landslides. The luminescence chronology indicates that valley aggradation took place in two phases: ∼49 –25 ka and 18 –11 ka. The incision of the fill started soon after 11 ka. Paleoclimatic records from marine sediments indicate that the aggradation and incision in the Alaknanda–Ganga River has oscillated in-phase with global climatic variations. Glaciation –deglaciation processes in the upper catchment produced huge amounts of sediment between 63 and 11 ka, which was fluvially transferred to the lower valley via several cycles of erosion and deposition, leading to extensive aggradation. The climatic amelioration at ∼11 ka and the completion of deglaciation processes led to increased fluvial discharge and decreased sediment supply, conditions conducive for incision of the alluvial fills. Records from the Indo-Gangetic plain and the Ganga Delta demonstrate that the phase of aggradation was regional but that incision in the foreland started at least 2 –3 ka later, after 7 ka. Bedrock incision rates, as calculated from dated alluvial covers of terraces that are separated by bedrock steps, are spatially variable and fall within the range of rates reported from across the Himalaya. These estimated rates, however, are higher than the basin average erosion rates calculated using isotopic mass balance in riverbed sediments. This study suggests that during the last 50 ka river dynamics in the Himalayas were dominated by monsoon variability and the role of tectonic activity was limited to bedrock incision in few reaches only.

Fish assemblage response to rehabilitation of a sand‐slugged lowland river

AbstractThe impact of excessive sediment supply on river channels has been described in many areas of the world. Sediment deposition disturbance alters habitat structure by decreasing channel depth, changing substrate composition and burying woody debris. River rehabilitation is occurring worldwide, but information is scant on fish assemblage responses to rehabilitation in sediment‐disturbed lowland rivers. Sediment removal and large woody debris (LWD) replacement were used to experimentally rehabilitate habitat along a 1500 m stretch of the Glenelg River in western Victoria, Australia. Using an asymmetrical before‐after control‐impact (BACI) design, fish were captured before and after the reach was rehabilitated, from two control reaches and from a ‘higher quality’ reference reach. After two years post‐rehabilitation monitoring, the fish assemblage at the rehabilitated reach did not differ from control reaches. Temporal changes in taxa richness and the abundance of Philypnodon grandiceps, Nannoperca spp. and three angling taxa occurred after rehabilitation (winter 2003) compared with the before period (winter 2002), but these effects did not differ between rehabilitated and control locations. Highest taxa richness and abundances occurred at the reference location. High salinity coincided with the timing of rehabilitation works, associated with low river discharges due to drought. The negative effects of other large‐scale disturbances may have impaired the effectiveness of reach‐scale rehabilitation or the effects of rehabilitation may take longer than two years to develop in a lowland river subjected to multiple environmental disturbances. Copyright © 2009 John Wiley & Sons, Ltd.

Short and long-term changes to bed mobility and bed composition under altered sediment regimes

Altered flow and sediment transport regimes from impoundment can result in significant channel bed composition changes that exacerbate the geomorphic and ecological effects of flow regulation. Using long-term discharge and cross-sectional data in combination with a two-fraction sediment transport model, we assess changes in the downstream bed of two flow-regulated rivers with equivalent dam-induced changes in flow but opposite changes in sediment flux. Supply limitation has led to incision and armoring in one case while supply excess has led to aggradation and embeddedness in the other. Under limited sediment supply, bed elevation variability decreases soon after impoundment, while excess sediment supply results in a decades-long gradual decrease in both bed elevation variability and depth of incision. Although the balance of sediment supply and transport differs between dam management styles, in both cases both the immediate and more gradual changes can be explained within the framework of a two-fraction sediment transport model. Our results demonstrate the importance of considering bed composition on sediment transport predictions and the development of management strategies for ecosystem maintenance.

Modeling sediment transport in the lower Yellow River and dynamic equilibrium threshold value

A major problem in the lower Yellow River is the insufficient incoming water and excessive sediment supply, which results in serious deposition, continuous rise of the river bed, and austere flood control situation. To understand the sediment transport regularity of the lower Yellow River and determine the relationship between sedimentation, incoming water and sediment, and zone water diversion, a mathematical model of the sediment suitable for the characteristics of the lower Yellow River has been developed. This model is first rated and verified by large quantity of observed data, and it is then used to analyze silting reduction for the lower Yellow River by Xiaolangdi Reservoir’s operation, the relationship between zone water diversion and channel sedimentation, and critical equilibrium of sedimentation in the lower Yellow River. The threshold values of equilibrium of sedimentation in the lower Yellow River are estimated and they suggest that deposition in the lower Yellow River can be effectively reduced by the operation of regulating flow and sediment from Xiaolangdi Reservoir, water-soil conservation, and controlling water diversion along the lower Yellow River.

Sedimentology, seismic facies and stratigraphy of a Holocene spit–platform complex interpreted from high-resolution shallow seismics, Lysegrund, southern Kattegat, Denmark

Abstract Eighty kilometres of high-resolution seismic profile data (8 km/km 2 ) were collected in the Lysegrund area, southern Kattegat, Denmark to reconstruct the seismic stratigraphy of a Holocene spit–platform complex. Comparison of two sets of seismic data shows a significant difference in reflection pattern. By using lap style analyses, four Holocene units were recognised in boomer profiles (0.8–2.0 kHz). They are closely examined in this paper and their geometry is illustrated in a series of isopach maps, drawn to understand the development of the deposit in time and space. Three vibra cores provide the stratigraphic control and demonstrate the presence of three lithofacies with gradational boundaries in the interval that correlates with the three upper boomer units. The three lithofacies are part of a single lithofacies succession. In contrast to the boomer profiles, the subbottom profiles (3.5 kHz) demonstrate continuous accretion surfaces within the lithofacies succession. Comparisons of the boomer and subbottom profile data therefore indicate that the three boomer units are genetically coupled and that there are no significant time gaps between units. The three boomer units are interpreted to represent a spit–platform complex that developed in three phases during a general rapid rise of relative sea-level (>10 mm/a) under conditions of excess sediment supply. The source materials were mobilised from glacial–fluvial sediments in a headland west of and close to the spit–platform. During phase 1, spit development occurred in shallow water. This acted as a core to the development of a spit–platform during phase 2 that consisted of three seismic facies identifiable on 3.5 kHz subbottom profiles. During phase 3, true foresets and bottomsets developed. Growth of the platform surface kept pace with sea-level rise and the entire spit–platform complex formed in less than a thousand years. Focusing on differences in the seismic data helps avoid pitfalls in choosing methods to interpret data. The closely spaced seismic line-net forms the basis of a 3D dynamic model of the spit–platform that resembles the model of Meistrell [Meistrell, F.J., 1972. The split–platform concept: laboratory observations of spit development. In: Schwartz, M.L. (Ed.), Spits and Bars, Dowden, Hutchinson and Ross, Stroudsberg, pp. 225–283.].

Slope-shelf-nearshore depositional sequences with transgressive conglomerate in a Miocene backarc and arc-junction setting, central Japan

Several third-order slope-shelf-nearshore depositional sequences are found in the Middle to Late Miocene Aoki and Ogawa Formations in the southern part of the Northern Fossa Magna region, central Japan. Each depositional sequence consists of a transgressive systems tract, varying in thickness from several tens of centimetres to about 10 m, a falling stage systems tract several tens of metres in thickness and a highstand systems tract, 20–80 m thick. The highstand systems tract and falling stage systems tract are bounded by the regressive surface of marine erosion. Each depositional sequence mainly consists of coarse-grained sediments. The materials of the transgressive conglomerate beds were derived from the gravelly foreshore and upper shoreface by transgressive shoreface erosion. The thin and conglomeratic transgressive systems tract indicates that these slope-shelf-nearshore sequences were deposited on the steep slope area under a rapid transgression. Therefore, these sequences were formed in response to excess sediment supply and a rapid rise in relative sea-level at an active margin of the backarc side of the arc-trench system.

Response of the Connecticut River estuary to late Holocene sea level rise

The morphology of the modern Connecticut River estuary has been inherited from an earlier tidal river. The Holocene evolution of the estuary has been dictated by the relative submergence of the coastline and the sediment trapping efficiency of the estuary and tidal river system. The mid-Holocene tidal river system extended to the modern mouth of the estuary and subaerial floodplains stretched across the modern submerged shoals of the estuary. Submergence in excess of the long-term sedimentation rate converted these floodplains into open backwater coves beginning about 4000 yr BP in the southern estuary and about 2700 yr BP in the upper estuary and it also initiated estuarine circulation in the tidal river. Approximately 1700 yr BP an abrupt decrease in the submergence rate caused freshwater marshes to prograde across the coves and across the remaining floodplain surfaces. At 1000 yr BP salt marsh peat at the mouth of the estuary indicates estuarine circulation similar to that of t he modern estuary. The marshes in the northern estuary have supported freshwater vegetation since their inception. Thus the continuum of modern environments present along the length of the estuary models the evolution of a single reach of the estuary through time. Continued submergence will increase the tidal prism and intensify estuarine circulation. This effect will be regulated by the large excess sediment supply available to fill the volume of the estuary created by relative sea level rise.

An error function solution of sediment transport in aggrading channels

The sediment transport law for the unsteady and/or non-uniform flow prevailing during aggradation arising from excess sediment supply over and above the equilibrium sediment transport capacity has been formulated. This law has been tested against the data collected in the laboratory under the same boundary conditions, and a very good agreement has been found. The uniform sediments tranport law in the presence of macro-morphological features — ripples and dunes — is based on two parameters, namely the mean height of sand waves and average bed wave velocity, whereas the unsteady transport law obtained in the presence of the macro-morphological feature — aggradation — is dependent upon a single parameter. As a by-product a theoretical value of the aggradiator lenght has been obtained.