Article

Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015

January 2018
Geomorphology 300(2)

DOI:10.1016/j.geomorph.2017.09.041

Authors:

Bo Wang

Brown University

Y. Jun Xu

Louisiana State University

Channel bars are a major depositional feature in alluvial rivers and their morphodynamics has been investigated intensively in the past several decades. However, relatively less is known about how channel bars in alluvial rivers respond to river engineering and regulations. In this study, we assessed 30-yr morphologic changes of 30 large emerged bars located in a 223 km reach of the highly regulated Lower Mississippi River from Vicksburg, Mississippi, to the Mississippi-Atchafalaya River diversion. Landsat imagery and river stage data between 1985 and 2015 were utilized to characterize bar morphologic features and quantify decadal changes. Based on bar surface areas estimated with the satellite images at different river stages, a rating curve was developed for each of the 30 bars to determine their volumes. Results from this study show that the highly regulated river reach favored the growth of mid-channel and attached bars, while more than half of the point bars showed degradation. Currently, the mid-channel and attached bars accounted for 38% and 34% of the total volume of the 30 bars. The average volume of a single mid-channel bar is over two times that of an attached bar and over four times that of a point bar. Overall, in the past three decades, the total volume of the studied 30 bars increased by 110,118,000 m3 (41%). Total dike length in a dike field was found mostly contributing to the bar volume increase. Currently, the emerged volume of the 30 bars was estimated approximately 378,183,000 m3. The total bar volume is equivalent to ~ 530 million metric tons of coarse sand, based on an average measured bulk density of 1.4 t/m3 for the bar sediment. The findings show that these bars are large sediment reservoirs. [See full text at http://www.sciencedirect.com/science/article/pii/S0169555X17304142]

Geographical location of the Lower Mississippi River (left). The study reach is located between RK 737 (River Mile 458) and RK 515 (River Mile 320), in which 30 large emerged bars are identified (right). The U.S. Army Corps of Engineers operates four gauging stations at Greenville (RK 855), Vicksburg (RK 701), Natchez (RK 585), and Red River Landing (RRL, RK 487). The long-term river stage records from these stations were used for determining bar surface area at different river water levels. A portion of the Mississippi River is diverted into the Atchafalaya River through the Old River Control Structure (ORCS) at RK 505.

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The cumulative volume of the 30 emerged bars in the river reach from Vicksburg to the Old River Control Structure along the Lower Mississippi River. Based on the change rates, the 223 km reach can be divided into three sub-reaches (A, B, and C). Dashed blue lines are the slope of the curves in reaches A, B, and C. Most of the sediment are currently stored in the middle of this river reach.

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Volume changes of the studied 30 emerged bars from Vicksburg to the Old River Control Structure on the Lower Mississippi River between 1985 and 2015.

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The rapid growth of bar 15 from 1985 to 2015 because of the construction of a dike field. River stages were very similar (i.e. 17.3 – 17.5 m) on these four days when the satellite images were taken.

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Morphologic changes of bars 7, 8, and 9 from 1985 to 2015. There were two dike fields near bars 7 and 9. Bar 8 was a mid-channel bar in 1985. However, it was fully unrecognized in 2015 because of the construction of the dike field around bar 7. The river channel narrowed by about 700 m over the past three decades owing to the growth of bar 9.

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Figures - uploaded by Y. Jun Xu

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Comparing the Capability of Sentinel-2 and Landsat 9 Imagery for Mapping Water and Sandbars in the River Bed of the Lower Tagus River (Portugal)

Article

Full-text available

Apr 2023

Mapping river beds to identify water and sandbars is a crucial task for understanding the morphology and hydrodynamics of rivers and their ecological conditions. The main difficulties of this task so far have been the limitations of conventional approaches, which are generally costly (e.g., equipment, time- and human resource-demanding) and have poor flexibility to deal with all river conditions. Currently, alternative approaches rely on remote sensing techniques, which offer innovative tools for mapping water bodies in a quick and cost-effective manner based on relevant spectral indices. This study aimed to compare the capability of using imagery from the Sentinel-2 and newly launched Landsat 9 satellite to achieve this goal. For a segment of the Lower Tagus River (Portugal) with conditions of very low river discharge, comparison of the Normalized Difference Water Index, Modified Normalized Difference Water Index, Augmented Normalized Difference Water Index, and Automated Water Extraction Index calculated from the imagery of the two satellites shows that the two satellites’ datasets and mapping were consistent and therefore could be used complementarily. However, the results highlighted the need to classify satellite imagery based on index-specific classification decision values, which is an important factor in the quality of the information extracted.

Impact of sand mining on the changes of morphological and physical dynamics in Sa’dang River, Pinrang District, Indonesia

Article

Full-text available

Sep 2020

The research aimed to investigate the morphological changes and physical dynamics conditions of the Sa’dang River associated with river sand mining activities. The research is expected to benefit the efforts of the management of sand mining in this area and enrich knowledge about the issues of river sand mining. The study was conducted in Sa’dang River sand mining area, which includes villages of Pincara, Massewwae and Mangki of Pinrang District in South Sulawesi Province, Indonesia. A field survey was conducted in October to December 2019 to see the condition of the river along with the mining activities that were taking place. Field data analysis was combined with google earth imagery data for 2006, 2014, 2017 and 2019 using ArcGIS 10.5 software to see changes in river morphology, formation and extent of point bar and channel bar, as well as changes in the area of river water bodies and non-water river bodies. The results showed that the changes in morphology and physical dynamics conditions of the Sa’dang River showed their own pattern. If sediment deposits forming point bars or channel bars were reduced, the area of river water bodies tended to increase. Or in other words, the width of the river is inversely proportional to the increase in the area of the river water body if the area of sediment deposits that form the point bar and channel bar tends to decrease. © 2020 Universitat Autonoma de Barcelona and Universitat de Girona. All rights reserved.

Sediment transport and channel dynamics of the Lowermost Mississippi River in recent decades

Presentation

Full-text available

Sep 2019

Sediment transport and channel dynamics of large alluvial rivers in their lower reaches can strongly influence physical, chemical and biological conditions in their deltaic regions. In this paper, we report the latest assessment on sediment transport and riverbed dynamics of the last 500-km reach of the Mississippi River before entering the Gulf of Mexico. This river reach is also termed as the Lowermost Mississippi River (LmMR), which begins downstream of its division node to the Atchafalaya River. Our assessment shows that in the past 2-3 decades, suspended sediment and bed material loads in the LmMR were significantly higher at the upstream location than those at the downstream locations. Nearly 70% of the riverine sand is trapped within the LmMR. Continuous riverbed aggradation (up to 10 m) occurred below the Mississippi-Atchafalaya diversion, presenting favorable backwater conditions for an avulsion. Backwater effects have mainly controlled riverbed deformation in the LmMR, while flow reduction may have also contributed to channel aggradation in the lowermost reach. Our assessment reveals the considerable complexity of morphodynamical responses of a large alluvial river to human interventions, strongly suggesting that future river engineering and management of the world large alluvial rivers need to look for effective strategies that will improve sediment transport.

Sediment Transport and Channel Dynamics of the Lowermost Mississippi River in Recent Decades

Conference Paper

Full-text available

Sep 2019

Mechanisms of bar adjustments in the Jingjiang Reach of the Yangtze River in response to the operation of the Three Gorges Dam

Article

Nov 2022
J HYDROL

Dams alter downstream river flow and sediment regimes, causing significant changes in river morphologies. The middle Yangtze River downstream of the Three Gorges Dam (TGD) has experienced rapid erosion in recent years, and the associated morphodynamic changes have negatively impacted the bank stability, navigation waterways and ecological functioning. Earlier studies have analyzed recent channel adjustments in the Yangtze River; however, our understanding of changes in the bar morphodynamics remains incomplete. In this study, we collected and analyzed flow and sediment data (1991∼2016) and river bathymetry data (1975∼2017) and investigated the mechanisms of bar adjustments along the Jingjiang Reach in the post-TGD period. The results indicate that most steady bars with a higher elevation and better vegetation coverage have experienced lateral erosion, while their elevations have remained stable overall. The unvegetated migrating bars with a lower elevation have experienced severe surficial erosion and area shrinkage. A new assessment method of the dominant discharge range on downstream bars is provided after dam closure. The new dominant discharge range that determines bar adjustments corresponds to the flow stages between the submersion of migrating bars (∼12,500 m³/s) and the overtopping of vegetated steady bars (∼27,500 m³/s) in the Jingjiang Reach. A quantitative relationship exists between bar erosion in response to changed dominant discharge regimes and sediment decline. The significant bar erosion after the operation of the TGD can be attributed to the increased flow duration and the sharp decline in the suspended sediment concentration (SSC) of the dominant discharge range, and changes in SSCs play a primary role. The bar area will decrease by ∼0.003 km² when the annual cumulative duration of the dominant discharge increases by one day and by ∼0.234 km² when the annual average SSC of the dominant discharge decreases by 0.01 kg/m³. Furthermore, vegetation encroachment and colonization play a positive role in stabilizing bar morphologies and limiting surficial erosion, whereas vegetation cannot prevent the lateral erosion of steady bars. These findings suggest that multiple controls, including flow, sediment and vegetation, shape the evolution of fluvial bars and have important implications for river management and ecological evaluation in response to the operation of large dams.

Geomorphologic changes around a mid-river bar system at a meandering reach in the lower Yangtze River, China: Impacts of the three Gorges dam (TGD) and human activities

Article

May 2022
CATENA

Mid-river bars are important agents that impact the flow and sediment dynamics of rivers and are commonly used for the development of human society. This study focused on the geomorphologic change (riverbed and bar morphology) around a large-scale river bar system in the lower Yangtze River (namely, Heishazhou) that was possibly impacted by the Three Gorges Dam (TGD) and other river engineering works. Relevant data sources including field surveys, hydrological station records, satellite images and historical practice reports were gathered for the investigation. The Heishazhou bar system previously consisted of two major subbars and recently merged as an entirety. Rapid bar growth occurred after the construction and operation of the TGD, leading to the closure of the Middle Channel and the South Channel dominating water conveyance at the reach. With the examination of another three bar systems in the lower Yangtze River, the lower-reach bar systems generally grew due to sediment deposition around bars, which differred from those in the middle reach that generally shrunk. The underlying cause may be that the middle-reach riverbed still supplies sediment to the lower reach. The riverbed at the Heishazhou reach, however, experienced apparent erosion after the construction of the TGD. Combining historical investigations and satellite images, a conceptual description was created to understand the geomorphologic cycle of river bar systems at a meandering reach and found that the formation of an inner-bank bar shall be a major force driving the geomorphologic cycle. Sediment deficits caused by the TGD and human activities such as dikes, bank revetments and riverbed dredging are highly likely to cause a slow-down of the geomorphologic cycle of these river bar systems with distinct mechanisms.

Relationship between the equilibrium morphology of river islands and flow-sediment dynamics based on the theory of minimum energy dissipation

Article

Dec 2021
INT J SEDIMENT RES

River islands are vital geomorphic units in alluvial rivers, and the variation of their morphology and position plays a significant role in regulating flow-sediment transport and channel stability. Based on the theories of minimum energy dissipation theory of fluid movement and river morphodynamics, this study uses the river islands in anabranching channels to analyze the relationship between the shape coefficient of river island and the flow-sediment dynamics under stable equilibrium conditions. Results indicate that the river islands could adjust their lengths and widths to reach an appropriate allocation of flow diversion ratio and sediment separation ratio in the two anabranching channels to ensure flow-sediment being smoothly transported downstream and achieve the minimum energy dissipation. Under the minimum energy dissipation condition of flow-sediment transport balance, the ratio of “sediment separation ratio” to “flow diversion ratio” in short and straight branching channels is between 1 and 1.1. When the proportion of flow-sediment transport reaches its maximum, the steady island morphology coefficient is approximately 0.2; that is, the island length is five times the width. We verify the steady island morphology coefficients using two typical islands in the middle and lower reaches of the Yangtze River and find the relative error is within ± 3%.

Examining the fluvial alteration hypothesis amidst recovery of the Interior Least Tern ( Sternula antillarum )

Article

Full-text available

Jul 2021

The world's large river systems are under increasing stress to support growing populations and economies. Balancing inevitable fluvial alteration with protections of river ecosystems will require awareness of lessons learned in big river systems where system‐scale alterations have already occurred. In 1985, the Interior Least Tern (Sternula antillarum; terns) was listed as endangered, citing “alteration of natural river dynamics” as cause for population declines. We refer to this as the “fluvial alteration hypothesis” and examine it by attempting to answer two primary questions: [Q1] What are the first‐order physical and hydrologic metrics associated with tern nesting reach selection at the regional scale? [Q2] Did human‐induced changes in these metrics differ between regions where tern breeding populations have recovered and where it has been stable or declined? We compiled metrics of sandbar nesting habitat across the nesting region for modern conditions and used regression modeling to assess the relative effects of each metric on nesting reach selection. At the scale of a nesting region, terns selected for nesting reaches that are wider have greater potential to braid, and are less likely to flood during nesting season. A comparison between subregions indicates that modern tern nesting conditions are substantially better in the lower Mississippi River and tributaries (LMRT) systems than the Missouri River and tributaries (MORT) system. However, both subregions had nearly equivalent sandbar nesting habitat quality prior to major human modifications. The fluvial alteration hypothesis was consistent with geomorphic assessments for the MORT system, but overlooked potential for human alterations to have differing, even beneficial effects to tern nesting habitat in the LMRT system.

Four-decades of bed elevation changes in the heavily regulated upper Atchafalaya River, Louisiana, USA

Article

Full-text available

Sep 2021
GEOMORPHOLOGY

Previous studies have established that under natural conditions, alluvial river confluence zones experience channel scour followed by mid-channel bar development. Less attention is given to bed evolution downstream of large alluvial river confluences under engineered conditions, such as discharge regulation and levee confinement. Here we present four decades of morphologic changes along the 69-km uppermost Atchafalaya River, a downstream distributary of the confluence of the Mississippi River Outflow channel and the Red River. We aim to find the answer to one critical question: how does the channel bed downstream of an engineering-controlled confluence respond to regulated flow? We utilize single-beam bathymetry data collected in 1967, 1977, 1989, 1998, and 2006 to quantify riverbed deformation of the reach after the flow regulation began in 1963. Suspended sediment load and stream power during the four periods are calculated. Results show that between 1967 and 2006, extensive bed degradation occurred and the average bed elevation reduced by 3.8 m. A total volume of 105 ± 26 × 10⁶ m³ sediment was scoured from the uppermost Atchafalaya riverbed over this 40-year period, implying that channel erosion in this river reach has contributed to downstream coarse sediment delivery and associated deltaic growth in the Atchafalaya Bay. Bed aggradation only occurred during 1989–1998 in response to excess sediment input from the Mississippi River, likely attributable to the 1993 long-lasting Mississippi River flood. But the same amount of riverbed deposit eroded in the following eight years, demonstrating how quickly a large regulated river can diminish a perturbation owing to excess sediment input. These findings not only reveal the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for future modeling studies and management plans for the Atchafalaya and other large, engineered alluvial river confluences.

The “Problem” of New Orleans and Diminishing Sustainability of Mississippi River Management—Future Options

Article

Full-text available

Mar 2021

Climate change forcings are having significant impacts in coastal Louisiana today and increasingly affect the future of New Orleans, a deltaic city mostly below sea level, which depends on levee and pumps to protect from a host of water-related threats. Precipitation has increased in the Mississippi River basin generally, increasing runoff, so that in recent years the Mississippi River has been above flood stage for longer periods of time both earlier and later in the year, increasing the likelihood that hurricane surge, traditionally confined to summer and fall, may compound effects of prolonged high water on river levees. The Bonnet Carré Spillway, just upstream of New Orleans has been operated more often and for longer periods of time in recent years than ever before in its nearly 100-year history. Because all rain that falls within the city must be pumped out, residents have been exposed to interior flooding more frequently as high-intensity precipitation events can occur in any season. A sustainable path for New Orleans should involve elevating people and sensitive infrastructure above flood levels, raising some land levels, and creating water storage areas within the city. Management of the lower Mississippi River in the future must include consideration that the river will exceed its design capacity on a regular basis. The river must also be used to restore coastal wetlands through the use of diversions, which will also relieve pressure on levees.

Three decadal morphodynamic evolution of a large channel bar in the middle Yangtze River: Influence of natural and anthropogenic interferences

Article

Apr 2021
CATENA

The study focused on the question of how the natural and human forcing has affected the morphodynamic evolution of bars in the middle Yangtze River. A large channel bar was investigated using satellite remote sensing data during 1986-2017. A novel algorithm was proposed to select images acquired at the similar river stage. The bar was delineated from selected images and surface area, volume and centroid were quantified to examine bar morphodynamics. The results indicated a four-stage evolution process, including a steady extension (1986-1998), a quick shrinkage (1998-2002), a fluctuated shrinkage (2002-2012), and a fluctuated extension (2012-2017). The first stage was characterized by the steady growth at a rate of 0.087 km 2 /a, and a centroid translation towards the inner bend by 400 m. The second stage was the most dynamic episode, with the bar degradation at a rate of 0.71 km 2 /a, and a downstream migration by 850 m. In the third stage, the bar was eroded at a rate of 0.05 km 2 /a, and migrated towards the inner bend by about 80 m. In the fourth stage, the bar was deposited at a rate of 0.12 km 2 /a, and traveled upstream by around 400 m. The bar volume exhibited the similar trend. The analysis showed that the four stages were separated by the consecutive floods, the closure of Three Gorges Dam and the installation of two river engineering works. The results suggested that the bar evolved much more dynamic under the natural flow (1985-2003) than that of the regulated (2003-2012). The findings revealed that the evolution was largely governed by the flow and sediment supply while the human forcing can affect the process by altering these regimes. This study also demonstrated the great usefulness of combining multi-temporal imagery and river stage records in the assessment of bar morphodynamics.

THE EFFECT OF POINT-BAR FORMATION AND BANK EROSION ON THE MORPHOLOGY OF THE LOWER TISZA RIVER, HUNGARY

Conference Paper

Apr 2018

Alluvial rivers have been a widely studied subject of recent researches in geomorphology (Knox and Latrubesse, 2016; Wang and Xu, 2018), as their morphological changes affect their human use, and also the risks and hazards. Key points in the morphological evolution of meandering lowland rivers are the bank erosion and the formation of bars, as they could indicate channel processes and equilibrium conditions of a reach. In recent years, several researches focused both on bank erosion (Piégay et al., 2005; Schuurman et al., 2016) and on the development of point-bars (Wang and Xu, 2016; Wang and Xu, 2018). According to Schuurman et al. (2016), spatially alternating bank erosion and point-bar growth are crucial in the development of meanders in rivers. While bars serve as sinks for sediments in alluvial rivers, bank erosion on the other hand is often the dominant source of sediments especially in modified watersheds (Shields et al, 2009). Bank erosion resulting from mass failure of steep banks is one of the most serious forms. The state of bars on the other hand, are representative of the equilibrium of channels (Church and Rice, 2009). The Tisza River in Hungary has been altered by various human interventions since the 19th century. It began with construction of embanked levees to control floods, artificial meander cutoffs and channelization to improve shipping and shorten the flood waves, as well as recovering land for agricultural purposes. Later works included the construction of revetments and groins to control the lateral erosion of the river. In the last fifty years, dams have also been constructed to support irrigation and to generate electrical power. As a result of these intensive human impacts, the channel processes seem to change. In a natural state (pre-19th c.), the Tisza had a meandering pattern with alternating point-bars. However, only some point-bars remain nowadays, the channel incises and bank erosional processes alter. The aim of the present research is to reveal the driving factors of recent point-bar formation and bank erosion, to determine the rate of these processes, and finally to give an evaluation on the channel evolution of the studied reach. Our study was performed on the 89-km-long section of the Lower Tisza River (between Csongrád and the Hungarian-Serbian border). Here, the regulation works doubled the slope (2-4 cm/km) and reduced floodplain (from 10-20 km wide floodplain to just 1-5 km; Kiss et al., 2008). To understand the role of point-bars and bank erosion in the morphological evolution of the Lower Tisza River, two approaches were applied. Firstly, to evaluate the long-term morphological changes of the river, military maps (since late 18th c.), hydrological surveys (1891-1999) including the planform and vertical cross-sectional parameters were applied. Based on this series of maps, the sinuosity, the width, the number and the length of point-bars were determined. Secondly, in selected study sites, the recent evolution of point-bars (at Csongrád and Ányás), and the rate of bank erosion (Csanytelek 1-2, and Ányás) were measured and analyzed between 2013

Formation, Migration, and Morphodynamic Alteration of 50 Channel Bars in Darjeeling Himalayan Piedmont Zone, India

Article

Full-text available

Aug 2020

Channel bars are common, striking fluvio-geomorphic depositional features of alluvial rivers. The study of this article has aimed to investigate the formation, migration, and morphodynamic alteration of channel bars (n = 50) in gravel braided alluvial rivers in Darjeeling Himalayan Piedmont zone. Dynamics of micro to meso bar deposition is mainly accomplished by the channel gradient, huge upstream landslide, and variation of discharge. Multi-criteria analysis method has been used to explain the variation of principal component analysis (PCA1, PCA2, and PCA3), both monsoon to post-monsoon. Bridge scouring, empirical aggradation, and degradation values are 0.1 to 1.05 m and [Formula: see text]. Migration, re-generation, and degradation of the micro bars with their optimum morphology show changes during the rainy season. In the upper ([Formula: see text]), middle ([Formula: see text]), and lower ([Formula: see text]) piedmont region, the bar dynamic area are 0.12 to 0.71, 0.12 to 2.83, and 0.10 to 5.43 km ² , respectively. In total, 58% to 72% stability of the channel bars is observed in the upper piedmont region. The coefficient of determination ( R²) of channel bar area and width shows positive (0.63) relation.

Bar dynamics in a sandy braided river: Insights from sediment numerical simulations

Article

Nov 2019
SEDIMENT GEOL

Bar dynamics in sandy braided rivers are keys to better understand the morphological processes of braided rivers and sediment distributions. Limited by insufficient quantitative information and satellite images that only show the larger-scale morphology and do not reveal the sand bars in the deeper channels, most previous studies have qualitatively assessed bar dynamics, but the ways in which the hydraulic parameters of sandy braided rivers control bar dynamics remains poorly understood. Here, we present sediment numerical simulations on sandy braided rivers to investigate the quantitative controls of river parameters on bar dynamics. Three bar dynamic forms (downstream migration, lateral variations and bar tail elongation) are observed. Unit bars are prone to downstream migration following formation, and the migration rate shows a deceasing trend during braided river evolution. This gradually decreasing migration rate is proven to be strongly connected to the decreasing flow velocity of braided channels that confine bar growth. The lateral variation in bar dynamics exhibits narrowing, migration and widening of sectional morphologies. We suggest that braided channel curvature dominantly controls lateral sediment transportation around the bar and thus results in several lateral variation forms of bars. The bar tail elongation difference between the two sides of the bar is associated with bar asymmetry. Strong bar asymmetry can make apparent differences in bar tail elongation.

Morphological adjustments of the Yamuna River in the Himalayan foothills in response to natural and anthropogenic stresses

Article

Jul 2023

Rivers can adjust to natural and anthropogenic stresses through changes in flow regime and sediment dynamics. Flow regulation due to dam construction can modify sediment transport processes, causing morphological adjustments and changes in conveyance capacity. In this study, we explored morphological adjustments along a 46 km segment of the Yamuna River in the Himalayan foothills from Dakpathar Barrage to Hathni Kund Barrage using a combination of remote sensing approaches. The cloud computing platform Google Earth Engine (GEE) was used to analyse multi-temporal collections of Landsat satellite imagery acquired between 1989 and 2021. Active river channels (including water and exposed sediment) were classified using an MNDWI and NDVI thresholding approach from annually resolved temporal composite images. Declassified CORONA satellite imagery (acquired in 1965, 1973 and 1976) was manually digitized to provide longer-term insights into channel change. Morphological adjustments were assessed using the RivMAP toolbox, providing quantitative information on the rates of bank erosion and accretion, bank line shifts and changes in active channel width. Results showed a substantial narrowing of the active channel after 2013, which coincided with the construction of the Lakhwar-Vyasi Dam. We calculated a 67% reduction in mean active channel width, narrowing from $800 m in 1989 to $250 m in 2021. During the same period, evidence of sand mining in the active channel indicates a substantial increase in mining activities after 2015. The concept of stasis was explored under different flow regulation scenarios to suggest that the river has undergone a state of inactivity, where the river is doing nothing for much longer durations. We suggest that the combined effects of flow regulation and increased mining activities during the same period have altered river morphology, and further stresses due to a combination of human activities can be damaging for the river regime. This study sheds light on the potential implications of multiple stresses acting on the Yamuna River at the same time, which can inform future sustainable river management activities. K E Y W O R D S flow regulation, fluvial geomorphology, remote sensing, river channel change

Disproportional erosion of the Middle-lower Yangtze River following the operation of the Three Gorges Dam

Article

Nov 2022
SCI TOTAL ENVIRON

The operation of the Three Gorges Dam (TGD) modifies downstream flow and sediment regimes, triggering disproportional fluvial responses at different distances downstream. However, our understanding of the downstream geomorphic changes in the middle-lower Yangtze River remains incomplete due to the complexity of the river responses across temporal and spatial scales. Here, we leverage data on discharge, suspended sediment concentration (SSC), riverbed grain size, cross-sectional profiles and high-resolution channel bathymetric maps at different locations downstream of the TGD to investigate geomorphic responses. The results show that the magnitude of fluvial erosion decreases downstream, with the Yichang-Luoshan Reach (the first ~500 km downstream) experiencing the most severe erosion in 2003–2020 (~9.05 × 10⁴ t/km/yr). Local changes in riverbed morphology include channel bar erosion, channel incision (~0.43 m/yr in CS1 near the dam site over 2002–2019), riverbank retreat and bed material coarsening (an increase in D50 from 0.175 to 43.1 mm at Yichang station from 2002 to 2017). Such marked erosion is caused by the sharply reduced SSC in the dominant discharge range (10,000–30,000 m³/s) and the extended duration of this dominant discharge range. The sediment erosive magnitude in the Luoshan-Datong Reach is relatively small (3.85 × 10⁴ t/km/yr) in 2002–2020. The Luoshan-Hukou Reach (~500–1000 km downstream) exhibits moderate channel incision, minor bed material coarsening and moderate mid-channel bar lateral erosion. The Hukou-Datong Reach (below 1000 km downstream) experienced minor geomorphic change without significant evidence of bed material coarsening. The relatively small impact of the TGD on the lower Yangtze River from Luoshan to Datong can be mainly attributed to the progressive SSC recovery along the river induced by upstream channel erosion providing sediment replenishment. These findings have significant implications for estimating geomorphic changes in response to upstream damming and thus could inform better river management and ecological assessment in other similar alluvial rivers.

Long term trends of streamflow, sediment load and nutrient fluxes from the Mississippi River Basin: Impacts of climate change and human activities

Article

Full-text available

Nov 2022
J HYDROL

Under the influences of global climate change and intense human activities, the hydrological and biogeochemical processes have been undergoing profound changes in most of the world's watersheds. However, the long-term changes in streamflow, sediment load, and nutrient fluxes, and quantitative attribution of these changes from a basin-wide perspective are incompletely understood. Here we examined the trends and major causes of streamflow, sediment load, and total nitrogen (TN), total phosphorus (TP), and dissolved inorganic carbon (DIC) fluxes in the Mississippi River Basin (MRB) and its four major tributary basins during the 1930s-2010s. The streamflow increased significantly (p < 0.05) in the entire basin except for the Arkansas River Basin, while the sediment load significantly decreased except in the Arkansas River Basin and Upper MRB. Generally, the TN flux reduced except for an increasing trend in the Upper MRB. While the TP flux rose except for a decrease in the Ohio River Basin, and the DIC flux increased significantly. These nutrient fluxes were more strongly correlated with the corresponding streamflow than with sediment load. Over the past 80 years, the increasing precipitation in the MRB has made the most contribution (67% on average) to the river discharge increase, whereas human activities contribute the most (>58%) to the changes in the sediment load and nutrient fluxes. The sediment yield coefficient decreased linearly with the increasing impermeable area proportion, river revetment length, and dam storage capacity (p < 0.05) in the MRB. The decrease in TN flux probably resulted from sedimentation and denitrification in reservoirs and wetlands, and increases in farm fertilizer and manure use were the two main factors contributing to the increased TP flux. In addition, the agricultural practices, increased precipitation, and raised temperature jointly led to an increase in the DIC flux. This study provides quantitative results of water-sediment-nutrient pattern in the MRB and a systematic and basin-wide perspective for the sustainable development management.

Flow resistance adjustments of channel and bars in the middle reaches of the Yangtze River in response to the operation of the Three Gorges Dam

Article

Full-text available

Oct 2022

Since the Three Gorges Dam (TGD) was put into operation, the flood water level at an identical discharge rate has not displayed a decreasing trend along the middle reaches of the Yangtze River (MYR). The flow resistance variations of the channel and bars in response to the operation of the TGD remain poorly understood, despite the importance of understanding these for water disaster mitigation and water environment regulation. Herein, the impacts of the TGD on the downstream flow resistance of the channel and bars in the MYR were analyzed using systematic surveys of hydrological datasets, cross- sectional profiles, sediment datasets, and remote sensing images, during different periods. Under the actual natural conditions in the MYR, a modified semi-empirical formula, which considered the grain, dune resistance, as well as the topographic features of the riverbed, was proposed to predict the channel resistance. Furthermore, the effect of various dam-control flow and sediment elements on the variation in different flow resistance components, and the corresponding relationships among them were investigated. Results showed a decline in the comprehensive, channel, and bar resistances as the discharge increased, whereas there was a slight increase when reaching the bank-full discharges. Notably, the bar resistance occupied 65%, while the channel resistance, in which dune resistance was much larger than grain resistance, contributed 35% to the comprehensive resistance. In addition, while flow resistance rose over time, there was a decline as the distance from the TGD increased. In conclusion, the increased dune and bar resistances, interpreted by the fluctuated channel longitudinal profile and growing vegetated area on bars, were the dominant factors preventing the flood water level from dropping.

Variability of Carbon Export in the Lower Mississippi River during an Extreme Cold and Warm Year

Article

Full-text available

Sep 2022

The Mississippi River (MR) discharges on average 474 km3 of water annually into the Northern Gulf of Mexico (NGOM) with a large quantity of carbon, playing a vital role in the ecosystem’s food chain and water quality. In this study, we analyzed exports of dissolved inorganic (DIC) and or-ganic carbon (DOC) from January 2021 to December 2021, during which the contiguous United States experienced one of the coldest winters as well as the hottest summer on record. Bi-weekly in-situ river measurements and water sampling were conducted in the lower MR at Baton Rouge in Louisiana, USA, approximately 368 km from the river’s mouth. We found that the MR transported 12.61 Tg C of DIC and 4.54 Tg C of DOC into the NGOM during the study period. Much of the DOC mass export occurred during the winter (~38%), while much of the DIC mass export took place in the spring months (~35%). The seasonality of DOC and DIC exports was affected by their con-centrations, water temperature, and discharge. DIC concentrations were significantly higher in the fall (32.0 mg L−1) than those during the winter (20.4 mg L−1), while DOC concentrations were highest during the winter months (11.3 mg L−1) and varied seasonally, however, not significantly. Partial pressure of dissolved carbon dioxide (pCO2) in the MR averaged 1703 ± 646 µatm peaking in the summer at 2594 µatm and reaching a low in the winter at 836 µatm. Outgassing of CO2 (FCO2) peaked in the spring averaging 3.43 g C m2 d−1 and was lowest in the winter at 1.62 g C m2 y−1. Our findings validate our initial hypotheses that seasonal variability and weather extremes strongly affect terrestrial-aquatic carbon transfer, and that climate change will likely intensify carbon export from the Mississippi River Basin.

Remote sensing of broad-scale controls on large river anabranching

Article

Sep 2022
REMOTE SENS ENVIRON

River patterns reflect complex geomorphological processes and affect ecosystems and human development along floodplains. Physical controls on anabranching development have been studied primarily at local scales on relatively small rivers (i.e., discharge <1000 m3 s−1). However, there has been little systematic quantification of river anabranching (development of stable multiple channels) for large rivers globally. Here, we use remote sensing, cloud computing, and geospatial analysis to explore the importance of water surface slope, floodplain topography, sediment supply, substrate lithology, and permafrost to anabranching throughout 20 of the world's largest river basins. We use Landsat-derived surface water extent to compute an anabranching index (Ai) for ∼1 M km of river reaches, and compare it with global datasets of water surface slope, available floodplain extent, substrate lithology, and permafrost. We find that ∼49 ± 19% of large-river mainstems are anabranching, ranging from at least ∼17% (Mekong) to as much as ∼84% (Ob’). At the basin scale, anabranching channels comprise at least ∼17% (Yenisey) to as much as ∼55% (Kolyma) of all Landsat-observable river reaches, with a mean global value of 35 ± 11%. Anabranching channel patterns are most commonly associated with low water surface slope (normalized slope < 0.2; absolute slope < 0.2 m km−1) accompanying wide floodplains (normalized floodplain width > 0.6; absolute width > 42 km). Cross-sectionally averaged channel width increases in the most intensely anabranching reaches, suggesting net sediment storage due to reduced stream power. Unconsolidated sedimentary substrates promote both the prevalence and intensity of anabranching, whereas the presence of permafrost enhances intensity only. Overall, our results identify pervasive channel anabranching and some important control mechanisms throughout the world's large river basins, with strong implications for satellite-based discharge retrievals. We conclude that global analyses afforded by remote sensing and cloud computing offer additional, complementary approaches to traditional field-based approaches for understanding large river geomorphic processes and channel forms.

Review and state of the art for the hydro-morphological modeling of transboundary rivers, Tigris River as a case study

Article

Jun 2022

Worldwide, many transboundary rivers suffer from massive changes in morphology due to climate change and increase in the number of hydraulic structures by the riparian countries. In Iraq, the sever hydro-morphological change in the Tigris River during the last 50 years intensified the need to conduct a comprehensive review and assessment for the available methods and modeling tools and carried out studies to investigate the morphological changes in this river. To this end, the main hydro-morphological theories, methods, modeling approaches, and tools were reviewed, as well as the hydrodynamic and morphological studies of the river. This review indicated that most of studies concerned with the Tigris River followed the traditional procedures such as field measurements and field geometry surveying or analyzing an old data obtained from other studies. Also, integral employment of the field measurements with recent modeling techniques and tools was rarely applied. Furthermore, there is no scientific cooperation between the riparian countries and their contribution in common studies almost non-existent. Consequently, most of the Tigris River’s reaches were not included in the measurements and field surveys. However, only the reaches within the main cities along the river were included in these surveys. This paper indicates that high priority should be given to the comprehensive field survey and morphological characteristic measurements. As well as recent investigation and modeling tools must be integrally used. These findings highlight the main challenges facing this issue. This is very important to determine the aspects of support and cooperation between the riparian countries to ensure the sustainable management of the transboundary rivers’ basins.

Ecologie et restauration de la Petite massette (Typha minima Hoppe)

Thesis

Dec 2021

Nadège Popoff

Spatial modeling of river bank shifting and associated LULC changes of the Kaljani River in Himalayan foothills

Article

Full-text available

Feb 2022
STOCH ENV RES RISK A

Channel dynamics is an inherent characteristic of river in the floodplain region. It has some significant impacts on the ecosystem and human life. GIS-based, DSAS and CA-Markov models are efficient techniques to measure historical and predictive changes following channel shifting and LULC change. In this study, forty-eight years (1972-2020) of earth observatory data were taken to demarcate and detect the channel bank position and LULC change along the Kaljani River, located at the eastern Himalayan foothill. During 1998-2008, a very high rate of erosion has been taken place on both the bankline, which are about −4.48 m/y (left bank) and −3.48 m/y (right bank), respectively. The overall result of the predicted bankline represents that the bulky expansion will occur along the left bank, and sediment accretion will take place at the right bank. Among the three zones, both banks of zone 'A' (lower part of the river) are worst affected in the past and present and will follow the same trends in future. The LULC change of all six classes from 1972 to 1998 is very high compared with the changes between 1998 and 2020. Moreover, long profile, hypsometric curve value, and the Soil Conservation Service Curve Number (SCS-CN) value have a significant help in understanding and identifying consequences reasons. The accuracy level is validated by the actual bankline positions (2020) with predicted bankline (2020) and actual LULC (2020) to predicted LULC (2020) empirically with RMSE and statistical test. The accuracy level of this study is conducted with the Kappa statistics for LULC map of 2020, and the result is 87.57%, and bankline shifting RMSE varies from 0.007 to 0.176. Therefore, the prediction output serves as the spatial guidelines for monitoring future trends of channel shift and land use planning management.

Ecologie et restauration de la petite massette (Typha minima Hoppe) (Ecology and restoration of the Dwarf bulrush (Typha minima Hoppe))

Thesis

Full-text available

Dec 2021

Nadège Popoff

La dynamique de la végétation riveraine est étroitement liée aux régimes de perturbation du cours d’eau, notamment les crues, ainsi qu’aux processus physiques et biologiques qui en découlent. L ’impact des activités humaines sur les rivières comme l’endiguement, la chenalisation ou l’exploitation hydroélectrique et sédimentaire, a fortement modifié ces régimes. Cela se traduit, entre autre, par une stabilisation du milieu et une régression des stades pionniers de l’habitat riverain due au manque de rajeunissement de celui-ci par les crues . A l’échelle de l’arc alpin, Typha minima Hoppe (la petite massette) est une espèce herbacée clonale qui colonise les habitats pionniers des cours d’eau de piémont. Cette espèce a vu ses populations fortement décroître au cours du siècle dernier, lui valant le statut d’espèce protégée dans plusieurs pays européens. En France, de nombreuses menaces pesant sur cette espèce persistent tel que la chenalisation, la modification du régime de crue et la destruction de son habitat par des travaux menés dans le lit et sur les berges des cours d’eau. C’est notamment le cas des travaux menés depuis 2004 par le Symbhi (Syndicat Mixte des Bassins Hydrauliques de l'Isère) dans le cadre du projet « Isère Amont » qui ont impacté des stations de T. minima sur l’Isère .Des mesures de compensations en ont résulté et ont accompagné la mise en place d’un projet de restauration de l’espèce dans la zone d’emprise des travaux. L’objectif de ce travail de thèse est de produire des connaissances sur la niche écologique et la dynamique spatiotemporelle d’une espèce clonale colonisant le stade pionnier de l’habitat riverain : la petite massette (Typha minima Hoppe), afin de servir de base théorique aux futurs projets de conservation et de restauration. Pour ce faire, la problématique suivante a été formulée : Quelles sont les variables environnementales et les processus écologiques qui affectent la dynamique spatio temporelle de T. minima et en quoi ces connaissances peuvent elles être appliquées à sa restauration et sa conservation? i) La première étape est de déterminer quels sont les facteurs biotiques et abiotiques liés aux processus sédimentaires et de succession écologique qui affectent l a dynamique locale des taches de T. minima . Les résultats montrent que la progression des taches de T. minima est liée au caractère pionnier des bancs (sédiments fins , faible altitude et couvert végétal). Le maintien et la régression des taches sont au contraire liés à la maturité de l’habitat, induit s par un processus de succession écologique plus avancé, avec un développement de la végétation et une amplification du phénomène d’accrétion. ii) Dans une seconde partie nous avons testé les effets de la compétition interspécifique avec des saules (Salix alba) et de l’ensevelissement par les sédiments sur la compétitivité et l’investissement dans la reproduction de T. minima . Bien que souvent mentionnée comme la principale cause de régression de l’espèce, la compétition interspécifique avec les saules n’affecte que peu T. minima alors que l’effet de l’ensevelissement par les sédiments est important. iii) Enfin, dans le but d’améliorer les protocoles de restauration de T. minima, plusieurs plusieurs expérimentations visant à tester l’effet de différentes biomasses initiales transplantées, la hauteurs par rapport à l’eau, le types de berge, la forme des placettes et l'association avec d’autres espèces pionnières ont été réalisées entre 2013 et 2016. Les résultats montrent que transplanter T. minima avec une biomasse initiale importante sous forme de placette linéaire parallèle au cours d’eau à des hauteurs comprises entre +1.00m et +1.55m est optimal. Ces travaux caractérisent caractérisent d’une part d’une part les facteurs environnementaux et les processus en jeu dans dans la dynamique de T. minima et d’autre part, optimisent les protocoles de restauration contribuant ainsi à alimenter les futurs projets de restauration et de conservation de l’espèce. (Riparian vegetation dynamics are linked to river disturbance regimes including floods as well as physical and biological processes. Human activities such as containment, channelization, hydroelectric dam and sand gravel extraction have modified this regime. The main consequences are the stabilization of environmental parameters and the regression of pioneer stage driven by a lack of habitat rejuvenation by floods. In the Alps, Typha minima Hoppe (the dwarf bulrush) is a clonal herbaceous plant species that colonizes pioneer stage habitat of piedmont rivers. It is a threatened species in the Alps because of the drastic decrease of its population during the last century. In France, the river channelization, the modification of flood regime and the works inside riverbed and banks still threaten relict populations. The Symbhi (Syndicat Mixte des Bassins Hydrauliques de l’Isère) initiated river works in 2004 through the “Isère Amont” project, impacted T. minima populations along the Isère and was mandated to set up a restoration project. The main thesis objective is to produce knowledge about the ecological niche and spatiotemporal dynamics of a clonal plant species (T. minima) that colonizes pioneer riparian habitat. The following question is formulated: What are the environmental variables and ecological processes that affect the spatio-temporal dynamics of T. minima, and how could this knowledge be applied to its restoration and conservation? i) The first step was to determine the biotic and abiotic parameters, linked to ecological succession and sediment accretion, involved in T. minima’s local patch dynamic. The results showed that local patch progression is linked to pioneer bar characteristics (fine sediment, low elevation and low vegetation cover). Patch maintenance and regression are related to more mature bar characteristics, induced by the progress of ecological succession and the accretion process amplification. ii) Second, we experimented ex-situ the effects of interspecific competition and sediment burial on the competitive and reproduction abilities of T. minima. Although considered as the main cause of T. minima regression, we highlighted that interspecific competition with Salix alba has lower effect on T. minima than sediment burial. iii) Last, to improve the restoration methods of T. minima we implemented in-situ experiments between 2013 and 2016. The objectives were to test the effects of initial transplanted biomass, transplantation elevation related to the mean water level, bank type, plot form and association with other pioneer species. The results show that transplantation with high or medium initial biomass in linear plots parallel to the water flow between +1.00m and +1.55m optimizes T. minima population restoration. This work, that characterizes the environmental factors, the processes involved in T. minima’s local patch dynamics and that optimize the design of restoration protocols, contribute to improve future pioneer species restoration and conservation projects.)

Morphological dynamicity and sediment characteristics of channel bars in the piedmont rivers: a study of Darjeeling Himalayan foothill region, India

Article

Full-text available

Mar 2022

The processes of the development of channel bars, their evolution history, morphological dynamicity, varying sediment characteristics along the reaches of the river are governed by the flow regime, channel gradient and bank character of the piedmont zone of the Darjeeling Himalayan foothill region (DHFR). The area covers 80 km × 50 km river corridor of Tista, Jaldhaka, Torsa and tributaries of Lish, Gish, Chel, Mal, Neora, Murti, Kurti, Kumlai and Damdim. Lithofacies, XRD data, bridge scouring, critical stream power, shear stress and sediment grain size samples were collected from the field studies, conducted in the upper, middle and lower piedmont zone of the foothill rivers. Past stream power, rates of aggradation as well as the age of the bars have been estimated following William’s empirical equations and Wilson and Goodbred’s methods. Results of the analysis depicting the rate of hydro-morphological behaviour of the bars vary between monsoonal and non-monsoonal period and the major affected zones are the middle and lower reaches of the piedmont rivers. The past stream power has reported > 90.3841ω to > 510.0668ω values by William's empirical equations. The expected average relative ages of the channel bars are between ± 120 and ± 125 years, ± 60 and ± 65 years and ± 45 and ± 50 years in the upper, middle and lower piedmont zone of DHFR, respectively. Exposed lithofacies analysis of the depositional bar has revealed the fact that the grain sizes of the depositional layers vary from pebble, gravel to sand and silt according to the hydro-morphological character of the channel reach sectors. The mineral composition also changes from quartz, silica, feldspar, iron oxide to silica along the upper to the lower piedmont. Moreover, the effect of hydro-technical activities like road and railway bridges on the transformational character of the channel bars cannot be ignored. Detailed long-term study through measurements is important for the optimum utilization of the stable bars and for the site selection of the roads and railway bridges.

Alluvial Channel Dynamic Associated with LULC Change in Himalayan Foothill

Chapter

Full-text available

Dec 2021

Dynamicity of the channel is the main characteristic of the Kaljani River in the Himalayan foothill. The present work intends to document the historical changes in the land use and land cover (LULC) pattern driven by channel migration during 1987–2020 at the Kaljani River adjacent village area. In this study, the sinuosity index, the radius of curvature, meander wavelength, amplitude, meander width, channel width, arc angle, direction angle, rate of channel migration, and direction of migration have been calculated for the years of 1987, 2004, and 2020. The historical positions of both bankline and dynamic channel width and meander width indicate that a large portion of the floodplain area depicts an erosion-accretion sequence with time. This work also investigated LULC changes in the Kaljani River adjacent village area using supervised image classification with an overall accuracy ranging between 85 and 89%. This research has demonstrated the application and capability of RS and GIS technology and generated a detailed evaluation of temporal and spatial changes in river channel processes and adjustment of LULC types. The LULC result revealed that their water bodies and dense forest are decreased and sandy area and built-up areas are increased. The LULC changes by the direct effect of bankline migration have a bad impact on the dwellers of the floodplain adjacent village area of the Kaljani River. The results of this study can represent an important indicator of the vulnerability of the Kaljani River adjacent village area and also provide information about geomorphological instabilities of the study area.

SIG et télédétection pour la dynamique du lit d'un fleuve : cas de la zone inondable du Sambirano

Conference Paper

Full-text available

Sep 2019

The flow is strongly dependent on the bed which conveys it. On the other hand, this same flow can modify this bed by the phenomenon of erosion or deposition according to the variation of the speed of the flow. This complex and dynamic phenomenon is manifested especially during a flood. A technique using a Geographic Information System (GIS) and a simple remote sensing (RS) are used to characterize the bed change of the Sambirano River (Madagascar) near its mouth towards the Mozambique Channel. The method is applied on a test section to compare the sequential change of its bed during two periods:. The results are maps in which we can clearly see the dynamics. This sequential comparison shows a considerable change after each passage of an intense cyclone, ie a five-year and / or ten-year return period flood. This article shows the results of the passage of two remarkable cyclones: GAFILO (2004) and ELIACKIM (2018). These results illustrate the last modifications of this bed during the last two decades, which are used for different management approaches and / or measures against the risks of floods.

Spatial and temporal variation of fluvial islands and sandbars in River Ganga from Bhagalpur to Farakka during 1955–2019

Article

Full-text available

Jun 2021

The lower portion of Ganga River is wide and braided with numerous fluvial islands and sandbars. This paper investigated the spatial and temporal variation of fluvial islands and sandbars in the Ganga River from Bhagalpur to Farakka during 1955–2019. Historical topographic maps and Landsat data from 1955 to 2019 have been used for the study. Normalized Difference Water Index (NDWI) followed by the classification of the image has been used to produce water and non-water area. Accuracy assessment of the classified image has been done through an error matrix and attained overall accuracy, ranging from 86 to 94%. Islands and sandbars have experienced considerable variation in due course of time. Island at the confluence of the Kosi river has expanded manifold. Areas of attached sandbars have increased with an increase in meander in the river. A sandbar has developed in front of Farakka Barrage, which has now transformed into a vegetated island. This paper also analyzed the correlation between morphological parameters (width, braiding index (BI), sinuosity index (SI)) and hydrological factors (annual maximum discharge and annual high-water level) influencing variation in the area of islands. The result shows a strong correlation between the width of the river and the BI, whereas the weak correlation between the SI and BI. The area of islands and sandbars was inversely related to annual maximum discharge as well as maximum water level. Although variation in the island is a function of several natural factors, the construction of Farakka barrage plays an increasingly important role behind this variation.

Evolution of erosive and hydrodynamic impacts on water quality in tidal channels at the mouths of the Araguari and Amazon rivers

Preprint

Full-text available

Oct 2020

Dynamics of mid-channel bars in the Middle Vistula River in response to ferry crossing abutment construction

Article

Full-text available

Jul 2020

Anna Sosnowska

The investigation focused on the non-migrating mid-channel bars that emerged in the river channel as a consequence of river engineering. A reach of around 280 km of Middle Vistula River (Poland) was taken into account as the study site. The most vivid examples of the river engineering works potentially influencing the river channel are the ferry crossings. Along the Middle Vistula River reach, there are 17 ferry crossings that were constructed from 1970s to 1990s. A cartographic analysis was carried out to determine the influence of the ferry crossing construction over the river channel. It was performed on the basis of aerial photographs (traditional aerial photography and UAV photography). In 3 out of 17 potential localizations, the emergence of large non-migrating mid-channel bars was observed. A study of cartographic materials (from 1950s to 2000s) allowed determination of the dynamics of those bars, including time of emergence and the changes of their size.

Hydrodynamic and geomorphic adjustments of channel bars in the Yichang-Chenglingji Reach of the Middle Yangtze River in response to the Three Gorges Dam operation

Article

Oct 2020
CATENA

The evolution of channel bars in response to upstream damming has significant impacts on channel stability, navigation, and aquatic habitats. Here, the effects of the Three Gorges Dam (TGD) operation on downstream channel bars in the Yichang-Chenglingji Reach (YCR) were comprehensively analyzed using remote sensing images, cross-sectional profiles, and hydrological datasets. The morphodynamic adjustments of channel bars in the YCR were significantly different during the pre- and post-TGD periods (i.e., before and after the construction of the TGD). Specifically, the total area of channel bars did not exhibit any significant trend in the pre-TGD period, but displayed a significant reduction following the construction of the TGD, although the morphodynamic response of each sub-reach was different. The channel bars in the YCR were relatively stable in the pre-TGD period, but became more erodible in the post-TGD period. The length/width ratio (LWR) of the bars showed an overall increase trend during the whole period from 1992 to 2017, not changing before and after the dam construction. The water discharge that led to the greatest channel bars adjustment was 27,000–30,000 m³/s (corresponding to bankfull discharge) in the pre-TGD period and 15,000–18,000 m³/s (corresponding to the medium discharge that can submerge the surface of the bars) in the post-TGD period. In addition, the grain size of non-uniform sediment with the highest replenishment degree gradually reduced downstream; these finer sediments were the main sources of material for the channel bars in the YCR. Quantitative relationships between bars area, the most effective bar-forming discharge and suspended sediment load with the highest replenishment degree, were proposed based on the improved Delayed Response Model (DRM). Results indicate that geomorphic adjustments of channel bars in the YCR are closely related to the previous four-year flow and sediment regimes, implying a delayed response of the fluvial system to damming.

Remote sensing and GIS techniques to monitor morphological changes along the middle-lower Vistula River, Poland

Article

Full-text available

Mar 2020

Michael Nones

Nowadays, the combined use of Remote Sensing data and Geography Information System (GIS) tools is becoming mainstream in evaluating the river channel evolution at many spatiotemporal scales, thanks to the low costs of such techniques and the increased availability of high-resolute satellite images. These methods can be applied to investigate large ungauged basins and accounting for relatively long periods, becoming a precious support tool for water management strategies at the reach scale. The medium-term planform changes of an 80-km reach of the Vistula River downstream of Warsaw, Poland, were evaluated considering the Modified Normalized Difference Water Index (MNDWI) of four Landsat images acquired in 1985, 1994, 2004 and 2017, which referred to similar hydrological conditions. The study highlighted that, by combining satellite information with semi-automatic digital processing procedures developed in GIS, it is possible to see that the reach resulted quite stable in the whole period under a morphological point of view, even if a reduced depositional trend along the banks is visible, especially in the last decade, probably because of a slight increment in the frequency of high water events. The results of this research can be helpful in better designing the future management strategies of this middle-lower reach, also in the light of the inland navigation projects started among Europe and affecting the Vistula River.

Long-Term (1986-2018) Evolution of Channel Bars in Response to Combined Effects of Cascade Reservoirs in the Middle Reaches of the Hanjiang River

Article

Full-text available

Jan 2020

Channel bars are essential landforms and their evolution is crucial to aquatic and riparian biodiversity, river's water-sediment process, and economic development. With the development of water conservation facilities and hydropower projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade dams across the entire downstream area. Using Landsat remote sensing images from 1986-2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1-DX7, XZ1, and XZ2), and three shoal group (XZ3-XZ5) dynamics. Results showed that the mid-channel bars' area in the reach from Danjiangkou to Xiangyang (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars' area change was mainly influenced by backwater from the Cuijiaying Reservoir with high water level after 2010 (r = −0.93, p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel bar area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = −0.79, p < 0.05), which was partly attributed to upstream precipitation according to the high correlation between runoff and precipitation (R 2 = 0.65). In general, the DX section was under equilibrium between scouring and deposition compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986-2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, revetments, flood events, sand mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.

Downstream geomorphic impact of the Three Gorges Dam: With special reference to the channel bars in the Middle Yangtze River

Article

Full-text available

Jun 2019

A review of sediment diversion in the Mississippi River Deltaic Plain

Article

May 2019

One of the proposed methods for restoring the disappearing Mississippi Delta is sediment diversion which uses channels and structures to divert water and sediment from the Mississippi and Atchafalaya Rivers into adjacent basins. This study presents a comprehensive review of geological and physical aspects of sediment dynamics in the Mississippi River Deltaic Plain (MRDP), with special reference to diversion studies over the past two decades. We synthesize these studies, present the current understanding of sediment diversions in the context of sediment dynamics, identify multiple key knowledge gaps, and make recommendations for future studies. To maximize net land building in the MRDP, management strategies should be focused on (a) enhancing river sediment delivery (both mud and sand), (b) increasing sediment retention in receiving basins and (c) minimizing erosion in bays and estuaries. Compared with extensive studies of land building, there have been relatively fewer studies of erosional processes. A heterogeneous coastal geological framework, cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the MRDP. Sediment retention rates are highly sensitive to spatial and temporal scales, types of sediments and delivery season. Sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes. Structures and devices that improve sediment retention, trap sediments, dissipate waves, and build living shorelines should be explored and cost-to-benefit analysis is needed. Long-term planning should consider more landward diversions, strategic community relocation, and nonlinear response of the complex sedimentary system of the MRDP.

Analysis of River Bank Erosion–Accretion Process Using Geospatial approach—A Case Study of the Upper Segment of Yamuna River

Chapter

Jun 2023

River bank erosion and accretion is one of the concerning intrinsic and dynamic phenomena of any alluvial river that initiates the migration process leading to loss of valuable land adjoining the floodplains. The present study quantifies the extent of bank erosion and accretion along the upper segment of Yamuna River by utilizing time series Landsat images of 40 years’ duration (i.e., from 1979 to 2019 with time interval of 10 years). For analysis, the 180 km river segment spanning between Tajewala Barrage and the Wazirabad Barrage was divided into 18 fixed sections at equal intervals of 10 km (i.e., S1 to S18). Results exhibit a decreasing trend in the average areal extent of erosion and an increasing trend in the average areal accretion along both west bank and east bank during the entire assessment period when evaluated with respect to base year 1979. The maximum extent of erosion covering 1200 Ha along west bank was observed at section S10 during 1999 and the same section unveiled lowest erosion coverage of 58.77 Ha during 2019. Likewise, maximum accretion coverage of 1800 Ha was observed at section S6 during 2019 when evaluated with respect to 1979. Moreover, east bank depicted maximum erosion coverage of 1104.06 Ha at S2 during 2019 and maximum accretion coverage of 995.27 Ha at S4 during 1989 evaluated with respect to 1979. The analysis of results further testifies the river behavior as aggrading type and is passing through active accretion phase. The study outcomes can form the basis for setting up spatial guidelines for monitoring and assessing erosion and accretion trends in erodible river banks for sustainable planning and management.KeywordsYamuna RiverTajewala BarrageWazirabad BarrageBank erosion and accretionGIS framework

Upstream River Erosion vis-a-vis Sediments Variability in Hugli Estuary, India: A Geospatial Approach

Article

Full-text available

Mar 2023

Satellite data shows that the Bhagirathi-Hugli River's riverbank has faced severe erosion during the last decades (1990 to 2020), with the middle stretch of the river being more prone to erosion. This huge sediment load derived from upstream erosion is coming to the estuary. The suspended sediment concentration dynamics of the Hugli estuary were calculated using in-situ data and remote sensing reflectance by establishing a linear regression. A continuous huge sediment load is found in the estuarine water. The sediment concentration was higher pre-monsoon than post-monsoon as the region is highly influenced by monsoonal rainfall and runoff. The sediment concentration was also higher in the estuary's southwestern section than in the northern part. The impact of this high sediment load contributes to the deposition. This depositional area assessment was performed using an object-based classification approach called Support Vector Machine utilizing Grey Level Co-occurrence Matrix to create cluster textural indices. Despite the impact of continuous sea level rise in the estuary, the result shows that effective island and Chars areas have increased in the past decade due to the upstream erosion-driven sediments.

Impacts of Dam Operation on Vegetation Dynamics of Mid-Channel Bars in the Mid-Lower Yangtze River, China

Article

Full-text available

Oct 2021

Vegetation dynamics on mid-channel bars (MCBs) is essential for supporting ecosystem functions and associated services in river systems, especially in dammed large rivers. Generally, there are two possible changing patterns that vegetation of MCBs downstream a dam would experience. On one hand, the vegetation area may shrink because of a decrease in the MCB area in the post-dam period, which has been observed in many rivers around the world. On the other hand, the MCB vegetation area may expand because flood disturbances would be weakened by dam operation. However, little evidence has been reported to clarify such confusion. Therefore, vegetation dynamics of MCBs in the mid-lower Yangtze River downstream the Three Gorges Dam (TGD; the world’s largest dam) is selected as a case study to address the issue. Using long-term (1987–2017) Landsat archive images, this study reveals the spatiotemporal variations of vegetation area change intensities (VACIs; indicated by dynamic trends) on MCBs in the mid-lower Yangtze River. Results show that an overall VACI in the post-dam period (2003–2017) is about three times faster than that in the pre-dam period (1987–2002). In other words, the rate of vegetation colonization accelerated after the TGD operation began in 2003. Moreover, the VACIs in the post-dam period are size dependent, where large size MCBs are likely to gain higher VACIs: Small-sized MCBs (0.33 km2/yr), medium-sized MCBs (1.23 km2/yr), large-sized MCBs (1.49 km2/yr). In addition, VACIs of individual MCBs in the post-dam period are distance dependent, where the further a MCB was from the TGD, the higher the VACI. It is also suggested that the weakened flood disturbances in the post-dam could explain the rapid vegetation growth and colonization. This work is not only beneficial for managing and protecting MCBs downstream the TGD after its operation, but is also helpful in understanding vegetation dynamics of MCBs in other dammed river systems around the world.

Environmental Pollution and Natural Resource Management

Book

Jul 2022

Monitoring of Channel Morphology of Ganga River Using Remote Sensing and GIS Data

Chapter

Full-text available

Jul 2022

The present study deals with the dynamics of channel morphology of river Ganga in Haridwar region during the last two decades (1992–2019). Land Use/Land cover classification and channel geometry characteristics of river Ganga were assessed using remote sensing and GIS analysis approach. The LULC classification was divided into three classes: River, Dry Riverbed and River Ganga catchment or floodplain covering an area of 8366.22 ha. Moreover, channel geometry was calculated using sinuosity index and geomorphological features. The study area is divided into 10 segments showing the different types of channel patterns. It was found that LULC classification of river banks depends on water flow and seasonal ecological and anthropogenic activities. The channel pattern of river Ganga is modified by natural processes such as heavy rainfall, floods, bank erosion and anthropogenic processes such as dams, riverbed mining and agricultural activities near the banks.

Recent geomorphological evolution of channel bar in Magdalena River (Colombia) due to natural and anthropogenic interferences

Article

Jun 2022
ESTUAR COAST SHELF S

The growing establishment of port structures in estuarine regions has resulted in alternated sedimentation patterns and new geoforms. The primary purpose of this study is to understand the recent formation and geomorphological evolution of the fluvial bar in the final portion of the Magdalena River. The geomorphological evolution analysis was made possible using Geographic Information Systems tools, satellite images, and bathymetric data. The studied bar formed and stabilized through anthropic and natural factors such as port structures (obstacles to sediment transport), channel morphology, and natural- and anthropic-caused variation to the sediment supply. The lateral bar formation, adjacent to port structures, began in 2017 with surface runoff accumulating and anchored on the riverbank since 2018. At present, the bar covers 45% of the original river width, and an approximate sediment volume worth 4,000,000 m³ has accumulated. Changes in the navigable channel result in slope modification from 2.02° to 5.36°. The construction of port structures, i.e. new pile bridges, changes the natural sediment flow, because they are obstacles, trapping sediments downstream. This period coincides with the end of a weak El Niño cycle, subsequent to a La Nina period also of low intensity, with reduced precipitation rates resulting in a decrease in flow, and therefore contributing to an increase in sedimentation. The observations suggest that it is crucial to understand the channel bars' morphology and origin to avoid possible economic and social impacts due to the disruptions in navigation.

Anthropogenic pressures induced hydromorphodynamic changes of riverine islands in the Upper Jingjiang reach along the Changjiang (Yangtze) River

Article

Oct 2022
CATENA

Riverine islands are widespread fluvial landforms with exceptional economic and environmental values. However, anthropogenic pressures have further induced drastic changes worldwide in fluvial islands, which endanger fluvial organism habitation and enable potential ecological degradation. Here, the hydromorphodynamics on these prominent riverine landforms of the seriously regulated Upper Jinjiang reach (UJR) of the Changjiang (Yangtze) River were detected based on the Google Earth Engine (GEE) platform between 1988 and 2019. The results indicated that total vegetated islands area in the UJR experienced expansion, while bars area exhibited a dramatic decrease by 15% between 1988 and 2019. Given the total areas and their subaquatic bars' volume variations, the island's evolution could be divided into “deposition–erosion–stabilization” stages. During the deposition phase from 1988 to 1999, the bars volumes showed a gradually increasing trend, and then it shifted to erosion between 1999 and 2010. After 2010, the islands maintained stability with a total volume of approximately 177 × 10⁶ m³. The multi-year sediment and water discharge variations contributed to the evolution of riverine islands. In particular, resulting from the Three Gorges Dam (TGD) project in 2003, the fluvial sediment load and duration of overbank discharges (25000 m³/s) decreased significantly, which were the dominant factors inducing islands with their subaquatic bars shrinkage. Recent multiple island protection projects were responsible for defending islands with their subaquatic bars against erosion and maintaining their stability. This work reveals the islands' evolution in the UJR response to the TGD effect and local artificial engineering, which is vital for managing riverine islands in global rivers affected by nature and human activities.

Influence of the Slot Configuration on Its Stability (On the Example of the Ob River)

Chapter

Mar 2022

The article discusses the issue of assessing the stability of dredging slots in the Ob River section. An important challenge in the dredging and straightening works production is the issue of the dredging slots stability. The experience of researching the issue is considered, a brief review of the literature is presented. The necessity of developing regional strategy for assessing the slots stability has been substantiated. Field data were obtained and analyzed in the course of track works on the Ob River cripples. A regional strategy for assessing the working width loss in dredging slots made on the Ob River “Novosibirsk - Tom river mouth” reach cripples is proposed. The dependences of the width loss in the slot on time after the work production on the roll, which make it possible to estimate the value of such losses, were obtained. The ways of further research of the dredging and navigable slots stability issue are outlined. River transport, having a number of advantages over other modes of transport, is an important link in the network of transport communications in Russia. It is of particular importance for the Siberian regions. Normal functioning of inland waterways transport in general, and river transport, in particular, is impossible without the implementation of a complex of track works and, above all, dredging works. An important task in maintaining the existing or increasing the fairway overall dimensions is the efficient production of dredging and river training works.

Role of Active Tectonism and Geomorphic Drivers on Channel Oscillation of the Raidak-I River in the Eastern Himalayan Foothills, India

Chapter

Full-text available

Apr 2022

The dynamicity of the channel is the main characteristic of the Raidak-I River in the eastern Himalayan foothill. The present study evaluates riverbank migration in association with erosion-deposition changes along the river Raidak-I using DSAS models. The present work intends to evaluate the relationship between the riverbank erosion-deposition and geomorphological and tectonic adjustment. For the study, earth observatory data like MSS, TM, ETM+ and OLI datasets of 1972, 1979, 1987, 1995, 2003, 2011, and 2020 have been used to demarcate the bankline position. Temporal analysis reveals that the river has changed its bank position by extensive erosion-accretion processes and modified its floodplain area uses significantly. The historical positions of both banklines indicate that a large portion of the floodplain area depicts an erosion-accretion sequence with time. In the timeframe of the last 48 years, the Raidak-I River has an average erosion-deposition at -0.23 m/year on the right bank and 1.57 m/y on the left bank. A general observation from this research is that the most dynamic or migrant part of the river is zone A and zone B compared to zone C which is relatively stable. In this study, the river course in zone C (both banks) is the most dynamic part of this entire river. The changes by the direct effect of banking migration have a bad impact on the dwellers of the floodplain adjacent village area of the river. The results of this study can represent an important indicator of the vulnerability of the Raidak-I River buffer area and also provide information about the geomorphological instabilities of the study area.

Estimation and Prediction of Riverbank Erosion Susceptibility and Shifting Rate Using DSAS, BEHI, and REBVI Models: Evidence from the Lower Ganga River in India

Preprint

Full-text available

Nov 2021

The process of riverbank erosion (RE) is often accelerated by natural events and anthropogenic activities leading to the transformation of this natural process to natural hazard. The present study aims to estimate bank erosion rate and prediction of the lower Ganga River in India using digital shoreline analysis system (DSAS) model. The prediction of RE susceptibility mapping has been generated using three ensemble models such as DSAS, bank erosion hazard index (BEHI), and river embankment breaching vulnerability index (REBVI). For the study satellite images and field data (bank materials, geotechnical parameters, embankment structure, hydraulic pressure etc.) have been used to recognize the river bank position and BEHI and REBVI scores. During 1973-2020, the average bank erosion and accretion rate was found 0.059 km/y and 0.022 km/y at the left bank while 0.026 km/y and 0.046 at the right bank respectively. The prediction results illustrated that the very high vulnerable condition of 06 villages and 21 villages for high vulnerable due to left bank erosion. BEHI and REBVI scores have been the significant performance of understanding and identification of RE vulnerable areas. The long-term (2020-2045) average erosion and deposition rate was predicted at 0.135 km/y and 0.024 km/y at the left bank and 0.043 km/y and 0.045 km/y at the right bank respectively. The prediction accuracy and validation of models were measures by statistical techniques such as student’s t-test, RMSE, and R ² values. This study would be help planners and decision makers the spatial guidelines to understanding future trends of bank erosion and shifting rate for land-use planning and management strategies to protect riverbank.

Geological evolution of the Mississippi River into the Anthropocene

Article

Full-text available

Nov 2021

The Mississippi River maintains commercial and societal networks of the USA along its >3700 km length. It has accumulated a fluvial sedimentary succession over 80 million years. Through the last 11,700 years of the Holocene Epoch, the wild river shaped the landscape, models of which have become classic in geological studies of ancient river strata. Studies of the river were led by the need to develop infrastructure and to search for hydrocarbons, through which, these models have become quite sophisticated. However, whilst the models demonstrate how the wild river behaves, a monumental shift in fundamental controls on the entire fluvial system, broadly coinciding with the proposed mid-20th century onset of the Anthropocene Epoch, has generated new geological patterns that are becoming globally ubiquitous, and which the Mississippi River typifies. As such, whilst classic Holocene river models may be compared to human-modified systems such as the Lower Mississippi River (and others worldwide), locally the models may now only directly apply to its fossilized components preserved in the sub-surface. Such river models need adapting to better understand the present dynamics, and future evolution of these landscapes.

Quantifying delta channel network changes with Landsat time-series data

Article

Jul 2021
J HYDROL

Delta channel networks (DCNs) are highly complex and dynamic systems that are governed by natural and anthropogenic perturbations. Challenges remain in quickly quantifying the length, width, migration, and pattern changes of deltaic channels accurately and with a high frequency. Here, we develop a quantitative framework, which introduces a water occurrence algorithm based on Landsat time-series data and spatial morphological delineation methods, in order to measure DCN structures and associated changes. In examining the Pearl River Delta (PRD) and Irrawaddy River Delta (IRD) as case studies, we analyze their conditions and trends between 1986–2018 at ten-year intervals. Both study areas have undergone various human interventions, including dam construction, sand mining, and land use change driven by urbanization. Our results show the following: (1) the use of a 0.5 water occurrence extraction based on Landsat time-series data, morphological delineation, and spatial change analysis methods can quantify the morphodynamics of DCNs effectively with a root-mean-square error of 15.1 m; (2) there was no evident channel migration in either PRD or IRD with average channel widths of 387.6 and 300.9 m, respectively. Most channels in the PRD underwent remarkable shrinkage, with average rates of 0.4–6.4 m/year, while there were only slight changes in the IRD, which is consistent with observed trends in sediment load variation. The results of this research have the potential to contribute to sustainable river management in terms of flood prevention, riparian tideland reclamation, and water and sediment regulation. Moreover, the proposed framework can be used to develop a new global river width dataset and can be generalized to remotely sensed water discharge and river depth estimation.

Spatial Modeling of River Bank Shifting and Associated LULC Changes of the Kaljani River in Himalayan Foothills

Preprint

Full-text available

Mar 2021

Channel dynamics is an inherent characteristic of the river in the floodplain region. The river bankline shifting and associate land use land cover (LULC) change is not only geomorphological but also an environmentally vital hazardous issue. It is a significant impact on the ecosystem and human life. GIS-based, DSAS and CA-Markov models are efficient to accurately measure historical and predictionevaluation of the relation between channel shifting and LULC change. In this study, forty-eight years (1972-2020) of earth observatory data have been used to demarcate the channel bank position and LULC change detection along the Kaljani River at the eastern Himalayan foothill. During 1998-2008, very high erosion rate on both bankline, which are about -4.48 m/y and -3.48 m/y at the left and right, respectively compared the others time frame. The overall result of the predicted bankline represents that the bulky expansion will occur along the left bank and sediment accretion will take place at the right bank. Among the three zones, both banks of zone ‘A’ (lower part of the river) is the worst affected part in the past, present, and future time. The LULC change of all six classes from 1972 to 1998 was very high when compared with the change between 1998 and 2020. Moreover, the long profile, hypsometric curve value, and the Soil Conservation Service Curve Number (SCS-CN) value have been a significant help in understanding and identification of consequences reasons. The level of accuracy is validated by the observed bankline positions (2020) with predicted bankline (2020) and observed LULC (2020) to predicted LULC (2020) empirically with RMSE and statistical test. Therefore, the output of the prediction not only serves as the spatial guidelines for monitoring future trends of channel shifting and land use planning management.

River Meandering

Chapter

Jan 2020

J. M. Hooke

Since early quantification of equilibrium relations of meander morphology to discharge and sediment, research has been pursued empirically, theoretically, and experimentally. The theoretical approaches have sought to provide fundamental explanations of meander development and produced numerical simulations. Empirical work, using field, map, and remote sensing evidence, has demonstrated variations in meander morphology, and stability and the evolution of meanders over time to compound forms and cut-offs; it has elucidated process mechanisms and interactions. Flume work has investigated the effects of particular conditions. Technological advances are enabling acquisition of high-resolution data and more sophisticated modelling, facilitating a convergence of approaches, and providing increased insights into the complexity and variability of meander morphology, changes and mechanisms.

Estimating bed material fluxes upstream and downstream of a controlled large bifurcation - the Mississippi-Atchafalaya River diversion

Article

Full-text available

Apr 2020

Bed material transport at river bifurcations is crucial for channel stability and downstream geomorphic dynamics. However, measurements of bed material transport at bifurcations of large alluvial rivers are difficult to make, and standard estimates based on the assumption of proportional partitioning of flow and bedload transport at bifurcations may be erroneous. In this study, we employed a combined approach based on observed topographic change (erosion/deposition) and bed material transport predicted from a one‐dimensional model to investigate bed material fluxes near the engineering‐controlled Mississippi‐Atchafalaya River diversion, which is of great importance to sediment distribution and delivery to Louisiana's coast. Yang's (1973) sediment transport equation was utilized to estimate daily bed material loads upstream, downstream, and through the diversion during 2004–2013. Bathymetric changes in these channels were assessed with single beam data collected in 2004 and 2013. Results show that over the study period, 24% of the Mississippi River flow was diverted into the Atchafalaya River, while the rest remained in the mainstem Mississippi. Upstream of the diversion, the bed material yield was predicted to be 201 million metric tons (MT), of which ~35 MT (i.e., 17%) passed through the bifurcation channel to the Atchafalaya River. The findings from this study reveal that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%); Conversely, the diversion channel receives a disproportionate amount of flow (24%) relative to bed material supply (17%). Consequently, severe bed scouring occurred in the controlled Outflow Channel to the Atchafalaya River, while riverbed aggradation progressed in the mainstem Mississippi downstream of the diversion structures, implying reduced flow capacity and potential risk of a high backwater during megafloods. The study demonstrates that Yang's sediment transport equation provides plausible results of bed material fluxes for a highly complicated river diversion and that integration of the sediment transport equation with observed changes in sediment storage is a valuable approach to investigate sediment dynamics at controlled river bifurcations. This article is protected by copyright. All rights reserved.

Long-term geomorphic response to flow regulation in a 10-km reach downstream of the Mississippi - Atchafalaya River diversion

Article

Full-text available

Dec 2016

A recent study reported considerable sediment trapping by three large channel bars downstream 18–28 km of the Mississippi–Atchafalaya River diversion (commonly known as the Old River Control Structure, ORCS) during the 2011 Mississippi River flood. In this study, we analyzed 3-decadal morphological changes of the 10-km river channel and the three bars to elucidate the long-term effects of river engineering including diversion, revetment and dike constructions. Satellite images captured between 1985 and 2015 in approximate 5-year intervals were selected to estimate the change of channel morphology and bar surface area. The images were chosen based on river stage heights at the time when they were captured to exclude the temporal water height effect on channel and bar morphology. Using a set of the satellite images captured during the period of 1984–1986 and of 2013–2014, we developed rating curves of emerged bar surface area with the corresponding river stage height for determining the change in bar volume from 1985 to 2013. Two of the three bars have grown substantially in the past 30 years, while one bar has become braided and its surface area has shrunken. As a whole, there were a net gain of 4,107,000 m2 in surface area and a net gain of 30,271,000 m3 in volume, an equivalent of approximately 36 million metric tons of sediment assuming a bulk density of 1.2 t/m3. Sediment trapping on the bars was prevalent during the spring floods, especially during the period of 1990–1995 and of 2007–2011 when large floods occurred. The results suggest that although revetments and dikes have largely changed the morphology of the channel and the bars, they seem to have a limited impact on the overwhelming trend of sediment deposition caused by the river diversion.

Sediment Trapping by Emerged Channel Bars in the Lowermost Mississippi River during a Major Flood

Article

Full-text available

Nov 2015

The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR). However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m2. Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m3. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.

Spatial and temporal adjustment of the Lower Mississippi River channel to major human impacts

Article

Full-text available

Jun 2006

The Mississippi is the largest river in North America and is considered one of the most humanly manipulated large river system on Earth. Analysis of a series of high-resolution hydrographic surveys documents the morphologic adjustment of the Lower Mississippi River thalweg to land use changes and engineering modifications from the late nineteenth century through the twentieth century. Extensive land degradation from the late 1800s to early 1900s in the Upper Mississippi is associated with channel aggradation in the Lower Mississippi River. Following the mid-1900s, reduction in sediment input to the Lower Mississippi due to dams and land management in the upper basin resulted in an opposite response, as channel incision ensued. Incision caused by main-stem channel engineering (cut-offs) is spatially distinguished from incision caused by a reduction in sediment supply. Since about the mid-1970s the channel has reached a new period of stability manifest by a stable thalweg profile with defined pools. The findings of this study are significant because they define the maximum extent of channel degradation possible while under human influence, the time-scale for returning to stability, and the range of variability that can be expected under the present regulated regime. Large river systems subjected to significant human disturbance may not return to a "natural" morphology, but can develop a new stable form within several decades.

Rapid Subsidence and Historical Wetland Loss in the Mississippi Delta Plain: Likely Causes and Future Implications

Technical Report

Full-text available

Jan 2005

Estimation of sedimentation rates in the distributary basin of the Mississippi River, the Atchafalaya River Basin, USA

Article

Full-text available

Jul 2013

The Atchafalaya River Basin (ARB) is the largest distributary basin of the Mississippi River composing anastomosing channels, backwater swamps, freshwater marshes, and wetland forests. Sedimentation in the ARB has presented management issues concerning habitat changes from open water areas to bottomland hardwood forests. A thorough understanding of sediment transport and deposition in the basin is not only required for proper management of the ARB, but is crucial for regional sediment budgets that affect the Mississippi River Delta Plain. In this study, we calculated 31 years (1980-2010) of total suspended sediment (TSS) inflow and outflow of the Atchafalaya River to quantify the long-term sediment retention in the basin. We then estimated sedimentation rates in the basin by spatially relating the retention with changes of turbid water area derived from Landsat imagery. The study found an annual average TSS inflow of 54.0 megatonnes (MD and an annual average TSS outflow of 48.7 MT, resulting in an average annual retention of 5.3 MT. Spatially derived mean sedimentation rates were estimated between 0.06 and 0.153 mm d(-1). The spatial estimates for sedimentation proved promising and with more sediment data available could become an invaluable tool for managing the ARB in the future.

A Hydrograph-Based Sediment Availability Assessment: Implications for Mississippi River Sediment Diversion

Article

Full-text available

Mar 2014

Y. Jun Xu

The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980–2010) suspended sediment yield as affected by different hydrologic regimes to determine actual suspended sediment availability and how this may affect diversion management. A stage hydrograph-based approach was employed to quantify total suspended sediment load (SSL) of the lower Mississippi River at Tarbert Landing during three river flow conditions: Peak Flow Stage (stage = 16.8 m, discharge >32,000 m3 s−1), High Flow Stage (stage = 14.6 m, discharge = 25,000–32,000 m3 s−1), and Intermediate Flow Stage (Stage = 12.1 m, discharge = 18,000–25,000 m3 s−1). Suspended sediment concentration (SSC) and SSL were maximized during High Flow and Intermediate Flow Stages, accounting for approximately 50% of the total annual sediment yield, even though duration of the stages accounted for only one-third of a year. Peak Flow Stage had the highest discharge, but significantly lower SSC (p < 0.05), indicating that diversion of the river at this stage would be less effective for sediment capture. The lower Mississippi River showed significantly higher SSC (p < 0.0001) and SSL (p < 0.0001) during the rising than the receding limb. When the flood pulse was rising, Intermediate Flow and High Flow Stages showed greater SSC and SSL than Peak Flow Stage. Together, Intermediate Flow and High Flow Stages on the rising limb annually discharged 28 megatonnes over approximately 42 days, identifying this to be the best period for sediment capture and diversion.

A new longitudinal approach to assess hydrologic connectivity: Embanked floodplain inundation along the lower Mississippi River

Article

Full-text available

Jul 2013

Hydrologic connectivity is fundamental to understanding floodplain processes along meandering river corridors. This study contributes to understanding hydrologic connectivity by utilizing a high resolution Light Detection and Ranging DEM with a new GIS-based approach for identifying the precise elevation of flood stage. The method created a high-resolution longitudinal channel bank profile, enabling a detailed examination of embanked floodplain hydrologic connectivity. A simple channel cross-section approach is likely to result in a large underestimation of floodplain inundation and hydrologic connectivity along meandering river floodplains because of differences in the elevation of the natural levee surface relative to floodplain bottoms and the considerable variability in the elevation of the channel bank profile. There is a large disparity in discharge duration associated with floodplain inundation in comparison to river channel bank inundation. A discharge duration of 10% is associated with inundation of 87% of the floodplain surface whereas only 53% of the channel bank profile is overtopped. An apparent threshold in inundation occurs as the discharge duration decreases below about 35%. While a duration of 25% results in very little of the channel bank being overtopped, it inundates 50% of the floodplain surface. Floodplain borrow pits, which are created in association with dike construction, represent a constant anthropogenic influence on the lower Mississippi embanked floodplain morphology, and are inundated by low discharge magnitudes. The results of the investigation shed new light on the topic of hydrologic connectivity for an important embanked fluvial system that has previously received little attention concerning its physical floodplain processes. Copyright © 2013 John Wiley & Sons, Ltd.

Sinking Deltas due to Human Activities

Article

Full-text available

Sep 2009
NAT GEOSCI

Many of the world’s largest deltas are densely populated and heavily farmed. Yet many of their inhabitants are becoming increasingly vulnerable to flooding and conversions of their land to open ocean. The vulnerability is a result of sediment compaction from the removal of oil, gas and water from the delta’s underlying sediments, the trapping of sediment in reservoirs upstream and floodplain engineering in combination with rising global sea level. Here we present an assessment of 33 deltas chosen to represent the world’s deltas. We find that in the past decade, 85% of the deltas experienced severe flooding, resulting in the temporary submergence of 260,000 km2. We conservatively estimate that the delta surface area vulnerable to flooding could increase by 50% under the current projected values for sea-level rise in the twenty-first century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.

Flood Trends and River Engineering on the Mississippi River System

Article

Full-text available

Dec 2008

Along >4000 km of the Mississippi River system, we document that climate, land-use change, and river engineering have contributed to statistically significant increases in flooding over the past 100-150 years. Trends were tested using a database of >8 million hydrological measurements. A geospatial database of historical engineering construction was used to quantify the response of flood levels to each unit of engineering infrastructure. Significant climate- and/or land use-driven increases in flow were detected, but the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction. In the area of the 2008 Upper Mississippi flood, for example, about 2 m of the flood crest is linked to navigational and flood-control engineering. Systemwide, large increases in flood levels were documented at locations and at times of wing-dike and levee construction.

Trends in flood stages: Contrasting results from the Mississippi and Rhine River systems

Article

Full-text available

Dec 2006
J HYDROL

An obstacle to testing human impacts on flooding is that anthropogenic and natural hydrologic changes occur simultaneously and are often indistinguishable. We attempted to isolate the impacts of overlapping mechanisms on flooding by analyzing 73–188 years of stage measurements from the Mississippi River system and the German Rhine. Although typically under-utilized in hydrologic analysis, stage data document the cumulative impacts of both upstream changes such and climate and land use as well as modifications of the river channel and floodplain. Analysis of trends in peak stages and flood frequencies shows contrasting results on the Mississippi and Rhine: increasing trends at most Mississippi stations, but no significant trends at most Rhine stations. On the Mississippi and its tributaries, statistically significant changes in peak stages and peak-stage frequencies were identified at 10 of 14 stations, with all but one exhibiting increased flooding. On the Rhine River, systematic change occurred at only 1 of 8 stations analyzed. On both river systems, channel and floodplain modifications dominated net hydrologic response, overwhelming the effects of climate change, land-use shifts, and dam construction. Specific-gage analysis, which isolates the impacts of instream river modifications, documented declining flow conveyance at all stations where flood levels and frequencies increased. Increased flooding at the Mississippi River sites appears to be driven by the history of aggressive channel engineering, in particular channel constriction to increase navigation depths. In contrast, navigation infrastructure on the Rhine has not substantially degraded the river’s capacity to efficiently convey flood flows.

Mid-Channel bar growth and its relationship to local flow strength and direction

Article

Full-text available

Feb 1996
EARTH SURF PROC LAND

Philip J. Ashworth

Anabranches of braided rivers typically migrate and avulse across the floodplain to produce new channel junctions, scour and subsequent mid-channel bar growth immediately downstream. Few quantitative studies have been made of this bar development process and the link to change in channel geometry and local flow strength and direction. This paper provides data on the spatial and temporal pattern of surface velocity as mid-channel bar growth is initiated downstream of a fixed junction scour in a generic scale flume model. The sequence of channel changes is: (i) development of a confluence scour with flow convergence and maximum velocity in the channel centre; (ii) exceedance of the local transport capacity and initial stalling of coarse sediment in the channel thalweg downstream of the scour; (iii) bar growth through entrapment of all sizes of bedload; (iv) change from velocity maximum to minimum and flow convergence to divergence when the bar height is approximately 55 per cent of the thalweg depth; (v) broadening of the bartop platform, a drop in local competence and bankward migration of the two distributaries whose cross-section and velocity remains approximately constant. These flume data and interpretations are compared to descriptions in the literature of the braiding process with particular reference to the flume work of Leopold and Wolman (1957) and Ashmore (1991, 1993). A new model for mid-channel bar growth is presented which helps explain the long-term development of the confluence–diffluence unit.

Dam impacts on the Changjiang (Yangtze) River sediment discharge to the sea: The past 55 years and after the Three Gorges Dam

Article

Full-text available

Apr 2006

1] In 5 recent years (2000–2004), the Changjiang (Yangtze) River has discharged past Datong (600 km from the river mouth) an average of $250 million tons (mt) of sediment per year, a decrease of more than 40% since the 1950s and 1960s, whereas water discharge at Datong has increased slightly. Water and sediment discharge data from the upper, middle, and lower reaches of the river suggest that the reduction of the Changjiang sediment load has occurred in two phases between 1950 and 2002: following the closure of the Danjiangkou Reservoir on the Hanjiang tributary in 1968 and following the installation of numerous dams and water-soil conservation works in the Jialingjijang catchment after 1985. As the Three Gorges Dam (TGD) started operating in 2003, the Changjiang entered a third phase of sediment reduction with annual sediment loads at Datong less than 200 mt/yr. Upon completion of the Three Gorges Dam (TGD) in 2009, the sediment load at Datong will decrease to $210 mt/yr for the first 20 years, then will recover to $230 mt/yr during 2030–2060, and will reach $310 mt/yr during 2060–2110. From the sediment budget and sediment erosion data for the Changjiang subaqueous delta, it can be assumed that the delta will be eroded extensively during the first five decades after TGD operation and then will approach a balance during the next five decades as sediment discharging from TGD again increases.

Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment

Article

Full-text available

Oct 2008
GEOMORPHOLOGY

Flood management alters fundamental fluvial processes that have geomorphic consequences for rivers and floodplains. The Lower Mississippi and Rhine Rivers (The Netherlands) are two important examples of intensively regulated large rivers. Understanding the magnitude and direction of change caused by flood management requires a long-term perspective. This is particularly true of large lowland fluvial systems because of substantial lag-times required for adjustment to be manifest in the floodplain geomorphology. This study is a historical analysis and synthesis of the impacts of flood management on the Lower Mississippi and Rhine Rivers (The Netherlands), and investigates the interrelations of flood management with floodplain geomorphology.

Anthropogenic Sediment Retention: Major Global Impact from Registered River Impoundments

Article

Full-text available

Oct 2003
GLOBAL PLANET CHANGE

In this paper, we develop and apply a framework for estimating the potential global-scale impact of reservoir construction on riverine sediment transport to the ocean. Using this framework, we discern a large, global-scale, and growing impact from anthropogenic impoundment. Our study links information on 633 of the world's largest reservoirs (LRs) (≥0.5 km3 maximum storage capacity) to the geography of continental discharge and uses statistical inferences to assess the potential impact of the remaining >44,000 smaller reservoirs (SRs). Information on the LRs was linked to a digitized river network at 30′ (latitude×longitude) spatial resolution. A residence time change (ΔτR) for otherwise free-flowing river water is determined locally for each reservoir and used with a sediment retention function to predict the proportion of incident sediment flux trapped within each impoundment. The discharge-weighted mean ΔτR for individual impoundments distributed across the globe is 0.21 years for LRs and 0.011 years for SRs. More than 40% of global river discharge is intercepted locally by the LRs analyzed here, and a significant proportion (≈70%) of this discharge maintains a theoretical sediment trapping efficiency in excess of 50%. Half of all discharge entering LRs shows a local sediment trapping efficiency of 80% or more. Analysis of the recent history of river impoundment reveals that between 1950 and 1968, there was tripling from 5% to 15% in global LR sediment trapping, another doubling to 30% by 1985, and stabilization thereafter. Several large basins such as the Colorado and Nile show nearly complete trapping due to large reservoir construction and flow diversion. From the standpoint of sediment retention rates, the most heavily regulated drainage basins reside in Europe. North America, Africa, and Australia/Oceania are also strongly affected by LRs. Globally, greater than 50% of basin-scale sediment flux in regulated basins is potentially trapped in artificial impoundments, with a discharge-weighted sediment trapping due to LRs of 30%, and an additional contribution of 23% from SRs. If we consider both regulated and unregulated basins, the interception of global sediment flux by all registered reservoirs (n≈45,000) is conservatively placed at 4–5 Gt year−1 or 25–30% of the total. There is an additional but unknown impact due to still smaller unregistered impoundments (n≈800,000). Our results demonstrate that river impoundment should now be considered explicitly in global elemental flux studies, such as for water, sediment, carbon, and nutrients. From a global change perspective, the long-term impact of such hydraulic engineering works on the world's coastal zone appears to be significant but has yet to be fully elucidated.

Sedimentation in the Three Gorges Dam and the future trend of Changjiang (Yangtze River) sediment flux to the sea

Article

Full-text available

Nov 2009

The Three Gorges Dam (TGD) on the upper Changjiang (Yangtze River), China, disrupts the continuity of Changjiang sediment delivery to downstream and coastal areas. In this study, which was based on 54 years of annual water and sediment data from the mainstream and major tributaries of Changjiang, sediment deposition induced by the TGD in 2003–2008 was quantified. Furthermore, we determined the theoretical trapping efficiency of the cascade reservoir upstream of the TGD. Its impact on Changjiang sediment flux in the coming decades is discussed. Results show that about 172 million tons (Mt) of sediment was trapped annually by the TGD in 2003–2008, with an averaged trapping efficiency of 75%. Most of the total sediment deposition, as induced by the TGD (88%), accumulated within the region between the TGD site and Cuntan. However, significant siltation (12% of the total sediment deposition) also occurred upstream of Cuntan as a consequence of the upstream extended backwater region of the TGD. Additionally, the Changjiang sediment flux entered a third downward step in 2001, prior to operation of the TGD. This mainly resulted from sediment reduction in the Jinshajiang tributary since the late 1990s. As the cascade reservoir is put into full operation, it could potentially trap 91% of the Jinshajiang sediment discharge and, therefore, the Jinshajiang sediment discharge would most likely further decrease to 14 Mt/yr in the coming decades. Consequently, the Changjiang sediment flux to the sea is expected to continuously decrease to below 90 Mt/yr in the near future, or only 18% of the amount observed in the 1950s. In the presence of low sediment discharge, profound impacts on the morphology of estuary, delta and coastal waters are expected.

Estimation of sedimentation rates in the distributary basin of the Mississippi River, the Atchafalaya River Basin, USA

Article

Jan 2015

The Atchafalaya River Basin (ARB) is the largest distributary basin of the Mississippi River composing anastomosing channels, backwater swamps, freshwater marshes, and wetland forests. Sedimentation in the ARB has presented management issues concerning habitat changes from open water areas to bottomland hardwood forests. A thorough understanding of sediment transport and deposition in the basin is not only required for proper management of the ARB, but is crucial for regional sediment budgets that affect the Mississippi River Delta Plain. In this study, we calculated 31 years (1980-2010) of total suspended sediment (TSS) inflow and outflow of the Atchafalaya River to quantify the long-term sediment retention in the basin. We then estimated sedimentation rates in the basin by spatially relating the retention with changes of turbid water area derived from Landsat imagery. The study found an annual average TSS inflow of 54.0 megatonnes (MT) and an annual average TSS outflow of 48.7 MT, resulting in an average annual retention of 5.3 MT. Spatially derived mean sedimentation rates were estimated between 0.06 and 0.153 mm d-1. The spatial estimates for sedimentation proved promising and with more sediment data available could become an invaluable tool for managing the ARB in the future.

Analysis of the Impacts of Dikes on Flood Stages in the Middle Mississippi River

Article

Oct 2013

The US Army Corps of Engineers has constructed numerous river engineering structures in and along the Middle Mississippi River. River training and bank stabilization measures include pile dikes, stone dikes, bendway weirs, chevrons, and revetments. Concerns have long been voiced about the effects of these structures on flood stages. Recent debate concerning the scientific basis for these concerns has been invigorated by specific gauge analysis that appears to show a rising trend in flood stages over the past 150 years. This paper attempts to advance the debate by providing an objective review of the specific gauge analysis technique that explains how the method should be performed and the results interpreted; identifies strengths and limitations; examines the uncertainties associated with application to the Middle Mississippi River given the available data; and reassesses the conclusions that can and cannot reasonably be drawn regarding the impacts of dikes and levees on flood stages, based on specific gauge analysis of the Middle Mississippi River. The application of specific gauge analysis to records from the St. Louis gauge, by using methods that carefully follow the guidelines set out in this paper, provides no evidence that dike construction has raised flood stages, but indicates that rises in flood stages are more probably related to levee building.

Sand as a stable and sustainable resource for nourishing the Mississippi River delta

Article

Apr 2014

The Mississippi River delta is undergoing a catastrophic drowning, whereby 5,000 km2 of low-lying wetlands have converted to open water over at least the past eight decades, as a result of many anthropogenic and natural factors. Continued net land loss has been thought inevitable due to a decline in the load of total suspended sediment--both sand and mud--carried by the river. However, sand--which accounts for ~50-70% of modern and ancient Mississippi delta deposits but comprises only ~20% of the sampled portion of the total load--could be more important than mud for subaerial delta growth. Historically, half of the Mississippi River sediment load is supplied by the Missouri River. Here we analyse suspended sediment load data from two locations downstream from the lowest Missouri River dam to show that the measured sand load in the lower 1,100 km of the Mississippi River has not significantly diminished since dam construction. A one-dimensional numerical model of river morphodynamics predicts that the sand load feeding the delta will decrease only gradually over the next several centuries, with an estimated decline from current values of no more than about 17% within the coming six centuries. We conclude that the lower Mississippi River channel holds a significant reservoir of sand that is available to replenish diminished loads via bed scour and substantially mitigate land loss.

Land loss rates: Louisiana Coastal Plain

Article

Jan 1993
J COASTAL RES

Land loss mapping and rate curve development for 62 quadrangles in the Mississippi River deltaic and chenier plains shows that land loss rates and trends vary significantly throughout coastal Louisiana. Land loss rates were defined for each quadrangle for four time periods (1930's to 1956-1958, 1956-1958 to 1974, 1974 to 1983, and 1983 to 1990). Differences in land loss rates among the individual quadrangles are a function of the geologic and hydrologic setting and the factors which contribute to land loss such as subsidence, storm induced erosion, channelization of streams and rivers, and canal dredging. On a regional scale, the land loss rate for the entire Louisiana Coastal Plain has decreased from an average yearly rate of 41.83 square miles in the 1956-1958 to 1974 period to 25.34 square miles during the 1983 to 1990 period. The regional land loss rate will probably continue to decrease slowly until a background rate is reached. -from Authors

The Mississippi Delta Region: Past, Present, and Future

Article

May 2012

The Mississippi delta region of south Louisiana houses a wealth of resources within a dynamic, subsiding landscape. Foundations for the delta region reflect Neogene evolution of the depocenter, whereas details of the modern landscape reflect late Pleistocene to Holocene evolution of the alluvial-deltaic plain. The Holocene delta plain was constructed by cyclical growth of deltaic headlands, followed by avulsion and relocation of the fluvial sediment source. Abandoned headlands were then submerged and reworked while a new headland was created at the site of active fluvial sediment input. Historic-period levees have decoupled the delta plain from its fluvial sediment source at the same time global sea-level rise was accelerating, which has accelerated delta-plain submergence. Diversions of Mississippi River water and sediment are necessary to achieve delta plain sustainability, but upstream dams trap ∼50% of the total sediment load, and there is not enough supply to keep pace with subsidence and accelerated...

Drowning of the Mississippi Delta due to Insufficient Sediment Supply and Global Sea-Level Rise

Article

Jun 2009

Over the past few centuries, 25% of the deltaic wetlands associated with the Mississippi Delta have been lost to the ocean1. Plans to protect and restore the coast call for diversions of the Mississippi River, and its associated sediment, to sustain and build new land2, 3. However, the sediment load of the Mississippi River has been reduced by 50% through dam construction in the Mississippi Basin, which could affect the effectiveness of diversion plans4, 5, 6. Here we calculate the amount of sediment stored on the delta plain for the past 12,000 years, and find that mean storage rates necessary to construct the flood plain and delta over this period exceed modern Mississippi River sediment loads. We estimate that, in the absence of sediment input, an additional 10,000–13,500 km2 will be submerged by the year 2100 owing to subsidence and sea-level rise. Sustaining existing delta surface area would require 18–24 billion tons of sediment, which is significantly more than can be drawn from the Mississippi River in its current state. We conclude that significant drowning is inevitable, even if sediment loads are restored, because sea level is now rising at least three times faster than during delta-plain construction.

Channel bar dynamics on multi‐decadal timescales in an active meandering river

Article

Nov 2011
EARTH SURF PROC LAND

Occurrence and development of channel bars are major components of the morphodynamics of rivers and their relation to river meandering has been much explored through theory and experimentation. However, field and documentary data of characteristics and evolution over timescales from years to several decades are lacking. Four sets of aerial photographs in the period 1984–2007 were used to map and quantify bar numbers and areas in GIS on an active meandering reach. Bar types were classified. Additional temporal resolution was provided by annual ground photography and mapping for 1981–2010. Analysis was extended backward by use of large scale Ordnance Survey maps from 1873 onwards. As expected, point bars are the most common type but ‘free’ bars of several types are major components of bar deposition. Point bars and attached bars are significantly larger in size than mid-channel and side bars. Spatial distribution of bars varies down the reach and over time but is related to channel sinuosity, gradient and mobility and to bend evolution. Different types of bar occur in distinctive channel locations, with point and concave-bend bars in zones of high curvature. Bar activity shows a relation with discharge events and phases and possibly with changing riparian conditions, but superimposed on this is a common sequence of bar evolution from incipient gravel mid-channel bars to full floodplain integration. This life-cycle is identified as 7–9 years on average. No evidence for mobility of free bars within the course is found. The results are compared with bar and bend theory; the bars are forced and conform in general to bend theory but detailed variation relates to geomorphic factors and to autogenic sequences of bends and bars. Mid-channel bars are width induced. Variability of bar occurrence needs to be taken into account in river management and ecological evaluation, including for the EU WFD. Copyright © 2011 John Wiley & Sons, Ltd.

Modelling mid‐channel bars in meandering channels

Article

Jun 2010
EARTH SURF PROC LAND

An analytical modelling framework is proposed to reproduce the frequently observed but poorly studied occurrence of mid-channel bars in meandering channels. Mid-channel bars occur in meanders and may characterize transitional morphologies between pure meandering and braided rivers. Based on existing field and experimental observations, we propose that two different mechanisms can generate central topographical patterns in meanders. A former mechanism (‘width-forced’) is related to spatial width oscillations which determine a laterally symmetrical bed shear stress pattern that promotes mid-channel bars. A second mechanism (‘curvature-forced’) can take place also in curvilinear equiwidth streams since also longitudinal variations of channel curvature can produce laterally symmetrical alterations of the sediment transport capacity. A perturbation approach is employed to model both mechanisms within a common framework, allowing reproduction, at least qualitatively, of several observed features. While width-forced mid-channel bars are a symmetric linear altimetric response, to reproduce curvature-forced mid-channel bars requires modelling nonlinear flow-bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to field cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of mid-channel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the flow against the two banks, a process which is proposed as a possible cause of cross-sectional overwidening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this first modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between mid-channel bars and width variations in river meanders. Copyright © 2010 John Wiley & Sons, Ltd.

Morphological changes of the Lower Mississippi River: Geomorphological response to engineering intervention

Article

Dec 2005
RIVER RES APPL

During the twentieth century, the planform and profile of the Lower Mississippi River from Cairo, Illinois, to New Orleans, Louisiana, have been transformed by a series of engineering modifications. These include steepening of the long profile by removal of the most sinuous bends, extensive bank stabilization, and regulating sediment movement by dyke field construction. Prior to these modifications, the Lower Mississippi River adjusted its morphology in the planform, long profile and cross-section. Planform adjustment has, however, effectively been negated and other adjustments are now constrained. Nevertheless, analysis of hydrographic surveys between 1949 and 1989 demonstrates that geomorphological response during the post-cutoff period remained complex. Morphological adjustments involved phased patterns of aggradation and degradation, together with changes in cross-sectional form, and in the number, size, location and shape of pools and crossings. Greatest changes occurred in the early post-cutoff period (1949–64) upstream from Vicksburg, Mississippi, but were accompanied by complementary changes elsewhere which propagated downstream. The combined set of responses may be interpreted with respect to a dynamic equilibrium in which the river responded to additional energy created by the cutoffs by increasing and adapting flow resistance over various scales and time periods. This study helps resolve paradoxes from previous analyses, and has significance for interpreting past engineering impacts and for suggesting future management strategies for the Lower Mississippi River. Copyright © 2005 John Wiley & Sons, Ltd.

Human modifications to the sediment regime of the Lower Mississippi River flood plain

Article

Dec 2003
GEOMORPHOLOGY

Richard H. Kesel

The Mississippi River is one of the most regulated rivers in the world. Human modifications constructed mainly after 1920 include dams and reservoirs, artificial levees, dikes, concrete revetments and a series of channel cutoffs. This paper examines some of the effects of these modifications on the channel and sediment budget of the river. In particular, the changes to the thalweg profile and the size of channel bars are examined in detail. It is concluded, that prior to the 1930s, when major modifications were introduced, the Lower Mississippi River was an aggrading meandering river. The role of the flood plain has also changed. Prior to modifications, the flood plain was the major sediment source as the result of bank caving. Today the flood plain provides only a minor amount of sediment. It can be shown that major degradation to the channel including the growth of channel bars has occurred as a result of these engineered modifications. The data also indicates that the different geomorphic regions respond to modifications in different ways. D 2003 Elsevier Science B.V. All rights reserved.

50,000 dams later: Erosion of the Yangtze River and its delta

Article

Nov 2010
GLOBAL PLANET CHANGE

Using 50 years of hydrologic and bathymetric data, we show that construction of ~ 50,000 dams throughout the Yangtze River watershed, particularly the 2003 closing of the Three Gorges Dam (TGD), has resulted in downstream channel erosion and coarsening of bottom sediment, and erosion of the Yangtze's subaqueous delta. The downstream channel from TGD reverted from an accretion rate of ~ 90 Mt (1Mt = 1000 000 t)/yr between the mid-1950 s and mid-1980 s to an erosion rate of ~ 60 Mt/yr after closing of the TGD. The delta front has devolved from ~ 125 Mm 3 (1 Mm 3 = 1000 000 m 3)/yr of sediment accumulation in the 1960 s and 1970 s, when river sediment load exceeded 450 Mt/yr, to perhaps 100 Mm 3 /yr of erosion in recent years. As of 2007 erosion seemed to have been primarily centered at 5–8 m water depths; shallower areas remained relatively stable, perhaps in part due to sediment input from eroding deltaic islands. In the coming decades the Yangtze's sediment load will probably continue to decrease, and its middle-lower river channel and delta will continue to erode as new dams are built, and the South-to-North Water Diversion is begun.

An approximation of the sediment budget of the lower Mississippi River prior to major human modification

Article

Nov 1992
EARTH SURF PROC LAND

Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015

Abstract and Figures

No full-text available