Recent morphological evolution of the Lower Mississippi River (original) (raw)
Related papers
Channel Evolution Models as Predictors of Sediment Yield
JAWRA Journal of the American Water Resources Association, 2017
This paper recounts our predictions of channel evolution of the Black Vermillion River (BVR) and sediment yields associated with the evolutionary sequence. Channel design parameters allowed for the prediction of stable channel form and coincident sediment yields. Measured erosion rates and basin-specific bank erosion curves aided in prediction of the stream channel succession time frame. This understanding is critical in determining how and when to mitigate a myriad of instability consequences. The BVR drains approximately 1,062 km 2 in the glaciated region of Northeast Kansas. Once tallgrass prairie, the basin has been modified extensively for agricultural production. As such, channelization has shortened the river by nearly 26 km from pre-European dimensions; shortening combined with the construction of numerous flow-through structures have produced dramatic impacts on discharge and sediment dynamics. Nine stream reaches were established within three main tributaries of the BVR in 2007. Reaches averaged 490 m in length, were surveyed, and assessed for channel stability, while resurveys were conducted annually through 2010 to monitor change. This work illustrates the association of current stream state, in-channel sediment contributions, and prediction of future erosion rates based on stream evolution informed by multiple models. Our findings suggest greater and more rapid sedimentation of a federal reservoir than has been predicted using standard sediment prediction methods.
Geomorphology, 2005
Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events.
Professional Paper, 1994
Channelization of alluvial channels in West Tennessee has increased energy conditions along main stems and tributaries and initiated systematic trends in channel adjustment. Gradation processes and adjustment trends are a function of the magnitude and extent of an imposed disturbance on a stream channel and the location of the adjusting reach in the fluvial network. Degradation at a site is described by power-decay equations. Exponents denoting the nonlinear rate of downcutting with time decrease with distance upstream from the area of maximum disturbance, and generally range from-0.002 to-0.04. Aggradation occurs in reaches immediately downstream from the area of maximum disturbance and in upstream reaches following overadjustment by degradation, and also can be described by power equations. Aggradation rates increase linearly with distance downstream and reach a maximum of 0.12 meters per year. Adjustment of channel width by mass-wasting processes follows degradation and continues through the aggradational phases. Bank instabilities are induced after downcutting creates bank heights and angles that exceed the critical conditions of the material. Piping in the loess-derived bank materials enhances bank-failure rates. Development of the bank profile is defined in terms of three dynamic and observable surfaces: vertical face (70° to 90°), upper bank (25° to 50°), and slough line (20° to 25°). Both the vertical face and upper bank sections represent major failure planes, and masses of failed bank material often come to rest on the upper bank. The slough line develops from additional flattening and downslope movement by low-angle slides and fluvial reworking, and is the initial site of reestablishing riparian vegetation and stable bank conditions. A six-stage, semiquantitative model of channel evolution in disturbed channels is developed by quantifying gradation trends, by interpreting process-response relations during stages of bank-slope development, and by interpreting changes over space as changes over time.
Extreme sediment pulses generated by bend cutoffs along a large meandering river
Nature Geoscience, 2011
In meandering rivers, bend cutoffs have long been recognized as an important mechanism of change in the path of the channel. Meander bend cutoffs can develop by the progressive migration of an elongated bend onto itself, which forms a neck cutoff, or by the erosion of a new channel across the neck of the bend, which is known as a chute cutoff 1 . River cutoffs affect channel navigation 2 , and form meander scars and oxbow lakes in river floodplains 1,3,4 , which are important habitats for riparian ecosystems 5 . The importance of cutoff processes in meander dynamics is well established 1,3,4,6-8 , but the effects of cutoffs on overall sediment flux are poorly characterized. Here we use aerial imagery, global positioning system mapping and measurements of channel bathymetry to estimate the amount of sediment released by two chute cutoffs on the Wabash River in the Midwestern USA. We find that each event triggered the rapid delivery of sediment into the river, at rates that are one to five orders of magnitude larger than those produced by lateral migration of individual bends. We find that much of this material was deposited immediately downstream, at the confluence of the Wabash and Ohio rivers, which led to significant changes in channel morphology. This sedimentation ultimately impeded barge traffic and necessitated extensive dredging.
A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River
/ The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d 95 values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.
Evolutionary trajectory of channel morphology and controlling factors in a large gravel-bed river
Geomorphology, 2012
Interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology, a quantitative analysis of controlling factors and, finally, identification of links between evolutionary trajectories and controlling factors. The aim of this paper is to explain channel adjustments and controlling factors in the Tagliamento River, a large gravel-bed river in northeastern Italy. Traditional methods for studying historical channel changes (i.e., use of aerial photos, topographic data) were employed, but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution. River channel underwent three main phases of adjustment over the last 200 years. The first two phases, from the end of the nineteenth century to the early 1990s, were characterized by narrowing (channel width decreased from 1250 to 540 m) and incision (about 1 m); and the third phase, from the 1990s to present day, by widening (from 540 to 600 m) and slight aggradation (about 0.2 m). Combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long-term channel evolution of the Tagliamento River was driven mainly by human intervention at the reach scale (i.e., sediment mining and channelization). Changes in sediment supply in the catchment area had no, or minor, effects in the study reach. The most recent changes (i.e., widening and slight aggradation) are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach. Magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes, while this was not the case during the previous phases of adjustment.
Science of The Total Environment
Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km2 Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998-2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.
Geomorphology, 2003
Three-dimensional morphological adjustment in a chute cutoff (breach) alluvial channel is quantified using Digital Elevation Model (DEM) analysis for a ca. 0.7 km reach of the River Coquet, Northumberland, UK. Following cutoff in January 1999, channel and bar topography was surveyed using a Total Station on five occasions between February 1999 and December 2000. Analysis of planform change coupled with DEM differencing elucidates channel and barform development following cutoff, and enables quantification of sediment transfers associated with morphological adjustment within the reach. This exercise indicates an initial phase of bed scour, followed by a period characterised by extensive bank erosion and lateral channel migration where erosion (including bed scour) totalled some 15,000 m 3 of sediment. The channel in the post-cutoff, disequilibrium state is highly sensitive to relatively low-magnitude floods, and provision of accommodation space by bank erosion encouraged extensive lateral bar development. Bar development was further facilitated by infilling of channels abandoned by repeated within-reach avulsion and large-scale aggradation of sediment lobes deposited by higher magnitude floods. Calculations indicate that at least 6600 m 3 of sediment was deposited on emerging bars within the reach over the survey period, and >2300 m 3 deposited within the channel. Sediment losses from the reach may have exceeded 6500 m 3 .