Cohesive Bank Research Articles

Putting down roots: Afforestation and bank cohesion of Icelandic Rivers

AbstractRiparian vegetation is widely recognized as a critical component of functioning fluvial systems. Human pressures on woody vegetation including riparian areas have had lasting effects, especially at high latitude. In Iceland, prior to human settlement, native downy birch woodlands covered approximately 15%–40% of the land area compared to 1%–2% today. Afforestation efforts include planting seedlings, protecting native forest remnants, and acquiring land areas as national forests. The planted and protected nature of vegetation along rivers within forests provides a unique opportunity to evaluate the various taxa within riparian zones and the channel stabilizing characteristics of the vegetation used in afforestation. We investigated bank properties, sediment textures, and root characteristics within riparian zones along four rivers in forests in Iceland. Bank sediment textures are dominantly sandy loam overlying coarser textures. Undercut banks are common because of erosion of the less cohesive subsurface layer. Quantitative root data indicate that the woody taxa have greater root densities, rooting depths, and more complex root structures than forbs or graminoids. The native downy birch has the highest root densities, with <1 mm roots most abundant. Modeling of added bank cohesion indicates that willow provides up to six times and birch up to four times more added cohesion to the coarse sediment textures comprising stream banks compared to no vegetation. We conclude that planting and protecting the native birch and willow helps to reduce bank erosion, especially where long‐term grazing exclusion can be maintained.

Meandering river evolution in an unvegetated permafrost environment

Meandering river sedimentary facies are associated with the rise of terrestrial vegetation in the rock record, but modern examples of highly sinuous, single-threaded streams are also found in many modern non-vegetated environments. Prior workers have speculated that permafrost or ice-cemented sediment could provide the cohesion needed to generate single-threaded, meandering channels, however, the presence of bank vegetation in the overwhelming majority of terrestrial permafrost sites in which meandering rivers are present has made it impossible to disentangle the relative impact of bank cohesion provided by frozen sediments from that provided by roots. Here, we examine topographic change over 13 years at the Onyx River, an unvegetated, permafrost-affected, sand-and-gravel-bottomed river in the McMurdo Dry Valleys (MDV) of Antarctica, supported by ∼40 years of image data. On the basis of lidar change detection and satellite/airborne image analysis, we find that meandering river processes (cut bank erosion, point bar growth, and meander migration) can occur in this ice-cemented, unvegetated permafrost river system. We find that the Onyx River's upper, single-threaded reach is meandering, and that bank cohesion is provided by ice-cemented permafrost. Meander migration rates within the Onyx's meandering reach are extremely close to values predicted based on observations of meander migration rates in other unvegetated rivers, however, the short (∼70 day) duration of Onyx River discharge suggests these migration rates should be considered low estimates, which could be up to ∼5 times greater during the short, summer erosion season. Together, these observations suggest that ice-cemented permafrost is a weak form of bank cohesion that can result in rapid landscape change in response to thermokarstic fluvial erosion, producing dynamic river channels, especially where warm meltwater encounters ice-rich permafrost deposits.

Quantitative analysis of riverbank collapse mechanisms based on BSTEM

ABSTRACT Riverbank collapse is a very common phenomenon in natural rivers that plays an important role in their evolution. In this study, we used the Bank Stability and Toe Erosion Model (BSTEM) to study quantitatively the effects of soil properties and bank inclination on its stability. For this, we investigated the stability of homogeneous banks under different bank inclination angles and binary structure banks composed of different materials. Our findings show differences between the cohesive and non-cohesive bank collapse modes, under the condition of ignoring the water table. We find the internal friction angle to be the dominant factor affecting non-cohesive banks, whereas cohesive banks are affected by the effective cohesion. The safety factor and water depth variations follow a quadratic polynomial law, more pronounced in non-cohesive banks. A higher inclination reduces the riverbank stability and safety factor. The latter is sensitive to bank slope changes for larger effective internal friction angles in non-cohesive banks. The material composition and thickness of the upper and lower layers of the riverbank with a binary structure affect the bank stability. The thickness and mechanical properties of the lower material play a key role in riparian stability.

Riparian Groundwater Level Variation and Its Impacts on Bank Erosion in the Middle Yangtze River

AbstractThe interaction between groundwater flow and river flow plays an important role in the bank erosion process, but field measurements on riparian groundwater flows remain very limited, especially in large river systems. Three monitoring wells of groundwater flow were constructed at two typical sites of the Middle Yangtze River (MYR) in 2021, with the hydrographs of the riparian groundwater level and river stage being obtained. Results indicated that (a) the variation in the groundwater level generally followed with the river stage during the flood peak period, but decoupled from the river stage during the recession period; (b) the delayed response of the groundwater level to river stage variation was quantified by the ratio of their change rates, which linearly increased with the magnitude of the river stage, but decreased with the change rate of the river stage as a loose power function; and (c) the increasing ground water level resulted in a 61% loss of the cohesion of upper cohesive bank soil and a complete loss of the apparent cohesion of the lower noncohesive bank soil. The major mechanism underlying the influences of the groundwater flow on bank erosion in the MYR is probably the development of pore water pressure that changes the forces acting on soil block and reduces the soil shear strength, whereas seepage erosion is seemingly not intensive. The current study can provide a direct evidence for the response of the riparian groundwater level variation to the river stage change in a large river system.

The problem of paleo-planforms

Abstract Reconstructing river planform is crucial to understanding ancient fluvial systems on Earth and other planets. Paleo-planform is typically interpreted from qualitative facies interpretations of fluvial strata, but these can be inconsistent with quantitative approaches. We tested three well-known hydraulic planform predictors in Cretaceous fluvial strata (in Utah, USA) where there is a facies-derived consensus on paleo-planform. However, the results of each predictor are inconsistent with facies interpretations and with each other. We found that one of these predictors is analytically best suited for geologic application but favors single-thread planforms. Given that this predictor was originally tested using just 53 data points from natural rivers, we compiled a new data set of hydraulic geometries in natural rivers (n = 1688), which spanned >550 globally widespread, sand- and gravel-bed rivers from various climate and vegetative regimes. We found that the existing criteria misclassified 65% of multithread rivers in our data set, but modification resulted in a useful predictor. We show that depth/width (H/W) ratio alone is sufficient to discriminate between single-thread (H/W > 0.02) and multithread (H/W < 0.02) rivers, suggesting bank cohesion may be a critical determinant of planform. Further, we show that the slope/Froude (S/Fr) ratio is useful to discriminate process in multithread rivers; i.e., whether generation of new threads is an avulsion-dominated (anastomosing) or bifurcation-dominated (braided) process. Multithread rivers are likely to be anastomosing when S/Fr < 0.003 (shallower slopes) and braided when S/Fr > 0.003 (steeper slopes). Our criteria successfully discriminate planform in modern rivers and our geologic examples, and they offer an effective approach to predict planform in the geologic past on Earth and on other planets.

Achieving Equilibrium as a Semi‐Alluvial Channel: Anthropogenic, Bedrock, and Colluvial Controls on the White Clay Creek, PA, USA

AbstractBased on well‐developed hydraulic geometry relations for width and depth, classic studies initially interpreted the Mid‐Atlantic White Clay Creek (WCC) as a quasi‐equilibrium, alluvial channel. Subsequent studies document the legacy of colonial‐age watershed disturbances and urban development, confounding earlier classifications. To investigate this matter, we contribute new data from reach‐scale geomorphic mapping, and observations and modeling of bed material transport. WCC's longitudinal profile reflects a history of bedrock incision, while hydraulic geometry equations for width and depth indicate quasi‐equilibrium cross‐sectional adjustment. Alluvial landforms such as pools and riffles, bars, and actively forming floodplains occur at all 12 study sites, but exposures of bedrock and colluvium are also common. The ratio of bankfull to threshold Shields stress averages 1.41 (range: 0.41–2.63), suggesting that WCC is an alluvial, threshold, gravel‐bed river. However, a numerical model of WCC bed material transport and grain size, calibrated to bedload tracer data, demonstrates that 22% (range: 8%–73%) of bed material is composed of immobile, locally sourced cobbles and boulders, while the remaining bed material represents mobile, sand to cobble‐sized alluvium; this leads us to classify WCC as a semi‐alluvial river. Additional computations suggest that channel morphology is insensitive to bed material supply. Field observations imply that bankfull Shields stresses do not represent channel adjustments to achieve stable banks; rather, width adjustment likely reflects cohesive bank processes. Despite the numerous and contradictory labels applied to WCC (i.e., quasi‐equilibrium, Anthropocene, bedrock, semi‐alluvial, and gravel‐bed), each term contributes insight that any single conceptual model would be unable to provide alone.

Critical transition of incising gravel channel to evacuate alluvial lateral supply

ABSTRACT Incising gravel channels respond with a robust critical transition to the lateral alluvial supply. Previous experiments have studied how supply affects bedload transport, but I studied a setup with converging banks, where the supply–transport relation implies feedback. While increasing flows, a short range of transitional flows showed sharp increase of bedload, accompanied by channel response via longitudinal homogenization (connectivity) and maximum sediment storage. To slightly vary initial bed mixing (channel history), I repeated the experiments three times, which validated the robustness of the transition. This transition synchronized hydraulics and transport along the channel, leading to a wetted width consistent with downstream hydraulic geometry that allowed a critical sediment evacuation with minimum energy and bed alteration. For flows higher than the transitional, longitudinal connectivity persisted, as most of the landslide material was redistributed by fluvial action. Finally, the largest flows dissipated energy excess via coarsening and channel migration, akin to non-incising rivers with floodplain. A conceptual model of the geomorphic cycle of gravel river reaches might contain the proposed critical transition, but only if this transition proves to be robust to bank cohesion and to spatial and temporal size of the experimental setup.

Riverbank stabilization based on the modulation of the near bank turbulence scales

Sundarban area of the lower Gangetic plain experiences embankment failure almost every year due to the formation of toe undercuts. Waves generated due to the continuous action of wind and tidal currents are paramount in the growth and development of these undercuts. Existing literature suggests that grids are effective in modulating the scales of turbulence. The present investigation is carried out with the objective of looking into whether grids can be effective in controlling the undercutting process and thereby restraining the failure of these embankments. To explore this artefact, laboratory flume experiments were carried out using cylindrical grid placed near the toe of a cohesive bank. Turbulent 3-D velocity was measured using micro-Acoustic Doppler velocimeter (ADV) at the junction of the grid and at the near bank region to get insight into the governing mechanism of grid-influenced flow on the bank. The present investigation revealed that the installation of the cylindrical grids at the toe region of the bank is effective in modulating the scales of turbulence. The scales of Reynolds shear stress fluctuations showed a reduced magnitude due to the presence of near bank grid. The results show that the grid breaks the large scales (larger eddies) of turbulence into smaller eddies through eddy cascading process and thereby modulate the turbulent shear stresses. This probably slowed down the erosion rate preventing the formation of the undercuts and thus restraining bank/embankment failure. Thus, it is envisaged that the proposed methodology on implementation at the riverbanks would serve as an effective measure in the protection of these embankments.

Characterization of Downstream Channel Morphology of a Monsoon Dominated Dwarkeswar River in West Bengal

Abstract Downstream changes in channel morphology and its flow were investigated over a monsoon dominated Dwarkeswar river from the western part of West Bengal. The basin has developed over the Proterozoic Granite Gneiss Complex to recent Holocene alluvium, forming three distinctive geomorphological regions of the basin e.g., dissected plateau, plate-fringe and alluvial plain area. Sixty cross-sections along the entire main stream were surveyed and bankfull channel parameters were measured. Sediment samples from each location were collected and Manning’s roughness (n) value for respective reaches were estimated and flow velocity, discharge, stream power and shear stress were calculated. The result has shown that channel width and channel capacity in the dissected plateau are increasing downstream, in the plateau-fringe area the trends are relatively constant. But in alluvial plain, width (87.3%), flow area (81.3%), discharge (84.1%), W/D ratio (91.3%) decreases downstream. Downstream decreasing channel width and channel capacity are strongly related to elevation, slope, HSI, TSI and SSI. Extremely low slope, channel switching, cohesive bank materials and vast flood plain width facilitates the river flow to spill over from the channel and spread to the surrounding area resulting diminishing discharge and thereby the channel morphology in downstream. Frequency of flow crossing bankfull limit has increased downstream resulting frequent flood diminishing channel morphology with distance down the valley.

Experimental study of effects of riverbed composition on the riverbank erosion process

Riverbank erosion is a common natural river process that threatens the security of instream structures, as well as public and private property. In this study, two sets of tests were performed (direct shear tests and flume tests) to study the effects of riverbed soil composition on the stability of riverbanks. The results of the direct shear tests demonstrated that the riverbank material became increasingly cohesive with increasing clay content and decreasing water content. The internal friction angle decreased monotonically with increasing clay content, but increased towards a peak value, before decreasing thereafter, with increasing water content. The results of the flume tests showed that, for each riverbank material, an increase in bed mobility (non-cohesive bed material > cohesive bed material > fixed bed) increased the equilibrium channel width, riverbank erosion volume and bed degradation. In the first 25 min of the test, the temporal channel width was greater when the bed mobility was lower; after 45 min, the temporal channel was wider when the bed mobility was higher. For each bed material, the riverbank erosion volume and temporal channel width were greater with a non-cohesive bank than with a cohesive bank.

Downstream Decreasing Channel Capacity of a Monsoon-dominated Bengal Basin River: A Case Study of Dwarkeswar River, Eastern India

Downstream changes in channel morphology and flow over the ephemeral Dwarkeswar River in the western part of the Bengal Basin, eastren India were investigated. The river stretches from the Proterozoic Granite Gneiss Complex to the recent Holocene alluvium, forming three distinctive geomorphological regions across the river basin: the pediplane and upper and lower alluvial areas. Sixty cross-sections from throughout the main trunk stream were surveyed and the bankfull width, depth, cross-sectional area, and maximum depth were measured. Sediment samples from each location were studied and the flow velocity, stream power, Manning’s roughness coefficient, and shear stress were estimated. The results show that the bankfull channel cross-section area, width, width-to-depth ratio, and channel capacity increased between the beginning and middle of the river. Thereafter, the size of the river started to decrease in the lower alluvial area. This was characterized by gentle gradients, cohesive bank materials with grass cover, and channel switching. Within the lower part of the river, the channel capacity was observed to diminish as the drainage area increased. This increased the bankfull flow frequency and accelerated large floodwater losses in the floodplain via overbank flows and floodways.

Decreasing Downstream Channel Capacity of a Low-Lying Ephemeral River of Bengal Basin, Eastern India

This paper discusses various aspects of channel morphology on an ephemeral Dwarkeswar River from the western part of the Bengal Basin. Geologically, it extends from the Proterozoic Granite Gneiss Complex to recent Holocene alluvium, creating three typical geomorphic regions, e.g. dissected plateau, plateau-fringe and alluvial plain. Sixty cross sections from the source to the mouth of the river were surveyed, and the bankfull channel parameters were measured. Consequently, sediment samples were collected and Manning’s roughness coefficient were determined to estimate velocity, discharge and stream power. The exponents of hydraulic geometry (width, depth and velocity) vary significantly with respect to physiographic divisions of the study area. Width, width–depth ratio and channel capacity enlarge up to the plateau-fringe area. Thereafter, reduction of channel capacity, width and W/D ratio has been observed in the alluvial plain area by 81.3%, 87.3% and 91.3%, respectively, which is associated with lowering of sea level, high topographic sinuosity index, extremely elongated basin, very low slope, wide flood plain area and cohesive bank materials. So, channel capacity reduces in the downstream direction and floods became an unavoidable part of this region.

Cohesive river bank erosion mechanism under wave-current interaction: A flume study

The effect of hydrodynamic forces due to combined action of surface waves and current on the riverbank is critical to understand sediment entrainment, transport and bank line retreatment process. In understanding the temporal effect of turbulent structures under induced wave-current flow, a series of laboratory experiments were carried out. Micro-Acoustic Doppler Velocimeter (ADV) and Ultrasonic Ranging System (URS) were used simultaneously for the measurement of velocity fluctuations and bank undercut depth increment. Modulation of the turbulent flow characteristics and the benefaction of turbulent bursting structures at the initiation of erosion process and before the failure of the cohesive bank due to undercut progression are discussed. The results show that velocity and Reynolds shear stress have direct dependence on the size and rate of the entrainment of cohesive aggregates from bank face. The effect of wave-current motion leads to an increase in shear stress at the lateral bank giving rise to erosion and transportation of sediment particles/aggregates. Quadrant analysis of the random velocity fluctuation under wave-current flow at the initiation of erosion process shows strong presence of ejection and sweep events. Findings from the present study may provide a better understanding on the design of cohesive bank erosion control measures.

Geomorphic variability of submarine channelized systems along continental margins: Comparison with fluvial meandering channels

Understanding submarine sediment conduits is a challenging yet rewarding task given its applications in offshore industries and source-to-sink studies. Despite flow structure dissimilarities, sinuous submarine sediment conduits display morphologies similar to alluvial meandering rivers. An extensive quantification of degrees of (dis)similarity is however lacking. This study combines (i) a new geomorphic classification of submarine sediment conduits relying on cross-sectional data only, complemented by (ii) measurements of planform (sinuosity, wavelength, amplitude) and cross-sectional (bankfull width, depth, section area) parameters to compare alluvial meandering and submarine systems. We apply the methods to a worldwide dataset including 254 measurements from 35 modern systems. We identify four types of submarine sediment conduits including composite (i.e., canyon and valley) and unit (i.e., incised or leveed channel) types. The following findings arise. (i) Submarine sediment conduit geomorphology is strongly controlled by slope, mostly depending on location along the continental margin. Composite and erosive submarine sediment conduits are located on the continental slope and unit and constructive channels extend down to the basin floor. (ii) Submarine unit leveed channels form a consistent group of constructive, higher-order, smaller-size and more laterally mobile submarine sediment conduits. (iii) Submarine unit leveed channels are the most analogous to alluvial meandering rivers. (iv) Without discharge contribution from tributaries, the width and the area of long-running submarine sediment conduits increase as they migrate towards the abyssal plains. Such behavior is similar to rivers, although it is better explained for submarine sediment conduits by progressive flow deconfinement, and decreasing bank cohesion and friction angle.

Equilibrium Cross Section of River Channels With Cohesive Erodible Banks

Predicting the equilibrium cross section of natural rivers has been widely investigated in fluvial morphology. Several approaches have been developed to meet this aim, starting from regime equations to the empirical formulations of Parker et al. (2007) and Wilkerson and Parker (2011), who proposed quasi‐universal relations for describing bankfull conditions in sand and gravel bed rivers. Nevertheless, a general physics‐based framework is still missing, and it remains an open issue to better clarify the basic mechanisms whereby a river selects its width. In this contribution we focus our attention on lowland rivers with cohesive banks, whose resistance to erosion is crucial to control the river width. In particular, we formulate a theoretical model that evaluates the equilibrium width of river cross sections modeling the interaction between the core flow in the central part of the section and the boundary layer that forms in the vicinity of the cohesive banks. The model computes the cross‐section equilibrium configuration by which the shear stresses on the banks equal a critical threshold value. These stresses are computed by partitioning the total shear stress into an effective grain roughness component and a form component (Kean and Smith, 2006a). The model is applied to a large data set, concerning both sand and gravel bed rivers, and it is used to determine the relations expressing the channel width and the bankfull flow depth to the bankfull discharge, which appear to provide a unitary description of bankfull hydraulic geometry.

Fluvial Erosion Rate of Cohesive Streambanks Is Directly Related to the Difference in Soil and Water Temperatures

Despite decades of research in the field of cohesive soil scour, a major challenge in water resource engineering is an understanding of the fundamental processes governing the erosion of cohesive streambank soils. Given that cohesive soil erodibility is affected by many factors simultaneously, it is necessary to study these factors independently to obtain insights into the erosion process. Three natural soils with different mineralogies were chosen for this study: montmorillonite‐dominated fat clay, vermiculite‐dominated lean clay, and kaolinite‐ and illite‐dominated silty sand. The soils were remolded at maximum dry densities and optimum moisture contents and subjected to 15‐min erosion tests in a laboratory flume. Erosion tests were performed at water temperatures of 15 and 25°C and corresponding soil temperatures of 0, 15, and 25°C, and 15, 25, and 40°C. Test results show that, irrespective of soil type, erosion rate increased with an increase in water temperature but decreased with an increase in soil temperature. When soil and water temperatures were equal, there was no significant change in erosion rate (α = 0.05). Further analyses showed that, irrespective of soil type, erosion rate was a function of the difference in soil and water temperatures and not either temperature alone, indicating that the important thermal factor in the erosion process was the difference in soil and water temperatures. These results show the importance of accounting for soil and water temperatures in erosion studies and suggest that the use of stormwater control measures to control runoff temperatures may be necessary to combat streambank degradation resulting from urbanization.Core Ideas Fluvial erosion of cohesive streambanks is affected by soil and water temperatures. Soil and water temperatures have a coactive effect on cohesive streambank erosion. Increases in stream temperature increase fluvial erosion rates of cohesive banks. Increased streambank temperature reduces fluvial erosion rates of cohesive soils. Urban stormwater runoff temperatures should be reduced to maintain channel stability.

Sidewall shear stress distribution effects on cohesive bank erosion in curved channels

A series of experiments were conducted to reveal the effects of the sidewall shear stress distribution (SSSD) on homogeneous cohesive bank erosion processes in a U-shaped flume. The three-dimensional flow velocities were measured in detail and the turbulent kinetic energy method was employed to estimate the SSSD. The experimental results showed that the SSSD changed with the discharge and the point of the maximum shear stress varied within a range of 0·15–0·75 of the relative water depth. The variation of underwater bank topography was found to be consistent with the SSSD and the location of the maximum erosion point was not at the bank toe. The dominant failure mechanisms were observed to be tensile and toppling failures. Furthermore, a simplified bank erosion model (SBEM) was developed with consideration of the SSSD. Compared with the bank stability and toe erosion model, the SBEM can provide a more accurate simulation of bank erosion processes. This study is expected to enrich general understanding of cohesive bank erosion processes in curved channels, which will help to formulate effective strategies for river regulation.

Investigations on undercutting processes of cohesive river bank

River bank failure is the result of prolonged and repetitive erosion processes caused by the interplay between the flow turbulence with that of bank sediment. These interactions of affecting factors become more complex when the river bank sediment is cohesive in nature. The present study attempts to understand the erosion of cohesive river banks under controlled laboratory conditions, starting from an uneroded bank to the state just before failure. Experimental runs are carried out over the cohesive river bank, fabricated in a laboratory flume. Instantaneous real-time transverse erosion depth within the bank is recorded using ultrasonic ranging system (URS) and the corresponding velocity data by acoustic Doppler velocimeter (ADV). The results from the experimental runs with varying flow characteristics and cohesive sediment properties reveal that the turbulent shear stress combined with velocity fluctuations close to the bank increases with the flow Reynolds number. The size of the dislodged cohesive aggregates from the cohesive bank depends on the shear stress fluctuations. The transverse erosion depth as a function of time and flow Reynolds number are the significant parameters on which fluctuations of shear stress depend. These fluctuations in shear stress and flow velocity are high near the upstream and downstream sides of the undercuts, resulting in a longitudinally elongated undercut zone.

Fluvial Geomorphology, Root Distribution, and Tensile Strength of the Invasive Giant Reed, Arundo Donax and Its Role on Stream Bank Stability in the Santa Clara River, Southern California

Arundo donax (giant reed) is a large, perennial grass that invades semi-arid riparian systems where it competes with native vegetation and modifies channel geomorphology. For the Santa Clara River, CA, changes in channel width and intensity of braiding over several decades are linked in part to high flow events that remove A. donax. Nevertheless, the area of A. donax at the two study sites increased fivefold over a period of 28 years at one site and fourfold over 15 years at the second site. Effects of A. donax on bank stability are compared to those of a common native riparian tree—Salix laevigata (red willow)—at two sites on the banks and floodplain of the Santa Clara River. There is a significant difference of root density of A. donax compared to S. laevigata and the latter has a higher number of roots per unit area at nearly all depths of the soil profile. Tensile root strength for S. laevigata (for roots of 1–6 mm in diameter) is about five times stronger than for A. donax and adds twice the apparent cohesion to weakly cohesive bank materials than does A. donax (8.6 kPa compared to 3.3 kPa, respectively). Modeling of bank stability for banks of variable height suggests that S. laevigata, as compared to A. donax, increases the factor of safety (FS) by ~60% for banks 1 m high, ~55% for banks 2 m high and ~40% for banks 3 m high. For 3 m high banks, the FS for banks with A. donax is <1. This has geomorphic significance because, in the case of A. donax growing near the water line of alluvial banks, the upper 10–20 cm has a hard, resistant near-surface layer overlying more erodible banks just below the near-surface rhizomal layer. Such banks may be easily undercut during high flow events, resulting in overhanging blocks of soil and A. donax that slump and collapse into the active channel, facilitating lateral bank erosion. Therefore, there is a decrease in the lateral stability of channels if the mixed riparian forest is converted to dominance by A. donax.

Evidence of, and a proposed explanation for, bimodal transport states in alluvial rivers

Abstract. Gravel-bedded rivers organize their bank-full channel geometry and grain size such that shear stress is close to the threshold of motion. Sand-bedded rivers, on the other hand, typically maintain bank-full fluid stresses far in excess of threshold, a condition for which there is no satisfactory understanding. A fundamental question arises: are bed-load (gravel-bedded) and suspension (sand-bedded) rivers two distinct equilibrium states, or do alluvial rivers exhibit a continuum of transport regimes as some have recently suggested? We address this question in two ways: (1) reanalysis of global channel geometry datasets, with consideration of the dependence of critical shear stress upon site-specific characteristics (e.g., slope and grain size); and (2) examination of a longitudinal river profile as it transits from gravel to sand bedded. Data reveal that the transport state of alluvial riverbed sediments is bimodal, showing either near-threshold or suspension conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural riverbed sediments. Sand readily forms near-threshold channels in the laboratory and some field settings, however, indicating that another factor, such as bank cohesion, must be responsible for maintaining suspension channels. We hypothesize that alluvial rivers adjust their geometry to the threshold-limiting bed and bank material, which for gravel-bedded rivers is gravel but for sand-bedded rivers is mud (if present), and present tentative evidence for this idea.