Bed Armoring Research Articles

An analysis of nonlinearity effects on bed load transport prediction

Because bed load equations are nonlinear and because parameters describing the flow and the bed can have large variance, different results are expected when integrating bed load over a cross section with respect to spatially variable local data (2D), or when computing bed load from cross‐section‐averaged data, which reduces the problem to uniform conditions (1D). Evidence of these effects is shown by comparing 1D (flume‐derived) equations with 2D field measurements, and by comparing a 2D (field‐derived) equation with 1D flume measurements, leading to the conclusion that different equations should be used depending on whether local or averaged data are used. However, whereas nonlinearity effects are considerable for low‐transport stages, they tend to disappear for higher‐flow conditions. Probability distribution functions describing the variance in flow and bed grain size distribution (GSD) are proposed, and the width‐integrated bed load data (implicitly containing the natural variance in bed and flow parameters) are used to calibrate these functions. The method consists of using a Monte Carlo approach to match the measured 2D bed load transport rates with 1D computations, artificially reproducing the natural variance associated with the mean input parameters. The Wilcock and Crowe equation was used for the 1D computation because it was considered representative of 1D transport. The results suggest that nonlinearity effects are mostly sensitive to the variance in shear stress, modeled here with a gamma function, whose shape coefficient α was shown to increase linearly with the transport stage. This variance in shear stress suggests that even for very low flow conditions, shear stress can locally exceed the critical shear stress for the bed armor, generating local armor breakup. This could explain why the bed load GSD is usually very similar to subsurface GSD, even in the presence of complete armor.

Development and application of 2-D mobile-bed model with bedrock river evolution mechanism

Steep slope and severe bed change are the characteristics of mountain rivers. These characteristics often cause the bed armour layer flushed away, make the bedrock exposed, and then increase the channel incision. Most mobile-bed models of past few decades aimed at the sediment transport of alluvial channel. In this paper, a bedrock river evolution mechanism is proposed, and included in a 2-D mobile-bed model, called the explicit finite analytic model (EFA). The EFA model can consider both incision and deposition over the bedrock, by combining a new stream power type of bedrock erosion rate formula with the flow and sediment transport modules. An uplifted reach of Taan River, Taiwan, caused by the Chi–Chi earthquake occurred in 1999 is chosen as the study site. Rapid bedrock incision since the uplift provides a rare chance to explore its mechanism and evolution process. The field and numerical results show that the proposed model has the capability of simulating morphological changes for bedrock rivers.

Dam Design can Impede Adaptive Management of Environmental Flows: A Case Study from the Opuha Dam, New Zealand

The Opuha Dam was designed for water storage, hydropower, and to augment summer low flows. Following its commissioning in 1999, algal blooms (dominated first by Phormidium and later Didymosphenia geminata) downstream of the dam were attributed to the reduced frequency and magnitude of high-flow events. In this study, we used a 20-year monitoring dataset to quantify changes associated with the dam. We also studied the effectiveness of flushing flows to remove periphyton from the river bed. Following the completion of the dam, daily maximum flows downstream have exceeded 100m(3)s(-1) only three times; two of these floods exceeded the pre-dam mean annual flood of 203m(3)s(-1) (compared to 19 times >100m(3)s(-1) and 6 times >203m(3)s(-1) in the 8years of record before the dam). Other changes downstream included increases in water temperature, bed armoring, frequency of algal blooms, and changes to the aquatic invertebrate community. Seven experimental flushing flows resulted in limited periphyton reductions. Flood wave attenuation, bed armoring, and a shortage of surface sand and gravel, likely limited the effectiveness of these moderate floods. Floods similar to pre-dam levels may be effective for control of periphyton downstream; however, flushing flows of that magnitude are not possible with the existing dam infrastructure. These results highlight the need for dams to be planned and built with the capacity to provide the natural range of flows for adaptive management, particularly high flows.

Evaluating channel response to an extreme sedimentation event in the context of historical range of variability: Upper Colorado River, USA

ABSTRACTIn May 2003, a breach in a large irrigation ditch within Rocky Mountain National Park (RMNP) initiated a debris flow that entered Lulu Creek and the Colorado River, where 36 000 m3 of sediment substantially altered channel forms and processes. We present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability (HRV), and whether the recovery potential of the system is sufficient to adapt to the disturbance. Flow and sediment regimes, and channel morphology and stability were monitored on Lulu Creek and the Colorado River from 2004 to 2011. Dominant channel response following the debris flow within Lulu Creek included step development, bed armoring, and channel widening. Step height‐to‐length ratios (H/L) for three reaches on Lulu Creek are outside the HRV of reference channels, with one reach approaching reference conditions. Erosion of approximately 23% of the debris fan volume occurred as a result of the long duration 2011 peak flow. Sediment within the Lulu Creek fan will persist for ~30–190 years, assuming current maximum and mean removal rates. Planform changes on the Colorado River since the debris flow include an increase in single‐thread geometries, with braided reaches where bar deposition occurred. Bedload transport and grain‐size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches. Lulu Creek is returning to a condition of natural variability, but the Colorado River is outside the HRV expected for steep‐gradient, pool‐riffle channels. Applying HRV to a situation where management questions require a longer term perspective, and pre‐disturbance baseline data are limited, is a useful approach. The HRV analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals. Copyright © 2012 John Wiley & Sons, Ltd.

Channel morphology and bed-load yield in fluvial, formerly-glaciated headwater streams of the Columbia Mountains, Canada

This study examines channel-reach morphology and bedload yield dynamics in relation to landscape structure and snowmelt hydrology in headwater streams of the Columbia Mountains, Canada. Data collection relies on field surveys and geographic information systems analysis in conjunction with a nested monitoring network of water discharge and bedload transfer. The landscape is characterized by subdued, formerly-glaciated upland topography in which the geomorphic significance of landslides and debris flows is negligible and fluvial processes prevail. While the spatial organization of channel morphology is chiefly controlled by glacially imposed local slope in conjunction with wood abundance and availability of glacigenic deposits, downstream patterns of the coarse grain-size fraction, bankfull width, bankfull depth, and stream power are all insensitive to systematic changes of local slope along the typically stepped long profiles. This is an indication that these alluvial systems have adjusted to the contemporary snowmelt-driven water and sediment transport regimes, and as such are able to compensate for the glacially-imposed boundary conditions. Bedload specific yield increases with drainage area suggesting that fluvial re-mobilization of glacial and paraglacial deposits dominate the sedimentary dynamics of basins as small as 2km2. Stepwise multiple regression analysis shows that annual rates of sediment transfer are mainly controlled by the number of peak events over threshold discharge. During such events, repeated destabilization of channel bed armoring and re-mobilization of sediment temporarily stored behind LWD structures can generate bedload transport across the entire snowmelt season. In particular, channel morphology controls the variability of bedload response to hydrologic forcing. In the present case studies, we show that the observed spatial variability in annual bedload yield appears to be modulated by inter-basin differences in morphometric characteristics, among which slope aspect plays a critical part.

Strong seasonality in sand loading and resulting feedbacks on sediment transport, bed texture, and channel planform at Mount Pinatubo, Philippines

ABSTRACTDeclining sand inputs to a channel with bimodal bed sediment can lead to degradation, armoring, and reduced bedload transport rates. Where sand loading is episodic, channels may alternate between high‐sand and low‐sand conditions, with ensuing responses in bed texture and bedload transport rates. The effects of episodic sand loading are explored through flow, grain size, and bedload transport measurements on the Pasig‐Potrero River, a sediment‐rich channel draining Mount Pinatubo, Philippines. Sand loading on the Pasig‐Potrero River is highly seasonal, and channel adjustments between seasons are dramatic. In the rainy season, inputs from sand‐rich 1991 eruption deposits lead to active, sand‐bedded, braided channels. In the dry season, many precipitation‐driven sand sources are cut off, leading to incision, armoring, and significantly lower bedload transport rates. This seasonal transition offers an excellent opportunity to examine models of degradation, incision, and armoring as well as the effectiveness of sediment transport models that explicitly encapsulate the importance of sand on transport rates. During the fall 2009 seasonal transition, 7·6 km of channel incised and armored, carving a 2–3 m deep channel on the upper alluvial fan. Bedload transport rates measured in the August 2009 rainy season were over four orders of magnitude greater than gravel‐bedded dry‐season channels surveyed in January 2010, despite having similar shear stress and unit discharge conditions. Within dry‐season incised channels, bed armoring is rapid, leading to an abrupt gravel‐sand transition. Bedload transport rates adjust more slowly, creating a lag between armoring and commensurate reductions in transport. Seasonal channel incision occurred in steps, aided by lateral migration into sand‐rich banks. These lateral sand inputs may increase armor layer mobility, renewing incision, and forming terraces within the incised seasonal channel. The seasonal incised channel is currently being reset by precipitation‐driven sand loading during the next rainy season, and the cycle begins again. Copyright © 2012 John Wiley & Sons, Ltd.

Sediment mobility and bed armoring in the St Clair River: insights from hydrodynamic modeling

ABSTRACTThe lake levels in Lake Michigan‐Huron have recently fallen to near historical lows, as has the elevation difference between Lake Michigan‐Huron compared to Lake Erie. This decline in lake levels has the potential to cause detrimental impacts on the lake ecosystems, together with social and economic impacts on communities in the entire Great Lakes region. Results from past work suggest that morphological changes in the St Clair River, which is the only natural outlet for Lake Michigan‐Huron, could be an appreciable factor in the recent trends of lake level decline. A key research question is whether bed erosion within the river has caused an increase in water conveyance, therefore, contributed to the falling lake level. In this paper, a numerical modeling approach with field data is used to investigate the possibility of sediment movement in the St Clair River and assess the likelihood of morphological change under the current flow regime. A two‐dimensional numerical model was used to study flow structure, bed shear stress, and sediment mobility/armoring over a range of flow discharges. Boundary conditions for the numerical model were provided by detailed field measurements that included high‐resolution bathymetry and three‐dimensional flow velocities. The results indicate that, without considering other effects, under the current range of flow conditions, the shear stresses produced by the river flow are too low to transport most of the coarse bed sediment within the reach and are too low to cause substantial bed erosion or bed scour. However, the detailed maps of the bed show mobile bedforms in the upper St Clair River that are indicative of sediment transport. Relatively high shear stresses near a constriction at the upstream end of the river and at channel bends could cause local scour and deposition. Ship‐induced propeller wake erosion also is a likely cause of sediment movement in the entire reach. Other factors that may promote sediment movement, such as ice cover and dredging in the lower river, require further investigation. Copyright © 2012 John Wiley & Sons, Ltd.

Near Real-Time Scour Monitoring System: Application to Indian River Inlet, Delaware

Scour hole monitoring is widely used by engineers in reaction to bridge scour. Current monitoring methods lack the ability to observe wide areas on operational time scales. It is imperative that wide areas be observed after channel bed armoring countermeasures are taken because the armoring occupies an area larger than can be observed by traditional single-point scour monitors. The present bridge piers within the Indian River Inlet, Delaware, are adjacent to deep scour holes that threaten the bridge. A new scour monitoring system (SM) using two three-dimensional profiling sonars was installed on the Indian River Inlet Bridge to observe more than 19,000 m2 of bathymetry daily. The system components, configuration, and operation are described and example data are presented. Bathymetric data collected by the SM compare favorably with historic high-quality multibeam data from the U.S. Army Corps of Engineers. Quantitative correlations with temporally consistent data from a single-beam personal watercraft sur...

A seismic signature of river bedload transport during storm events

A seismic signature of river bedload transport during storm events

Advances in sediment transport modelling

A detailed description of how recently-developed sediment dynamics formulations are incorporated into the United States Environmental Protection Agency's Environmental Fluid Dynamics Code is presented. The new approach is an extension of previous models and accounts for multiple sediment size classes, has a unified treatment of suspended load and bedload, and appropriately replicates bed armouring. The resulting flow, transport, and sediment dynamics model is an improvement to previous models because it may directly incorporate site-specific data, while maintaining a physically consistent, unified treatment of bedload and suspended load. Experimental data from a noncohesive sediment erosion experiment in a straight channel help validate the numerical model. In this simulation, unknown parameters representing the active layer thickness and the erosion rates of the two largest sediment size classes when they are newly deposited are identified from the available data.

Secular Sediment Waves, Channel Bed Waves, and Legacy Sediment

AbstractThe concept of sediment waves is reviewed and clarifications are proposed for nomenclature concerning vertical channel responses to large fluvial sediment fluxes over a period of a decade or longer. Gilbert’s (1917) original sediment waves are re‐evaluated at their type locale and used to develop a consistent set of definitions. A ‘sediment wave’ represents a transient sediment flux that includes but is not necessarily identical to a ‘channel bed wave’ that represents the rise and fall of the bed in response to sedimentation. A large‐scale sediment wave results when a major sediment‐delivery event generates an aggradation–degradation episode (ADE). It may leave a legacy of sediment deposited on valley bottoms. Gilbert’s classic sediment‐wave model was empirically based on changes in bed elevations but was described as a sediment flux. He described downstream translation of an attenuating symmetrical wave although it was too large and disjointed to have a coherent waveform. Rapid return of bed elevations to pre‐sedimentation levels in Gilbert’s wave should not be mistaken for exhaustion of sediment. The common concept of a symmetrical sediment wave representing the time series of sediment loads is not accurate for large‐scale sedimentation events that store and slowly release sediment on floodplains. Large sediment waves composed of relatively fine material and limited bed armoring tend to be right‐skewed owing to rapid vertical readjustments relative to slow lateral recruitment of stored sediment. A revised right‐skewed conceptual model of large sediment and bed waves is presented that fits modern observations and incorporates stochastic elements of flood events. Large sediment waves may be linked to legacy sediment; that is, large repositories of anthropogenic alluvium stored on valley bottom floodplains, wetlands, mill ponds, and reservoirs. This illustrates the ubiquity and importance of secular sediment waves to river management.

Channel change at Toudaoguai Station and its responses to the operation of upstream reservoirs in the upper Yellow River

The application of dams built upstream will change the input conditions, including water and sediment, of downstream fluvial system, and destroy previous dynamic quasi-equilibrium reached by channel streamflow, so indispensable adjustments are necessary for downstream channel to adapt to the new water and sediment supply, leading the fluvial system to restore its previous equilibrium or reach a new equilibrium. Using about 50-year-long hydrological, sedimentary and cross-sectional data, temporal response processes of Toudaoguai cross-section located in the upper Yellow River to the operation of reservoirs built upstream are analyzed. The results show that the Toudaoguai cross-section change was influenced strongly by upstream reservoir operation and downstream channel bed armoring thereafter occurred gradually and extended to the reach below Sanhuhekou gauging station. Besides, median diameter of suspended sediment load experienced a three-stage change that is characterized by an increase at first, then a decrease and an increase again finally, which reflects the process of channel bed armoring that began at Qingtongxia reservoir and then gradually developed downstream to the reach below Sanhuhekou cross-section. Since the joint operation strategy of Longyangxia, Liujiaxia and Qingtongxia reservoirs was introduced in 1986, the three-stage change trend has become less evident than that in the time period between 1969 and 1986 when only Qingtongxia and Liujiaxia reservoirs were put into operation alone. In addition, since 1987, the extent of lateral migration and thalweg elevation change at Toudaoguai cross-section has reduced dramatically, cross-sectional profile and location tended to be stable, which is beneficial to the normal living for local people.

Maximum scour depth at piers in armor-beds

The paper presents a design method to determine the maximum equilibrium scour depth at piers embedded in a sand-bed overlain by a thin armor-layer of gravels using the experimental data of Dey and Raikar and those of Ettema. The proposed equation is in terms of empirical relationships, termed K-factors, which account for the effects of flow depth, pier shape, flow intensity, bed sediment size and armor gravel size on scour depth for individual cases of scour holes as identified by Dey and Raikar. These K-factors are determined by fitting envelope curves to the experimental data. The estimated largest possible scour depths that can occur at circular and square piers with an armor-layer are 3.15 and 3.47 times the pier width, respectively.

새만금 해역에서 연직 1차원 퇴적물 확산모델 검증

The sediment resuspension and diffusion model is an integral part of a sediment transport and morphologic change model. We examined a vertical one-dimensional sediment resuspension and diffusion model using field data collected at about 10-m depth off the Saemangeun 4 th dike. The field data include waves, currents and suspended sediment concentration near the bed for about a day in May, 2007. The suspended sediment concentration obtained from the 1D model overestimated the observation about two orders of magnitude with single grain size and multiple grain sizes. The incorporation of the bed armoring effect, which adjusts the amount of suspended sediment with the available bed sediment, improved the agreement between the model and observation within a factor of two.

Impact of natural (waves and currents) and anthropogenic (trawl) resuspension on the export of particulate matter to the open ocean: Application to the Gulf of Lion (NW Mediterranean)

Modern sediment deposits on continental margins form a vast reservoir of particulate matter that is regularly affected by resuspension processes. Resuspension by bottom trawling on shelves with strong fishing activity can modify the scale of natural disturbance by waves and currents. Recent field data show that the impact of bottom trawls on fine sediment resuspension per unit surface is comparable with that of the largest storms. We assessed the impact of both natural and anthropogenic processes on the dispersal of riverborne particles and shelf sediments on the Gulf of Lion shelf. We performed realistic numerical simulations of resuspension and transport forced by currents and waves or by a fleet of bottom trawlers. Simulations were conducted for a 16-month period (January 1998–April 1999) to characterise the seasonal variability. The sediment dynamics takes into account bed armoring, ripple geometry and the cohesive and non-cohesive characteristics of the sediments. Essential but uncertain parameters (clay content, erosion fluxes and critical shear stress for cohesive sediment) were set with existing data. Resuspension by waves and currents was controlled by shear stress, whereas resuspension by trawls was controlled by density and distribution of the bottom trawler fleet. Natural resuspension by waves and currents mostly occurred during short seasonal episodes, and was concentrated on the inner shelf. Trawling-induced resuspension, in contrast, occurred regularly throughout the year and was concentrated on the outer shelf. The total annual erosion by trawls (5.6×10 6 t y −1, t for metric tonnes) was four orders of magnitude lower than the erosion induced by waves and currents (35.3×10 9 t y −1). However the net resuspension (erosion/deposition budget) for trawling (0.4×10 6 t y −1) was only one order of magnitude lower than that for waves and currents (9.2×10 6 t y −1). Off-shelf export concerned the finest fraction of the sediment (clays and fine silts) and took place primarily at the southwestern end of the Gulf. Off-shelf transport was favoured during the winter 1999 by a very intense episode of dense shelf water cascading. Export of sediment resuspended by trawls (0.4×10 6 t y −1) was one order of magnitude lower than export associated with natural resuspension (8.5×10 6 t y −1). Trawling-induced resuspension is thought to represent one-third of the total export of suspended sediment from the shelf. A simulation combining both resuspension processes reveals no significant changes in resuspension and export rates compared with the sum of each individual process, suggesting the absence of interference between both processes.

Effect of Coarse Surface Layer on Bed-Load Transport

Existing bed-load transport formulas may overestimate the transport rate in mountain rivers by two orders of magnitude or more. Recently published field data sets provide an opportunity to take a fresh look at the bed-load transport relationship and it is hypothesized that the overestimate is due to a failure to account for the effect of a coarse surface layer of bed material inhibiting the release of fine subsurface material. Bed-load transport is determined as gs=aρ(q−qc) where q=water discharge per unit width; qc=critical value for initiation of bed material movement; ρ=water density; and a=coefficient. The gs∕q relationship is typically piecewise linear, characterized by two transport phases with, respectively, low and high rates of change. Twenty-one flume and 25 field data sets were used to quantify the relationship for Phase 2. The flume data confirm the dependence of a on S1.5, where S=channel slope, in agreement with earlier studies. The field data additionally show that a varies inversely with the degree of bed armoring, given by the ratio of surface to subsurface bed material size. The finding is consistent with the hypothesis and suggests the need to account for the bed material supply limitation in the bed-load transport formula. However, the available data are not entirely sufficient to rule out an alternative dependency, or codependency, on flow resistance. The critical conditions for initiation of Phase 2 transport are also quantified as a function of bed material size and channel slope. The resulting set of equations allows a more accurate estimation of Phase 2 bed-load transport rates. However, the equations are empirical and should be restricted for use within the range of conditions used in their development, to determine mean rather than instantaneous transport rates and to determine bulk transport rates, not transport by size fraction.

Optimising flow–sediment transport parameters for rivers

The unsteady flow–sediment transport equations for a real river–reservoir system contain unknown hydraulic, sediment and geometric parameters. Knowledge of these parameters and input data comprising initial and boundary conditions is required to simulate flow–sediment transport, water surface and river bed profiles. The sensitivity analysis and optimisation procedure, which involves determining these parameters by fitting a model with real river field data, is an important tool for finding the best-fit values of each parameter in a river–reservoir sediment transport model. Sensitivity analysis and optimisation methods have been found to be very useful when applied to sediment routeing problems in river–reservoir systems. This technique was employed for optimisation of some of the important parameters involved in graded sediment routeing and bed armouring processes using a non-linear coupled model (NCMG). The particular parameters studied in this investigation are degradation, bed armouring and grain size distribution of bed material in a river bed downstream of a dam. The effects of bed roughness, sediment parameters and thickness of the active layer on bed level changes are studied. A combined Gauss–Newton and modified Newton method are employed to calculate optimised parameters.

Physique du sédiment et des eaux interstitielles :stratégies de modélisation pour les besoins en environnement côtier

After a brief reminder on the sediment composition and facies, the paper reviews water-sediment exchanges that require to account for sediment transformation in Coastal Environment modelling. Physical processes (consolidation, bed armouring, liquefaction) and bio-chemical processes (organic matter mineralisation, adsorption/desorption, precipitation/dissolution, bioturbation…) are briefly reviewed, as well as usual modelling methods. The need for a guide book on the required minimal modelling, depending on the arisen questions, is put forward.

River sediment and flow characteristics near a bank filtration water supply: Implications for riverbed clogging

Riverbed clogging is an important issue related to the sustainable exploitation of riverbank filtration well fields. In this research, several complementary field techniques are employed to assess the current state and possible evolution of riverbed clogging at a site in the Saint John River, New Brunswick. The study is conducted in regions of the riverbed that have previously been identified as allowing recharge to the semi-confined aquifer that has been used since 1955 to supply water to the City of Fredericton. Flow velocity measurements, video imaging, and suspended sediment and bed sediment analyses conducted during the low flow (summer) period indicate that part of the recharge area closest to the well field, about 20% of the total area, is affected by bed armoring with cobbles and boulders. Consistent with previous studies, with increasing distance from the riverbank the sediment size decreases and the armor layer disappears. Previous research indicates that turbulent impacting of fine particles into the voids between the cobbles and boulders of the armor layer may reduce infiltration by up to 95%; however, the suspended sediment load in the river is mainly composed of organic matter, and the measured concentrations of suspended sediment (up to 3 mg/L) are not considered high enough to create such large reductions in infiltration. Additionally, the mineral fraction of the suspended sediment would not be expected to settle under the calculated average shear velocity of 0.012 m/s. Other sources of particulate matter, such as the degradation of aquatic vegetation on the riverbed, may be more significant with respect to riverbed clogging; however, annual peak flows may also create bed shear stresses that serve to limit long-term clogging effects.

Numerical modeling of mixed sediment resuspension, transport, and deposition during the March 1998 episodic events in southern Lake Michigan

A two‐dimensional sediment transport model capable of simulating sediment resuspension of mixed (cohesive plus noncohesive) sediment is developed and applied to quantitatively simulate the March 1998 resuspension events in southern Lake Michigan. Some characteristics of the model are the capability to incorporate several floc size classes, a physically based settling velocity formula, bed armoring, and sediment availability limitation. Important resuspension parameters were estimated from field and laboratory measurement data. The model reproduced the resuspension plume (observed by the SeaWIFS satellite and field instruments) and recently measured sedimentation rate distribution (using radiotracer techniques) fairly well. Model results were verified with field measurements of suspended sediment concentration and settling flux (by ADCPs and sediment traps). Both wave conditions and sediment bed properties (critical shear stress, fine sediment fraction, and limited sediment availability or source) are the critical factors that determine the concentration distribution and width of the resuspension plume. The modeled sedimentation pattern shows preferential accumulation of sediment on the eastern side of the lake, which agrees with the observed sedimentation pattern despite a predominance of particle sources from the western shoreline. The main physical mechanisms determining the sedimentation pattern are (1) the two counter‐rotating circulation gyres producing offshore mass transport along the southeastern coast during northerly wind and (2) the settling velocity of sediment flocs which controls the deposition location.