Bed Armoring Research Articles

Sediment-transport modeling on Southern Californian shelves: A ROMS case study

Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System ( roms). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size.

A simple mathematical model to predict the particle size distribution of the armour layer

A simplified method to predict the particle size distribution of the armour coat in the case of parallel degradation has been proposed. Using all available data, the particle size distribution of the armour layer was found to follow a normal probability distribution.A mathematical formulation to predict the median size of the armour coat has also been given. Some of the earlier methods for the prediction of the same have been studied and their accuracy has been compared to that of the proposed method. The time evolution of the river bed armouring process has also been investigated and the findings discussed.

Prediction of alluvial river bed variation by SDAR model

In this research, a mathematical model is developed to predict bed elevation variation of alluvial rivers. Scour and Deposition model of Alluvial Rivers (SDAR) is a newly developed model with two principal modules of hydraulics and sediment transport. SDAR is a semi-two-dimensional (S-2D) model in which lateral variation of velocity, flow stresses and sediment transport rate are achieved by subdividing the main channel into several stream tubes, each carrying equal discharges. In order to overcome the existing limitations, a new idea of reachwise stream tube concept is introduced. This enhancement allows to link branch connections and withdrawal points with stream tubes for lateral mass exchanges. Sediment routing and bed variation calculations are accomplished separately along each tube strip designated by virtual interfaces. A new method, based on the advection–dispersion (A–D) principles, is also developed for distributing inflowing sediment from branches among stream tubes. Furthermore, in SDAR model a controlling parameter Ωis included to distinguish between scour and deposition phases. The range of Ωis found to vary between ±1 depending on whether deposition or scouring prevails.A new formula for calculating active layer thickness is also introduced in the model. To test the model behavior, comprehensive calibration scenarios were conducted. During calibration runs, the ability of the model in longitudinal and transversal bed profile simulation and armor layer development prediction were especially detected. Results of simulation are also compared with the outcomes of well-known models, e.g. HEC-6, GSTARS-2, and FLUVIAL-12. It was found that the model's ability in simulating bed variation is noticeably increased when the S-2D concept is introduced. Indeed, the comparative validity tests confirm SDARS's promising role in facing with complex real engineering cases

The impact of shrimp trawling and associated sediment resuspension in mud dominated, shallow estuaries

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm 2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

A bed‐load transport model for rough turbulent open‐channel flows on plane beds

AbstractData from flume studies are used to develop a model for predicting bed‐load transport rates in rough turbulent two‐dimensional open‐channel flows moving well sorted non‐cohesive sediments over plane mobile beds. The object is not to predict transport rates in natural channel flows but rather to provide a standard against which measured bed‐load transport rates influenced by factors such as bed forms, bed armouring, or limited sediment availability may be compared in order to assess the impact of these factors on bed‐load transport rates. The model is based on a revised version of Bagnold's basic energy equation ibsb = ebω, where ib is the immersed bed‐load transport rate, ω is flow power per unit area, eb is the efficiency coefficient, and sb is the stress coefficient defined as the ratio of the tangential bed shear stress caused by grain collisions and fluid drag to the immersed weight of the bed load. Expressions are developed for sb and eb in terms of G, a normalized measure of sediment transport stage, and these expressions are substituted into the revised energy equation to obtain the bed‐load transport equation ib = ω G 3·4. This equation applies regardless of the mode of bed‐load transport (i.e. saltation or sheet flow) and reduces to ib = ω where G approaches 1 in the sheet‐flow regime. That ib = ω does not mean that all the available power is dissipated in transporting the bed load. Rather, it reflects the fact that ib is a transport rate that must be multiplied by sb to become a work rate before it can be compared with ω. It follows that the proportion of ω that is dissipated in the transport of bed load is ibsb/ω, which is approximately 0·6 when ib = ω. It is suggested that this remarkably high transport efficiency is achieved in sheet flow (1) because the ratio of grain‐to‐grain to grain‐to‐bed collisions increases with bed shear stress, and (2) because on average much more momentum is lost in a grain‐to‐bed collision than in a grain‐to‐grain one. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Breakup and reestablishment of the armour layer in a large gravel-bed river below dams: The lower Ebro

Changes in armour layer during floods under supply limited conditions are little known. This paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel-bed Ebro River during a flooding period. The study was conducted over a 28-km study reach from 2002 to 2004. The surface, subsurface and bed load grain size distribution constitute the bases for the analysis of bed-armouring dynamics. The results indicate that the magnitude of floods controlled the degree of armouring of the river bed. The initial mean armouring ratio was 2.3, with maximum values reaching 4.4. Floods in the winter of 2002–2003 ( Q 8) caused the breakup of the armour layer in several sections. This resulted in the erratic bed load pattern observed during the December 2002 flushing flow and in the increase in bed load transport during successive events. Most grain size classes were entrained and transported, causing river bed incision. The mean armouring ratio decreased to 1.9. In contrast, during low magnitude floods in 2003–2004 ( Q 2), the coarsest fractions (64 mm) did not take part in the bed load while finer particles were winnowed, thus surface deposits coarsened. As a result, the armour layer was reestablished (i.e., the mean armouring ratio increased to 2.3), and the supply of subsurface sediment decreased. The supply and transport of bed material appear to be in balance in the river reach immediately below the dam. In contrast, the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream, a phenomenon that progressively reduced their availability in the river bed surface, hence the armour layer reworking.

Effect of Bed Armoring on Bed Topography of Channel Bends

The two-dimensional numerical model previously developed by the writers for modeling the bed variations in a channel bend with uniform sediment is upgraded to incorporate the nonuniformity of sediment particles as well as bed armoring. In this model, the two-dimensional, depth-averaged, unsteady flow equations along with the bed-load mass conservation equation are solved in a body-fitted coordinate system by using the Beam and Warming alternating-direction implicit (ADI) scheme. A one-dimensional bed surface armoring approach is extended herein for application to a two-dimensional domain. The model is applied to a 180° bend with a constant radius under unsteady flow conditions. Numerical simulations are carried out to study the effect of bed armoring on the bed deformations in channel bends. Results show that bed armoring reduces scour in channel bends.

Sediment mining in alluvial channels: physical effects and management perspectives

AbstractBased on a review of a number of documented case studies from various countries and a detailed analysis of sediment exploitation from five rivers in Italy and Poland, we discuss alluvial river response to extensive sediment mining. A sediment deficit caused by in‐stream mining typically induces upstream‐ and downstream‐progressing river incision, lateral channel instability and bed armouring. The resultant incision alters the frequency of floodplain inundation along the river courses, lowers valley‐floor water tables and frequently leads to destruction of bridges and channelization structures. Mining also results in the loss or impoverishment of aquatic and riparian habitats. In the rivers of Italy and southern Poland studied, where mining coincided with other human activities that reduce sediment delivery to the channels, deep river downcutting, changes in channel pattern and, in one case, transformation from alluvial to bedrock boundary conditions were recorded over recent decades.The type and magnitude of channel response to sediment mining depend mainly on the ratio between extraction and sediment replenishment rates. The effects of mining will be especially severe and difficult to reverse: (i) where material is extracted at a rate greatly exceeding the replenishment rate; (ii) in single‐thread rivers, that are generally associated with relatively low rates of catchment sediment supply; (iii) in channelized reaches; (iv) where rivers are underlain by a thin cover of alluvium over bedrock; and (v) where mining coincides with other human activities that reduce upstream sediment delivery. With a large number of detrimental effects of instream mining, the practice should be prohibited in most rivers except aggrading ones. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

A benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors (ABS) documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off Duck, North Carolina. Sediment concentration was higher in the wave boundary layer (WBL) during storm conditions but higher ∼40 cm above the bed (cm ab) during swell conditions. To test the applicability of a diffusive balance during storm versus swell, ABS data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab. During the storm period, diffusivity derived from the ABS up to ∼40 cm ab agreed well with viscosity derived above the WBL from observed current profiles and from the Grant‐Madsen‐Glenn (GMG) model. During the swell period, diffusivity derived from the ABS up to ∼40 cm ab did not agree with observed mean current shear above this level nor with the GMG model. Diffusivity did agree with viscosity derived from shear stress due to waves within the WBL extrapolated to a height greater than the modeled WBL. We speculate that during swell conditions, shedding vortices enhanced mass and momentum exchange, extending the eddy viscosity associated with waves above the predicted WBL; during storm conditions, strong currents prevented vortices from penetrating beyond the predicted WBL. Rouse diffusion models with two‐ and three‐layered eddy diffusivity and combined diffusion‐advection models with one and three‐layer were applied to the observational data set. During the storm the two‐ and three‐layered Rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well. During swell (weak current conditions) all the models considered underpredicted the observed concentration if applied with a standard WBL thickness. To correct this, enhanced vertical exchange was represented by a thickened WBL whenever mean currents were weak relative to the estimated jet velocity associated with wave‐induced vortex shedding. The two‐layer Rouse model then reproduced the concentrations observed during swell remarkably well. This implies that mean sediment suspension dominated by wave‐induced advection may still be approximated by a diffusion‐like process under some circumstances.

A geomorphologist's criticism of the engineering approach to channelization of gravel-bed rivers: case study of the Raba River, Polish Carpathians.

River engineers use sediment transport formulas to design regulated channels in which the river's ability to transport bedload would remain in equilibrium with the delivery of materials from upstream. In gravel-bed rivers, a number of factors distort the simple relationship between particle size and hydraulic parameters at the threshold of sediment motion, inherent in the formulas. This may lead to significant errors in predicting the bedload transport rates in such streams and hence to instability of their regulated channels. The failure to recognize a nonstationary river regime may also result in unsuccessful channelization. Rapid channel incision has followed channelization of the main rivers of the Polish Carpathians in the 20th century. A case study of the Raba River shows that incision has resulted from the increase in stream power caused by channelization and the simultaneous reduction in sediment supply due to variations in basin management and a change in flood hydrographs. Calculations of bedload transport in the river by the Meyer-Peter and Müller formula are shown to have resulted in unrealistic estimates, perhaps because the different degree of bed armoring in particular cross-sections was neglected. It would have been possible to avoid improper channelization if the decreasing trend in sediment load of the Carpathian rivers had been recognized on the basis of geomorphological and sedimentological studies. Allowing the rivers to increase their sinuosity, wherever possible without an erosional threat to property and infrastructure, and preventing further instream gravel mining are postulated in order to arrest channel incision and reestablish the conditions for water and sediment storage on the floodplains.

Concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content

Concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover (Rc) on sediment yield for an initially wet and an initially air-dry loamy topsoil. The experimental results indicate that Rc reduces concentrated flow erosion rates ( E) in an exponential way (i.e., E= e − bRc ), which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet-rill erosion measured on runoff plots. The decay rate ( b) of this exponential relationship increased throughout the experiments because of scour-hole development and bed armouring. The concentrated flow erosion rates and b-values also depend on the initial moisture content of the topsoil. Depending on Rc, mean concentrated flow erosion rates were 20% to 65% less on initially wet compared to initially air-dry topsoils. The mean value for b was 0.032 for the initially wet, but only 0.017 for the initially air-dry topsoil, indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air-dry than when it is initially wet. The results help explain the data scatter in reported relationships between Rc and interrill–rill erosion rates. They also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils, which are very common in Mediterranean environments. Event-based water erosion models should incorporate effects of antecedent soil moisture content as well as those of Rc on concentrated flow erosion rates.

Modeling sediment entrainment and transport processes limited by bed armoring

A sediment-transport model including a bed-armoring algorithm is used to ascertain the potential importance of bed-armoring processes. The parameters that control the bed-armoring effects on the suspension of granular sediments in the marine environment are examined using the numerical model and data from the Eel shelf. Results for the shelf indicate that bed armoring controls the total suspended-sediment load and affects the across-shelf gradients of suspended material. The armoring effects are too significant to be neglected in most analyses of shelf and nearshore sediment transport, and this is especially relevant for models that are developed to represent large-scale and long-term sedimentary regimes. A principal controlling parameter is the thickness of the active layer, and the dynamics of this layer are not well known.

Across-shelf variations in bed roughness, bed stress and sediment suspension on the northern California shelf

Field measurements of bed micromorphology (roughness), benthic flow, bed stress, and suspended-sediment flux were made on the northern California continental shelf in connection with the STRATAFORM program. A sediment profiling camera and side-scan sonar were used to observe bed features in December 1995. Biogenic roughness with ∼2 cm relief prevailed at both sites. Bottom-boundary-layer tripods were deployed on the 60-m and 70-m isobaths over the period 5 January to 7 March 1996, during which time two high-energy events occurred. Skin-friction shear stresses were subcritical under `average' conditions but appreciably exceeded the threshold for sediment suspension during storms. For those high-energy events, near-bed suspended-sediment concentrations reached 2 g/l at 60 m (15 cm above bed) and 1g/l at 70 m (27 cm above bed) and suspended-sediment-induced stratification significantly affected bed stress estimates. At those times, increases in current shear were accompanied by increases in suspended-sediment-concentration gradients, causing the gradient Richardson number within the log layer to remain near the critical value of 1/4. This suggests suppression of turbulence by sediment-induced stratification. The abundance of under-consolidated fine sediment on the shelf to the north of the Eel River mouth presumably allows increases in stress to be accompanied by progressive increases in suspended-sediment concentration within the log layer to maintain the critical Richardson number. This contrasts to situations where bed armoring limits the total amount of fine sediment in suspension. Applications of a wave–current boundary-layer model, with stratification effects included, reduces estimates of current friction velocities by (u ∗ c ) about 24% relative to results from fitting the von Karman–Prantdl equation without stratification. The model suggests that the total wave–current friction velocity (u ∗ cw ) reached 3.0 cm/s at 70 m and 3.5 cm/s at 60 m. Depth-integrated across-shelf suspended-sediment fluxes were seaward at the 60-m isobath and near zero to weakly landward at the 70-m isobath during high-energy periods, implying flux convergence. This is consistent with the conclusions of other STRATAFORM investigators that rapid long-term accumulation of mud is occurring on the mid-shelf.

Physical characterisation of microform bed cluster refugia in 12 headwater streams, New Zealand

Recent sediment transport research has demonstrated that microform bed clusters (MBC) are particularly resistant to entrainment during floods and preliminary biological surveys have shown that such structures could be providing important refugia for benthos in streams. We therefore surveyed MBC in a selection of headwater streams, South Island, New Zealand to determine how common such structures are and then related their occurrence to flow variability, relative armouring, reach gradient, and sediment geology/ geometry to obtain a greater understanding of hydrological and hydraulic stream‐scale factors affecting their density and composition. MBC were present in all streams and ranged in density from 0.067 to 0.279 m−2. They occupied up to 4.4% of the surface area of the survey reaches, generally had 2–3 particles (a maximum of 7), and the average size for the largest particles ranged from 18.5 to 42.8 cm. MBC density and percentage site cover was significantly related to the relative armouring, but not to flow variability. The number of sediment particles per cluster was significantly correlated with reach gradient, although sediment geology, and associated geometry of particles, also appeared to be important. Thus, density and structure of MBC appeared to be primarily controlled by the interaction of relative armouring and reach gradient with highest densities and the largest number of cluster particles occurring in steep streams with high bed armouring. These structures were sufficiently common to be playing a significant role as flood refugia for benthos in some high gradient, headwater streams.

Sediment transport modelling in combined sewer

The classical solid transport theory has been used to analyse the experimental results obtained from the No. 13 combined sewer trunk of Marseille for more than two years. This study demonstrates that the sediment transport phenomena in a combined sewer trunk are nothing other than the classical ones. A numerical model has been established according to the analyses. Based on a permanent flow regime, this model considers not only the effects of the real channel geometry, non-uniform particles size, but also the coexistence of mineral and organically materials. Some particular sediment transport phenomena such as the armouring of bed have also been taken into account. It also shows that although the influence of the suspension particles is not necessarily considered, the simulation including the variations of particle density with each granular fraction may be improved.

Sediment transport modelling in combined sewer

The classical solid transport theory has been used to analyse the experimental results obtained from the No.13 combined sewer trunk of Marseille for more than two years. This study demonstrates that the sediment transport phenomena in a combined sewer trunk are nothing other than the classical ones. A numerical model has been established according to the analyses. Based on a permanent flow regime, this model considers not only the effects of the real channel geometry, non-uniform particles size, but also the coexistence of mineral and organically materials. Some particular sediment transport phenomena such as the armouring of bed have also been taken into account. It also shows that although the influence of the suspension particles is not necessarily considered, the simulation including the variations of particle density with each granular fraction may be improved.

Sediment resuspension and bed armoring during high bottom stress events on the northern California inner continental shelf: measurements and predictions

Geoprobe bottom tripods were deployed during the winter of 1990–1991 on the northern California inner continental shelf as part of the STRESS field experiment. Transmissometer measurements of light beam attenuation were made at two levels and current velocity was measured at four levels in the bottom 1.2 m of water. Intervals of high measured bottom wave velocity were generally correlated with times of both high attenuation and high attenuation gradient in the bottom meter of the water column. Measured time series of light attenuation and attenuation gradient are compared to values computed using a modified version of the Smith [(1977) The sea, Vol. 6, Wiley-Interscience, New York, pp. 539–577] steady wave-current bottom-boundary-layer model. Size-dependent transmissometer calibrations, which show significantly enhanced attenuation with decreasing grain size, are used to convert calculated suspended sediment concentration to light attenuation. The finest fractions of the bed, which are the most easily suspended and attenuate the most light, dominate the computed attenuation signal although they comprise only about 5–7% of the bed sediment. The calculations indicate that adjusting the value of the coefficient γ 0 in the expression for near-bed sediment concentration cannot in itself give both the correct magnitudes of light attenuation and attenuation gradient. To supply the volumes of fine sediment computed to be in suspension during peak events, even with values of γ 0 as low as 5 × 10 −5, requires suspension of particles from unreasonably large depths in the bed. A limit on the depth of sediment availability is proposed as a correction to suspended sediment calculations. With such a limit, reasonable attenuation values are computed with γ 0 ≈ 0.002. The effects of limiting availability and employing a higher γ 0 are to reduce the volume of the finest sediment in suspension and to increase the suspended volumes of the coarser fractions. As a consequence, the average size and settling velocity of suspended sediment increases as bottom shear stress increases, with accompanying increases in near-bed concentration gradients. Higher concentration gradients produce larger stratification effects, particularly near the top of the wave boundary layer at times when wave shear velocities are high and current shear velocities are low. These are the conditions under which maximum attenuation gradients are observed.

Stream and floodplain restoration in a riparian ecosystem disturbed by placer mining

Techniques for the hydrologic restoration of placer-mined streams and floodplains were developed in Denali National Park and Preserve Alaska, USA. The hydrologic study focused on a design of stream and floodplain geometry using hydraulic capacity and shear stress equations. Slope and sinuosity values were based on regional relationships. Design requirements include a channel capacity for a 1.5-year (bankfull) discharge and a floodplain capacity for a 1.5- to 100-year discharge. Concern for potential damage to the project from annual flooding before natural revegetation occurs led to development of alder ( Alnus crispa) brush bars to dissipate floodwater energy and encourage sediment deposition. The brush bars, constructed of alder bundles tied together and anchored laterally adjacent to the channel, were installed on the floodplain in several configurations to test their effectiveness. A moderate flood near the end of the two-year construction phase of the project provided data on channel design, stability, floodplain erosion, and brush bar effectiveness. The brush bars provided substantial protection, but unconsolidated bank material and a lack of bed armour for a new channel segment led to some bank erosion, slope changes and an increase in sinuosity in several reaches of the study area.

A simplified method to simulate the time evolution of the river bed armoring process

Abstract A simplified procedure for determining the lime evolulion of the armor coat is presented. The method is developed for a parent bed material characterized by a log-normal grain size distribution function. The interaction of the armoring process with flow resistance and sediment transport are also discussed. Experimental data are used for the calibration of the model.

A simplified method to simulate the time evolution of the river bed armoring process

Abstract A simplified procedure for determining the lime evolulion of the armor coat is presented. The method is developed for a parent bed material characterized by a log-normal grain size distribution function. The interaction of the armoring process with flow resistance and sediment transport are also discussed. Experimental data are used for the calibration of the model.