Bed Armoring Research Articles

The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors

A method is proposed for estimating the concentrations of the constituents of nonhomogeneous sediment suspensions using optical backscatter sensors. The method is predicated on the assumption that constituent interaction (e.g. grain shielding and multiple scattering) does not significantly affect sensor response. This fundamentally linear approach was verified by laboratory tests of a backscatter sensor in a range of silt/sand mixtures. Implementation of the method will require further development of the optical backscatter sensor or possibly the pairing of an acoustic backscatter sensor with an optical sensor. The method has the potential for resolving small-scale spatial and temporal changes in the suspended-sediment population, and therefore could be employed in studies of winnowing and bed armouring, for example.

Channel response to large‐scale river training works: hunter river, Australia

AbstractExtensive river training works were undertaken on the upper Hunter River, Australia between 1956 and 1978 in response to rapid lateral migration and channel widening caused by a series of large floods between 1949 and 1955. River training involved artificial cutoffs, extensive realignment, structural bank protection works and tree planting. Channel length and hence sinuosity (ratio of channel length to valley length) decreased throughout each of six reaches located in the 78.9 km long study area as a direct result of the artificial cutoffs and realignment. Although the overall length reduction was less than 5%, one section experienced a 17% reduction. Localized channel straightening directly increased slope, decreased roughness and consequently increased flow velocity. The structural works have generally succeeded, converting an actively migrating stream into a laterally stable channel. Largely because of channel straightening and reduced sediment supply by floodplain reworking, the river has responded by eroding its bed by up to 1.1 m since the 1950s. Bed erosion preferentially removed the sand and fine gravel fraction producing an armour layer of coarse gravel. This armour layer halted further erosion by protecting the underlying fine material and is now only episodically mobilized by floods with return periods greater than 5.6 years on the annual maximum series. Unfortunately degradation was not predicted before the river training works were undertaken and it was only the fortuitous formation of the bed armour layer which prevented the degradation from becoming a major problem.

A gravel/diamicton lag on the south Albertan prairies, Canada: Evidence of bed armoring in early deglacial sheet-flood/spillway courses

A clast-supported, massive diamicton to poorly sorted, massive gravel (Gm/Dcm) bed in the area around Dinosaur Provincial Park, southern Alberta, Canada, provides a means of identifying early deglacial flood courses. This is especially useful where flood and spillway courses have very subtle geomorphic definition and are difficult to map from aerial photographs. The Gm/Dcm bed conformably overlies a brown, late Wisconsinan till and rarely overlies bedrock and lacustrine sediments. The geographic distribution of the bed is restricted to flat to gently undulating prairie on either side of the Red Deer River where shallow floods and spillway melt waters were concentrated upon deglaciation. The bed has also been observed along the banks of the South Saskatchewan River at Medicine Hat, 90 km to the south, and is therefore regionally extensive. Because the Gm/Dcm bed has none of the attributes of supraglacial sediment and possesses a clast fabric and sphericity very similar to those of underlying till, it is interpreted as a winnowed gravel/diamicton lag, which was produced by low-competence melt-water floods along shallow depressions in the prairie surface after the drainage of local proglacial lakes. After enough of the fine-grained matrix of the late Wisconsinan till was removed to create a lag

Modèles d'évolution de lits alluvionnaires soumis à charriage : Tri granulométrique et pavage

An unidimensional model of alluvial river is developed to take account of temporal and spatial evolutions of bed (and bed load) grain compositions. The set of equations is solved in a fully coupled manner; numerical coefficients of the implicit scheme allow to optimize the time step value. A bed load loading law is included in the model. Some examples derived from the general features of the middle third of the Rhone river make clear how the flow unsteadiness affects the bed evolutions, through grain sorting and bed armouring.

Estimates of the suspended sediment reference concentration ( C a) and resuspension coefficient (γ 0) from near-bottom observations on the California shelf

Near-bottom current and suspended sediment measurements above a sandy clayey silt bed on the California continental shelf are used to estimate the near-bed suspended sediment reference concentration, C a , and the bed shear stress, τ 0. A proposed model contends that C a should be proportional to the normalized excess shear stress acting on the sediment grains and to the availability of erodible grains in the bed. The coefficient of proportionality (γ 0) in this model is estimated for 10 erosion events and is found to range from about 1.5 × 10 −5 to 3 × 10 −4. Moreover, the estimated γ 0 values appear to be inversely proportional to the excess bed shear stress. This result suggests that other factors, such as bed armoring and grain cohesion, inhibited the resuspension of particles at the relatively higher shear stresses.

A note on cluster bedform behaviour in a gravel-bed river

The behaviour of cluster bedforms was investigated in the Farma R., Central Italy. A study reach was surveyed and the three components of the pebble clusters occuring on the surface of a gravel bar were painted using different colours. Before and after each flood all the clusters were photographed and a comparison was made. This simple methodology allowed the evolution of the painted clusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones. The experiment shows that the mobility of the wake grains is higher than that of the stoss particles. A pebble cluster generally loses the wake grain first, followed by the stoss pebbles. The obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged. Different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed.

Rapid accumulation of fluvially derived sands and gravels in a tropical macrotidal estuary: the Pioneer River at Mackay, North Queensland, Australia

Sediments are accumulating rapidly in the Pioneer Estuary and adjoining foreshore. It has been estimated that approximately 4.0 × 10 6 m 3 of sediments have accumulated within the 92-year period between 1887 and 1979. Furthermore, the bar of the river, as depicted by the −3-m contour which is approximately Low Water Spring Tide level, has prograded by 600 m in the same 92 years. The Pioneer River has a catchment of only 1375 km 2 yet has recorded flood flows of up to 9800 m 3 s −1. It is a short flashy coastal stream which rises in largely granitic coastal highlands. Owing to the topography and location of the region exceptionally heavy rainfalls may occur in the catchment, the heaviest recorded being 530 mm in five hours. The flood flows produced during these heavy rainfall events supply large quantities of sand, gravel, and cobbles up to 250 mm long, to the present estuary and the bar. Extensive sand dunes have built up from these deposits on the northern side of the estuary causing restriction at the mouth. During historical time, an island composed of sands and gravels has developed within the estuary. Its growth was associated with flood events in the river. The maximum tidal range along this part of the coast is approximately 6.5 m which generates tidal flows in the estuary greater than 1000 m 3 s −1. The tidal processes have an over-riding influence on sediment distribution even in the very dynamic fluvial conditions found in the Pioneer Estuary. The substantial quantity of gravel and cobbles supplied to and present in the estuary has led to the development of gravel banks and bed armouring. The tidal currents are not competent to move these deposits but they may be moved by subsequent river floods. The deposition of flood-borne material in the estuary is affected by the state of the tide at the time of flood. Deposition occurs higher up the estuary when the tide is high; conversely coarse material can be carried right through the estuary and deposited on the bar if the tide is ebbing. The main channels in the estuary are now restrained by rock training walls. Prior to their construction it is probable that there was substantial channel migration, particularly at times of river flood. The Pioneer Estuary is an atypical macrotidal estuary containing immature fluvial sediments together with tidal depositional features. It exhibits characteristics of a wave-dominated shore line and it is considered that this is a result of the high rate of coarse sediment input. Substantial quantities of sediments of fluvial aspect have been deposited in the estuarine and near-shore environments and these sediments exhibit tidal distribution patterns. These observations are relevant to the interpretation of environmental conditions associated with the deposition of ancient clastic sediments.

Discussion of “ Simulation of Bed Armoring in Alluvial Channels ” by Hong‐Yuan Lee and A. Jacob Odgaard (September, 1986, Vol. 112, No. 9)

Discussion of “ <i>Simulation of Bed Armoring in Alluvial Channels</i> ” by Hong‐Yuan Lee and A. Jacob Odgaard (September, 1986, Vol. 112, No. 9)

Closure to “ Simulation of Bed Armoring in Alluvial Channels ” by Hong‐Yuan Lee and A. Jacob Odgaard (September, 1986, Vol. 112, No. 9)

Closure to “ <i>Simulation of Bed Armoring in Alluvial Channels</i> ” by Hong‐Yuan Lee and A. Jacob Odgaard (September, 1986, Vol. 112, No. 9)

Numerical Simulation of Degradation of Alluvial Channel Beds

The bed degradation in alluvial channels resulting from sediment interruption is simulated numerically by solving the one‐dimensional momentum and continuity equations for water and sediment. A simple “bed layer” model is proposed to simulate the plane bed armoring process; the model does not contain any parameter that needs calibration. The agreement between the numerical results and the available experimental data is satisfactory.

Selective Grain Entrainment by a Current from a Bed of Mixed Sizes: A Reanalysis

The selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain-sorting processes that lead to the formation of placers in sands and to armored beds in gravels. Existing field and laboratory measurements of selective entrainment depart systematically from the standard threshold curves (such as that of Shields) which are based on experiments with nearly uniform grain sizes. Entrainment measurements from beds of mixed sizes form trends which obliquely cross the threshold curves for uniform grains, the crossing point being roughly at the median diameter of the size distribution. Due to this crossover, the coarser size fractions of the distribution require lower-flow stresses for their entrainment than if they formed uniform beds, while the finer-size fractions require higher stresses than uniform beds. With deposits having medians in the range of medium sand through gravel, the larger the individual particle within the mixed sizes, the greater the flow stress required for its entrainment. In this case the smaller grains are preferentially entrained, possibly leading to bed armoring. In contrast, if the median is in the fine-to-medium sand range, the smallest grains in the bed of mixed sizes are the most difficult to entrain due to sheltering by the larger grains; where grain density is also involved, this sorting can lead to the formation of placers. A variety of empirical relationships is considered to quantify the evaluation of selective entrainment from deposits of mixed sizes, but no single formulation is satisfactory for all data sets.

Simulation of Bed Armoring in Alluvial Channels

A numerical procedure for simulating the temporal variation of sediment‐transport rate and corresponding variation of bed‐material composition in a straight alluvial channel has been developed. The procedure is based on a simple model for the exchange of grain sizes between a surface and a subsurface layer of bed sediment together with a standard bed‐load function. Verification was made with laboratory data. The procedure should be a useful tool for estimates of the effect of alternative flow regulation measures on a channel bed's long term stability.

Armoring and Sorting Simulation in Alluvial Rivers

Formulations for bed armoring and hydraulic sorting of bed sediments in alluvial streams are developed, and an application to Missouri River bed evolution is presented. A relation for the areal coverage factor for armoring on a plane bed is first formulated as a function of degradation depth and bedmaterial characteristics, and is then modified by introducing correction factors for the effect of bed forms and the stochastic nature of sediment motion. For the simulation of hydraulic sorting, bed sediments are assumed to be mixed homogeneously in a mixed‐layer depth approximately equal to average bedform height, and the composition of mixed‐layer materials is adjusted to satisfy the continuity equation for each sediment size fraction. These formulations are applied in a 20‐year simulation of bed degradation in the middle Missouri River reach downstream of Gavins Point Dam. It is found that the simulated bedsediment composition, including the two‐layer structure comprising the top armor coat and the underlying finer sediments, is in good agreement with the observed Missouri River bed characteristics.

Modeling alluvial channels

The present state‐of‐the‐art of modeling of flow of water and transport of sediment in alluvial channels is assessed. The characteristics of several presently available models are described, along with their similarities and differences. The general problems in present models are found to be in their choice of sediment transport function, assessment of resistance to flow and how it varies with changes in flow and cross section, modeling of bed armoring and its effect on sediment transport, the allocation of net scour and fill in a stream cross section, and the handling of upstream and downstream boundary conditions. Current work in these areas is described. The need for the inclusion of bank scour, stream migration, and meander development is discussed, along with present methods for approaching the assessment of their effects. Needed areas of research are outlined for the improvement of prediction of scour and fill in alluvial channels.

Discussion of “ Development and Prediction of Bed Armoring ” by Hsieh W. Shen and Jau‐Yau Lu (April, 1983)

Discussion of “ <i>Development and Prediction of Bed Armoring</i> ” by Hsieh W. Shen and Jau‐Yau Lu (April, 1983)

Closure to “ Development and Prediction of Bed Armoring ” by Hsieh W. Shen and Jau‐Yau Lu (April, 1983)

Closure to “ <i>Development and Prediction of Bed Armoring</i> ” by Hsieh W. Shen and Jau‐Yau Lu (April, 1983)

Erratum. Temporal variations in bedload transport rates: The effect of progressive bed armouring

Erratum. Temporal variations in bedload transport rates: The effect of progressive bed armouring

Development and Prediction of Bed Armoring

The major purposes of this study are to use available data to predict: (1) The development of bed armoring; and (2) the final composition of grain size distribution. First, the different factors influencing the incipient motion for nonuniform sediment size distribution are examined. Then, the procedures for simulating the grain size distribution of the armor layer are developed based on modifications of Gessler’s procedure and Einstein’s hiding factor. Three regression equations are derived to predict the median size, d\d3\d0 and d\d8\d4, of the final armor bed, given the initial sediment bed size distribution and flow conditions. A modified Shields diagram for incipient motion for nonuniform sediment sizes is also suggested.

Temporal Variations in the Particle Size Distribution of Surficial Bed Material: The Effect of Progressive Bed Armouring

Temporal Variations in the Particle Size Distribution of Surficial Bed Material: The Effect of Progressive Bed Armouring

Teaching unit for blood pressure screening and control for secondary schools: Jim Glick, Health Education Division, State of Washington Department of Social and Health Services, Olympia, Washington

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/cm2 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.