Bed Degradation Research Articles

Effects of channel restabilization on habitat diversity, twentymile creek, mississippi

AbstractTwentymile Creek, a sand‐bed stream draining a 450 km2 catchment in northeast Mississippi, was channelized prior to 1910, in 1938, and in 1966. Straightening and enlargement in 1966 was followed by channel instability—rapid bed degradation (2‐4 m) and cross‐section enlargement by 1.4 to 2.7 times. Grade control structures (GCS) (weirs with stoneprotected stilling basins) and various types of streambank protection were constructed along the channel in the early 1980s to restore stability. Other investigators have suggested that habitat recovery in incised, channelized streams is facilitated by construction of GCS because they create stable scour holes and promote natural formation of a low‐flow channel flanked by vegetated berms. Effects of restabilization of Twentymile Creek on aquatic habitats were assessed in four ways. The fraction of the bank line covered by woody vegetation was mapped from aerial photographs taken in 1981 and 1985; physical habitat (depth, velocity, substrate, and cover) and fishes were sampled at base flow; and the existence and size of a low‐flow channel was ascertained from cross‐section surveys taken in 1980 and 1989. Woody vegetation, physical aquatic habitat, and fishes were also sampled from Mubby‐Chiwapa Creek, a similar‐sized unstable channel with no GCS. Physical habitat variables and fishes were sampled concurrently at five stations on Twentymile Creek, and four stations on Mubby‐Chiwapa. Four of the five Twentymile stations were either above or below a GCS. Bank‐line woody vegetation cover increased 8 per cent between 1981 and 1985 along Twentymile Creek but was stable along Mubby‐Chiwapa. Reaches above and below GCS were deeper with slower current velocities than elsewhere. Mean Shannon diversity indices based on physical data were similar for both streams, but were 58 per cent higher for stations immediately above and below GCS than for other stations. Since construction of the GCS and bank protection measures, longitudinal berms have formed within the enlarged Twentymile Creek channel, creating a low‐flow channel. Low‐flow channel capacity was equivalent to a mean daily discharge equalled or exceeded 30 per cent of the time, and was considerably lower than the effective discharge. Differences in aquatic habitat diversity among the stations sampled were primarily due to the scour holes below the GCS and the low‐flow channel. Thirty‐nine fish species were collected from Twentymile Creek, but only 22 from Mubby‐Chiwapa. Fourteen species were collected exclusively at GCS. Principal component analyses of the abundance of the eight numerically dominant fish species indicated similar faunas at most stations, but Twentymile Creek GCS stations were faunistically distinct. Abundance of several of the numerically dominant species was positively influenced by greater depths and lower velocities found near Twentymile GCS. The mean fish diversity index for Twentymile Creek was 29 per cent higher than for Mubby‐Chiwapa, and fish diversity was positively correlated with substrate diversity and mean depth.

Impacts of River Training on the Quality of Bank-Filtered Waters

The impacts of river training and gravel dredging on the quality of bank filtered waters are considered along the Danube sections upstream and downstream of Budapest, where important sources of drinking water are situated. Case studies are presented to show that training structures and dredging operations affect the hydraulic conditions in the river that are conducive to silting in areas with reduced flow velocities. Adverse hydrochemical changes occur in the silted filter layer, especially the dissolution of iron and manganese,and higher concentrations of ammonium-ion are observable. Dredging tends to disrupt the biologically active filter layer, while the ensuing bed degradation causes changes in the inflow ratio, increasing the proportion of polluted groundwater from the background areas in the wells.

STREAMBANK EROSION DUE TO BED DEGRADATION A MODEL CONCEPT

ABSTRACT Processes of fluvial erosion which operate on the banks of alluvial streams are examined by considering mechanisms of bed and bank erosion and mass failure of drained, homogeneous, cohesive banks. These concepts are used to formulate a mathematical model to evaluate bed degradation for the case in which bed lowering causes bank instability. Application of the model to a laboratory experiment verifies the behavior of the bed degradation submodel. Analysis of a more complex scenario demonstrates the importance of considering streambank erosion in streambed degradation analyses.

Evolution naturelle d'un remplissage sédimentaire intramontagnard et impacts des aménagements contemporains : L'exemple de la vallée de l'Arve (74)

The present valley floors of rivers in mountainous regions represent a particular phase of postglacial and holocene dynamics. Over an extended period i.e. at a geological or historical timescale, alluvial masses have experienced changes in their facies and volume, which are linked to change in the character of mountain erosion and fluvial dynamics. At a contemporaneous timescale, floodplains have probably experienced a shortage of sediment inputs: the resulting degradation of the river bed was canalized by nineteenth century enbankments and gravel extraction. These rapid changes have affected the long profiles and reduced the transport of bedload; conversely, the movement of suspended matter appears to have been less affected.

Gravel transport, gravel harvesting, and channel-bed degradation in rivers draining the southern olympic mountains, Washington, U.S.A.

The potential for gravel extraction to adversely affect anadromous fish habitat in three gravel-bed rivers of southwestern Washington, U.S.A., prompted the need to determine sustainable rates of gravel removal. This was accomplished by evaluating the components of a long-term sediment mass balance for the three rivers. Average annual gravel transport was determined by three independent methods. The closely agreeing results indicate that annual bedload supply decreases downstream through deposition and storage in response to declining gradient and from attrition during transport, as confirmed by laboratory experiments. A survey of gravel-bar harvesting operations indicates that the annual replenishment rate has been exceeded for up to three decades, often by more than tenfold. Analysis of data from nine stream gauging stations over a 55-yr period indicates degradation of about 0.03 m/yr in these reaches and suggests that bed degradation has produced the difference between the replenishment rates and the volumes of gravel harvested from the river beds and bars.

Guide for Estimating Riverbed Degradation

The rate and extent of riverbed degradation resulting from sediment interruption are determined by means of computer-based numerical experiments. Momentum and continuity equations for water and sediment flows in a wide prismatic channel are solved numerically using a recently verified model for the bed-armoring process. The bed profiles for different times are found to be similar and are coalesced to a single normalized curve. The changes in bed levels and mean sediment size, and the length of degradation, determined from the numerical experiments, are correlated by multiple regression analysis to the independent dimensionless variables. The depth of degradation depends on the Froude number, the geometric standard deviation, and the normalized median size of the bed material. The proposed relationship for bed degradation is verified with the field data of Missouri River. The computation procedure is illustrated by an example.

Channel response to grade‐control structures on Muddy Creek, Mississippi

AbstractGrade‐control structures are commonly employed to prevent bed degradation and concomitant bank instability of channelized reaches of rivers. A study of a 20 km reach of a coastal plains stream was conducted in 1985 to determine the effects of 12 rock‐lined grade‐control structures that were installed between 1977 and 1983 prior to channel excavation. An Allowable Tractive Stress Method (TR.25) was used to determine the placement of grade‐control structures, and the gradients between them, for a trapezoidal‐shaped channel designed to convey the 1 year recurrence interval peak flow. The design was successful in preventing bed degradation and bank erosion over the period of observation. However, unpredicted channel responses have occurred. Aggradation is apparent between control structures, and a two‐stage compound channel has formed as a consequence of berm development, especially in the lower, older subreach. In the lower subreach, the vegetated berms have constricted the cross‐sectional area at the design discharge (99 m3 s−1), and as a result, water‐surface slope, shear stress, and unit‐stream power have incresed. Bed material has become coarser and better sorted, which has increased shear intensity values. These unexpected changes are attributed to the lack of adequate consideration of the requirement for balance between sediment supply and transport in the TR.25 procedure; seven tributaries contribute sediment to the main channel of the study reach. It appears that the increased energy is utilized in bedload transport rather than channel erosion. This study demonstrates that the TR.25 method is too conservative if significant amounts of sediment are being supplied to the design channel. In such cases, adjustments in channel morphology will increase the energy to accommodate bedload transport.

Riverbank Stability Analysis. I: Theory

In this paper, a slope stability analysis for steep banks is used in conjunction with a method to calculate lateral erosion distance, to predict bank stability response to lateral erosion or bed degradation. The failure plane angle, failure block width, and volume of failed material per unit channel length may be calculated for the critical case. These parameters define the bank geometry following failure and form the starting point for subsequent analyses. The calculation procedure is illustrated by a worked example. Following mass failure slump, debris accumulates at the bank toe. The debris is removed by lateral erosion prior to further oversteepening or degradation generating further mass failures. Any process‐based model for channel width adjustment must account for the combined effects of lateral erosion and mass instability in producing bank instability. The approach adopted here represents a marked improvement over earlier work, which does not account for changes in bank geometry due to lateral er...

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.

The distribution and management of channelized streams in Denmark

AbstractThe Danish Watercourse Law of 1982 states that major river works and maintenance procedures must be planned and undertaken with regard to water quality and the physical form of the river channel. Fluvial geomorphology is an essential component of alternative procedures of river management which work with nature rather than against it. A survey of Danish stream channels has been produced by collating data obtained from maps, field surveys and engineering documents. This indicates that 97.8 per cent of Danish watercourses have been artificially straightened and that only 2.2 per cent (880 km) have natural morphological characteristics. The density of channel works is 300 times greater than in the U.S.A. and 15 times greater than in England and Wales. It is suggested that since there are only a few naturally sinuous channels in Denmark these should be given special consideration with regard to stream management practices. Measurements undertaken at approximately 300 sites in Denmark indicate five major processes of adjustment within straightened reaches. These adjustments include degradation of the channel bed with associated bank slumping, armouring of the bed, the development of a sinuous thalweg by the deposition of sediment at low gradients, and restoration of meanders. Channel stability is related to specific stream power and the results may be useful in predicting potential adjustments following channel straightening. Deposition in natural reaches below straightened channels often obliterates pool and riffle sequences but several channel adjustments may provide a greater variability of physical habitats than the initial uniform straightened channel; their significance for the biology of the watercourse is best considered before traditional management practices are carried out to control the adjustment.

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.

Simulation of Missouri River Bed Degradation

The future course of bed degradation in the middle Missouri River has been predicted using numerical simulation techniques. The simulation required development of the new Total Load Transport Model (TLTM), which incorporates the interdependence of friction factor and sediment transport through data‐based empirical relations. TLTM was implemented in a mathematical simulation model called IALLUVIAL, which computes quasi‐steady water and sediment flow in natural rivers having nonuniform bed sediments. IALLUVIAL also incorporates bed‐sediment sorting and armoring, which are processes of fundamental importance to the future course of Missouri River degradation. IALLUVIAL was first validated through simulation of the 1960–1980 severe degradation in the Missouri River between Sioux City, Iowa and Omaha, Nebraska. Subsequently IALLUVIAL was used to predict 1980–2000 degradation for several river‐management scenarios. The simulations suggest that the worst of the degradation is now over, and that it is the channelization, rather than upstream regulation, which is primarily responsible for the degradation. A companion paper describes the details of IALLUVIAL's armoring and sorting simulation procedures.

Diffusion models applied to channel degradation

The application of the diffusion model to bed degradation due to the sudden cessation of sediment supply to an alluvial system has been investigated and limitations of the model noted. Experiments indicate that local maximum scour depths do not occur at the upstream end of an erodible reach. This was accounted for by modifying the diffusion model to include a mobile upstream boundary scheme. The modified model was calibrated using bed load sediment transport data and diffusion coefficient values of best fit obtained. Significant differences between calibrated and predicted diffusion coefficient values were obtained. Measured and predicted bed profiles were also compared and conclusions about the applicability of the diffusion model to bed degradation drawn.

Changes in river regime after the construction of upstream reservoirs

AbstractThis article presents and analyses many years of investigations in China on the fluvial processes downstream of impounding and detention reservoirs. The study covers the change in hydrograph, the recovering of sediment concentration along the river course, the degradation of stream bed, the adjustment of longitudinal profile, the coarsening of bed material, the change in channel width, and the trend of channel pattern variation for alluvial streams downstream of impounding reservoirs. Without confluence of major tributaries, the degradation may extend to a great distance below the dam. In the process of reducing the sediment carrying capacity of the flow to match the diminished sediment supply, the coarsening of bed material is a factor of equal, if not greater, importance as compared with the flattening of channel gradient. In places where the flow has not been sufficiently cut down and the bank is erosive non‐resistant, a receding of banklines may take place in concurrence with the deepening of the river bed. Below detention reservoirs, even if the total runoff and sediment supply remain essentially unchanged, the modification of the hydrograph is sufficient to enhance the deterioration of the downstream channel.

Meandering channel response to altered flow regime: Milk River, Alberta and Montana

Mean and peak flows, channel width, and rates of meander migration are examined for two sandy meandering reaches of the Milk River in south Alberta and north Montana. One reach, upstream of the international boundary near Onefour, Alberta, is affected by the St. Mary diversion. The other reach near Havre, Montana is below the Fresno Dam and reservoir. Comparisons are possible because of the ample and long‐term availability of maps, air photos, and hydrometric records preceding and succeeding the two altered flows for both reaches. Understanding the natural river regime and effects of diversion and dam storage are critical for predicting channel responses to a proposed additional dam on the Milk River. Results indicate that increased discharge due to a diversion has increased channel width 5.5 m and increased the rate of meander migration by 0.85 m/year. Downriver, reservoir storage has decreased channel width 16.8 m and the rate of meander migration by 1.3 m/year and caused channel bed degradation up to 1.5 m near the dam.

Computation of 3‐D Flow and Shear in Open Channels

A mathematical modeling technique is presented which is capable of simulating various patterns of primary flow velocity distribution which may have the point of maximum velocity on or below the water surface. The simulated primary flow isovels are then used as a coordinate system in hydrodynamic analysis, to develop equations to compute corresponding distributions of shear stress and secondary flow. The mathematical models developed can be used to determine interactions among the primary flow velocity, shear stress and secondary flow velocity which all have a three‐dimensional distribution. The approach is essentially semi‐empirical which uses a measured or simulated distribution of primary flow velocity. It is designed to facilitate practical applications in studying various process in open channels, such as diffusion and dispersion of sediment, local scour and deposit, bank erosion, changes in channel geometry, aggradation and degradation of channel bed, which require three‐dimensional modelings. An application to a flow in a bend of a laboratory flume has shown that results of simulating the distributions of primary flow velocity and shear stress agree well with the measured data. Large discrepancies are found to exist between the computed and measured secondary flow velocities. However, it is still uncertain whether the discrepancies are entirely due to modeling and computational errors as the accuracy level of measured data is unknown.

Causes of river bed degradation

River bed degradation can proceed downstream as well as upstream depending upon the cause of degradation. The causes of downstream progressing degradation are primarily related to changes in independent river channel variables, such as increase in water discharge, decrease in size of bed material, and decrease in bed material discharge. The causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man‐made changes. Study of various case histories indicates that river slopes are increased by lowering a base level, by decreasing the length of a river, or by removal of a control point. Case histories also show that downstream and upstream progressing degradation can act in combination along a river system: downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary.

On the erosion of consolidated clay material by a fluid containing sand

Erosion of a consolidated clay material by fluid containing sand was tested in a laboratory facility. Two modes of erosion were distinguished: "sand blasting" or impact of individual sand particles moving by saltation and "milling" by sand particles forming part of a moving bed layer of discontinuous dunes. Sand blasting resulted in the more severe erosion and this process was adjusted during the experiments to yield maximum erosion rates. It was found that the overall bed degradation rate was a result of many local, shallow erosional features which coalesced. The maximum overall rate of bed degradation was found to be of the order of 0.2 mm/h or 1.75 m/year for a clay material consolidated to 345 kPa.

Impact of the 1978 floods on the channel and floodplain of the lower Macdonald River, N.S.W.

Summary Moderate to heavy rainfall over the lower catchment of the Macdonald River in March and May/June 1978 resulted in two moderate floods at St Albans with return periods of 11.11 and 3.63 years, respectively. The March flood resulted in about 0.30 m of bed aggradation, substantial river bank erosion and deposition of sand splays. The June flood resulted in about 0.10 m of bed degradation. This divergence in river response to the two events is due to the episodic supply of large volumes of sediment from river bank erosion during bankfull floods and the subsequent reworking of the sediment by lesser floods.

Local Scour due to River Bed Degradation

[in Japanese]