Bed Sills Research Articles

Prediction of scouring around an arch-shaped bed sill using Neuro-Fuzzy model

In this study, Adaptive Neuro-Fuzzy Inference System (ANFIS) has been used to model local scouring depth and pattern scouring around concave and convex arch shaped circular bed sills. The experimental part of this research study includes seven sets of laboratory test cases which were performed in an experimental flume under different flow conditions. A data set consists of 2754 data points of scouring depth were collected to use in the ANFIS model. The ratio of arch diameter, D, to flume width, W, is used as a non dimensional variables in all test cases. The results from ANFIS model were compared with the results of ANN model obtained by Homayoon et al. [24] and previously presented models. The results indicated that for D/ W equal to 1 and 1.2, the ANFIS models produced a good performance for convex and concave bed sills. As a result, the ANFIS models can be used as an alternative to ANN for estimation of scour depth and scour pattern around a concave bed sill installed with a bridge pier.

Modelling time varying scouring at bed sills

ABSTRACTIn this paper a modelling approach is presented to predict local scour under time varying flow conditions. The approach is validated using experimental data of unsteady scour at bed sills. The model is based on a number of hypotheses concerning the characteristics of the flow hydrograph, the temporal evolution of the scour and the geometry of the scour hole. A key assumption is that, at any time, the scour depth evolves at the same rate as in an equivalent steady flow. The assumption is supported by existing evidence of geometrical affinity and similarity of scour holes formed under different steady hydraulic conditions. Experimental data are presented that show the scour hole development downstream of bed sills due to flood hydrographs follow a predictable pattern. Numerical simulations are performed with the same input parameters used in the experimental tests but with no post‐simulation calibration. Comparison between the experimental and model results indicates good correspondence, especially in the rising limb of the flow hydrograph. This suggests that the underlying assumptions used in the modelling approach are appropriate. In principle, the approach is general and can be applied to a wide range of environments (e.g. bed sills, step‐pool systems) in which scouring at rapid bed elevation changes caused by time varying flows occurs, provided appropriate scaling information is available, and the scour response to steady flow conditions can be estimated. Copyright © 2011 John Wiley & Sons, Ltd.

Environmental protection stability of river bed and banks using convex, concave, and linear bed sills

River bed scourings are a major environmental problem for fish and aquatic habitat resources. In this study, to prevent river bed and banks from scouring, different types of bed sills including convex, concave and linear patterns were installed in a movable channel bed in a laboratory flume. The bed sills were tested with nine different arrangements and under different flow conditions. To find the most effective bed sill pattern, the scouring depth was measured downstream of the bed sill for a long experimental duration. The scour depth was measured at the middle and at the end of each experimental test for different ratios of the arch radius to the channel width [r/w]. The experimental results indicated that the convex pattern with r/w=0.35 produced minimum bed scouring depth at the center line whereas the concave pattern with r/w=0.23 produced the minimum scour depth at the wall banks. Therefore, the convex pattern was the most effective configuration for prevention of scouring at the center line of the river while the concave pattern was very effective to prevent scouring at the river banks. These findings can be suggested to be used in practical applications.

Application of Artificial Neural Network, Kriging, and Inverse Distance Weighting Models for Estimation of Scour Depth around Bridge Pier with Bed Sill

This paper outlines the application of the multi-layer perceptron artificial neural network (ANN), ordinary kriging (OK), and inverse distance weighting (IDW) models in the estimation of local scour depth around bridge piers. As part of this study, bridge piers were installed with bed sills at the bed of an experimental flume. Experimental tests were conducted under different flow conditions and varying distances between bridge pier and bed sill. The ANN, OK and IDW models were applied to the experimental data and it was shown that the artificial neural network model predicts local scour depth more accurately than the kriging and inverse distance weighting models. It was found that the ANN with two hidden layers was the optimum model to predict local scour depth. The results from the sixth test case showed that the ANN with one hidden layer and 17 hidden nodes was the best model to predict local scour depth. Whereas the results from the fifth test case found that the ANN with three hidden layers was the best model to predict local scour depth.

Effect of Flood Recession on Scouring at Bed Sills

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.

Scouring at Bed Sills as a Response to Flash Floods

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth at a bed sill for a given hydrograph can be estimated as a function of the identified temporal parameter.

Control of Scour at Bridge Piers by a Downstream Bed Sill

Results are presented from laboratory experiments to investigate the effectiveness of bed sills as countermeasures against local scouring at a smooth circular bridge pier, for flow conditions near the threshold of uniform sediment motion. The bed sill was located downstream of the pier, and its effectiveness with the distance between pier and sill was evaluated. The dependence of the scour depth on different dimensionless groups was defined. The results showed that a bed sill placed at a short distance downstream of the pier reduces the scour depth, area, and volume. In particular, the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. The bed sill seems to take effect some time after the beginning of the test, as the scour hole downstream of the bridge pier develops sufficiently and interacts with the countermeasure.

Laboratory study of maximum scour depth downstream of sills

Bed sills have been widely used as a sequence of grade-control structures in mountain streams where incision has to be prevented. Major local scouring downstream of these structures is, however, often observed. In order to investigate the characteristics of local scouring, a number of experiments have been carried out with 105 cases in different situations. The initial bed slopes were 0·176 and 0·268; sediments were coarse sand, fine gravel and medium gravel. The sill spacings were 0·4, 0·6, 0·8 and 0·9 m, and the sill heights above initial stream bed level were 0, 0·02, 0·04 and 0·06 m, respectively. Three stages of scour evolution could be classified. The size of scour hole depends on the bed slope, sediment diameter, sill height and sill spacing. The slope of the stream bed was found to be the most important parameter affecting the maximum scour depth. Using dimensional analysis, the relative maximum scour depth at the equilibrium stage was formulated as functions of the relative morphological jump to mobility of sediment and the relative energy loss to flow energy. Compared with the measured data at the Maso di Spinelle River in the Italian Alps, the proposed formulae showed satisfactory results.

Channel degradation and slope adjustment in steep streams controlled through bed sills

AbstractGeneral, parallel and rotational degradation and the effect of bed sills are investigated with the aid of historical field data and laboratory experiments. The case under consideration is a system of two tributaries and a main river in northeast Spain, characterized by its short length, steepness and ephemeral nature. The comparison of bed profiles since 1962 revealed an incision greater than 5·5 m. The main causes of degradation are thought to be channelization (width reduction) and urban growth. The equilibrium slopes with and without sills were obtained in 111 live‐bed flume tests. It is demonstrated that the bed slope is milder with sills than without sills under the same flow and sediment rates. The equations for the calculation of equilibrium slopes, scaled to prototype, are applied to the case‐study, with good agreement. Copyright © 2008 John Wiley and Sons, Ltd.

Affinity and similarity of local scour holes at bed sills

Excessive erosion of gravel bed rivers represents a hazard, which is often controlled by the construction of bed sills. Shape and volume of equilibrium local scour holes at bed sills in high‐gradient streams were investigated by analyzing steady flow experimental data. Scour profiles were normalized by scaling the depths and the longitudinal distance from the upstream sill with the maximum dimensions of the scour hole. Results exhibited a universal geometrical affinity of the scour. Geometrical similarity was also found to occur, but only in a limited range of physical conditions. Evidence on the scaling nature of the scour hole shapes has important implications on the prediction of the eroded volume. In turn, a correct evaluation of the eroded volume is necessary for the optimization of the design of the intersill distance. Prediction formulae are proposed which link the eroded volume to the geometrical, hydraulic, and sediment properties.

Disturbance Caused by Bed Sills on the Slopes of Steep Streams

Live-bed tests on steep bed slopes are run in a laboratory flume to investigate the slope of equilibrium. The tests simulate a general degradation until a new equilibrium is reached. Subsequently, different bed sill settings are tested as a countermeasure against degradation. Evidence is presented that, in some cases, the bed slope with sills is milder than the one without sills under the same flow and sediment rates. This fact implies that the efficiency of the bed sill system to stop degradation decreases as the sills are placed closer together. Tests are on the verge of sheet flow, and friction factors are heavily dependent on sediment transport. Reaches in uniform and gradually varied flow behave very differently. Bed profiles are not straight, and grain size is linked to this fact. Other detailed information on testing equipment, experimental program, time development and local scour in a total number of 111 tests is given.

Scour holes downstream of bed sills in low-gradient channels

An experimental study on long local scouring downstream of bed sills in a monogranular sand bed was carried out in the hydraulic laboratory flume at the Mediterranean Agronomic Institute of Bari (Italy). The main objectives of this study were to determine scour hole dimension, with its maximum scour depth as a function of time and at the equilibrium stage, the scour hole shapes and the investigation of the influence of sills on the distribution of the three-velocity components through the scour hole at the same stage. Four experimental configurations were tested, the main difference between them being the distance between sills. Based on experimental data, the classical dimensional analysis of the variables that influence the development of the scour hole has been carried out in order to obtain two empirical formulas predicting the maximum scour depth and the length of the scour hole at the equilibrium stage. Moreover, it was observed that the distance between sills influences the scour hole dimension and shape. The three-velocity components of the flow, measured with an acoustic Doppler velocimeter, show that in the scour hole, at the equilibrium stage, the three components of the flow turbulence intensities are very high. Near-bed flow vortexes in addition to secondary currents are also observed.

Sediment supply and local scouring at bed sills in high‐gradient streams

This paper presents the results of an experimental study about the pattern of local scouring generated at the toe of bed sills, which are commonly built in steep, gravel bed mountain streams. The aim of this study is to evaluate the effect of upstream sediment supply on the scour depth and shape. The experiments simulated conditions of a steady upstream sediment supply which had the same grain size composition as the sediment deposit placed on the bed of the flume. The geometric characteristics of three scour holes developed under conditions of steady flow discharge and steady upstream sediment supply were measured during 48 different tests. This provided a data set containing measurements of 144 scour holes. Three different sediment feed rates were used, and tests were also performed with clear water conditions. Geometric interference between sills was not observed. A natural river gravel with a limited grain size was used. The collected data were analyzed using nondimensional groups developed in previous clear water studies. It is shown that the imposed sediment transport does not require the introduction of new dimensionless parameters into existing scour depth and length prediction formula. The effects of sediment feed are shown to be incorporated in the existing dimensionless parameters. The new data set was used to recalibrate existing scour depth formula. The influence of jet erosion on scour geometry is also discussed. Inspection of the data indicates a possible two‐stage behavior, with the threshold between behaviors related to the ratio of the value of the morphological jump created at a scour hole and the flow specific energy at the bed sill.

Morphological effects of local scouring in step–pool streams

AbstractStep–pool morphology characterizes many high‐gradient streams in a variety of natural settings, but formative processes and evolutionary dynamics are still poorly understood. In this paper, natural step–pool geometry is compared with steep alluvial channels where grade‐control structures such as check‐dams and bed sills make the stream profile resemble a natural stepped stream. Along these channels, local scouring due to falling jets forms plunge pools under each structure, analogous to natural steps determining the formation of pools. In order to test the hypothesis that natural pools are analogous to pools formed below grade‐control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. Pools below grade‐control works featured a transitional zone between the scour hole and the downstream sloping bed, marked by a depositional berm. When geometric parameters such as maximum pool depth, length and step–berm distances are normalized to the jet virtual energy, no statistically significant differences were detected between natural and artificial systems. These results lend support to an upstream‐forced cascade model for step–pool formation, where the energy of falling jets controls the geometry of the pools, and is therefore regarded as the most important scaling‐independent variable. Copyright © 2005 John Wiley & Sons, Ltd.

Local Scouring at Bed Sills in a Mountain River: Plima River, Italian Alps

The main characteristics of local scouring below 26 bed sills constituting a sequence of grade-control structures in a mountain river (Plima River, Italian Alps) have been surveyed. Bed sills were built with concrete strengthened boulders and other loose boulders were placed in the river bed to enhance habitat diversity for fish. The most likely formative water discharge is used to evaluate specific flow energy at each bed sill. Measured maximum scour depths are well predicted by an empirical equation developed through laboratory results, showing an average relative error of 0.15.

Computation of transcritical steady flow over a curved bed with lateral contraction

The St. Venant equations are adequate in simulating the free-surface flow over lightly curved beds, but are insufficient when applied to some moderately to highly curved flows caused by bed sills and lateral contractions. Many other studies have only considered effects of the changes in the channel bottom elevation. In comparison, the effect of the lateral contraction combined with the bottom change has not been thoroughly investigated. In this study, with the consideration of the pressure modification induced by the flow curvature, a one-dimensional equation is derived to compute the water surface profile for transcritical flows. The situation investigated is limited to the case of flows with a moderately curved bed combined with lateral contraction. The computational results show that the predicted flow profiles compare well with experimental data.

Local scouring at grade‐control structures in alluvial mountain rivers

Scour holes below 73 grade‐control structures (check dams and bed sills) in six mountain rivers located in the eastern Italian Alps have been surveyed. The most likely formative water discharge is used to evaluate jet thickness at each structure, which along with drop height, appears to determine scour hole dimensions, as shown by the consistent trends observed for nondimensional plots of both maximum scour depth and length versus the respective drop ratio. Sediment differences regarding size and lithology apparently play a minor role in determining scour hole dimension. Measured maximum scour depths are well predicted by a semiempirical equation developed through laboratory results, showing an average relative error of 0.13. Scour hole geometry is described by several ratios that are thought to represent approximately invariant characteristics. A new energy‐based normalization for scour hole dimensions is proposed as the most suited to evaluate the role of jet geometry and aeration upon scouring efficiency. It is noteworthy that the ratio between the maximum scour depth and the total energy available at a drop tends toward an asymptotic value around unity for increasing drop heights.

Time evolution of scouring downstream of bed sills

Results of laboratory experiments conducted on time evolution of the scouring phenomenon at bed sills are presented. Starting from previous findings on long-term depth and length of the local scour hole downstream of bed sills, 12 long-duration tests were designed and performed in the Sloping Sediment Duct at HR Wallingford Ltd., using uniform sand. No sediment feeding system was adopted. Video-recordings of the scour holes through the glass-sided duct allowed successive image analysis to obtain water surface and bed profiles over time. Results confirmed the validity of the assessment method of the maximum scour depth, y s, at equilibrium; uncertainties arose about the length of the scour hole, l s. A cyclic phenomenon perturbing the scour hole was also observed and described. The investigation of the time evolution of \\\\ showed that the scour hole develops rapidly and conditions close to equilibrium are achieved in a short time. The introduction of a morphological time. t s permitted the non-dimensional description of the increasing of y s over time through a unique curve. The short- and the long-term local scour regions are clearly distinguished. An application of the results is shown in a numerical example.

Interference processes on scouring at bed sills

AbstractThe local scouring downstream of bed sills forming a sequence for bed stabilization in steep channels has been investigated in a laboratory flume. The initial bed slopes ranged from 0·078 to 0·148. The bed alluvium was characterised by a non‐uniform grain size distribution. The results show that when the ratio between the critical water depth hc and the sill spacing L rises above a characteristic value the scouring dynamics become heavily affected by the presence of the downstream sill, associated with the onset of a form of “interference” which renders the scouring process less effective. The difference with an “undisturbed” case is demonstrated. Self‐affinity of scour holes is confirmed and the scour length appears to be the reference parameter from which the scour depth might be evaluated. Copyright © 2003 John Wiley & Sons, Ltd.

Local scouring in low and high gradient streams at bed sills

The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two equations are proposed for the estimation of the maximum scour depth and length. The equations are from previous data sets on low-gradient tests and a new one of experimental results on high-gradient cases.