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Rockfill dams are a common dam type, but breach development in these dams is not very well understood, particularly rockfill dams with placed or dumped riprap protections on the crest and downstream slope. In this paper, we look at several experimental setups investigating the overtopping of rockfill dam models with and without riprap protections and how these experimental results compare with existing empirical models for breach development in embankment dams. We describe the model setups, data acquisition, and data analysis as well as the comparison with existing empirical equations for breach development, focusing on the following parameters: breach width, breach formation time, and peak outflow. The discussion brings to light how the parametric breach models can predict breach development in rockfill dam models. It also highlights how the existing models are wholly unsuited to predict breach development in protected dam models. Breach initiation and development in protected dams differs so much from unprotected dams that new models are needed. Such models could integrate new parameters that describe the resistance against erosion the crest and slope protection provides, as well as the sudden breach development that ensues once the protection fails. [ABSTRACT FROM AUTHOR]

Improving the knowledge of the failure mechanism of riprap exposed to overtopping is valuable to enhance the construction and reinforcement techniques of rockfill dam defense systems. In this research, four experimental overtopping tests were carried out on 1∶10 rockfill dam models with placed riprap protection layers supported at the toe. Axial reaction loads were measured and 3D models were built using the structure from motion technique to study the displacement of riprap stones exposed to overtopping. The importance of throughflow in the rockfill shoulder is brought to light by comparing these results with previous data from a placed riprap model built on a ramp without throughflow. The structure from motion technique demonstrates to be very effective and highlights the existence of a buckling phenomenon, along with the compaction of the riprap. The displacement data show that the compaction mechanism is more important (both horizontally and vertically) and appears at lower overtopping discharges for less packed placed riprap layer. Finally, similar trends for vertical and horizontal riprap displacements are demonstrated between the initial position and the last position before failure. Such information suggests that the critical discharge value is not as critical as the riprap displacement to predict dam failure under overtopping conditions. [ABSTRACT FROM AUTHOR]

The effect of turbulence intensity on riprap stability was studied experimentally. Riprap stones were placed on a flat streambed and turbulence intensity was changed with a turbulence generator. An acoustic Doppler velocimeter (ADV) was used to measure turbulence intensity. Results showed that in a constant flow condition, by increasing the near-bed turbulence intensity from 10% to 25%, the stable riprap diameter increased about 2.5 times. Based on the results of the present work and using all existing data, an empirical equation was developed for calculating stable riprap stone size considering the time averaged near-bed velocity and near-bed turbulence intensity. [ABSTRACT FROM AUTHOR]

The large pier of an emblematic bridge built in 2008 in the Ebro River (Zaragoza, Spain) obstructs the flow in high floods. Clear-water scour experiments in a scale model were conducted to anticipate maximum local scour depths and design riprap protections. These proved to be effective during a large flood event in 2015, but bed aggradation under the left bridge span and deep scour under the right one, not mirroring the bed deformation observed in the model, raised concerns about the bridge safety. The effects of the protected pier on the changes in the aftermath of the 2015 flood are discussed. It is shown that a large meander upstream generated an imbalance in the spanwise bedload distribution, leading to sedimentation on the left and contraction scour on the right. The paper argues for the need to take into account the effects of large piers on river morphology at the bridge planning phase. The case study shows that using a clear-water model to design the riprap protection is adequate, but more importantly, that the fluvial processes during a flood could only be studied with a live-bed model with geometrical detail of the full river reach, namely, the upstream meander. [ABSTRACT FROM AUTHOR]

This study introduced a new riprap placement design for the purpose of reducing bridge pier scour. The design consisted of embedding a layer of the riprap around the pier, flush with the surrounding bed, with a depression to help guide the horseshoe vortex. The new countermeasure design was tested among other conventional riprap placement designs in clear-water flow conditions and transition-flow conditions. The transition-flow condition is the evolution period from clear water to live bed where the bed has yet to reach an equilibrium state. The new countermeasure design was found to reduce greatly the depth and volume of scour in both flow conditions, up to 81.0% and 92.3%, respectively, which was more than any other riprap countermeasure tested. In addition, the new design used 18% less riprap material while also experiencing less shape deterioration when compared with a traditional flat riprap cover. Therefore, installing riprap in this specified shape yields an improved scour countermeasure that could help preserve infrastructure and increase public safety. [ABSTRACT FROM AUTHOR]

Acknowledged as a significant cause of bridge failure is abutment scour around bridge foundations. Because of the complexity of flow structures and scour mechanisms around an abutment, abutment scour remains the focus of extensive research. In this study, to elucidate the complex flow physics and resulting sediment transport around an abutment, time development of abutment scour in three flow types of free, submerged orifice, and overtopping were examined by conducting laboratory experiments in a compound channel. Different lengths of erodible abutment were used to explore the effect of flow contraction on the abutment scour. Detailed bed contours at selected time intervals, as well as velocities, for the initial conditions were measured with acoustic Doppler velocimeters during each experiment. From observations, the scouring process and the resulting scour hole shape and scour depth around an abutment over time were found to be inconsistent depending on the flow type and degree of flow contraction. Tracking the maximum scour location over time explained the complex scouring process around an abutment, which is a simultaneous effect of local turbulence and flow contraction. [ABSTRACT FROM AUTHOR]

Bendway weirs are submerged, in-stream rock structures that redirect impinging flow away from outer-banks toward the center of a channel. Riprap weirs are composed of stone and sized using the average channel velocity in conjunction with existing design guidance. A physical model study and field study were conducted to determine the magnitude of the convective velocity that accelerates around the tip or toe of the weir. The ratio of the maximum tip velocity to the average channel velocity (prior to weir installation) was determined to be approximately 1.70. Stone sizing criteria that significantly increase the riprap median size over traditional approaches were presented for consideration. [ABSTRACT FROM AUTHOR]

A stability parameter for rock in bed protections under nonuniform stationary flow is derived. The influence of the mean flow velocity, turbulence, and mean acceleration of the flow are included explicitly in the parameter. The relatively new notion of explicitly incorporating the mean acceleration of the flow significantly improves the description of stone stability. The new stability parameter can be used in the design of granular bed protections using a numerical model for a large variety of flows. The coefficients in the stability parameter are determined by regarding the measured low-mobility entrainment rate of rock as a function of the stability parameter. Measurements of flow characteristics and stone entrainment of four different previous studies and many configurations (uniform flow, expansion, contraction, and sill) are used. These configurations have different relative contributions of mean flow, turbulence, and stationary acceleration. The coefficients in the parameter are fit to all data to obtain a formulation that is applicable to many configurations with nonuniform flow. [ABSTRACT FROM AUTHOR]

Laboratory experiments were conducted in order to investigate the effectiveness of a new scour countermeasure consisting of a horizontal extension plate (EP) downstream of a gated spillway-stilling basin structure. These experiments were performed on a 1:30 scale model of one of the South Florida Water Management District (SFWMD) gated spillway structures, the S65E gated spillway, because it has experienced worrisome downstream scour. Currently, riprap is the only scour countermeasure documented for these structures. EPs of various lengths were installed downstream of the stilling basin, and the equilibrium scour was measured. It was found that a plate length of approximately 1.88 times the exiting flow depth was the shortest length that resulted in significant scour reduction and downstream scour translation. Measured flow patterns reveal that the EP altered the flow patterns by transferring the plunging flow downstream and eliminating the near-bed reattachment and surface recirculation, possibly due to a decreased interaction between the plunging flow and the stilling basin face. [ABSTRACT FROM AUTHOR]

Liu's theory of 1957 to estimate the point of incipient motion, derived assuming a flat bed slope and uniform flow, states that for large particles the movability number (defined as the ratio of the shear velocity to the settling velocity), is constant. Data collected from 12 undistorted and unscaled laboratory tests was used to investigate the applicability of the theory on steep riprap slopes. The tests indicate that the dimensionless movability number (MN) is constant for large particle Reynold numbers. For design purposes, the recommended movability number which emerged from this study is 0.18, provided that the steep bed slope is taken into account, the energy slope is considered in MN, and that the theoretical settling velocity is calculated using an accurate drag coefficient and the d90 sieve size of the riprap. [ABSTRACT FROM AUTHOR]

The present study focuses on riprap protection at single piers under live-bed conditions. The riprap mattress is displayed so that its top is leveled with the surrounding river bed, for ease in the construction process. Over 48 laboratory experiments have been carried out to understand the trend of the required riprap mattress width to protect single piers against edge and bedform undermining failures. The influence of bedform height on the required width is emphasized. A simple expression is given to calculate the required riprap mattress width; for low sediment transport rates the expressions given in the literature for clear-water conditions are recovered. [ABSTRACT FROM AUTHOR]

Using riprap stones is a common method to prevent scouring around bridge piers. Many parameters affect stable riprap size. In the present work, stability of riprap stones around rectangular bridge piers are studied experimentally under clear-water conditions. Round-nose rectangular piers with three different aspect ratios of 3, 5, and 7 were tested. Furthermore, each pier was tested aligned with the flow and skewed at 5, 10, and 20° to the flow. Failure of the riprap layer occurred either because of high shear forces at the upstream face of the pier or under a combination of forces attributable to wake vortices and shear stresses downstream of the pier. Analysis of experimental data showed that relative flow depth, relative stone size, and effective pier width are the important factors affecting the stability of the riprap layer. At different stone sizes and pier skew angles, the region at which the riprap layer fails varies. Based on more than 140 sets of experiments, a design equation is presented for stable riprap size. [ABSTRACT FROM AUTHOR]

The Shields and the Isbash equations for predicting the stability of rocks exposed to a turbulent flow were both proposed in 1936, and since then, both equations have been used widely in the analysis of transport thresholds for coarse sediment. These two equations were obtained using two very different approaches, but as demonstrated in this paper, the equation developed by Isbash is consistent with the relation formulated by Shields for predicting the motion of sediments either in the flume or in the field. Comparison of the two equations suggests that standard approaches tend to oversize riprap in mountain streams. [ABSTRACT FROM AUTHOR]

The article discusses a study which determined the dimensions of a canal having side walls lined with loose riprap rock and channel bottom unlined. It offers information on another method of solution that applies the particle swarm optimization (PSO) technique with the changed objective function. The article also discusses a hydraulically efficient channel and the slope of channel side walls and bottom width.

The article comments on an article which determined the dimensions of a canal having side walls lined with loose riprap rock and channel bottom unlined. The author of the article minimized the nondimensional cross section area as an objective function. The article also discusses another method of solution that applies the particle swarm optimization (PSO) technique with the changed objective function.

Small drainage channels lined with rock riprap are often designed using Manning's formula for uniform flow to calculate water depths and velocities. Resistance to flow is quantified by the Manning roughness coefficient n, which is strongly dependent on water depth when flows are shallow in comparison with the size of rock particles protecting the channel beds. To provide reliable formulas for evaluating n in riprap-lined channels, particularly for relatively shallow flows, 142 measurements of flow resistance carried out by others in several experimental studies were assembled and analyzed. Two formulas founded on a theoretical assessment of flow resistance are presented in which roughness heights are calculated based on measures of the distribution of riprap particle diameters. The formulas are compared with other equations that have been developed specifically to predict n in riprap-lined drainage channels, and they are shown to provide significantly more accurate estimates of flow resistance. [ABSTRACT FROM AUTHOR]

The sides and bottoms of earthen irrigation channels are erodible. The main design criterion for the earthen channel is to minimize or eliminate channel erosion under design flow conditions. The Manning formula and the tractive force equation were used as governing equations to design erodible and riprap channels. The traditional design approach involves a tedious trial procedure to find the bottom width of trapezoidal channels. Some hydraulic engineering problems require the solution of the bottom width from the Manning formula for nonerodible channels. An approximate equation is available for determining channel bottom width, but it is not applicable for channels with side slopes z<1 (1 vertical and z horizontal), which are encountered in practice for nonerodible channels. This technical note presents a direct and accurate equation for determining the channel bottom width (with errors less than 0.1%) using the tractive force concept and a dimensionless form of the Manning formula. The proposed direct solution facilitates the design of erodible and riprap-lined channels. Moreover, the proposed equation can be used to accurately determine the bottom width of rigid (nonerodible) channels for any side slope. [ABSTRACT FROM AUTHOR]

Scour hole monitoring is widely used by engineers in reaction to bridge scour. Current monitoring methods lack the ability to observe wide areas on operational time scales. It is imperative that wide areas be observed after channel bed armoring countermeasures are taken because the armoring occupies an area larger than can be observed by traditional single-point scour monitors. The present bridge piers within the Indian River Inlet, Delaware, are adjacent to deep scour holes that threaten the bridge. A new scour monitoring system (SM) using two three-dimensional profiling sonars was installed on the Indian River Inlet Bridge to observe more than 19,000 m2 of bathymetry daily. The system components, configuration, and operation are described and example data are presented. Bathymetric data collected by the SM compare favorably with historic high-quality multibeam data from the U.S. Army Corps of Engineers. Quantitative correlations with temporally consistent data from a single-beam personal watercraft survey vessel yield an r2 correlation coefficient of 0.84 with 93% of the absolute value of elevation differences between the two data sets less than 3 m. [ABSTRACT FROM AUTHOR]

Mass angle of repose (that is, the angle at which a mass of sliding particles will come to rest) is a fundamental material property that is needed to design drainage channel linings composed of open-graded riprap stone, also called dumped rock riprap, which protect earthen slopes from erosion by flowing water. Multiple regression analysis of measurements at 74 stockpiles of dumped natural and crushed rock shows the angle to depend primarily on rock particle angularity and, to lesser extents, on the gradation and the median particle diameter of the stone mixture, the angle increasing with angularity, mixture nonuniformity, and particle size. Uncomplicated expressions are developed from the data to calculate the expected mass angle of repose of open-graded rock riprap along with prediction intervals. [ABSTRACT FROM AUTHOR]

Various methods to control scour around bridge piers have been proposed, including application of riprap and installing a collar around piers. In the present study application of riprap alone and a combination of riprap and collar were examined experimentally for scour control around rectangular bridge piers. Piers aligned with the flow and skewed at 5, 10, and 20° to the flow were tested. Piers with three different aspect ratios equal to 1:3, 1:5, and 1:7 were used in this study. A collar three times wider than the piers’ width was installed around the piers at the streambed level. All experiments were conducted at the threshold of motion of the bed material. The size and extent of stable riprap stones for prevention of scouring around the piers was found by experiment with and without the collar. A method previously given for calculating stable riprap size around circular piers is extended for rectangular piers with different aspect ratios and skew angles with and without collar protection. The extent of stable riprap layer in all tests is also presented. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to determine the influence of length, width, and protrusion of noncontinuous riprap protections on shear failure conditions. The incipient motion of particles as a failure criterion and the reference transport method as the threshold of motion were used. In each test, riprap transport rates were measured at different time intervals using a sediment trap placed immediately downstream from the test reach so that time dependence could be well described. Results reveal that incipient motion conditions of transverse (cross-sectional) protections are strongly influenced by both the protrusion and length of bed protection, which indicates that stability significantly increases as protection length increases and decreases as protrusion increases. In the case of longitudinal protections, almost the same failure conditions were found as in the case of continuous protection. Furthermore, these conditions are unrelated to the width of the protection. A coefficient to correct design formulas obtained by other authors is proposed to take into account the effect of the geometry of transverse protections on their stability. [ABSTRACT FROM AUTHOR]

This study addresses the design of riprap mattresses as a scour countermeasure near vertical-wall bridge abutments under clear water flow conditions. It specifically deals with the diameter of riprap, Dr50, the lateral extent of mattresses, w, and their thickness, t. Experiments were performed in a rectangular, sand-bed open channel using different abutment lengths, three riprap stone sizes, and two different sands. The minimum size of stable stones as well as the mattress dimensions depend on the ratio between the abutment length and the flow depth. New equations for the evaluation of Dr50 and w are suggested. The geometric properties of the scour holes which develop at the edge of riprap mattresses are similar to those reported in the literature for spill-through abutments. Although it is not possible to fully arrest scour by winnowing, the corresponding scour depth is negligible when the mattress layer thickness is at least 6Dr50. [ABSTRACT FROM AUTHOR]

The Gupo Bridge crosses the Nakdong River near the city of Busan, South Korea. During Typhoon Maemi in 2003, the old Gupo Bridge collapsed due to excessive pier scour. More recently, the highway construction on the left-bank floodplain required right-bank channel widening to restore the channel flood-carrying capacity. This 7 m deep floodplain excavation is expected to cause significant local scour around the 8–10 m wide and 3 m thick spread footings of Piers 11 and 12 of the Subway Bridge and Piers 15 and 16 of the Gupo Bridge. Three design options are examined for retrofitting floodplain bridge piers with massive spread footings. A solution with sheet piles and riprap was recommended in 2006 as the most appropriate design, but Plan III with a conical riprap structure around the footings was ultimately constructed in 2007 for economic reasons. Laboratory experiments also highlight the need to place gravel and synthetic filters under the designed riprap. [ABSTRACT FROM AUTHOR]

Highway drainage channels often approach slopes of 0.5 (50%). Single layer riprap-lined channels at this slope have been constructed for highway drainage and appear to perform satisfactorily, but no quantitative information is available to guide their hydraulic design. A 1/6 scale laboratory hydraulic model, using a single layer of sized crushed limestone, was constructed to determine the relationship between depth of flow and discharge. The flow range investigated took place at depths near or below the top of the riprap, since riprap instability occurred at greater depths. Standing waves and localized hydraulic jumps dominated the surface of the flow, which held streamwise-averaged Froude numbers in a narrow range near the critical condition for those flow depths for which the channel is designed. This resulted in a simple relationship between unit discharge and depth of flow that applies to the model and prototype channels. [ABSTRACT FROM AUTHOR]

The paper reports on an experimental investigation concerning two important issues: (1) local scour and (2) riprap stability at a 45° wing-wall abutment in a degrading river bed of noncohesive sediment. The abutment considered was short (that is abutment length/flow depth <1). From the experimental observations, no influence of abutment inclusion on bed degradation was evident, as bed profiles with and without abutment were quite identical apart from the immediate vicinity of the abutment. Total scour depth at an abutment is found to be the maximum abutment scour depth in addition to the reduction of bed elevation due to bed degradation. The maximum abutment scour depth can be estimated from the equation given by Kandasamy and Melville in their 1998 paper. For scouring time beyond 24 h, the local abutment scour depth remains independent of time. In a degrading bed, the bed forms cause edge failure of the riprap at an abutment when the dunes propagate over the riprap layer. Initially, the dune height is significant causing the maximum damage of riprap layer. As the flow velocity reduces, the resulting bed-shear stress diminishes with the degrading bed and gradually the formation of dunes ceases. An additional experiment reveals that the damaged riprap layer is significantly vulnerable against a subsequent flood accompanied by large dunes. [ABSTRACT FROM AUTHOR]

A series of 26 tests was performed on a riprap layer in which median round-shaped stones ranging from 32.3 to 99.1 mm (from 1.27 to 3.90 in.) were subjected to overtopping flow conditions. Embankment slopes range from 10 to 45% with median stone sizes ranging from 23.9 to 104.2 mm (from 0.94 to 4.1 in.). The database was analyzed, yielding a unique expression in which the median size of a round-shaped stone can be predicted as a function of the unit discharge, embankment slope, and coefficient of uniformity. It was determined that the requisite round-shaped stones range from 5 to 42% larger than angular stones to stabilize the riprap layer for similar flow conditions with unit flows of 0.2 cm/s and slopes of 40%. The maximum deviation between rounded and angular-shaped stone may approach 70% as flow and bed conditions are extrapolated. [ABSTRACT FROM AUTHOR]

Experimental studies on the hydraulics of flow through and over gabion-stepped weirs are presented. Two flow components were observed, i.e., base flow through the void between filled stones and overflow on the gabions. The energy loss ratios in the gabion-stepped weirs are greater than those in the corresponding horizontal stepped weirs by approximately 7, 10, and 14% for weir slopes of 30, 45, and 60°, respectively. As a result, the velocity at the outlet is lower. Stone size and shape have little influence on the energy loss and flow velocity as compared to the increasing effect of the weir slope. The pressure acting on the step face for the gabion-stepped weirs is less than that of the horizontal steps owing to the cushioning effect generated by filled stones. To reduce pressure on a step face at a fixed discharge for different weir slopes, the void ratio of the filled stones should be low to allow a small amount of base flow. The pressure distribution pattern on the horizontal face of a step is provided. [ABSTRACT FROM AUTHOR]

Laboratory tests using a large-scale model of a spill-through bridge abutment led to important findings about the performance of a riprap apron as an abutment scour countermeasure. Riprap stone is widely used for protecting side slopes of embankments against erosion, and several design guidelines are available in the literature. In contrast, only a few guidelines exist for the design of a riprap apron around an abutment. These guidelines focus only on the armoring effect of riprap, neglecting other effects. This study shows that apron performance involves several mechanisms: armoring the bed, dissipating large-scale turbulence shed from the abutment, reducing the peak unit discharge, reducing the average shear stress, and shifting the scour region away from the abutment. Together, these mechanisms substantially reduce the maximum scour depth. The test findings are compared with those from a much smaller model of riprap-apron performance. [ABSTRACT FROM AUTHOR]

The results of an experimental investigation of riprap stability at wing-wall abutments are reported. The aim of the experiments was the determination of the size of riprap stones necessary to resist shear failure by the flow. The results, which were obtained under a wide range of mobile-bed conditions, are compared with predictions of riprap stone size given by existing equations. Two existing methods, with the addition of suitable factors of safety, are shown to give stable stone size for riprap layers at wing-wall abutments under mobile-bed conditions. It is noted that consideration needs also to be given to filter protection, edge effects, and the effects of bed-form undermining in the design of such riprap layers. [ABSTRACT FROM AUTHOR]

Block ramps or rock chutes are stream restoration structures used in many hydraulic projects. If the base material presents stability problems, a reinforcement can be used; it consists of placing over the ramp rocks greater in size than the base material in order to increase the flow resistance and reduce the shear stresses on the base material. The presence of these boulders also changes the energy dissipation of the flow over the structure. This work shows the results of experimental tests conducted on reinforced block ramps with slopes varying from 0.08 to 0.33. The effect of the various geometric parameters has been investigated. The aim is to extend existing relationships for a simple block ramp and define a new equation that can be applied to a block ramp with or without the presence of boulders. [ABSTRACT FROM AUTHOR]

Riprap of bridge piers is placed to prevent scour and to secure the pier from failure. Riprap is therefore an addition to a pier to increase its performance against scour. The present research intends to present three basic scour mechanisms associated with circular-shaped bridge piers in rivers first, to introduce then a number of selected experiments for a range of hydraulic, geometric, and sedimentologic conditions, and finally to describe a novel procedure for assessing the safety of these river elements against failure. This procedure is based on the Shields diagram relating to sediment entrainment in a uniform and flat sediment bed subjected by a water flow. The Shields approach is extended for the presence of a circular-shaped pier that is protected by a circular-arranged riprap layer of equal size elements. The design procedure presented in the following thus reduces to the entrainment condition of a pier for equal riprap and the sediment sizes and to the Shields entrainment condition when the pier diameter degenerates to 0. [ABSTRACT FROM AUTHOR]

The present study examines the use of independent and continuous pier collars in combination with riprap for reducing local scour around bridge pier groups. The efficiency of collars was studied through experiments. The data from the experiments were compared with data from earlier studies on single piers with collars and bridge pier groups without collars. The data showed that in the case of two piers in line, combination of continuous collars and riprap results in the most significant scour reduction of about 50 and 60% for the front and rear piers, respectively. In other cases for two piers in line, independent collars showed better efficiency than a continuous collar around both piers. It was also shown that efficiency of collars is more on a rectangular pier aligned with the flow than two piers in line. Experiments however, indicated that collars are not so effective in reduction of scouring around two transverse piers. [ABSTRACT FROM AUTHOR]

As a new alternative countermeasure to riprap for scour protection around bridge piers, wire gabions were investigated experimentally for failure mechanisms, effects of significant parameters on failure and its sizing in a clear-water condition. The dominating failure mechanism was found to be a shear failure. Based on the experimental data, the controlling factors for the stability of wire gabions as a scour countermeasure at the pier are flow depth relative to pier diameter, length to thickness ratio, coverage, alignment and placement depth of wire gabions. An equation for sizing of a wire gabion is proposed in terms of Froude number and factors reflecting both the effect and limit of significant parameters. Comparison of the equation with those of ripraps shows that smaller wire gabions than ripraps provide an equivalent protection implying cost effective and improved stability. [ABSTRACT FROM AUTHOR]

The experimental study examines local scouring and riprap stability at bridge piers in rivers subject to bed degradation. The data show that the equilibrium bed profile associated with that with or without a pier is essentially the same, except for the obvious section around the pier. Total scour depth is shown to be the sum of bed degradation and pier scour depth. The latter can be computed from the time-average live-bed scour depth associated with the undisturbed velocity ratio before bed degradation. The experimental data also show that pier-scour depth is invariant with time, for t>=24 h. In a degrading channel, riprap around a pier will eventually develop into a stable mound when the bed shear stresses reduce with bed degradation. An auxiliary test shows that the mound is very vulnerable to another designed flood flow accompanied by large dunes. This type of riprap instability may be called bed-degradation induced failure. [ABSTRACT FROM AUTHOR]

Discusses the cause of failure in riprap revetments, especially on alluvial streambanks and rivers. Failure mechanism in riprap revetments; Analysis of seepage under and through riprap; Factors that affect filter performance of riprap revetments.

Investigates the destabilizing influence of bed forms on pier riprap and assesses the effects of riprap layer placement with respect to the surrounding bed level. Failure mechanisms in riprap protection; Experiments conducted in two laboratory recirculating flumes; Effects of riprap placement at bridge piers; Proposal of a riprap size-prediction equation.

Presents a study which showed that a riprap layer at a cylindrical bridge pier will fail in either disintegration or embedment. Effect of the formation of scour hole around bridge piers; Types of erosion under live-bed conditions; Effect of riprap stones interaction with bed sediment on riprap degradation; Classification of riprap failure mode; Conclusions.

Studies the roughness of rock riprap laid on step slopes. Median diameters of the angular riprap; Prediction of the Manning roughness coefficient as a function of D50 and S0; Use of equations developed from observations on natural mountain streams; Bed elevations and discharge through riprap.

Describes a rapid statistical procedure using the Weibull distribution as a probabilistic model of rock particle resistance developed for sizing loosely dumped rock riprap. Fraction of particles on the surface of a revetment displaced by a specified hydrodynamic shear stress; Relation of fitted parameters to the relative size of an individual particle.

Engineering methods to deal with pier-scour problems include placing coarse riprap stones around the bridge pier. This study aims to investigate the stability of a riprap layer around a cylindrical bridge pier. Experiments conducted in a laboratory flume identified three different modes of riprap failure. They can act singly or combine to render a complete failure of the riprap layer. These modes of failure are riprap shear failure, winnowing failure, and edge failure. The study proposes a semiempirical method to size stones for riprap protection. The procedure for this method was illustrated with field data from three different bridge sites. The experimental data show that a thick riprap layer can prevent winnowing in the absence of a filter layer. The thick riprap layer can also sustain a partial breakup of the layer with the capability of rearmoring the scour hole, preventing a total disintegration of the riprap layer. Finally, the study proposes two empirical relationships, which describe the effect of riprap thickness and cover on the stability of the riprap layer. [ABSTRACT FROM AUTHOR]

A design procedure for sizing gabion mattresses is presented that is based on depth-averaged velocity in the same manner as the Corps of Engineers' 1991 riprap-design guidance. This allows variation of gabion-mattress rock size and thickness with cross-section shape, channel alignment, and side slope. Stability is based on size of rock in the gabion rather than thickness, and stability coefficients are determined from flume-test results. Recommended mattress thickness and rock gradation are also presented. [ABSTRACT FROM AUTHOR]