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An experimental investigation of riprap stability at spill-through abutments has been carried out in two similar horizontal-bed flumes, avoiding riprap failure modes other than shear failure and erosion failure. Tests were conducted for two common abutment side slopes, subcritical flow regime, short to intermediate abutment lengths and high bed roughness. The aim of the experiments was to determine the size of stone riprap necessary to resist (a) shear failure in aprons placed at the base of spill-through abutments, and (b) erosion failure over their side slopes. Results are compared with predictions of stone riprap size given by expressions found in the literature. The predictor of Pagán-Ortiz is confirmed as being appropriate for shear failure at abutment aprons and can be adapted to account for erosion failure over abutment side slopes. Alternatively, two new expressions, written in terms of the (critical) approach flow intensity required to make the riprap stones move, are suggested. [ABSTRACT FROM AUTHOR]

This paper presents laboratory tests using riprap as a countermeasure for river bend scouring. The bank is a vertical wall in a sharp bend. The riprap is a horizontal apron extended at the toe of this wall. The aims of the tests are to describe the behaviour of riprap in sharp bend flow and to examine how the main flow characteristics are affected by the riprap behaviour. The experimental results show that flexible riprap is able to adapt to bed deformation in river bends by tilting, spreading and paving a transverse slope which ensures that the bank remains protected. Even a blanket located at the initial bed elevation provides efficient protection if it is thick enough. The tests also show that the mainstream velocity and shear stress above the riprap increase after riprap adaptation. [ABSTRACT FROM AUTHOR]

Abstract: Various methods are proposed to control scouring around bridge piers. In the present study application of riprap alone and combinations of riprap and collar were examined experimentally for scour control around cylindrical bridge piers. Tests were conducted with seven riprap sizes and with two different sizes of collars. Empirical equations were developed for stable riprap diameter for two cases of piers with and without collar protection. Extent of riprap layer is also presented for these two cases. The results showed that in the range of b/dR =7.5 (where b is pier diameter and dR is riprap size) using a collar reduced the stable riprap size. It was also concluded that using collar reduced the riprap layer extension in front and sides of the pier. [ABSTRACT FROM AUTHOR]

Abstract: Unfavorable erosion to a revetment can affect the stability of the bank and may jeopardize the safety of adjacent structures, thus improvement work is needed to increase the stability of the revetment as well as reducing the possibility of failure. The use of geotextiles as a protection material for banks is not only environmentally friendly, but also stable in the long run. However, improper design of geotextiles may cause considerable loss of soil, which might result in failure. The actual flow behavior in revetments using geotextiles is rather complicated and can be categorized into three zones, namely, the uni-directional flow zone, the cyclic flow zone, and the tangential flow zone. In this study, a wave flume experiment was performed on model revetments using two kinds of geotextiles as the filter material to prevent erosion induced by cyclic flows. Soil migration behaviors were monitored. Furthermore, two kinds of cover blocks, riprap and concrete blocks, were carefully placed on the revetments in order to avoid puncture and abrasion of geotextiles during construction of revetments. The main purpose of this study is to elucidate the erosion control and filtration performance of soil-geotextile filtration systems under wave action. Two nonwoven needle punched geotextiles were tested. The geotextiles both have the same characteristic opening size, but have a different number of constrictions and different structures. One is a thin double-layer nonwoven material consisting of continuous filaments and the other is a thick one-layer nonwoven material consisting of short fibers. The test result reveals that two different erosion mechanisms occur during wave action. The cyclic wave loadings triggered higher excess pore-water pressure in the upper part of the model and resulted in soil collapse. The geotextile with a higher number of constrictions produced more serious collapse than that with a lower one. At the middle part of the revetments, soil was eroded by the up-and-down drag force of the flow along the bank. In this zone, the thick geotextile displayed better performance in retarding the drag force on the soil surface. In addition, the opening size and thickness of the geotextile as well as the coverage condition provided by the geotextile were the key factors controlling soil erosion. [Copyright &y& Elsevier]

This study addresses the design of riprap aprons as a scour countermeasure near abutments under clear-water conditions. It deals with the lateral extent of riprap aprons and the geometry of the scour hole prevailing at the apron edge. The study applies to riprap aprons acting as granular filters. The scour depth appears to be independent for a sufficiently long relative abutment length. Scour holes develop farther away from spill-through abutments than from vertical-wall abutments; the distance between the point of maximum scour depth and the abutment increases with the relative abutment length. The effect of contraction on this distance was not identified. The angle defining the position of the deepest scour point is close to 30°. Neither the abutment shape nor the flow contraction seems to influence the minimum stable apron width. [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]

Abstract: Armoring countermeasures, such as riprap stones, are the primary method used to protect bridge piers against scouring; however, these methods have not had definitive success. Recently, flow-altering countermeasures, such as sacrificial piles, have been tested as an alternative to armoring countermeasures. This study investigated the mechanics of an innovative flow-altering countermeasure device, frames in the shape of tetrahedrons that act as a pier-scour countermeasure. Results of measured characteristics for turbulence flow showed that the flow around the tetrahedral frames can be divided into three regions: (1) a deceleration region near the sediment bed; (2) an acceleration region in the middle of water depth region; and (3) a restoration region near the water surface. The velocity magnitudes, turbulent intensities and vorticities decreased in the deceleration region, increased in the acceleration region and reverted to that of the unprotected condition in the restoration region. This pier-scour countermeasure is innovative because of its ability to dissipate energy associated with the downflow and the horseshoes vortex generated around the bridge pier. The scour tests revealed that the frames protected the foundation of bridge piers against scour. The experimental results showed that the percentage reduction of scour depth decreased as the velocity ratio, U/Uc , decreased, reaching a value of 50% for the range of parameters tested in this study. Moreover, its efficiency was dependent on the placement density of the frames. The data showed that the frames were more effective when η was larger. However, their influence becomes less significant when η ≥ 0.16. [Copyright &y& Elsevier]

Severson, J.P., J.R. Nawrot and M.W. Eichholz. 2009. Shoreline stabilization using riprap breakwaters on a Midwestern reservoir. Lake Reserv. Manage. 25:208–216. Shoreline erosion causes shoreline habitat loss and degradation and contributes to sedimentation, a major impairment in many lakes throughout the United States. Various shoreline stabilization techniques have been employed, but many are unsuccessful under high wave stress, do not contribute to shoreline habitat, or are too expensive to install on a large scale. Extensive erosion and lack of shoreline habitat on Kinkaid Lake in southern Illinois prompted lake managers to design and install riprap breakwaters to protect the littoral zone and bank as well as enhance habitat. The offshore breakwaters were shown to decrease wave height and associated erosion, allowing banks to start stabilizing and the protected littoral zone to begin sequestering sediment. Terrestrial area inside the protected zones was regressed against age since protection, bank height, and distance from bank to produce a terrestrialization predictive model. Vegetation richness was much greater at protected sites than unprotected sites, and vegetation cover increased with age since protection. The riprap breakwaters were successful at bank stabilization and habitat enhancement and should therefore be considered for use where these attributes are desired. [ABSTRACT FROM PUBLISHER]

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]

Increasing human populations along marine coastlines has lead to increasing urbanization of the marine environment. Despite decades of investigations on terrestrial ecosystems, the effect of urbanization on marine life is not well understood. Riprap is the rocky rubble used to build jetties, breakwaters, and armored shorelines. Roughly 30% of the southern California shoreline supports some form of riprap, while 29% of the shoreline is natural rocky substrate. Astonishingly few studies have investigated this anthropogenic rocky habitat even though it rivals a natural habitat in area on a regional scale along a coastline that has been extensively studied. In this study, I compared the diversity and community structure of exposed rocky intertidal communities on four riprap and four natural sites in southern California. I ask the following questions: (1) does diversity or community composition differ between intertidal communities on riprap and natural rocky habitats in southern California, (2) if so, which organisms contribute to those differences, (3) which physical factors are contributing to these differences, and (4) do riprap habitats support higher abundances of invasive species than natural habitats? On average, riprap and natural rocky habitats in wave exposed environments in southern California did not differ from each other in diversity or community composition when considering the entire assemblage. However, when only mobile species were considered, they occurred in greater diversity on natural shores. These differences appear to be driven by wave exposure. The presence of invasive species was negligible in both natural and riprap habitats. [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 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]

Even though electrofishing is commonly used to sample Neogobius spp. and other swimbladder-lacking benthic fishes, its efficiency is considered poor especially in habitats with abundant interstitial spaces. To determine the efficiency of electrofishing Neogobius spp. and quantitatively estimate sampling bias in a riprap (shot rock used to armor shorelines against water erosion) mesohabitat, riprap fragments were set up in a natural riverine environment. The experimental setting enabled us to collect all fish remaining in the riprap fragments after these areas had been electrofished. The sampling efficiency of electrofishing Neogobius spp. (dominated by Neogobius melanostomus) varied between 17.6 and 47.4% (mean 29.7%), while percids (possessing a well-developed swim bladder) were collected with 74.6% efficiency. Fish size had no effect on the probability of capturing Neogobius spp. by electrofishing. Within Neogobius spp., N. melanostomus was less susceptible to electrofishing than Neogobius gymnotrachelus (23.7% and 50.1%, respectively). Decreased electrofishing efficiency in areas of rocky substrate should be considered in estimates of total abundance of Neogobius spp., especially if they are to be compared with catches of other species possessing swim bladders. [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]

1. The shores of many lakes have been substantially altered by human developments such as erosion control structures or recreational beaches. Such alterations are likely to increase in the future, yet almost nothing is known about their impacts on the littoral macroinvertebrate community. 2. Macroinvertebrates were studied in seven German lowland lakes exhibiting natural shorelines (reference), retaining walls, ripraps and recreational beaches to examine impacts on the eulittoral (0–0·2 m water depth) and infralittoral (0·2–1·2 m water depth) communities associated with the three types of shoreline development. 3. Among sites, eulittoral species richness and abundance of Coleoptera, Gastropoda, Trichoptera, shredders and xylophagous species were lowest on beaches and retaining walls but ripraps did not differ significantly from natural shorelines. Retaining walls and ripraps had no significant impact on the infralittoral macroinvertebrate community. Conversely, beaches had significantly lower infralittoral species richness and abundance of Ephemeroptera, Trichoptera and shredders than natural shorelines. Furthermore, species richness was correlated positively with habitat heterogeneity expressed as number of habitat types. 4. Among lakes, whole-lake littoral macroinvertebrate density increased with increasing proportion of developed shorelines due to increasing abundances of Chironomidae. The remaining macroinvertebrate major groups decreased with increasing proportion of shoreline development. 5. Synthesis and applications. The biological impacts of shoreline development in lowland lakes depend upon the extent to which structural complexity and heterogeneity of littoral habitats are reduced. Hence, we recommend that management programmes focus upon the conservation of littoral habitat complexity and habitat heterogeneity. The biological effects of shoreline development may be assessed efficiently by combining an assessment of the morphological status of lakeshores and information on macroinvertebrate indicator species with a defined response to the loss of their preferred habitats. [ABSTRACT FROM AUTHOR]

In Germany small dams (<10 m) on flood retention reservoirs are often provided with an overtoppable dam section for flood relief. In case of overtopping a protection of the dam body is absolutely mandatory as the mainly cohesive dam material is not capable to withstand the affecting erosive forces of the flow. There is a big number of possibilities how to protect the dam whilst overtopping. Some of the most common construction types of slope protection layers for a sufficient dam protection such as riprap and placed stones, have been tested in large scale physical models at the Hydraulic Laboratory at Universität Stuttgart. Thereby different experiments for the failure scenarios “erosion of single stones”, “sliding of the protection layer” as well as “disruption of the protection layer” have been conducted and analysed aiming to find some design criteria for the construction of such dams. [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]

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Accurate estimation of the maximum possible depth of scour at bridge abutments is important in decision-making for the safe depth of burial of footings. Besides, investigation of the geometric features of scour holes around abutments provides useful information for the degree of scour counter-measure to be implemented against excessive scouring. Experiments have been performed to investigate time-dependent characteristics of scour holes around vertical wall abutments under clear water conditions with uniform bed materials. Temporal variations of scour depth and scour contours were measured. Using this information, an empirical relation was developed for temporal variation of scour depth. Additional relations were also derived for time-dependent volume and surface area of the scour holes around abutments. The findings of this study may provide useful information for preliminary design of abutment footings and placement details of armoring counter-measures, such as riprap. [ABSTRACT FROM AUTHOR]

An investigation on the effects of deepwater outfall discharges on the status of rocky reef communities was carried out. The sanitation system was found to be an environmentally suitable option for the protection of those habitats situated in high energy coastal environments. Sediments occurring between the predominating rocky substrates showed low values of the fine fraction (<63μm) and organic matter content. In addition, high average concentrations of Cd, Hg and Zn were found in these sediments, though these values were similar to those registered in non-affected sites, far away from the outfall. On the other hand, those assemblages typical of hard substrates that settled near the outfall showed an increase in total richness and abundance of macroinvertebrates. Moreover, the average number of species of each taxonomic group, a good indicator of the maintenance of the previous trophic structure, only varied considerably over time at the rip-rap protection. In a global context, those changes were not directly related to the discharge disturbances, but to the natural variability or the successional processes occurring within those communities. Only communities dwelling in the rip-rap protection area were affected by the proximity of the discharges. [Copyright &y& Elsevier]

Abstract: The rockfill dam of this study has a clay core, filter zone, and sandy gravel shell, and it is located in the Nakdong River basin in Korea. Filling the reservoir began in August 1994 and it was full by April 1998. When the reservoir level was approximately 150 m, three settlement holes were found in the dam crest. To determine possible seepage paths and potential damaged areas within the rockfill dam, 15 tracer tests were performed during two periods. During the two test periods, six and nine tracer tests were conducted when the average water levels of the reservoir were 145 m and 142 m, respectively. Rhodamine WT (RWT) and bromide ion were used as tracers. For each test, 1800 to 4000 L of tracer solution were injected into the riprap for about 5 h. The 15 injection points were uniformly distributed from the right to left sides of the embankment and the two tracers were injected alternately. Tracer concentrations were monitored at 26 observation wells, which were divided into four groups by their geographical positions: right side of the crest, left side of the crest, right side of the dam toe, and left side of the dam toe. For each tracer test, more than 30 water samples were taken during a period of 96 h at each observation well. Based the breakthrough curves obtained from the sequential tracer tests, it was inferred that the most probable seepage paths and potential damaged area were in the left side of the crest (tongue wing zone) and, to a lesser extent, in the right side of the dam abutment. [Copyright &y& Elsevier]

Abstract: Rock armourstone is unusual amongst geomaterials because of the extremely wide range of intrinsic rock strengths that often have to be used and the range of end use conditions in which it must serve. The authors'' experience has been distilled to formulate suitable guidance as demanded by practicing coastal and hydraulic engineers for the forthcoming update to the 1991 Rock Manual (CIRIA, CUR, CETMEF. in press. The Rock Manual. The use of rock in hydraulic engineering (second edition). C683, CIRIA, London.). The armourstone evaluation theme of earlier work by Lienhart, and abrasion testing and degradation modelling of armourstone by Latham are revisited. A general in-service degradation model for wear of armourstone, based on a progressive fractional mass loss with time, is redeveloped. The innovation is to introduce two new methods that are both practical and simple to implement. One method uses abrasion resistance, water absorption and block integrity in a system that couples the rock susceptibilities to various common site conditions. The other treats intrinsic rock properties from the quarry all together to achieve a global armourstone quality designation, AQD, of the rock, that is independent of the actual conditions of the proposed site of application. Depending on the assessment of site aggressiveness, the degradation with time on site is generated by retarding or accelerating the standard mass loss plot invoked from the AQD value, but adjusted by semi-qualitative engineering and/or scientific judgement of the rating geologist. Examples are used to illustrate and compare the two methods. Discussion is included on the need for more rigorous future research approaches based on breakage prediction and on the need for further case histories. [Copyright &y& Elsevier]

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]

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d95 values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered. [ABSTRACT FROM AUTHOR]

The use of riparian land buffers to protect water quality for human consumption and wildlife habitat has become an important conservation tool of both government and non-government agencies. The funds available to acquire private lands for riparian buffers are limited, however, and not all land contributes to water quality goals in the same way. Conservation agencies must therefore identify effective ways to allocate their scarce budgets in heterogeneous landscapes. We demonstrate how the acquisition of land for a riparian buffer can be viewed as a binary optimization problem and we apply the resulting model to a case study in New York. [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.