Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Chapter 8

OTHER COUNTERMEASURES AND CASE HISTORIES OF PERFORMANCE

8.1 INTRODUCTION

Design Guidelines 1 through 19 contain specific design procedures for a variety of stream instability and bridge scour countermeasures that have been applied successfully on a state or regional basis. Other countermeasures such as retarder structures, longitudinal dikes, bulkheads, and even channel relocations may be used to mitigate scour at bridges or stream bank erosion. Some of these measures are discussed and general guidance is summarized in this chapter. Chapter 4 (Section 4.3.6) illustrates the use of the concept of radial stress on a meander bend to evaluate the performance of fence, dike, and retarder type structures in protecting an eroding bankline.

Case histories of hydraulic problems at bridge sites can provide information on the success (or failure) of the various countermeasures used to stabilize streams. This chapter also summarizes the evaluation of countermeasure performance compiled for FHWA from case histories at 224 bridge sites (Brice and Blodgett 1978).

8.2 HARDPOINTS

Hardpoints consist of stone fills spaced along an eroding bank line, protruding only short distances into the channel. A root section extends landward to preclude flanking. The crown elevation of hardpoints used by the USACE at demonstration sites on the Missouri River was generally at the normal water surface elevation at the toe, sloping up at a rate of about 1 ft in 10 ft (1 m in 10 m) toward the bank. Hardpoints are most effective along straight or relatively flat convex banks where the streamlines are parallel to the bank lines and velocities are not greater than 10 ft/s (3 m/s) within 50 ft (15 m) of the bank line. Hardpoints may be appropriate for use in long, straight reaches where bank erosion occurs mainly from a wandering thalweg at lower flow rates. They would not be effective in halting or reversing bank erosion in a meander bend unless they were closely spaced, in which case spurs, retarder structures, or bank revetment would probably cost less. Figure 8.1 is a perspective of a hardpoint installation. Hardpoints have been used effectively as the first "spur" in a spur field (see Design Guideline 2).

8.3 RETARDER STRUCTURES

Retarder structures are permeable or impermeable devices generally placed parallel to streambanks to reduce velocities and cause deposition near the bank. They are best suited for protecting low banks or the lower portions of streambanks. Retarder structures can be used to protect an existing bank line or to establish a different flow path or alignment. Retards do not require grading of the streambank, and they create an environment which is favorable to the establishment of vegetation.

Figure 8.1. Perspective view of hardpoint installation with section detail (after Brown 1985).

8.3.1 Jacks and Tetrahedrons

Jacks most commonly consist of three linear members fixed together at their midpoints so that each member is perpendicular to the other two. Wires are strung on the members to resist distortion and to collect debris. Cables are used to tie individual jacks together and for anchoring key units to deadmen. Tetrahedrons consist of six members of equal length fixed together so as to form three faces, each of which is an equilateral triangle, i.e., a tetrahedron. The tetrahedron unit may be braced as shown in Figure 8.2 and wire mesh added to enhance flow retardance. Tetrahedrons are not as widely used as are jacks.

Jacks and tetrahedrons are effective in protecting banks from erosion only if light debris collects on the structures thereby enhancing their performance in retarding flow. However, heavy debris and ice can damage the structures severely. They are most effective on mild bends and in wide, shallow streams which carry a large sediment load.

Where jacks are used to stabilize meandering streams, both lateral and longitudinal rows are often installed to form an area retarder structure rather than a linear structure. Lateral rows of jacks are usually oriented in a downstream direction from 45° to 70°. Spacing of the lateral rows of jacks may be 50 to 200 ft (15 to 75 m) depending on the debris and sediment load carried by the stream. A typical jack unit is shown in Figure 8.3 and a typical area installation is shown in Figure 8.4.

Outflanking of jack installations is a common problem. Adequate transitions should be provided between the upstream bank and the structure, and the jack field should be extended to the overbank area to retard flow velocities and provide additional anchorage. Jacks are not recommended for use in corrosive environments or at locations where they would constitute a hazard to recreational use of the stream.

Figure 8.2. Typical tetrahedron design (after Brown 1985).

Figure 8.3. Typical jack unit (after Brown 1985).

Figure 8.4. Retarder field schematic (after HDS 6, Richardson et al. 2001).

8.3.2 Fence Retarder Structures

Fence retarder structures provide protection to the lower portions of banks of relatively small streams. Posts may be of wood, steel, or concrete and fencing may be composed of wood planks or wire.

Scour and the development of flow channels behind linear structures are common causes of failure of longitudinal fences. Scour at the supporting members of the structure can be reduced by placing rock along the fence or the effects of scour can be overcome by driving supporting members to depths below expected scour. Tiebacks can be used to retard velocities between the linear structure and the streambank, thus reducing the ability of the stream to develop flow channels behind the structure.

8.3.3 Timber Pile

Timber pile retarder structures may be of a single, double, or triple row of piles with the outside of the upstream row faced with wire mesh or other fencing material. They have been found to be effective at sharp bends in the channel and where flows are directly attacking a bank. They are effective in streams which carry heavy debris and ice loads and where barges or other shipping vessels could damage other countermeasures or a bridge. As with other retarder structures, protection against scour failure is essential. Figure 8.5 illustrates a design.

8.3.4 Wood Fence

Wood fence retarder structures have been found to provide a more positive action in maintaining an existing flow alignment and to be more effective in preventing lateral erosion at sharp bends than other retarder structures. Figure 8.6 is an end view of a typical wood fence design with rock provided to protect against scour.

Figure 8.5. Timber pile bent retarder structure (after Brown 1985).

Wire fence retarder structures may be of linear or area configuration, and linear configurations may be of single or multiple fence rows. Double-row fence retards are sometimes filled with brush to increase the flow retardance. Figures 8.7 and 8.8 illustrate two types of wire fence retarder structures.

8.4 LONGITUDINAL DIKES

Longitudinal dikes are essentially impermeable linear structures constructed parallel with the streambank or along the desired flow path. They protect the streambank in a bend by moving the flow current away from the bank. Longitudinal dikes may be classified as earth or rock embankment dikes, crib dikes, or rock toe-dikes.

8.4.1 Earth or Rock Embankments

As the name implies, these dikes are constructed of earth with rock revetment or of rock. They are usually as high or higher than the original bank. Because of their size and cost, they are useful only for large-scale channel realignment projects.

Figure 8.6. Typical wood fence retarder structure (modified from USACE 1981, after Brown 1985).

Figure 8.7. Light double row wire fence retarder structure (after Brown 1985).

Figure 8.8. Heavy timber-pile and wire fence retarder structures (Modified from USACE 1981, after Brown 1985).

8.4.2 Rock Toe-Dikes

Rock toe-dikes are low structures of rock riprap placed along the toe of a channel bank. They are useful where erosion of the toe of the channel bank is the primary cause of the loss of bank material. The USACE has found that longitudinal stone dikes provide the most successful bank stabilization measure studied for channels which are actively degrading and for those having very dynamic beds. Where protection of higher portions of the channel bank is necessary, rock toe-dikes have been used in combination with other measures such as vegetative cover and retarder structures.

Figure 8.9 shows the typical placement and sections of rock toe-dikes. The volume of material required is 1.5 to 2 times the volume of material that would be required to armor the sides of the anticipated scour to a thickness of 1.5 times the diameter of the largest stone specified. Rock sizes should be similar to those specified for riprap revetments (see Design Guideline 4). Tiebacks are often used with rock toe-dikes to prevent flanking, as illustrated in Figure 8.10. Tiebacks should be used if the toe-dike is not constructed at the toe of the channel bank.

Rock toe-dikes are useful on channels where it is necessary to maintain as wide a conveyance channel as possible. Where this is not important, spurs could be more economical since scour is a problem only at the end projected into the channel. However, spurs may not be a viable alternative in actively degrading streams (Design Guideline 2).

8.4.3 Crib Dikes

Longitudinal crib dikes consist of a linear crib structure filled with rock, straw, brush, automobile tires or other materials. They are usually used to protect low banks or the lower portions of high banks. At sharp bends, high banks would need additional protection against erosion and outflanking of the crib dike. Tiebacks can be used to counter outflanking.

Crib dikes are susceptible to undermining, causing loss of material inside the crib, thereby reducing the effectiveness of the dike in retarding flow. Figure 8.11 illustrates a crib dike with tiebacks and a rock toe on the stream side to prevent undermining.

8.4.4 Bulkheads

Bulkheads are used for purposes of supporting the channel bank and protecting it from erosion. They are generally used as protection for the lower bank and toe, often in combination with other countermeasures that provide protection for higher portions of the bank. Bulkheads are most frequently used at bridge abutments as protection against slumping and undermining at locations where there is insufficient space for the use of other types of bank stabilization measures, and where saturated fill slopes or channel banks cannot otherwise be stabilized.

Bulkheads are classified on the basis of construction methods and materials. They may be constructed of concrete, masonry, cribs, sheet metal, piling, reinforced earth, used tires, gabions, or other materials. They must be protected against scour or supported at elevations below anticipated total scour, and where sections of the installation are intermittently above water, provisions must be made for seepage through the wall. Some bulkhead types, such as crib walls and gabions, should be provided with safeguards against leaching of materials from behind the wall.

Bulkheads must be designed to resist the forces of overturning, bending and sliding, either by their mass or by structural design. Figure 8.12 illustrates anchorage schemes for a sheetpile bulkhead. Because of costs, they should be used as countermeasures against meander migration only where space is not available to construct other types of measures.

Figure 8.9. Typical longitudinal rock toe-dike geometries (after Brown 1985).

Figure 8.10. Longitudinal rock toe-dike tiebacks (after Brown 1985).

Figure 8.11. Timber pile, wire mesh crib dike with tiebacks (modified from USACE 1981, after Brown 1985).

Figure 8.12. Anchorage schemes for a sheetpile bulkhead (after Brown 1985).

8.5 CHANNEL RELOCATION

At some locations, it may be advantageous to realign a stream channel, either in combination with the use of other countermeasures against meander migration or in lieu of other countermeasures.

Figure 8.13 illustrates hypothetical highway locations fixed by considerations other than stream stability. To create better flow alignment with the bridge, consideration could be given to channel realignment as shown in this figure (parts a and b). Similarly, consideration for realignment of the channel would also be advisable for a hypothetical lateral encroachment of a highway as depicted in part c of the figure. In either case, criteria are needed to establish the cross-sectional dimensions.

Figure 8.13. Encroachments on meandering streams (after HDS 6, Richardson et al. 2001).

Before realigning a stream channel, the stability of the existing channel must be examined. The stream classification, recent and older aerial photographs, and field surveys are necessary. The realigned channel may be made straight without curves, or may include one or more curves. If curves are included, decisions regarding the radius of curvature, the number of bends, the limits of realignment (hence the length and slope of the channel) and the cross-sectional area have to be made. Different streams have different historical backgrounds and characteristics with regard to bend migration, discharge, stage, geometry, and sediment transport, and an understanding and appreciation of river hydraulics and morphology is important to decision making. It is difficult to state generalized criteria for channel relocation applicable to all streams. HEC-20 (Lagasse et al. 2001a) provides quantitative techniques for evaluating and predicting lateral channel migration and analyzing vertical channel stability, as well as an introduction to channel restoration concepts that should be considered for channel relocation projects.

Based on a study of the stability of relocated channels, Brice presented the following recommendations and conclusions regarding specific aspects of planning and construction of channel realignment (Brice 1981):

• Channel Stability Prior to Realignment.
  
  Assessment of the stability of a channel prior to realignment is needed to assess the risk of instability. An unstable channel is likely to respond unfavorably. Bank stability is assessed by field study and by stereoscopic examination of aerial photographs (see HEC-20). The most useful indicators of bank instability are cut or slumped banks, fallen trees along the bank line, and exposed wide point bars. Bank recession rates are measured by comparison of time-sequential aerial photographs. Vertical instability is equally important but more difficult to determine. It is indicated by changes in channel elevation at bridges and gaging stations. Serious degradation is usually accompanied by generally cut or slumped banks along a channel and by increased debris transport.
  
  
• Erosional Resistance of Channel Boundary Materials.
  
  The stability of a channel, whether natural or relocated, is partly determined by the erosional resistance of materials that form the wetted perimeter of the channel (see HEC-20). Resistant bedrock outcrops in the channel bottom or that lie at shallow depths will provide protection against degradation, but not all bedrock is resistant. Erosion of shale, or of other sedimentary rock types interbedded with shale, has been observed. Degradation is not a problem at most sites where bed sediment is of cobble and boulder size. However, degradation may result from the relocation of any alluvial channel, whatever the size of bed material, but the incidence of serious degradation of channels relocated by DOTs is small in number. The erosional resistance of channel beds tends to increase with clay content. Banks of weakly cohesive sand or silt are clearly subject to rapid erosion, unless protected with vegetation. No consistent relation has been found between channel stability and the cohesion of bank materials, probably because of the effects of vegetation.
  
  
• Length of Realignment.
  
  The length of realignment contributes significantly to channel instability at sites where its value exceeds 250 channel widths. When the value is below 100 channel widths, the effects of length of relocation are dominated by other factors. The probability of local bank erosion at some point along a channel increases with the length of the channel. The importance of vegetation, both in appearance and in erosion control, would seem to justify a serious and possibly sustained effort to establish it as soon as possible on graded banks.
  
  
• Bank Revetment.
  
  Revetment makes a critical contribution to stability of relocated stream channels at many sites. Rock riprap is by far the most commonly used and effective revetment (see Design Guideline 4). Concrete slope paving is prone to failure. Articulating concrete block is effective where vegetation can establish within the open cells of the blocks (Design Guideline 8).
  
  
• Check Dams (drop structures).
  
  In general, check dams are effective in preventing channel degradation in relocated channels. The potential for erosion at a check dam depends on its design and construction, its height and the use of revetment on adjoining banks. A series of low check dams, less than about 1.5 ft (0.5 m) in height, is probably preferable to a single higher structure, because of increased safety and reduced potential for erosion and failure. By simulating rapids, low check dams may add visual interest to the flow in a channel. One critical problem arising with check dams relates to improper design for large flows. Higher flows have worked around the ends of many installations to produce failure (see Design Guideline 3).
  
  
• Maintenance.
  
  Problems which could be resolved by routine maintenance were observed along relocated channels. These were problems with the growth of annual vegetation, reduction of channel conveyance by overhanging trees, local bank cutting, and bank slumping. The expense of routine maintenance or inspection of relocated channels beyond the highway right-of-way may be prohibitive; however, most of the serious problems could be detected by periodic inspection, perhaps by aerial photography, during the first five to ten years after construction. Hydraulic engineers responsible for the design of relocated stream channels should monitor their performance to gain experience and expertise.
  
  

8.6 CASE HISTORIES OF COUNTERMEASURE PERFORMANCE

Case histories of hydraulic problems at bridge sites can provide information on the relative success of the various countermeasures used to stabilize streams. The following case histories are taken from Brice and Blodgett (1978), Brice (1984), and Brown et al. (1980). Site data are from Brice and Blodgett (1978). This compilation of case histories at 224 bridge sites is recommended reference material for those responsible for selecting countermeasures for stream instability. Additional case histories are given in HDS 6 (Richardson et al. 2001).

8.6.1 Flexible Revetment

Rock Riprap. Dumped rock riprap is the most widely used revetment in the United States. Its effectiveness has been well established where it is of adequate size, of suitable size gradation, and properly installed. Brice and Blodgett (1978) documented the use of rock riprap at 110 sites (Volume 1, Table 2). They rated the performance at 58 sites and found satisfactory performance at 34 sites, partially satisfactory performance at 12 sites, and failure to perform satisfactorily at 12 sites. Keeley concluded that riprap used in Oklahoma performed without significant failure and provides basic and efficient bank control on the meandering streams in Oklahoma (Keeley 1971). Additional discussion of riprap revetment failure modes and inspection guidance can be found in Chapter 5 and Appendix D.

A review of the causes of failure at the sites studied by Brice and Blodgett (1978) is instructive. They found the absence of a filter blanket clearly the cause of the failure at a site subject to tides and wave action. The riprap was placed on a fill of sand and fine gravel which eroded through the interstices of the riprap.

Internal slope failure was the cause of failure of riprap at the abutment of bridges at two sites. At one site, failure was attributed to saturation of a high fill by impounded water in a reservoir. Wave action also probably contributed to the failure. The other site is difficult to include as a riprap failure because the rock was not placed as riprap revetment. Thirty-three freight car loads of rock were dumped as an emergency measure to stop erosion at a bridge abutment during high-flow releases from a reservoir. The rock was displaced, and the high streambanks and highway fill were still susceptible to slumps. At both sites, riprap failed to prevent slumps in high fills.

Inadequate rock size and size gradation was given as the cause of failure at eight sites. All of these sites are complex, and it is difficult to assign failure to one cause, but rock size was definitely a factor.

Channel degradation accounted for failure at three sites in Mississippi. Channel degradation at these sites is due to channel straightening and clearing by the SCS (NRCS) and USACE. Riprap installations on the streambanks, at bridge abutments and in the streambed have failed to stop lateral erosion. At one site, riprap placed on the banks and bed of the stream resulted in severe bed scour and bank erosion downstream of the riprap.

Failure of riprap at one site was attributed to the steep slope on which the riprap was placed. At this site, rock riprap failed to stop slumping of the steep banks downstream of a check dam in a degrading stream.

Successful rock riprap installations at bends were found at five sites. Bank erosion was controlled at these sites by rock riprap alone. Installations rated as failing were damaged at the toe and upstream end, indicating inadequate design and/or construction, and damage to an installation of rounded boulders, indicating inadequate attention to riprap specifications. Other successful rock riprap study sites were sites where bank revetment was used in conjunction with other countermeasures, such as spurs or retards. The success of these installations was attributed more to the spurs or retards, but the contribution of the bank revetment was not discounted.

Broken Concrete. Broken concrete is commonly used in emergencies and where rock is unavailable or very expensive. No specifications were found for its use. Performance was found to be more or less unsatisfactory at three sites.

Rock-and-Wire Mattress and Gabions. The distinction made between rock-and-wire mattress and gabions is in the dimensions of the devices (see Design Guideline 10). Rock-and-wire mattress is usually 1.0 ft (0.3 m) or less in thickness and a gabion is thicker and nearly equidimensional. The economic use of rock-and-wire mattress is favored by an arid climate, availability of stones of cobble size, and unavailability of rock for dumped rock riprap. Corrosion of wire mesh is slow in arid climates, and ephemeral streams do not subject the wire to continuous abrasion. Where large rock is not available, the use of rock-and-wire mattress may be advantageous in spite of eventual corrosion or abrasion of the wire.

Rock-and-wire mattress performance was found to be generally satisfactory although local failure of the wire mesh and spilling out of the rock was not uncommon. Mattresses are held in place against the bank by railroad rails at sites in New Mexico and Arizona where good performance was documented (see Design Guideline 6). This is known locally as "railbank protection." The steel rail supported rock-and-wire mattress stays in place better than dumped rock riprap on the unstable vertical banks found on the ephemeral streams of this area. Mattress held in place by stakes has been found to be effective in Wyoming.

The use of rock-and-wire mattress has diminished in California because of the questionable service of wire mesh, the high cost of labor for installation, and the efficiency of modern methods of excavating for dumped riprap toe protection. The Los Angeles Flood Control District, however, has had installations in-place for 15 years or more with no evidence of wire corrosion. On the other hand, Montana and Maryland reported abrasion damage of wire. These experiences illustrate that economical use of countermeasures is dependent on the availability of materials, costs, and the stream environment in which the measure is placed.

Several sites were identified where gabions were installed, but the countermeasures had been tested by floods at only one site where gabions placed on the downstream slope of a roadway overflow section performed satisfactorily.

Other Flexible Revetment. Favorable performance of precast-concrete blocks at bridges was reported in Louisiana. Vegetation is reported to grow between blocks and contribute to appearance and stability. Vegetation apparently is seldom used alone at bridges. Iowa relies on sod protection of spur dikes, but Arkansas reported failure of sod as bank protection.

8.6.2 Rigid Revetments

Failure of rigid revetment tends to be progressive; therefore, special precautions to prevent undermining at the toe and termini and failure from unstable soils or hydrostatic pressure are warranted.

Concrete Pavement. Well-designed concrete paving is satisfactory as fill slope revetment, as revetment on streams having low gradients, and in other circumstances where it is well protected against undermining at the toe and ends. The case histories include at least one location where riprap launching aprons were successful in preventing undermining at the toe from damaging the concrete pavement revetment. Weep holes for relief of hydrostatic pressure are required for many situations (see Design Guideline 4).

Documented causes of failure in the case histories are undermining at the toe (six sites), erosion at termini (five sites), eddy action at downstream end (two sites), channel degradation (two sites), high water velocities (two sites), overtopping (two sites), and hydrostatic pressure (one site). Good success is reported with concrete slope paving in Florida, Illinois, and Texas.

Sacked Concrete. No DOT reported a general use of sacked concrete as revetment. California was reported to regard this as an expensive revetment almost never used unless satisfactory riprap was not available. Sacked concrete revetment failures were reported from undermining of the toe (two sites), erosion at termini (one site), channel degradation (two sites), and wave action (one site) (see Design Guideline 4).

Concrete-Grouted Riprap. Fully-grouted riprap permits the use of smaller rock, a lesser thickness, and more latitude in gradation of rock than in dumped rock riprap. No failures of grouted riprap were documented in the case histories, but it is subject to the same types of failures as other rigid revetments (see Chapter 5, Section 5.6 and Design Guideline 12).

Concrete-Filled Fabric Mat. Concrete-filled fabric mat is a patented product (Fabriform) consisting of porous, pre-assembled nylon fabric forms which are placed on the surface to be protected and then filled with high-strength mortar by injection. Variations of Fabriform and Fabricast consist of nylon bags similarly filled. Successful installations were reported by the manufacturer of Fabriform in Iowa, and North Dakota reported successful installations (see Design Guideline 9).

Soil Cement. In areas where any type of riprap is scarce, use of in-place soil combined with cement provides a practical alternative. The resulting mixture, soil cement, has been successfully used as bank protection in many areas of the Southwest (see Design Guideline 7). Unlike other types of bank revetment, where milder side slopes are desirable, soil cement in a stairstep construction can be used on steeper slopes (i.e., typically one to one), which reduces channel excavation costs. For many applications, soil cement is generally more aesthetically pleasing than other types of revetment.

8.6.3 Bulkheads

A bulkhead is a steep or vertical wall used to support a slope and/or protect it from erosion (See Section 8.4). Bulkheads usually project above ground, although the distinction between bulkheads and cutoff walls is not always sharp. Most bulkhead applications were found at abutments. They were found to be most useful at the following locations: (1) on braided streams with erodible sandy banks, (2) where banks or abutment fill slopes have failed by slumping, and (3) where stream alignment with the bridge opening was poor, to provide a transition between streambanks and the bridge opening. It was not clear what caused failures at five sites summarized in Brice and Blodgett (1978), but in each case, the probable cause was undermining.

8.6.4 Spurs

Spurs are permeable or impermeable structures which project from the bank into the channel. Spurs may be used to alter flow direction, induce deposition, or reduce flow velocity. A combination of these purposes is generally served. Where spurs project from embankments to decrease flow along the embankment, they are called embankment spurs. These may project into the floodplain rather than the channel, and thus function as spurs only during overbank flow. According to a summary prepared for the Transportation Research Board, spurs may protect a streambank at less cost than riprap revetment, and by deflecting current away from the bank and causing deposition, they may more effectively protect banks from erosion than revetment (Richardson and Simons 1984). Uses other than bank protection include the constriction of long reaches of wide, braided streams to establish a stable channel, constriction of short reaches to establish a desired flow path and to increase sediment transport capacity, and control of flow at a bend. Where used to constrict a braided stream to a narrow flow channel, the structure may be more correctly referred to as a dike or a retard in some locations (see Design Guideline 2).

Several factors enter into the performance of spurs, such as permeability, orientation, spacing, height, shape, length, construction materials, and the stream environment in which the spur is placed.

Impermeable Spurs. The case histories show good success with well-designed impermeable spurs at bends and at crossings of braided stream channels (eight sites). At one site, hardpoints barely projecting into the stream and spaced at about 100 to 150 ft (30 to 45 m) failed to stop bank erosion at a severe bend. At another site, spurs projecting 40 ft (12 m) into the channel, spaced at 100 ft (30 m), and constructed of rock with a maximum diameter of 1.5 ft (0.5 m) experienced erosion between spurs and erosion of the spurs. At a third site, spurs constructed of timber piling filled with rock were destroyed. Failure was attributed to the inability to get enough penetration in the sand-bed channel with timber piles and the unstable wide channel in which the thalweg wanders unpredictably. Spurs (or other countermeasures) are not likely to be effective over the long term in such an unstable channel unless well-designed, well-built, and deployed over a substantial reach of stream. Although no failures from ice damage were cited for impermeable spurs, North Dakota uses steel sheet pile enclosed earth fill spurs because of the potential for ice damage. At one site, such a spur sustained only minor damage from 2.5 ft (0.75 m) of ice.

Permeable Spurs. A wide variety of permeable spur designs were also shown to successfully control bank erosion by the case histories. Failures were experienced at a site which is highly unstable with rapid lateral migration, abundant debris, and extreme scour depths. Bank revetments of riprap and car bodies and debris deflectors at bridge piers, as well as bridges, have also failed at this site. At another site, steel H-pile spurs with wire mesh have partially failed on a degrading stream.

8.6.5 Retardance Structures

A retardance structure (retard) can be a permeable or impermeable linear structure in a channel, parallel with and usually at the toe of the bank. The purposes of retardance structures are to reduce flow velocity, induce deposition, or to maintain an existing flow alignment. They may be constructed of earth, rock, timber pile, sheet pile, or steel pile. Steel jacks or tetrahedrons are also used (see Section 8.3).

Most retardance structures are permeable and most have good performance records. They have proved to be useful in the following situations: (1) for alignment problems very near a bridge or roadway embankment, particularly those involving rather sharp channel bends and direct impingement of flow against a bank (ten sites), and (2) for other bank erosion problems that occur very near a bridge, particularly on streams that have a wandering thalweg or very unstable banks (seven sites).

The case histories include a site where a rock retardance structure similar to a rock toe dike was successful in protecting a bank on a highly unstable channel where spurs had failed. There were, however, deficiencies in the design and construction of the spur installation. At another site, a rock retardance structure similar to a rock toe-dike has reversed bank erosion at a bend in a degrading stream. The USACE (1981) reported that longitudinal rock toe dikes were the most effective bank stabilization measure studied for channels having very dynamic and/or actively degrading beds.

8.6.6 Dikes

Dikes are impermeable linear structures for the control or containment of overbank flow (see Section 8.4). Most are in floodplains, but they may be within channels, as in braided streams or on alluvial fans. Dikes at study sites were used to prevent flood water from bypassing a bridge at four sites, or to confine channel width and maintain channel alignment at two sites. Performance of dikes at study sites was judged generally satisfactory.

8.6.7 Guide Banks

The major use of guide banks (formerly referred to as spur dikes) in the United States is to prevent erosion by eddy action at bridge abutments or piers where concentrated flood flow traveling along the upstream side of an approach embankment enters the main flow at the bridge (see Design Guideline 15). By establishing smooth parallel streamlines in the approaching flow, guide banks improve flow conditions in the bridge waterway. Scour, if it occurs, is near the upstream end of the guide bank away from the bridge. A guide bank differs from dikes described above in that a dike is intended to contain overbank flow while a guide bank only seeks to align overbank flow with flow through the bridge opening. An extension of the usual concept of the purpose for guide banks, but not in conflict with that concept, is the use of guide banks and highway fill to constrict braided channels to one channel. At three sites studied, guide banks only or guide banks plus revetment on the highway fill were used to constrict wide braided channels rather severely, and the installations have performed well.

Guide bank performance was found to be generally satisfactory at all study sites. Performance is theoretically affected by construction materials, shape, orientation, and length.

Most guide banks are constructed of earth with revetment to inhibit erosion of the dike. At two sites, guide banks of concrete rubble masonry performed well. Riprap revetment is most common, but concrete revetment with rock riprap toe protection, rock-and-wire mattress, gabions, and grass sod have also performed satisfactorily. Since partial failure of a guide bank during a flood usually will not endanger the bridge, wider consideration should be given to the use of vegetative cover for protection. Partial failure of any countermeasure is usually of little significance so long as the purpose of protecting the highway stream crossing is accomplished.

Guide banks of elliptical shape, straight, and straight with curved ends performed satisfactorily at study sites, although there is evidence at one site that flow does not follow the nose of the straight guide bank. Clear evidence of the effect of guide bank orientation was not found at study sites although the conclusion from a study of guide banks in Mississippi that guide banks should be oriented with valley flow for skewed crossings of wooded floodplains was cited (Colson and Wilson 1973). There was evidence at one site that a guide bank may be severely tested where a large flow is diverted along the roadway embankment, as at a skewed crossing or on a wide floodplain which is severely constricted by the bridge. At these locations, embankment spurs may be advisable to protect the embankment from erosion and to reduce the potential for failure of the guide bank.

Guide banks at study sites tended to be longer than recommended by Bradley (1978) at most sites, except at five sites where they ranged from 16 to 75 ft (5 to 23 m). All guide banks appeared to perform satisfactorily. Not enough short guide banks were included in the study to reach conclusions regarding length.

8.6.8 Check Dams

Check dams are usually used to stop degradation in the channel in order to protect the substructure foundation of bridges (see Design Guideline 3). At one site, however, a check dam was apparently used to inhibit contraction scour in a bridge waterway. The problem with vertical scour was resolved, but lateral scour became a problem and riprap revetment on the streambanks failed (Brice and Blodgett 1978).

Scour downstream of check dams was found to be a problem at two sites, especially lateral erosion of the channel banks. Riprap placed on the streambanks at the scour holes also failed, at least in part because of the steep slopes on which the riprap was placed. At the time of the study, lateral erosion threatened damage to bridge abutments and highway fills. At another site, a check dam placed at the mouth of a tributary stream failed to stop degradation in the tributary and the delivery of damaging volumes of sediment to the main stream just upstream of a bridge.

No structural failure of check dams was documented. Failures are known to have occurred, however, and the absence of documented failures in this study should not be given undue weight. Failure can occur by bank erosion around the ends of the structure resulting in outflanking; by seepage or piping under or around the structure resulting in undermining and structural or functional failure; by overturning, especially after degradation of the channel downstream of the structure; by bending of sheet pile; by erosion and abrasion of wire fabric in gabions or rock-and-wire mattress; or by any number of structural causes for failure.

8.6.9 Jack or Tetrahedron Fields

Jacks and tetrahedrons function as flow control measures by reducing the water velocity along a bank, which in turn results in an accumulation of sediment and the establishment of vegetation. Steel jacks, or Kellner jacks which consist of six mutually perpendicular arms rigidly fixed at the midpoints and strung with wire are the most commonly used (see Section 8.3). Tetrahedrons apparently are not currently used by DOTs. Jacks are usually deployed in fields consisting of rows of jacks tied together with cables.

Four sites where steel jack fields were used are included in the case histories. At two sites, the jack fields performed satisfactorily. Jacks were buried in the streambed and rendered ineffective at one site, and jacks were damaged by ice at one site, but apparently continued to perform satisfactorily. From Keeley's (1971) observations of the performance of jack fields used in Oklahoma and findings of the study of countermeasures by Brice and Blodgett (1978), the following conclusions were reached regarding performance:

• The probability of satisfactory performance of jack fields is greatly enhanced if the stream transports small floating debris and sediment load in sufficient quantity to form accumulations during the first few years after construction.
• Jack fields may serve to protect an existing bank line, or to alter the course of a stream if the stream course is realigned and the former channel backfilled before the jack field is installed.
• On wide shallow channels, which are commonly braided, jack fields may serve to shift the bank line channelward if jacks of large dimensions are used.

8.6.10 Special Devices for Protection of Piers

Countermeasures at piers have been used to combat abrasion of piers, to deflect debris, to reduce local scour, and to restore structural integrity threatened by scour. Retrofit countermeasures installed after problems develop are common. The usual countermeasure against abrasion consists of steel armor on the upstream face of a pier in the area affected by bed load. At one site, a pointed, sloping nose on a massive pier, called a special "cutwater" design, and a concrete fender debris deflector has functioned to prevent debris accumulation at the pier. At another site, a steel rail debris deflector worked until channel degradation caused all countermeasures to fail.

Countermeasures used to restore structural integrity of bridge foundations included in the case histories include underpinning, sheet pile driven around the pier, and a grout curtain around the pier foundation.

8.6.11 Channel Alterations

Although channel alterations or modifications have been curtailed due to environmental concerns, their judicious use can be a viable countermeasure not to be dismissed. It is recognized that extensive channelization projects, usually made to reduce floodplain damage, have resulted in serious channel degradation and lateral erosion. However, there is little documentation of upstream or downstream environmental damage of an alteration of a short reach in the vicinity of a bridge (Brice and Blodgett 1978).

In a United States Geological Survey study for FHWA of 103 stream channels that were altered for purpose of bridge construction mostly during the period of 1960-1970, the stability of the relocated channel was rated as good at 36 sites, as fair to good at 42 sites, as fair at 15 sites, and as poor at 7 sites. In comparison with bank stability of the channels where such data was available before and after relocation, bank stability was about the same at 45 sites, better at 28 sites, and worse at 14 sites. At sites where the value of channel relocation length to channel width was below 100, the effects of length of channel relocation were dominated by other factors (Brice 1981) (see Section 8.5).

8.6.12 Modification of Bridge Length and Relief Structures

A countermeasure for contraction scour and lateral movement of stream banks that may not always be considered for an existing bridge but may be needed is to increase its length. Increased bridge length was used at 11 sites and increased freeboard was provided at 2 sites. Other techniques that have been used by State DOTs include the design of abutments as piers so that the bridge may be extended to accommodate future movement of the stream. Other means of providing additional relief to flow would be the use of a relief bridge.

8.6.13 Investment in Countermeasures

While it may be possible to predict that bank erosion will occur at or near a given location in an alluvial stream, one can frequently be in error about the location or magnitude of potential erosion. At some locations, unexpected lateral erosion occurs because of a large flood, a shifting thalweg, or from other actions of the stream or activities of man. Therefore, where the investment in a highway crossing is not in imminent danger of being lost, it is often prudent to delay the installation of countermeasures until the magnitude and location of the problem becomes obvious. In many, if not most, of the case histories collected by Brice et al., DOTs invested in countermeasures after a problem developed rather than in anticipation of a problem (Brice and Blodgett 1978, Brice 1984).