Countermeasures to Protect Bridge Abutments from Scour (2007)

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191 10.1 Introduction This chapter presents design guidelines for scour counter- measures for use at bridge abutments. The guidelines use the findings of the laboratory experiments discussed in Chapters 7 through 9. Additionally, the guidelines use information obtained from the survey of state DOT countermeasure prac- tice as well as from existing literature on scour control. The guidelines are structured in terms of a simple selection process described in Section 10.1. This process identifies the countermeasure concepts that may be appropriate for addressing a scour concern, indicates possible construction options, and then provides the design relationships associated with the layout and dimensioning of the countermeasures developed in this project. Because the project focuses on countermeasures for miti- gating scour at bridge abutments, the countermeasure con- cepts do not address in detail countermeasure concepts for mitigating scour of channels at locations away from a bridge waterway. Instead, the guidelines identify these countermea- sures and refer to current design monographs that give the pertinent design guidelines. The design guidelines detailed in this chapter also address the set of criteria mentioned at the outset of this report: 1. Technical effectiveness (including no substantial adverse effects), 2. Constructability, 3. Durability and maintainability, 4. Aesthetics and environmental issues, and 5. Cost. It must be stated that the guidelines can only address Cri- teria 2 through 5 in relative terms. Though the criteria of constructability and durability and maintainability were expressly considered when identifying the countermeasures detailed in this chapter, the present project did not dwell on the criteria of aesthetics and environmental issues or cost. 10.2 Countermeasure Selection and Construction Options The several processes creating a scour concern for an abut- ment require different countermeasure concepts, and each countermeasure concept can be constructed and imple- mented in a variety of ways. As indicated in Table 10-1, and as discussed in Chapter 3, there are three main scour concerns: • General bed degradation, or overall scour, which results from a reduction in the bed-load supply of sediment to the bridge site (i.e., degradation progressing from upstream to downstream) or a steepening of channel slope owing to head-cutting of the channel bed (i.e., degradation pro- gressing from downstream to upstream). • Approach-flow scour, which results from channel shifting or thalweg shifting. • Localized scour at the abutment, which results from local- ized vortices. Table 10-1 shows the corresponding countermeasure con- cepts for each of the three concerns, as well as construction options available for implementing the countermeasure concepts. The steps for proceeding through the guidelines given herein are as follows: 1. Identify the process causing the scour concern. 2. Select a countermeasure concept. 3. Select a construction method for the countermeasure concept (using Criteria 1 through 5 above). 4. Design the countermeasure. 5. Review the design in terms of Criteria 1 through 5 above. Although these steps encompass the main design consider- ations, the steps are not meant to be prescriptive. It is antici- pated that each design office undertaking the design of a C H A P T E R 1 0 Design Guidelines

192 Abutment Scour Concern Countermeasure Concept Construction Option General bed degradation Use a bed-control structure 1. Place weir across channel to maintain bed level at bridge waterway 2. Place sheet pile around abutment to maintain bed level at abutment Channel or thalweg shift Use a channel-control structure 1. Use a channel-control structure to guide flow away from a bank 2. Use a bank-control structure to armor the bank and thereby prevent further channel shifting 3. Shift the abutment back and add a bridge span Modify the flow field at the abutment 1. Align approach-channel bank(s) 2. Shift the abutment back and add a bridge span 3. Add a relief bridge 4. Add a parallel wall or guidebank(s) (Ch. 9) 5. Place flow-deflection spur dike(s) or groin(s) (Ch. 9) Armor the abutment boundary 1. Place riprap or cable- tied blocks at spill- through abutments located on floodplain (Ch. 8) 2. Place riprap, cable-tied blocks, parallel walls, or spur dikes at wing- wall abutments at main channel bank at narrow crossings (Ch. 7) 3. Armor the outflow region of lateral drains and the adjacent channel bank Localized scour at abutment Increase the geotechnical stability of the abutment 1. Place sheet pile around the abutment to retain the embankment Table 10-1. Scour concerns, countermeasure concepts, and construction options. bridge abutment, or taking responsibility for the maintenance of the abutment, will have its own design procedure. 10.3 Channel Bed Degradation Countermeasures to control bed degradation aim to maintain the channel bed level at the bridge waterway or around the foundations of bridge abutments and piers. If the degradation is progressing from upstream (e.g., because a dam has greatly reduced bed sediment transport in the channel): • Place a weir across the channel at a location close to the downstream end of the bridge waterway or • Place sheet-piling around the abutment. If the degradation is progressing from downstream (usu- ally owing to head-cutting): • Place a low weir across the channel at a location down- stream of the bridge waterway or • Place sheet-piling around the abutment. According to Lagasse et al. (1995) and others, channel lin- ing with riprap and concrete has proven unsuccessful in stop- ping degradation. Therefore, it is normal to use a check dam or low weir. Design information on check dams and rock weirs is given by Breusers and Raudkivi (1991).

10.3.1 Low Weirs In recent years, considerable effort has been devoted to developing low weirs that do not block fish and aquatic crea- tures from moving along channels. The structures typically have replicated the form and flow features of rock riffles, like small-scale rapids. A weir is typically constructed of sheet piles, rock mound, or a combination of the two. The rock weir concept is illustrated in Figure 10-1. The top photo shows an example, and the bottom sketch shows a conceptual drawing. Such weirs are favored by biologists because, in addition to stopping head-cutting, they resemble a natural rock riffle and enable fish and aquatic creature migration upstream or downstream. A rock weir can be con- structed to halt bed degradation by head-cutting, but can enable the passage of aquatic creatures. Figure 10-2 shows an example of the combination of sheet-piling and rock weir for a small channel. In this figure, riprap is not protected with 193 (b) Conceptual drawing on a small waterway where the rock weir was needed to arrest head-cut migration upstream toward a bridge (a) Example on a wide river where a barge was needed for construction Figure 10-1. Rock weir concept. grout, so rocks can be dislodged during high flows. Figure 10-3 shows a weir built only of sheet-piling but including a fish ladder. Sheet pile weirs, though technically effective, are less favored by biologists because of their less-than- appealing appearance. The following construction and constructability issues should be considered for rock weirs: • A rock weir should be grouted to ensure that the rock remains in place during high flows. Although grout will crack over time, it still has a longer life than riprap. Grout- ing may also help protect against the effects of freeze-thaw breakdown of rock. • The contractor should ensure that the weir’s rocks are placed carefully so as not to include large protrusions that may be moved during high flows. • For larger channels, the weir will have to be constructed with the aid of a barge (see Figure 10-1a).

The following construction and constructability issues should be considered for sheet pile weirs: • A sheet pile weir will need scour protection along its down- stream side because of flow passage over the weir. • The channel bank adjoining the sheet pile weir will need concrete or riprap scour protection. • The sheet piles will have to be located deep enough that they will not fail owing to scour. 10.3.2 Sheet-Piling Around Abutment A countermeasure practice sometimes used is to place a sheet pile skirt around an abutment base. Figure 10-4 indicates the extent of sheet pile placement around a spill-through abut- ment. Figure 10-5 illustrates an example where the sheet pile skirt prevented the possible failure of a spill-through abutment located on a floodplain. The scour condition shown is localized scour consequent to flow contraction and the local flow field generated by the abutment. The following construction and constructability issues must be considered: • The sheet-piling, which is not load bearing, should be placed to a depth exceeding the estimated depth of scour. 194 Figure 10-4. Sheet pile skirt around a spill- through abutment. Figure 10-5. Sheet pile skirt placed to protect a spill- through abutment from collapsing into a large scour hole formed in the floodplain at the abutment. Figure 10-2. A combination of sheet pile and rock weir located below a bridge to stop knickpoint migration. Figure 10-3. Use of a sheet pile weir, fitted with a fish ladder, to prevent further stream-bed degrada- tion due to head-cutting and to enable fish to migrate upstream.

• The sheet-piling should extend around the front and sides to the end of the bridge. • The sheet-piling should be a short distance out from the toe of the face slope of the abutment. • For an existing abutment, sheet-piling can be retrofitted by various means, depending on local site conditions. If access beneath the deck of a bridge is difficult for pile-driving, piles could be driven close to the sides of the deck, then piles at the central portion beneath the deck could be formed with an infill panel placed in an excavated trench, or an infill of large riprap stone could be placed. 10.4 Channel Control The purpose of approach-channel control is to ensure that the approach flow passes directly through the bridge opening in a manner that does not expose the bridge’s abutments, approach embankments, or piers to severe scour. Flow- control methods seek to guide the flow and/or to protect the banks of a channel. In terms of reducing abutment scour, it is important that channel-control countermeasures align the axis of the approach channel so as to be perpendicular to the bridge axis. There are extensive publications concerning the design of flow-control structures and bank-protection structures. Chapter 5 provides an extensive discussion of these publi- cations. Among the pertinent publications regarding chan- nel-control measures are Acheson (1968), Ahmad (1951), Copeland (1983), Farsirotou et al. (1998), Grant (1948), Khan and Chaudhry (1992), Kuhnle et al. (1997, 1998, 1999), Mayerle et al. (1995), Maza Alvarez (1989), Molinas et al. (1998a, 1998b), Molls et al. (1995), Muneta and Shimizu (1994), Neill (1973), Richardson et al. (1998), Shields et al. (1995a, 1995b, 1995c), Soliman et al. (1997), Strom (1962), Suzuki et al. (1987), Tominaga et al. (1997), United Nations Economic Commission for Asia and the Far East (1953), Wu and Lim (1993), and Zhang and Du (1997). Richardson and Simons (1984) give design recommenda- tions based on the literature. Lagasse et al. (1995) and Richardson et al. (1991) give design guidelines for imper- meable and permeable spur dikes, guide banks, and riprap stability factor design. 10.4.1 Flow Control The options for flow control vary according to the extent to which the approach flow has to be aligned and guided through the bridge opening. In most cases, the layouts of flow-control structures have to be determined on a site-by- site basis. Sometimes, determining the layout requires inves- tigation by means of a hydraulic laboratory model or a two-dimensional, depth-averaged numerical model. Flow control typically requires the use of one or more of the following structures for the purpose indicated: • Spur dikes are fitted to force the realignment of a channel and/or to increase flow velocities. Channel realignment may be needed when an approach channel is shifting later- ally, as shown in Figure 10-6. Increased flow velocities may be needed in situations where a channel has widened, flow velocities have decreased, and the approach channel is aggrading. Channel aggradation may reduce the flow area of the bridge opening; • Bendway weirs or barbs are fitted to stop lateral shifting of a channel and thereby to redirect the channel optimally through a bridge opening, as shown in Figure 10-7; and, 195 Figure 10-6. Spur dikes placed to narrow a widened approach channel and to ensure desired alignment of approach channel.

• Vanes are an alternative to spur dikes, bendway weirs, or barbs for use in improving approach channel alignment, as shown in Figure 10-8. These flow-control structures are used in rather specific applications that often have to be tailored to fit local condi- tions of channel alignment and morphology, as well as bridge extent and alignment. However, because the use of guide- banks and spurs varies from one form to another (e.g., barbs, bendway weirs, and wing dams), it is useful here to mention briefly a couple of preliminary notes regarding their use. For use of impermeable and permeable spurs, guidebanks, and riprap stability factor design to control the approach channel to a bridge opening, the following design guidelines are useful to keep in mind: • The flow field around a typical, straight spur causes bed scour at the spur’s tip and sediment deposition (i.e., silta- tion) close to where the spur adjoins the river/stream bank, as illustrated in Figure 10-9a. A spur is useful for defining the local path of flow thalweg (i.e., line of deepest flow), as well as providing local bank protection. • If the spur points downstream, flow can be drawn to the river/stream bank because the scour hole is moved closer to the river/stream bank. An attracting spur is illustrated in Figure 10-9b. 196 Figure 10-7. Barbs placed to stop lateral migration of an approach channel. Figure 10-8. Vanes placed to stop lateral migration of an approach channel and to narrow the approach channel to match the width of the bridge opening.

• If the spur points upstream, the scour hole is shifted away from the bank, and flow accordingly is deflected away from the bank. A deflecting spur is illustrated in Figure 10-9c. • Spurs of low crest elevations (e.g., barbs or bendway weirs) are sometimes used for sites where concerns exist about excessive depth produced by a spur, spur retarding of higher flow discharges (e.g., bankfull flow), and debris accumulation on spurs. Additionally, flow passage over the submerged spurs reduces the amount of sediment deposi- tion around the spur. • Spurs in series are spaced so that the space between spurs just accommodates the wake eddy formed by flow around a spur, as illustrated in Figure 10-10. There is no need to space the spurs more closely. The spurs are spaced too widely if flow is drawn in toward the face of the down- stream spur. Figure 10-11 shows spur dike installation along the outside bank upstream of an approach channel to a bridge. Spurs and their variants can be built from placed rock or from timber posts driven into a stream or river bed. A great variety of sizes and construction methods have been employed in building spurs. Vanes have been used for erosion reduction on river bends and for stopping river bend migration at a bridge waterway. Vanes are small panels placed in the riverbed at an angle of attack to the flow, which creates a vortex downstream that can be used to manage sediment and alter flow. When placed in an array, vanes deflect water current and bed sediment toward the desired orientation through a bridge waterway. 10.4.2 Bank Protection The literature on bank protection is extensive and does not need to be elaborated on here. In general, the customary approaches for bank protection are as follows: 1. Armor the bank. Place a protective lining to ensure that flow does not erode the surface of the bank. Various forms of armoring are used, notably riprap, rock gabions, and concrete lining. 197 Figure 10-9. Flow, scour, and siltation features for spurs (spur dikes, groins, exposed barbs, and bendway weirs). Figure 10-10. Typical spur layout along the convex bank.

2. Hardpoints. Place resistant nodes along the bank to make sure that the bank holds its alignment in situations where the approach flow may otherwise tend to shift the channel laterally. The nodes, commonly called hard- points, are usually formed from rock placed in relatively close spacing. Sometimes, hardpoints are formed from a combination of timber posts and rock. Figure 10-12 illustrates this option, and Figure 10-13 illustrates a typ- ical application. 10.5 Design Guidelines for Localized Abutment Armoring The construction choice between riprap, cable-tied blocks, or geobags is largely up to the designer and should be based on a life-cycle cost assessment of the structure and/or coun- termeasure. One exception is that some designers find geobags not particularly pleasing aesthetically and may, therefore, consider geobags a temporary countermeasure. 10.5.1 Wing-Wall Abutments Riprap The design parameters for riprap as an abutment scour coun- termeasure at wing-wall abutments are riprap size and size gra- dation, riprap layer thickness, filter requirements, and riprap layer extent. Figure 10-14 shows the pertinent parameters. Riprap size, d50. Riprap size selection can be based on sta- bility against shear and edge failure if the other possible modes of failure are also addressed appropriately. Either of the following Pagan-Ortiz (1991) and Lagasse et al. (2001) equations, with appropriate factors of safety, are suitable for predicting riprap stone sizes that are resistant to shear failure at wing-wall abutments. Pagan-Ortiz (1991): (10-1) Lagasse et al. (2001): (10-2) Where: d50 median size of the riprap stones, Umean velocity in the contracted bridge section, d y K S Fr Frs s 50 1 0 8= −( ) > 0.28 . d y K S Fr Frs s 50 2 1 0 8= −( ) ≤ . d U y S gs 50 2 0 23 0 811 064 1 = −( ) ⎛ ⎝⎜ ⎞ ⎠⎟ . . . 198 Figure 10-11. Spur dike installation along the outside bank upstream of an approach channel to a bridge. Figure 10-12. Hardpoints placed to keep an approach channel from eroding its banks.

y  depth of flow in the contracted bridge section, Fr Froude number in the contracted bridge section, Ss  specific gravity of the riprap material, g gravitational acceleration, and Ks  shape factor. Riprap size selection is appropriately based on stability against shear and edge failure, although consideration of the possibility of winnowing or bed-form undermining is also important in design. Riprap layer thickness. The criterion given by Lagasse et al. (2001) (discussed in Section 5.6.3) is recommended—that is, the riprap layer thickness should be at least the larger of 1.5 times d50 or d100. Riprap gradation.The Brown and Clyde (1989) criteria (dis- cussed in Section 5.6.3) for correctly grading riprap for bridge abutment protection are recommended. The criteria were shown in Table 5-7 and are shown again here in Table 10-2. Filter Requirements. As discussed in Section 5.6.3, filters are used to prevent winnowing of bed sediment from between the riprap voids. Filters can be granular (which use the filter- ing effect of graded sediments) or synthetic (commonly known as geotextiles). Filters are placed beneath riprap layers to meet the following objectives: • To prevent the groundwater seepage behind the riprap from transporting the underlying sediment through the riprap, commonly known as piping failure. The filter should be fine enough to prevent the base sediment from passing through it, but more permeable than the base sediment being pro- tected to prevent build-up of any excess pore-water pressures. • To prevent the high level of turbulence in front of the riprap layer from winnowing the underlying material through the riprap. It is recommended that filters be placed beneath riprap at wing-wall abutments whenever practicable. Riprap layer extent. Under mobile-bed conditions, riprap aprons placed at wing-wall bridge abutments are subject to undermining due to localized scour and bed-form propaga- tion through the bridge section. Typically, the riprap apron settles (i.e., the outer edge of the riprap layer tends to settle most). If appropriately designed, the riprap layer will remain intact as it settles. The limiting condition for design is when Wmin is zero. For this situation, the following expression was developed in Section 7.2.4: (10-3) Where: W apron width; ds2  scour depth (i.e., layer settlement depth) at the outer edge of the riprap; db  placement (i.e., burial) depth of the riprap; d50 median size of the riprap stones; and C1  1.68 and 1.19 at the upstream and downstream cor- ners of the riprap layer, respectively. Equation 10-3 is recommended for determination of the lateral extent of the riprap apron. Furthermore, the apron should extend at least 1.5W upstream and 1.0W downstream from the wing-walls. Design steps. Design steps are as follows: 1. Estimate the maximum likely scour depth, ds. 2. Select the riprap size (using Equations 10-1 or 10-2), grad- ing, filter, and layer extent (using Equation 10-3). 3. Sketch the abutment/countermeasure/scour hole geome- try (in a cross section) that is likely to appear after scour. 4. Assess the geotechnical stability of the abutment, as shown in Figure 10-15. Cable-Tied Blocks The design parameters for cable-tied blocks as an abut- ment scour countermeasure at wing-wall abutments are block size and shape, cable design, filter requirements, and cable-tied block layer extent. Cable-tied block aprons are subject to two observed flow- induced failure modes, as described by Parker et al. (1998). The failure modes are overturning and rolling-up of the leading edge of a cable-tied block mat (which can occur in the absence of sufficient anchoring or toeing-in of the leading edge) and uplift of the inner mat (which can occur at higher flow veloci- ties when the leading edge is sufficiently anchored). Block size. In order to avoid failure by uplift, the weight per unit area, , of the block mattress as a whole, should be greater than the value given by the following equation, which was proposed by Parker et al. (1998): W C d d ds b= − +1 2 50( ) 199 Figure 10-13. Rock hardpoints placed along a bank approach to a bridge.

(10-4) Where: cb  block density,  fluid density, and U approach-flow velocity.     = − 0 2 2. cb cb U 200 Table 10-2. Rock riprap gradation (Brown and Clyde, 1989). Figure 10-14. Riprap or cable-tied blocks at a wing-wall abutment. Stone Size Range Percent of gradation smaller than the stone size range 1.5d50 to 1.7d50 100 1.2d50 to 1.4d50 85 1.0d50 to 1.15d50 50 0.4d50 to 0.6d50 15

The minimum required block height, H, can be calculated as follows: (10-5) Where: p volume fraction pore space within the mattress. Block shape. Cable-tied blocks are typically manufactured in a truncated pyramid shape with a square base and top. Parker et al. (1998) recommend that the spacing between cable-tied block units be adequate to allow the mattress to have a sufficient degree of flexibility and that block shape not inhibit mat flexibility. Filters. Synthetic filters are recommended for use beneath cable-tied block mats. Cable-tied block layer extent. Under mobile-bed condi- tions, cable-tied block aprons placed at wing-wall bridge abut- ments are subject to undermining due to localized scour and bed-form propagation through the bridge section. Typically, the apron settles (i.e., the outer edge of the cable-tied block mat tends to settle most). The cable-tied block mat will remain intact as it settles. The limiting condition for design is when Wmin is zero. For this situation, the following expression is developed in Section 7.2.4: (10-6) Where: W apron width, ds  scour depth (i.e., mat settlement depth) at the outer edge of the mat, and db  placement (i.e., burial) depth of the mat. Equation 10-6 is recommended for determination of the lateral extent of the cable-tied block apron. Furthermore, the apron should extend at least 1.5W upstream and 1.0W down- stream from the wing-walls. W d ds b= −1 55. ( ) H g pcb = −   ( )1 Edge protection. As discussed in Section 7.2.4, cable-tied block mats are stable against overturning of the leading edge when Equation 7-5 is satisfied: (10-7) Where: Scb  specific gravity of the blocks and n Manning coefficient. Equation 10-7 provides a simple means of estimating block size to resist failure due to overturning and rolling-up of the leading edge. In use of Equation 10-7, care needs to be taken to ensure that the leading edge of the mat remains buried. Design steps. Design steps are as follows: 1. Estimate the maximum likely scour depth, ds. 2. Select the cable-tied block size (using Equations 10-4, 10-5, and 10-7), filter, and layer extent (using Equation 10-6) 3. Sketch the abutment/countermeasure/scour hole geome- try (in a cross section) that is likely to appear after scour. 4. Assess the geotechnical stability of the abutment (using Figure 10-15) Geobag Countermeasure Guidelines are briefly presented here for the geobag layout, sizing, and post-geobag scour location as an apron around a single abutment or as a mat extending across the full bridge waterway. Each design necessarily is tailored to the site, but the following design steps should be incorporated as much as possible. Design steps. Design steps are as follows: 1. Sizing of the geobags should be such that the thickness of an individual geobag is equivalent to or exceeds that of a riprap stone sized for the abutment site. Several methods for sizing H Y S Fr n y b cb = −( ) 158 1 2 2 0 33. 201 Figure 10-15. Sketch of geotechnical stability of an abutment.

riprap are available (e.g., U.S. Army Corps of Engineers, 1989; Richardson and Davis, 1995; Austroads, 1994). 2. After the riprap size is calculated, the geobags can be sized with overall plan dimensions that enable convenient assembly of bags as an apron whose extent is comparable to the apron extents commonly used for riprap (e.g., Fig- ures 7-60 and 7-62) or to an apron extent found necessary for a particular bridge site. 3. It is necessary to link (e.g., by tying) the geobags placed as an apron around an abutment. So doing enables the sys- tem of geobags to function as a moderately flexible armor cover that stays intact when the channel bed scours around the abutment. 4. The maximum slope of the geobag apron should be about 2:1 (H:V). The geobag apron should have a toe or skirt at the bottom end that extends deeper than the mattress by at least two thicknesses of geobag. 5. The region beneath, and immediately behind, the pile cap of a wing-wall abutment should be protected so as to pre- vent loss of embankment soil. Protection can be by means of geobags and/or riprap. Because the earthfill region of the embankment adjoining the abutment typically is poorly compacted and prone to erosion, it is important to ensure that it is protected. 6. If the aprons are linked so as to form a protective mat, the geobags should be of double layer thickness, but need not be tied together. However, it is necessary to provide toe and heel protection of the mat. 10.5.2 Spill-Through Abutments Riprap The design parameters for riprap as an abutment scour countermeasure at spill-through abutments are riprap size, riprap layer thickness, riprap gradation, filter requirements, riprap layer extent, and scour hole geometry. Figure 10-16 shows riprap or cable-tied blocks at a spill-through abutment. Riprap size, d50. The riprap used for the experiments undertaken for this study was selected using the following equation presented by Lagasse et al. (2001). The riprap based on Equation 10-8 was observed to be stable in all cases. (10-8) Where: yf  flow depth for the flood channel (i.e., adjacent to the abutment) in the contracted bridge section, Ks  shape factor, Fr U/(gyf)0.5  Froude number in the bridge contracted section, U  characteristic mean velocity in the contracted sec- tion, and Ss  specific gravity of the riprap stones. The characteristic velocity, U, depends on the setback distance (i.e., position of the abutment toe with respect to the main channel near bank). The setback ratio (SBR) is d y K S Fr f s s n50 1 = −( ) 202 Figure 10-16. Riprap or cable-tied blocks at a spill-through abutment on a floodplain.

defined as the setback distance divided by the average chan- nel flow depth. For SBRs less than 5, U is evaluated as the total discharge divided by the total flow area of the con- tracted section. For SBRs greater than 5, U is evaluated as the discharge in the flood channel upstream from the bridge divided by the flow area for the flood channel in the bridge section. For spill-through abutments, values of Ks and n are given in Table 10-3. Riprap layer thickness. The criterion given by Lagasse et al. (2001) in Section 8.6.3 is recommended—that is, the riprap layer thickness should be at least the larger of 1.5 times d50 or d100. Riprap gradation. The Brown and Clyde (1989) criterion in Section 8.6.3 for correctly grading riprap for bridge abut- ment protection is recommended. This criterion was shown in Table 10-2. Filter requirements. As discussed in Section 8.1.4, filters are used to prevent winnowing of bed sediment from between the riprap voids. Filters can be granular (which use the filter- ing effect of graded sediments) or synthetic (commonly known as geotextiles). Filters are placed beneath riprap layers to meet the following objectives: • To prevent the groundwater seepage behind the riprap from transporting the underlying sediment through the riprap, commonly known as piping failure. The filter should be fine enough to prevent the base sediment from passing through it, but more permeable than the base sediment being pro- tected to prevent build-up of any excess pore-water pressures. • To prevent the high level of turbulence in front of the riprap layer from winnowing the underlying material through the riprap. It is recommended that filters be placed beneath riprap at spill-through abutments whenever practicable. Riprap layer extent. Based on the experimental work pre- sented in Section 8.3.4, the minimum apron width to ensure adequate toe protection (i.e., for Wmin  0 in Figure 10-16) is (10-9) Where: W apron width and dsf maximum scour depth measured with respect to the level of the floodplain. W y d yf sf f = ⎛ ⎝⎜ ⎞ ⎠⎟0 5 1 35 . . Equation 10-9 is recommended for calculation of the width of riprap apron needed to adequately protect the toe of spill-through abutments. It is recommended that the protec- tion extend around the curved portions of the abutment to the point of tangency with the plane of the embankment slopes, as was illustrated in Figure 10-14. Scour hole geometry. As discussed in Section 8.3.4, the scour-hole geometry at a spill-through abutment featuring riprap apron protection can be described by the following set of equations: (10-10) Where: dsf  scour depth relative to the bed level in the flood chan- nel; ym  flow depth in the main channel; yf  flow depth in the flood channel; L abutment length; and F a function that depends on the position of the scour hole in a compound channel, as was illustrated in Fig- ure 10-14 and as given by (10-11) Where:   position of the outer edge of the scour hole and Bf  width of the flood channel (as was shown in Figure 10-14). When the scour hole forms entirely in the flood channel (i.e., when  Bf), Equation 10-11 reduces to dsf  ds. When L  Bf, the scour depth forms mostly in the main channel, F 1 and dsf  ds  (ym – yf). The position of the center of the scour hole is defined by R and  30 degrees; R is given as follows: (10-12) Design steps. Design steps are as follows: 1. Estimate the maximum likely scour depth, dsf, and scour hole position, R, using Equation 10-12. 2. Select riprap size with Equation 10-8 and grading, filter, and apron extent with Equation 10-9. 3. Sketch the abutment/countermeasure/scour hole geome- try (in a cross section) that is likely after scour. 4. Assess the geotechnical stability of the abutment using Figure 10-15. R y L y W yf f f = ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎛ ⎝⎜ ⎞ ⎠⎟4 1 0 2 0 4. . F L B B F f e Bf e f = − − ⎛ ⎝⎜ ⎞ ⎠⎟ > = − ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 1 0 2 1 α α when when αe fB ≤1 d d F y ysf s m f= + −( ) 203 Fr Ks n ≤ 0.8 0.89 2 >0.8 0.61 0.1 Table 10-3. Values of Ks and n in Equation 10-8.

Cable-Tied Blocks The design parameters for cable-tied blocks as a scour countermeasure at spill-through abutments are block size, block shape, filters, layer extent, edge protection, and scour hole geometry. Cable-tied block aprons are subject to two observed flow- induced failure modes, as described by Parker et al. (1998). The failure modes are overturning and rolling-up of the lead- ing edge of a cable-tied block mat (which can occur in the absence of sufficient anchoring or toeing-in of the leading edge) and uplift of the inner mat (which can occur at higher flow velocities when the leading edge is sufficiently anchored). Block size. In order to avoid failure by uplift, the weight per unit area, , of the block mattress as a whole should be greater than the value given by the following equation, which was proposed by Parker et al. (1998): (10-13) Where: cb block density,  fluid density, and U  approach-flow velocity. The minimum required block height, H, can be calculated from (10-14) Where: p volume fraction pore space within the mattress. Block shape. Cable-tied blocks are typically manufactured as a truncated pyramid shape with a square base and top. Parker et al. (1998) recommend that the spacing between cable-tied block units be adequate to allow the mattress to have a sufficient degree of flexibility and that block shape not inhibit mat flexibility. Filters. Synthetic filters are recommended for use beneath cable-tied block mats. Layer extent. Based on the experimental work presented in Section 8.3.4, the minimum apron width to ensure adequate toe protection (i.e., for Wmin  0 in Figure 10-14) is (10-15) Where: W apron width and dsf maximum scour depth measured with respect to the level of the floodplain. Equation 10-15 is recommended for calculation of the width of cable-tied block apron needed to adequately protect W dsf= 1 4. H g pcb = −   ( )1     = − 0 2 2. cb cb U the toe of spill-through abutments. It is recommended that the protection extend around the curved portions of the abutment to the point of tangency with the plane of the embankment slopes, as was illustrated in Figure 10-14. Edge protection. As discussed in Section 7.2.4, cable-tied block mats are stable against overturning of the leading edge when the following expression is satisfied: (10-16) Where: Scb  specific gravity of the blocks and n Manning roughness coefficient. Equation 10-16 provides a simple means of estimating block size to resist failure due to overturning and rolling-up of the leading edge. In the use of Equation 10-16, care must be taken to ensure that the leading edge of the mat remains buried. Scour hole geometry. As discussed in Section 8.3.4, the scour-hole geometry at a spill-through abutment featuring cable-tied block apron protection can be described by the fol- lowing set of equations: (10-17) Where: dsf  scour depth relative to the bed level in the flood channel, ym  flow depth in the main channel, yf  flow depth in the flood channel, L abutment length, and F a function that depends on the position of the scour hole in a compound channel, as illustrated in Fig- ure10-14 and as given by (10-18) Where:   position of the outer edge of the scour hole and Bf  width of the flood channel (as was shown in Figure 10-14). When the scour hole forms entirely in the flood channel (i.e.,   Bf), Equation 10-18 reduces to dsf  ds. When L  Bf, the scour depth forms mostly in the main channel, F  1 and dsf  ds  (ym – yf). The position of the center of the scour hole is defined by R and  30 degrees (Figure 10-14); R is given by F L B B F f e Bf e f = − − ⎛ ⎝⎜ ⎞ ⎠⎟ > = − ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 1 0 2 1 α α when when αe fB ≤1 d d F y ysf s m f= + −( ) H Y S Fr n y b cb = −( ) 158 1 2 2 0 33. 204

(10-19) Design steps. Design steps are as follows: 1. Estimate the maximum likely scour depth, dsf, and scour hole position, R, using Equation 10-19. 2. Select the cable-tied block size using Equations 10-13 and 10-14, and select the filter and apron extent using Equa- tion 10-15. 3. Sketch the abutment/countermeasure/scour hole geome- try (in a cross section) that is likely after scour. 4. Assess the geotechnical stability of the abutment using Figure 10-15. Guidance for Estimating Scour Depth, ds Use of Equations 10-15 and 10-17 to estimate apron extent requires knowledge of expected scour depth. A number of equations exist for prediction of localized scour depth at bridge abutments. However, the scour depth where protection is in place differs from that traditionally measured in labora- tory experiments, so that existing scour equations may lead to excessively large aprons using Equations 10-15 and 10-17. In Equation 7-6, it is shown that the settlement depth, due to bed-form propagation, at the outside edge of aprons at wing-wall abutments is given by (10-20) Where: H-max maximum bed-form height and C2  1.2 and 1.0 for the upstream and downstream corners of the riprap layer, respectively. Equation 10-20 can be used in situations where local scour is relatively insignificant. This is often the case due to the riprap protection inhibiting scour near the abutment walls. 10.6 Design Guidelines for Localized Flow Field Modification 10.6.1 Parallel-Wall Countermeasure The design parameters for parallel-wall scour counter- measures are the wall length, wall side angle, wall height, wall base width, wall protrusion, wall plan form, and apron. See Figure 10-17 for a sketch of the design dimensions. Wall Length, Lw The length of the parallel wall should be 0.5aL, where La is the abutment length (perpendicular to flow direction). d Cs H= −2 max R y L y W yf f f = ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎛ ⎝⎜ ⎞ ⎠⎟ 0 2 0 9 1 . . Wall Side Angle, The maximum steepness of the side wall angle should be the angle of repose for the rock employed. Wall Height, Hw The height of the wall should be sufficient to have the top of the wall be above the highest flow depth that the bridge will experience. Wall Base Width, 2Hw The wall should be wide enough to accommodate the wall height and the sidewall angle of the rock wall. Wall Protrusion The bottom of the rock wall should be even with the abut- ment such that no part of the wall protrudes out into the main channel. Wall Plan Form The wall should be parallel to the river banks. Thus, if the river section is straight, then the wall should be straight as well, but if the river section is curved, then the wall should also be curved and parallel to the river banks. See Figure 10-18 for a sketch of a curved wall. Apron The thickness of the apron should be at least two times the diameter of the size of rocks used for the wall. The width of the apron should be at least four times the wall height. The apron should extend the full length of the wall. At the upstream end, the apron should join the floodplain. 10.6.2 Spur Dike Countermeasure The design parameters for spur dikes as abutment scour countermeasures are number of dikes, dike length, dike height, dike spacing, dike face angle, and dike width. See Figure 10-19 for a definition sketch. Number of Dikes There should be at least three dikes used: two shorter dikes at the upstream and downstream corners of the abut- ment and a longer dike upstream of the abutment. For wide abutments parallel to the flow, there may need to be addi- tional short dikes, as well (see the discussion on dike spac- ing below). 205

Dike Length The top length of the dike (perpendicular to the flow) should be equal to the abutment length, La (perpendicular to the flow). For the shorter dikes, this length extends from the abutment face out into the main channel. For the longer dikes upstream of the abutment, the length is longer than La. The dike should extend the same distance into the river that the shorter dikes do and extend back onto the floodplain a distance far enough not to affect the river flow. The bottom dike length is determined by the angle of the wall face. Care should be taken, however, on nar- rower rivers not to block too much of the river width with the dikes. Therefore, the dikes should not extend farther out into the main channel than one-fourth of the river width. 206 Floodplain Lw V ym Apron Parallel Rock Wall Hw=5.2m yf Floodplain Section B-B Ltw=35m Apron Parallel Rock Wall Abutment yf ym Flow Riverbed ym θ=45o Riverbed Top View Flow Main Channel Parallel Rock Wall B B A A Abutment La θ=45o Section A-A Abutment Top and Bottom of Bridge Deck Figure 10-17. Design dimensions for parallel-wall countermeasure using piled rocks. Dike Height The top elevation of the dike should be higher than the highest expected water level. Dike Spacing Dikes should be located at the abutment corners and extend out into the main channel. Since dike spacing should be less than the abutment length, La, an intermediate dike may be needed if the abutment width (parallel to flow direction) is longer then the abutment length, La. Dike Face Angle The steepness of the side wall angle should be the angle of repose for the rock employed.

Dike Width Dike width is determined by the dike face angle, which should be less than the angle of repose of the rock used to construct the dike. 10.7 Relation to Existing HEC Guidelines The set of HEC guidelines that currently address bridge scour and stream instability countermeasures is HEC-23 (Lagasse et al., 1997), which describes design guidelines for the following countermeasures: bendway weirs/stream barbs, soil cement, wire-enclosed riprap mattresses, articulated concrete block systems, articulating grout-filled mattresses, toskanes, grout- or cement-filled bags, and rock riprap at abutment and piers. These countermeasures are applicable to preventing bank erosion and therefore addressing stream instability. They are not addressed in the context of pier or abutment protection except for rock riprap. None of them address compound-channel flow, in which there is flow in the floodplain as well as in the main channel. It is suggested, therefore, that this report be used to supple- ment HEC-23 and be used specifically for the design of coun- termeasures at abutments with compound-channel flow conditions, which is when most of the critical scouring occurs. 207 Parallel-wall countermeasure located parallel to curved riverbank. Bridge Figure 10-18. Curved parallel-wall countermeasure located on a river bend. Main Channel Floodplain Upstream Spur Dike Upstream-Corner Spur Dike Downstream-Corner Spur Dike Additional Spur Dike for Wide Abutments Abutment Figure 10-19. Spur dike countermeasure design.