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

Design Guideline 14 Rock Riprap at Bridge Abutments

14.1 INTRODUCTION

Scour occurs at abutments when the abutment and embankment obstruct the flow. Several causes of abutment failures during post-flood field inspections of bridge sites have been documented (Parola et al. 1998):

• Overtopping of abutments or approach embankments
• Lateral channel migration or stream widening processes
• Contraction scour
• Local scour at one or both abutments

Abutment damage is often caused by a combination of these factors. Where abutments are set back from the channel banks, especially on wide floodplains, large local scour holes have been observed with scour depths of as much as four times the approach flow depth on the floodplain. As a general rule, the abutments most vulnerable to damage are those located at or near the channel banks.

The flow obstructed by the abutment and highway approach embankment forms a horizontal vortex starting at the upstream end of the abutment and running along the toe of the abutment, and a vertical wake vortex at the downstream end of the abutment. The vortex at the toe of the abutment is very similar to the horseshoe vortex that forms at piers, and the vortex that forms at the downstream end is similar to the wake vortex that forms downstream of a pier. Research has been conducted to determine the depth and location of the scour hole that develops for the horizontal (so called horseshoe) vortex that occurs at the upstream end of the abutment, and numerous abutment scour equations have been developed to predict this scour depth.

Abutment failures and erosion of the fill also occur from the action of the downstream wake vortex. However, research and the development of methods to determine the erosion from the wake vortex has not been conducted. An example of abutment and approach embankment erosion of a bridge due to the action of the horizontal and wake vortex is shown in Figure 14.1. The types of failures described above are initiated as a result of the obstruction to the flow caused by the abutment and highway embankment and subsequent contraction and turbulence of the flow at the abutments.

14.2 DESIGN APPROACH

The preferred design approach is to place the abutment foundation on scour resistant rock or on deep foundations. Available technology has not developed sufficiently to provide reliable abutment scour estimates for all hydraulic flow conditions that might be reasonably expected to occur at an abutment. Therefore, engineering judgment is required in designing foundations for abutments. In many cases, foundations can be designed with shallower depths than predicted by the equations when they are protected with rock riprap and/or with a guide bank placed upstream of the abutment designed in accordance with this design guide and Design Guideline 15. Cost will be the deciding factor (Richardson and Davis 2001).

Figure 14.1. Scour of bridge abutment and approach embankment.

The potential for lateral channel migration, long-term degradation and contraction scour should be considered in setting abutment foundation depths near the main channel. It is recommended that the abutment scour equations originally presented in HEC-18 (Richardson and Davis 2001) be used to develop insight as to the scour potential at an abutment.

Where spread footings are placed on erodible soil, the preferred approach is to place the footing below the elevation of total scour. If this is not practicable, a second approach is to place the top of footings below the depth of the sum of contraction scour and long-term degradation and to provide scour countermeasures. For spread footings on erodible soil, it becomes especially important to protect adjacent embankment slopes with riprap or other appropriate scour countermeasures. The toe or apron of the riprap serves as the base for the slope protection and must be carefully designed to resist scour while maintaining the support for the slope protection.

In summary, as a minimum, abutment foundations should be designed assuming no ground support (lateral or vertical) as a result of soil loss from long-term degradation, stream instability, and contraction scour. The abutment should be protected from local scour using riprap and/or guide banks. To protect the abutment and approach roadway from scour by the wake vortex several DOTs use a 50-foot (15-meter) guide bank extending from the downstream corner of the abutment (see Design Guideline 15). Otherwise, the downstream abutment and approach should be protected with riprap or other countermeasures.

14.3 SIZING ROCK RIPRAP AT ABUTMENTS

The FHWA conducted two research studies in a hydraulic flume to determine equations for sizing rock riprap for protecting abutments from scour (Pagán-Ortiz 1991, Atayee 1993).The first study investigated vertical wall and spill-through abutments which encroached 28 and 56% on the floodplain, respectively. The second study investigated spill-through abutments which encroached on a floodplain with an adjacent main channel (Figure 14.2).

Encroachment varied from the largest encroachment used in the first study to a full encroachment to the edge of main channel bank. For spill-through abutments in both studies, the rock riprap consistently failed at the toe downstream of the abutment centerline (Figure 14.3). For vertical wall abutments, the first study consistently indicated failure of the rock riprap at the toe upstream of the centerline of the abutment.

Figure 14.2. Section view of a typical setup of spill-through abutment on a floodplain with adjacent main channel.

Figure 14.3. Plan view of the location of initial failure zone of rock riprap for spill-through abutment (Pagán-Ortiz 1991).

Field observations and laboratory studies reported in HDS 6 (Richardson et al. 2001) indicate that with large overbank flow or large drawdown through a bridge opening that scour holes develop on the side slopes of spill-through abutments and the scour can be at the upstream corner of the abutment. In addition, flow separation can occur at the downstream side of a bridge (either with vertical wall or spill-through abutments). This flow separation causes vertical vortices which erode the approach embankment and the downstream corner of the abutment.

For Froude Numbers (V/(gy)^1/2)#0.80, the recommended design equation for sizing rock riprap for spill-through and vertical wall abutments is in the form of the Isbash relationship:

(14.1)

where:

For Froude Numbers >0.80, Equation 14.2 is recommended:

(14.2)

where:

In both equations, the coefficient K, is a velocity multiplier to account for the apparent local acceleration of flow at the point of rock riprap failure. Both of these equations are envelope relationships that were forced to over predict 90% of the laboratory data.

The recommended procedure for selecting the characteristic average velocity is as follows:

1. Determine the set-back ratio (SBR) of each abutment. SBR is the ratio of the set-back length to channel flow depth. The set-back length is the distance from the near edge of the main channel to the toe of abutment.
   
   SBR = Set-back length/average channel flow depth
   
   
   1. If SBR is less than 5 for both abutments (Figure 14.4), compute a characteristic average velocity, Q/A, based on the entire contracted area through the bridge opening. This includes the total upstream flow, exclusive of that which overtops the roadway.

Figure 14.4. Characteristic average velocity for SBR<5.

2. If SBR is greater than 5 for an abutment (Figure 14.5), compute a characteristic average velocity, Q/A, for the respective overbank flow only. Assume that the entire respective overbank flow stays in the overbank section through the bridge opening.
3. If SBR for an abutment is less than 5 and SBR for the other abutment at the same site is more than 5 (Figure 14.6), a characteristic average velocity determined from Step 1a for the abutment with SBR less than 5 may be unrealistically low. This would, of course, depend upon the opposite overbank discharge as well as how far the other abutment is set back. For this case, the characteristic average velocity for the abutment with SBR less than 5 should be based on the flow area limited by the boundary of that abutment and an imaginary wall located on the opposite channel bank. The appropriate discharge is bounded by this imaginary wall and the outer edge of the floodplain associated with that abutment.

2. Recent research results published by the Transportation Research Board as NCHRP Report 587, "Countermeasures to Protect Bridge Abutments from Scour," endorse the use of the SBR approach for sizing riprap at spill-through abutments (Barkdoll et al. 2007). NCHRP Report 568, "Riprap Design Criteria, Recommended Specifications, and Quality Control," recommends an additional criteria for selecting a characteristic average velocity when applying the SBR method (Lagasse et al. 2006). Based on the results of 2-dimensional computer modeling of a typical abutment configuration NCHRP Report 568 concludes:
   1. Whenever the SBR is less than 5, the average velocity in the bridge opening provides a good estimate for the velocity at the abutment.
   2. When the SBR is greater than 5, the recommended adjustment is to compare the velocity from the SBR method to the maximum velocity in the channel within the bridge opening and select the lower velocity.
   3. The SBR method is well suited for estimating velocity at an abutment if the estimated velocity does not exceed the maximum velocity in the channel.
3. Compute rock riprap size from Equations 14.1 or 14.2, based on the Froude Number limitation for these equations. A recent study of riprap size selection for wing wall abutments (Melville et al. 2007) verified that these equations give stable stone size for riprap layers at wing wall abutments under subcritical mobile-bed conditions. Based on experimental results, this study concluded that with the SBR approach riprap size selection is appropriately based on stability against shear and edge failure. It is noted that stability against winnowing or bed-form undermining (see HEC-23, Volume 1, Chapter 4) is also important in design; however, adequate filter layer protection can prevent winnowing.
4. Determine extent of rock riprap.
   1. The apron should extend from the toe of the abutment into the bridge waterway a distance equal to twice the flow depth in the overbank area near the embankment, but need not exceed 25 ft (7.5 m) (Atayee et al. 1993). There may be cases where an apron extent of twice the flow depth is not adequate (Melville et al. 2006). Melville's findings are based on data collected for NCHRP 24-18. Therefore, the engineer should consider the need for a greater apron extent. The downstream coverage should extend back from the abutment 2 flow depths or 25 ft (7.5 m), whichever is larger, to protect the approach embankment (Figure 14.7).

Figure 14.5. Characteristic average velocity for SBR>5.

Figure 14.6. Characteristic average velocity for SBR>5 and SBR<5.

Figure 14.7. Plan view of the extent of rock riprap apron (Lagasse et al. 2006).

2. Spill-through abutment slopes should be protected with the rock riprap size computed from Equations 14.1 or 14.2 to an elevation 2 ft (0.6 m) above expected high water elevation for the design flood. Several States in the southeast use a guide bank 50 ft (15 m) long at the downstream end of the abutment to protect the downstream side of the abutment.
3. The rock riprap thickness should not be less than the larger of either 1.5 times D₅₀ or D₁₀₀. The rock riprap thickness should be increased by 50% when it is placed under water to provide for the uncertainties associated with this type of placement. Figure 14.8 illustrates the recommendation that the top surface of the apron should be flush with the existing grade of the floodplain (Lagasse et al. 2006). This is recommended because the layer thickness of the riprap (1.5 d₅₀ or d₁₀₀) could block a significant portion of the floodplain flow depth (reducing bridge conveyance) and could generate significant scour around the apron. The apron thickness may also be increased to protect the edge of the apron from contraction scour, long-term degradation and/or channel migration.
4. The rock riprap gradation and potential need for underlying filter material must be considered (see Design Guidelines 4 and 16).

Figure 14.8. Typical cross section for abutment riprap (Lagasse et al. 2006).

5. It is not desirable to construct an abutment that encroaches into the main channel. If abutment protection is required at a new or existing bridge that encroaches into the main channel, then riprap toe down or a riprap key should be considered. In cases where the abutment extends into the main channel and dune-type bed forms may be present, it is strongly recommended that only a geotextile filter be considered for the riprap protection.

14.4 DESIGN EXAMPLE FOR RIPRAP AT BRIDGE ABUTMENTS

Riprap is to be sized for an abutment located on the floodplain at an existing bridge. The bridge is 650 ft (198.12 m) long, has spill-through abutments on a 1V:2H side slope and 7 equally spaced spans. The left abutment is set back from the main channel 225 ft (68.58 m). Given the following tables of hydraulic characteristics for the left abutment, size the riprap.

Step 1. Determine the SBR (set-back distance divided by the average channel flow depth)

Step 2. Determine characteristic average velocity, V. SBR is greater than 5, therefore overbank discharge and areas are used to determine V.

V = Q/A = 7720/613.5 = 12.6 ft/s (3.84 m/s)

Step 3. Check SBR velocity against main channel velocity

Velocity in channel is greater than SBR velocity, therefore, use SBR velocity.

Step 4. Determine the Froude Number of the flow.

Fr = V/(gy)^1/2 = 12.6/(32.2(2.7)) ^½ = 1.35

Step 5. Determine the D₅₀ of the riprap for the left abutment. The Froude Number is greater than 0.8, therefore, use Equation 14.2.

D₅₀ = 0.4(2.7) = 1.1 ft (0.33 m)

Step 6. Determine riprap extent and layout.

• Extent into floodplain from toe of slope = 2(2.7) = 5.4 ft (1.66 m)
• Vertical extent up abutment slope from floodplain = 2.0 ft + 2.7 ft = 4.7 ft (1.4 m)
• Downstream face of the embankment should be protected a distance of 25 ft (7.5 m) from the point of tangency between the curved portion of the abutment and the plane of the embankment slope.
• Riprap mattress thickness = 1.5 (1.1) = 1.7 ft (0.5m). Also, the thickness should not be less than D₁₀₀.
• Riprap gradation and filter requirements should be designed using Design Guideline 12. This portion of the design is not conducted for this example.

14.5 SPECIFICATIONS FOR BRIDGE ABUTMENT RIPRAP

14.5.1 Size, Shape, and Gradation

Riprap design methods typically yield a required size of stone that will result in stable performance under the design loadings. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum allowable representative size. For abutment scour protection, the designer specifies a minimum allowable d₅₀ for the rock comprising the riprap, thus indicating the size for which 50% (by weight) of the particles are smaller. Stone sizes can also be specified in terms of weight (e.g., W₅₀) using an accepted relationship between size and volume, and the known (or assumed) density of the particle.

For the shape, weight, density, and gradation of bridge abutment riprap, specifications developed for revetment riprap are applicable (Lagasse et al. 2006). These specifications are provided in Design Guideline 4 of this document (see Section 4.2.4).

Design Guideline 4 recommends gradations for ten standard classes of riprap based on the median particle diameter d₅₀ as determined by the dimension of the intermediate ("B") axis. These gradations were developed under NCHRP Project 24-23, "Riprap Design Criteria, Recommended Specifications, and Quality Control." The proposed gradation criteria are based on a nominal or "target" d₅₀ and a uniformity ratio d₈₅/d₁₅ that results in riprap that is well graded. The target uniformity ratio is 2.0 and the allowable range is from 1.5 to 2.5 (Lagasse et al. 2006).

14.5.2 Recommended Tests for Rock Quality

Standard test methods relating to material type, characteristics, and testing of rock and aggregates recommended for revetment riprap are applicable to bridge abutment riprap (see Design Guideline 4). In general, the test methods recommended are intended to ensure that the stone is dense and durable, and will not degrade significantly over time.

Rocks used for riprap should only break with difficulty, have no earthy odor, no closely spaced discontinuities (joints or bedding planes), and should not absorb water easily. Rocks comprised of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as riprap. The recommended tests and allowable values for rock and aggregate are summarized in Table 4.3 of Design Guideline 4.

14.6 REFERENCES

Atayee, A. Tamin, 1993, "Study of Riprap as Scour Protection for Spill Through Abutment," presented at the 72nd Annual TRB meeting in Washington, D.C., January.

Atayee, A. Tamin, Pagán-Ortiz, Jorge E., Jones, J.S., and Kilgore, R.T., 1993, "A Study of Riprap as a Scour Protection for Spill Through Abutments," ASCE Hydraulic Conference, San Francisco, CA.

Barkdoll, B.D., Ettema, R., and Melville, B.W., 2007, "Countermeasures to Protect Bridge Abutments from Scour," NCHRP Report 587, Transportation Research Board, National Academies of Science, Washington, D.C.

Lagasse, P.F., Clopper, P.E., Zevenbergen, L.W., and Ruff, J.F., 2006, "Riprap Design Criteria, Recommended Specifications and Quality Control, NCHRP Report 568, Transportation Research Board, Academies of Science, Washington, D.C.

Melville, B.W., van Ballegooy, S., Coleman, S., and Barkdoll, B., 2007, "Riprap Size Selection at Wing-Wall Abutments," Technical Note, ASCE, Journal of Hydraulic Engineering, Vol. 133, No. 11, November.

Melville, B.W., van Ballegooy, S., Coleman, S., and Barkdoll, B., 2006, "Countermeasure Toe Protection at Spill Through Abutments," ASCE Journal of Hydraulic Engineering, Vol. 132, No. 3.

Pagán-Ortiz, Jorge E., 1991, "Stability of Rock Riprap for Protection at the Toe of Abutments Located at the Floodplain," FHWA Research Report No. FHWA-RD-91-057, U.S. Department of Transportation, Washington, D.C.

Parola, A.C., Hagerty, D.J., and Kamojjala, S., 1998, NCHRP Report 417, "Highway Infrastructure Damage Caused by the 1993 Upper Mississippi River Basin Flooding," Transportation Research Board.

Richardson, E.V. and Davis, S.R., 2001, "Evaluating Scour at Bridges," Hydraulic Engineering Circular 18, Fourth Edition, FHWA NHI 01-001, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.

Richardson, E.V., Simons, D.B., and Lagasse, P.F., 2001, "River Engineering for Highway Encroachments - Highways in the River Environment," Report FHWA NHI 01-004, Federal Highway Administration, Hydraulic Design Series No. 6, Washington, D.C.