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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 2 Spurs
A spur can be a pervious or impervious structure projecting from the streambank into the channel. Spurs are used to deflect flowing water away from, or to reduce flow velocities in critical zones near the streambank, to prevent erosion of the bank, and to establish a more desirable channel alignment or width. The main function of spurs is to reduce flow velocities near the bank, which in turn, encourages sediment deposition due to these reduced velocities. Increased protection of banks can be achieved over time, as more sediment is deposited behind the spurs. Because of this, spurs may protect a streambank more effectively and at less cost than revetments. Furthermore, by moving the location of any scour away from the bank, partial failure of the spur can often be repaired before damage is done to structures along and across the stream.
Spurs are generally used to halt meander migration at a bend. They are also used to channelize wide, poorly defined streams into well-defined channels. The use of spurs to establish and maintain a well-defined channel location, cross section, and alignment in braided streams can decrease the required bridge lengths, thus decreasing the cost of bridge construction and maintenance.
Spur types are classified based upon their permeability as retarder spurs, retarder/deflector spurs, and deflector spurs. The permeability of spurs is defined simply as the percentage of the spur surface area facing the streamflow that is open. Deflector spurs are impermeable spurs which function by diverting the primary flow currents away from the bank. Retarder/deflector spurs are more permeable and function by retarding flow velocities at the bank and diverting flow away from the bank. Retarder spurs are highly permeable and function by retarding flow velocities near the bank.
Table 2.1 can be used as an aid in the selection of an appropriate spur type for a given situation (Brown 1985). The primary factors influencing the selection of a specific spur type are listed across the top, and primary spur types are evaluated in terms of those selection criteria. A scale from 1 to 5 is used to indicate the applicability of a specific spur for a given condition. A value of 1 indicates a disadvantage in using that spur type for given condition, and a value of 5 indicates a definite advantage. The table can be used by summing values horizontally for given site conditions to select the best spur type for the specific site. It should be recognized however, that adherence to the results of such a procedure assigns equal weight to each of the factors listed across the top of the table and places undue reliance on the accuracy and relative merit of values given in the rating table. It is recommended that values given in the table be used only for a qualitative evaluation of expected performance. Spur type selection should be based on the results of this evaluation as well as estimated costs, availability of materials, construction and maintenance requirements, and experience with the stream in which the spur installation is to be placed.
Spur design includes setting the limits of bank protection required; selection of the spur type to be used; and design of the spur installation including spur length, orientation, permeability, height, profile, and spacing.
The longitudinal extent of channel bank requiring protection is discussed in Brown (1985, 1989). Figure 2.1 was developed from USACE studies of the extent of protection required at meander bends (USACE 1991). The minimum extent of bank protection determined from Figure 2.1 should be adjusted according to field inspections to determine the limits of active scour, channel surveys at low flow, and aerial photography and field investigations at high flow. Investigators of field installations of bank protection have found that protection commonly extends farther upstream than necessary and not far enough downstream. However, such protection may have been necessary at the time of installation. The lack of a sufficient length of protection downstream is generally more serious, and the downstream movement of meander bends should be considered in establishing the downstream extent of protection.
Spur length is taken here as the projected length of spur normal to the main flow direction or from the bank. Where the bank is irregular, spur lengths must be adjusted to provide for an even curvature of the thalweg. The length of both permeable and impermeable spurs relative to channel width affects local scour depth at the spur tip and the length of bank protected. Laboratory tests indicate that diminishing returns are realized from spur lengths greater than 20% of channel width. The length of bank protected measured in terms of projected spur length is essentially constant up to spur lengths of 20% of channel width for permeable and impermeable spurs. Field installations of spurs have been successful with lengths from 3 to 30% of channel width. Impermeable spurs are usually installed with lengths of less than 20% while permeable spurs have been successful with lengths up to 25% of channel width. However, only the most permeable spurs were effective at greater lengths.
The above discussion assumes that stabilization of the bend is the only objective when spur lengths are selected. It also assumes that the opposite bank will not erode. Where flow constriction or changing the flow path is also an objective, spur lengths will depend on the degree of constriction required or the length of spur required to achieve the desired change in flow path. At some locations, channel excavation on the inside of the bend may be required where spurs would constrict the flow excessively. However, it may be acceptable to allow the stream to do its own excavation if it is located in uniformly graded sand
Spur orientation refers to spur alignment with respect to the direction of the main flow current in a channel. Figure 2.2 defines the spur angle such that an acute spur angle means that the spur is angled in an downstream direction and an angle greater than 90° indicates that the spur is oriented in a upstream direction.
Permeable retarder spurs are usually designed to provide flow retardance near the streambank, and they perform this function equally as well without respect to the spur angle. Since spurs oriented normal to the bank and projecting a given length into the channel are shorter than those at any other orientation, all retarder spurs should be constructed at 90° with the bank for reasons of economy.
No consensus exists regarding the orientation of permeable retarder/deflector spurs and impermeable deflector spurs. There is some agreement that spurs oriented in an upstream direction do not protect as great a length of channel bank downstream of the spur tip, result in greater scour depth at the tip, and have a greater tendency to accumulate debris and ice.
Spur orientation at approximately 90° has the effect of forcing the main flow current (thalweg) farther from the concave bank than spurs oriented in an upstream or downstream direction. Therefore, more positive flow control is achieved with spurs oriented approximately normal to the channel bank. Spurs oriented in an upstream direction cause greater scour than if oriented normal to the bank, and spurs oriented in a downstream direction cause less scour.
It is recommended that the spur furthest upstream be angled downstream to provide a smoother transition of the flow lines near the bank and to minimize scour at the nose of the leading spur. Subsequent spurs downstream should all be set normal to the bank line to minimize construction costs.
Figure 2.3 can be used to adjust scour depth for orientation. It should be noted that permeability also affects scour depth. A method to adjust scour depth for permeability is presented in the following section.
The lateral extent of scour can be determined from the depth of scour and the natural angle of repose of the bed material [see HEC-18 (Richardson and Davis 2001)].
The expansion angle downstream of a spur, i.e., the angle of flow expansion downstream of the contraction at the spur is about 17° for impermeable spurs for all spur angles. The implication is that spur orientation affects the length of bank protected only because of the projected length of the spur along the channel bank.
The permeability of the spur depends on stream characteristics, the degree of flow retardance and velocity reduction required, and the severity of the channel bend. Impermeable spurs can be used on sharp bends to divert flow away from the outer bank. Where bends are mild and only small reductions in velocity are necessary, highly permeable retarder spurs can be used successfully. However, highly permeable spurs can also provide required bank protection under more severe conditions where vegetation and debris will reduce the permeability of the spur without destroying the spur. This is acceptable provided the bed load transport is high.
Scour along the streambank and at the spur tip are also influenced by the permeability of the spur. Impermeable spurs, in particular, can create erosion of the streambank at the spur root. This can occur if the crest of impermeable spurs are lower than the height of the bank. Under submerged conditions, flow passes over the crest of the spur generally perpendicular to the spur as illustrated in Figure 2.4. Laboratory studies of spurs with permeability greater than about 70% were observed to cause very little bank erosion, while spurs with permeability of 35% or less caused bank erosion similar to the effect of impermeable spurs (Richardson et al. 2001).
Permeability up to about 35% does not affect the length of channel bank protected by the spur. Above a permeability of 35%, the length of bank protected decreases with increasing permeability. Figure 2.5 shows the results of laboratory tests of the effects of permeability and orientation on the expansion angle of flow downstream of spurs. For this figure, spur lengths were 20% of the channel width projected normal to the bank (Brown 1985).
From the above discussion, it is apparent that spurs of varying permeability will provide protection against meander migration. Impermeable spurs provide more positive flow control but cause more scour at the toe of the spur and, when submerged, cause erosion of the streambank. High permeability spurs are suitable for use where only small reductions in flow velocities are necessary as on mild bends but can be used for more positive flow control where it can be assumed that clogging with small debris will occur and bed load transport is large. Spurs with permeability up to about 35% can be used in severe conditions but permeable spurs may be susceptible to damage from large debris and ice.
Impermeable spurs are generally designed not to exceed the bank height because erosion at the end of the spur in the overbank area could increase the probability of outflanking at high stream stages. Where stream stages are greater than or equal to the bank height, impermeable spurs should be equal to the bank height. If flood stages are lower than the bank height, impermeable spurs should be designed so that overtopping will not occur at the bank.
The crest of impermeable spurs should slope downward away from the bank line, because it is difficult to construct and maintain a level spur of rock or gabions. Use of a sloping crest will avoid the possibility of overtopping at a low point in the spur profile, which could cause damage by particle erosion or damage to the streambank.
Permeable spurs, and in particular those constructed of light wire fence, should be designed to a height that will allow heavy debris to pass over the top. However, highly permeable spurs consisting of jacks or tetrahedrons are dependent on light debris collecting on the spur to make them less permeable. The crest profile of permeable spurs is generally level except where bank height requires the use of a sloping profile.
The most common causes of spur failure are undermining and outflanking by the stream. These problems occur primarily in alluvial streams that experience wide fluctuations in the channel bed. Impermeable rock riprap spurs and gabion spurs can be designed to counter erosion at the toe by providing excess material on the streambed as illustrated in Figures 2.6 and 2.7. As scour occurs, excess material is launched into the scour hole, thus protecting the end of the spur. Gabion spurs are not as flexible as riprap spurs and may fail in very dynamic alluvial streams.
Permeable spurs can be similarly protected as illustrated in Figure 2.8. The necessity for using riprap on the full length of the spur or any riprap at all is dependent on the erodibility of the streambed, the distance between the slats and the streambed, and the depth to which the piling are driven. The measure illustrated would also be appropriate as a retrofit measure at a spur that has been severely undermined, and as a design for locations at which severe erosion of the toe of the streambank is occurring.
Piles supporting permeable structures can also be protected against undermining by driving piling to depths below the estimated scour. Round piling are recommended because they minimize scour at their base.
Extending the facing material of permeable spurs below the streambed also significantly reduces scour. If the retarder spur or retarder/deflector spur performs as designed, retardance and diversion of the flow within the length of the structure may make it unnecessary to extend the facing material the full depth of anticipated scour except at the nose.
A patented Henson spur, as illustrated in Figure 2.9, maintains contact with the streambed by vertical wood slats mounted on pipes which are driven to depths secure from scour. The units slide down the pipes where undermining occurs. Additional units can be added on top as necessary.
Spur spacing is a function of spur length, spur angle, permeability, and the degree of curvature of the bend. The flow expansion angle, or the angle at which flow expands toward the bank downstream of a spur, is a function of spur permeability and the ratio of spur length to channel width. This ratio is susceptible to alteration by excavation on the inside of the bend or by scour caused by the spur installation. Figure 2.10 indicates that the expansion angle for impermeable spurs is an almost constant 17°. Spurs with 35% permeability have almost the same expansion angle except where the spur length is greater than about 18% of the channel width.
As permeability increases, the expansion angle increases, and as the length of spurs relative to channel width increases, the expansion angle increases exponentially. The expansion angle varies with the spur angle, but not significantly.
Spur spacing in a bend can be established by first drawing an arc representing the desired flow alignment (Figure 2.11). This arc will represent the desired extreme location of the thalweg nearest the outside bank in the bend. The desired flow alignment may differ from existing conditions or represent no change in conditions, depending on whether there is a need to arrest erosion of the concave bank or reverse erosion that has already occurred. If the need is to arrest erosion, permeable retarder spurs or retarder structures may be appropriate. If the flow alignment must be altered in order to reverse erosion of the bank or to alter the flow alignment significantly, deflector spurs or retarder/deflector spurs are appropriate. The arc representing the desired flow alignment may be a compound circular curve or any curve which forms a smooth transition in flow directions.
Next, draw an arc representing the desired bankline. This may approximately describe the existing concave bank or a new theoretical bankline which protects the existing bank from further erosion. Also, draw an arc connecting the nose (tip) of spurs in the installation. The distance from this arc to the arc describing the desired bank line, along with the expansion angle, fixes the spacing between spurs. The arc describing the ends of spurs projecting into the channel will be essentially concentric with the arc describing the desired flow alignment.
Now, establish the location of the spur at the downstream end of the installation. For a highway application, this is normally the protected abutment or guide bank at the bridge. Finally, establish the spacing between each of the remaining spurs in the installation (Figure 2.11). The distance between spurs, S, is the length of spur, L, between the arc describing the desired bank line and the nose of the spur multiplied by the cotangent of the flow expansion angle, θ. This length is the distance between the nose of spurs measured along a chord of the arc describing spur nose location. Remaining spurs in the installation will be at the same spacing if the arcs are concentric. The procedure is illustrated by Figure 2.11 and expressed in Equation 2.1.
At less than bankfull flow rates, flow currents may approach the concave bank at angles greater than those estimated from Figure 2.10. Therefore, spurs should be well-anchored into the existing bank, especially the spur at the upstream end of the installation, to prevent outflanking.
In general, straight spurs should be used for most bank protection. Straight spurs are more easily installed and maintained and require less material. For permeable spurs, the width depends on the type of permeable spur being used. Less permeable retarder/deflector spurs which consist of a soil or sand embankment should be straight with a round nose as shown in Figure 2.12.
The top width of embankment spurs should be a minimum of 3 ft (1 m). However, in many cases the top width will be dictated by the width of any earth moving equipment used to construct the spur. In general a top width equal to the width of a dump truck can be used. The side slopes of the spur should be 1V:2H or flatter.
Rock riprap should be placed on the upstream and downstream faces as well as on the nose of the spur to inhibit erosion of the spur. Depending on the embankment material being used, a gravel, sand, or geotextile filter may be required. The designer is referred to Design Guideline 4 on revetment riprap design for procedures for sizing riprap at spurs. If a revetment equation is used for sizing spur riprap, then either the factor of safety should be increased or a higher velocity (than the channel average) should be used in the design. To accomplish this, the EM 1601 equation can be used to size riprap at spurs by selecting a Cv value of 1.25 (see Design Guideline 4).
It is recommended that riprap be extended below the bed elevation to a depth as recommended in Design Guideline 4 (to the combined long-term degradation and contraction scour depth). Riprap should also extend to the crest of the spur, in cases where the spur would be submerged at design flow, or to 2 ft (0.6 m) above the design flow, if the spur crest is higher than the design flow depth. Additional riprap should be placed around the nose of the spur (Figure 2.12), so that spur will be protected from scour. Figure 2.13 shows an example of an impermeable spur field and a close-up of a typical round nose spur installation.
Figure 2.14 illustrates a location at which a migrating bend threatens an existing bridge (existing conditions are shown with a solid line). Ultimately, based upon the following design example, seven spurs will be required. Although the number of spurs is not known in advance, the spurs (and other design steps) are shown as dashed lines on Figure 2.14 as they will be specified after completing the following design example. Assume that the width of the river from the desired (north) bankline to the existing (south) bankline is 164 ft (50 m).
For this example, it is desirable to establish a different flow alignment and to reverse erosion of the concave (outside) bank. The spur installation has two objectives: (1) to stop migration of the meander before it damages the highway stream crossing, and (2) to reduce scour at the bridge abutment and piers by aligning flow in the channel with the bridge opening. Impermeable deflector spurs are suitable to accomplish these objectives and the stream regime is favorable for the use of this type of countermeasure. The expansion angle for this spur type is approximately 17 ° for a spur length of about 20% of the desired channel width, as indicated in Figure 2.10.
Step 1. Sketch Desired Thalweg
The first step is to sketch the desired thalweg location (flow alignment) with a smooth transition from the upstream flow direction through the curve to an approach straight through the bridge waterway (Figure 2.14). Visualize both the high-flow and low-flow thalwegs. For an actual location, it would be necessary to examine a greater length of stream to establish the most desirable flow alignment. Then draw an arc representing the desired bankline in relation to thalweg locations. The theoretical or desired left bank line is established as a continuation of the bridge abutment (and left bank downstream) through the curve, smoothly joining the left bank at the upstream extremity of eroded bank.
Step 2. Sketch Alignment of Spur Tips
The second step is to sketch a smooth curve through the nose (tip) locations of the spurs, concentric with the desired bankline alignment. Using a guideline of 20% of the desired channel width for impermeable spurs (see Section 2.2.2) the distance, L, from the desired bankline to the spur tips (Figure 2.14) would be:
Step 3. Locate First Spur
Step number three is to locate spur number 1 so that flow expansion from the nose of the spur will intersect the streambank downstream of the abutment. This is accomplished by projecting an angle of 17 ° from the abutment alignment to an intersection with the arc describing the nose of spurs in the installation or by use of Equation 2.1. Spurs are set at 90 ° to a tangent with the arc for economy of construction. Alternatively, the first spur could be considered to be either the upstream end of the abutment or guide bank if the spur field is being installed upstream of a bridge. Thus, the spur spacing, S, would be:
It may be desirable to place riprap on the streambank at the abutment. Furthermore, the size of the scour hole at the spur directly upstream of the bridge should be estimated using the procedures described in Chapter 4. If the extent of scour at this spur overlaps local scour at the pier, total scour depth at the pier may be increased. This can be determined by extending the maximum scour depth at the spur tip, up to the existing bed elevation at the pier at the angle of repose.
Step 4. Locate Remaining Spurs
Spurs upstream of spur number 1 are then located by use of Equation 2.1, using dimensions as illustrated in Figure 2.11 (i.e., the spacing, S, determined in Step 3). Using this spur spacing, deposition will be encouraged between the desired bank line and the existing eroded bank.
The seventh and last spur upstream is shown oriented in a downstream direction to provide a smooth transition of the flow approaching the spur field. This spur could have been oriented normal to the existing bank, and been shorter and more economical, but might have caused excessive local scour. Orienting the furthest upstream spur at an angle in the downstream direction provides a smoother transition into the spur field, and decreases scour at the nose of the spur. As an alternative, a hard point could be installed where the bank is beginning to erode. Hard points are discussed in Chapter 8. In this case the hard point can be considered as a very short spur which is located at the intersection of the actual and planned bank lines. In either case, spurs or hard points should be anchored well into the bank to prevent outflanking.
Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Stream Crossings-Executive Summary," FHWA/RD-84/099, Federal Highway Administration, Washington, D.C.
Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Engineers," FHWA/RD-84. 100, Federal Highway Administration, McLean, VA.
Brown, S.A., 1985, "Design of Spur-Type Streambank Stabilization Structures, Final Report," FHWA/RD-84-101, Federal Highway Administration, Washington, D.C.
Brown, S.A. and Clyde, E.S., 1989, "Design of Riprap Revetment," Hydraulic Engineering Circular No.11, FHWA-IP-89-016, prepared for the Federal Highway Administration, Washington, D.C.
Karaki, S.S., 1959, "Hydraulic Model Study of Spur Dikes for Highway Bridge Openings," Colorado State University, Civil Engineering Section, Report CER59SSK36, September, 47 pp.
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, National Academies of Science, 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 No. FHWA NHI 01-004, Hydraulic Design Series No. 6, Federal Highway Administration, Washington, D.C.
Richardson, E.V. and Davis, S.R., 2001, "Evaluating Scour at Bridges," Hydraulic Engineering Circular No. 18, Fourth Edition, FHWA NHI 01-001, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.
U.S. Army Corps of Engineers, 1981, "The Streambank Erosion Control Evaluation and Demonstration Act of 1974," Final Report to Congress, Executive Summary and Conclusions.