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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 15 Guide Banks
When embankments encroach on wide floodplains, the flows from these areas must flow parallel to the approach embankment to the bridge opening. These flows can erode the approach embankment. A severe flow contraction at the abutment can reduce the effective bridge opening, which could possibly increase the severity of abutment and pier scour.
Guide banks (formerly known as spur dikes) can be used in these cases to prevent erosion of the approach embankments by cutting off the flow adjacent to the embankment, guiding streamflow through a bridge opening, and transferring scour away from abutments to prevent damage caused by abutment scour. The two major enhancements guide banks bring to bridge design are (1) reduce the separation of flow at the upstream abutment face and thereby maximize the use of the total bridge waterway area, and (2) reduce the abutment scour due to lessening turbulence at the abutment face. Guide banks can be used on both sand- and gravel-bed streams.
Principal factors to be considered when designing guide banks, are their orientation to the bridge opening, plan shape, upstream and downstream length, cross-sectional shape, and crest elevation. Bradley is used as the principal design reference for this section.(1)
Figure 15.1 presents a typical guide bank plan view. It is apparent from the figure that without this guide bank overbank flows would return to the channel at the bridge opening, which can increase the severity of contraction and scour at the abutment. Note, that with installation of guide banks the scour holes which normally would occur at the abutments of the bridge are moved upstream away from the abutments. Guide banks may be designed at each abutment, as shown, or singly, depending on the amount of overbank or floodplain flow directed to the bridge by each approach embankment.
The goal in the design of guide banks is to provide a smooth transition and contraction of the streamflow through the bridge opening. Ideally, the flow lines through the bridge opening should be straight and parallel. As in the case with other countermeasures, the designer should consider the principles of river hydraulics and morphology, and exercise sound engineering judgment.
Guide banks should start at and be set parallel to the abutment and extend upstream from the bridge opening. If there are guide banks at each abutment, the distance between them at the bridge opening should be equal to the distance between bridge abutments. Best results are obtained by using guide banks with a planform shape in the form of a quarter of an ellipse, with the ratio of the major axis (length Ls) to the minor axis (offset) of 2.5:1.0. This allows for a gradual constriction of the flow. Thus, if the length of the guide bank measured perpendicularly from the approach embankment to the upstream nose of the guide bank is denoted as Ls, the amount of expansion of each guide bank (offset), measured from the abutment parallel to the approach roadway, should be 0.4 Ls.
The plan view orientation can be determined using Equation 15.1, which is the equation of an ellipse with origin at the base of the guide bank. For this equation, X is the distance measured perpendicularly from the bridge approach and Y is the offset measured parallel to the approach embankment, as shown on Figure 15.1.
It is important that the face of the guide bank match the abutment so that the flow is not disturbed where the guide bank meets the abutment. For new bridge construction, abutments can be sloped to the channel bed at the same angle as the guide bank. For retrofitting existing bridges modification of the abutments or wing walls may be necessary.
For design of guide banks, the length of the guide bank, Ls must first be determined. This can be easily determined using a nomograph which was developed from laboratory tests performed at Colorado State University and from field data compiled by the USGS (Karaki 1959, 1961, Neeley 1966). For design purposes the use of the nomograph involves the following parameters:
A nomograph is presented in Figure 15.2 (English) and Figure 15.3 (SI) to determine the projected length of guide banks. This nomograph should be used to determine the guide bank length for designs greater than 50 ft (15 m) and less than 250 ft (75 m). If the nomograph indicates the length required to be greater than 250 ft (75 m) the design should be set at 250 ft (75 m). It is recommended that the minimum length of guide banks be 50 ft (15 m). An example of how to use this nomograph is presented in the next section.
FHWA practice has shown that many guide banks have performed well using a standardized length of 150 ft (46 m). Based on this experience, guide banks of 150 ft (46 m) in length should perform very well in most locations. Even shorter guide banks have been successful if the guide bank intersects the tree line. If the main channel is equal to or less than 100 ft (30 m) use the total main channel flow in determining the guide bank discharge ratio (Qf/QA).
As with deflection spurs, guide banks should be designed so that they will not be overtopped at the design discharge. If this were allowed to occur, unpredictable cross flows and eddies might be generated, which could scour and undermine abutments and piers. In general, a minimum of 2 ft (0.6 m) of freeboard, above the design water surface elevation should be maintained.
The cross-sectional shape and size of guide banks should be similar to deflector, or deflector/retarder spurs discussed in Design Guideline 2. Generally, the top width is 10 to 13 ft (3 to 4 m), but the minimum width is 3 ft (1 m) when construction is by drag line. The upstream end of the guide bank should be round nosed. Side slopes should be 1V:2H or less.
In some states, highway departments extend guide banks downstream of the abutments to minimize scour due to rapid expansion of the flow at the downstream end of the abutments. These downstream guide banks are sometimes called "heels." If the expansion of the flow is too abrupt, a shorter guide bank, which usually is less than 50 ft (15 m) long, can be used downstream. Downstream guide banks should also start at and start parallel to the abutment and the distance between them should enlarge as the distance from the abutment of the bridge increases.
In general, downstream guide banks are a shorter version of the upstream guide banks. Riprap protection, crest height and width should be designed in the same manner as for upstream guide banks.
Guide banks are constructed by forming an embankment of soil or sand extending upstream from the abutment of the bridge. To inhibit erosion of the embankment materials, guide banks must be adequately protected with riprap or stone facing. Rock riprap should be placed on the stream side face as well as around the end of the guide bank. It is not necessary to riprap the side of the guide bank adjacent to the highway approach embankment. As in the case of spurs, a gravel, sand, or geotextile filter may be required to protect the underlying embankment material (see Design Guideline 16).
Because guide banks are designed to protect abutments from deep scour by providing a smooth flow transition through the bridge, it is reasonable to use the abutment riprap equations for guide banks. The designer is referred to Design Guideline 14 for design procedures for sizing riprap. Design guidance for riprap for countermeasures was investigated under NCHRP Project 24-23 (Lagasse et al. 2006). This study confirmed the applicability of the Set Back Ratio (SBR) approach for designing riprap at bridge abutments (Design Guideline 14) to riprap design for guide banks. It is recommended that the riprap size for guide banks be computed using 0.85 times the characteristic average velocity computed using the SBR approach discussed in Design Guideline 14.
Riprap should be extended below the bed elevation to a depth as recommended in Design Guideline 4 (below the combined long-term degradation and contraction scour depth), and extend up the face of the guide bank to 2 ft (0.6 m) above the design flow. Additional riprap should be placed around the upstream end of the guide bank to protect the embankment from scour.
As in the case of spurs, it is important to adequately tie guide banks into the approach embankment for guide banks on non-symmetrical highway crossings. Hydraulics of Bridge Waterways (Bradley 1978) states:
Therefore, for skewed crossings, the length of guide banks should be set using the nomograph for the side of the bridge crossing which yields the largest guide bank length.
In some cases, where the cost of stone riprap facing is prohibitive, the guide bank can be covered with sod or other minimal protection. If this approach is selected, the design should allow for and stipulate the repair or replacement of the guide bank after each high water occurrence. Other measures which will minimize damage to approach embankments, and guide banks during high water are:
For the example design of a guide bank, Figure 15.4 (English units) or Figure 15.5 (SI units) will be used. These figures show the cross-section of the channel and floodplain before the bridge is constructed and the plan view of the approach, guide banks, and embankments after the design steps outlined below are completed.
Step 1. Hydraulic Design Parameters
The first step in the design of guide banks requires the computation of the depth and velocity of the design flood in the main channel and in the adjacent overbank areas. These studies are performed by using step backwater computations upstream and through the bridge opening. The computer programs WSPRO or HEC River Analysis System (RAS) are suitable for these computations (Arneson and Shearman 1987, USACE 1998). Using these programs or by using conveyance curves developed from actual data, the discharges and depths in the channel and overbank areas can be determined.
To use the conveyance curve approach, the designer is referred to example problem number 4 in Hydraulics of Bridge Waterways (Bradley 1978) for methods to determine these discharges and areas. That publication also contains another example of the design of a guide bank.
For this example, the total, overbank, and channel discharges, as well as the flow area are given. We also assume that a bridge will span a channel with a bottom width of 230 ft (70 m) and that the abutments will be set back 148 ft (45 m) from each bank of the main channel.
The abutments of this bridge are spill-through with a side slope of 1V:2H. The design discharge is 12,360 cfs (350 m3/s), which after backwater computations, results in a mean depth of 11.8 ft (3.6 m) in the main channel and a mean channel velocity of 3 ft/s (0.91 m/s).
Step 2. Determine Qf in the Left and Right Overbank
The depth in each overbank area is given as 3.9 ft (1.2 m)and the widths of the left and right overbank areas are 295 ft (90 m) and 590 ft (180 m), respectively. Velocity in the overbank areas (assuming no highway approach embankment, i.e., at an upstream cross section) is 1.2 ft/s (0.37 m/s). The floodplain flow is equal to 1,413 cfs (40 m3/s) for the left overbank and 2,825 cfs (80 m3/s) for the right overbank.
Using the continuity equation and noting that the abutments are set back 148 ft (45 m) from each bank, the floodplain discharge intercepted by each approach embankment is:
Q = AV
(Qf) right = 2,825 - (148) (3.9) (1.2) = 2132 cfs (60 m3/s)
(Qf) left = 1,413 - (148 (3.9) (1.2) = 720 cfs (20 m3/s)
Step 3. Determine QA and Qf/QA for the Left and Right Overbank
The overbank discharge in the first 100 ft (30 m) of opening adjacent to the left and right abutments needs to be determined next. Since for this case the flow is of uniform depth [3.9 ft (1.2 m)] and velocity [1.2 ft/s (0.37 m/s)] over the entire width of the floodplain, and both abutments are set back more than 100 ft (30 m) from the main channel banks, the value of QA will be the same for both sides:
(QA) right = (100) (3.9) (1.2) = 468 cfs (13.3 m3/s)
(QA) left = (100 (3.9) (1.2) = 468 cfs (13.3 m3/s)
For the left and right overbanks the reference values of Qf /QA can be determined by simple division of the discharges determined in previous steps:
For design purposes, the largest value will result in the more conservative determination of the length of the guide banks, except where Step 4 indicates a guide bank is required for only one of the overbank areas.
Step 4. Determine the Length of the Guide Bank, Ls
The average channel velocity through the bridge opening can be determined by dividing the total discharge of the stream, Q, by the cross-sectional flow area at the bridge opening, An2, which in this case includes the main channel (2,714 ft2) plus 148 ft of the left and right overbank areas adjacent to the abutments at the bridge opening (1,154 ft2). Thus:
Vn2=3.2 ft/s (0.97 m/s)
For Qf /QA equal to 4.5 and an average channel velocity of 3.2 ft/s (0.97 m/s), the length of the guide bank is determined using the nomograph presented in Figure 15.2.
(Ls) right=138 ft (42 m)
For the left abutment, a Qf /QA of 1.5 and Vn2 of 3.2 ft/s (0.97 m/s) indicate that Ls would be less than 50 ft (15 m). Thus, no guide bank is required for the left overbank for this example.
Step 5. Miscellaneous Specifications
The offset of the guide bank is determined to be 55.2 ft (16.8 m) by multiplying Ls by 0.4. The offset and length determine the plan layout of the guide bank. Coordinates of points along the centerline can be determined using Equation 15.1, which is the equation of an ellipse with a major to minor axis ratio of 2.5:1. The coordinates for a 138 ft (42 m) long guide bank with a 55.2 ft (16.8 m) offset are presented in Table 15.2.
These coordinates would be used for conceptual level design. For construction, coordinates at an offset or along the toe of side slope would be necessary.
The crest of the guide bank must be a minimum of 2 ft (0.6 m) above the design water surface (elevation 1070.2 ft (326.2 m)). Therefore, the crest elevation for this example should be greater than or equal to 1072.2 ft (326.8) m. The crest width should be at least 3 ft (1 m). For this example, a crest width of 10 ft (3 m) will be specified so that the guide bank can be easily constructed with dump trucks.
Stone or rock riprap should be placed in the locations shown on Figure 15.4. This riprap should extend a minimum of 2 ft (0.6 m) above the design water surface (elevation 1070.2 ft (326.2 m)) and below the intersection of the toe of the guide bank and the existing ground to the combined long-term degradation and contraction scour depth.
Arneson, L.A. and Shearman, J.O., 1987, "User's Manual for WSPRO - A Computer Model for Water Surface Profile Computations," Office of Technology Applications, Federal Highway Administration, FHWA Report No. FHWA-SA-98-080, June 1998.
Bradley, J.N., 1978, "Hydraulics of Bridge Waterways," Hydraulic Design Series No. I U.S. Department of Transportation, FHWA.
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.
Karaki, S.S., 1961, "Laboratory Study of Spur Dikes for Highway Bridge Protection," Highway Research Board Bulletin 286, Washington, D.C., p. 31.
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 Research Council, Washington, D.C.
Neeley, B.L., Jr., 1966, "Hydraulic Performance of Bridges in the State of Mississippi," U.S. Geological Survey, Jackson, MS, June. (Unpublished report).
U.S.Army Corps of Engineers, 1998, "HEC-RAS River Analysis System," User's Manual, Version 2.2, Hydrologic Engineering Center, Davis, CA.