The recommended shape of a guidebank is a quarter ellipse with a major to minor axis ratio of
2.5. The major axis should be approximately parallel to the main flow direction. For bridge
crossings normal to the river, the major axis would be normal to the highway embankment.
However, for skewed crossings, the guidebank should be placed at an angle with respect to
the embankment with the view of streamlining the flow through the bridge opening. Design
guidelines and a design chart for guide banks are provided in HEC-23 (Lagasse et al. 2001).
The length of the guide bank, Ls, required depends upon quantity of flow on the floodplain,
width of bridge opening and skewness of the highway crossing. Shorter guide banks may be
used where floodplain flow is small, scour potential at piers and embankment ends is small, or
where trees or brush are intersected by the guidebank.
The crest elevation of guidebanks should be higher than the elevation of the design flood
taking into consideration the effect of the contraction of the flow; this is because the design
flow should not overtop the guidebank.
6.4.11 Drop Structures
Check dams or channel drop structures are used downstream of highway crossings to arrest
head cutting and maintain a stable streambed elevation in the vicinity of the bridge. Check
dams are usually built of rock riprap, concrete, sheet piles, gabions, or treated timber piles.
The material used to construct the structure depends on the availability of materials, the height
of drop required, and the width of the channel. Definition sketches for a vertical wall and a
sloping sill drop structure are shown in Figures 6.11 and 6.12. Design considerations for
vertical wall or sloping sill structures are given in texts by Rouse (1950), Chow (1959),
Peterson (1986), and Simons and Senturk (1992). Design guidelines for a vertical drop
structure and stilling basins for drop structures are given in HEC-23 (Lagasse et al. 2001).
Figure 6.11. Definition sketch for a vertical drop (Lagasse et al. 2001).
6.19
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