these situations it is often advantageous to integrate the grade control structure into the
planned improvements at the bridge. If the bridge is not in immediate danger of failing and
only needs some additional erosion protection, the grade control structure can be built at or
immediately downstream of the bridge with the riprap from the structure tied into the bridge
for protection. If the bridge is to be replaced, then it may be possible to construct the grade
control structure concurrently with the road crossing (Figure 12.2).
12.2.6 LOCAL SITE CONDITIONS
When planning grade control structures, the final siting is often adjusted to
accommodate local site conditions, such as the planform of the stream or local drainage. A
MAIN DITCH - MODIFIED DROP STRUCTURE AND ROAD CROSSING
Combination Grade Control Structure and Road Crossing (adapted from
U.S. Soil Conservation Service, 1976)
stable upstream alignment that provides a straight approach into the structure is critical.
Since failure to stabilize the upstream approach may lead to excessive scour and possible
flanking of the structure, it is desirable to locate the structure in a straight reach. If this is
not possible (as in the case in a very sinuous channel), it may be necessary to realign the
channel to provide an adequate approach. Stabilization of the realigned channel may be
required to ensure that the approach is maintained. Even if the structure is built in a straight
reach, the possibility of upstream meanders migrating into the structure must be considered.
In this case, the upstream meanders should be stabilized prior to, or concurrent with, the
construction of the grade control structure.