Placement of floodplain culverts.

At the top of this figure is a plan view of a road crossing a stream channel and its floodplain. The primary channel shown in a solid line with the primary culvert situated over the top of the primary channel. Floodplain culverts are situated within the floodplain but outside of the primary channel. A dashed line is used to indicate a floodplain channel and the floodplain culvert is placed to intercept the existing floodplain channel. A cross section view of the plan is shown below the top image and depicts a primary culvert wider than the bankfull channel width. Floodplain culverts are placed outside of the primary bankfull channel and are provided with vertical separation between the thalweg of the primary channel and the thalweg of the floodplain culverts. Material stable to the 100-year flood is shown filling in one floodplain culvert as an option to provide vertical separation.