At times it will be advantageous or necessary to realign or change the hydraulic characteristics of the channel. Reasons for altering the channel include improving culvert alignment, protecting roadways from erosion damage, reducing maintenance requirements, and eliminating hydraulic structures where the roadway recrosses the channel.
Plans for channel modifications must include a determination of what effect the change will have on the stream and the surrounding environment. Long — and short-term effects must be considered. The impact on the stream of the realignment or change in slope will vary from one site to another. At some sites, minor changes will have significant impacts, while at others the opposite may be true. Regardless of the magnitude of the effect on the stream and its environment that the change may have, plans should be developed to mitigate those effects.
Changes to a channel usually cause a decrease in the roughness and an increase in the slope. The resultant higher velocity may lead to increased scour and sedimentation buildup at the downstream end of the channel improvement, and may result in changes that affect the habitat in and around the stream. Any changes to existing streams that support fish or wildlife must be coordinated with the appropriate resource agencies early in the planning phase.
