Modification Guidelines
It may be necessary to modify a stream in order to make it more compatible with the highway facility and the physical constraints imposed by local terrain or land use. The modifications may involve changes in alignment or conveyance. Changes may be necessary to accommodate the highway requirements, but they must be evaluated to assess short-term and long-term effects on the stream system.
Background data on the existing stream should be available from previously completed planning and location studies, and a preliminary highway design should be available in sufficient detail to indicate the extent of required channel modifications.
Certain types of streams may have a very wide threshold of tolerance to changes in alignment, grade, and cross-section. In contrast, small changes can cause significant impacts on sensitive waterways. An analysis of the tolerance to change may reveal that necessary modifications will not have detrimental results.
If you recognize detrimental effects, develop plans to mitigate the effects to within tolerable limits. You can enhance certain aspects of an existing stream system, often to the economic benefit of the highway. The following are examples of ways to enhance stream systems:
- Control active upstream headcutting (degradation due to abrupt changes in bed elevation) with culverts or check dams so that many hectares of land along the stream banks will not be lost and the highway facility will be protected from the headcutting.
- Coordinate and cooperate with fish and wildlife agency personnel, adapt or modify stabilization measures necessary to protect the highway while improving aquatic habitat.