Appendix 1. GENERAL STATEMENT.

Urban development adjacent to natural stream channels poses complex problems for planning and regulatory agencies and developers. Due to the dynamic nature of most natural stream channels, it is imperative that appropriate steps be taken to protect adjacent structures and facilities from damage due to flooding and erosion. Lining channels with nonerosive or erosion-resistant material is a common protection method; however, the cost associated with this "hard lining," in terms of construction and maintenance costs, as well as degradation of the natural environment, may be unacceptable. Additionally, channels left in a natural or naturalistic condition can provide improved aesthetic value, better water quality conditions and opportunities for multi-use development, including bike trails, parks and greenways.

However, any natural channel, from a small swale to a major river, exists in a dynamic environment and can move both laterally and vertically with time. Therefore, when a channel is left in its natural condition it is necessary to limit how close development can occur next to the channel to minimize potential property damage as a result of future channel migration and flooding. This requires definition of a setback, or erosion risk boundary, within which development is not allowed. In some metropolitan areas, this setback or erosion risk boundary has been referred to as the "prudent line," since without major channel stabilization measures it would not be prudent for development to occur within this boundary.

The definition of a prudent line must recognize both the short-term impacts of flooding and erosion and the cumulative impacts of erosion over the long term. The physical processes involved with channel migration and the analysis of those processes are inherently complex. The procedure outlined in this addendum is a gross simplification of this process in an attempt to establish a procedure that is easily applied, yet provides reasonable definition of a "prudent line." The procedure was developed specifically for application to rural basins in El Paso County, Colorado, where the land use density is low and the application of the prudent line concept is justified given both engineering and economic considerations. Definition of a prudent line in other areas, particularly basins with higher density, should be based on a more comprehensive and detailed analysis than outlined in this addendum.

Engineers and designers applying the procedures outlined in this addendum should have training or experience in water resource engineering, including hydrology, hydraulic and sediment transport concepts. It is generally recognized that the calculation of a floodplain boundary requires a certain amount of specialized water resource engineering knowledge. This is equally true, if not more so, for the analysis and calculation of a prudent line boundary. Therefore, the application of this procedure should be completed under the responsible charge of an adequately qualified and licensed Professional Engineer.