The potential for landslide debris to enter a stream is an interpretation of the likelihood of bedload sized material and organic debris to colluvially enter a stream. An example classification and criteria are given in Appendix 2. This is based on a consideration of the hillslope gradient and slope morphology downslope from the polygon, evidence of landslide runout, presence or absence of a runout zone, length of the runout zone, and the presence of gullies that give direct access to the stream channel. The interpretation can be attached to the terrain stability symbol or provided on a separate map. Additional map symbols in the polygon label should be separated by hyphens (e.g. IV-2). This interpretation is made for polygons with terrain stability class IV, IVR or V.
This interpretation is best suited to site-specific assessments, for example, of short segments of road. Landslide runout estimation is sometimes difficult to apply to terrain polygons, however, many mappers have developed reasonable criteria and standardized procedures for making this interpretation. Where there is a specific need for assessing the likelihood of landslide debris entering a stream in a particular project, mappers are expected to develop and substantiate criteria relevant to their specific map area.
Interpretation for soil erosion potential may be required to be included in a terrain stability mapping project (e.g., in community watersheds). This interpretation can be attached to the terrain stability symbol or provided on a separate map. Additional map symbols in the polygon label should be separated by hyphens (e.g. IV-M).
Most fine sediment production from soil erosion is from surfaces exposed by roads and trails.
Classification of soil erosion potential should be based on terrain mapping. Derive a simple soil erosion potential rating from the terrain map, based on slope gradient, generic material, texture and soil drainage. Appendix 3 presents an example of classification criteria for assigning a soil erosion potential based on genetic material to terrain polygons. It is based on preliminary work done in the Nelson Forest Region, and requires modification for use in other areas. The slope classes given are hypothetical; they have not been tested1.
The "soil erosion hazard key" (as described in the Hazard Assessment Keys for Evaluating Site Sensitivity to soil Degrading Processes Guidebook) is not recommended for use in terrain mapping.
The risk of sediment delivery to streams indicates the likelihood that sediment derived from erosion sources in a specific terrain polygon will be transported or delivered to a stream. This interpretation is made for polygons that have a high or very high surface erosion potential.
This interpretation is better suited to site-specific assessments than to terrain mapping. It is difficult to apply to terrain polygons, because the risk of sediment delivery to streams often varies greatly throughout a polygon, and is highly dependent on future road alignment and gradient. Therefore, this interpretation is not recommended for most terrain stability mapping projects. Where there is a specific need for assessing the risk of sediment delivery in a particular project, mappers should use a classification and criteria such as the one given in Appendix 4.
1 A research project in the Nelson Forest Region is being conducted to produce a final field-tested classification in late 1999.