[Coastal Watershed Assessment Procedure Guidebook Table of Contents]

Appendix 4

Definition of areas with a potential for surface erosion

The potential for surface erosion in a watershed is determined, primarily, by the inherent erodibility of the material and the magnitude of erosive forces (Carr et al. 1991). The erodibility of a soil is determined by the material textural properties, such as particle size, structure and cohesion. The magnitude of erosive forces is determined by the quantity of surface runoff and its associated energy, which are related to precipitation and slope characteristics. Determining surface erosion potential for operational forest development planning has primarily been done through two processes: pedological assessment and terrain mapping.

Using soils maps and reports

Where soils maps and reports are available for a watershed, soil erodibility is best assessed from the classification of soil associations, based on similarities in soil development (and hence, erodibility). Many polygons on soil maps contain complexes of two or three soil associations. To assign one erodibility rating to the entire polygon, the simplest approach is to prorate the polygon area and road lengths according to the percentage of the polygon with erodible soil (e.g., if 30% of a polygon is mapped as erodible soil, then 30% of the logged blocks and road length in the polygon would be assumed to be located on erodible soils).

Where soils maps and reports are available

The potential for surface erosion in a watershed can be determined from reconnaissance soil mapping, available for a number of areas at scales of 1:50 000 to 1:126 720 (see Appendix 1). Most soils maps have accompanying reports which describe the soil associations in detail. Included in these reports are interpretations of the erodibility of the soil associations in the event of soil surface disturbance. Erodibility is assigned to soils in erodibility groups based on slope. In British Columbia a three-class erodibility classification associated with soils mapping is most commonly used.

Where soils maps without reports are available

In some cases, soils maps may not be accompanied by reports in which soils associations and potential erodibility are defined. In these cases, erodibility groups can be established for soil associations on the basis of slope and soil attributes identified in the soil polygon label. An example of a possible rating scheme is shown in Table 4-1. Note that such a rating scheme should be reviewed for the map area in question by an appropriately qualified professional (i.e., Ministry of Forests soil scientist or other professionally qualified soil scientist).

Table 4-1. Example of possible rating scheme for soil erodibility where soil mapping exists

Using terrain maps and reports

In a terrain mapping process, surface erosion potential is estimated on a mapped polygon basis, using, as defining criteria, slope angle, material texture, terrain type and geomorphological processes. Terrain maps provide information about the distribution and characteristics of surficial materials, landforms and geological processes in an area. The terrain classification system used for mapping in British Columbia is defined in Howes and Kenk (1988). The Resources Inventory Committee (1994) provides additional important information, including that on terrain survey intensity levels and interpretive products such as erosion potential classification. The Mapping and Assessing Terrain Stability Guidebook also contains information on surface erosion assessments as a derivative product of terrain mapping.

Terrain mapping and surface erosion classification must be done by a registered professional who has extensive experience in terrain mapping and interpretations. Junior mappers can do this work under the close supervision of such an individual.

An example of a five-class erosion potential classification, as a derivative product from terrain mapping, is presented in Table 4-2 (from Mapping and Assessing Terrain Stability Guidebook).

Where surface erosion potential classification has not been done, but terrain mapping is available, the terrain types shown in Table 4-3 can be considered as potentially highly erodible.

Where terrain mapping or surface erosion potential classification are not available for a watershed, areas of potentially high surface erosion hazard may be assessed at a reconnaissance level as:

Table 4-2. Surface erosion potential classes

Table 4-3. Examples of potentially highly erodible terrain types identified on terrain maps

References

Carr, W. W., W. R. Mitchell and W. J. Watt. 1991. Basic Soil Interpretations for
Forest Development Planning: Surface Soil Erosion and Soil
Compaction. Land Management Report Number 63. B.C. Ministry of
Forests, Research Branch, Victoria, B.C. 17 p.

Howes, D. E. and E. Kenk. 1988.Terrain Classification System for British
Columbia (Revised Edition). Ministry of Environment Manual 10,
Victoria, British Columbia.

Resources Inventory Committee. 1994. Guidelines and Standards to Terrain
Geology Mapping in British Columbia. Report prepared for: Terrain
Geology Task Group, Earth Science Task Force, Resource Inventory
Committee, British Columbia, 1993–1994.

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