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The erosional processes associated with mass wasting include two primary types:
- shallow, fast movements of debris avalanche/debris torrents and mudflows that generally move only once, and
- slow, deep-seated slump/earthflow erosional processes that move intermittently over varying time scales in response to infrequent events and/or disturbance factors.
The evidence of mass wasting can be detected on aerial photographs, thus frequency and magnitude of the slides can be observed over time-sequential aerial photos. Examples of debris torrents are shown in Figure 3 (below right) in the Idaho batholith, and Figure 4, associated with an A3a+ stream type in Colorado. Mass wasting slides are shown in Figure 5 (Willow Creek, Colorado) and Figure 6, in Marine Shale geology (Blue River, Colorado).
Figure 4. An A3a+ stream type depicting debris torrent mass wasting erosion process - Colorado.
Figure 6. Slump/earth flow erosional processes adjacent to Blue River - Colorado. Exposed erosional faces are now influenced by fluvial erosion. (below)
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Sediment delivery is often estimated by measuring the concave slope remnant less that portion of the slide mass that had been removed by fluvial entrainment. Annual sediment yield associated with mass wasting processes is extremely difficult to predict due to the episodic nature of climatic events that initiate movement. Often landslides occur many years following vegetation change and road construction due to complex interactions of root mass decay and soil saturation from major storms. The short-term impact to stream channels from landslides triggered by roads or other imposed land use activities may be of more importance for assessment than the long-term contribution to annual sediment yield.
Models to predict annual sediment yield from mass wasting presently do not exist. Any predictive assessment of these erosional processes must rely primarily on an experienced individual who can recognize the relative stability/instability of an area from soil and geology maps, aerial photographs, vegetation indicators and field observations. GPS, digital terrain models and other appropriate tools can speed up the mapping process of these features. An overlay of existing and proposed road systems over the landslide risk maps will provide valuable warning indicators of past and/or impending potential high risk for failure.
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Nilsen et al. (1976), Swanson and Swanson (1976), Dietrich and Dunne (1978), Caine (1980), WRENSS (USEPA 1980), Reid (1981), Kelsey (1982), Rice and Pillsbury (1982), Burroughs (1984), Wilson (1985), and Benda and Dunne (1987) have methodologies that assess landslide potential related to controlling variables, triggering events, and potential sediment contributions. A prediction methodology based on terrain mapping and landslide occurrence for quantitatively predicting landslide susceptibility following timber harvest was developed by Howes (1987). WARSSS users can select from this range of applied research methods for the identification and prediction of specific landslide processes appropriate to their assessment site.
Sediment delivery potential estimated from the methods presented in WRENSS (USEPA 1980) may be compared to direct field measurements of slide activity. Sediment delivery may be estimated by calculating a delivery ratio by dividing the slide mass removed from slide path or toe of slide into the channel by the initial mass involved in the landslide. Relations may be back-calculated from actual slides based on failure type, slope gradient, channel type, proximity to channel and vegetative cover to empirically generate values of sediment delivery potential.
Mitigation associated with various land uses in landslide-prone areas involves:
- Avoiding mapped "high hazard" slump-earthflow areas with large-scale vegetation conversions such as clearcut silvicultural treatments.
- Not constructing roads on high hazard debris-torrent and slump-earthflow terrain.
- For existing roads, not concentrating surface or intercepted sub-surface runoff onto high risk slopes, dispersing runoff.
- Keeping cut bank heights to a minimum, avoiding thru-cuts.
- Stabilizing abandoned roads by redistributing road fill on road prism and cut bank, and out-sloping the road.
These mitigation procedures reduce sub-surface flow interception, disperse surface runoff, discourages traffic, and allow for re-vegetation of disturbed areas. Drainage crossings should be cleared to prevent fill erosion and debris torrent "dams."
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Figure 3. Debris torrent form of mass wasting erosional process - North Fork Clearwater River - Idaho.
Figure 5. Slump/earth flow erosion process adjacent to stream - Colorado.
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