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Mass Wasting

The erosional processes associated with mass wasting include two primary types:

  1. shallow, fast movements of debris avalanche/debris torrents, and mudflows that generally move only once, and
  2. slow, deep-seated slump/earthflow erosional processes that move intermittently over a wide range of time trends in response to infrequent events and/or disturbance factors.
The evidence of mass wasting can be observed on aerial photographs, thus frequency and magnitude of the slides can be observed over time sequential aerial photos.

Examples of debris torrent/avalanche slides are shown at Hillslope Erosional Processes: Mass Wasting. Sediment delivery is often accomplished by measuring the concave slope remnant minus that portion of the slide mass that has been removed by fluvial entrainment. Prediction of annual rates of sediment associated with mass wasting processes is extremely difficult due to the episodic nature of events that initiate movement.

Often landslides occur many years following vegetation change, clearing, and/or road construction due to complex interactions regarding internal soil strength, particularly as defined by root mass, the decay of such roots, and the subsequently altered response to soil saturation from major storms. The impact to stream channels and associated beneficial uses by landslides triggered by roads or other human induced land use activities and land cover changes may be of more importance for assessment than the long-term contribution to annual sediment yield.

Current models do not accurately predict annual sediment yield from mass wasting. Any 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. The user should select tested methods for the identification and prediction of specific landslide process for the appropriate geologic type of the assessment site, and also should review the Mass Wasting chapter in WRENSS.

Sediment delivery potential is determined from general relations of slope gradient, shape irregularity and slope position of landslide activity, as well as soil strata/profile information. These relationships illuminate the likelihood and subsequent risk of converting hillslope erosion to instream sediment.

The general guidelines for identifying potentially high risk of slump/earthflow or debris slide/debris torrent processes are shown in Table 14 (PDF, 62 kb, 1 p.). This table identifies characteristics of landform features, soils, rock type, vegetative cover and precipitation that are associated with high risk of slump/earthflow and debris torrent/debris flow processes. The user must be familiar with mass wasting processes and be able to use photographic interpretations to identify high risk features.

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Sediment delivery potential (conversion of erosional debris to sediment in a conveyance channel) is key in this analysis, as the combination of high risk for erosion coupled with a high risk for sediment delivery prompts the requirement of the most detailed investigations and quantitative prediction of sediment yield at the PLA level of WARSSS. Sediment delivery potential is determined from slope gradient and slope shape as shown in Figure 80.



A relationship of delivery potential using slope position (Figure 81) is also used to get a numerical risk index as shown in Table 15.


Table 15. Combined mass wasting sediment delivery potential rating from Figures 80 & 81.
Very Low 2
Low 3-4
Moderate 5-6
High 7-8
Very High 9-10

Any land use activities such as roads, clearcuts, land clearing, control burns, etc. that occur on sites demonstrating high risk potential for these erosional processes that also have a high delivery potential (e.g., sites on the lower third of steep slopes adjacent to streams) indicate that the analysis should proceed to the PLA level.

Enter the values from Table 15 for each subwatershed into the Step 6 column of summary Worksheet 6.

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