Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 33

Board fences tilted,
pulled apart

Monuments tilted

Cracked foundation

Curved tree trunk

Tension cracks
tilted pavements

Road

Tilted guard rails,
utility poles

Tilted
retaining walls

Rock strata curved near surface,
blocks in soil downslope

FIGURE 2.11
Example of soil creep. (Reprinted from Bloom, A.L., Geomorphology: A Systematic Analysis of Late Cenozoic
Landforms , 3rd edn., Waveland Press, Long Grove, IL, 2004. With permission.)

• Soil creep—Slow movement of slope. Soil creep is the most common type of mass
wasting and is always present on any slope where there is soil. This process tends
to go unnoticed because the process is slow. Some of the effects from soil creep are
shown in Figure 2.11.

Gravity is not the sole force in operation during mass wasting. Other transport mediums
such as wind, flowing water, ice, or molten lava may be involved. Although flowing water is
excluded from the process by definition, water nevertheless plays an important role in mass
wasting. Flowing water can erode the base of a hillslope and weaken the mid-slope region,
leading to an increase in the effects of gravity. In a process known as piping, water migrat-
ing near the soil surface from seepage fronts of groundwater may subsequently produce
slope instability and increase the “lubrication” of a preexisting plane of weakness. More
frequently, however, water entering the soil simply increases the pore pressure, reducing
grain-to-grain contact of the soil particles and results in slope failures such as debris flows.

2.3.1.3 Stream Transport

After the downslope transport of material, streams carry a large portion of these solids
away (process #3, Figure 2.8). The larger and heavier rocks become part of the channel bot-
tom, while a small fraction of the solid material reacts chemically with water and becomes
dissolved load that takes the same journey as the stream. Most of the downslope material,
however, becomes suspended load and eventually ends up in the continental shelves as a
sedimentary deposit.