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5.1 Erosion and Sedimentation Processes

Sedimentation of surface water is the most common nonpoint source pollution concern related to forest management activities. Sedimentation is the end result of several processes, including erosion; sediment production, transport and deposition; and in-stream morphological processes. In-depth discussion of these processes can be found in Dunne and Leopold (1978); Leopold, Wolman and Miller (1992); Knighton (1993); or Rosgen (1996). This Chapter summarizes portions of those authors' work to provide background and context for the origin, purpose, and design of BMPs.

5.1.1 Hillslope Erosion

Erosion is the wearing away of the earth's surface by wind, water, ice, or gravity. For purposes of this Chapter, sediment is the mineral or organic material that is displaced by these forces and delivered to water bodies. Sedimentation is the settlement or deposition of sediment out of the water column.

Erosion and sedimentation are natural processes critical to developing and maintaining stream channel form and function. However, sedimentation at above geologic rates, especially fine inorganic sediment particles, can be of concern (Waters 1995).

Rainsplash, sheetwash, rills, and gullies associated with overland runoff account for most hillslope erosion. Other sources include mass wasting and soil creep (Dunne and Leopold 1978). Mass wasting usually occurs on steep slopes that slide, or slump, when saturated soils weaken to the point of failing to hold in place against gravity. Soil creep occurs on more gentle slopes where soil particles move downslope very slowly. While these are naturally occurring processes, human activities can cause or accelerate them.

Rainsplash erosion occurs when raindrops impact and displace exposed soil. Vegetation and litter cover on the ground absorb virtually all the kinetic energy of rainfall and prevent most rainsplash erosion. Thus, protection of soil cover is an important strategy for minimizing this type of erosion.

Sheet erosion occurs when overland flow travels downslope in an irregular, sheet-like fashion. This type of erosion actually occurs as tiny streams of water moving back and forth across the slope. It can transport already-detached sediment as well as dislodge soil particles. Several site characteristics including soil particle size and pore space, bulk density, and organic matter content affect sheet erosion processes by influencing soil infiltration capacity. The latter three can be directly affected by management activities.

Rill erosion occurs when sheetflow cuts small, separate channels as it moves downslope. Gullies are rills greater than 1 foot wide and 1 foot deep. Exposed soil in rills and gullies is especially vulnerable to rainsplash erosion, so rills and gullies can grow rapidly. Gully erosion can be dramatic, contributing large sediment loads to streams. Nevertheless, rainsplash and sheet erosion generally account for over 70 percent of total hillslope erosion (Leopold, Wolman, and Miller 1995).

5.1.2 Stream Channel Erosion Processes

Channel erosion can be caused by a variety of factors. Most stream channel erosion is caused by the action of in-stream water (Leopold, Wolman and Miller 1995). Water in motion exerts fluid stress, or applied stress, on the streambed and varies with velocity. When applied stress reaches the point that bed particles begin to move, channel erosion results.

The capacity of a stream to carry sediment also increases with stream velocity. At a given flow, velocity varies within channels longitudinally and in cross-section. Thus, channel erosion and sedimentation occur simultaneously. The magnitude of these processes is affected by flow rate; high flows increase channel erosion and low flows increase sedimentation, or deposition.

Rosgen (1996) discusses stream morphology in terms of channel balance, or equilibrium. Sediment size and load vary with stream discharge and stream channel slope, and all exist in a state of dynamic equilibrium. Changes in one variable lead to adjustments by one or more of the others. For example, when sediment delivery to a channel exceeds its transport capacity, sedimentation results. Conversely, reductions in sediment supply below a minimum limit deprive streamflow of sediment and channels can erode.

5.1.3 Hydrologic Responses

Stream equilibrium is also sensitive to hydrologic response of watersheds, especially peak flow. The most important peak flows for channel formation are associated with bankfull events. Bankfull reoccurs about every 1.5 years, on average. During bankfull floods, streambed material is mobile and channels experience change.

Factors affecting peak flow include the area of impervious material, soil infiltration capacity, time of concentration, drainage density, and antecedent soil moisture. Changes in any of these factors can alter peak flows.

5.1.4 Channel Alteration

Channel straightening effectively reduces total channel length over a given elevation change, resulting in increased stream channel slope. Increases in slope frequently increase stream velocity and can cause upstream channel erosion. The effect proceeds upstream until stream slope equilibrium is re-attained.

Constrictions at stream crossings (culverts, bridges) can increase downstream velocity (result in downstream channel scour) and decrease upstream velocity (increase sedimentation above the crossing).

Silviculture BMPs are designed to eliminate or mitigate impacts of management activities on these erosion and sedimentation processes. Natural watershed processes and flow regimes are encouraged and impacts to water quality are minimized by protecting soil cover and soil properties, minimizing channel disturbance, providing adequate road drainage to the forest floor, and designing and properly installing stream crossings.