This unit was developed in 1984 as part of Caps System work. The assistance of Susan Hamilton is appreciated. It is intended to supplement the major soil erosion unit and other soil information.

It related directly to BMPs or Best Management Practices in Virginia for soil and water management.

Soil does not move. It is moved. Soil movement is a complex process, one part of a large, complex soil system. Movement is largely driven by gravity. Understanding this fundamental force can prevent the selection of poor practices, the waste of limited funds, and the loss of fossil energy to do other work.

Gravity

Water and wind move soil too, but much of those subsystems are gravity driven. The reason for emphasizing gravity in The Trevey is not to oversimplify, but to emphasize the most profound force in the system, to state the use of it as the basis for most of the relationships used and for recommendations made, and to suggest that it be used first in creative modifications and adjustments in the prescriptions to fit unique situations.

Erosion

Erosion connotes "bad." It is a word that simply means soil movement, but, through use, has come to imply losses and damages. As a noun, it is used to describe undesirable conditions like nh erosion or gulley erosion.

Wind moves soil around. It may uncover some plant roots, and may add plant nutrients to others. There may be losses or gains, but movement of soil by wind is called wind erosion. There are rich valleys in Virginia. Scoured out of geologic land masses by streams and rivers, these valleys were flattened and rebuilt by the meandering rivers. Likewise, the rivers worked to flatten out a thousand beaver dams capturing the eroded sediments to form new crop-growing surfaces.

Every year some landowners await the spring floods that bring water and nutrients and a new layer of soil on which they can grow select crops. Certain forest and marsh communities have adapted to this normal sequence.

H. P. Odum described these as pulse communities, those that are dependent upon and respond with surges of plant and animal growth. Soil is a very complex system composed not only of rock fragments, but also of gases, water, dissolved substances, organic substances in water as well as among the soil particles, and living plants and animals. It is so complex and interactive that it has elements of the mysterious. Because of particle size, shape, electromagnetic attraction, cements, and organic molecules, soils differ in their innate stability. Without consideration of any of the elements of soil movement, the first aspect of management is to utilize the inherent characteristics to minimize the effectiveness of gravity. The most conspicuous technique is to assure a slope that, unique for each soil, assures the repose of the soil. In most situations, making a slope very gradual is very costly. Soil characteristics influence the costs of developing a stable slope. In some conditions that slope is zero.

Surface soil movement (called sheet, rill, on gully erosion depending on its magnitude and form) is determined by the inter-relations of

1. rainfall amount, duration, and intensity,
2. soil erodability,
3. topography, and
4. cover - namely litter and vegetation.

This statement can be symbolized in an equation known as the universal soil loss equation (USLE). This is used elsewhere in The Trevey.

One purpose of this report report is to assist in the control of soil erosion and sedimentation resulting from land disturbing activities (Section 21-89.3a Code of Virginia). Sediments are usually considered one of the non-point source pollutants. They cause turbidity that may be harmful to aquatic life and that does fill in stream and watershed structures.

It is stated in the General Criteria of the Virginia Soil and Water Conservation Commission (1980:1-4) that "properties and waterways downstream from development sites shall be protected from erosion due to increases in the volume, velocity and peak flow rate of storm water runoff."

One challenge of this system is to develop a site-specific analysis of erosion from any development. What is the likely assault downstream from any development? Maximum? Controlled? The relation of the two?

While pesticides, such as those applied to forests, attach to sediments, and sediments contribute to pesticides in water, in general such contributions are very small in intact forest areas. However, serious pollution may occur in regions where natural forest conditions are greatly altered (Cornell Aeronautical Laboratory 1972:3-42). Rice et al. (1979) observed timber cutting resulted in sedimentation ranging from 385 m³/km² to 1413 m³/km² (0.80 to 3.0 times that of the forested area). Logging and roadbuilding made sediment available for transport.

Sediment transport in undisturbed environments is related to supply of sediment; in disturbed areas it is related to stream power (80% occurs when velocity is > 40 ft³/s).

Logging and road building deliver sediment to the stream.

Sediment trapped at three weir ponds in the Cascade Mts. following a fire were 403, 262, and 119 kg/hectare. These were conservative estimates because some sediment was flushed through the traps and was not measured. Following the fire, sediment production exceeded researchers' ability to trap and measure it. In these cases, grab samples were taken and these contained (as is usually the case) suspended as well as bedload sediments. Even these yield conservative estimates because large and heavy particles which move along the stream bed are rarely sampled. Suspended sediment concentrations reached levels of 1300 mg/I and frequently were at 900 mg/I.

Phenomena

1. Sediment concentrations increase rapidly with increased stream discharge rates
2. Slight decreases in discharge cause large reductions in sediment concentrations
3. High water scours materials from the channels; stream energy is a factor
4. During low discharge, available materials to be moved have been reduced
5. The wetted perimeter increases after stream scouring
6. Sediment concentrations are higher during rising stage than falling stage of hydrograph
7. Frequency of rising and falling influences process.
8. Dry conditions allow sluffing of sediment into channel
9. Sediment production drastically declines when stream channel reaches bedrock
10. Debris dams can influence sediment estimates
11. Available materials decrease
12. Vegetation, especially along streams, increases decreasing sediment.

Creep

Creep is the show, average of 10 mm/year downslope movement of soil mantle materials as the result of longterm gravitational stress (Swanston, 1976). Most natural movement occurs near the surface but an underlying zone exists relating to geology, soil development, and ground water. Most creep occurs during the rainy season. But it may occur year around in areas where the water table level is constant.

The general relationship is: the higher the level, the higher the rate of creep. The relationship is exponential (Ter-Stepanian, 1963).

Creep is accelerated by clearcutting forests, increasing the land slope angle, and compacting fill materials. Road construction often has the above associated phenomena. Leopold et al. (1964) said that soil creep rates in the temperate forest is of the magnitude of 0.6 mm/year. Total movement of soil into stream channels in forested, mountainous regions is about 0.01 acre-foot/sq mi/year.

Clearcutting may result in much more sedimentation than the above, not only from loss of cover but also from accompanying roads, log skidding, slash burning and subsequent land use practices and and stream bank sluffing.

Brown and Sheu (1975) have developed a mathematical model of creep related to forest cutting. This may be explored for future refinements in the Trevey analyses.

Compaction

Compaction of soil from vehicles causes decreased infiltration, thus increased runoff and soil movement. Two important factors affecting compaction are soil texture and moisture content. Loam and silt barns compact to greater densities than do fine or coarse texture soils. Moisture contents of soils that compact most are midway between field capacity and permanent wilting point.

Whether soil moves due to rainfall is a function of whether it is covered or not. Rain droplets- impact on soil with great energy. Whether mulch or vegetation protects it is related to the use and management of the land. The falling droplet moves soil and thus it reduces water infiltration rates by filling the pore spaces (destroying soil structure) thereby increasing runoff. Rapid revegetation is needed to reduce the time during which bare soil is exposed.

An equation for slope and length factors of the universal soil loss equation allows for complex slopes

LSi = (L 0.5/100)(0.76 + 0.53 si + (0.076si²) ) where L is the slope length and si is the slope of the last increment of slope length. This gives better prediction of mean soil loss from concave and convex slopes that the assumption of a uniform slope. from:

Onstad, C.A., C.L. Larson, L.F. Hermsmeier, and R.A. Young. 1967. A method of computing soil movement throughout a field. Trans. Amer. Soc. Agricultural Engineers 10)6): 742-745

Town and Urban Areas

Although the wisdom of holding water in forests and on farms has been acknowledged, it is less well understood that the same concept applies in cities. The former absorb and impound water, releasing it gradually. In cities and residential areas, streets, curbs, and all of the impervious remainder create difficult water management problems in downstream areas. The small watershed idea is "stuck" on the farm (Clay 1965:23).

The options are to create storm water catchments, multiple-use parking areas that retain water during floods. In England "water meadows" have served for hundreds of years. These are open valleys and meadows adjacent to streams which tend to flood. They act as great sponges.

Areas adjacent to brooks can be left undeveloped with adjustable sluices to control their rate of filling. After flooding, the water can be released gradually to prevent downstream flooding. In most cases of land becomming more urbanized and conflicts increasing, planners can prevent these conflicts by zoning and land acquiring and using areas for parks, wilderness, zoos, space for churches, arboretums, nurseries and specialized agriculture. These are areas not to be zoned as simply "agriculture" but as water-retention areas, part of community and regional self-interest. These areas can be used to emphasize the form, shape, and beauty of neighborhoods while providing an essential engineering function--a subtle kind of low-cost water control.

In the uplands, slopes become greater each year (on the average) due to soil movement. In the lowlands, they similarly become less until they stabilize with only very slight slopes. There is a physical loss in the uplands, a physical gain in the lowlands, all impartially driven by universal gravity.

When the world was only for beavers and bullfrogs it probably did not make much difference about the losses and gains in soil. Of course it makes a great deal of difference now because people attach value to those physical changes. Some person's gain is another person's loss and vice versa. In a Constitutional society with objectives of justice and the general welfare, it is important that the greatest expected net gains be made, and that the ability to make future gains not be impaired.

In some cases, it will be appropriate to move soil onto an area by vehicle to create a growing medium. This is soil movement. If it is taken from a rich flood plain by truck up onto the hill, is it erosion? It probably produced scars in the flood plain. It probably helped immensely to improve the uphill site. The direction of the movement is irrelevant (water erosion connotes "down," wind erosion connotes "down," somewhat, but probably lateral movement). The means are irrelevant, for erosion is often defined as "wind and water."

Rather than continue to use the phrase "preventing erosion" (though it cannot always be avoided) staff within The Trevey takes the approach of managing soil movement to achieve maximum net social or landowner gains. This is not a mere quibble over a word, but an effort to allow objective analysis of what soil moves, where it will go, what will be its long-term resting state, and how much profit (or avoided loss) can be made from it for people. It may be in some situation very appropriate for soil to move naturally (probably producing a bad index of performance) into a series of terraces which, over time, produce highly productive grazing or cropping areas. Erosion occurred but the result was positive. The system had, however, been designed and managed to achieve net long-term benefits. This was not erosion but soil movement management.

As in any system, objectives are needed. How can you tell when you have achieved the optimum management of soil movement? The answer: When pre-stated objectives can be (or are) demonstrably achieved.

Management Prescriptions

The preliminary orientation within The Trevey is toward design of a site-specific prescription that will best achieve (given local constraints) the maximum expected net present estimated value of a set of resources over a specified period (e.g. 30 years).

Humans play a major role in the ongoing natural decision of whether soil moves or stays in place. They may inadvertently change conditions resulting in movement. Of course, conscious acts sometimes called "land or soil management" may be taken to assure soil stays in place. In some cases there is nothing that humans can do to prevent soil movement. Volcanic action is a spectacular example but more frequent and less conspicuous are the major slumps and earthfbow that are phenomena difficult to predict and usually too costly by most criteria to say they can be prevented.

When soil moves, it can degrade the quality of the existing site and cause at least temporary losses of productivity. It may increase the quality of downstream areas (the pulse communities that receive an annual recharge of nutrients). It may cover crops, of course, and cause loss of productivity. Large movements can destroy roads, dams, and houses. Lesser movements to streams can cause damage from aggradation and degradation of the channel, flooding, siltation of lowlands, destruction of fish spawning habitats, and deleterious changes to estuanine habitats by siltation and channel alterations (Swanston, 1976). In major portions of Virginia moving soil contributes to the river channel load that must be dredged at high cost to maintain ports for ocean-going ships. The proper subsequent disposal of this dredged material remains a controversial issue.

Under mature temperature forests such as those in Virginia, surface movement of soil is almost non-existent. It does occur on steep slopes but is often "caught" by down trees and natural barriers so that over the longrun the changes are small and slow. Suspended sediments from forested areas observed in high waters are from caving and slumping of undercut stream banks and road and other slope disturbances (Eagleson, 1970).

There is an interaction of the natural physical system (mainly soil, topography, precipitation, and wind), the biological system (mainly plants), and the human system (mainly financial resources, technology, knowledge, and objectives). There are 10 key factors; the factorial of these is 3,628,800 or 3.6 million possible sequences.The permutations greatly exceed this number. What the earth movement will be or might be is, thus, likely to be unique for each watershed, probably every spot on Earth.

A Legislative Base of Sediment Control is available.