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Private forests: commonsense erosion control.

There is hardly a process on earth more natural and inevitable than erosion. After all, without the erosive forces of wind and rain, temperature extremes and moving water, there wouldn't be any soil. Somewhere in my past I remember a fatalist's view of erosion-that soil is merely particles of rock on their way to the ocean. Yet, in spite of the fact that erosion is an earth process, when soil washes away from forest ecosystems at an accelerated rate it can downgrade water quality, harm fish habitat, lower stand productivity, and destroy access routes.

On a geologic timetable, our careless activities probably hurry the inevitable to an insignificant degree. But on a human timetable, excessive erosion caused by poorly designed logging roads and other practices that disturb the soil can destroy the potential of a site overnight. Excessive erosion is detrimental to the values we ascribe to forests. Look at it this way: when we allow soils to erode from carelessly placed skid trails, haul roads, and landings, we lose capital. When we lose capital, we become poorer.

Make no mistake about it-harvesting timber from forests increases erosion, although it is rarely the stand prescription that causes soil loss. Regardless of whether the site will be clearcut or lightly thinned, removing tree cover does not usually increase the rate of erosion. Virtually aU of the erosion caused by logging east of the Mississippi is attributable to the transport routes used to extract timber. Why? Because about 10 to 16 percent of the surface area of a site being harvested is in roads and skid trails, all of which are subject to the erosive forces of water. Ironically, water is the culprit on the site, soil the victim. However, when soil suspended as sediment in water reaches rivers and streams, the tables are turned-soil is the culprit and water quality the victim.

There is really very little that anyone needs to know about controlling soil erosion on forest sites that is not common sense. A little background in soil physics will help explain what happens when soil is washed away.

Soil is considerably more than particles of rock on their way to the ocean. It is an aggregation of particles bound in a matrix that is roughly half mineral and half water and air. A very small amount of organic matter is usually present as well. Therefore, about half of a soil's volume is pore space. When the matrix is destroyed either by the impact of rain or the physical action of water, soil will break into its components, which vary in size from small gravel to microscopic particles of mineral and organic matter. The smaller the particle, the more easily it is moved from the site and the farther it will go. Farmers in the Midwest say that when a river is holding as much sediment in suspension as it can, the water becomes 'too thick to drink but too thin to plow".

In spite of the fact that fine-textured soils with a high proportion of clay and silt have more total pore space, they are more highly erodible than coarse-textured soils, which have less total pore space but larger pores. Water cannot infiltrate as readily in a fine-textured soil and therefore has a tendency to "run off," taking with it very fine particles that are suspended in the water. Larger pores in coarse soils allow water to soak in. Moreover, since the particles are larger and heavier, they will tumble along with water but do not become suspended in it.

The ability of water to carry sediment increases as the fifth power of its velocity, and the maximum particle size that water can move (called its competence) increases as the sixth power of velocity. In other words, when the speed of water running down a roadbed doubles, it can carry 32 times as much sediment and particles that are 64 times the size of what it could carry previously. Rain has a phenomenal amount of energy as well. One source indicates that a two-inch rain falling at a velocity of 20 miles per hour exerts six million foot-pounds of kinetic energy on an acre of land. This is enough energy to lift a seven-inch layer of soil three feet ! So to minimize erosion potential on logging trails, we need to sap water of its energy in two ways-by protecting bare soils from the impact of rain and by decreasing the volume and velocity of water on road surfaces. This can be accomplished by ensuring that bare soil surfaces are revegetated and water is drained quickly off roads. Simple enough and relatively easy to accomplish, right? Bear in mind, however, that timing is crucial. One severe storm event can cause unprotected roads to wash away in a matter of a few hours, sometimes less.

The first and foremost erosion-control technique is implemented before any equipment shows up at the site-planning. Some experts believe that with good planning a manager can reduce the surface area in roads by 40 percent. All log landings, haul roads, and main skid trails should be mapped first on topographic sheets, then laid out and flagged in the field. During the layout process, the locations of water-control structures must be identified, and required amounts of culvert pipe and materials such as seed and hay mulch can be estimated. Potential buyers need to know before bidding on timber how much road work the job will require. They are much more receptive to a manager's demands regarding the condition of roads and trails during and after the sale if they know what is expected of them.

When foresters talk about road grades, they use the term percent' to quantify slope. A road's grade expressed in percent is its vertical rise divided by the horizontal run. Generally, forest roads and skid trails should be kept under 15 percent grade, or 15 feet of vertical distance for every 100 feet of horizontal distance. For example, a point on the road at eye level, up slope from where you stand, should be at least 40 feet away along the horizontal to qualify as a grade of 15 percent or less. The grade may be greater than this for short sections, but generally, the lower the overall grade, the less potential for erosion. However, in the case of haul roads and landings, a slight grade of three to five percent is desirable to improve drainage.

It is virtually impossible to harvest timber on a tract of any size without crossing a stream. Depending upon a stream's size and potential storm-flow, it may be crossed by a bridge (permanent or temporary) by routing the brook under the road in a culvert, or the stream may be forded (by routing the road across a shallow spot) at right angles to the banks. The important thing to remember is that in some states you must apply for a permit to cross a classified stream. Contact your local Soil Conservation Service office before crossing any perennial stream with heavy equipment.

Aside from crossings, roads and trails should be kept away from streams. If that's impossible, a filter strip, 50 to 80 feet wide, of undisturbed vegetation should be left between the road and the water's edge. Sediment traveling overland during a heavy rain will be filtered out before it gets to the stream. The strip also maintains shade, which keeps water temperatures cool and dissolved oxygen levels high. Even the smallest mountain brooks can support populations of feisty trout that like cold, clear water.

Keeping soil in its place is the key to preventing excessive erosion and sedimentation in streams. The trick is to impede the movement of water on road surfaces or, better yet, get the water off the road. Virtually all erosion-control techniques attempt to do one or both. Some of the most common techniques to deal with water on logging jobs are water bars, reversing grade, outsloping and insloping of roads, and broad-based dips. All are variations of a theme: Slow water down and get it off the road ! During the timber harvest, equipment operators are reluctant to install erosion-control structures that will need to be reconstructed at least once when the job is completed, sometimes at the end of every day. But it takes only one torrential downpour to cause considerable damage. What to do? The manager and woodland owner must assuage the operator's reluctance and insist that trails and roads be protected from erosion at all times. Otherwise, the woodland owner may be subject to civil or, in some states, criminal suits for causing excessive stream sedimentation.

After the harvest is over and all work on the roads has been completed, bare soil surfaces should be seeded and mulched. The cheapest and most readily available seed is the -Conservation mix"-a combination of annual and perennial grasses and a small amount of clover.

The seed should be covered with a hay mulch at a rate of 60 bales per acre. Mulch is extremely important for good germination of the seed, especially on dry, south-facing sites. In addition to keeping the seed moist, mulch will break the impact of raindrops and to some extent slow water down. After the grasses have become established, fertilizer can be applied at a rate of 150 to 200 pounds of 10-10-10 per acre. Fertilizers applied beforehand are washed away and, aside from the wasted expense, can ultimately end up in the stream.

Roads can be further protected by installing gates or travel obstacles, as long as they are not hazards that might hurt an unwary trespasser. Two deep water bars placed back-to-back (also known as a tank trap) at the foot of a trail will discourage all but the most foolhardy four-wheel trespassers. Remember, the obstacles must be obvious to avoid being held liable for damage, even trespassers.

Aside from the negative effects of excessive erosion on stand productivity and access, when soil is transported off the site it ends up someplace else. In most cases, it is someplace where it is not wanted. Of the four billion tons of sediment produced through all causes of erosion in the U.S. each year, nearly half gets into streams; half of that makes it to the ocean. Sediment fills in ditches, lakes, and harbors; it alters the navigable course of rivers and causes the early demise of reservoirs. It can severely pollute public water.

Excessive sedimentation also changes the ecology of streams and rivers by increasing water temperatures and decreasing oxygen supply. It can also blanket newly laid fish spawn, causing the eggs to suffocate before they hatch. And a stream full of sediment just doesn't look good.

Though logging is a relatively small contributor to the billions of tons of soil lost each year, for two reasons it is not an insignificant cause of soil erosion: it happens quickly, often before erosion-control measures are installed, and the disincentive to those who contribute to excessive erosion is not enough to cause them to be more conscientious. In most states, causing erosion is not against the law. However, causing excessive levels of sediment to accumulate in brooks, streams, rivers, and lakes is.

Note that I use the word excessive. just like any activity that disturbs the soil, logging will cause some sediment to get into streams. As long as the amount of sediment is not tremendous, does not get into streams during fish spawning in the spring and fall, and is unlikely to cause a human health hazard, logging should have a minimal impact.

If, after reading this far, you find the subject of erosion and timber harvesting intimidating, I suggest you employ the services of a good consulting forester. There are many aspects of erosion control that go beyond common sense, with which foresters are trained to deal. Moreover, their job of supervising timber sales for clients involves making sure that the roads, trails, and landings are properly constructed, and properly laid to rest when the sale is over. Foresters know when weather conditions should suspend the operation, and how to communicate with loggers.

So although there is very little that a woodland owner needs to know about controlling erosion that is not common sense, when you don't know what you're doing, hire a professional! That's common sense.
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Author:McEvoy, Thom J.
Publication:American Forests
Date:May 1, 1989
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