| Forestry training manual for the Africa region |
Total time 5 hours
- To acquaint the trainees with local soil erosion problems,
- To have the trainees build gully plugs for erosion control,
- The trainee who has taken this as a special project gives a brief lecture and describes activities,
- To investigate vegetation at erosion site.
In this session, the trainee who has taken soil erosion as a special project gives a brief lecture and describes the day's activities. The trainees go to the field and install gully plug(s) and investigate plant life on and near the erosion site for possible planting.
1. Practical Erosion Control
2. Watershed Management
Shovels, local brush, paste for weaving brush (possible to use trees that were thinned in earlier session).
Exercise 1 Practical Erosion Control
Total time 4 hours
The trainee who has previously taken soil erosion as a special project and who has field experience will give a short lecture and explain the activity before going into the field. Then the trainees will investigate vegetation and build gully plugs at the erosion site.
1. The trainee lectures on the site visit and describes the activity. The trainees break into groups with a forester in each group.
2. In groups, the trainees move to the erosion site: investigate and record the vegetation around the site. Each group records their observations.
3. Each group builds a gully plug as described earlier at the place designated by the special project trainee.
4. The technical trainer supervises the construction of the gully plugs. When the plugs are completed, he/she calls the groups together to discuss vegetation in the area and to explore methods to be applied in erosion control.
The following concentrates upon water erosion. This is not to belittle the power of wind as an eroder - and in flat lands with sporadic vegetation, wind has tremendous impact - but rather to keep the contents to a manageable size. Some of the controls outlined below affect both wind and water erosion.
Two aspects of water erosion will be dealt with: First, why erosion is occuring; and second, some control methods.
Why Erosion is Occuring
The first consideration in approaching an erosion problem is the determination of the cause of erosion. Before implementing any specific controls, one should look at as a many variables as possible. The conditions at the head of the drainage, the type and amount of vegetation in the drainage, slope, etc., in each situation is different. The best tools in determining the whys of erosion are basic knowledge of erosion and a pair of eyes that work.
Erosion begins with rain falling on land with incomplete cover of the soil (here, "cover" can be rock as well as vegetation). A falling drop has enough power to knock many soil particles to pieces, sometimes clogging surface pores. When it rains too fast for soil to absorb the water, overland erosion will occur, carrying off sediment. The finest particles, clays, are removed first, followed by coarser particles (silts and sand). The faster the water moves, the more sediment drops out. The larger particles drop first and the clays last. Almost any type of land manipulation in which humans engage will increase erosion; road-building has, by far, the greatest impact.
Some Methods of Control
First, decide what type of control is needed. Controls can protect the soil from raindrops and slow rampaging water and can divert water or catch water for plant growth.
Often, any one of these types can be used in a situation the decision as to which one is best depends on funds, time, manpower and importance of the problem. By the time erosion has produced large gullies, however, substantial work is usually required.
- Soil Protection - Methods of soil protection include rock coverings and the planting of grasses, shrubs, and trees. These methods can treat the problem at the root by keeping water in the soil and preventing it from running on the surface (a covering of organic matter serves the same purpose). In arid regions this is often not possible. Overall, protective vegetation is the best method.
- Slowing Water - Anything that stands in the way of water slows it gully plugs or scattered rock will allow ruts and gullies to fill. These erosion controls show relatively quick results, but unless they are coupled with methods of treating the cause of the problem, gully plugs, etc., are mainly stopgap measures which only delay the problem. They are quick and inexpensive to install.
- Diversions - Water bars in roads and trails, diversion dams, channelling of streams are considered to be diversions. These can be used to collect water or to divert it to flat land where it can infiltrate the soil or redirect it away from a critical area without diminishing its force. This method can easily cause as much damage somewhere downstream as it prevents at the present critical area.
- Catchments - Catchments are usually built on a slope. They include V-catchments, terraces, honeycomb type catchments, and trenches of various designs. They serve the dual purpose of erosion control and enhanced tree growth and can be used at the head of drainages.
It is important to consider the consequences of any kind of control. As already mentioned, a diversion may serve mainly to anger your neighbors. What good has a gully plug served when after the gully has filled, water runs elsewhere to form another gully?
Finally, the very beat way to control erosion is through practices that do not promote excessive erosion. The material in this section can be applied in the alleviation of existing problems as well as to the prevention of future difficulties.
Exercise 2 Watershed Management
Total time 1 hour
Soil erosion control, high quality water and control flooding are the topics of this lecture on watershed management.
1. The technical trainer lectures as follows:
Trainer’s Note: You may want to have an expert deliver this lecture if the technical trainers do not feel competent to cover this material. If the technical trainer does cover this lecture, you may want to use slides or movies to illustrate points.
Watershed management produces high quality water and controls flooding. This requires integrated management of cropland, range-land, pasture land, forest land and urban development. This objective for forest land often differs from that of timber management. Timber management is producing the most commercial wood that can be grown on a watershed, managed on a sustained yield basis, and harvested at regular intervals. In contrast, watershed management could be total protection with no timber harvesting or other disturbances. Generally, however, good timber management is compatible with watershed management.
Two aspects of watershed management are watershed protection an flood prevention.
Watershed Protection is accomplished by applying land treatment measures such as tree planting, contour farming, pasture planting, dam control, debris basins, streambank stabilization, etc.
Flood Prevention is accomplished with flood water retarding structures and stream channelizations. PCVs will be involved in diagramming and applying forest land treatment measures for watershed protection. They normally are not involved in flood prevention so this will not be discussed further. Since a large proportion of the Volunteers' tree planting will be done on Steep, badly eroded hillsides, the following two supporting conservation measures are valuable to know:
1. Gradonis (mine bench terraces) and,
2. Bench terraces.
Gradonis are small terraces that run level or nearly level across the slope. Trees are planted in the gradonis (see Fig. ). They can be built and maintained by hand, constructed by animal drawn implements or by machines. The purpose of gradonis is to change a steep slope to many continuous flat slopes and long slopes to a series of short slopes. This traps run-off, filters it into the soil and aids in seedling survival and growth. It also reduces erosion and sedimentation. (see fig. _ ).
Gradonis are needed at, but not limited to, planting sites that receive less than 800mm (32 inches) of rainfall per year. They can be built on slopes up to 35° (70%) but are better suited to slopes of less than 30' (58%).
Gradonis spacing is determined by the desired tree spacing usually 2 x 2 or 3 x 3 meters. The closer spacing is recommended on the critical eroding areas to obtain a quicker crown closure and litter (mulch deposit on the forest floor). The trees can be thinned as needed to maintain plant vigor. The thinnings can be utilized for fuel or other uses.
Due to slope conditions, the gradonis are often wider than the desired spacing. To have complete land utilization or a fully stocked stand, a better or substitute gradonis should be used as illustrated below.
Individual basins can be substituted for the gradonis. The basins are round (about 1 meter in diameter) with a 10% back slope. The trees are planted in the center of the basin.
Bench Terraces can be constructed by hand, thus providing additional employment for local inhabitants. Figure __ shows the cross sectional view of a bench terrace.
Bench Terraces should be no longer than 10 meters because runoff from longer terraces is difficult to manage. The horizontal grade of toe-drain is 1 percent to safe outlet (waterway).
If topsoil is available, it should be moved away and put to one side and then spread back on the finished terrace.
The top (first) terrace is built just below the ridge. Downhill spacings of the other terraces is three times the width of the previous bench terrace.
Run-off Disposal - Excess run-off is inevitable and a protected waterway is needed to drain it safely. A natural depression - not a large gully - can be used. It must be reshaped into a parabolic shape and sprigged with a soil binding grass (i.e., kukuyu grass). The sprigs should be about 15 cm apart. A light mulch and fertilizer will help to establish the grass.
A parabolic shaped bow (illustrated below) - used in shaping the waterway can be made from bamboo or other similar materials.
PARABOLIC SHAPED BOW USED IN SHAPING WATERWAYS
WIDTH (w) 2 m.
The rear part of the terrace, called the toe-drain, functions as a drainage ditch toward the waterway. In order to avoid overbuilding and excessive expenses, the following principles should be observed:
1. Divide rather than concentrate runoff, if possible,
2. Use adjacent, well protected grassland or forest land to diffuse run-off,
3. Use locally available materials, if possible,
4. Select suitable waterway sites carefully to reduce construction coot.
Structural measures may be needed on slopes over 20% or where flow velocity exceeds one meter/second.
Cost depends upon elope, soil, typo and width of the terrace, presence of rocks, etc., and the necessary tools. Intermittent bench terraces, including waterways, are estimated to coat about $250 per hectare (125 man-days). Gradonis cost about $180 per hectare (about 90 man-days). A man with hand tools can move, on the average of four cubic meters of soil in eight hours. Actual records, however, are needed before detailed costs can be estimated.