|Forestry Training Manual: Inter-America Region (Peace Corps, 1986)|
- Further discussions on soil erosion control.
- Flood control measures.
- Introduce high quality water concepts.
In this session, we discuss further soil erosion control, flood control measures, and high quality water production.
1. Lecture on watershed management.
2. Possible watershed management projects at trainees' site.
Flip charts, marker pens, tape.
Exercise I - Watershed Management
Soil erosion control, high quality water and control flooding are the topics of this lecture on watershed management.
1. Technical trainer gives lecture 1 hour as
Trainer's Note: You may want to bring in an expert to do this lecture if technical trainers do not feel competent to cover this session. If technical trainer does cover this lecture, you may want to use slides or movies to illustrate points.
Soil Erosion Control
Objective - To produce high quality water and control flooding.
To accomplish this requires integrated management of cropland, rangeland, pastureland, forest land and urban development.
For forest land, this objective often differs from that of timber management.
Timber management is producing the most commercial wood that can be grown on a W/S, managed on a sustained yield basis, and harvested at regular intervals.
In contrast, W/S management could be total protection with no timber harvesting or other disturbances.
Generally, however, good timber management is compatible with W/S management.
Two aspects of W/S management are:
(1) Watershed protection, and,
(2) 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 structure 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 much of the volunteers' tree planting will be done on steep, badly eroded hillsides, the following two supporting conservation measures (EMDEFOR is using troth in their forestry program) are valuable to know:
(1) gradonis 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. 61). They can be built and maintained by hand. They can also be constructed by animal drawn implements or by machines.
Their purpose is to change a steep slope to many continuous flat slopes and to change long slopes to a series of short slopes, thus (1) trapping run-off and infiltrating it into the soil to aid in seedling survival and growth, and (2) reducing erosion and sedimentation (see fig 61).
Gradonis are needed, but not limited to planting sites that received less than 800 mm (32 inches) of rainfall per year. They can be built on slopes up to 35° (70%) but are better suited to slopes of 1 ½ than 30 degrees (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 he utilized for fuel or other uses.
Due to slope conditions, the gradonis often come out wider than the desired spacing. To have complete land utilization or a fully stocked steno, a better or substitute gradonis should he used as illustrated below.
Bench Terraces - Bench terraces are too expensive to construct for forest plantings alone. They are, however, used in agro-forestry projects (EMDEFOR is experimenting with them). Crops are grown on the bench terraces; trees are grown on the slopes between the terraces. For additional soil stabilization, the trees should be planted on gradonis also. This practice affords needed W/S protection while allowing the landowner to farm part of the land. see Fig. 63.
Individual basins can be substituted for the gradonis. Round basins are constructed (about 1m in diameter) with a 10% back slope. The trees are planted in center of the basin.
General specifications for bench terraces - The terraces can be constructed by hand; this provides additional employment for local villagers. Figure 64 shows the cross sectional view of a bench terrace.
Bench Terraces should be no longer than 10 meters - 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 cut away and put to one side and spread back on the terrace when it is finished.
The top (first) terrace is built just below the ridge. Downhill spacings of the other terraces is 3 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 down the slope. 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 - used in shaping the waterway can be made from bamboo, or other materials. A parabolic shaped bow is illustrated below.
The rear part of 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 cost.
Structural measures may be needed on slopes over 20% or where flow velocity exceeds 1 meter/second.
Costs - Cost depends on slope, soil, type of terrace, width of terrace, presence of rocks, etc., and the tools used to build them. Intermittent bench terraces, including waterways are estimated to cost about $250 per hectare (125 man-days). Gradonis cost about $180 per hectare (about 90 man-days). A man with hand tools can move, in the average, 4 cm3 of soil in 8 hours. Actual records, however, are needed before detailed costs can be estimated.