|Regenerative Agriculture Technologies for the Hill Farmers of Nepal: An Information Kit (IIRR, 1992, 210 p.)|
|Natural resources and their enhancement|
Gullies are thought of as a curse by farmers. When managed and stabilised, however, they can be highly productive because of the advantageous micro-climate.
Only basic principles intended to serve as a general guide suitable for many gully situations in the hills are presented here. Large and complex gullies requiring additional facilities and deeper analysis should be referred to the soil and water conservation office.
A few technical terms are used as follows:
Profile - Longitudinal section down the length of the gully, along the stream course. Section Section across the gully at right angles to the stream.
When working in gullies, the golden rule is to build check dams and plant seedlings to the highest quality. Use the same care as if building ones own house. Follow design instructions carefully. Important points are highlighted. If you do not do this, the work is likely to fail and resultant problems can be much greater.
There are two stages in gully stabilisation which need to be considered.
1. Building checkdams to stop soil movement. (where a gully has naturally stabilised this may not be necessary).
2. Planting vegetation to protect surfaces to a depth of one metre in the soil horizon.
METHODS INVOLVED IN STABILISATION
1. Why checkdams: They help stop gully growth by preventing downcutting of the gully floor, and subsequent slumping of side.
2. Where to position checkdams:
a) At the knick point: resistant points along the gully marked by rock outcrops or bamboo clumps.
b) between these sites in strong areas, if the distance between "knick points" is over 15 ms, and the land between them is weak.
c) Do not put checkdams onto surface of gully. Dig well into existing material, setting foundations ½ metre into original ground and 1/2 metre into sides of gully.
d) The checkdams should curve back into the gully by a ratio of 5:1, e.g., if the gully checkdam is 5 metres wide, the centre should be 1 metre back into the slope.
e) The dimensions of the checkdams are difficult to recommend as they depend on the size of the gully however as a rule of thumb. From the back wall - 2/3rds of the checkdam should sit in the foundations of the gully and at the front of the gully, the foundation should be 1/2 metre.
f) Where neccesary, protect the floor of the checkdams with stone (cobble stone rip-rap)
Knick points and checkdams
3. How to build checkdams - design
a) Always curve top of checkdam so that water flows to center and does not cut or scour the sides of the gully.
How to build checkdams - a
b) Top 30 cm should be of strong, large stone faced back into the slope.
How to build checkdams - b
How to build checkdams - c
How to build checkdams - e
How to build checkdams - f
Pack the stones tightly and make a smooth surface. At edges, use extra large stones and bed them firmly into the ground to their full depth to prevent lifting out.
· The placing of vegetation and the choice of species used need to be carefully considered. Names and details of a large number of species of grasses, trees, shrubs and bamboos used by the Eastern Region Road Maintenance (ERRM) Project in the Koshi Zone of Nepal are given in Appendix D of the manual Vegetation Structure for Stabilising Highway Slopes which is available from ERRM.
· A gully has within it a range of micro climates and a range of areas susceptible to erosion, from relatively stable to very active areas.
· The choice of species, wherever possible, should be indigenous.
· It should aim to produce useful outputs, such as fodder trees and grass, bamboos and shrubs.
· The species should be carefully chosen to match the site characteristics, especially rainfall and soils.
· A variety of vegetation types should be selected to maximise both the cover by the overstory and the reinforcement effect of roots on the soil.
· The vegetable should be light in weight so that large trees do not overload the slope.
An example of how different species may be used in a gully is given in the figure below.
3) Watch gully in the rain.
4) Patch up areas as quickly as possible whenever erosion problems begin.
5) Prevent cutting, burning, or grazing of vegetation whilst it establishes.
An the above work should start only after the local landowners or users have been consulted and are involved in the scheme. They should be involved in: a) planning, b) protecting and c) using or managing the area. Initially, a management plan or simple contract that they produce themselves with the technical assistance of the government technical officer will be required.