Mitigation and Intervention Techniques Related to Micro-hydro Power (MHP) Projects

The identification of appropriate mitigation and intervention techniques on the basis of the Environment Assessment is crucial for the implementation of a successful Watershed Action Plan in order to achieve a sustainable use of the catchment area and thus the MHP project. Biophysical as well as political mitigation and intervention techniques are listed in the table below:

 

BIOPHYSICAL MEASURES
Gully control	Stone Checkdams Objectives: A stone checkdam is a structure across the bottom of a gully or a small stream, which reduces the velocity of runoff and prevents the deepening and widening of the gully. Sediments accumulated behind a checkdam could be planted with crops or trees/shrubs grass and thus provide additional income to the farmer. Brushwood Checkdams Objectives: Brushwood checkdams are vegetative measures constructed with vegetative materials, branches, poles/posts and twigs. Plant species which can easily grow vegetatively through shoot cuttings are ideal for this purpose. The objective of BWs is to retain sediments and slowdown runoff, and enhance the revegetation of gully areas. They are constructed either in single or double row. Some of the vegetation can be used for fodder. BWs are also ideal to stabilize conservation structures bunds, SS bunds, check dams, bench terraces, road sides, etc. Gully Resheping, Filling and Revegetation Objectives: Reshaping & Filling is an operation meant to decrease the gully erosion angle of incidence, create planting areas and encourage revegetation & stabilization, usually in small gullies or in other medium sized gullies from which most runoff has been diverted into a stable waterway or drainage line. When these gullies are shaped and smoothed, vegetation can be established over the levelled gullies. Some of these areas can be used for farming purposes. Revegetation of a gully is the plantation of the reshaped gully sides and bottom with multipurpose species so that it reduces runoff and control erosion and make the land productive. Gully revegetation implies reaching a dense vegetation cover over the entire gully surface, i.e. both sides and bottom by planting grasses, legumes and a variety of trees and shrubs. Sediment Storage and Overflow Earth Dams (SS Dams) for Productive Gulley Control Objectives: SS dams are water harvesting and conservation systems that convert unproductive large and active gullies into productive areas (fertile cultivated or fodder producing areas, mixed plantations, and fruit tree orchards). SS dams are stone-faced earth dams constructed across medium/large size gullies to trap sediments, collect water and divert excess runoff. SS dams accommodate the runoff generated by the catchment located above the gully. The structures are often constructed in series along the gully. It is just like creating a land that does not exist. Contribute significantly to protect cultivated lands, arrest gully expansion and recharge water tables. Huge potential in Ethiopia - can provide tens of thousand Km of gullies to poor households (small land holders and landless). Sediment Storage and Overflow Soil Bunds (SS Bunds) Objectives: SS bunds are large and strongly built soil embankments, constructed across gullies, often in series. As for SS dams, their purpose is to create a new field for cultivation by allowing and helping the sedimentation (filling) of the space behind each bund. After rains, the new fields fill up and retain enough soil and moisture to grow crops. The excess runoff move to the next structure downstream through a spillway.
Agro-forestry, Forage Development and Forestry	Area Closure Objectives: Strip cropping is a cropping practice where strips of two or more crops are alternately established on the contour or, it is a system of establishing more than one crop in alternate strips following a contour pattern for the purpose of erosion control, crop diversification, and decrease the risks associated to the use of single crops only. This cropping system is designed as a defence mechanism against soil erosion in areas where the cropping system is dominated by row/sparsely growing crops that exposes the ground to erosive forces. For instance, crops like sorghum and maize are susceptible to erosion and need to be grown alternately with soil conserving crops. Microbasins (MBs) Objectives: MBs are small circular & stone faced (occasionally sodded) structures for tree planting. Are suitable for medium and slightly low rainfall areas, stony areas and shallow soils. Based upon experience they are not very effective in low rainfall areas (where trenches, eyebrows, etc. are preferred). Eyebrow Basins (EBs) Objectives: EB are larger circular and stone faced (occasionally sodded) structures for tree and other species planting. Based upon experience they are effective in low rainfall areas to grow trees and harvest moisture. Can be constructed in slopes above 50% for spot planting. Controls runoff and contribute to recharge of water tables. Herring bones (HBs) Objectives: HBs are small trapezoidal structures (called also A structures) for tree and other species planting. HBs are suitable for both dry and medium rainfall areas, and medium soil depth. Based upon experience HBs are most effective in medium/low rainfall areas (500-900 mm). Can be constructed only on slopes < 5% and soils > 50 cm depth Micro-trenches (MTRs) Objectives: MTRs are rectangular and deep pits constructed along the contours - main purpose and effects are same as for trenches. Can support the growth of trees, shrubs, and cash crops. Can be constructed on slopes 3-30% max. gradient and soils at least 50 cm depth. Trenches Objectives: Trenches are large and deep pits constructed along the contours with the main purpose of collecting & storing rainfall water to support the growth of trees, shrubs, cash crops and grass or various combination of those species in moisture stressed areas (350-900 mm rainfall). Trenches can have FLEXIBLE DESIGN, to accommodate the requirements of different species. Therefore they can suit what the farmer want to grow. Trenches collect and store considerable amount of runoff water, thus vegetation grows faster and vigorous. Trenches protect cultivated fields located downstream from flood and erosion. Part of the water captured by the trenches reaches the underground aquifer. Therefore, water tables are recharged and supply springs and wells with good quality water and for a long period of time. Improved Pits (IP) Objectives: IPs are square shaped water collection pits constructed along the contours with a plantation pit in front of the main water storage pit - main purpose similar as micro-trenches. IP support the growth of trees and fodder shrubs, and can be used for cash crops like coffee. They are effective in medium and low-medium rainfall areas (above 600-700 mm). Multistorey Gardening Objectives: Multi-storey gardening is a way of planting a mixture of crops, shrubs and trees of different heights and different uses: food crops, cash crops, fruit trees, woody perennials, and forage plants. It makes the land more productive and improves soil fertility, reduces temperature, provides shade, and increase family income, particularly during a period of drought.Fields close or within homesteads can be converted into diversified productive units (decreasing risk factors, increased income, etc.). Such systems have higher resistance to pests and insects. A broad network of such homesteads can extend into open fields and constitute large erosion-protected zones or a “web” of green fences and mutually supporting units. Seed Collection Objectives: Seed collection is divided into two:(1) Seed collection of tree species: mostly for indigenous trees to grow in nurseries for specific and multipurpose uses. (2) Legume shrubs and grass seeds: mainly legume shrubs seeds and grass/plant species that can be used for stabilisation, homestead plantations, grazing lands improvement, support to nurseries, fencing, gully control. This activity is especially valuable for grass seeds collected outside the nursery areas and seed multiplication centers, particularly native grasses of particular value for their palatability and adaptation to local conditions. The collection of local seeds is an effective way to protect valuable planting materials from extinction and to replenish depleted areas with materials collected from other areas.
Soil Fertility Management and Biological Soil Conservation	Compost Making Fertilization and Manuring Live Checkdams Mulching and Crop Residues Management Grass Strips along the Contours Stabilization of physical Structures and Farm Boundaries Vegetative Fencing Ley Cropping Integration of Food/Feed Legumes into Cereal Cropping Systems Intercropping Crop Rotation Strip Cropping
Flood Control and Improved Drainage	Waterways (Vegetative and Stone Paved) Cut-off Drains Graded Soil Bund Graded Fanya Juu Improved Surface Drainage for Increasing Productivity of Vertisols and Soils with Vertic Properties
Physical Soil and Water Conservation	Level Soil Bunds Stone Bunds Stone Faced Soil Bunds Level Fanya Juu Bench Terracing Conservation Tillage using Maresha and Broad Bed and Farrows Maker (BBM) Hillside Terraces Hillside Terrace with Trenches
Water harvesting	Hand-dug Wells Low cost Water Lifting Low Cost Micro-ponds Underground Cisterns Percolation pit Percolation Pon Farm Pond Construction Spring Development Family Drip Irrigation System Roof Water Harvesting System Farm Dam Construction River-bed or Permeable Rack Dams Small Stone Bunds with Run-on and Run-off Areas Narrow Stone Lines Along the Contour Stone Faced/Soil or Stone Bunds with Run-off/ Run-on Areas Conservation Bench Terraces (s) (CBT(s)) Tie Ridge (s) The Zai and Planting Pit System Large Half Moons Diversion Weir Design and Construction
POLITICAL MEASURES
Education and training
Income generating measures
Participatory approach

  

Terracing in the Gobecho catchment (Ethiopia)	Gully control near Ambo (Ethiopia)

Box 1 shows three examples of how (rather simple) interventions can improve the environmental condition of the catchment. A more detailed list of mitigation and intervention techniques and the respective application can be found in DESTA, L. ET AL. (2005): Part 1: Community Based Participatory Watershed Development.  Addis Ababa: Ministry of Agriculture and Rural Development, pages 69 – 165 and DESTA, L. ET AL. (2005): Part 2: Community Based Participatory Watershed Development: Annex. Addis Ababa: Ministry of Agriculture and Rural Development, pages 43 - 48.

Box 1: Fenced vs. unfenced springs/wetlands Springs and wetlands not only guarantee the discharge in dry season, also they have a great retention capacity during and after rains. Springs are sensitive areas that react to disturbances easily. Protection of springs and wetlands is thus crucial for the sustainable use of the MHP projects. Protection measurements might include fencing (to protect the areas from cattle step and thus soil compaction), protection of natural vegetation around the springs, protection of a natural humus layer (in order to sustain sufficient infiltration and thus groundwater recharge). Furthermore fencing can help to improve the water quality, since pollution due to agricultural and pastoral use can be prevented. Buffered vs. unbuffered riverbanks/wetlands A buffer typically consists of a band of vegetation along a wetland or water body, preferably natural habitat, but including previously altered, stable native or introduced species. A buffer can perform a variety of functions, which can improve the environmental condition of a watershed, such as sediment removal and erosion control, runoff reduction through infiltration, reduction of human impacts by limiting easy access as well as barrier to invasion of exotic species. Thus buffers can benefit the sustainable use of the MHP and should hence be established and/or protected. Eucalyptus vs. indigenous plants Eucalyptus easily adapts to any soil and water condition, it is characterized by fast growth, high survival, long roots and hard leafs. Those features make it economically very beneficial. When it comes to watershed management, its negatives sides should be considered though: it does not only take a lot of nutrients and water and is hence a high competition to companion plants and decreases biodiversity, also soil erosion can occur which can negatively affect the MHP’s performance. FAO thus recommends that large scale monocultures of eucalypt plantations be excluded from watersheds, aforestation projects should avoid monocultures and the use of eucalypts. The only way to include eucalyptus is to adopt it in agroforestry systems, but the proportion of each species should be planned out carefully.

References

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