Landslide prevention using drainage trenches lined with fast growing trees. Tajikistan

Transcription

1 Landslide prevention using drainage trenches lined with fast growing trees. Tajikistan The construction of linear gravel bed ditches lined with local tree species, at angles across a hill slope to channel the surface water. A series of 80m long ditches are constructed at angles of approximately 30 degrees, across a hill slope at the base of the watershed. This land is prone to waterlogging, therefore several ditches approximately 0.5m deep, with a gravel bed to prevent erosion, drain the excess surface water away to the main tributary of the watershed. The edge of the ditches are further lined with fast growing tree species such as willow and poplar for stabilisation and afforestation purposes. The purpose of the ditches is two fold, firstly to channel the surface water to prevent waterlogging that had previously led to landslides and small mud flows. Secondly, to enable cultivation on land that was previously unusable. The technology is very simple and cost effective. Initially there needs to be an assessment of the amount of surface water that runs over the slope, this will determine the number of ditches required. The ditches are marked out, running at approximately 30 degrees perpendicular to the slope. The ditches are dug to a depth of 0.5m (or deeper) and filled with a base layer of stone to prevent the bed of the ditch from being eroded. Once the ditch is established, preferably in the spring time, fast growing and naturally available cuttings from trees such as poplars and willows are planted alongside the edge of the ditch at 0.2m intervals. These will stabilise the ditch bank and as the trees become established some thinning out will be required due to their close proximity to each other. The area used is a hillside slope, subject to substantial amounts of surface water run off due to its location at the base of the watershed. The region suffers from a lack of accessible irrigation water, especially during the hot summer months. This technology allows land to be brought into cultivation that has natural access to water and is able to sustain vegetation during the dry months. left: The photo shows the ditches running towards the main trench at the foot of the slope. (Photo: BONATI GIUSEPPE) right: The ditches run towards the river bed at the base of the valley. (Photo: Foteh Rahmatilloev) Location: Khatlon Region: Khovaling Technology area: 0,1 km 2 Conservation measure: structural Stage of intervention: mitigation / reduction of land degradation Origin: Developed through land user`s initiative, recent (<10 years ago) Land use type: Forests / woodlands: Natural Land use: Forests / woodlandsrests / woodlands: Natural (before), Cropland: Tree and shrub cropping (after) Climate: semi-arid, temperate WOCAT database reference: T_TAJ353en Related approach: Compiled by: Giuseppe Bonati, CESVI Date: Contact person: Giuseppe Bonati, CESVI, 84, Hakimzoda street, Dushanbe, Tajikistan; dushanbe@cesvioverseas.org Classification Land use problems: - Due to a lack of drainage in the existing soil, surface flow collects and becomes concentrated leading to top soil washing and gulleying. Animal paths across the slope exacerbate the erosion process. (expert's point of view) In Soviet times the land was not used due to waterlogging, but the land had good potential to plant trees and produce an income. (land user's point of view) Land use Climate Degradation Conservation measure Natural Forests / woodlandsrests / woodlands: Natural (before) Cropland: Tree and shrub cropping (after) semi-arid Soil erosion by water: loss of topsoil / surface erosion, mass movements / landslides Structural: Graded ditches / waterways (to drain and convey water)

2 Stage of intervention Origin Level of technical knowledge Prevention Mitigation / Reduction Rehabilitation Land users initiative: recent (<10 years ago) Experiments / Research Externally introduced Agricultural advisor Land user Main causes of land degradation: Direct causes - Human induced: overgrazing Direct causes - Natural: Heavy / extreme rainfall (intensity/amounts), other natural causes, Topological situation at the base of the watershed means it is susceptible to mud flows and other natural disasters Main technical functions: - control of concentrated runoff: drain / divert - stabilisation of soil (eg by tree roots against land slides) Secondary technical functions: - control of dispersed runoff: impede / retard - control of concentrated runoff: impede / retard Environment Natural Environment Average annual rainfall (mm) Altitude (m a.s.l.) Landform Slope (%) > 4000 mm mm mm mm mm mm mm mm < 250 mm > <100 plateau / plains ridges mountain slopes hill slopes footslopes valley floors flat gentle moderate rolling hilly steep very steep Soil depth (cm) >120 Growing season(s): 180 days(april - October) Soil texture: medium (loam) Soil fertility: high Topsoil organic matter: medium (1-3%) Soil drainage/infiltration: poor (eg sealing /crusting) Soil water storage capacity: low Ground water table: < 5 m Availability of surface water: good, medium Water quality: poor drinking water Biodiversity: medium Tolerant of climatic extremes: temperature increase, seasonal rainfall increase, seasonal rainfall decrease Sensitive to climatic extremes: heavy rainfall events (intensities and amount), floods, droughts / dry spells Human Environment Forests / woodlands per household (ha) < ,000 1,000-10,000 >10,000 Land user: groups / community, Small scale land users, common / average land users, mainly men Population density: persons/km2 Annual population growth: 1% - 2% Land ownership: state Land use rights: communal (organised) Water use rights: open access (unorganised) (Water user right refer to the water running over the land) Importance of off-farm income: less than 10% of all income: Access to service and infrastructure: low: health, technical assistance, employment (eg off-farm), market, energy, drinking water and sanitation, financial services; moderate: education, roads & transport Market orientation: subsistence (self-supply) Purpose of forest / woodland use: fruits and nuts

3 Technical drawing The drawing shows the layout of the trench and the lining with fast growing native trees. (BONATI GIUSEPPE) Implementation activities, inputs and costs Establishment activities - - Tree planting - Construction of the drainage ditches Establishment inputs and costs per ha Inputs Costs (US$) % met by land user Labour % Equipment - tools % Construction material - stone % Agricultural - seeds % TOTAL % Maintenance/recurrent activities - tree maintenance - clearance of ditches Maintenance/recurrent inputs and costs per ha per year Inputs Costs (US$) % met by land user Labour % TOTAL % Remarks: The cost of an 80m ditch is mainly determined by labour and seedlings. Labour can be provided by the land owner and seedlings for willows and poplars can be found locally and therefore are of no cost. The stone may occur naturally but there may be transport costs. The costs are calculated for an 80m trench based on 2010 prices in rural Tajikistan. Assessment

4 Impacts of the Technology Production and socio-economic benefits increased wood production increased irrigation water availability quality increased production area reduced risk of natural disasters increased crop yield increased farm income diversification of income sources Socio-cultural benefits Production and socio-economic disadvantages Socio-cultural disadvantages improved conservation / erosion knowledge Ecological benefits improved excess water drainage reduced hazard towards adverse events reduced soil loss improved harvesting / collection of water reduced surface runoff recharge of groundwater table / aquifer improved soil cover Off-site benefits Ecological disadvantages Off-site disadvantages improved buffering / filtering capacity reduced damage on neighbours fields movement of debris to adjacent land Contribution to human well-being / livelihoods Benefits /costs according to land user Benefits compared with costs short-term: long-term: Establishment slightly positive very positive Maintenance / recurrent positive very positive Acceptance / adoption: 100% of land user families (1 families; 100% of area) have implemented the technology voluntary. There is no trend towards (growing) spontaneous adoption of the technology. Concluding statements Strengths and how to sustain/improve Weaknesses and how to overcome It is a basic and easy technology that could be easily replicated, The trench may be inefffective if there is a heavy deluge of over larger areas. Further training on the correct angles to surface water, and may in fact concentrate the surface water build channels and dimensions run off. The trenches could be filled with gravel and rocks, and intertwined in a grid network, this is a common practice for The technogy is low cost and potentially quick to build. It railway embankements in Europe. The slope has to be shallow can be built on many different slopes types, angles and heights. enough to prevent the gravel material being washed away. It allows the land to be used for growing fruit trees and timber. There is the opportunity to sell the products. Training could be provided on tree cultivation. It is easy to build and cheap.

5 Copyright (c) WOCAT (2017)