1.6 Experience and challenges of spate irrigation projects in Tigray. (By Demisew Abate)

Transcription

1 1.6 Experience and challenges of spate irrigation projects in Tigray (By Demisew Abate)

2 Part I : Introduction What is spate irrigation? o Spate irrigation is a flood harvesting and management system, involving the diversion of flowing flood using some deflecting technologies (using simple deflectors of bunds constructed from earth, sand, stones, brushwood and recently gabions, masonry or concrete structures) on the beds of normally dry creeks or river channels in to a farmland. Spate irrigation is a type of river basin water management that is unique to the semi arid environment. Here water is harvested from river basins or large parts thereof by diverting water from ephemeral rivers/streams.

3 Introduction cont d Flood flows usually flowing for only a few hours with appreciable discharges, and with recession flows lasting for one to a few days, are channeled through short steep canals to bunded basins, which are flooded to depths of 0.5 m or more Subsistence crops, often sorghum, are planted only after irrigation has occurred Crops are grown from one or more irrigation using residual moisture stored in the deep alluvial soils formed from sediments deposited from previous irrigations

4 Limitations Some basic features of spate irrigation It requires high level of cooperation between farmers to divert and manage the distribution of flood flows. It requires very high labor input to maintain intakes, canals and field systems It is a subsistence activity with low returns, generating highly variable incomes between good and bad years

5 Limitations It is uncertain due to the unpredictable timing, frequency, duration and volume of floods the occasional very large floods that wash out diversion structures and frequent change to the stream channels at the diversion site At present little guidance concerning the improvement or development of spate systems is available, which makes the implementation of improved systems to be liable to some risks

6 Part II. Spate irrigation experiences in Tigray 2.1 Traditional spate irrigation Traditional spate irrigation has been experienced in the southern zone of Tigray (Raya Azebo and Alamata weredas) since long time ago. They construct simple deflector bunds out of sand, stones, earth, brushwood/tree branches, and other locally available materials to divert floods from different reaches of the ephemeral stream beds, convey it through earthen canals and spread over their fields.

7 Traditional Spate Advantages of traditional schemes Flexible and adjustable to changes in river bed topography & flow alignment Ease of operation and maintenance Equitable water use/water rights Appropriate & low cost Relatively efficient Limit diversion of high flows & high sediment loads

8 Traditional Spate Limitations Frequent Wash out by floods - high labor input Time consuming Diversion of flood not sustainable Erosion of canal intakes It encourage deforestation Wash out of traditional bunds and spurs after a flood (Raya area)

9 2.2 Modern Spate irrigation The regional government has been made efforts to improve or modernize the traditional spate irrigation systems in the Raya Valley area since the last decade. As a result, TWRME has implemented more than13 modern spate irrigation schemes so far ; among them

10 S. No Scheme Woreda Command Remarks 1 Hara Alamata 400 * 2 Tirki -do- 380 * 3 Fokisa -do- 250 constructed 4 Ula-ula -do do- 5 Tengago -do do- 6 Dayu -do do- 7 Burka R/ Azebo 280 -do- 8 Beyru -do do- 9 Haya -do do- 10 Buffe -do do-

11 Modern Spate irrigation o Modern spate irrigation schemes have been implemented since 1995 E.C in Alamata and Raya Azebo woredas o In the 1998 E.C rainy season, an evaluation of the performance of the schemes has been carried out o The evaluation study shows that some schemes are performing well and others have major physical and social problems that resulted in low performances

12 Modern Spate irrigation Limitations Failure of diversion head works to divert the required volume of flow due to: blockage of under sluices/off-takes by boulders, sediments and trash Erosion of downstream protection works mal-functioning of under sluice gates by high floods and sediment/boulder abrasion

13 Modern Spate irrigation Limitations Sedimentation and/or erosion of conveyance canals & farm lands Failure to divert the required volume of flood (low diversion efficiency) O & M complexities or poor O&M practices of farmers Social impacts, inequity of flood distribution between u/s and d/s users, not satisfying farmers preferences

14 Modern Spate irrigation Advantages of Modern Spate Irrigations The replacement of numbers of existing traditional diversion structures by a single permanent modernized diversion structure eliminates the need of farmers to rebuild diversions after floods It discourages deforestation as compared to the traditional once Effective and durable

15 Example of a site with observed problems Scheme name : Tirke spate irrigation Main problems: Erosion of d/s scour protection works (insufficient scour depth provided) Mechanical damage of Under sluice pillars by the impact of big boulders Silting up and/or blockage of closed pipe inlets High sediment accumulation in canal head reaches Lacks proper O&M by farmers Under sizing of canal inlets Wide & angular under sluice piers, that catch trash & boulders during floods All these problems lead to low diversion efficiency and mal-functioning of the scheme during the time of evaluation

16 Tirke spate irrigation o Farmers were consulted to express their ideas & they indicated that they would prefer the improved spate to have: An off-take or canal inlet with a deflection angle of less than 60 degrees for smooth diversion of flood A wide off-take (more than the existing 2.0 m) so as to divert more flood flow Open and un-gated inlets instead of the existing gated and closed pipe inlets for ease of silt removal No under sluices so as to avoid any loss of water downstream through leakage

17 Factors that contributed to the problems at Tirke scheme Problems related to design- no standard guidelines available so far Implementation approach with no active participation of farmers Poor construction quality No or poor operation & maintenance by farmers Absence of strong farmers organizations

18 Example of a spate irrigation with good performance Name of scheme: Fokissa spate irrigation o Efforts were made to incorporate some improvements following the experiences gained during the operation of previously implemented schemes and consultation of the farmers from same schemes

19 Fokissa spate irrigation The improvements include: o A relatively wider offtake (3.0 m by 1.0 m size) Off-take aligned at an angle of 60 degrees Un-gated, open offtake structure Good management by farmers, especially in regard to water distribution (equity of water rights)

20 Fokissa Fokissa spate irrigation spate cont As a result this scheme has been operating in a good condition although the following limitations were observed: Frequent sediment deposition of up to a depth of 70 cm in the upper reaches of the canal due to lack of proper operation of the de-silting basins at the canal head reaches & mild canal bed slopes

21 Fokisa spate Main canal

22 Fokisa spate irrigation area

23 3.0 Main lessens learned The planning & design of the rehabilitation and improvement works have been carried out with out effective participation of the farmers Farmers valuable knowledge regarding their preferences on the scope & type of works, changes in layout, location of diversion structure, type, alignment & size of off-takes, etc were not considered The investment costs have been very high and it is doubtful if they can be justified in purely economic terms

24 Main lessens The O&M of the larger schemes is difficult & expensive In particular sedimentation at intakes and in canals is often not properly controlled in modernized spate systems New structures have promoted larger inequity in the distribution of irrigation water due to the collapse of traditional water rights Modernized diversion structures give much larger control over spate flows to favoured groups of upstream farmers

25 Main lessens Smaller farmer managed schemes, where small improvements have been introduced to improve the reliability of existing traditional intakes, or improvements to intakes constructed using masonry or gabions, have been generally far more successful and cost effective

26 Main lessens As much of the available surface water is already effectively used, the major benefits from any investment must derive from increased productivity of water use Therefore, the emphasis in the development of spate irrigations should be focused first on the: Improvement of the reliability of existing systems of water diversion, allocation, distribution, water management & conserving soil moisture, within the framework of existing water rights and O&M practices

27 4.0 Recommended principles o The success of any intervention will largely depend on incorporating the following principles in the development approach In all but the largest and more technically complex schemes, farmers should drive the planning, design & execution of the rehabilitation and/or improvement works, as well as any amendment to existing water rights Engineers need to provide a range of technically & economically viable options, and then assist farmers in selecting the most appropriate improvements for their particular scheme

28 Recommended principles In most cases, low-cost, simple and maintenancefriendly technology should be used The replacement of numbers of existing traditional diversion structures by a single permanent modernized diversion structure eliminates the need of farmers to rebuild diversions after floods However there is a strong probability that concentrating diversion at one location will result in conflict between u/s & d/s farmers due to inequitable distribution of available spate water

29 Recommended principles.. It is suggested that this approach should only be adopted when: Downstream water users are not disadvantaged The sedimentation problems can be managed Appropriate & sustainable maintenance can be assured

30 5.0 Challenges (problem faced) It was believed that the rehabilitation of the traditional spate irrigation system by designing a proper diversion and conveyance structures would thereby increase the efficiency of traditional spate irrigation system. However it was found that designing, constructing and managing these projects was not an easy task.

31 Challenges Hence, This presentation will be winding up on the issues most challenging with design of spate irrigation and expect from the training a better approach and solutions by addressing and appreciating the most serious problems under listed

32 Challenges 1) The amount of runoff that has to be diverted (flood diversion ratio.doc) 2) Siltation problems ( off takes, canals and piped structures) -Estimating sedimentation rates on spate irrigated fields 3) Availability of Standard guidelines (at present little guidance concerning the improvement or development of spate systems is available, which makes the implementation of improved systems to be liable to some risks)

33