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

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.

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

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

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

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.

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

Limitations• Frequent Wash out by

floods - high labor input• Time consuming• Diversion of flood not

sustainable• Erosion of canal intakes• It encourage

deforestation

Traditional Spate…Wash out of traditional bunds and Wash out of traditional bunds and spurs after a flood (Raya area)spurs after a flood (Raya area)

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

S. No

Scheme Woreda Command Remarks

1 Hara Alamata 400 *

2 Tirki -do- 380 *

3 Fokisa -do- 250 constructed

4 Ula-ula -do- 250 -do-

5 Tengago -do- 250 -do-

6 Dayu -do- 200 -do-

7 Burka R/ Azebo 280 -do-

8 Beyru -do- 500 -do-

9 Haya -do- 250 -do-

10 Buffe -do- 200 -do-

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

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

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

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

Modern Spate irrigation…Modern Spate irrigation…

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

Example of a site with observed problems

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

Tirke spate irrigation…

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

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

The improvements include:o A relatively wider off-

take (3.0 m by 1.0 m size)• Off-take aligned at an

angle of 60 degrees• Un-gated, open off-

take structure• Good management by

farmers, especially in regard to water distribution (equity of water rights)

Fokissa spate irrigation

Fokissa spate irrigation 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

Fokissa spate cont…Fokissa spate cont…

Fokisa spate Main canal

Fokisa spate irrigation area

3.0 Main lessens learned3.0 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

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

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

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

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

Recommended principles In most cases, low-cost, simple and maintenance-

friendly 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

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

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.

5.0 Challenges (problem faced)

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

Challenges …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)

Challenges …Challenges …