6.1 BASIC ENGINEERING New ‘modernised’ diversion structures have promoted larger inequity in the distribution of irrigation water between upstream and downstream farmers due to the collapse of traditional evolving water rights.

Irrigation infrastructure Three types of engineering structures: Diversion and ephemeral river training structures Spate canals and water diversion structures Field structures

Importance of sedimentation Management of sedimentation: Less important in traditional systems More important in permanent diversion structures: Limit diversion of coarse sediments Cope with future rise in field levels

Diversion and ephemeral river training structures Requirements for successful intakes: Divert low and spate flows Prevent large flood flows Limit high coarse sedimentation concentrations Function over the longer term

Traditional intakes Two basic forms: Diversion spur extending into ephemeral river bed Cross bund across ephemeral river bed

Advantages of traditional diversion structures: Traditional intakes Advantages of traditional diversion structures: Flexibility Appropriate and low cost Relatively efficient Limit diversion of high flows and high sediment loads

Disadvantages of traditional diversion structures: Traditional intakes Disadvantages of traditional diversion structures: Enormous input of labour Not always possible to divert water where needed

Low cost improvements to traditional diversions Improvement options: Use of bulldozers and backhoes Bed sills More durable diversion spurs Improved diversion bunds Controlling the flows admitted to canals Basic gated intakes Rejection spillways

New permanent diversion structures Disadvantage multiple intake improvements: Increase in costs Disadvantage single intake improvement: Increase in inequity between water users

New permanent diversion structures Recommendations: Provide multiple diversion points for systems containing numerous intakes and canals Participation of farmers in design and development Introduction of spate irrigation to new areas on small tributary ephemeral rivers

Bank protection and river training Combination of substantial structures and natural vegetation Controlling river beds: Bunds constructed by bulldozers across the ephemeral river will cause siltation and avoid scour

Spate canals and water diversion structures Objective: divert maximum amount of water to the fields in a short time period

Traditional canals and water control structures Characteristics: No drop structures and steeper slope Unequal distribution of bed sediments Often not well-maintained – ‘everybody’s land’ Several types of water control structures

Improved traditional canals and water control structures Design based on slopes and cross sections of existing traditional canals Check and drop structures: Structure design for maximum canal discharge Expensive gates Upstream off takes allow down flow? Need to raise canal bed? Spill capacity?

Improved traditional canals and water control structures Flow splitting structures: Diversion of flows to manageable discharges Fixed shares?

Field-to-field irrigation Field off-takes Field-to-field irrigation Advantages Quick apply of water No investment in canal system Disadvantages Damage of crops Less water to downstream fields Requirement flow capacity of field off-takes: Sufficient for full incoming canal flow

In-field structures Fields form into series of level terraces: Drop structures between fields prevent erosion

Requirements for successful engineering interventions: Recommendations Requirements for successful engineering interventions: Be easy for farmers to operate Prevent large flood flows Distribute water in line with rules and rights Continue to function with sedimentation Cope with unstable ephemeral river channels