Intakes and Diversion Strucures Designing a Spate System Lesson 3 IanMcAnderson@aol.com Designing a Spate System - Diversion structures & intakes; - Spate canals - water control & dividing structures; - Bank protection - Wadi training structures. Traditional diversion and water distribution structures divert ephemeral rivers using only local materials and indigenous skills relatively high overall water diversion efficiency the high labour inputs needed to re-build the structures environmental problems resulting from unsustainable use of trees and brushwood What we have learnt For engineering successful interventions: – replicate as far as possible traditional diversion practices – reflect time commitments and technical knowledge of the farmers – facilitate the control of large flood – replicate water distribution in line with accepted rules and rights What we have learnt ensure a right balance between the needs of different water uses and users – improve the effectiveness of the systems to function with high rates of sediment transport – improve the ability to cope with frequent and large changes to the levels and alignments – rehabilitation and improvement works Farmers should be consulted and involved in the planning, design and execution Engineers provide a range of technically and economically viable options Comprehend the long years of experience that the farmers have of the systems and Wadi flow, Assisting farmers in selecting the most appropriate improvements that replicate and improve upon traditional schemes Disappointing performance attributed to: An increased inequity of water distribution Command and diversion problems due to high rates of sediment deposition reduced the WUAs/farmers’ role in diverting and distributing water unrealistic assumptions concerning levels and costs of operation and maintenance failures to achieve an expected increase in irrigated area Understanding each other Ensure what you have in mind is what the farmers envisage Compromise “perfect” engineering solutions with levels of risk acceptable to farmers What we have learnt ensure a right balance between the needs of different water uses and users – improve the effectiveness of the systems to function with high rates of sediment transport – improve the ability to cope with frequent and large changes to the levels and alignments – What we have learnt Do not upset balance of water distribution and inadvertently favour the better off at the tops of the systems Understand water rights and implications Traditional Local resources Large bund diverting all flow Traditional Diverting only part of flood flow Across wide Wadi Alongside narrower Wadi to collect only part of flow design of improved spate systems The over riding principle is that there is no single approach Specific requirements vary widely between and in some cases within schemes, Before proposals are finalised, it is essential that engineers fully understand the way in which the farmers system has operated and farmers truly understand and comprehend what the engineers are proposing for them Use local knowledge of floods and how they judge them to design improved structures Diversion structures and intakes Need to be able to divert short duration flood flows into gravity canal systems Ensure that sufficient amounts are abstracted in the time available that the canals systems ensure command over the fields to be irrigated. Diversion structures and intakes Have the capacity to convey the water with minimal sedimentation and scour Some lands will fall out of command or may not be economic to irrigate considering the technically most suitable sites for structures. Secondary and maybe more problematic sites should not be chosen just so that small additional areas can be conveniently added. Structures proposed need to be equipped with intakes that prevent large uncontrolled flows from entering canals only floods that can be contained within the canal capacities are allowed to pass down the system downstream damage to channels and field systems is minimised designed to limit the entry of the very high concentrations of coarse sediments Structures also need to act as erosion control devices in unstable Wadis, characterized by lateral movements of low/medium flow channels within the wider Wadi cross sections, streambank erosion and head-cutting, etc. In the flatter areas, often at the downstream limits of the spate system areas - command over the irrigated lands can be a problem Exacerbated over time with the deposition of silt on the lands, Intakes must also function to ensure that sufficient head is maintained over the longer term Cope with rising irrigation command levels at the field Traditional Intakes Advantages: – Flexibility • – Appropriate and low cost – Relatively efficient in water use and sharing between users – Restrict diversion of high flows with high sediment loads Traditional intakes fall into two main types spur deflection Wadi Deflecting Spur Wadi To fields bund diversion Bund A A Wadi A-A To Fields 2-5 m 5-7m Common Features Located at outside or just downstream from relatively mild Wadi bends Consist of low spurs flows extending at a slight angle out into the Wadi deep water channel is scoured in floods where lower flows are channelled during flood recessions; to intercept the low flow channel, diverting lower flows to an un-gated canal; Constructed from locally available materials damaged by larger flood flows – equitable use of flood flows can be maintained and re-constructed by farmers without or with limited significant external support (bulldozers are often available to farmers); Common Features Recognize force and damage caused by large and very large flood flows designed to breach or break when they occur, thereby reducing danger to the spate irrigation system; Are not “greedy” and do not try to extract all of the flow Designed to “coax” the flows into the intake taking as much as they dare without endangering the whole system. While the different forms of construction result in varying degrees of durability, mainly due to available labour and local materials, all are likely to be damaged or completely swept away by larger floods. Advantages of traditional intakes: Flexibility: – – Location and layout of traditional intakes easily adjusted to suit the changing Wadi bed condition; Deflecting spurs can be extended and diversion bunds moved upstream when sedimentation on the fields or in the canals starts to take fields out of command, or stream bank erosion occurs. Appropriate and low cost: – – traditional intakes are constructed from local materials using indigenous skills can be maintained indefinitely by farmers without outside support, apart from environmental problems resulting from unsustainable use of trees and brushwood. Relatively efficient in water use and sharing between users: Many traditional intakes are used along larger Wadis, each serving their own separate command area Can achieve high overall diversion efficiency and better water equality Large spate floods will destroy intakes located at the heads of each spate command area as they pass down a Wadi, with those upstream intakes breaking sooner than those further downstream. Once the flood peak (or rising limb of the flood hydrograph) has destroyed traditional diversion bunds at one location, the flow passes to the next one further downstream and so on, sharing water between many offtakes and not permitting the upstream site to “steal” all of the flow. Lower sites experience less severe floods, exposed for longer periods, thereby compensating for water not received from other smaller floods that only reach the upper intakes. Restrict diversion of high flows with high sediment loads: Failure of deflecting spurs, diversion bunds and breach sections of main canal at high Wadi discharges abruptly lower the water level at the canal intakes, reduces discharges that are diverted, limiting the damage to the downstream canals and field systems Preventing the incursion of high concentrations of coarse bed material sediments transported in large floods Important disadvantage associated with traditional diversion structures enormous input of labour needed to maintain and reconstruct intakes that are damaged, or washed out by large floods the continual use of new brushwood and tree material needed to reinforce the bunds. The timing of repairs and breaches means that there is often a shortage of suitable material around the diversion sites Time is water lost so need to rebuild the bunds as soon as possible High recession flood flows often prohibit access to river bed material Deposition of silt and wet area around bund means access even by machinery very difficult Improved diversion structures Raised weir; – Gated scour or under sluice; – Gated canal head regulator; and – Guide or divide wall. divert the maximum possible amount of water capacities per unit area being 10 to 20 times perennial irrigation schemes – Improved Intakes More durable diversion spurs and division groynes; Improved diversion bunds and check weirs; Controlling the flows admitted to canals; Basic gated or orifice control intakes; and Rejection spillways. Intake Capacity a limited number of major diversion structures large new canals that connect into and traverse the existing traditional canal network increase in the inequity between upstream and downstream users’ Intake Capacity conventional economic analyses are insufficient to meet the requirements of the previously commanded areas design duties too low numerous intakes and canals reveals consolidation into one system supplied An overall Water User Association Objective for improvement Make it easy for farmers to operate and maintain reducing the large inputs of labour or other resources to maintain; Prevent large and uncontrolled flood flows from damaging canals and field systems; Help maintaining the distribution of water within the system in line with accepted rules and rights, Providing flexibility to accommodate changing hydrological patterns, future changes in water distribution and cropping pattern Equality of water shares between upstream and downstream users; Ensure an appropriate balance between the needs of different irrigation water uses and other water users (agriculture, drinking water, downstream users, etc); Help maintaining the continuity to function with high rates of sedimentation, with as much as possible being deposited on the fields and not in the canal beds; and Help farmers to cope with frequent and sometimes large changes to intake invert levels and initial reaches of canal alignments due to significant changes in Wadi channels following floods. Diversion Spurs Durable spurs constructed on deep foundations and protected from scour can be successful provided that – – – the designer is sufficiently experienced with designing under spate flow conditions is familiar with the construction material and techniques Aware of process of scour under peak flood flows. Durability of improved forms of traditional intake, in terms of damage suffered and number of times re-construction is necessary in a “normal” spate season, presented below. Durability of traditional and improved gabion diversion spurs in Eritrea (Adapted from Haile, 1999) Number of times reType of Material used for construction is necessary diversion. spur during normal spate season (Average) Traditional Wadi bed material and brushwood Stone Gabion 2-4 0-1 Can last for up to 5 years – need concrete crest protection Intake Capacity Need to convey large volumes of water by gravity to fields during the short periods when Wadi flows occur. Hydrological information on Wadi flows are in almost all cases very limited. The timing, duration and maximum discharge of spate flows are thus unpredictable, Areas traditionally irrigated varying significantly from year to year. Water conveyance and distribution systems developed for perennial irrigation are not appropriate for spate systems. Intake Capacity Much larger capacities are needed (10 to 100 times greater than for perennial systems for a given area) Flow velocities will be higher than conventional canals as the water is heavily laden with sediment and generally pass through much coarser material that have been deposited over the years. Traditional intakes and their modern replacements can both fulfil these above functions, although by different means and with large differences in capital and maintenance costs and requirements Disadvantage of single new intake approach Gives upstream users control over diversion of a larger proportion of the annual flows in turn leads to an increase in the inequity between upstream and downstream users’ access to water. often resulted from the way in which systems are being operated in response to pressures from powerful local interests, rather than from inherent technical design deficiencies or misunderstanding of traditional water distribution arrangements. An equitable distribution of flows would have been possible in some upgraded systems if larger intake capacities had been provided. Disadvantage of single new intake approach - Wadi Mawr, Yemen Complicated system was devised for dividing flows - never used as the farmers did not understand the Engineers, Too theoretical – need to be tailored to farmers education/understanding All WUAs not properly involved from the start. Net result that upstream users control all the water that was able to enter the system, Not sufficient to meet all needs, downstream users in middle and end parts of the command area could not receive sufficient water. Those farmers who could afford it, changed to using groundwater for irrigation Those with less resources farmers have now succeeded in getting donor support to revive the older traditional intakes Reason for Inappropriate Designs Most upgrading and modernisation activities related to intake and diversion improvements - donor supported. Used conventional economic analyses to reach what they consider as cost effective designs Resulted in a diversion capacity for new intakes less than the combined capacity of the traditional intakes. An over riding problem in Wadi Siham in Yemen where all such intakes are insufficient to meet the requirements of the previously commanded areas. Designers did not seem comprehend the traditional means for sizing intakes or if they did, it was not made clear to farmers and local authorities that only part of the previously command and irrigated area would continue to be irrigated under the new intake system. Example of Better Approach In Wadi Zabid, designs adopted comprise a number of separate intakes built from gabions Based on traditional locations and design duties (15 l/s/ha up to about 60 l/s/ha ). The smaller the area, the larger the unit flow Gated intakes Provide a capability to regulate the flow into a canal Can be considered where improved more durable diversions such as weirs are used. Gates should be as wide as possible considering the intake requirements Electrical or hydraulic operation preferred to manual due to the very short time available after the start of flood flows to open and close the gates. If manual operation is considered, high gain gears must be included for fast gate operation. Vertical lift gates wider than 2m are not suitable for manual operation especially in spate systems. Intake gates should be provided with large trash diverters/excluders to trap the very large transported items such as trees, but not restrict flow to the intakes. located upstream from the intakes where possible to guide large debris over the weir sections or around the diversion bunds or spursto ensure that no blockage of the intake and loss of water for the farmers occurs. Access to clean these racks etc. must be included so that machines can be used for regular maintenance. As a safety feature, and assuming that gates may be left open when excessive floods occur, the gated opening must be designed to operate as an orifice as described for enhanced local intakes above. There is a danger that the breast wall is set to low, thereby reducing the actual flows that can enter the intake. Example - Barquqa diversion weir and intake on the Wadi Siham in Yemen, the breast wall was set so that less than the stated design flow (5 m3/sec for 3,700 ha) specific flow (l/s/ha) meant that the served command area had to be reduced to 1,700 ha and an additional new weir (Dabaishia Weir) and new main canal was built Essential that intake flows are related to command areas and downstream main canal capacities. Hydraulic calculations for free flow and submerged flow are needed to cross check the elevation of the breast wall and intake size Operation Ease of Rapid opening and closure Considers local conditions – no power; poor communication; time of floods; etc Canals in Spate Schemes traditional systems are diverted to short, steep canals split flows to reduce flood discharges to manageable flow rates Gates not used; control of flows by proportional dividers/farmer management New/improved canal structures Check and drop structures; Flow splitting structures; Field offtakes; and In-field structures Canals in Spate Schemes Bed slopes of traditional canals in the original (before modernization) Wadi Zabid system in Yemen. Canal Maximum capacity (m3/s) Average bed slope (m/km) Mansury 40 3.8 Rayyan 60 3.7 Bagr 40 3.7 Gerhazi 50 3.9 Mawi 60 4.8 QUESTIONS?