Sediment Transport in Wadi Systems Part 1: Overview

Managing sedimentation in spate irrigation schemes Part 1 – Overview (Today) Part 2 – Sediment transport in spate systems Part 3 – Sediment Control structures – features and design methods Part 4 – Review of Sediment Management options + exercise

Part 1 Overview of sedimentation issues in spate irrigation systems

Sediment concentrations carried by some perennial rivers

Compare with sediment concentrations transported by wadis

Typical sediment size range transported in wadis

High sediment concentrations are transported in flash floods

The contribution of high discharges to the total run off varies widely between wadis.

Fine sediments are transported through canals to the fields A major feature - spate systems build their own soils Field rise rates of 5 or more cms/ year observed in some systems, older spate irrigated areas have silt deposits many metres thick. Rising command levels are a feature of spate irrigation systems

Field sediment deposits after one irrigation – Wadi Tuban (Yemen)

Boundary between irrigated on non irrigated land – Wadi Zabid (Yemen)

Summary – features of sediment transport in wadis Very high sediment loads carried by flash floods, but in some wadis an appreciable proportion of the annual run off occurs at relatively low discharges in flood recessions and seasonal base flows. Total load sediment concentrations exceeding 10 percent by weight are transported by floods in some wadis. Wadi bed materials can range from boulders and cobbles to silts, but sediment transport is dominated by the finer sediment fractions, silts clays and fine sand transported in suspension. Large sediments, coarse sand, gravel, cobbles, and small boulders, typically represent only 5 percent or less of the annual sediment load. Very large quantities of fine silts delivered the fields

Sediment management in spate systems Silt wanted by farmers on the fields (fertility) But larger sediments transported at high wadi discharges flows will, if diverted, settle and block canals Ideally all fine sediments (silts) will be transported to the fields, while large sediments, coarse sand and larger, should be excluded from canals.

Traditional spate systems Intakes are washed away by large floods, preventing the ingress of the coarse sediments carried by high wadi flows.

Traditional Intake – Yemen

Traditional intake – Pakistan

Sediment management in traditional systems Traditional spate canals are very steep compared with conventionally designed perennial canals, flow at high velocities, and have a very high sediment transporting capacity for fine sand and silts.

Sediment management in traditional systems Usually all the flow in a canal is diverted to at a single point and then flows from field to field. A high sediment transporting capacity is maintained right through to the fields and flows are not headed up at water control structures.

Sediment management in traditional systems When command starts to be lost intakes can easily be moved further upstream, by extending a diversion spur and or moving the canal intake.

Summary of sediment management features in traditional systems Diversion spurs are washed away by large floods, preventing diversion of high concentrations of coarse sediments. (As multiple intakes are used diversion from floods usually still possible at downstream intakes) Diversion bunds spanning a wadi are only used in lower reaches of wadis where the coarse sediments have settled on the wadi bed and only fine sediments are transported. Canals are very steep compared with canals in conventional perennial irrigation schemes – providing a high sediment transporting capacity. Usually all the flow in a canal is diverted fields at a single point, a high sediment transporting capacity is maintained from the wadi through to the fields. When command starts to be lost by rising field levels intakes can easily be moved further upstream.

Sediment problems in modernised spate systems Permanent diversion structures enable much larger discharges to be diverted from spate flows. Diversion from wadi flows carrying very high sediment loads is possible. Sediment sluices, canal closures in flood peaks and sediment exclusion/extraction facilities are used to reduce the loads of coarser sediment fractions entering canals.

Wadi Rima Intake (Yemen)

Canal design In the first systems that were modernised lower canal slopes than observed in traditional systems were provided. Limited sediment transporting capacity in canals = severe canal sedimentation problems = high/unaffordable maintenance requirements.

Desilting a canal head reach

Water distribution In some early schemes water distribution systems similar to those used in perennial schemes were adopted where water is supplied to numerous field outlets at the same time. Farmers then head up flows at undersized outlets, promoting canal sedimentation.

Farmers check structure

Silted field outlet

Silted undersized crossing structure

Timely operation of manual sluice and intake gates in spate flows difficult or impossible Water levels change very rapidly at flood peak Operators do not how big a flood peak is going to be Multi peak spates common Farmers resist ”wasting” water by operating scour slices etc.

Manually operated gate

Rising command levels Adequate provision for rising command levels must be made when permanent diversion structures with reasonably long anticipated design lifetime are adopted. In Wadi Zabid (Yemen) the upstream weirs have been raised twice since the early 1980’s

Summary sediment management strategy for modernised schemes Limit the diversion of coarser sediments by appropriate intake sitting and design, use of scour sluices and, where feasible provide sediment control structures Transport fine sediments through canals to the fields, (steep canals, maintain high flow rates to fields, don’t head up flows at water control structures.) Make provision for the inevitable rise in command levels Anticipate and quantify the need for canal de-silting and plan for it

Exercise – differences between spate and conventional irrigation practice Make a list of, and discuss the key differences between spate and conventional irrigation, focussing on:  Sediment loads  Water availability  Canals and water control structure structures  Sediment management  Command issues  “Ownership” and O & M

Sediment loads Spate irrigationPerennial / seasonal irrigation Sediment concentrations up too and exceeding 10% by weight Sediment concentrations rarely exceed 0.2 % by weight

Water availability Spate irrigationPerennial / seasonal irrigation Spate floods with highly variable and unpredictable timing number and magnitude. In some cases supplemented with low seasonal base flows Reasonably predictable seasonal flows allowing continuous diversion at controlled flow rates.

Canals and water control structure structures Spate irrigationPerennial / seasonal irrigation Steep canals operating a high flow velocities Flows supplied to very limited number of field off- takes (only 1 at a time in most traditional systems) Low sediment transporting capacity provided by conventional canal design methods Water supplied to numerous outlets simultaneously

Sediment management Spate irrigationPerennial / seasonal irrigation Many sediment management options, particularly those involving sediment flushing, not appropriate for spate schemes Range of tried and tested sediment management options available.

Command issues Spate irrigationPerennial / seasonal irrigation Command levels in upstream areas that receive most water and the largest sediment loads may be rising by 5 cms./year or more Command levels usually fixed over the design life of diversion and water control structures.

“Ownership” and O & M Spate irrigationPerennial / seasonal irrigation Traditional systems are farmer constructed operated and managed. Some spate irrigation systems are the largest farmer managed systems in the world. Customary water rights and arrangements and obligations for O and M well developed. Larger systems often agency designed operated and maintained, with turnover to artificially created farmer groups at some point within the water distribution system.

More tomorrow !