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6.3 Sediment management options Phil Lawrence
Summary sediment management strategy Limit the diversion of coarser sediments Transport fine sediments through canals to the fields Make provision for the inevitable rise in command levels
Limiting the diversion of coarser sediments: four options 1. Locate intakes at outside of bends 2. Sediment excluding intakes 3. Limit diversion when wadi flows high – throttling structures or close gates 4. Secondary sediment control
(1) Location of intakes at bends The low flow channel carrying flood recession flows forms at the outside of a wadi bend. (Traditional intakes are placed at the outside of wadi bends for this reason.) In floods bed load sweep will move the largest sediments towards the inside of a bend and away from an intake.
Bed load sweep at a channel bend
Traditional intake showing location at the outside of a wadi bend
Limiting the diversion of coarser sediments Locate intakes at outside of bends (2) Sediment excluding intakes Limit diversion when wadi flows high – throttling structures or close gates Secondary sediment control
Example of a spate sediment excluding intake
Features of the spate intake No divide wall, flows can approach from any direction including parallel to the weir Intake aligned to minimise the diversion angle Curved channel with floor set lower than the intake gate sill encourages coarser sediments to move through the sluiceway In this case a fuse plug was used
Limitations of sediment excluding intakes in spate schemes Spate intakes divert all the wadi flows except for the short periods, sometimes only minutes, during flood peaks when wadi flows exceed the intake capacity. Sediment exclusion only effective during these periods. Sluice gates have to be operated in response to rapidly varying spate flows – mechanised gates desirable but not often affordable. It is not unusual for farmers to block the sluice gate as they do not want to loose water – in some systems however the downstream farmers take their water from the sluice gate and this makes the sluice gate operated
Downstream area irrigated from scour sluice
Limiting the diversion of coarser sediments Locate intakes at outside of bends Sediment excluding intakes (3) Limit diversion when wadi flows high – throttling structures or close gates Secondary sediment control
Limit diversion from flood peaks For simple un-gated intakes use flow throttling structures with a rejection spillway to limit the flows entering a canal. For gated intakes consider closing canal gates during short periods of high flow. (There are problems of responding to rapidly varying flows, and farmers reluctance to waste water) Flow throttling structures with a rejection spillway are also used with gated intakes to ensure that canals are not damaged by excessive flows if the gates are left open during very large floods.
Limitation to throttling structures May not be like by farmers – too much of the flow is lost May attract trash and become clogged
Limiting the diversion of coarser sediments Locate intakes at outside of bends Sediment excluding intakes Limit diversion when wadi flows high –throttling structures or close gates (4) Secondary sediment control
Secondary sediment control Settling basins Canal sediment extractors
Wadi Mawr settling basins
Models are used design settling basins/gravel traps Model predictions include: Variation in sediment concentrations and grain sizes passing through a basin it fills with sediment. Estimates of the frequency of sediment sluicing or de-silting operations. The time period required to flush the basin and the volume of water needed for flushing. The dimensions of an escape channel to convey sediment flushed from a basin to the river or disposal point.
Minimising trapping fine sediments A disadvantage of settling basins in spate schemes is their high trap efficiency for fine sediments at low flows or when basins are empty. To minimise the trap efficiency for fine sediments: Basins should be relatively narrow, with sediment storage obtained by increasing the length, rather than the width or depth of the basin. If it is considered necessary substantial reductions in the trap efficiency for fine sediments can be made if the tail water level in the basin is lowered for very low basin discharges. One possibility is to provide a notched weir at the basin exit, so that tail water levels are substantially lowered when the basin discharge is very low.
Secondary sediment control for spate schemes Settling basins – Mechanically excavated or flushed basins can provide high sediment trap efficiencies with a low, or in the case of mechanically excavated basin, zero, wastage of water for sediment flushing. But sediment trap efficiency varies as a basin fills, and also with the basin discharge which varies from zero to full supply discharge in spate schemes. Canal sediment extractors – Trap coarse sediment with a relatively constant trap efficiency but require continuous flushing flows of between 10% and 15 % of the canal discharge. Conventional extractors not suitable for use in spate schemes.
Limitations to secondary sedimentation control Not easy to get it right – i.e. catching the coarse and not the fine sediment Requires space, esp. if sediment loads are high – this space may not be there Cleaning requires adequate organization In case of flushing systems, farmers may object on water being used for this