6.12b Belilo Spate Irrigation System Discussion Case on Sedimentation

History Earlier only perennial irrigation – dried up Weir crest increased – now spate irrigation Serves 500 ha = approximately 750 families Sedimentation problems

Sedimentation problems

Headworks Intake on the right

Headworks Intake on the right

Intake channel Intake with stoplog Some sediment Washing in No from the sides No undersluice Intake channel filled with sediment

Gate – including stoplog - at the intake

Fine sediment in the river Stilling basin filled with sediment

View immediately upstream of the weir Main river course Side flow

Analysis The intake may be located on the inside of a shallow bend- not recommended as this will enhance the sediment loads diverted to the canal. The weir crest is still above the river bed, so presumably diversion of low flows is possible. However depending on the river slope the upstream bed level can be expected to rise, and if (when) this occurs the intake location will make diversion of low flows more difficult. The intake is aligned at 90 degrees to the river channel, this will enhance the coarse sediment load diverted to the canal and a far shallower angle is preferred. (I.E. the existing intake is probably not optimally designed )

Solution? Undersluice on intake channel before the gate Conical abuttment (as in Yemen) and soil bund guiding flow to intake channel – increasing slope and providing safety valve

Expert opinion 1) Assume you are moving the intake upstream to gain command, enabling the canal slope to be increased? If so then a more favourable location at the outside of river bend may be possible. 2) Increasing the canal slope, particularly of the lined section, will reduce the quantities of sediment settling in the head reach, but of course slopes have to be increased through the system if the coarser sediments are to moved down to fields, ( where they are probably not wanted by farmers.) If exclusion cannot be improved then regular de silting at a milder slope section away from the intake could be considered? 3) You are advocating replacing the fixed weir with a soil bund. If the objective is to reduce sedimentation then the bund should wash out at high river flows. Are farmers prepared to replace the bund, perhaps several times a year? In this case an under sluice will provide little benefit as the bund will fail when the high river flows occur, and this is the only times when an under sluice would be operated. 4) If on the other hand you intend to bund to fail only when very large (historical) floods occur then a properly designed under sluice “fixed” to a hard point (the conical pier) could proved some benefits, (if operated) but it would be essential to limit flows entering the canal with an orifice/rejection spillway to prevent excessive flows being diverted.