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Spreading floodwaters in Iran and checking salinity

Submitted by Nitya Jacob 29th November 2022 6:50
Semi arid part of Iran


People in arid areas around the world innovate with water conservation, especially rainwater harvesting. One such is the Fars Province in Iran, a desert landscape. Since 1983, an extraordinary programme of floodwater harvesting has been implemented that has turned area lush green. This floodwater spreading programme has turned silt-laden floods into an asset. It has been combined with upstream water management to control the salinity in the Helleh River.

The programme has used water and sediment from occasional floods in this arid environment to recharge groundwater by spreading floodwater gently over a large area, develop lands for spate irrigation – using the silt to build up soil and prepare land for direct irrigation and introduce integrated farming systems of field crops, tree crops, honey bees and livestock. Additionally, plantations of Eucalyptus camaldulensis have been developed on the newly-formed lands to function as windbreaks and shelterbelts, sequester carbon, produce honey, and serve the urban markets with timber.

This programme has turned a threat into an opportunity. Flood waters carry up to 5% by volume of sediment that can choke conventional dams and water infrastructure. Instead, by spreading and slowing the water flow, the silt is deposited and becomes an asset by building up fertile land in a sandy desert that is under the constant threat of wind erosion. The floodwater is also used to recharge groundwater and irrigation.

A combination of conveyance spreading canals and level-silled channels (LSCs) are used to achieve the effect of spreading water and sediment over a large area. The simplest version of this is a single, level-silled channel that allows floodwater and sediment to spread gently over the debris cone and alluvial fan. A full floodwater spreading system has a diversion weir, conveyance canal, wasteway, conveyance-spreader channel (CSC), many number LSCs, flood outlets (sometimes with masonry drops), trail dikes and a tail drain. If the system also functions for artificial recharge, an infiltration pond is added to the end.

LSCs are the main feature in floodwater spreading systems. Visualise these as long stilling basins, closed at both ends, with the down slope edge following the contours of the land. It converts small, concentrated flows into sheet flows. The control section of the LSC is a level sill adjacent to its down slope edge, which allows the floodwater to spread and allows the sediment to settle. The soil excavated from the channel forms the bank immediately on its upstream side. Water enters the LSC through the gaps in the bank at 100 to 400 m intervals. The water loses most of its kinetic energy after entering the basin. When the channel is filled up, the overflow spills along the entire length of the sill as a thin sheet. LSCs are not intended to impound but slow and spread both water and sediment.

The size of the floodwater systems depends on the expected normal flood levels and the use of the floodwater. There are no calculations for determining the layout and space between consecutive channels, but two criteria are important. Firstly, the flowing water should not gain erosive velocity. Secondly, water should be distributed evenly over the space between the channels.

The other component, conveyance spreader channels (CSCs) are larger than LSCs and can be many kilometres long. The main function of the conveyance spreader channel is to convert concentrated flows from the upland to sheet flow. CSCs receive fast-flowing floodwater with heavy sediment loads that can hinder the working of the spreading channels. Sometimes earth or rock buffers are used to slow the water down before it enters the CSCs. The structure of CSCs is similar to LSCs except they have a shallower slope, have a larger cross-section and are straight.

Knowing what to do where

This technique of water harvesting requires skilled management. The method does not quite follow conventional wisdom of ‘catch the water where it falls’, as some catchments produce floods rather than recharge. Professor Sayyed Ahang Kowsar, the lead scientist of the artificial recharge programme said, ‘Basins with impermeable outcrops are different from ones with permeable surfaces. Iran can consider itself fortunate to be blessed with flood-producing impermeable watersheds. Without this it would be impossible to live in such dry environments. The natural recharge for the alluvial aquifers materializes only by floods. The diffuse recharge is utterly insignificant.’

For example, the Helleh River has a drainage basin of 8,600 sq km, making it the second largest river in the Fars and Bushehr provinces. The discharge of the Shapur River, the most important tributary of the Helleh, however, is saline that limits the use of the Helleh River water for irrigation. The source of the Shapur River is supplied by springs and is fresh but salinity builds up as a number of tributaries join the river. There were plans to divert these tributaries through pipelines to the Persian Gulf but these were too expensive to implement. Similarly, the idea of constructing a large reservoir on the Helleh River to store floodwaters and dilute the saline inflows was rejected; the reservoir would silt up rapidly and there would be high evaporation losses.

A better way to reduce salinity is the careful management of the water flows in the Shapur River to increase its baseflow and eliminate the saline ingress. A large part of the Shapur River’s catchment is a 770 sq km area upstream of the Tchegan Gorge. Intensive recharge here would increase the freshwater baseflow of the Shapur River. This could be achieved through floodwater management some distance from the river’s channel, to ensure that the subsurface flow reached the Shapur River when it is most needed. For example, if the recharge occured in December and the irrigation season started in April, then the distance should be such that the stored water should not reach the Shapur River’s baseflow before April. This required an understanding of the hydrogeology of the region.

This method could be used for harnessing floodwater in other tributaries. Additionally, in semi-arid, hot areas, it is cost-effective to store water in shallow aquifers than in surface reservoirs.

Smart water management would be able to check the saline outflows if the saline springs in the Jareh and Dalaki tributaries were diverted to evaporation ponds to prevent them from joining the Shapur River. In addition to the evaporation ponds closure of the saline outflows may be considered. The Shekastian Drainage Basin is covered by an impermeable formation and the saline discharge of the rivers is probably from underground karstic streams flowing through local faults and dissolving salt plugs. The streams could be intercepted before reaching the salt plugs.

(Excerpted from Transforming landscapes, transforming lives : the business of sustainable water buffer management).