Discharge diversions and returns

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Discharge diversions and returns

02. Flow regulations

General description

Removal and downstream return of water from the river through a man-made diversion structure called a bypass often results in significant flow reduction in the intervening section of the river’s course. This is a typical pressure that affects rivers used for hydropower, whereby flow is diverted from the river by a weir at higher altitude and conducted through a near horizontal bypass channel into turbines that are located downstream at a much lower altitude. A similar pressure occurs in association with irrigation of farmlands located in the floodplain and near the river margins, but in this case the return flows are greatly reduced by plant water consumption, evaporation and infiltration, and may also suffer from a reduction in water quality. Diversion also takes place to supply urban areas and industries with water, and in these cases the return flow is affected by significant reductions in both water quality and quantity. Flood diversion is a special case of flow diversion and return that is designed to alleviate flooding.

Effect/Impact on (including literature citations)

Although in temperate regions water abstractions may have relatively minor impacts, in Mediterranean countries they can represent major alterations with the potential to turn perennial rivers into intermittent rivers and to severely degrade physico-chemical conditions, if base flow becomes limited in relation to emissions or discharge of effluents (Prat & Munné, 2000; Menció et al., 2010). A review of low flow river conditions during dry periods of the year, as well as the problem of changing minimum river flows as a consequence of climate variability is presented by Smakhtin (2001).

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Menció, A., Folch, A., Mas-Pla, J., 2010. Analyzing Hydrological Sustainability Through Water Balance. Environmental Management 45: 1175–1190.

Prat, N., A. Munne, 2000. Water use and quality and stream flow in a Mediterranean stream. Water Research 34: 3876–3881.

Smakhtin, V.U., 2001. Low flow hydrology: a review. Journal of Hydrology 240: 147– 186.

Other relevant information