Discharge diversions and returns

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. 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 & Munne, 2000; Mencio & Mas-Pla, 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).

Effect/Impact on (including literature citations)

• HYMO (general and specified per HYMO element)
• physico - chemical parameters
• Biota (general and specified per Biological quality elements)

Case studies where this pressure is present

• Tordera_-_Restoration_of_a_secondary_channel_of_the_Tordera_River
• Blashford_-_Demonstrating_strategic_restoration_and_management_STREAM_(LIFE05_NAT/UK/000143)
• Restoration_and_remeandering_of_the_Müggelspree_-_downstream_Mönchwinkel
• Mörrumsån_-_Hemsjö

Possible restoration, rehabilitation and mitigation measures

• Reduce surface water abstraction with return
• Increase minimum flows
• Water diversion and transfer
• Recycle used water
• Reduce water consumption
• Ensure minimum flows
• Establish environmental flows / naturalise flow regimes
• Reduce anthropogenic flow peaks
• Favour morphogenic flows
• Create low flow channels in over-sized channels