Run-of-River Hydro » The environment
Water Abstraction Regime
At an intake, water is abstracted from the river in accordance with the guidelines agreed with the relevant statutory authority. Run-of-river schemes can be environmentally sensitive, though a sensible proportion of the river flow is always allowed to continue downstream (so-called hands-off flow). Abstraction begins once the flow in the river exceeds a predetermined value chosen by the local statutory body, who take into account factors such as fish migration and river morphology. The hands-off flow is determined solely by the relative positions and sizes of the fixed river and intake weirs, and therefore cannot be affected by the operating regime of the commissioned scheme.
Figure 1 shows a typical intake arrangement. A small weir less than 1m high is placed across the river. Water flows through a brass V-shaped plate embedded in the river weir, allowing accurate calculation of the flow progressing downstream for a given water level. A second weir (intake weir) is constructed on one of the banks immediately upstream of the river weir, behind which is the pipeline inlet. As abstraction increases from its minimum to maximum level the residual downstream flow increases by a factor again specified by the local statutory authority. The maximum amount of water abstracted is typically about 4-5% of the highest flows occurring in the river. Flow events large enough to modify the riverbed therefore remain largely unaffected.