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02. Flow regulations

General description

The production of electricity by hydropower plants is often implemented to satisfy peaks in electricity demand. For this reason these plants work intermittently, creating periodic and extremely rapid and short-term fluctuations in flow in the receiving water body. These fluctuations are called hydropeaking and usually show a marked weekly and daily rhythm.

Effect/Impact on (including literature citations)

Hydropeaking is a unique form of flow regulation, in that it introduces frequent, short duration, artificial flow events to the river. The impacts of hydropeaking on channel size and morphology are highly dependent on the size and frequency of hydropeaks in relation to size of geomorphological effective flows prior to regulation. Where extreme hydropeaking leads to frequent geomorphological-effective flows (i.e. flows close to the bankfull stage), the hydropeaks dominate channel size and form because they readily mobilize bed sediments until the bed becomes heavily armoured by very coarse particles or scoured to bed rock. Such severe hydropeaking leaves few alluvial habitats and in-channel vegetation, and an abrupt transition between unvegetated and vegetated surfaces at the channel boundary. However, where hydropeaks are smaller and thus less competent to move bed sediment and the pre-existing geomorphologically-effective flows are significantly less frequent or no longer occur under the new hydropower dominated regime, physical adjustments may occur within the pre-existing channel. Sear (1995) describes an example of the latter situation, where the observed adjustments to the gravel-bed North Tyne River are indicative of channel narrowing and retention of fine sediment in the bed, in response to diurnal adjustments of river stage by approximately 0.6 m from hydropeaking: a) the development of fine sediment berms along channel margins, b) the aggradation of pools, c) the encroachment of vegetation on former gravel shoals d) the growth of tributary confluence bars. e) degradation of riffle spawning grounds: characterized by higher percentages of fines within spawning gravels, coarsening of surface gravels and the development of a stable, strong bed fabric. Hydropeaking has been found to have other more indirect hydromorphological impacts beyond those on channel morphology and sediments. Thus, Curry et al. (1994) found that short-term flow fluctuations affected hyporheos dynamics. Fluctuating flow levels altered groundwater pathways, chemistry, and flow potentials within the river bed. Rising river levels introduced river water into the bank where various degrees of mixing with groundwater occurred. Subsequent recessions of river levels increased the potentials for groundwater flow into the river channel. They found that these effects were significant at spawning and incubation sites and were thus potentially important for river ecology. Further indirect hyporheic effects were identified by Arnzten et al. (2007) who found empirical evidence of how changes in flow regime and in bed sediment permeability, relating to ingress of fine sediments, altered the vertical hydraulic gradient and water quality of the hyporheic zone within the Hanford Reach of the Columbia River in response to 2 m daily water level fluctuations due to hydropower generation. A range of ecological consequences of hydropower development have also been recorded. Nilsson et al. (1991) evaluated the effects of an old hydropower reservoir on river margin vegetation. While he found no difference in mean annual discharge, number of types of substrate, and width and height of the river margin (relative to the summer low-water level), the regulated river had fewer frequent and more infrequent species, and the proportion of annual plus biennial species-richness was higher, while the proportion of perennial species-richness was lower. Also, vegetation cover was lower in the regulated river. Reduced biotic productivity in reaches below hydroelectric reservoirs may be due directly to flow variations or indirectly to a variety of factors related to flow variations, such as changes in water depth or temperature, or scouring of sediments. Many riverine fish and invertebrate species have a limited range of conditions to which they are adapted. The pattern of daily fluctuations in flow imposed by hydropeaking is not one to which most species are adapted; thus, such conditions can reduce the abundance, diversity, and productivity of these riverine organisms (Cushman, 1985). Thus, Trotzky & Gregory (1974) focussed specifically on the effect of severe low flows associated with flow fluctuations on the upper Kennebec River. In particular they observed that the very slow currents during low flows between hydropeaks appeared to limit the diversity and abundance of swift-water aquatic insects (Rhyacophila, Chimarra, Iron, Blepharocera, Acroneuria, and Paragnetina) on the river-bottom below the dam. In relation to high flows, Robertson et al. (2004) subjected Atlantic salmon par to simulated short term flow fluctuations and found that fish habitat use was not affected; there was little effect on fish activity within diel periods; and stranding rates during flow reduction were also very low. However, most research has indicated significant impacts of hydropeaks on both macroinvertebrates and fish. Bain et al. (1988) found that the fish guild of small-species and size classes that occupied habitats characterized as shallow in depth, slow in current velocity, and concentrated along stream margins, were eliminated or reduced in abundances at a study site subject to large flow fluctuations. Furthermore, Moog (2006) identified significant impacts of intermittent power generation on the fish fauna and benthic invertebrates of several Austrian rivers. Hydropeaking was found to disturb long sections of rivers, with a breakdown of the benthic invertebrate biomass of between 75 and 95% within the first few kilometres of river length, and a reduction of between 40 and 60% of biomass within the following 20–40 km. The reduction of the fish fauna was of the same order of magnitude and correlated well with the amplitude of the flow fluctuations. Similar biotic effects were detected in a Pyrenean river affected by a hydropower impoundment by Garcia de Jalón et al. (1988). At a single sampling station 2.4 km below the dam, there was a significant (p < 0.05) decrease in total macrophyte biomass, although the species composition remained dominated by two species (Myriophyllum verticillatum and Ranunculus fluitans), and the macroinvertebrate community exhibited a significant (p < 0.05) decrease in taxonomic richness, total density and total biomass. In general, planarians, ephemeropterans, coleopterans, plecopterans and trichopterans disappeared or decreased their abundances. Scrapers (as relative biomass) were the functional feeding group most adversely affected by the new flow regulation. With regard to the fish community, the most significant change was the absence of all resident coarse fishes (cyprinids, primarily) at the sampling site during the 1990 and 1991 sampling surveys.

Conceptual framework of hydropeaking hydromorphological (HYMO) processes and variables.

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Other relevant information