This review has highlighted a lack of studies and peer reviewed publications on the physical and ecological impacts of low head weirs and low head ‘on weir’ hydropower and has attempted to evaluate potential implications from other riverine infrastructure and personal observations. It is apparent that low head weirs have the potential to alter the physical and chemical habitat conditions and the phytobenthic biofilm both above and below the weir. However the magnitude of change and extent to which the biofilm will respond is uncertain. The majority of weirs have been part of our riverine environment for hundreds and even thousands of years and little is known about pre-disturbed conditions or how the area above and below the weir compare to natural undisturbed reference or control reaches. The area above the weir is likely to consist of higher water depths, reduced flow velocity and flow variability and high levels of sedimentation. The area below the weir is likely to consist of highly complex flow patterns, turbulent flow, higher flow velocities and a bed of coarse large cobbles and boulders. Yet effects on the phytobenthic biofilm are likely to differ between rivers according to a combination of environmental variables and will be seasonally driven by preceding discharge, grazing density, light regimes and nutrient concentrations.
Only one study could be identified that had evaluated the impact of low head ‘on weir’ hydropower on the aquatic environment and many hydro schemes have been installed with little pre- and post-installation monitoring (Mould et al., 2015a). It is hence unclear as to how adding a hydro scheme to a weir will further impact the riverine environment. The study by Mould et al., (2015) concludes that low head ‘on weir’ hydro is unlikely to affect biotic communities but conclusions are based on “expert opinion” rather than empirical evidence and are not validated using in field evidence. A low head ‘on weir’ hydro scheme could cause changes in the area above and below the weir but changes in the area below the weir are likely to be more prevalent. The intake which is situated directly on top or directly alongside the weir will cause a re-distribution of flow. Instead of the majority of flow being distributed over a single long weir crest, it will be split between the weir and a turbine channel. This channel will create a pathway for sediment movement which might increase water depths above the
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weir but in many respects could have a greater impact on the area below the weir by decreasing sediment deprivation. The area below the weir will experience a complete re- distribution of flow, changes in spatial flow patterns and flow velocities and could experience hydraulic and morphological zones of impact, similar to that at river confluences. As such this study concentrates on the area below the weir from here in.
As with low head weirs, the effects of adding a scheme to a weir, on the phytobenthic biofilm, are likely to differ between rivers according to a combination of environmental variables and will be seasonally driven by preceding discharge, grazing density, light regimes and nutrient concentrations (Tett et al., 1978; Biggs and Price, 1987; Suren et al., 2003b; Berthon et al., 2011; Tang et al., 2013). Moreover changes created by a hydro scheme are likely to alter in accordance with the amount of flow diverted through the turbines (i.e. the magnitude of change). It is hence important to consider a range of inter-related environmental variables, preceding flow conditions and the magnitude of change when evaluating how the phytobenthic community might respond to the weir and hydro scheme. The hydro scheme could cause changes in biomass and shifts in community structure as a result of changes in the spatial distribution of flow velocities creating a difference between communities on the hydro and non- hydro side of the channel and between hydraulic zones of impact. While extreme changes can be detrimental for the whole community, small changes might not be biologically relevant i.e. will not be great enough to effect water quality or general ecosystem functioning. Based on issues described above this PhD will concentrate on advancing understandings of the influence of existing low head weirs on the aquatic environment by using temporal and spatial surveys; in the most part using a comparative approach. As pre-disturbed conditions are often unknown comparing reference sites uninfluenced by an installation to impacted sections along the same river reach is the best option (Demars and Britton, 2011). This design could be echoed across numerous rivers with differing conditions to broaden results (Demars and Britton, 2011). Furthermore collecting a wide variety of environmental variables alongside species and using detailed spatial surveys could also help to improve understandings.
Based on the review presented above and gaps identified, the objectives of this PhD are following:
1. Assess the influence of an existing obsolete low head weir on longitudinal changes in phytobenthic biomass and community structure in the weir pool below the weir through the growing season.
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2. Assess the influence of an existing low head ‘on weir’ hydro scheme on longitudinal changes in phytobenthic biomass and community structure in the weir pool below the weir through the growing season.
3. Compare the influence of an existing obsolete low head weir to the influence of an ‘on weir’ hydro scheme on longitudinal changes in phytobenthic biomass and community structure through the growing season.
4. Assess the influence of an ‘on weir’ hydro scheme on the spatial distribution of physical and chemical habitat conditions and the phytobenthic biofilm below the weir.
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Chapter 3: Methods
This chapter introduces the sites investigated in this thesis, the method used to assess the influence of the hydro scheme and weir, field measurements and other generic methods used. The three results chapters that follow contain details of experimental design specific to those parts of the thesis.