Effects of changes in flow velocity on the phytobenthic biofilm below a small scale low head hydropower scheme

This study presents a spatial analysis of physical and biotic river conditions below a low head hydro scheme in the River Goyt, UK. The overall aim was to assess whether changes in localized hydrological features, introduced by low head hydro, affect phytobenthic biomass. Single point and profile measurements of flow velocity and velocity vectors, elevation, depth and biofilm biomass were mapped. Results showed evidence of high flow velocity on the hydro side of the river and low flow velocity on the non-hydro side of the river. A number of distinct hydrological and morphological features were defined. Biofilm biomass appeared lower on the hydro side of the channel but no obvious relationship with flow velocity was observed. Future analysis will include evaluation of phytobenthic species on either side of the river channel and an investigation into the combined effects of a number of variables on phytobenthic biomass. To date review studies have derived the potential implications from the relatively well known impacts of high head schemes. Conclusions are often based on expert opinion rather than experimental data and the impact of the weir alone is typically separated from the additional impacts of the scheme (Anderson et al., 2014). As schemes are often installed on existing weirs understanding the impacts of the weir on natural river conditions is crucial. The hydrological, morphological and biotic impacts of weirs are relatively well researched but the additional impacts or added benefits of the scheme are still uncertain. Weir structures by their very nature can change natural river conditions. They can create a weir pond directly above the weir and a scour pool directly below the weir. This paper is concerned with the area directly below the weir where the cascading water erodes the river bed and banks. This area typically consists of a deep, wide pool with high energy, complex flow and a mid-channel island or tail riffle were the bed material is deposited as energy in the flow decreases (Mould et al., 2015). This area is often associated with specialized aquatic communities adapt to the complex environments. In line with the Environment Agency (Mould et al., 2015) this whole area will be referred to as the weir pool from herein. Adding a scheme to a weir could cause changes in the weir pool environment. Robson et al., (2011) suggests that by changing the distribution of flow a scheme might cause changes in energy dissipation, flow pattern, morphology and aquatic communities. A modelling study conducted by the Environment Agency (Mould et al., 2015) supports this theory suggesting that there could be changes in the spatial variation of flow velocities and depths. Although they do conclude that such changes are unlikely to alter habitats. A case study at Romney Weir on the River Thames revealed higher flow velocities along the river bank closest to the turbine and lower velocities on the opposite side of the river up to 20 meters beyond the structure (Mould et al., 2015). In this particular instance the Environment Agency (Mould et al., 2015) concluded that the changes in flow were not “ecologically significant” but such conclusions are based on expert opinion and species preference rather than observed evidence. In-situ biotic investigations need to be carried out before such claims can be accepted. Different communities could potentially develop either side of the river, especially sessile benthic communities, like the phytobenthos which are unable to move. Particular changes in communities might occur were the flow from the main river channel and turbine outlet collides. There could be changes in hydrological and morphological features which in turn could alter biotic communities. Similarities can be drawn to the interface of two flows at river confluences and tributaries. However this is most likely to occur at ‘by weir’ schemes where the flow is diverted through a turbine forebay, a small channel in which the turbine is situated. Where the water is discharged back into the main river channel and the two flows collide any of the following features could occur; 1 Stagnation at the upstream junction corner between the outlet and main channel, 2 Mixing, development of shear layers and scouring of benthic communities where the two flows combine, accelerate and scour the river bed, 3 Separated flow below the downstream outlet channel junction corner and bar formation, 4 Deflection where the flows collide and change path, 5 Advanced recovery downstream (adapted from Szupiany et al., 2009). Further morphological changes could occur were the sediment that would have built up behind the weir will pass through the turbine forebay. This suggested by Anderson et al., (2014) and has been named the draw down effect. This study will explore the potential aquatic implications of a low head ‘by weir’ schemes in a bid to improve knowledge and understandings, update and improve empirical evidence and to inform scheme designs to reduce their impact on the environment. The main motivation is to identify distinct hydrological, morphological and biotic features which could be attributed to the scheme.