The Loxley and Rivelin are eastward flowing impounded tributaries of the River Don. The two rivers join at Malinbridge, approximately two kilometres upstream of the confluence with the River Don in the Owlerton district of northwest Sheffield. At the confluence of these two rivers the compensation flow level is set at 30.6 Thousand Cubic Metres per Day (TCMD). However, for historical reasons, this compensation flow is not evenly divided between the two rivers, as the Loxley receives 28 TCMD whilst the Rivelin receives only 2.6 TCMD.
Over recent years there has been an increasing drive towards environmental optimisation, in which the maximum economic benefit is gained for the least environmental cost. In this case there is a need to determine whether either of the rivers is adversely affected by the compensation flow regime, and if it is possible in the longer term to alter the compensation flow regimes to increase the ecological value of the rivers. There is also the need for optimisation in terms of money as a single TCMD over a year is worth around £1 million to Yorkshire Water.
The differences in discharge will lead to differences in flow hydraulics by altering flow velocity, flow depth and wetted perimeter (Order 1 impacts). As a consequence of this there may be differences in sedimentology generated by the difference in compensation flows, both in terms of substrate and fine sediment behaviour within the bed (Order 2 impacts). Also there may be changes in other environmental parameters such as; dissolved oxygen, temperature, turbidity and PH (also Order 2 impacts). Differences in these parameters will lead to differences in habitat between the two rivers. It is therefore of interest to investigate the potential impacts on macroinvertebrates and fish (mainly brown trout) of altering the compensation flow regime.
It is possible to split the research up into three sections. Firstly the Order 1 study will seek to establish the hydrological regimes under both the existing and the altered compensation flows. With these established, the regimes will be used to simulate flow hydraulics (wetted area, velocity, depth, flow duration) under both the existing and altered regimes, as these are parameters of ecological interest. Instrumentation and field survey will allow assessment of both hydrological and hydraulic changes as the flows are changed. A hydrodynamic model will be developed to allow these changes to be simulated for a wider range of possible compensation flows, and also for application to other catchments. Data from instrumentation and records will be used to calibrate and to validate the model.
The aim of the Order 2 study is to identify the changes in system geomorphology and sedimentology associated with the changes in compensation flow regime. These changes include bed morphology and cross-section shape, fine sediment transport patterns, and patterns of both coarse and fine sediment deposition. Other environmental parameters such as temperature, dissolved oxygen, PH and turbidity will also be included. As with the Order 1 study, field data will be acquired before and during trials in order to assess basic impacts of existing and altered compensation flow regimes. Evidence from these will be used to extend the hydrodynamic model to include key Order 2 parameters which, after calibration and validation, will be used to simulate Order 2 impacts.
The Order 3 study will seek to link existing and changed compensation flows, through hydrological, hydraulic and environmental changes, to ecosystem processes, notably those associated with aquatic macroinvertebrates and fisheries. The core aim here is to link observed changes in ecosystem processes to possible explanatory variables in order to identify controlling factors. There is an extensive literature on such links, but they have been much less well-developed for upland-draining catchments in the U.K. The study will be based upon the field data acquired from before and after the change in compensation flows. This will then be used to identify the basic model structure required to extend the Order 1 and Order 2 simulation models to the prediction of Order 3 impacts.
This PhD has four main objectives.
1) To establish the inter-relationship between the existing compensation flow regime in the Rivelin-Loxley systems and system hydrology, sedimentology, and geomorphology (Order 1 and Order 2 impacts).
2) Assess changes to the system as a result of trials to be conducted over the 4 year project period, in relation to Order 1 and Order 2 considerations.
3) Link the Order 1 and Order 2 impacts identified under (1) and (2) to ecological and fisheries data that is being collected as part of the project in order to identify Order 3 impacts during the trial.
4) Develop a generic tool for assessing the consequences of changes to compensation flow regimes that can be applied to catchments characteristic of upland environments, based upon simulation modelling. This will include Order 1, 2 and 3 impacts.