One of the key environmental issues on hydroelectric schemes with diverted river stretches, such as the lower River Shannon, is that discharges are at a consistent uniform low flow for sustained periods of time. This is called the base compensation flow rate (or discharge). On the Old River Shannon this flow is set at 10 cubic meters per second (cumecs). This discharge rate is far too low and, with the exception of occasional winter flood spills, there is generally no variation in flows on this channel. The compensation flow of 10 cumecs is equivalent to a 1 in 50 year natural drought flow only.
This scenario deviates significantly from the conditions that occur on an unregulated river; or what would have occurred on the lower River Shannon prior to the Shannon scheme. Aquatic and riparian ecological communities have evolved and adapted to a natural hydrograph with seasonal and unpredictable changes in flow influenced by climatic and seasonal variation. The unnaturally low and consistent flow rate which predominantly prevails on the lower River Shannon has significantly reduced the ecological and fluvial gemorphological functioning of this waterbody to the detriment of its ecological status, and has increased flood risk. The low and unvaried compensation flow was set in the 1920s, and needs to be urgently reviewed and brought into line with sustainable water management requirements. Current management of water on the Old River Shannon is clearly not compatible with the requirements of the EU Habitats and Water Framework Directives.
Water management on this waterbody is currently not compatible with the objectives of the either the Habitats Directive or Water Framework Directive, or even more general aspirations of sustainable water management
Although water is occasionally spilled from Parteen Weir (i.e. when flows exceed the 400 cumec capacity at Ardnacrusha), this is a relatively unusual event (<5% of the time). For the majority of the year, and sometimes for many years in a row, the Old River Shannon only has this constant base flow of 10 cumecs. It is a totally unnatural hydrological regime for a river to have the same low flow throughout the year. This has therefore resulted in a profound deterioration of the ecology and hydromorphology of the river, with encroachment of vegetation, siltation and increased flood risk. Misguided attempts to remodel the river with the movement and introduction of hundreds of tonnes of rocks and boulders, and even concrete and iron girders, to compensate for the reduced flows have further degraded the river corridor physically, ecological and aesthetically. A catastrophic decline in salmon stocks and other native fish species has occurred, and the current regime may also be favouring the establishment of a range of non-native invasive species both in and along the river corridor.
There are also problems with the way that increases in water are currently released in the river during flood events. Inevitability, significant ecological impacts occur when the rate of water discharge is significantly increased suddenly; for example in a matter of hours from 10 cumecs to a few hundred cumecs as occurred recently at the peak of the salmon spawning season. Under current management protocols the sudden discontinuation of spilling on the Old River Shannon also occurs quickly and can result in significant impacts such as the stranding of fish, invertebrates etc. In well managed rivers freshets are tapered up and down over a period of days in an attempt to mimic a natural hydrograph. If water releases need to be reduced significantly over a short period of time this should be done at night and as slowly as possible to minimise ecological impacts.
A catastrophic decline in salmon stocks and other native fish species has occurred, and the current regime may also be favouring the establishment of a range of non-native invasive species both in and along the river corridor
It is clear that the current hydrological management regime and base compensation flow on the Old River Shannon is not sustainable and needs to be urgently reviewed. Water management on this waterbody is currently not compatible with the objectives of the either the Habitats Directive or Water Framework Directive, or even more general aspirations of sustainable water management. The Old River Shannon is part of the Lower Shannon Special Area of Conservation, but is in serious ecological decline and is not currently at Good Ecological Status (GES) when the current hydrological regime is taken into account. It is accepted that ecologically appropriate hydrological regimes are necessary to meet GES (Acreman et al, 2009). Implementing sustainable environmental flows is a key measure for restoring and managing river ecosystems.
International experience has shown that by utilising a good process of establishing environmental flow objectives, it may be possible to find ways to address these objectives without significant loss of hydroelectric generating potential. The Old River Shannon Research Group (ORSRG) believe that a sustainable compensation flow, involving increased and variable flows, can be introduced on the Old River Shannon while not significantly affecting generation capacity at Ardnacrusha. We are not looking for the removal of the Shannon scheme or anything drastic like that. We are simply looking for a change in the way the river is managed to balance the requirements for hydroelectricity generation with the interests of ecology, hydrology, fluvial geomorphology, and of course the catchment residents and varied user groups on the river. The current management regime was set in the 1920’s; the scientific knowledge to manage things better is there now and needs to be acted on.
There is a range of guidance to show the way forward. One of these papers is Acreman et al (2009) and we recommend that guidance such as this is used in the review of the current management of hydrology on the Old River Shannon.
- Acreman et al (2009) Environmental flows from dams: the water framework directive. Proceedings of the Institution of Civil Engineers Engineering Sustainability 162 March 2009 Issue ES1 Pages 13–22
A review of the Old River Shannon compensation flow, in the context of an overall management plan for the waterbody, is urgently required. Campaigning for the delivery of a plan, that will allow the sustainable management of this waterbody, is a key aim of the ORSRG.