The Lower (old) River Shannon downstream of Parteen Regulating Weir is usually provided with a ‘compensation’ flow of just 10 m3 sec-1 (cumecs). However, during the winter of 2015/16 flows in the old channel were increased up to over 40 times this ‘compensation’ flow. Prior to the Shannon Scheme a flow of around 400 cumecs would have been just a normal flood on the river. However, during the winter of 2015/16 this volume of water caused widespread flooding. In addition to the water being released at at Parteen Regulating Weir, 400 cumecs was also being abstracted from the river at Parteen Regulating Weir and diverted through Ardnacrusha Hydroelectric Station during this period. It is clear that it would currently be impossible for the old River Shannon to cater for the full expected flows in the Lower Shannon due to the reduction of conveyance capacity over the 85 years since the Shannon Scheme was constructed.
The reduction of conveyance capacity which has occurred in the old river channel since the construction of the Shannon Scheme is a consequence of the generally low and unvaried flows which this waterbody normally receives (e.g. 10 cumec compensation flow). The low and unvaried flow has favoured the encroachment of riparian vegetation and siltation. The ESB have also installed numerous in-channel fisheries features to remodel the old river channel to cater for the reduced post-Shannon scheme flows. These structures have furthermore reduced channel conveyance capacity by narrowing the river channel and encouraging vegetation encroachment and siltation. This has all significantly increased flood risk.
The current water management regime was set in the 1920’s; the scientific knowledge to manage things better is there now and needs to be acted on. Overall, it is clear that there needs to be a change in the way this Natura 2000 river is managed to balance the requirements for hydroelectricity generation with the interests of ecology, fisheries and of course the catchment residents and varied user groups on the river. It is time to urgently review the compensation flow provided to this water body and restore a more naturalised flow regime. It will also be necessary to remove many of the fisheries development features that have been used to modify the river. A change in management such as this would increase the ability of the river to maintain itself through natural hydrogeomorphological processes and reduce flood risk.
Sustainable water management
Since the Shannon scheme, the “old” River Shannon usually receives a compensation flow of just 10 cumecs. This is equivalent to only a 1 in 50 year drought flow on the river prior to the construction of Parteen Regulating Weir. This has had a significant morphological impact on this river corridor. It is clear that there needs to be increased and variable flows in the Old River Shannon to be delivered within the context of a sustainable management plan for the Lower River Shannon. Spillages of water during flood events should also be tapered up and down slowly mimicking a natural hydrograph.
Removal of fisheries weirs and deflectors
The instream physical modification along the river would also need to be removed to restore the river back to a more natural physical state. Rather than enhancing the river’s ecology, the intrusive instream physical modifications on the Lower Shannon are working against the rivers natural fluvial geomorphological and ecological processes, increasing encroachment of vegetation, reducing the channel’s conveyance capacity, and preventing the water body from reaching good ecological status.
New fish passes and bypasses
Fish passage requirements on the Lower River Shannon also need to be reviewed. It is clear that new upstream fish passes and downstream bypasses are required for salmon, eels and lampreys. The River Shannon above Parteen Regulating Weir currently meets less than 5% of its salmon conservation escapement target. Traps to facilitate the upstream movements of juvenile eels are not operated regularly or efficiently. Thousands of downstream migrating silver eels and salmon smolts are killed each year in the turbines. It is noteworthy that even at the peak of the floods and water spillages during the winter of 2015/16 that turbine killed silver eels continued to turn up on the Lower River Shannon. Increased used of spillways and trap and transport will not solve this problem and silver eel (and salmon smolt) bypasses will have to be installed.
PS: It is noted that the restoration of sustainable water management in the Lower River Shannon, along with the provision and fish bypasses and passes and, needs to be addressed before considering any new major developments. The current proposal for the Dublin Water Supply scheme is fundamentally flawed because it assumes that going forward the old River Shannon will just receive its current 10 cumec compensation flow (as set in the 1920’s), and this assumption is incorrect. An abstraction of 4 cumecs from Parteen Reservoir is not just 2% of the available water as recently claimed. This is 20% of the natural 95%ile flow of a Natura 2000 river so would not be an acceptable abstraction under either the Water Framework or Habitats Directive. This water belongs in the old River Shannon and – regardless of what the ESB considers to be its entitlements – is required to restore the Lower River Shannon SAC and the fisheries of the River Shannon. We believe that it is possible to abstract water from the River Shannon, but the future requirements of sustainable water management on the Lower River Shannon will have to be fully provided for first, and sorting the fish passage problems in the lower Shannon will have to come ahead of any major new development on the river.
For further reading please see the following posts:
- Management plan for the Lower River Shannon
- A sustainable compensation flow is needed for the Old River Shannon
- Sustainable management required for the alluvial woodlands of the Lower River Shannon
- Fisheries modifications on the Old River Shannon – time to get them removed?
- Then and now : The “Bush Hole”, Castleconnell Fishery
- Aerial Surveys of the Lower River Shannon
Also, if you have any queries please contact us.