Guest Post by David Whitehead. BA(Mod. Nat.Sc.)TCD, FIMMM, C.Eng.
"The fact that it has manifested in different locations and it's now moving into the Midlands and it's an evolving situation over a number of days... it may well be associated with climate change - Irish Independent, 9th Dec 15.
Has climate change played a role? Although it can’t be said with certainty that the latest flooding was caused by climate change, scientists have for years been predicting increasingly frequent extreme weather. When more than 100 homes flooded in Athlone in 2009, homeowners were told that it was a once-in-100-years event. Six years later a similar event has unfolded. Locals say that the River Shannon was at a record low for October in Athlone just two months ago - Irish Times, 12th December, 2015.
There are several things that need to be borne in mind when listening to the rhetoric about the causes of Flooding in the Shannon basin- which includes about 20% of the country.
The first is to remember that much of the Basin lies in the western half of the country which receives a much higher annual rainfall than the east and, not surprisingly, has a higher frequency of high and extreme rainfall events.
The second is to be aware that the altitude at Lough Allen hydro dam sill is 50m above sea level, where it has been artificially held by the ESB about 5m higher than its natural outlet level as a holding pond to control the level at Ardnacrusha Hydro power station. The river falls to 45m above sea level at Leitrim and then only another 12m to Killaloe in Co.Clare. Thus the gradient is 17m in 170 km or 10cm per 1km. This slight gradient has to carry the waters of all the river basin to O’Briensbridge in Co. Limerick . Only five shallow locks are required in all this distance to allow navigation for the full 170 km.
From Castleconnell (in North Limerick) to Limerick the fall is 33 m and it is this fall that provides the head for the Ardnacrusha hydro scheme. More important is the fact that during the last glacial period the natural course of the Shannon was diverted south eastwards (by an ice dam ) from its natural course from O’Briensbridge to Parteen (in Co.Clare) – which is more or less followed by the Ardnacrusha head race- to its present course where it cut down through and crosses a ridge at Castleconnnell. At this point the river bed is at 25m above sea level while the surrounding land rises to 35 on the east side and 65m on the west side of the present river channel. This constriction as well as those further upstream at Shannonbridge and Clonmacnoise, where esker ridges transect the river's course, together with the low- lying shores of the river and the very slight gradient mean it is not possible for the river to carry away even a normal level of winter rainfall – so it expands onto the surrounding callows which flood most years.
When there has been a prolonged period of rainfall and the major lakes are at their highstand and the callows are flooded and the ground is saturated, a heavy rainfall event will inevitably result in extensive flooding of the river banks.
Add to this the urban riparian developments at e.g. Athlone and the extensive covering of previous drainage catchments with tarmac by Supermarkets, car parks industrial estate and and domestic residential estates which carry water very swiftly to the river – much more so than agricultural land does and the scene is set for the sort of thing that happened in recent weeks.
I am sure most Irish readers all learned at primary school that the Shannon basin is a saucer shaped depression – which is a roughly accurate description and explains why the old cry of “dredge the Shannon” is not a workable solution unless the gorge at Castleconnell is blasted out to form a much wider and deeper channel extending all the way to Parteen – a huge, disuptive and very expensive civil engineering work - it will remain saucer shaped and there will be floods at frequent intervals.
Building windmills to generate electricity and punitively bringing the climate under human control by reducing CO2 emissions will not change these unpalatable realities!