Sunday 20 December 2015

Shannon Floods - Climate or Contour ?

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!

1 comment:

  1. The Shannon rises in west Co. Cavan and flows through mountainous terrain to the village of Dowra to Lough Allen. South of Dowra village there are meadows adjoining the river. In 1970, very heavy rain fell in August and it swept away and destroyed the cocks of hay to the dismay of farmers. Large areas of Westmeath, Offaly and Longford was covered with blanket bog, a peat layer up to 20 meters deep. This soaked water, delaying its release for several days after a heavy rain. The State Body, Bord Na Mona, cut away most of this peat for fuel/electricity generation, leaving rocky artificially drained waste land behind. State subsidised upland sheep grazing hasn't helped.

    Farmers around Lough Allen always blamed the dams for summer flooding of their land. The fall of the lower Shannon is 13 times less than it is for the river Glyde which adjoins my farm. Flooding never happens on it. All rivers receive clay, silt, sand, wood, gravel and boulders from their banks, but the river stays the same width over decades. It is obviously a natural process which works if man stays out of it.

    If done to day the Shannon Hydro and the peat extraction schemes (state works) would require compliance with the SEA Directive and the Aarhus Convention on public participation. The EU and Ireland's renewable energy plans also require compliance. A few brave individuals are trying to force that compliance. Unfortunately those with resources, like Donald Trump. failed to incorporate these legal breaches on their various judicial reviews.

    Dublin Engineer Pat Swords's, commenced a High Court application over a year ago to quash Ireland's National Renewable Action Plan, last week, Judge Keane, further adjourned the case to March, 2016. There must be something substantial to it. The complaints from a number of Irish people, including Mr Swords to the UN Compliance Committee are progressing very well.

    A nice little exercise is to go on Google maps and check altitude levels of the Shannon basin (get the levels above sea level). Of course, the eco green warriors will blame it all on climate change, conveniently ignoring the 1970 flood as an inconvenient truth.

    The Shannon is a problem going forward, the experience makes it highly important to comply with the laws for assessment of all public plans and programmes which impact on the environment. This with public participation. Otherwise we will repeat the same mistakes for our energy plans and what other plans governments may think up after that. Thankfully, those making submissions to wind farms and pylons this past year have cited this non compliance. This effectively means these projects cannot be finalised until Justice Keane rules. It takes only one success to bring the whole thing to a halt.