The Slieve Aughty mountains are one of the critical factors of the overall flooding situation in South Galway. Our management of these mountains is completely regardless of, and more often than not, contributory toward flooding issues and this is something that needs a fundamental change if we are really serious about alleviating the flooding situation in South Galway.
The situation is not good. In South Galway we have a highly karstified landscape that includes 10s of kilometers of underground rivers responsible for bringing water from mountains to the sea. This has been explained previously in Presentation #1 : Slieve Aughty Drainage.
Because of the underground rivers, there is a finite limit to the flow that can be handled by the system.
If the underground river channels can’t handle the flow they rise and move overground and this causes overground flooding.
One of the critical factors that determines flooding is not just the volume of water but the rate of which water enters the system. It’s like using a funnel when adding fuel to your lawnmower – you’ve got a particular volume of fuel that will end up in the tank but if you try and fill all the fuel at once it will spill over. If you slow the flow down through the funnel can take in you get a good flow but no spills!
The Slieve Aughty’s are the opening funnel of South Galway, the main catchment area of the 500 km.sq or so of the total catchment. Slieve Aughty mountains are the main highland within this and rise to over 300m.
The undisputed fact is that water is coming off the Slieve Aughty mountains quicker than ever before. I’m a native of the South Galway area and a these days rivers rise in Kiltartan 5-6 hours after heavy rain where as in my youth, it would take over 24 hours for a flood to fully rise. I’ve heard plenty of other locals saying that ‘back-in-the-day’, it would have taken days. I’ve also looked at some of the dynamics between rain-fall and river levels ( Rain vs. Rivers in South Galway) that shows that certain rivers can rise over 1 metre just 3-4 hours after heavy rainfall.
The Slieve Aughty mountains rather than acting like a sponge seem to behaving like an irrigation channel with water is flowing off the slopes unnaturally quickly
What do you mean unnaturally quickly?
The Slieve Aughty mountains are mainly a wetland comprising of a large bog area. This type of area has an ability to soak and hold huge quantities of water. Over the past 50 years there has been a lot of human interaction with the Slieve Aughty mountains and this interaction seems to have upset the natural drainage patterns of the mountains. The main types of interaction are :
- More intensive farming + turf cutting in the area
- Windmill construction
- Forestry plantation
Along with this increase in activity there is also new infrastructure such as forestry access roads that will affect drainage patterns.
The effects of these can be seen on river level height after rainfall . What we can see in South Galway is that water comes off the mountain very quickly leading to increases of over 1 m in some rivers in a few hours – e.g. Storm Gertrude (Feb 2016) below:
- Killafeen is the input to Lough Cutra on the Derrybrien river
- Ballycahalan is the Bollyneendorish river
- Gortmacken is on the Ownshree river at Kilchreest
Getting a grip on ‘grips’!
What is a grip? A grip is a man-made channel that is cut into wetland area that runs in straight lines and occur in regular patterns. They can be readily seen on aerial photographs. The main purpose of grips is to drain wetlands (which, when your catchment area is prone to severe flooding may not be the best thing to do!) and seem to have an added benefit for forestry to provide better bedding material for new plantations.
The following is an aerial photograph of the area just south of the windfarm close to Derrybrien. This was take on 14th Jan 2016 where South Galway floods had fully peaked. A light snowfall the night before provide us a good contrast to see the grips.
The following video [Courtesy of Sean Brady] shows some of the grips and how the connect and go downhill.
Impacts of Grips
It is intuitively obvious that combing the top of a mountain with 3-4ft rains every 12 yards aiming downhill – will have an massing impact on the drainage dynamics. That’s why they are put there in the first place. According to the Forestry Commission of England, when establishing new forests trees need to be planted on dry ground where their roots are not constantly waterlogged. – to the Slieve Aughty bogs are being drained.
There are some studies that argue that the grips don’t make the water flow more, but they help to bring the water flow earlier – Sounds like they are describing that overflowing funnel to me.
Changes to the hydrological management of upland habitats can be detrimental to the characteristic vegetation and species of the uplands, as well as increasing the risk of soil erosion and flash flooding. [RSPB/DEFRA]
Rather than a square kilometer of land soaking water slowly you could have 100s of these narrow channels that transport the water instantly off the mountain. T
Who is responsible for the grips?
One body – Coillte! Coillte manages about 7% of land cover in Ireland. Its core purpose is “to enrich lives locally, nationally and globally through the innovative and sustainable management of natural resources – but it’s not a charity – It’s is a commercial company operating in forestry, land based businesses, renewable energy and panel products. So what about the Slieve Aughty Mountains? you can see the main highland catchment area for the Slieve Aughty mountains highlighted below:
Its tagline is :
Coillte manages its forests to deliver social, economic and environmental benefits.
I think it’s fair to say that from its tagline, Coillte should be in the position to deliver positive environmental benefits from managing the land of the Slieve Aughty’s. Are Coillte actually engaging in practices that dramatically affect the hydrological dynamics of the mountain that will then affect flooding probability in South Galway?
Also, come to think of it, where is our National Parks and Wildlife Service (NPWS) again? Forestry works require an Environmental Impact Assessment (EIA) before commencing disturbing operations including harvesting, establishment , engineering and other development works prior to commencement. Why has the NPWS not anticipated the downstream impacts and devastation that rapid run-off will cause?
Forestry Access Roads
Many forestry access roads in the Slieve Aughty’s are constructed below bog level and have drains for carrying run-off water and can become the main conduits to the system. It is a common occurrence to see the floor of the a bog with planted forest 3-4 ft above road level. This again improves drainage but leads to rapid run-off during severe weather.
Impacts of Windfarms
I haven’t found any detailed studies (yet) on the impact of windfarms on drainage and flooding. Recently Dr Professor Johnston highlighted the likely hydrological impacts of a windfarm development in the Finn valley in Donegal and was concerned that, if the development was to go ahead, flooding events of past week would be tenfold in their extremity. It makes sense that if you strip the top of a mountain of 500,000 cubic meters of peat and soil, it will have an impact on the drainage dynamics. We’ve already seen a landslide caused by the construction of the windmills close to Derrybrien.
Running off the mountain
Other studies have shown that in heavy rain fall, water will cascade overland on grasses and contribute as much as drains. A project called the Pontbren project prompted the formation of a government-funded Flood Risk Management Research Consortium’ to do more in-depth analysis on water flows and came up with proven metrics on the effects of tree belts across the mountain.
Slowing the flow!
One of the best very recent examples of really effective flood management is what happened in the town of Pickering, in the UK. This town was flooded four times between 1999 and 2007 – and being refused a conventional flood defense as too expensive – the local people took matters into their own hands, “working with nature, not against it”. They formed a committee and ran a project called ‘Slow the flow at Pickering’ and used a range of land management measures to help slow the rush of water. These included:
- Constructing low-level bunds (reservoirs or catchment pools)
- Planting more trees, especially along streamsides and in the floodplain
- Restoring woody debris dams in small streams
- Targeted blocking of moorland drains
- Restoring wetlands
Bunds create additional flood storage capacity and can be designed to only operate during flood events by storing overflow water. After flooding events, the water slowly dissipates back into the system. Surely there are areas within the Slieve Aughty mountains where it would be possible to create these bunds/resevoirs to store 100s of 1000s of cubic metres of water?
Planting more trees
A hydrology department (Centre for Ecology and Hydrology) initially took measurements comparing the speed at which the soils under the pasture and those in the fenced
off woodland could absorb water.
The results were striking, showing that the infiltration rates inside the woodland were 60 times those on the pasture ten metres away, and that the beneficial effect extended beyond the edge of the woodlands.
This prompted the formation of a government ‘Flood Risk Management Research Consortium (FRMRC) , to do more indepth analysis. The FRMRC setup various studies:
- They measured infiltration and flow rates and build up a picture of how water moved from fields to streams.
- They examined the way rainwater flowed through small channels in the soil (using simulated rain, coloured blue with food dye)
- They investigated the influence of tree shelterbelts (thin strip of woodland across pasture/grassland) on the amounts and patterns of rainfall reaching the ground both directly underneath, and on the leeward side of the plantings.
Tree Drainage : Research found that water flowed along the pathways provided by living tree roots, rather than through earthworm channels and other cracks in the soil. This overturned existing theories that suggested water flowed only along channels left by dead tree roots. The experiment with dyed water showed that the pathways tree roots provided for water were strongly influenced by tree species. Birch trees have a spherical root structure and this caused the water to be channelled into a bulblike area at the top of the soil; ash trees have horizontal surface roots from which vertical roots extend down into the soil. Water here was channelled along these vertical roots in finger-shaped patterns to a greater depth.
Tree Shelterbelts : It was shown that in winter, high wind speeds blow small
raindrops in an almost horizontal direction across the shelterbelt. This sheltering effect led to less rainfall both directly underneath the tree canopy and over a width
equal to the average tree height from the leeward edge. In contrast, low wind speeds and large raindrops in summer fall almost vertically, directly onto the canopy causing little or no rainfall depletion on the leeward side:
The Pontbren results have shown conclusively that strategically planted narrow, fenced shelterbelts of trees across slopes capture surface run-off from the pasture land above and allow it to soak more rapidly into the soil.
Restoring woody debris dams in small streams
Woody debris dams are accumulations of trees, branches, root boles, twigs etc that are seen as a vital component of our mountain watercourses. Their function as ‘leaky’ dams can reduce peak flow rates and increases floodwater storage
Woody debris helps regulate the energy of running water by decreasing the velocity.
Thus the ‘travel time’ of water across the catchment is increased.
There a lot of articles on Grip/Gully blocking that outline the positive effects that they have on the
Grip blocking can help to restore natural drainage patterns, encourage re-vegetation, reduce erosion, and minimise the knock-on effect of hydrological change downstream. [DEFRA, UK]
One paper was looking at the hydrological affects of blocking drains and they were able to reduce peak flows substantially.
There are assessment guidelines, implementation guides lines but the best guideline would be not to have downward facing drains in the first place.
The following are they key facts:
- The key catchment area for the South Galway is the Slieve Aughty mountains
- Coillte, who aim to deliver social, economic and environmental benefits, are the owners of much of this land.
- The water is coming off the mountains very fast, leading to rapid rise in river levels in hours.
- There are many proven solutions (resevoirs, drain blocking, planting native trees) to restoring natural hydrological flows and these solutions are within our reach
- NPWS should be enabling these solutions
The good news is that there are solutions within reach that mean that we technically don’t have to ‘move mountains’. The only mountain in our way is that of bureaucracy which will need quite a few grips to streamline!
We need to ‘Slow the Flow‘ in South Galway