Flooding Solutions – ‘The Coole Lake Channel’

Let’s take a deep dive into looking at how to solve the majority of South Galway flooding by providing an overground channel from Coole to the sea.  This is a solution that has been mentioned over many years and the subject of many reports and speculation.  But what exactly is this ‘channel’? What effect would it have? How big should it be?  I’m not an ‘expert’ but I’ve compiled this information from many sources and local knowledge and hopefully it can give a clear perspective on a viable solution for flooding in South Galway.

[Usual disclaimer – I’m not an expert but if you are I’d love to hear from you!]

Acknowledgements to Sean Brady for use of his Aerial photographs and for Owen Naughton, NUIG for providing vital research papers.

The South Galway ‘Basins’

What we’ve learned from previous blogs is that a fast network of underground rivers brings water from the Slieve Aughty’s to the sea. The Ownshree river may go underground over 7 times as it emerges and disappears in the South Galway limestone landscape.  A a reminder here is the high-level view of Slieve Aughty with the rivers draining to the west and the green dashed lines are the underground connections.


The unique drainage characteristics of the South Galway area mean that in normal conditions, many of the flows are underground.  The peak water flow in an underground is limited by the size of the channel and the water pressure through it.  In severe rainfall, the amount of water flowing through the system can be much more than what the underground system can handle.  When this happens the land starts to flood and in dramatic flooding situations this will resolve into a series of basins that flow into each other – something like this.


So for instance:

  • Blackrock will overflow through Skehana,
  • Ballylee will overflow through Castletown
  • Kiltartan will overflow through Corker
  • Kilmacduagh will overflow through Tierneevin
  • Coole Overflows through Ballynastaig
  • Caherglassaun overflows through Cahermore

What is very noteworthy is how the effect of rising levels on the underground systems can affect the rate of flow underground coming into the basin and reduce it. This leads to the upstream basin rising until the pressure builds up again but sometimes the levels rise too much to match the inflow capacity and eventually flow overground. The effects are shown in the diagrams below:


The more we let each basin fill up,  the bigger the impact it will have upstream

We’ve seen these dynamics in Kiltartan where the high-level of Coole lake drastically reduces the pressure in the main swallow hole. Kiltartan levels rise and flood until eventually there is a flow overground into Corker.  The high levels in Kiltartan mean reduced pressure in Castletown and Ballylee. We’ve also seen these dynamics in Tierneevin where the levels of Coole lake impede the underground flows and eventually the levels rise and saturate the culvert beside Tierneevin church, flooding houses.

We already have a ‘channel’ – what’s wrong with it?

It seems as with severe rainfall water does get to this sea at Kinvara.  So there is a natural overground flow –  What’s the big deal then? Why do we need a channel?

With the current topography/levels – when these basins fill up to these levels and cause the overground flows – many communities in South Galway are in trouble. These levels will have already caused major devastation and distress in the area. Houses, farms and roads are flooded.  In winter 2015, Coole lake expanded and flooded many areas :

  • Crannagh
  • Glenbrack
  • Tierneevin
  • Hawkhill
  • Garyland
  • Roo
  • Raheen
  • Corker

Caherglassaun expanded and flooded many areas

  • Ballynastaig
  • Killomoran
  • Shigaunagh

Cahermore expanded and flooded the area on Cahermore east and west. The communities around the Coole, Caherglassaun and Cahermore basins had a major flooding crisis.


The downstream basins like Coole and Caherglassaun levels will also have major effects upstream in areas like Kiltartan, Tierneevin, Gort, Ballylee etc.

The effects are not just immediate flooding of the areas but because of the lack of a proper overflow channel, these area can remain underwater for months.

The main solution to South Galway Flooding that would solve the majority of issues is to manage the ‘lip’s of these key basins especially on the peak level of Coole lake.

There are some other dynamics to consider when we see how Coole, Caherglassun, and Cahermore rise and flows.

How does Coole flood?

When Coole lake keeps rising – where does it overflow?  It overflows in elbow shape into Caherglassaun beside Connolly’s house in Ballynastaig. This is what it looks like :

Coole overflowing into Caherglassaun (Courtesy of Sean Brady Aerial Photography)

In the top of the photograph below, you can see Caherglassaun  area flow into Cahermore Turlough:

Caherglassaun flowing into Cahermore (Courtesy of Sean Brady Aerial Photography)

On an aerial photograph it looks like this:  coole1.JPG

There is one dynamic that is important there. When Coole starts to overflow here it flows under a road through a culvert beside Connolly’s house.  This is the ‘lip’ of Coole basin and it pours into Caherglassaun lake here.  The ‘lip’ of Caherglassaun basin however, is in fact higher than the lip of Coole and eventually not only does Caherglassaun basin expand and fill, Coole lake levels now continue to rise with it until it overflows into Cahermore. So, in order to lower the level of Coole Lake peak levels, it’s not just about lowering the lip of Coole basin, but in fact it’s more about lowering the lip of the Caherglassuan-Cahermore link.

Coole lake peak levels can be controlled by lowering the lip of Caherglassaun-Cahermore section


I sent an email to Ciaran Canon early in Dec 2015 warning him about Cahermore and that it was going to be very serious this time.  In 2009 it took 4 weeks after the heavy rains for the peak in Cahermore.   In Dec 2015, there was more rain and it was getting into Coole quicker than ever (culverts in Kiltartan). The one difference between Cahermore and Coole/Caherglassaun is that Cahermore doesn’t have an overflow channel – basically the water ends up there and unfortunately for the community,  there is no dedicated channel.

There are minor flows into Caherawoneen at Adrian Glynn’s house where as the water levels rise, they sink through rocks and come out behind the house and flow into Caherawoneen. This was eventually where, through some brave decision making (Joe Bynre)  m, Galway County Council, and with permission from a great neighbour (Adrian Glynn),  a temporary channel was cut.


This channel was was able to take down the peak levels in Cahermore and avert disaster to the Cahermore community. Unfortunately, some housesholds experienced a lot of distress as their houses flooded and many were pumping to keep water out.


The temporary channel at Cahermore

Whatever the skeptics may say – this channel had a significant impact on reducing the flooding situation in Cahermore.  The only problem with this channel is that it is too high. Again, like the Coole overflow, when it starts flowing here – it is too late. This means there is major flooding in the Coole region. An ideal channel would be lower. Also, 4 weeks is too long. After severe rains when Coole starts to overflow, we want Cahermore to also start flowing. We need water flowing two weeks earlier than it does at present!

Another issue is that this channel placed significant pressure on the downstream drainage infrastructure around Caherawoneen and at Kinvara where this additional capacity was saturating the current flow capability.

Cahermore is 13M above sea level and just 3.75 KM to the sea so this natural incline is ideal for enabling a significant water flow, as long as the drainage infrastructure is sized properly.

Basins, Basins, Basins.

It’s all very well looking at flat maps and aerial photographs but it can be difficult to visualize the basins. Here is a conceptual view with the green line showing some underground connectivity also (note, many of the underground channels go well below sea level)


This has been compiled from various sources and it should be ‘fairly’ accurate and there are a few things that stand out for me:

  • Blackrock is fairly well landlocked close to 29m (this is when it overflows at Skehana)
  • The level of Coole lake is no determined by the boundary of Coole/Garryland but by the link between Caherglassaun and Cahermore.

After severe winter rains, the basins start to fill up and the levels rise as the underground systems are saturated. The basins may fill with just underground flows but as levels rise in the system, the underground rivers may become redundant leading to the basins overflowing e.g. through Skehana, Ballylee, Kiltartan, Caherglassaun etc

The basins start to fill and as they fill, they

Coole lake starts overflowing into Caherglassaun at Ballynastaig, under a culvert close to Connolly’s house. This starts to fill Caherglassaun more but a serious problem then emerges.

The water cannot progress out of Caherglassaun and it starts to fill and rise and eventually, the culvert is overrun and Connolly’s house becomes flooded

It was very heartbreaking for many people to see the Connolly’s build major flood defenses around their house and very distressful for them as the water levels eventually swamped the house.  This was also the situation which eventually saw major flooding for many other communities in South Galway. Eventually the levels peaked and equalized  all around Coole, Caherglassaun, Tierneevin close to 14.78m


In the 1st week in January 2016, the water level bound by Caherglassaun reached 14.68m with Coole, Tierneevin just above it at 14.78m and it flowed into Cahermore. Cahermore then rose and flooded and then with a temporary channel it continued into Caherawoneen and then into Kinvara.

With the Coole, Caherglassuans rising to 14.78 m, Coole levels reach and impacted other communities (e.g.Tierneevin, Tawnagh, Corker, Raheen) and made many underground river systems redundant.

An opinion that I mention to many people around the Coole area is that when they mention clearing out swallow-holes as the high-priority item for the area, I highlight the fact that if the level of Coole lake is allowed to rise as much as it is, it renders swallow holes useless. In my own area of Kiltartan there is a swallow hole that connects Kiltartan to Coole and in medium winter flooding/s the water flowing through it is substantial.  In winter 2015 however, the levels of water between Kiltartan and Coole were just 0.22 m and this wasn’t going to be enough for any underground river to sink a substantial flow.

In summary we cannot have Coole lake rising to these levels and if we want to do something about it we need to focus on the lip of the basin between Caherglassaun and Cahermore and mange these levels to the sea at Kinvara.

How big are these ‘flows’?

In a report called “Groundwater flood risk mapping and management: examples
from a lowland karst catchment in Ireland“, 2015, by O. Naughton, P.M. Johnston, T. McCormack and L.W. Gill, there was some very good analysis of flood levels during 2009. They produced the following graph of the November 2009 rainfall and the corresponding levels of the lakes/turloughs:

Graph from “Groundwater flood risk mapping and management: examples from a lowland karst catchment in Ireland“, 2009,

The most dramatic levels seen here are in Blackrock where the turlough rose 7m  in just over 3 days.   Focusing back on Coole/Garryland we can see

  • Coole/Garryland rose 5.5m in 12 days from low levels (4m) =  0.45m/Day
  • Coole/Garryland rose 4m in 12 days at higher levels = 0.35m/day

We can also see the long tail and the levels slowly falling where it is averaging only a a 0.15m (15 cm) fall per day and this is despite the fact that there is water from underlying areas continuing to flow into it.


So we have some metrics on Coole lake height increase per day and in 2015 these seemed the same (taking to locals) but the height went up an additional metre than 2009.  But what about volume?  I did a quick estimate on maps and calculated the area of Coole lake + Caherglassaun to be at a maximum of 12 sq km. What type of volume are we looking at for Coole/Caherglassaun to rise 0.35m?  – Simple :

  • 1000m x 1000m x 12 x 0.35m =  4,200,000 m3 (Note : m3 is a cubic metre)

So 4,200,000 m3/Day =175,000m3/hour =2917m3/Minute = 48.61m3/Second and that’s the magic number. Let’s round it up to 50m3/s.

The magic number for how much water is needed to rise Coole, Caherglassaun at 0.35M/Day is 50M3/Second.

This is a great number because it doesn’t tell me how much water is flowing in, our how much water is flowing out. This magic number It’s telling me the difference between the two!

This magic number is telling me that if I want to STOP this Coole/Caherglassaun basin from rising then I need to ‘Channel’ water at a rate of a mere 50m3/s!

Before focusing on this number, there are some things to considerations.

I’m using the 0.35m/day rather than the initial rate of 0.45m/day because at lower-levels there isn’t as much land to fill e.g. 3-4 square km. So I’m using 0.35m/day. If the lake was filling at this level in 2009 then this would equate to a magic number closer to 70m3/s.

What about the flows draining the lake?  According to an excellent white paper called ‘Quantification of submarine/intertidal groundwater discharge and nutrient loading from a lowland karst catchment Coole Lake‘, 2014, T. McCormack ⇑, L.W. Gill, O. Naughton, P.M. Johnston. When Coole lake is 12m high, the discharge at Kinvara (West) is 17m3/s and approx 2m3/s at Kinvara (East). We can estimate outflow for Coole Lake @14m height is as follows:

  • Approx 20 m3/s : Kinvara (West)
  • Approx 2 m3/s : Kinvara (East)
  • 10 m3/s : Culverts near Dunguaire
  • 3 m3/s :  Kinvara (Central) : Close to  Pier Head

This means that in peak levels, the underground drainage (+culvert) to Kinvara could be close to 35m3/s.  Remember that Coole Lake is rising by 0.35m/day by what we’ve calculated at 50m3/s so that means the overall inflow into Coole lake would be close to 85m3/s on average.  This doesn’t seem too far off the mark.

Again, the 50m3/s is the number to watch because this is roughly the flow that we will need to Kinvara to equalize levels.  The figures are approximate but if the OPW wants to give me €2.5million then I’ll do my best to come up with more accurate ones 🙂

Now, we come to the nub of the problem. Water doesn’t flow uphill so next we need to look at the levels.

Managing the levels

In an ideal world what is the maximum height do we want to keep Coole lake? We don’t want to drain Coole lake but we don’t want it to have an adverse effect on the drainage dynamics.  From the previous report the following dynamics were the ‘good times’ in Coole Lake.


From 2010-2013, Coole lake peaked at 12m and there were no drastic flooding crisis so let’s use this for discussion.

We want to keep Coole Lake peak levels below 12m

If we do this then the diagram below shows out the level in the main basins. The orange line shows us our ideal level at 12m and this the level we  would need a channel at.


This is interesting for a number of reasons.

  • If this level was maintained then the culvert at Ballynastaig would keep working (we want it to!)
  • There are are just two land areas that need to be managed.
    • Caherglassaun to Cahermore
    • Cahermore to Caherawoneen

Now, most of the locals in the area could have told you this but a picture paints a thousand words.

Interestingly enough this level has some good news for upstream also, it means that Ballylee and upstream would flow better because the new levels would promote better flow from Ballylee and consequently Blackrock.

If we go back to the 2009 maps there’s some other dynamics that we could consider.



Some interesting potential dynamics here on the 2009 floods.

  • Water Peaked in Cahermore on Christmas day in 2009, almost a month after the heavy rain. With the channels, in place water would have been flowing swiftly through Cahermore 2-3 weeks earlier
  • With the height at 12 m, the channel would in theory only need to be flowing for a week
  • Because the channel would have brought away the peak levels the return to ‘normal’ could be accelerated by 3-4 weeks.

The estimated duration of flow would need to be increased for the 2015 flooding as there was a more substantial volume of water involved.

What size would the channels need to be and how long?

OK, firstly we’re not talking about the Suez canal here. We are not even talking about water super-highways. We are talking about 2 channels in the order of a 5-7 metres wide and 2-3 metres deep (We’ll let the experts calculate these). These would probably in the order to 700m from Caherglassaun to Cahermore and 200m from Cahermore to Caherawoneen.

There are other factors to be taken into account e.g. the culverts should be oversized to account for regional growth, climate change  but something in the order of 50m3/s-80m3/s would be substantial progress.  [Note OPW are futureproofing the culverts under the M18 motorway at Kiltartan at 100m3/s so this type of figure should be taken under consideration]- We have to consider if we find a way of streamlining Blackrock and Castletown then this will increase flow rates into Coole. 

Also, this is not the only work, the channel would need to be streamlined toward  Kinvara and this means putting in properly sized culverts and land profiling getting the water out to the sea.

What would this cost?

That depends. The OPW report on the channel to Coole was planning a deeper channel (7.5m deep) and a longer channels (6.5km). The OPW costing for the channel were ‘€48,230,759’ (?) and of course with ‘Cost-Benefit’ analysis was deemed unfeasible]

Other than that I think smaller channels as discussed would be substantially lower cost and w.r.t. ‘Cost-Benefit’ analysis,  even if it cost €10 million, this would probably pay for itself in alleviating 1 flooding crisis.


In this blog post, I’ve gone through the basins and outlined how they flow. I’ve picked a level of 12 m as Coole Lake as a good level to work from and with basic calculations a number of 50m3/s has been calculated as rate that will get the water moving and not allow the levels to build up beyond this

If we imagine starting at the sea and walking backwards – Here is what we should see.

Caherawoneen-> Kinvara

At the sea at Kinvara, not far from Dunguaire castle there is now a large culvert capable of draining flood water from Caherawoneen.  This culvert may be in addition to the two 3ft  pipes there currently or it could be a precast culvert – It would deliver 3 times the capacity that is currently there.  It reminds me of some of those storm channels that you see in any modern country where a water flow seems tiny compared to the large capacity of the culverts – In modern countries this would be a real bridge.

The capacity on the culverts on the Ardrahan road and the other road at Caherawoneen are also 3 times the current capacity. Some of the land has been reformed to smooth the flow and some walls have been replaced with railings to aid flows

This allows Caherawooneen to drain efficiently into the sea. This will have added benefit to allow Cahermore to sink underground more efficiently.

Cahermore -> Caherawonneen

The temporary channel close to Glynn’s house has been replaced with a more substantial channel that is much lower then the temporary one.  A new channel is now in it’s place 2-3 m below the temporary channel and 3 m wide. The road at Cahermore has been raised but a large culvert can take a substantial flow from Cahermore across to Caherawoneen.


There is a new channel from Caherglassaun to Cahermore South – 700-800m long. It is an open channel and it’s designed to take the lip of Caherglassaun and lower the peak levels by 2-3 m. There is a large culvert beneath the road (Leech’s) that brings water to Cahermore.


Water is flowing out from Coole and going under the Culvert at Ballynastaig, The water keeps flowing as Caherglassaun is no longer backing up.

Overall the levels of Coole lake are maintained at 12m and there is not of the catastrophe that we have seen in recent times!

This is not rocket science, there is nothing technically blocking on this. The numbers may need to be tweaked a bit but they are probably in or around the ballpark. If the experts want to add more volume or height then let them  – the important thing is to get a good solution in place before this community suffers again.

That’s what South Galway wants!

-David Murray

[Note : Please find a list of references here:]