Flooding in Grannagh, Blackrock and Skehanna

This article covers some of the flooding scenarios within the Ownshree catchments where the communities of Roxborough, Castleboy, Grannagh, Blackrock and Skehanna are vulnerable to flooding.

Location and Hydrology

This area that we are focusing on is mainly the area west and south west of Loughrea, along the main Loughrea to Gort Road.  The main river flowing through this area is called the Owenshree river which makes it’s grand appearance flowing under the Seven-Eye Bridge close to Kilchreest.

seveneye.JPG

The Owenshree river is part of 3 main rivers in the Galway Bay South East Catchment and this area in particular is called the Kilchreest Catchment which is sourced from the North Western region of the Slieve Aughty mountains. It can flow as close to 1km from Loughrea lake but will eventually end up in Kinvara bay (Note : Loughrea and easterly Slieve Aughties drain into the Shannon)

kilchreest

The following diagram shows the profile of the river flow from Slieve Aughties to Kinvara and you can se that Blackrock is at the end of a land-locked basin but has underground connectivity.

owneshreeprofile

Some of the main areas within the downstream catchment are Kilchreest, Castleboy, Grannagh, Blackrock and Skehanna.   The main upstream sources are Sonnagh, Gortnamannagh, Killeenadeema and lower Glenaclara

sonnagh2

The area of the main sources of the Owenshree river is Sonnagh and one of the highest sources is Lough Belsrah which has a Windfarm (Sonnagh) located next to it.

sonnagh2.JPG

Lough Behsrah is an unusual lake located in a valley with steep sides and supposed to be quite deep.  Its about 500m Long and average of 50m across.

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Lough Belsrah (Courtesy of Marco Leen)

Lough Belsrah and small channels from the windfarm flow downstream and are joined from small channels from Gortnamannagh and flow toward the ‘Soldiers bridge’

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Stream coming down from the Sonnagh windfarm

The Sonnagh, Gortnamnnagh areas represent a significant portion of the uplands catchment and these flow to the Soldiers bridge in Gortnamannagh.owenshree2.JPG

There are also areas of Killeenadeema and lower-lying Glenaclara that flow into the Owenshree river and this flows north-west under the Seven-Eye bridge at Kilchreest.

From there the river changes direction from north-west to south-west and heads parallel to the Loughrea-Gort road toward Castledaly and Peterswell.   It first flows through Roxborough (Lady Gregory’s birthplace) ruins and past the old stewards house.  From here the river flows into  Isterkelly and Castleboy where it disappears into swallow holes in the streambed in Ballybackage.  There is connectivity between these swallow holes and to the Coole Demense river rising.

In a normal summer the river fully disappears into the swallow holes and in fact in a normal winter this can be also be the case.  When there are heavy winter rains, these football type of swallow holes overflow downstream into Blackrock Turlough which rises and flows through a number of small swallow holes.

Blackrock Turlough

Blackrock Turlough is located on the left side of the N66 between Gort and Loughrea close to Peterswell.

blackrock.JPG

In heavy winter rain, it can rise significantly.

djotcexxuaar8bp
Courtsey of

Blackrock can contain a significant amount of water in wintertime e.g. 4.1 million cubic metres (2006), 4.0 million cubic metres (2007) and 2.47 million cubic metres (2008) and can rise to depths of over 12 metres in some places [1].

One thing about Blackrock is that it’s regarded as a flashy Turlough as it is one of the closest to the Slieve Aughty Mountains – After heavy rains, in over 36 hours, Blackrock Turlough can rise over 10 metres and extend over 2km.

flash.JPG
A single night’s rain can cause Blackrock Turlough to rise (Courtesy : Sean Brady Aerial Photography)

Blackrock Turlough swallow holes have a significant connectivity into the underground river system. The connect to neighbouring Lough Coy and Ballylee and can connect to Kinvara east (close to Dunguaire) and Kinvara Central springs (Pier).

blackrock turlough connectivity

Blackrock connects underground to its close neighbour Lough Coy which then connects to the Ballylee (Ballycahalan  river) and in severe cases, the depth difference and pressure between Blackrock and Ballylee is great (>10m heights) that one of the Ballylee swallow holes starts to reverse and become a rise (Also known as an estevelle) .  The following diagram shows a conceptual model [1] and the type of connectivity from mountain to sea.

swallowholes

Severe Winter Flooding

As we have seen on many previous blogs, in normal summers and normal winters, these underground systems work perfectly.  Here is a snapshot of the height over 3 years. [1]

blackrock2.JPG

Another way to look at Blackrock is to look at average inflows and outflows and in the above period in ‘filling phase’ is we can see an average inflow to Blackrock is 10m3/S while at the same time it’s outflow is just on average 2m3/s [1].  Therefore, if the 10m3/sec inflow is sustained over a prolonged period, the Turlough fills and starts to become a flood risk.

When you get severe winter rains in the Slieve Aughty mountains, the small swallow holes that can normally sink 2m3/s cannot handle this and like a basin the Turlough levels start to rise,  leading  to significant flooding.  While the maximum volume of Blackrock Turlough in Sept-2006-Sept 2009 as 4.1 Million cubic metres of water, in 2009 the volume reached of 15.9 Million cubic meters at its peak and covered an area in excess of 3 km2.

This Turlough rose to close to 29 m and this caused significant flooding impacts. Here is a profile of Blackrock Turlough during the 2009 winter flooding. [1]

blackrock3.JPG

The level of Blackrock peaks around 29m because it then finds an over ground route which will be discussed below. Similarly with Lough Coy, once it reaches 17m, it will also overflow over ground.

Flooding Dynamics

There are 4 main flooding dynamics when the Owenshree catchment has high levels of rainfall.

owenshree_flooding_dynamics.JPG

These are as follows:

  1. Flashflooding in Roxborough
  2. Backup and flooding in Blackrock and upstream
  3. Turlough Overflow (Underground)
  4. Turlough Overflow (Overground)

These will be dealt with separately.

Flash-flooding in Roxborough

The first casualty of flooding happens around Roxborough.  The Owenshee river powers under the 7-Eye bridge and turns South-West and into Roxborough.

roxborough_map

The river goes through a narrow bridge and then the river itself narrows and causes a restriction in very high water flows.  With rapid run-off the Slieve Aughty’s these restrictions lead to a water backup and this floods Paddy McGlynns home and farm buildings. Note : This house is a protected building constructed around 1820. (Details here)  and unfortunately floods to about 2ft of water

roxborough_flood.JPG
The area in Roxborough, close to the old Roxborough House which is prone to flash flooding (McGlynns home – Stewarts house inset)

roxborough_flooding.JPG
The dynamics around flooding in Roxborough. The Bridge and a restriction in the river cause the water to back up and overflow into McGlynns – This only lasts for a few hours but floods a home and farm buildings.

As this is close to the Slieve Aughty mountains, the flooding is flashy.  The flood can come within a few hours and be gone 20 hours later. This type of flooding has happened several times over the past 20 years only.

Backup and flooding in Blackrock and upstream

When there is a large water volume coming down the Owenshree catchment (from Sonnagh, Gortnamannagh and Killeennadeema) then Blackrock Turlough can’t sink all of this water (coming on average at  and starts to rise. If the volume of water is too great then the entire area from Blackrock back to Grannagh starts to fill up. This causes flooding in Castleboy, Grannagh and around Blackrock

flood1.JPG

flood3
Here Blackrock Turlough has filled back as far as Grannagh and flood homes. Castleboy i
grannaghFlood.JPG
Looking directly west toward Blackrock. With Grannagh on right. Courtesy of Sean Brady Ariel Photography.

Eventually, if the water runnoff the mountain is voluminous enough, the Blackrock Turlough level reaches a limit (29m above see level) and overflows into Skehanna.

Turlough Overflow (Overground through Skehanna)

Once the level of Blackrock approaches 29 metres (above sea level) – then it finds a outlet close to Limepark house, beside Rahealy.  Some of Turlough is over 16m deep at this stage.

It overflows across the Skehanna to Blackrock road and flows through Colm Burke’s farmyard and in through the village of Skehanna causing severe flooding to the community.  It then makes its way to the Lough Coy overflow.

ske4
The devastation of the overflow close to Rahealy as it comes through Skehanna.
FLOOD4
Blackrock expanding to threaten Coleman’s house (left of Centre). Courtesy of Sean Brady Ariel Photography.
ske1
Blackrock Turlough spilling over into Colm Burke’s farm yard. Courtesy of Sean Brady Ariel Photography.
ske2
The overflow contining into Skehanna and cutting off access. Temporary scaffolding is in place to help with access. Courtesy of Sean Brady Ariel Photography.
ske3
Capturing this from the other side,  with Blackrock Turlough in the background. the flow through Skehanna village can be seen here. Courtesy of Sean Brady Ariel Photography.

Blackrock Turlough Overflow (underground)

As Blackrock gathers a head of volume of water 16m deep in some places, it can put a significant pressure (underground) on the downstream system in Lough Coy and Ballylee.  It causes swallow holes to become springs. It causes Ballylee swallow hole north (Pollaleen) to  reverse direction and rather than being a swallow hole – it becomes a rise and water flows out of it (this is called an estevelle).  Blackrock overflow and underflows now pouring into Ballylee, through Skehanna and Lough Coy join with the the swelling Ballycahalan river and starts to flood the Ballylee, Deerpark and Rinerush areas.

ballylee_flood2.JPG

Impacts

There were several houses flooded in Roxborough, Castleboy, Grannagh, Blackrock and the Skehanna area in the 2009 and 2015/16 floods.

In Roxborough, 1 house and farmyard was flooded.  In Grannagh 3 homes, were flooded and 4 were badly threathened and 11 were inacessible and many septic tanks were flooded. In Skehana/Blackrock – 2 homes were flooded badly, 2 flooded but were protected (sandbags/pumps) and 3 were badly threathened.

Over 38 farms were affected in Grannagh, Blackrock and Skehanna and over 800 acres of farmland was flooded, many of these farms were flooded for several months. Over 10 farms were in accessible and 8-9 farmyard slurry tanks were washed out into the area.

Access

In Castleboy, acces was limited from Castledaly.  In Skehanna and Blackrock 16 families were isolated and required a pedestrian bridge to get out. The following roads were closed:

  • Labane-Peterswell road closed for 50 days
  • Skehanna-Limepark road closed for 50 days
  • Skehanna-Peterswell road closed for 99 days

Overall, the flooding dealt a very serious blow to the communities of Roxoborough, Grannagh, Castleboy, Blackrock and Skehanna.

What can be done?

This area is one of the high-risk areas as it is so close to the Slieve Aughty Mountains and is very sensitive to changes in land practice.  There are real concerns that mountain land management, farming, forestry and windfarms have increased the mountain run-off over the past few decades. There are several potential mitigation measures including.

  • Slowing the flow
  • Flow Diversion
  • Underground drainage
  • Keep flowing

For slowing the flow, attenuation would attempt to hold water on the mountains.  There may also be methods to ensure mountain land management doesn’t overly accelerate water off the mountain e.g.. land drains, forestry drains, forestry roads, windfarm drains and roads.  There is a potential to use natural flood management techniques to slow the flow – remember a dynamic of blackrock is that normally it can only drain at 2m3/s to anything to slow the peak flows into is are useful.

Diverting the flow

One of the interesting things about the Owenshree river (which can rise 1M in 4 hours),  is that soon after the Seven-Eye Bridge close to Kilchreest where the river swings left (west) it is only a few hundred metres from the source of the Aggard. Here it is in the red circle below:

Linkup.JPG

This means that there is a potential solution to be able to divert some of the Ownshree into the Aggard. This solution has been details in an OPW report Termon Mannin Kilchreest Final Report 231210.  This solution is feasible, but only on the back of delivering an improved drainage scheme on the Dunkellin river which is currently being developed.  Temporarily alleviating some of the flow from the Owenshree river would lessen the build up of water on Castledaly, Grannagh and Blackrock and be an overall benefit on the whole South Galway flooding.

Protecting from flash flooding

Once the water is travelling unders the 7 Eye bridge – It’s almost at full flow and there is no stopping it on its way to Roxborough toward Blackrock.  An obstruction (Bridge and river narrowing)  holds water back but floods McGlynns as the river is very close his house.  Could the house, farmyard be bunded?  Could the bridge be increased? Will it have an impact downstream?  It will have to be analyzed by our hydrologists but it seems very reasonable to try to protect a house that is threatened by water for a several hours, after which things are back to normal again and that extra water is already in the Blackrock Turlough.

Underground Drainage

Blackrock Turlough is the farthest from the sea and it becomes more challenging to drain off additional water through the current system, particularly where there are 2 other rivers into South Galway system that have flooding dynamics.  From talking to locals in the area, I’ve heard rumors that there were some river diversions on the Owenshree river many years ago, something to do with Limepark house. If you check out the 1836 OS map here and move the OVERLAY Slider and you can see the overlay of then (1936) and now  and you will see a river appear and disappear

diversion

These (and other) swallow holes may be covered but it may be possible to clear some out so they can sink their maximum flow.

 

Another dynamic that could be investigated is the results of keeping water flowing downstream. For instance if Coole Lake has a more streamlined outlet to the sea that stops it reaching 15m (over sea level) then in theory Kiltartan and Ballylee will flow better and this would have an impact on the underground flow from Blackrock. It’s hard to say how much this would be but it would probably not be major.  Hydrology experts are currently studing these effects and will be able to give a scientific answer.

Overall the flooding mitigation of these communities is a high prioritiy for the South Galway- Gort Lowlands floods scheme and the design consultants will be tasked with coming up with solutions designs within the next 18 months.

 

References/Acknowledgements

A huge credit goes to Own Naughten, Lawerence Gill and professor Johnston who have done a lot of studies around the South Galway Area.  The Turlough data and some concept images were taken from the following paper

Also big credit to Sean Brady, from Sean Brady Aerial Photography as his flooding images captured from his drown have really helped us tell the story of South Galway Flooding

Thank to Tommy Fahy for details on Grannagh

Thanks to Pauric Collins and Colm Burke for providing details on Skehanna and Blackrock.
-David Murray

 

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3 thoughts on “Flooding in Grannagh, Blackrock and Skehanna”

  1. Wonderful article. It would be great to organise a tour so we could visit these places aystematically. Also a public talk with slides etc. could help
    Build up a public understanding of what is happening and contribute to a public will to encourage and support more action.

    Liked by 1 person

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