Flooding Solutions #1: Overview

I’ve done some detailed presentations on the flooding situation happen in South Galway and the good news is that for the majority of the areas affected by flooding there are solutions – they are not rocket science.  It’s time to have an overview of the key solutions available.

From a previous video presentation, I summarized the key items to flooding in this  South Galway area.  These are detailed below:

CriticalItems.jpg
Critical items influencing flooding in South Galway
Conceptually this can be viewed like this:

  

These are as follows:

  1. Manage the rapid run-off the Slieve Aughty Mountains
  2. Manage the pinch point at Kiltartan
  3. Manage the Level of Coole lake and its path to the sea

I will try and document these in detail in the coming weeks but here is a summary of the main solution approaches.

The rapid run-off the Slieve Aughty Mountains

There is no doubt that there is a serious increase in how fast and how much water is coming off the mountain during heavy rainfall.  In a previous post I’ve described the correlation between heavy rainfall and an immediate increase in river water levels. Within 4 hours, some of the Slieve Aughty rivers can rise over 1 metre. With 36 hours, Blackrock turlough can rise over 10 metres and extend over 2km.  This rapid run-off has an immediate impact on the area around Castledaly, Grannagh,  Peterswell, Skehana, Blackrock, Ballylee, Deerpark, Rinrush, Castletown. There are three viable solutions to this:

Slow the flow

There are well documented cases about slowing the flow coming down from mountain.  In English towns such as Pickering 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. A lot of this is to bring back some of the more natural drainage to the mountains.

IMG_1500
A ‘Grip’ cut into the mountain in the Slieve Aughty’s

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 [Subject to approval on 29th-Feb-2016].    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.

Underground Drainage

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. Check out the 1836 OS map here : Move the OVERLAY Slider and you can see the overlay of then (1936) and now  and you will see a river appear and disappear

Was there a swallow hole covered in around here or the OS guys just come across a summer? If we zoom with on aerial photographs around Bullaunagh- we see the old path of the river in 1836 … and now today it does not exist [but you can still make out the old river path]

Does this swallow hole still exist?   This needs more investigation. In general it is essential to ensure that the swallow-holes throughout the system are in good condition and can keep the water flowing.

The pinch point at Kiltartan

As detailed before, Kiltartan is the pinch point in the Slieve Aughty drainage. All flows must go through Kiltartan.  The water needs to be able to flow freely through Kiltartan and into the Coole Basin.

motorway2
Kiltartan Flows
The key aspects of the solutions in the Kiltartan Area are :

  1. Keep the level of Coole Lake down – If Coole Lake rises then the swallow-hole at Kiltartan will not be as efficient and will result in an overflow across Corker.
  2. A properly managed Kiltartan-Corker overflow.
    1. Properly sized culverts to manage the Kiltartan- Corker flood overflow across the local road
    2. Properly sized culverts underneath the new M18 motorway
    3. Properly sized culverts to transfer water over to Raheen and Coole
  3. Alternate overflow channel beneath Coole Ridge, again, under the new M18 motorway.

Areas such as Skehana,  Rinrush will also need to have well managed overflow paths.

The Level of Coole lake and its path to the sea

Coole lake level probably has the biggest impact on flooding in the South Galway area.  Its levels have an indirect affect on the levels in Kiltartan and consequently the majority of the underground river system.  Its levels also have a direct impact on the neighboring communities including Raheen, Glenbrack,  Roo, Tierneevin, Garryland, Ballynastaig, Caherguassuan and Cahermore. The key in minimizing the tremendous amount of flood damage that current Coole lake levels cause, is in the the lowering of Coole lake maximum levels.

The following diagram shows the relative heights of Coole lake and other areas during this years flooding:

lakelevels.JPG
Highlights of some of the key levels in South Galway on 4th Jan 2016
As Coole lake fills, Kiltartan levels will raise as the swallow-hole pressure drops and this impacts Ballylee etc also.  Also note, that the blocking point to the sea is not the level of Coole lake but the level of Caherglassaun. This lake needs to raise very high before it overflows into Cahermore, so consequently if you want to control the level of Coole Lake, you need to control the level of Caherglassaun. The follow on from that is that you then need to ensure Cahermore has a path to the sea.

Creating a proper overflow channel from Caherglassaun to the sea via Cahermore will have a major impact on the reduction of flooding in South Galway

The temporary river channel that was dug from Cahermore to Caherawoneen was a very good example of allowing the water to flow and reducing the peak flood levels in Cahermore.  A more permanent solution is currently being looked at to create a proper overflow channel.

No silver bullets

These solutions should complement each other to reduce flooding affects in South Galway.  If Coole lake levels can be kept down this will allow water to flow more freely through the system but if we can ‘Slow the Flow’ and divert some of the rapid-runoff the Slieve Aughty Mountains then this will reduce the overall peak volume of water in the system.  We also have to manage areas where water dynamics will lead to significant overflows in areas like Skehana, Rinrush and Kiltartan.

 

-David Murray

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One thought on “Flooding Solutions #1: Overview”

  1. Yes David, the swallow hole at Grannagh exists It was cleaned late 2014 and seemed to be working very well until the December 2015. The Owenshree disappears into the Blackrock turlough as it leaves Bullanagh This is a very accurate report Please keep up your good work. I trust that the OPW are taking note of your work and respond accordingly.

    Like

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