Limerick County Council has published the findings of a Severity and Impact report into the flooding events of August 1st 2008 in Newcastle West, County Limerick.
Details of the report, which was compiled by JBA Consulting, were revealed at a public meeting in the West Limerick town this evening. The meeting was attended by members of the local community affected by the floods, as well as representatives of the Office of Public Works and JBA Consulting, and Limerick County Council's Transport and Infrastructure, Environment and Housing sections.
Commenting on the background to the report, Mr. Gerry Behan, Deputy County Manager, Limerick County Council, stated, ‘Following the devastating flooding in Newcastle West in the early hours of the 1st August 2008, JBA Consulting was commissioned to compile a timely report to provide a basis for informed discussion and analysis between all stakeholders. The aims of the report were to assess the severity of both the rainfall event in the upstream catchment and the resulting flood in the River Arra, to provide a factual record of the impact of the flood and to identify probable ‘root causes’ of the flooding in order to make recommendations for future analysis.’
‘I would like to thank JBA Consulting for producing this detailed report. Limerick County Council, in conjunction with the Office of Public Works, now looks forward to following through on the key recommendations outlined within the report. The report will be made available in full on the Limerick County Council website at www.lcc.ie from tomorrow morning’, added Mr. Behan.
EXECUTIVE SUMMARY:
Data Collection
Two walkover surveys were undertaken; the first immediately after the flood event, and the second once water levels had dropped and the clean up had begun. Data collected included river levels and rainfall totals, weather reports and radar images and photographs and video footage. Interviews were conducted with eyewitnesses to the event, including local residents and emergency personnel, and local engineers. Questionnaires were also distributed to gather the experiences of local residents. The collation and analysis of this data is to be undertaken following this report.
Mechanisms of Flooding
The investigations followed the source-pathway-receptor model, with particular attention paid to the pathways of flooding. The pathways through the catchment above the affected area are important because flooding is not always caused by rain which falls on the location affected directly. It can be caused by rain falling on the upstream catchment which then reaches the town via the drains, rivers and overland flow. The speed with which rain is transferred through and over the ground is determined by features of the catchment such as size, gradient, permeability and wetness, as well as the intensity and duration of the rainfall.
Newcastle West and the Arra Catchment
Newcastle West lies on the downstream reach of the River Arra, which is a tributary of the River Deel. The catchment is relatively small, at 43km2, and extremely steep in its upper reaches. The ground falls from over 350 mAOD to around 80 or 90 mAOD from the summit to the foot of the mountains lying on the western margins of the catchment. The ground then slopes more gently to Newcastle West, which is at an elevation of around 45 mAOD. The underlying geology of the catchment is impermeable, and is overlain by poorly draining soils. There are three main rivers in the Arra catchment which meet in Newcastle West; the Arra, the Dooally and the Mash.
Rainfall
The weather in the days leading up to the 1st August was very wet. Persistent rainfall had resulted in a saturated catchment, revealed by the soil moisture deficit for the area as calculated by Met Éireann.
Rain gauges in the vicinity of the catchment are all daily gauges, meaning they record total rain for the previous 24 hours. Thiessens analysis of the gauge data allowed a catchment average total rainfall to be calculated for the period 09:00 GMT 31st July to 09:00 GMT 1st August. The catchment average rainfall for the period was 85.9mm.
Eyewitness accounts and rainfall radar images indicate that that rain began at around 19:30 on the 31st July and continued until approximately 01:30 on the 1st August, giving a storm duration of 6 hours. Assuming a storm duration of five to six hours, this depth of rain equates to up to a 0.15% AEP (or a return period of up to 1 in 650 years).
River Level
As there are no recording gauges on the Arra a hydraulic model will be required to calculate the exact return period. However, there are two river level recording gauges on the Deel near the Arra at Deel Bridge and Grange. The levels and severity information for the Deel gauges show that water levels were well in excess of any previously recorded event; by 600mm at the Deel Bridge and 400mm at the Grange Bridge.
Description of the Event
Eyewitness accounts and evidence gathered on the site visit were used to establish the flooding sources and pathways. The saturated catchment was unable to absorb the rain and a deluge of water ran off the steep mountain side and over-whelmed the field drains and ditches, before reaching the main watercourses through the catchment, namely the Dooally, the Mash and the Arra. Water flowed through the town, both via the channel and the roads, inundating many properties and moving debris and vehicles in its path. Water depths of up to 1.8m above road level were recorded in the town.
Impacts of the flood
A large number of properties suffered from flood damage, both internally and externally. A more detailed count of those affected will be established following analysis of the questionnaires, but initial estimates suggest over 200 properties may have suffered from some degree of flooding. Although a number of people were rescued from property or vehicles, no-one was seriously injured during the event. Power and water supplies were also affected for a time after the event. Road closures, both as a direct result of the flooding and due to subsequent land slides, remain in place in some areas.
Conclusions
The Arra catchment is very steep with a dense river network upstream of the town. Newcastle West sits at the confluence of three main rivers; the Dooally, the Arra and the Mash;
- July was an exceptionally wet month which resulted in fully saturated ground conditions prior to the storm;
- During the storm, 10% of the expected annual rainfall fell in 5-6 hours giving a rainfall return period of up to 1 in 650 years;
- Water sheeted off the upper slopes, over-whelming field drains and ditches and forming overland flow routes upstream of Newcastle West town Some properties were flooded as a result of water returning to the channel;
- In the town itself, the river channel and structures were also overwhelmed. In response to public concern, it is noted that Mill Falls Apartment and the new Bridge of Tears have greater capacity to pass flow than other structures on the channel.
RECOMMENDATIONS:
- Available data on the channel and structures should be supplemented with additional channel topographic survey and structure survey as required;
- Further detailed hydraulic analysis should be undertaken, in conjunction with the OPW, to assess the capacities of the channel and individual structures in order to identify actual risk associated with each of them. This includes in particular Cullenagh Bridge, the Bridge Street crossing, the culvert below the Bedding Store, Mill Falls Apartments, Courtenay Bridge and the Bridge of Tears (both structures);
- JBA Consulting visually recorded wrack marks throughout the whole town. It is recommended that these marks be surveyed at a later date to give a more detailed analysis of flood levels;
- Flood outlines for planning should be produced for the 1% AEP and 0.1% AEP events to align with forthcoming Government Flood Planning Policy;
- Development of a detailed Digital Elevation Model (DEM), ideally using LiDAR, to produce a detailed flood event outline;
- Compilation and analysis of questionnaires, including further interviews should they be deemed necessary.
Details of the report, which was compiled by JBA Consulting, were revealed at a public meeting in the West Limerick town this evening. The meeting was attended by members of the local community affected by the floods, as well as representatives of the Office of Public Works and JBA Consulting, and Limerick County Council's Transport and Infrastructure, Environment and Housing sections.
Commenting on the background to the report, Mr. Gerry Behan, Deputy County Manager, Limerick County Council, stated, ‘Following the devastating flooding in Newcastle West in the early hours of the 1st August 2008, JBA Consulting was commissioned to compile a timely report to provide a basis for informed discussion and analysis between all stakeholders. The aims of the report were to assess the severity of both the rainfall event in the upstream catchment and the resulting flood in the River Arra, to provide a factual record of the impact of the flood and to identify probable ‘root causes’ of the flooding in order to make recommendations for future analysis.’
‘I would like to thank JBA Consulting for producing this detailed report. Limerick County Council, in conjunction with the Office of Public Works, now looks forward to following through on the key recommendations outlined within the report. The report will be made available in full on the Limerick County Council website at www.lcc.ie from tomorrow morning’, added Mr. Behan.
EXECUTIVE SUMMARY:
Data Collection
Two walkover surveys were undertaken; the first immediately after the flood event, and the second once water levels had dropped and the clean up had begun. Data collected included river levels and rainfall totals, weather reports and radar images and photographs and video footage. Interviews were conducted with eyewitnesses to the event, including local residents and emergency personnel, and local engineers. Questionnaires were also distributed to gather the experiences of local residents. The collation and analysis of this data is to be undertaken following this report.
Mechanisms of Flooding
The investigations followed the source-pathway-receptor model, with particular attention paid to the pathways of flooding. The pathways through the catchment above the affected area are important because flooding is not always caused by rain which falls on the location affected directly. It can be caused by rain falling on the upstream catchment which then reaches the town via the drains, rivers and overland flow. The speed with which rain is transferred through and over the ground is determined by features of the catchment such as size, gradient, permeability and wetness, as well as the intensity and duration of the rainfall.
Newcastle West and the Arra Catchment
Newcastle West lies on the downstream reach of the River Arra, which is a tributary of the River Deel. The catchment is relatively small, at 43km2, and extremely steep in its upper reaches. The ground falls from over 350 mAOD to around 80 or 90 mAOD from the summit to the foot of the mountains lying on the western margins of the catchment. The ground then slopes more gently to Newcastle West, which is at an elevation of around 45 mAOD. The underlying geology of the catchment is impermeable, and is overlain by poorly draining soils. There are three main rivers in the Arra catchment which meet in Newcastle West; the Arra, the Dooally and the Mash.
Rainfall
The weather in the days leading up to the 1st August was very wet. Persistent rainfall had resulted in a saturated catchment, revealed by the soil moisture deficit for the area as calculated by Met Éireann.
Rain gauges in the vicinity of the catchment are all daily gauges, meaning they record total rain for the previous 24 hours. Thiessens analysis of the gauge data allowed a catchment average total rainfall to be calculated for the period 09:00 GMT 31st July to 09:00 GMT 1st August. The catchment average rainfall for the period was 85.9mm.
Eyewitness accounts and rainfall radar images indicate that that rain began at around 19:30 on the 31st July and continued until approximately 01:30 on the 1st August, giving a storm duration of 6 hours. Assuming a storm duration of five to six hours, this depth of rain equates to up to a 0.15% AEP (or a return period of up to 1 in 650 years).
River Level
As there are no recording gauges on the Arra a hydraulic model will be required to calculate the exact return period. However, there are two river level recording gauges on the Deel near the Arra at Deel Bridge and Grange. The levels and severity information for the Deel gauges show that water levels were well in excess of any previously recorded event; by 600mm at the Deel Bridge and 400mm at the Grange Bridge.
Description of the Event
Eyewitness accounts and evidence gathered on the site visit were used to establish the flooding sources and pathways. The saturated catchment was unable to absorb the rain and a deluge of water ran off the steep mountain side and over-whelmed the field drains and ditches, before reaching the main watercourses through the catchment, namely the Dooally, the Mash and the Arra. Water flowed through the town, both via the channel and the roads, inundating many properties and moving debris and vehicles in its path. Water depths of up to 1.8m above road level were recorded in the town.
Impacts of the flood
A large number of properties suffered from flood damage, both internally and externally. A more detailed count of those affected will be established following analysis of the questionnaires, but initial estimates suggest over 200 properties may have suffered from some degree of flooding. Although a number of people were rescued from property or vehicles, no-one was seriously injured during the event. Power and water supplies were also affected for a time after the event. Road closures, both as a direct result of the flooding and due to subsequent land slides, remain in place in some areas.
Conclusions
The Arra catchment is very steep with a dense river network upstream of the town. Newcastle West sits at the confluence of three main rivers; the Dooally, the Arra and the Mash;
- July was an exceptionally wet month which resulted in fully saturated ground conditions prior to the storm;
- During the storm, 10% of the expected annual rainfall fell in 5-6 hours giving a rainfall return period of up to 1 in 650 years;
- Water sheeted off the upper slopes, over-whelming field drains and ditches and forming overland flow routes upstream of Newcastle West town Some properties were flooded as a result of water returning to the channel;
- In the town itself, the river channel and structures were also overwhelmed. In response to public concern, it is noted that Mill Falls Apartment and the new Bridge of Tears have greater capacity to pass flow than other structures on the channel.
RECOMMENDATIONS:
- Available data on the channel and structures should be supplemented with additional channel topographic survey and structure survey as required;
- Further detailed hydraulic analysis should be undertaken, in conjunction with the OPW, to assess the capacities of the channel and individual structures in order to identify actual risk associated with each of them. This includes in particular Cullenagh Bridge, the Bridge Street crossing, the culvert below the Bedding Store, Mill Falls Apartments, Courtenay Bridge and the Bridge of Tears (both structures);
- JBA Consulting visually recorded wrack marks throughout the whole town. It is recommended that these marks be surveyed at a later date to give a more detailed analysis of flood levels;
- Flood outlines for planning should be produced for the 1% AEP and 0.1% AEP events to align with forthcoming Government Flood Planning Policy;
- Development of a detailed Digital Elevation Model (DEM), ideally using LiDAR, to produce a detailed flood event outline;
- Compilation and analysis of questionnaires, including further interviews should they be deemed necessary.
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