Dr. Owen Naughton:

I will give an introduction to the nature and characteristics of groundwater flooding. I will speak about how it differs from other forms of flooding and the influence it has on the flooding at Ballycar. Historically, groundwater flooding has been relatively poorly understood and overlooked. The standard of data on this form of flooding is very poor, especially by comparison with that on river flooding. There are no established methods for risk management or frequency analysis of this kind of flooding.

Groundwater flooding in Ireland is strongly linked with geology. It occurs mainly in the west in areas of pure well bedded limestone that are susceptible to a process known as karstification which occurs when limestone dissolves to form caves and conduits. Such areas tend to have discontinuous or absent surface water drainage. This means that it does not tend to be the case that there are continuous rivers draining rainfall out to sea. Instead, the groundwater system plays an important role in drainage. The movement of water through the landscape occurs mainly through the groundwater system, rather than through the surface water system. Groundwater flooding tends to require sustained rainfall of relatively long duration. Fluvial flooding tends to occur after hours, days or, in some cases, weeks of rainfall, whereas groundwater flooding tends to be caused by weeks or months of rainfall. The documentation I have furnished to the joint committee provides some examples of this. It includes three representative hydrographs for groundwater flooding. As members can see, there is quite a range. The red dot on the hydrograph shows where the annual peak takes place.

In terms of flood management, it is the highest level reached in any given flood season. The top turlough responds quite quickly so the annual maxima is quite early in the season. That is responding to rainfall in the order of two to three weeks. We get a range of those from weeks, to two to three months, all the way to maybe six months.

The second and third graphs show that the flood peak happens later in the season. The flood extent in any given year is a function of the amount of rainfall that fell in the previous three to six months. Ballycar appears to be in this range. Flooding tends to occur due to sustained rainfall over months. This also gives rise to the longer durations as it takes quite a while for flood waters to build up and it also takes quite a while for them to recede.

The other element of complexity with groundwater flooding is that there tends to be a range of flood mechanisms. This is the way in which a flood manifests. Groundwater flooding is more related to volume than discharge. In rivers, the flood is the peak discharge so it is the peak flow. In groundwater it is more an accumulation of water. It tends not to be flowing water; it is more hydrostatic water. It is just a build-up of volume within the landscape. We have developed some conceptual models for this. The most common form of groundwater flooding in Ireland is the turlough. These are topographic depressions in the landscape. They tend to dry out during summer but during winter, because there is a discontinuous surface drainage system, there is not enough storage in the landscape. What happens is excessive recharge is stored in these depressions so there is a build-up of flood water over the winter period which is then discharged back into the ground water system when the capacity becomes available. In most years this would not present a flood risk but in some cases, in periods of extreme sustained rainfall events, it can pose a flood risk to surrounding areas.

Another flood mechanism of particular relevance to Ballycar is backwater flooding. This is where there is point recharge, where a stream or river is discharging into the groundwater system. Under normal conditions, the capacity of the groundwater system to drain this water away is sufficient not to cause an excessive build-up of floodwaters so there would not be a flood risk. Under extreme circumstances, such as in the example in the second graph, there can be a large build-up of floodwater that can pose a flood risk. What causes this is the balance between the rate at which water enters the sink and the rate at which it exits. The rate at which it exits is a function of the size of the conduit draining it. It can be a function of downstream hydrological conditions. That is the most relevant groundwater flood mechanism with regard to Ballycar.

That is the background. I will now hand over to my colleague, Dr. Ted McCormack. The purpose of our programme is mainly to address the lack of historic data and provide more relevant information on how, why and when groundwater floods occur.