The Office of Public Works (OPW) has responsibility for approximately 11,500 km of arterial drainage channels, about 2200 km being subject to routine maintenance each year as part of an ongoing programme of arterial drainage maintenance. This leads to an average cycle of maintenance taking place just over once every 5 years. This work is carried out to ensure that the State's investment in the Arterial Drainage schemes continues to provide the intended benefits. These schemes were primarily focused on facilitating an increase in agricultural output. In doing this two criteria were considered, namely, the reduction in the frequency of flooding and the increase in drainage outfall thus reducing water logging. The former tends to be the overriding criterion on larger channels while the latter tends to dictate the depth and size of the minor channels. If maintenance is not carried out then most channels would return over a number of years to their pre-drainage state.
It should be noted that there are many other rivers and watercourses for which the OPW does not have any maintenance responsibility. Local Authorities have statutory responsibility for maintenance of rivers and watercourses which form part of Drainage Districts. For other watercourses falling outside of the OPW and Local Authority areas of responsibility it is private landowners generally that have responsibility for the maintenance of these.
The attenuation referred to relates to the reduction of the flood peak by storage of the floodwater somewhere where it will do less damage. To do this, somewhere that is not already flooded has to be flooded or somewhere that is flooded has to be flooded to a greater depth. On larger channels this option would therefore require that considerable areas of land would have to be flooded to a greater extent in a controlled fashion. For example, on the Blackwater in Fermoy, 25 square kilometres of land flooded to an extra metre would have been required. This option is examined on all of our urban flood relief schemes and is only rarely found to form part of the preferred solution, generally where the flows are relatively low and a suitable attenuation area is available. In a rural setting it is almost never beneficial to transfer the flood from one landowner to another. Another negative that arises with storage solutions is in the area of residual risk. When an event in excess of the design event is encountered there can be a step change in the effect on the benefiting area when the storage is exhausted and the integrity of the storage area also poses a risk as the excess flood water is stored upstream of the benefiting area. In contrast, if the design capacity of the drained channel is exceeded it continues to confer benefit and the increase in flood is more gradual.
It has been suggested that bogs might be used to attenuate the effects of flooding. A recent (February 2012) study "The Use of Wetlands for Flood Attenuation" carried out by the Aquatic Services Unit, UCC commissioned by An Taisce, stated in its executive summary that "The value of wetlands for flood attenuation is, however, often exaggerated and many wetlands in fact play only a very weak role, if at all, in attenuating floods". It further stated that "it (the influence of wetlands in reducing flood peaks) is least for large events, particularly following a long period of prior rainfall, when soil and wetland storage are saturated."
