Flooding
Legislative Background
Local Authorities have the authority to respond to and provide flood protection works against fluvial and/or tidal flooding under the Local Authorities Works. This authority has been used to good effect on recent schemes such as the River Tolka Flood Study and the Baldoyle Flood Relief Scheme. At present, there are no statutory obligations for Local Authoities to prevent fluvial/tidal flooding.
Flood Embankment Under Construction, River Tolka
The Office of Public Works is responsible for numerous flood relief schemes throughout Ireland, deriving authority from the Arterial Drainage Act. There are several of these schemes within the administrative area of Fingal County Council, including the Rivers Matt, Delvin, Ward and Broadmeadow.
Under the Planning and Development Act, Fingal County Council considers the proper planning and sustainable development of planning applications. When an application is submitted, the onus is on the applicant to illustrate to the Water Services Department that there is not a significant flood risk to new and existing development.
Managing Flood Risk for New Development
Climate change, in combination with the ongoing urbanisation of catchments means that the flood risk to property is continually changing. In order to minimise the impact of an increased future flood risk, there are various steps that Local Authorities can take. These include flood protection works, stormwater attenuation and more significantly, control of development in flood plains.
In order to offset (partially) the effect of continued urbanisation, the attenuation of runoff from large developments can be included as a condition of planning. Climate change is best accounted for by anticipating the effect it would have over the design life of the development and allowing for this in the design and predictive model calculations.
In order to manage the risk of flooding to new and established development in historical floodplains the recommendations made in the River Tolka Flood Study (2003) should be followed.
The Impact of Climate Change
The Greater Dublin Strategic Drainage Study produced a policy document assessing the impact of climate change. It was concluded that the expected change in rainfall and sea levels would have a significant impact on future flood risk. Current thinking is that the average annual rainfall depths will increase, with typically drier summers and wetter winters. This would see a decrease in river base flows and a corresponding rise in flood flows. Sea levels are expected to rise gradually for approximately the next thousand years due to global warming, which will have an impact on low lying coastal areas.
In the Dublin region for the 2080 horizon, there will be an expected sea level rise of 400mm-480mm. The design return period for coastal flooding is to be 200 years, which equates to a design level of 3.4m AOD for the Dublin region. The rainfall depths for duration’s can be modified by using different return period multipliers to account for the expected increase in rainfall.
Flooding of Existing Development
If a property is subject to repeat flooding, various measures can be taken by the owner as an individual in order to reduce the damage and costs associated with floodwaters. Where ground floors are subject to flooding, it is recommended that flood resistant materials be installed. For example, instead of carpeting install ceramic tiling. Significant and costly damage also occurs when electrical sockets and appliances are inundated by floodwaters. If possible, they should be raised to a height above the expected flood level.
Small removable proprietary flood barriers can be installed across doors and vents. An alternative to sandbags in an emergency would be to use bags filled with earth. When flooding occurs and the council staff are unable to assist due to demand, the public can collect sand bags from the depots, either in Swords or Ballycoolen.
General Comments Regarding Flood Risk
There is some knowledge within the Water Services Department of historical flooding in the County, but this is limited and not always accurate. It is not always possible for Water Services to provide an accurate assessment and hence, not policy to issue letters assessing the flood risk to property. From time to time there may be reports produced and made publicly available that investigate flooding in more detail for specific locations. The onus rests with insurance companies and the owners to assess the risk to property based on the best available information.
It is important to understand that where flood protection works have been provided, they only serve to reduce the risk of flooding and do not eliminate it. Most works are sized to provide protection against a particular size event with some freeboard. If this event is significantly exceeded or there is a failure, flooding will more than likely reoccur.
Section 50 Consent
Any bridge built across a watercourse can have a serious impact on the flow, particularly during times of flood. It is important that the hydraulic performance of these structures is assessed and that a satisfactory level of performance is achieved. Section 50 of the Arterial Drainage Act requires that consent be obtained from the Office of Public works and applies to any new bridge and alterations to or the reconstruction/restoration of an existing bridge.
A Section 50 approval does not grant consent to construct a structure. Rather, it is a requirement of the Arterial Drainage Act used to assess the impact that the structure will have on flood risk.
Flooding on the River Tolka, November 2002. Photo courtesy of Mr R Roberts
The Section 50 requires that an application form is completed and all pertinent design data is included with the submission. For more detail, the OPW have made available a brochure listing all requirements. The information guide for Section 50 consent is available on the Internet at: http://www.opw.ie/services/eng_ser/eng_sec50.htm
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