Criteria for Flood Defence Schemes

In determining if a flood defence scheme is to be implemented, the OPW have regard to the following broad criteria:

(a) the scheme must be technically feasible;

(b) the scheme must generally be cost beneficial (a cost benefit analysis is undertaken to determine the economic merits of the project); and

(c) the scheme must also be environmentally compatible (an Environmental Impact Study is normally undertaken for each scheme and the scheme must satisfy the requirements of the EIS).


Flood defence schemes can incorporate any measure that reduces the risk or impacts of flooding. For example, some of the options which may be considered include the following: flood containment (permanent walls, embankments, demountable flood barriers, etc.); increasing the flow capacity of a river through widening, deepening, re-alignment or maintenance of channels, removal or modification of structures (bridges, weirs, etc.); retention or storage upstream by means of dams or controlled flooding of a natural floodplain; diversion of part of the flood flow (through a flood relief channel or by-pass channel) or the redirection of the entire river (full channel diversion); flood warning systems which provide adequate warning of an impending flood and allow residents and local authorities to take action to reduce the impacts of the flood (an effective flood warning system is essential in the case of the use of demountable flood barriers).

Implementation Process

There are a number of basic stages in the process of implementing a single flood defence scheme. Normally, the process is initiated in response to a request from a Local Authority.

Firstly, there is a preliminary assessment, which reviews the cause and scale of the flood risk and determines if OPW involvement is appropriate. This is followed by a pre-feasibility study to determine if a scheme is likely to be viable. If so, there then follows the feasibility and outline design stage when a detailed assessment, involving much data collection and surveys, of possible flood relief options is carried out culminating in the selection, with justification, and outline design of a recommended flood relief scheme.

A catchment-wide approach is now being followed, in accordance with current policy , in the assessment of flood risk and the development of risk management measures. This approach is being implemented through Catchment Flood Risk Assessment and Management (CFRAM) Studies, which will, for most at-risk locations, deliver on the objectives of the pre-feasibility and feasibility and outline design stages, although some localised confirmation and design work will be required before moving to the next stage of preparing for Public Exhibition (see below).

This is followed by the document preparation and Public Exhibition stage. This involves the preparation of the scheme documentation (schedules setting out details and benefits of the scheme, including names of the proprietors, owners and occupiers of the lands with which the proposed scheme will interfere; maps, drawings, plans, sections setting out the technical detail; Environmental Impact Statement, if required; and Interference Notices sent to each affected person detailing the extent of works proposed on their respective lands or property and any proposed compulsory interference with, or acquisition of, these lands and property).

All of the Scheme Documents are forwarded to the relevant Local Authority and they are also placed on formal Public Exhibition in a public building(s) in the area typically over a period of 4 weeks when interested parties and the public have the opportunity to study the proposals and make comments, observations, objections, etc. OPW staff and/or consultancy staff are available at Public Exhibition to answer queries and offer clarification. Interference Notices are also forwarded to affected parties in advance of the Exhibition period.All observations received are responded to and, if necessary, the scheme may be revised as a result of them.

Following Public Exhibition, the scheme is submitted to the Minister for Public Expenditure and Reform for Confirmation of the Scheme.

Once confirmed by the Minister, the OPW are empowered to implement the scheme and the process moves to the detailed design and construction stage.Normally, OPW appoints a firm of consulting engineers to act as the Engineer (detailed design, construction supervision, etc.) and a firm of engineering contractors for the actual construction of the scheme.When all works have been completed, a Completion Certificate is issued by the Minister for Public Expenditure and Reform. Before the issue of the Certificate there is a set time frame for the receipt of any objections that must be dealt with before the Completion Certificate is signed and sealed.

Finally, there is the monitoring and maintenance stage when the OPW assumes responsibility for the ongoing monitoring and maintenance of the scheme.

Further Information