Part A - Guidance for Submission of Flood Risk Assessment (FRA) and Drainage Assessment (DA)

When is a Flood Risk Assessment required? 

Due to the nature of the Strategic Flood Map for Northern Ireland the geographical extent of predicted flood areas cannot be precisely defined. In some cases reservoir inundation maps may not be available.  A FRA is required: 

  • When the proposed development is located close to the edge of an area of flood risk in order to determine a more accurate extent of flooding, irrespective of whether the site lies just outside or just inside (wholly or partially) the extent as depicted on the Strategic Flood Map.  
  • When the proposed development is located within a flood plain but constitutes an exception listed under Policy FRD1. In such cases, an FRA is required to be submitted as part of the planning application, so as to ensure the identification of all sources of flooding, the resulting flood extents and the means by which flooding is to be controlled and mitigated.  A FRA should not be undertaken when a proposal is clearly unacceptable in principle under the policy as this will invariably result in nugatory work and expense on the part of the developer.

What should a Flood Risk Assessment contain? 

  • A location plan to a suitable scale, which clearly illustrates geographical features and identifies the catchment, watercourses in the vicinity and the built development.
  • A site plan (and where appropriate, cross sections) showing existing levels related to Ordnance Datum Belfast), existing structures, watercourses in or bounding the site, internal site drainage and drainage outfalls.
  • Data on historical flooding events, including photographs and media reports, supported by information on rainfall, flood return periods and the probability of storm surge occurrences, where appropriate. Evidence on trends in flood occurrences and changes in the local environment since the last event is particularly valuable.
  • A plan of the site showing the extent of the predicted flood plain, and/or in the case of a reservoir, the extent of the predicted flood inundation area. This may require a local hydraulic model based on the topographical information, historical flood events and the assessment of design flow discharges at the site using industry standard methodologies.

When the proposed development is within the fluvial or coastal floodplain 

  • A location plan as detailed above.
  • A site plan (and where appropriate, cross sections) showing pre-development and post development levels related to Ordnance Datum Belfast, existing structures, development proposals, watercourses in or bounding the site, internal site drainage and drainage outfalls.
  • Details of any existing or proposed flood alleviation measures or flood defence structures that may influence the site including information on their structural condition, level of protection and maintenance regime.
  • The identification of all sources of flooding pre and post-development.
  • An assessment of the hydraulic capacity and structural integrity of all drains and sewers within or bounding the site. The methodologies for assessment must be clearly identified.
  • Data on historical flooding events, including photographs and media reports, supported by information on rainfall, flood return periods and the probability of storm surge occurrences, where appropriate. Evidence on trends in flood occurrences and changes in the local environment since the last event is particularly valuable.
  • A plan of the site showing the extent of the predicted flood plain and/or in the case of a reservoir, the extent of the predicted flood inundation area. This will involve the production of hydraulic models requiring longitudinal / cross sections of the watercourse and the site, assessment of flood discharges using industry standard methodologies, and the inclusion of information such as finished floor levels, access road and car park levels, estimated flood water levels, flood depths and velocities and associated probability of flooding.
  • A plan and description of features which may influence local hydraulics. For example, bridges, pipes or ducts crossing watercourses, culverts, embankments and walls.
  • An assessment of the likely speed of potential flooding, the sequence in which various parts of the site may flood, the likely duration of a flood event, the potential consequences of a flood event, the depth and velocity of flood water.
  • Where appropriate, the likely impact of any displaced water or increased runoff from the development site should be estimated and the consequences for neighbouring or other locations assessed.

Where the proposed development is located within the fluvial or coastal flood plain (or reservoir flood inundation area), the FRA will also be required to provide details of flood control and mitigation measures as well as safety procedures that will address the flood risks identified. The following considerations may be relevant: 

  • Flood Control Measures:
    • Infrastructure and drainage design where it may be possible to limit the flow and duration of flood water to the proposed development by diversion of flow paths, culvert upgrading and introduction of control structures such as sluices, weirs and sealed manholes.
    • Management of residual flood risk through keeping development a safe distance away from flood defence structures and introducing sacrificial flood storage areas at the rear of defences.
    • Suitable maintenance and management procedures.
    • Ground water control and pumping. 
  • Flood Mitigation Measures:
    • Site design and layout such as siting built development so as to avoid areas of the site liable to flooding and flood flowpaths. 
    • Raising finished floor levels of new buildings. 
    • Coastal infilling / land raising.
    • Flood resistant and resilient construction. 
  • Safety Procedures: 
    • Flood and weather warning systems. 
    • Clear communication lines between those at flood risk and those with flood risk responsibilities; Emergency evacuation plans and procedures including safe access and egress for emergency rescue services.
    • Capacity and procedures for the rapid movement of furniture and goods to locations outwith of the flood risk area.
    • Safe shutdown of electrical supply for domestic and industrial use. 
    • Pollution control procedures.

When is a Drainage Assessment required? 

A Drainage Assessment (DA) must be submitted along with the planning application for development proposals located outside the fluvial flood plain, in any of the circumstances outlined in Policy FRD3.   The DA, as well as addressing surface water flooding, should also identify control measures for storm water discharge from the site, including the use of sustainable drainage systems to manage and limit site discharges to pre-development run-off rates.

What information is required in a Drainage Assessment? 

A Drainage Assessment will typically be required to contain the following information relating to the assessment of surface water flood risk:

  • A location plan to a suitable scale, which clearly illustrates geographical features and identifies the catchment, watercourses in the vicinity and the built development.
  • A site plan (and where appropriate, cross sections) showing pre-development and post development levels related to Ordnance Datum Belfast, existing structures, development proposals, watercourses in or bounding the site, internal site drainage and drainage outfalls;
  • Confirmation as to whether the proposed development is to be located on previously developed land (that may have minimal impact on the existing drainage network).
  • Indication as to whether the local area has past flooding problems, which may limit site discharge to the local drainage and watercourses to pre-development run-off rates.
  • Identification of likely overland flow paths including depth, velocities, timing and sequence of inundation.
  • An assessment of hydraulic capacity and structural integrity of all drains and sewers within or bounding the site, which may result in out of sewer flooding. The methodologies for assessment must be clearly identified.
  • Data on historical flooding events, including photographs and media reports, supported by information on rainfall, flood return periods and the probability of storm surge occurrences, where appropriate. Evidence on trends in flood occurrences and changes in the local environment since the last event is particularly valuable.
  • The likely impact of any displaced water or increased run-off from the development site should be estimated and the consequences for neighbouring or other locations assessed.
  • Details of how sustainable drainage systems (SuDS) have been used as the preferred drainage solution where appropriate/feasible.

Part B - Sustainable Drainage Systems (SuDS)

Careful design and incorporation of SuDS into new development or redevelopment schemes will deliver effective drainage while at the same time avoiding increased flood risk downstream. Sustainable drainage effectively delivers on the three ‘pillars’ that define the concept, i.e. water quantity, water quality and amenity and biodiversity, as depicted below:

  • Water Quantity (Manage rainfall to mimic natural drainage):
    • Reduce run-off rates. 
    • Reduce additional run-off volumes and frequencies. 
    • Encourage natural groundwater recharge. 
    • Reduce the impact of short duration intense storm events, in particular helping to reduce the impact of ‘out of sewer’ flood / pollution events.
  • Water Quality (Minimise adverse impacts on water quality):
    • Reduce pollution and protect the quality of receiving waters. 
    • Prevent direct discharge of spillage - SuDS used at the construction stage for a development is considered as ‘best practice’. 
    • Reduce the volume of surface waste runoff to sewers and so reduce storm overflows 
  • Amenity and Biodiversity: 
    • Contribute to the amenity and aesthetic value of the development and the wider environs 
    • Provide habitat for wildlife and enhance biodiversity

Sustainable Stormwater Management Techniques 

There is a wide range of sustainable drainage techniques available to developers which can be applied, individually or in combination. A combination of techniques will deliver the best results – for example, a housing development where downpipes are fitted with water butts, permeable paving driveways, stormwater separation piping, and potentially swales which may in turn be linked to a pond or wetland area. 

Benefits of Sustainable Drainage 

Whilst the focus of SuDS may be on flood risk management benefits, it should also be noted that sustainable drainage offers a wide range of environmental, economic and social benefits.  

  • Flood Risk Management Benefits 

    With climate change predictions for more extreme rainfall events, sustainable drainage systems will provide more drainage capacity and will incorporate a design capacity considerably greater than traditional pipes. Accordingly, they offer greater flood protection. The main flood risk management benefits are outlined below: 

    • SuDS reduce peak flows through the use of appropriate sustainable drainage techniques and will reduce the impact of localised surface water flooding.
    • The reduction of peak flows from new development sites incorporating SuDS means that less stormwater will discharge to downstream drainage networks or watercourses, thereby reducing flood risk.
    • Effective sustainable drainage systems can reduce the demand for and cost of flood emergency response and preparedness procedures.
    • Sustainable drainage promotes a joined up approach to flood risk management as it requires input from a range of responsible bodies (e.g. DfI Rivers, DfI WDPD, Council, NI Water and other statutory undertakers) and a wide variety of disciplines (engineers, planners, architects / designers, hydrologists, water quality expertise and ecologists). 
  • Environmental Benefits 

    While the flood risk element of the disposal of surface water and the impact on human health and safety has long been a material consideration in the determination of planning applications; environmental considerations such as amenity, ecology and water resource issues have historically had limited influence on drainage system design and the determination of development decisions. However, the EC Water Framework Directive now requires urban diffuse pollution to be regulated through the implementation of the Directive. This means that continuing to drain built up areas without taking due account of wider environmental impacts, particularly on water quality, is no longer an option. Sustainable drainage provides opportunity for the realisation of a number of environmental benefits. These include:

    • Improved water quality. This can be delivered in a number of ways, including: (a) natural treatment provided within the SuDS component – for example by filtering drainage thus reducing the level of sediment discharging to watercourses; (b) absorbing of nutrients by plants growing within the SuDS system; and (c) reduced volumes within the combined piped wastewater systems will mean fewer spills of storm sewage to watercourses. 
    • Increased capacity for water storage through retention of storm water, for example in basins, ponds and water butts provides opportunities for this water to be reused. This in turn creates potential for households and businesses to reduce their consumption of potable water. 
    • Conservation of biodiversity and ecology will be supported through the incorporation of SuDS features such as ponds and wetlands. 
    • A well designed SuDS system can connect into and support the existing drains and waterways located beyond the development site, thus extending biodiversity via new nature corridors. 
  • Economic Benefits 

    Economic benefits likely to accrue from sustainable drainage include the following: 

    • The increased application of on-site sustainable drainage solutions by developers should mean that less investment will be required in the provision and maintenance of traditional piped infrastructure.
    • The removal of storm water from combined wastewater systems will reduce the running costs of sewage treatment works and costs associated with pollution of watercourses.
    • Developer savings can accrue through the combination and integration of sustainable drainage with open space provision, particularly on residential sites where the latter is usually required for amenity reasons. 
    • Developer costs associated with designing and installing a sustainable drainage system are invariably less than with a traditional piped system.
    • The retention of stormwater as a consequence of sustainable drainage may offer scope for rainwater harvesting and the reuse of this water can result in economic benefits. Considerations such as long term water resource security and improved water supply efficiency are assuming greater economic importance in the face of continually increasing demands upon water resources. 
    • Buildings overlooking water features generally command higher than average premiums. 
    • Although difficult to quantify, the benefits to societal health and wellbeing (see below) associated with sustainable drainage, particularly within urban areas, are likely to reduce public expenditure in such sectors as health and social services. 
  • Social / Amenity Benefits 

    Sustainable drainage also offers scope for the realisation of significant social, recreational and health / quality of life benefits. Examples include the following: 

    • The potential of some elements, such as swales, basins, ponds and wetlands to contribute to the provision and integration of ‘green and blue infrastructure’ within the urban fabric helps to deliver the quality of life benefits associated with such infrastructure. 
    • The potential use of some elements, such as ponds, for active and passive recreational purposes and educational purposes. 
    • Improved water quality generally will benefit public health and enhance the enjoyment of water based recreational activities such as angling. 
    • The risks to those suffering from respiratory conditions, notably asthma, resulting from air pollution, have been shown to reduce through the chemical effect of certain sustainable drainage systems in trapping pollutants. Such benefits are particularly realised in large urban areas where levels of air pollution are usually highest.

Feasibility and Design Considerations 

There are a number of considerations which may influence the choice and design of sustainable drainage solutions for specific sites. These include the following, although the list is not exhaustive: 

  • The surface structures that may be needed can use more space than conventional systems, although it is usually possible for them to be integrated into the surrounding land use, for example in public open space or road verges.
  • Infiltration may not be possible or is likely to be restricted in a number of circumstances; for example, if the permeability of the soil is limited, or the water table is high, or the land is contaminated or where there is ground instability. However, in all such situations, alternative SuDS solutions are usually available.
  • Safety and access considerations associated with surface water will always need to be considered as part of the overall design of the development in general and surface water SuDS features in particular, so as to minimise risks. 

Applicants should familiarise themselves with the ‘CIRIA SuDS manual’ (Construction Industry Research and Information Association, 2015) ‘Managing Stormwater – A Strategy for Promoting the Use of Sustainable Drainage Systems (SuDS) within Northern Ireland’ (NIEA, 2011) and the various references therein to other available SuDS documentation.  

Part C - Controlled Reservoirs within Mid and East Antrim Council Area (as of June 2019) 

NAME OF RESERVOIR

LOCATION

OWNERSHIP

Dungonnell

Cargan

NI Water

Killylane

Larne

NI Water

Kilwaughter Pond

Larne

Private

Lough Mourne

Carrickfergus

NI Water

North Woodburn

Carrickfergus

NI Water

Quoile Lower

Broughshane

NI Water

Quoile Upper

Broughshane

NI Water

South Woodburn Lower

Carrickfergus

NI Water

South Woodburn Middle

Carrickfergus

NI Water

South Woodburn Upper

Carrickfergus

NI Water

Wild Life Ponds

Carrickfergus

Council

Carrickfergus Mill Ponds
(Sullatober Flood Storage Reservoir)

Carrickfergus

Council

Copeland

Carrickfergus

NI Water

Dorisland

Carrickfergus

NI Water

Peoples Park

Ballymena

Council

Woodford Fishery

Carrickfergus

Private

Killyglen

Larne

Private

Ballymullock

Larne

NI Water

Beltoy

Carrickfergus

NI Water

Crebilly Ballymena South

Ballymena

NI Water

North Road Carrickfergus West

Carrickfergus

NI Water

Sullatober

Carrickfergus

NI Water

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