Case Study:

DLRD - Green Roofs

Background

Recent flood events across Dún Laoghaire Rathdown, particularly as a result of intense summer rainfall, have resulted in hundreds of properties being flooded and key road/rail infrastructure being impacted. In addition to riverbanks bursting and the resultant overland flows flooding homes/businesses, many properties were impacted from pluvial flooding caused by ‘monster’ rainfall events. Whilst work is underway in many key locations on river-based flood alleviation schemes, there is a clear need to seek to minimise the risk of localised flooding during these intense downpours. SuDS are a key tool in tackling the effects of increased stormwater runoff. However, often in urbanised areas, some SuDS measures are not practicable (e.g., swales, ponds, etc.) and thus other measures are required. Green (and/or blue) roofs are one approach to SuDS that can be incorporated into the majority of new (and in some cases existing) developments given that most developments have roofs. Thus, these can be readily modified/adapted to store rainfall either in vegetated areas (green roofs) or simply in ‘open basins’ (blue roofs).

Solution

Dún Laoghaire Rathdown County Council has been very proactive in the area of green roofs, and is the first local authority with a mandatory policy on green roofs embedded within its County Development Plan. This policy has a requirement for green roofs on the majority of larger developments. Green roof types range from simple ‘sedum moss’ type roofs, to intensive green roofs, which can be planted with shrubs, flowers, and lawn areas (Figure 1).

Benefits of solution

Environmental
The primary benefit to Dún Laoghaire Rathdown County Council is the attenuation effect that green/blue roofs have on rainwater runoff from large developments. This significantly reduces the effects of localised flooding (up to 80% in some cases). Other benefits include biodiversity enhancement, particularly if the green roof is designed with this in mind. A wide range of planting material can be utilised including wildflower meadows, etc. Other benefits can include reducing the ‘heat island’ effect by cooling the upper floor of a building in summer and reducing the amount of reflected heat. Equally, a green roof can provide an element of insulation during freezing conditions.

Social
Amenity benefits are often overlooked with green roofs, yet they can provide significant mental and emotional well-being benefits as is similar to other nature-based environments.

Figure 1: Green roof – Dún Laoghaire Rathdown County Council.

Project Details:

Timeframe
The installation of green roofs will continue to be a core element of SuDS in Dún Laoghaire Rathdown County Council.

Local authority project contact
Austin Baines
Senior Executive Officer
Dún Laoghaire Rathdown County Council

Joe Craig
Senior Engineer
Dún Laoghaire Rathdown County Council

Report: Implementation and Analysis of an Experimental Green Roof System with Runoff Reduction Modeling

This study, led by Prof. Francesco Pilla at UCD, evaluates the hydrological performance of green roofs in Dublin using real-time data and machine learning models. It focuses on how green roofs reduce runoff and delay peak flows, which is crucial for urban flood mitigation.

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