Insufficient storage tank could have caused town centre flood
INADEQUATE flood prevention measures taken at the time of the South Eastern Motorway's construction may have contributed to the flooding of Dundrum Town Centre two weeks ago.
As engineers for Dun Laoghaire-Rathdown County Council continue their investigations into the tsunami-like flooding, the Sunday Independent can reveal that a storage tank designed to handle just two hours of heavy rainfall was put in place to prevent flooding of the Slang River in the area of the shopping centre site.
More conventional attenuation measures were ruled out on the basis that engineers considered them to be "impractical" for the Slang River, a part of which flows directly through the site of the Dundrum Town Centre.
Referring to the flood prevention proposals for the Slang River, the environmental impact statement drawn up in 1997 in advance of the South Eastern Motorway works stated: "Due to a history of flooding on the Slang River, attenuation of this run-off is considered necessary.
"Options for attenuation on the Slang River appear to be impractical, so a dedicated storage tank is proposed. Provisional sizing of this storage is based on two hours' retention at 20mm/ hour rainfall average intensity."
The provision for rainfall of 20mm per hour and a storm duration of two hours would appear to have been wholly insufficient to deal with the monsoon conditions experienced in Dublin and along the east coast on October 24.
According to Met Eireann, 82.2mm of rain was recorded at Casement Aerodrome that day, with 60mm of this amount falling over the course of a four-hour period between 2pm and 8pm alone.
The adequacy of the rainfall storage tank, which is located at the Ballinteer interchange of the M50, will almost certainly feature prominently in the report now being prepared by engineers for Dun Laoghaire-Rathdown County Council.
Early indications from the council's investigations into the flooding, which saw the emergency evacuation of panicked shoppers from the south Dublin retail mecca, are pointing to an "overtopping" of the Slang River at a location above the centre's site.
Asked by the Sunday Independent if the Ballinteer interchange rainfall storage tank was being considered by engineers as a contributing factor to the flooding, a spokeswoman for Dun Laoghaire-Rathdown County Council said: "As the causes of the flooding at Dundrum are still under investigation by council engineers, it just isn't possible at this time to comment on this specific question."
As revealed in this newspaper last week, the site of the Dundrum Town Centre has been flooded by the Slang River in the past.
Another environmental impact statement prepared in 2000 at the time of the Dundrum Town Centre's planning application stated that: "Flooding has occurred on the Slang stream in the past and the disused Pye factory at the Ballinteer Road end of the site was flooded in May 1993.
"This arose when the channel of the stream was blocked by debris close to the inlet to the Mill Race and resulted in waters flowing alongside the north of the Castlebrook access road over the existing bridge and into the forecourt of the Pye buildings."