Dunboyne Combined Residents Association
Home - About DCRA - Contact DCRA -Archive - News- Politicians Directory - Planning - Environment - Links Page - Newsletters - DCRA Meetings - Soapbox - Waste & Recycling
An Bord Pleanala Oral Hearing: 20/21.04.04 ( Appeal Ref:PL 17.204690 P.A.Reg Ref DA/30236)
Presented by (Prof.) Susan McKenna-Lawlor
Rainfall Records and Flooding at Dunboyne
Seventeen flood events at Dunboyne are recorded in the available historical records.
Prior to November 2000, the floods of 1880 (estimated flow at the outlet of the Tolka 71 m3/s) and 1954 (85 m3/s) were the largest on record. The floods of 2000 (76 m3/s) and 2002 (97 m3/s) which occurred within two years of each other and caused major local distress, suggested to the Dublin Drainage Consultancy Consortium, then engaged in producing the Greater Dublin Strategic Drainage Study (GDSDS) for Dublin City Council, that a critical review of the statistical model for flood prediction was called for.
It transpired in this review that the data set available does not fit the typical statistical profile of extreme flood flows in Irish rivers (contained in the 1975 Flood Studies Report).
In particular, the ratio of extreme floods to typical mean annual floods is significantly higher for the Tolka than for Irish rivers in general, so that a particular combination of circumstances can give rise to a disproportionately high run-off extreme event as compared with a typical storm event.
A statistical analysis of the data available yielded design flows at Drumcondra as follows
Once in 100 year return period 90 m3/s
Once in 50 year 75
Once in 25 year 60
An alleviation scheme was recommended aimed at protecting existing developed areas in the study region of the rive from at least a one in 100 year flood event, with built in capacity to deal with projected urban development (unspecified) up to 2031. The safety margin of the scheme was adjusted to accommodate a significant degree of rainfall increase (300 mm) associated with possible climate change impacts (up to 10%)
Comments on the Analysis
The flood of 15th November 2002 at Dunboyne, Co. Meath followed 2 days of very heavy rainfall. A previous rainfall event on 8-10 November had resulted in a very wet catchment which, combined with winter vegetation conditions, meant that a very high level of run off took place on November 15th with little, if any, infiltration/soakage in the ground
The associated total rainfall depth measured at Dublin Airport over 52 hours was 87 mm. Again at Casement 72 mm of rainfall was recorded over a period of 35 hours. Met Eireann estimated that a rainfall of this duration has a return period of about 50 years - supposing a steady background climate unaffected by anthropological activity and no change in local land use. Later modeling studies showed that the rainfall recorded at Dublin Airport and at Casement must, in actuality, have been significantly exceeded (over 100 mm of rain) to produce the observed flood flows at Dunboyne and this was confirmed by reference to radar data.
The Intergovernmental Panel on Climate Change (IPCC) has considered the problem of projecting precipitation patterns and found that winter precipitation in northern Europe may increase by as much as 20% in this century according to most models (IPCC 1996 WG 1, Fig. 3.62). In a later study published in 1997 entitled The Regional Impacts of Climate Change (Chapter 5, Europe) it was stated " annual precipitation trends in this century are characterized essentially by enhanced precipitation in the northern half of Europe with increases ranging from 10% to close to 50%". Since there are still many uncertainties, related work is ongoing at the IPCC.
In view of the peculiar association of high run-off extreme events with flooding characterizing the behavior of the Tolka River, it is too simplistic to take the statistics of typical storm events as a baseline when estimating flood risk. It is noted that Dunboyne has already experienced in practice two serious floods in the past 4 years and three within a period of 50 years. While it is too early to state that these floods represent a signature of changing precipitation patterns. a rising trend in accordance with the IPCC results quoted above is suggested by the data, with a flooding response that was probably exacerbated by increasing levels of building on the flood plain.
Rather than consider the kind of special precipitation events to which the Tolka (with rising frequency in this century) responds by flooding, the GDSDS report, and also the Castle River study contained in the Environmental Impact Study (EIS) prepared by Fenton Simons Ltd. on behalf of Menolly Homes, Ltd. (which purports to provide a basis for the proposed attenuation of surface water at Dunboyne Castle following Meath County Council guidelines), have based their predictive arguments on past storm events. This is not reliable.
Also it should be remembered that an important objective of the Tolka River Study prepared by the GDSDS was to develop a sophisticated model that would constitute the basis of an informed assessment of the environmental impact of proposed contiguous developments. This robust model was to represent the hydrological and hydraulic characteristics of the River Tolka catchment and have the ability to map flood risk
Unfortunately, because of recent work carried out on the bed of the River Tolka and its environs to alleviate flood risk, the model concerned cannot now be calibrated using historical measured data. The Tolka is in effect a different river from that which produced the design flows quoted in the various documents submitted to An Bord Pleanala and 'ground truth' figures are not available.
A safety margin of 10% to cover the possible impact of climate change is noted in the GDSDS Strategic Study Report for flood management in the Dublin area to be included in their figures.. This percentage is based on climate change scenarios developed for the UK (the Hadley Center Regional Climate Model). This British model depends on a complex mix of local factors and the philosophy that it can be applied in the Dublin area as a basis for estimating possible, medium - long term impacts in the River Tolka catchment is not tenable.
Conclusion
The statistics of typical storm events are not adequate to provide information concerning those high run-off extreme events in the river Tolka catchment area that are understood to pose a particular risk of flooding. When considered against IPCC reports of changing precipitation patterns in northern Europe, such events can be expected to significantly increase in frequency with time (we already have had 3 major flooding episodes in Dunboyne in the last fifty years). The IPCC is presently actively studying the relevant changing precipitation trends but more statistics and modeling needs to be done.
The Model of the Tolka River so long under preparation by the GDSDS cannot presently be calibrated using actual measurement data due to remedial work recently carried out on the river.
Since reliable predictive tools are not at this time available, neither the Dublin Drainage Consultancy Consortium that informs Dublin County Council nor Fenton Simons Ltd are in a position to reliably anticipate future flooding patterns in the Dunboyne area.