A SELECTION OF EXTREME FLOOD EVENTS – THE IRISH EXPERIENCE A.M. Cawley (Hydro Environmental Ltd. Galway), J. Fitzpatrick (ESBI Dublin), C. Cunnane (Dept of Hydrology NUI Galway) and T. Sheridan (Met Eireann, Dublin). 1. INTRODUCTION This paper presents a technical discussion of six significant flood events in Irish Catchments over the past 50 years. These catchments and flood events are the River Dodder Dublin - August 1986, the River Lee Co. Cork – Aug 86 and Nov 00, the River Brosna Mullingar - November 1965, the River Deel at Crossmolina Co. Mayo October 1989, the River Clare Co. Galway November 1968 and the River Griffeen Dublin November 2000. Details as to the severity and rareness of the flood event along with information on rainstorm conditions are presented. Estimated flood runoff rates, flood and storm return periods and percentage storm runoff are presented. 2. RIVER DODDER FLOODING General Description The Dodder River which rises in the Dublin Mountains and drains northwards through south Dublin to Ringsend has long been noted for its flashy response to rainstorms. In a paper by F.G. Dixon (1953) to the Old Dublin society he made reference to floods in the Dodder in Sept 1787, Nov 1794, Dec 1802(ranked as the most serious flood), Sept 1807, Jan 1851, Sept 1880, Oct 1891, etc. In a report to the Commissioners of Drainage of Ireland in 1844 by Robert Mallet, he referred to the effect of flooding the river had on the lands along its banks as follows: (source Dublin City Council, 1986) “One of the frequent floods on the river occurred so suddenly at night, of such magnitude, and attended with such appalling circumstances of destruction to property and danger to human life, as to have aroused the public concern for the damage resulting. ” Over the past century flooding events resulting in overtopping of river banks occurred in 1905 (ranked as second largest flood this century), 1912, 1915, 1931, 1946, 1958, 1965 and the most recent and disastrous 25 th /26 th August 1986 (known as Hurricane Charlie) following exceptionally heavy rainfall. During construction of the Bohernabreena reservoirs in the 1880’s two flood events were noted in September 1883 and October 1886 when 96.5 and 93.5mm of rainfall fell during storms which lasted for approximately 14hours with runoff at a scale of 23.3 l/s per ha (2.23cumec per km2). The River Dodder rises near Kippure in the Dublin Mountains at an elevation 751m OD. The principal tributaries are the Owendoher River, the Little Dargle, the Dundrum River and the Tallaght stream. The Dodder has a total catchment area of 113.3 km 2 to its confluence with the River Liffey at Ringsend. Its upper reaches stretch from Kippure to Bohernabreena where it forms a reservoir system which is an integral part of the Dublin City Supply (Bohernabreena Reservoir created in 1883). It flows northwards to Tallaght and then north-eastward through Rathfarnham/Terenure, Donnybrook, and Ballsbridge before discharging into the Liffey Estuary at Ringsend. The total length of the river from source to outfall is 27km. The average channel gradient between the source of the river at Kippure and the spillway at Bohernabreena is 1 in 15. From Bohernabreena Lower reservoir downstream to Ringsend the channel has a gentler gradient of 1 in 115 (this gradient would still be considered significant). The lower 16km of the river contains 13 weirs of varying height. Downstream of the weir at Ballsbridge the river is tidal. The river is gauged by the DCC/EPA at Orwell Weir (95km 2) and at Bohernabreena (28km 2). Historical Flood Flows A list of the 15 largest flood flows on the Dodder gauged at Orwell Bridge Station (09010) over at least 100years is presented in Table 1 below. This information was derived from a combination of previous floods in the Dodder (1880 – 1986) compiled by Jack Keyes (1987) and the EPA annual 14 National Hydrology Seminar 2005 A.M.Cawley & C.Cunnane maximum (AM) flow series (MacCárthaigh EPA, 2005). The Hurricane Charlie Flood represents the historical maximum flood in the Dodder in at least 100years if not significantly longer. Table 1 List of highest Ranked floods on the Dodder at Orwell Weir Date Peak Flow (cumec) 25 August 1986 232 25 August 1905 198 05 November 2000 156 03 September 1931 153 17 November 1965 139 19 December 1958 116 02 December 2003 112 11 June 1993 110 05 November 1982 106 09 April 1998 87 02 November 1968 85 December 1983 82 11 June 1993 81 26 August 1912 80 25 September 1957 74 The number of properties flooded by the Dodder during Hurricane Charlie was estimated to be 340. The main areas of Flooding were from Lower Dodder Road, Orwell Gardens, Dartry Cottages, Clonskeagh Road, Simmonscourt Terrace, Eglinton Road, Anglesea Road, Merrion Road, Wilfield Road, Gilford Road, Shelbourne Road, Ballsbridge Avenue and Beatty’s Avenue. Flooding was also observed on the Poddle (85 properties flooded), Camac (30 properties flooded) and Tolka (10 properties flooded) Rivers (Keyes, 1987). Rainfall During Hurricane Charlie it was estimated based on 11 rain-gauging stations using Thiessen’s Polygons that the catchment areal rainfall to Orwell Gauging Station was 134mm in 24hours and 137mm in 31hours. Slight adjustment in the rain gauge representation gave a rainfall estimate for the catchment of 142mm in 24hours and 145mm in 31hours. These rain depths averaged over a catchment area of 95km 2 to Orwell Gauge represent substantial rainfalls well in excess of 100years. Table 2. Comparison of Rainfall amounts at specified gauge locations between 1905 and 1986 events (Extracted from O Reilly Met Report, 1987.) Location 1986 Storm 1905 Storm Glenasmole 165.3mm 145.3mm Bray 86.0mm 113.0mm Dunlaoghaire (Harbour yard) 77.5mm 80.5mm Dunlaoghaire (Peoples Park) 67.7mm 79.5mm Phoenix Park 85.1mm 85.1mm Stillorgan 89.0mm 81.8mm Glasnevin 73.5mm 90.9mm Antecedent conditions prior to the arrival of Hurricane Charlie were reasonably wet resulting in relatively low SMD. In the upper Dodder catchment, approximately 180mm of rainfall fell in a 24hour period from 9:00am Monday to 9.00am Tuesday 26 th August. This exceeded the previously largest 24hour rainfall amount recorded in 1905 and practically doubled that recorded during the 1965 flood. The 24 hour rainfalls in the lower catchment are similar to previous recorded highs and would 15 National Hydrology Seminar 2005 A.M.Cawley & C.Cunnane be of the order of 100year return period storm event. At the Kippure summit 1day fall of 250mm was registered. The MET Eireann 100-year 24hour duration rainfall amount for Casement rain gauge is 91mm and 128mm for Glenasmole gauge. During Hurricane Charlie 89.2mm fell at Casement and 162.5mm fell at Glenasmole in 24hrs . Flood Flow Estimate and Return Period The peak flow rate for Hurricane Charlie was estimated at that time to be between 215 and 254cumec with 232cumec being used (Hennigan et al., 1988). These estimates were based on a rating equation derived from flow measurements having a peak measured flow rate of only 33cumec (not very reliable given the extrapolation length involved). A flow estimate of 228cumec was produced using Horton’s (1907) Weir equation for Orwell Weir. Flood profile modelling of the Orwell weir and upstream reach gave a flow estimate of 201cumec (Hennigan et al, 1988). The EPA’s most recent estimate for the Hurricane Charlie flood peak is a flow rate of 269cumec. This is derived from present day rating relationship which includes a maximum gauged flow rate of 106.5cumec (measured on the 3 rd Feb 1994). A flow estimate of 264cumec was obtained by fitting a best fit curve to the gauged flows above 20cumec, refer to Figure 1. Deriving the Hurricane Charlie flow represents a significant extrapolation from the reliable range of 107cumec with ±30cumec also possible. In the following flood frequency analysis to estimate the flood return period the original flow estimate of 232cumec is retained. A flow rate of 232cumec represents a runoff rate of 2.44cumec per km 2 or 24.4 l/sec per ha over the entire catchment and represents approximately 70% storm runoff. Using the gauged annual maximum flow series for Orwell gauge 1949 to 2004 (8 AM years were missing) and fitting an EV1 and an EV2 (k=-0.15) probability distribution gives a return period for the Hurricane Charlie peak flow of 469years and 142 years respectively, refer to Figure 2. The EV1 fit is poor and thus the return period estimate of 469years is not considered accurate bearing in mind the high flow recorded in 1905. The EV2 fit (i.e. the k value) is considerably influenced by the Hurricane Charlie flow estimate. If this estimate represents an outlier or is substantially over-estimated it will skew upwards the return period estimates. A censured flood frequency analysis using the documented 15 highest AM floods in 100years is presented in Figure 3. This approach allows inclusion of the highest floods in the past century and gives a return period estimate of 158years which is similar to the EV2 estimate. 1000 y = 19.837x 2.2882 100 10 Discharge (cumec) Discharge 1 0.1 0.1 1 Hurricane 10 Stage m above 27m OD Malin Charlie Figure 1 Rating Data and best fit Rating Curve for River Dodder Flood Flows at Orwell Bridge (Station 09010) 16 National Hydrology Seminar 2005 A.M.Cawley & C.Cunnane 400 EV1 (k=0.0) EV2 (k=-0.15) 350 AM Flows Return Period 300 250 200 150 Discharge (cumec) Discharge 100 50 5 10 25 50 100 200 500 years 0 -2-10 1 2 3 4 5 6 7 8 EV1 Yvariate Figure 2 Dodder Flood Frequency analysis and estimate of Return Period for Hurricane Charlie Peak flow 350 Censured AM Flows 300 Censured EV1 Fit Return Period 250 200 150 Flow Rate (cumec) Flow Rate 100 50 10 25 50 100 200 0 2 2.5 3 3.5 4 4.5 5 5.5 6 Return Period (years) Figure 3 Censured Flood Frequency EV1 analysis using largest 15 AM flows in 100years (period 1905 – 2004).