On the Meteorological and Hydrological Mechanisms Resulting

Australian Meteorological and Oceanographic Journal 61 (2011) 31-42 On the meteorological and hydrological mechanisms resulting in the 2003 post-fire flood event in Alpine Shire, Victoria Jillian Gallucci1, Lee Tryhorn2, Amanda Lynch3 and Kevin Parkyn4 1Department of Sustainability and Environment, Victoria, Australia. 2Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA. 3School of Geography and Environmental Science, Monash University, Victoria, Australia 4Australian Bureau of Meteorology (Manuscript received May 2007, revised November 2010) This paper describes an analysis of the factors surrounding a post-fire flooding event that occurred in the Alpine Shire, Victoria, Australia in late February 2003. Flash flooding occurred because of highly localised thunderstorms and was enhanced by the burned landscape. In addition, our analysis of the resulting situation suggests that other contributing factors included the storm cells likely being pulse wet microburst, cell regeneration over the same area, and the steepness of the Buckland River Catchment. The synoptic conditions surrounding the event suggest that the major drivers of the extreme rainfall event were the high levels of precipitable water in the atmosphere and enhanced atmospheric instability (including associated high CAPE values) from increased surface heating due to the reduction in surface albedo and soil moisture of the recently burned fire surface. In addition, weak upper atmospheric winds during the flash flood event contributed to slow storm movement. Hydrological modelling of the flash flooding event further indicated that fire-induced soil hydrophobicity was likely to have caused an enhancement of the hydrological response of the catchment. With an increase in frequency of fires expected in the Alpine Shire associated with anthropogenic climate change, the relationship between fire and flood, even for rare events, has implications for emergency managers and Alpine Shire residents. It is intended that the results of this study will assist in emergency preparedness and thereby contribute to a reduction in the vulnerability of fire-affected communities to post-fire flash floods. Introduction February 2003 saw a post-fire flash flood event occur in the Alpine Shire of Victoria (Fig. 1). This flash flood followed the Flash flood events arise from high intensity rainfall events longest campaign to that time in the recorded history of fire in regions where the hydrological conditions are favourable fighting in Victoria, with fires that could not be controlled (Doswell et al. 1996). In order for heavy precipitation to for nearly 60 days. On 7 January 2003, a day of Total Fire occur, high rainfall rates must be sustained. Slow movement Ban across the entire state of Victoria, a weather system of rain-producing systems is required to maintain high generating many thunderstorms swept across eastern rainfall rates over a long duration. Flash floods represent Victoria and southern New South Wales. Lightning associated an acute problem in fire-prone mountainous regions, such with these thunderstorms started over 80 individual fires in as the Victorian Alps of southeastern Australia. For areas Victoria. Most of these were quickly controlled, but those in recently burned by fires, flash floods can be caused by lower remote, inaccessible terrain came together to form Victoria’s rainfall rates than would normally be required, even to the largest bushfire since the fire of 1939. No lives were lost in extent of a ten-year rainfall event causing a 100- to 200-year Victoria as a direct result of the 2003 fires, but 75 000 hectares flood event (Conedera et al. 2003). of farmland, 241 buildings and 110 000 head of stock were destroyed. Significantly, localised flash flooding immediately following the fires in the Dingo Creek area of Alpine Shire Corresponding author address: Jillian Gallucci, Department of Sustain- caused a firefighter to lose her life when her utility truck was ability and Environment, Locked Bag 3000, Box Hill, Vic 3128, Australia Email: [email protected] swept away as she attempted a bridge crossing. 31 32 Australian Meteorological and Oceanographic Journal 61:1 March 2011 Fig. 1 The three-nested model domains used to produce the Cerro Grande, New Mexico 2000 (Cannon et al. 2001b); and MM5 simulations. (a) Domains 1 (bold solid line) and Buffalo Creek, Colorado 1996 (Chen et al. 2001c; Elliot and 2 (dashed line) and (b) Domain 3 (bold dashed). The Parker 2001). In the Alpine Shire specifically, while both fires extent of the 2003 bushfires are shown in grey and the Alpine Shire boundary is shown as a solid line. and floods are not uncommon, a comprehensive survey of all the data available, including local newspapers, interviews, 145°0'0"E 150°0'0"E historical records, and precipitation and streamflow records, (a) failed to identify another definitive post-fire flash flood event over the past five decades. However, the poor data quality in the Alpine Shire leaves open the possibility that other similar New South Wales events in the region have gone unrecorded. For example, the 1939 bushfires were followed by state wide rain, resulting in erosion along the Macalister River that community members 35°0'0"S 35°0'0"S report is still visible today. Flash floods may occur immediately following a fire or be Victoria delayed by several weeks, but nonetheless can be linked to the fire event through two categories of mechanism. First, Tasman Sea bushfires have the potential to affect the hydrogeological response of catchments. This occurs through the destruction 40°0'0"S Bass Strait 40°0'0"S of vegetation, which leads to decreases in canopy interception, surface roughness, transpiration and ground Tasmania litter storage (Scott 1997; Liu et al. 2004). The response can also be affected by the alteration of soil structure and properties, leading to increased hydrophobicity (Krammes 145°0'0"E 150°0'0"E and DeBano 1965; Conedera et al. 1998; Huffman et al. 2001; Legend Letey 2001; Martin and Moody 2001; Shakesby and Doerr CCoastlineoastline 2006). In the context of a rainfall event, these changes can SStatetate B oBoundaryundary AAlpinelpine Sh iShirere Bou nBoundarydary lead to increases in runoff and erosion and hence lower 22003003 bu Bushfireshfire the amount and rate of precipitation necessary to cause a DDomainomain 1 1 DDomainomain 2 2 flood event (Giovannini 1994; Marxer et al. 1998; Robichaud DDomainomain 3 3 and Brown 1999; Giovannini et al. 2001; Conedera et (b) al. 2003). Second, precipitation can be linked with fire- associated meteorological mechanisms. Surface heating has the potential to enhance the development of localised thunderstorms, which can be of sufficient intensity to initiate a flood event (Tryhorn et al. 2008). In combination with the steep slopes of mountainous regions and hydrogeological Myrtleford modifications, this process can create an enhanced potential Mt Buffalo Harris Lane Chalet Bright for severe flash flooding. Halls The aim of this study was to use both a meteorological and Falls Creek Mt Selwyn Mt Hotham a hydrological model to provide the Alpine Shire community Mt Hotham Airport with information on the basic mechanisms behind flash Alpine Shire 2003 Bushfire flooding immediately after fire. Overall, this research was Domain 3 River/Creek Buckland C’ment motivated by community need and aims to support their common interest. This paper describes an analysis of two of 0 10000 40000 60000metres the mechanisms contributing to this flash-flooding event. It is In interviews conducted by Amanda Lynch and Lee known from the limited available observations that the flood Tryhorn of the Monash Regional Climate Group in 2005, event was preceded by very intense, localised rainfall totals. residents of the Alpine Shire expressed concern over the Previous work on modelling extreme rainfall and surface 2003 post-fire flood event and many wanted to know whether hydrology has shown that the appropriate representation they should typically expect a flood after a fire. Concerns of a burned fire area in a model can cause an enhancement were exacerbated by a similar event in nearby Licola in 2007. of the convection (Chen et al. 2001c; Tryhorn et al. 2008) or This type of post-fire flash flood event has occurred in an increase in runoff (Beeson et al. 2001). The candidate other mountainous regions around the world including: San mechanisms focused upon in this study are associated with Bernardino County, California 2003 (Barkley and Othmer both of these sets of mechanisms: the surface modifications 2004); Riale Buffaga, Switzerland 1997 (Conedera et al. 2003); and their impact on the meteorological conditions preceding Storm King Mountain, Colorado 1994 (Cannon et al. 2001a); the flooding event, and the hydrological response during Gallucci et al.: Meteorological and hydrological mechanisms resulting in the 2003 post-fire flood event 33 and after the flooding event. UTC 27 February 0000 UTC 28 February 2003. This system The introduction has provided an overview of the produced widespread rainfall across northeast Victoria, motivation for this research and the linkages between including 21 mm at Falls Creek (24 hour total to 2200 UTC 27 recently burnt areas and flash flooding. The section that February), the majority of which fell between 1700 and 1900 follows (‘The observed event’) describes the observed flood UTC. Other rainfall totals in the surrounding area included event and a second rainfall event that occurred 36 hours 13.4 mm at Mount Hotham Airport, 26.6 mm at Mt Buffalo later. Data and model descriptions and methodology discuss Chalet, 20.8 mm at Bright, and 20 mm at Halls. The response the models and methodology used in this study. The results on the Buckland River at Harris Lane streamflow gauge was section details an analysis of the results. The conclusions of minor compared to the flash flood event (Tryhorn et al. 2008). this investigation can be found at the end of this paper.

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