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Erosion and Deposition During the January 2011 Lockyer Flood

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Erosion and Deposition During the January 2011 Lockyer Flood Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. Erosion and Deposition during the January 2011 Lockyer Flood. 1 2 3 4 Jacky Croke , Fiona Watson , Chris Thompson and Peter Todd 1 Australian Rivers Institute, Griffith University Nathan Campus, Queensland 4111 Email: [email protected] 2 Department of Environment and Resource Management, Remote Sensing Centre, Ecosciences Precinct, Dutton Park Q 4102 3 Integrated centre for Catchment Management (ICaM) Australian national University, ACT. 4 Department of Environment and Resource Management, Land Centre, Woolloongabba QLD 4102 Key Points • The Lockyer Catchment lies in an area of recognized hydrological variability. • For the January 2011 event, average peak flood transmission speeds varied from 24 kmh-1 to 2.9 kmh-1 between the upper and middle reaches. • Net erosion occurred only in the inner channel and macro-channel Banks. • Dominant form of channel adjustment was channel widening through Bank slumping and removal of within-channel Benches. Abstract This paper presents a preliminary assessment of the nature and spatial extent of erosion and deposition that occurred as a result of the January 2011 flood event in the Lockyer Valley, Queensland. Research methods include analysis of Both pre- and post-flood, high-resolution (LiDAR) digital elevation data, aerial photography, and field validation of selected sites. Results indicate that extensive channel widening (3xincrease) occurred in the steep bedrock confined river reaches of the upper valley. Dramatic examples of whole-scale re-organisation of channel morphology and geometry are apparent in these high-energy reaches. In the middle and downstream reaches, the dominant process of channel adjustment is bank slumping which has left significant and spatially widespread erosional scars. Processes such as floodplain erosion, channel avulsion and meander cut-offs were rarely apparent. Over the entire study area, the floodplain is a net sink with a mean elevation difference pre- and post-flood of 0.07m. ConsideraBle variaBility in channel adjustment occurred along the energy gradient, between confined and unconfined settings, and with distance away from the source of precipitation in the headwater catchments. Keywords Lockyer Valley, Queensland, January 2011 Flood, channel adjustment, bank erosion, slumping. Introduction In January 2011, a cloud burst over the Great Dividing Range in the BrisBane River Catchment, Queensland saw the development of an extreme flood event that resulted in the loss of human life and extensive damage to infrastructure and facilities estimated to cost in excess of $30 Billion. The January 2011 flood event has been rated as the second highest flood of the past 100 years, after January 1974, with an event peak of 648mm (BOM, 2011). Gauging station hydrographs indicate a rapid and extreme rise in discharge in the upper part of the Lockyer Catchment which has been referred to as a ‘wall of water’, to describe both the speed and force of the resultant floodwaters. Spectacular examples of large-scale channel and floodplain adjustment following extreme or catastrophic flood events have Been descriBed worldwide (Baker, 1973; Church, 1978; Costa, 1983; House et al., 2002). In Editors names. (2011). Proceedings of the 6th Australian Stream 1 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX. Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. Australia, the most well documented studies of channel adjustment processes come from NSW notably the Hunter River and its tributaries (Erskine, 1993, Erskine and Saynor, 1996, Erskine and Warner, 1988, Erskine 2011). Studies revealed that channel adjustment due to extreme events can include modifications to channel planform, sinuosity, slope, channel metamorphoses, and catastrophic floodplain stripping (Nanson, 1986). Existing research confirms, however, that catchment response cannot be considered as simply the product of the intensity and magnitude of the rainfall event But rather the interaction of other factors (topography, flood sequencing, catchment management history) that contriBute to the ‘susceptiBility’ of a given catchment. Landscape susceptiBility to a given process response may occur at spatially distinct locations in a catchment and over a rapid or slow response time. Such differences have previously been described in terms of a ‘pulsed’ and a ‘ramped’ input (Brunsden and Thornes, 1979) and as ‘response gradients of adjustment’ (Fryirs et al., 2009). Pulsed disturbance can produce more catastrophic adjustments such as avulsions, channel shortening and cut-offs, whereas ramped disturbance produces incremental erosion and adjustment over time. The major aim of this paper, therefore, is to present a preliminary assessment of the broad spatial patterns of erosion, deposition and channel adjustment following the Lockyer Creek January 2011 flood. Study Area Background The Lockyer Valley lies to the east of ToowoomBa on the Great Dividing Range with a catchment area of 2600km2 accounting for approximately one-quarter of the BrisBane River catchment. The rich suB-coastal land and alluvial plains of the catchment are one of the state’s most important centres of diversified agriculture centred around the principal towns of Gatton (pop. 6,000) and Laidley (pop.3,500) (Fig 1). Lockyer Creek flows generally east for ~100km and enters the Brisbane River at Lowood (Fig 1). Climate in the region is classified as suB-tropical and suB- humid. Geology The Lockyer Valley is part of the Moreton Basin which was a large shallow fresh water lake during the Mesozoic Era. During the Jurassic, Figure 1. Lockyer Valley catchment the sandstones, shales and conglomerates of the Helidon and Marburg in SE Queensland. formations were deposited in Beds that now dip in a southerly direction (Powell, 1987). The main stem Lockyer Creek drains west to east into the Brisbane River and is aligned with the Gatton Sandstone beds. Geomorphology Lockyer Creek has undergone several phases of incision and aggradation since the Tertiary with the river system cutting through the Tertiary volcanics and into the underlying Mesozoic sedimentary rocks to reveal the present day exposures (Powell, 1987). Several erosional surfaces have Been identified including an early Miocene Upper Erosion Surface (Helidon sandstone 450-600 m above sea level (a.s.l.) and an early Pliocene Middle Erosion Surface (Lower MarBurg at 120-210 m a.s.l.) (McTaggart, 1963). A later phase of renewed incision is thought to have occurred at some time close to the last glaciation (18,000-20,000 yrs) and again around c.3,000 B.P. that resulted in the development of the present-day entrenched river and its triButaries (Powell, 1987). The present-day main stem of Lockyer Creek is a single, meandering channel of medium-high sinuosity (1.3- 1.5) with some tightly curving, locally incised meander Bends. The upper reaches are Bedrock confined high- Editors names. (2011). Proceedings of the 6th Australian Stream 2 Management Conference. Canberra, Australian Capital Territory, pages XX - XXX. Erosion, deposition and channel adjustment in Lockyer Creek, South East Queensland post the January 2011 Floods. energy channels (see Thompson et al., this volume) But once the river emerges onto the unconfined alluvial plain around Helidon, the present channel is inset within a large (~100m wide and 20m deep) macro-channel. Some alluvial cutoffs are preserved on the valley floor But there is little topographical evidence of recent lateral migration of the river in the form of remnant scroll bars or extensive point-bar development. Preliminary investigations of some exposed floodplain stratigraphy reveal the dominance of vertical accretion processes with horizontally-bedded alternating sands and fine silts suggesting very slow rates of lateral migration. Levees are also notaBle features of the current entrenched Lockyer Creek with floodplain surfaces which slope steeply away from the present channel. Floodplain width varies with distance downstream from the confined, high energy reaches of the headwater triButaries where floodplain pockets occur on alternating sides of the valley floor, through to the wide alluvial plain at the lower end of the catchment. Regional Hydrological Characteristics. The general region of the BrisBane and Lockyer River systems is characterized By patterns of floods and droughts which have been linked to the inter-annual rainfall variations of the El Nino-Southern Oscillation (ENSO) and the inter-decadal Pacific Oscillation (IPO) (Kiem et al., 2003, Rustomji et al., 2009). In NSW, these patterns of floods and droughts have been used to explain associated adjustments in channel morphology (Erskine and Warner, 1988). In a recent analysis of hydrological variaBility in all Basins east of the Dividing range, Rustomji et al.,(2009) concluded that flood variaBility increased southwards through Queensland to reach a maximum in the Burnett Catchment, north of the study area. Flash Flood Magnitude Indices (FFMI) for the four gauging stations along the Lockyer range from 0.68-1.07 (Rustomji et al., 2009). While this index does not capture all components of hydrological variability, such high values indicate a propensity of alternating extremes. Erskine (1999), for example, found that a value of at least 0.6 characterised Australian rivers with a known high flood potential. Hydrology
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