Defining the floodplain in hydrologically‐variable settings: implications for flood risk management

Abstract

Flood risk management is an essential responsibility of state governments and local councils to ensure the protection of people residing on floodplains. Globally, floodplains are under increasing pressure from growing populations. Typically, the engineering‐type solutions that are used to predict local flood magnitude and frequency based on limited gauging data are inadequate, especially in settings which experience high hydrological variability. This study highlights the importance of incorporating geomorphological understanding into flood risk management in southeast Queensland (SEQ), an area badly affected by extreme flood events in 2011 and 2013. The major aim of this study is to outline the hydrological and sedimentological characteristics of various ‘inundation surfaces’ that are typical of catchments in the sub‐tropics. It identifies four major inundation surfaces; within‐channel bench [Q ~ 2.33 yr average recurrence interval (ARI)]; genetic floodplain (Q = 20 yr ARI); hydraulic floodplain (20 yr < Q ≤ 200 yr ARI) and terrace (Q > 1000 yr ARI). These surfaces are considered typical of inundation areas within, and adjacent to, the large macrochannels common to this region and others of similar hydrological variability. An additional area within genetic floodplains was identified where flood surfaces coalesce and produce an abrupt reduction in channel capacity. This is referred to here as a Spill‐out Zone (SOZ). The associated vulnerability and risk of these surfaces is reviewed and recommendations made based on incorporating this geomorphological understanding into flood risk assessments. These recommendations recognize the importance to manage for risks associated with flow inundation and sediment erosion, delivery and deposition. The increasing availability of high resolution topographic data opens up the possibility of more rapid and spatially extensive assessments of key geomorphic processes which can readily be used to predict flood risk. Copyright © 2016 John Wiley & Sons, Ltd.

Beyond the gorge: Palaeoflood reconstruction from slackwater deposits in a range of physiographic settings in subtropical Australia

“…Preliminary alignment of broadly synchronous dates, therefore, points to some coincidences has been referred to as Spill-out Zones (SOZ) (Croke et al, 2016a). These areas typically occur downstream of enlarged macrochannel systems and coincide with a notable reduction in channel capacity.…”

“…Macrochannels, or compound channel-in-channels, have been described in a range of hydrologically variable settings globally (Woodyer, 1968;Graf, 1988;Gupta et al, 1995Gupta et al, , 1999Van Niekerk et al, 1995) and are a common channel form in SEQ (Croke et al, 2013). Macrochannels are characterized by a small inner channel and associated benches set within a much larger channel that operates as a conduit for high magnitude floods (Croke et al, 2016a). They have been shown to have large channel capacities, with bank top capacities approaching a 50-year annual recurrence interval (ARI), and are laterally stable because of adjacent, highly resistant, clay-rich Pleistocene alluvium.…”

“…Like previous studies (e.g., Kale et al, 2010;Zhang et al, 2012, Huang et al, 2013Wasson et al, 2013), there now seems to be general consensus that SWD can move beyond the bedrock gorge, where well-preserved flood units are found in flow expansive and backwater settings, serving as a useful addition to sample settings. Importantly, the macrochannel settings of SEQ fulfil the stable boundary conditions required to provide robust discharge estimates because they have experienced minimal channel adjustments over the past 200 years (Fryirs et al, 2015;Thompson et al, 2016) and even over the longer timescale of the Holocene (Croke et al, 2016a(Croke et al, , 2016bDaley et al, 2016). In a more detailed assessment of how changes in withinchannel sedimentation rates may affect predicted palaeoflood discharges, Lam et al (2017) ACCEPTED MANUSCRIPT…”

“…Australia also experienced numerous floods equalling, or exceeding, the largest floods on record, including the 2011 and 2013 floods across southeast Queensland (SEQ) (Croke et al, 2016a(Croke et al, , 2016b. The 2011 flood was the most expensive natural disaster in Australia and cost the economy ~A$30 billion (Australian Government, 2015).…”

Extreme flood events in South East Queensland: evidence and implications

“…This extreme rain event was followed by ex-tropical Cyclone Oswald in 2013 which produced another major flooding event in SEQ. The geomorphic, hydrological and landscape impacts and responses have now been widely researched in the Lockyer Valley basin (Croke et al 2016a), a key tributary of the Brisbane River upstream of Brisbane city.…”