Channel–floodplain connectivity during an extreme flood event: implications for sediment erosion, deposition, and delivery

Abstract

The term connectivity has emerged as a powerful concept in hydrology and geomorphology and is emerging as an innovative component of catchment erosion modeling studies. However, considerable confusion remains regarding its definition and quantification, especially as it relates to fluvial systems. This confusion is exacerbated by a lack of detailed case studies and by the tendency to treat water and sediment separately. Extreme flood events provide a useful framework to assess variability in connectivity, particularly the connection between channels and floodplains. The catastrophic flood of January 2011 in the Lockyer valley, southeast Queensland, Australia provides an opportunity to examine this dimension in some detail and to determine how these dynamics operate under high flow regimes. High resolution aerial photographs and multi-temporal LiDAR digital elevation models (DEMs), coupled with hydrological modeling, are used to assess both the nature of hydrologic and sedimentological connectivity and their dominant controls. Longitudinal variations in flood inundation extent led to the identification of nine reaches which displayed varying channel-floodplain connectivity. The major control on connectivity was significant non-linear changes in channel capacity due to the presence of notable macrochannels which contained a > 3000 average recurrence interval (ARI) event at mid-catchment locations. The spatial pattern of hydrological connectivity was not straight-forward in spite of bankfull discharges for selected reaches exceeding 5600 m 3 s -1. Data indicate that the main channel boundary was the dominant source of sediment while the floodplains, where inundated, were the dominant sinks. Spatial variability in channel-floodplain hydrological connectivity leads to dis-connectivity in the downstream transfer of sediments between reaches and affected sediment storage on adjacent floodplains. Consideration of such variability for even the most extreme flood events, highlights the need to carefully consider non-linear changes in key variables such as channel capacity and flood conveyance in the development of a quantitative 'connectivity index'.

Suspended sediment transport dynamics in rivers: Multi-scale drivers of temporal variation

“…by landslides or gully formation) can act both as a blockage to sediment movement or as an additional sediment source. Contrarily, sediment concentrations in the river can also decrease during an event when the sediment supply gets exhausted or when dilution occurs as a result of persistent high river discharges (Bracken et al, 2015;Croke et al, 2013;Fryirs, 2013). Finally, human activities can also have an impact on the SS concentration during events, especially in urban areas.…”

“…Previous studies have used concepts such as 'sediment coupling', 'sediment connectivity', 'jerky conveyor belt' and 'sediment cascade' to describe the field-catchment-river sediment transfer system, all of which emphasize the variable, non-linear linkages across temporal and spatial scales that eventually determine SS transport (Bracken et al, 2015;Croke et al, 2013;De Vente et al, 2007;Ferguson, 1981;Fryirs, 2013;Hollister et al, 2008;Koiter et al, 2013a). While these conceptual frameworks have helped researchers to better comprehend the dimensions of the sediment transfer system, important gaps remain in actually linking spatial and temporal scales of SS transport in rivers.…”

Topographical change caused by moderate and small floods in a gravel bed ephemeral river – a depth-averaged morphodynamic simulation approach

“…Thus, the modelled fluid was considered vertically homogeneous. These well-known equations were found in the user manual of the model (Deltares, 2011).…”

“…Also, data collection over large channel bars has been enhanced with these methods. Digital elevation models produced from airborne laser scanning data have also been applied for detecting the geomorphic effects of different discharge events (Hauer and Habersack, 2009;Croke et al, 2013;Thompson and Croke, 2013;Nardi and Rinaldi, 2015) and for recording and calibrating sediment transport models (RodriguezLloveras et al, 2015). These reveal the reorganization of the channel morphology due to flood events even over large ar- eas with great detail .…”

Topographical changes caused by moderate and small floods in a gravelly ephemeral river – 2D morphodynamic simulation approach

“…Mobile laser scanning, for example from car, backpack and boat, provide practical scanning angle to survey channel banks, bar lobes and vertical surfaces, and also data collection over large channel bars has been enhanced with the method (Vaaja et al, 2011;Kasvi et al, 2015;Kukko et al, 2015). Digital elevation models produced from LiDAR data have been applied for detecting the geomorphic effects of different discharge 5 events (Hauer and Habersack, 2009;Croke et al, 2013;Thompson and Croke, 2013;Nardi and Rinaldi, 2015) and for recording and calibrating sediment transport models . These reveal the reorganization of the channel morphology due to flood events even over large areas with great detail .…”