Natural levee banks are formed by the process of overbank flood sedimentation. Sediment sorting during overbank conditions result in lateral fining of the flood deposits producing a landform that slopes away from the channel toward the lower lying floodplain. These floodplain features are scale dependent generally increasing in size with drainage area due to increases in sediment supply and discharge. In laterally stable rivers, the height of levee development is assumed to reach some maximum whereby continued aggradation reduces overbank flooding. Large floods are required to overtop the levees and such large events increase the risk of significant geomorphic change such as bank erosion, removal on inset floodplains/benches and channel avulsion.
Lockyer Creek in SEQ is ~ 3000 km2 subcatchment of the Brisbane River and is an Australian horticultural food bowl. Geomorphic assessment of the catchment identified that natural levees have evolved over its lower third. Since the 2011 and 2013 flood events, uncontrolled artificial levee development has occurred on top of existing natural levee banks for landholder flood protection measures. The addition of one – two metres of artificial levee has rapidly progressed the natural evolution of the channel to a state where adjacent floodplains are basically disconnected while the channel conveyance capacity (bankfull discharge) is increased. A 1-D hydraulic model was constructed to assess the flow hydraulics of bankfull discharge along a reach of Lockyer Creek with natural levee banks and anthropogenically enhanced levee banks. Results show that bankfull discharge flows through the channel with natural levee banks produce unit stream power less than 300 W m-2. However, modelling of reaches with anthropogenically enhanced levee banks show significant increases in unit stream power (> 600 W m-2) which exceed the threshold value of geomorphic change. These findings are discussed in terms of the potential management implication for water quality protection for downstream reservoirs and the need for regulation of artificial levee development.