The relationship between riparian vegetation, bank erosion and channel pattern, Magela Creek, Northern Territory. Thesis submitted in part fulfilment BSc Hons, School of Geosciences, University of Wollongong, 2002
Internal Report 419
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts
About the report
The relationship between riparian vegetation and flow are poorly understood for rivers in general, and particularly for anabranching rivers. Riparian vegetation along the lowland anabranching section of Magela Creek within the Ranger Mine Lease Area, Northern Territory, greatly influences bank strength, flow hydraulics, sediment erosion and deposition, and bar and island formation. This study presents a comparative investigation of vegetation composition, bank erodibility and flow characteristics for contrasting anabranching and single-thread reaches of Magela Creek. The well-defined anabranching reaches are generally characterised by dense monsoon forest growing on the channel banks and large islands, and the channels are narrow and deep with their steep sides protected by mature tree trunks and thick roots. The forest here is self-propagating with numerous juveniles and is resistant to penetration by fire. The single-thread reaches are generally characterised by relatively low-density melaleuca forest and partly treed and grassy banks. Numerous trees grow within the channel forming multiple bars and small islands. Consequently the channel here is wider and shallower than in the anabranching reaches. Bank hydraulic erodibility on three differently vegetated banks was measured with an instrument specifically constructed for this study and termed a ‘hydro jet’. The most resistant banks are protected by a dense melaleuca root mat at the head of bars and small islands around within-channel trees. The next most resistant are the relatively finegrained and steep banks within the anabranching reaches. The grass-lined banks on the single-thread reaches were the most easily eroded of the three surfaces. Velocity fields measured close to these three bank types show that the anabranching banks can withstand very high bank shear, as do the heads of bars, whereas the grassy bank experience least. However, the anabranching reach exhibits bank alcoves that generate strong upstream eddies that, along with the retarding effect of dense island vegetation during overbank flow, restrict channel velocities occurring at greater than bankfull to less than those that occur at below bankfull. Such a velocity reversal does not occur in the less densely treed single-thread reaches and illustrated the significant role played by flow-momentum transfer associated with the densely monsoonforested islands. Clearly, vegetation, bank erodibility and channel pattern are strongly interrelated variables on Magela Creek.