Sydney Chapter

Tree Roots and the Slope Stability of River Banks in Eastern Australia

Tom Hubble

Riverside vegetation is a significant factor influencing the occurrence and progress of streambed and riverbank erosion. Recent riparian management practice in Australia has focused on re-establishing or maintaining native riparian vegetation in order to control or prevent erosion as well as regenerate or preserve the complex variety of in-stream and riverside habitats. An integrated review of field and experimental studies conducted in eastern Australia will be presented that evaluates native vegetation’s role in mass failure of riverbanks.

These studies have general application and have conclusively demonstrated the following:

  1. The presence of riparian forest on riverbanks significantly reduces the likelihood of erosion by mass failure due to reinforcement of riverbank soils by tree roots and this reduced likelihood of mass failure enables a narrower channel cross-section than would otherwise be the case for many Australian coastal streams.
  2. A number of Australian tree species have apparently evolved roots that seek the permanent, summer water table in order to survive prolonged dry spells, these root systems are particularly effective in mass failure mitigation due to rooting depths that are commonly greater than 5m and are sometimes well in excess of 20 m.
  3. Wu and Waldron’s so-called “Root-Area-Ratio method” of calculating the shear strength of root-reinforced soil using root tensile strength data leads to significant overestimation of the actual root-reinforcement due to (a) breakage or pull-out of roots that taper and narrow beneath the shear plane such that individual roots do not achieve the tensile strength calculated on the basis of root diameter at the shear plane; and/or (b) the fact that the soil mass fails progressively along the length of potential shear plane rather instantaneously across the entire shear plane.

A general root-reinforcement model for estimating the stability of eastern Australian coastal river banks will be presented and the potential for applying this model to the evaluation of the stability of eastern Australian hillslopes will also be discussed.

Speaker biography

Associate Professor Tom Hubble has worked on projects involving collecting and interpreting geological samples and geophysical data sets in marine, estuarine and riverine settings. He has published a number of papers on river-bank collapse and the stabilisation of river-banks against collapse by tree-roots as well as works on submarine land-sliding on the eastern Australian seaboard.

Current projects include riverbank collapse studies on the Murray River and further work on the large-scale, submerged- slope collapses on the eastern Australian seaboard; including a two-week research cruise onboard the CSIRO managed Marine National Facility vessel the RV Southern Surveyor which he led in January 2013.

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