Oral Presentation 7th Australian Stream Management Conference 2014

Floodplains are an important source of material for the construction industry.  Around 80% of concrete and 90% of asphalt are composed of aggregate, one source of which are floodplain gravels.  In countries such as Australia, the annual demand for aggregate is around 7 tonnes/person/year and mining of aggregate, including hard rock and gravel extraction, is second only to coal mining in terms of the quantity of material that is moved.  Demands for floodplain gravels are increasing as populations grow and urban areas expand.

Floodplain gravel mining can cause change in the riverine environment, both locally and distant to the mining site, and in the short and long term. Immediate effects of mining include loss of land and vegetation.  Longer term effects include:

• The low resistance flow path provided by the open area of a gravel mine can alter floodplain hydraulics during high flows
• Stockpiles of overburden and gravel on the floodplain may divert or change flow paths under flood conditions and may lead to water quality issues and downstream sedimentation
• Mining on floodplains may reduce groundwater levels where water is removed by pumping and may affect groundwater quality
• Floodplain mines may lead to river channel changes that include erosion, bed degradation and damage to infrastructure.

Impacts to river stability are caused when gravel mining pits affect flood hydraulics or where piping failure causes collapse of pit walls. During floods, gravel pits provide an area of decreased resistance to flow and often a shortened flow path for floodwaters. This can result in high velocities and scour. Erosional processes associated with floodplain pits can result in river channel changes where the river diverts into, and through, gravel mines. This can lead to bed degradation, bank erosion and channel widening. Infrastructure may be damaged or destroyed.

Local, national and international case studies show that pit capture and subsequent river channel changes, are a common consequence of floodplain mining. There are at least 37 documented examples where rivers have diverted through mining pits.  

Sustainable gravel extraction requires development of an appropriate planning framework.  There are a number of existing guidelines for floodplain mining and mine rehabilitation that may be adapted for Australian conditions. Approaches that have been used by others to minimise impacts include: protecting stockpiles from floodwaters to reduce water quality risks, and extracting gravel from features, such as terraces, that are higher in elevation than the active floodplain.