This conference paper outlines the primary reasons why Gold Coast City Council developed a categorisation strategy for urban drains and channels. This particular Local Government Area has hundreds of open channels on low lying coastal floodplains, the ocndition of which can have significant impact on the condition of major waterways.
The paper describes how open drainage systems have been traditionaly managed within the LGA. These approaches have focused on reducing localised perceived flooding risks by reducing channel roughness through channel widening and the removal of vegetation from the channel and its banks (often undertaken several times a year).
However the past decade has seen some changes to this operational context:
- There are increased licensing requirements for channel drainage maintenance works (as it is scheduled as an 'Environmentally Relevant Activity' under Queensland state environmental legislation).
- There are increased understanding of the natural processes that are inherent to riparian systems - for example around managing channel flow to improve channel stability and prevent bank scouring; and water quality improvements to preserve asset condition and performance.
- There is an increase interest in the environmental and recreational values of urban creeks, evidenced through a number of community based bushcare groups that are active across the LGA.
To satisfy such objectives whilst ensuring that hydraulic function is preserved an adaptive approach to open channel maintenance was applied.
The report briefly described how a series of channel "categories" were developed to help classifying each channel and provide a framework to improve the consistency of decision making across the city around potential intervention triggers and service levels. The resulting framework includes interventions such as maintaining established, good condiotion vegetation in select riparian zones to help preserve the 'ecosystem services' they currently offer.
This paper details the various parameters that were considered when determining a framework to categorise all open drainage channels across the city. Parameters considered include:
- The location of flood prone properties and road (sensitivity of surrounding catchment to flooding),
- Channel orientation (to help consider exposure and shading of the bed and banks),
- The value of existing instream and riparian vegetation and fauna,
- Proximity of the channel in question to existing bushland
- The significance of the channel as a "link" between existing bushland remnants,
- Degree of impact from surrounding urban drainage inputs,
- Available access to channel features for maintenance purposes and
- Compatibility with surrounding land uses.
Table 1 (page 32) outlines the definitions and descriptions of the 4 categories of open drainage channel that resulted from this project. They range from highly modified channels with low ecological value to natural channels with an established riparian zone.
The strategy identified that for high-flood risk areas, channels were assumed to require regular vegetation (and other) maintenance to minimise roughness and optimise hydraulic capacity.
Alternatively, channels that drain low risk areas (i.e. usually higher gradient, elevated catchments) were afforded higher roughness values and opportunities to protect and/or enhance riparian vegetation structure. The strategy identified that regular removal of vegetation along some streams is unnecessary, however others may benefit from a 'bush regeneration' approach to managing existing riparian vegetation.
Open channel maintenance expenditure has been stable during the implementation of the strategy and is expected to decrease as many channels with establishing riparian zones in low flood risk areas mature and become increasingly "self maintaining". Community education and consultation was run in parallel with the strategy implementation, which may contribute to the decrease in residential complaints received around channel condition.