Grass chutes

A grass chute is a relatively short, steep earthen channel arrangement that involves excavation works to reduce the slope (battering). The works usually occur at a head-cut or knickpoint feature associated with channel/floodplain deepening processes. The reduced slope chute improves bed slope stability, riverbank stability and creates more favourable conditions for vegetation establishment. Vegetation may comprise native and/or introduced species, however, native species are often preferred as introduced species risk becoming invasive weeds. A suitable vegetation composition may include grasses, rushes, reeds and/or shrubs.

Potential benefits from this intervention:

• Allows for medium to long-term stabilisation of bed deepening processes (preventing knickpoints/erosion heads from propagating in an upstream direction in either a channel or floodplain), while minimising potential site disturbance and costs.

Potential negative implications from this intervention:

• Battering disturbs soil and results in the bank being more vulnerable to erosion in the short-term, while the revegetation establishes. This may not only impact on bank stability but also water quality.
• High disturbance during construction including vegetation removal and heavy machinery within rivers.
• May be more prone to failure compared to other bed grade control management options including rock chutes and timber sleeper drop weirs.

Intervention considerations:

• Seek appropriate specialist advice and check legal obligations (e.g. permits).
• This technique generally has limited application and is most suitable for ephemeral systems or floodplain flow re-entry points where the grass chute will be subject to infrequent and short duration flow events.
• Grass chutes typically require specialist technical input to account for site and reach based physical processes, topography and hydrologic and hydraulic characteristics.
• Without vegetation establishment on the battered bank face, the site is unlikely to be successful in the long term.
• Alternative/complementary forms of channel stability may be required (e.g. some rock armouring) to provide long term channel stability.
• The disturbance and cost associated with battering may not justify physical intervention.
• Safety of volunteers and employees including seasonal exposures (e.g. heat) and high risk areas (e.g. crocodile presence in waterways or areas with soil contaminant risks).

Last updated: 23 June 2022