What is a concrete lined channel: As the title implies it is a concrete lined channel usually trapezoidal or rectangular in shape. The process typically involves removing all in bank vegetation straightening, re-profiling the channel and utilising reinforced concrete to build the channel. Concrete lined channels were utilised a lot in the past to convey more water through relatively narrow areas. The negatives of this practice include increasing the velocity, timing and severity of flooding downstream, environmental impacts such as riverine habitat destruction, loss of important breeding areas and ecosystems, stream bed and bank stability issues and significant decreases in water quality.
A concrete lined channel is generally neutral in improving community access and recreational use. Why? A concrete channel can replace over grown vegetation and limit the occurrence of overbank flooding, potentially increasing the recreational use of surrounding public space. However, often concrete lined channels have to be closed to public access due to their high hazard as small amounts of rain can cause life threatening situations for people within them and their rescuers. In addition concrete lined channels can reduce access across a watercourse and reduce the streams “natural” aesthetics and recreational usability.
A concrete lined channel slightly disadvantages individual members of the community that may have utilised the existing stream for recreational purposes such as fishing and access. Why? Although concrete lining a stream is usually straight forward and does not cause significant disruption, members of the community that utilised the stream previously would no longer be able to do so, as the stream becomes unusable and access is often restricted.
Concrete lining a channel is slightly negative in providing safety to the community during flooding. Why? Although concrete lined channels can reduce flood levels in critical locations they can significantly increase the velocity, timing and severity of flooding downstream. This can cause loss of life to both people that become trapped in fast flowing channels and their potential rescuers. In addition, this increased conveyance can cause cars to become lodged in channels redirecting critical flow paths and causing flooding elsewhere.
A concrete lined channel does not raise community awareness and understanding of the local flood risk. Why? A concrete lined channel is usually perceived as a local drainage measure, or a mechanism to convey all flood water causing a false sense of security and creating potentially life threatening situations.
Concrete lined channels have significant negative environmental and ecological impacts. Why? A concrete lined channel clears all habitat and is designed to reduce the streams roughness, which in effect quickens the flow of water through the stream channel. This increased conveyance of water and habitat destruction causes major negative environmental and ecological impacts including pushing the flood peak downstream, increased erosion and scour leading to bed and bank stability both upstream and downstream of the channel, extensively altering plant and animal communities and destroying their homes.
Concrete lined channels have significant negative water quality impacts. Why? As noted above concrete lined channels are designed to increase conveyance (move water downstream quicker) which in turn diminishes the capacity of the river to regulate its flows, which causes scour and erosion, increases turbidity, reduces the ability for the floodplain to recharge, increases pH and limits the ability of the system to buffer nutrient and pollutant loads which all decrease water quality.
A concrete lined channel has major initial costs to council Why? Concrete lined channels are simple to construct, have moderate cost materials and are easy to maintain but overall have major initial costs to council. In NSW the construction costs for a concrete lined channel are typically around $1,000 per linear metre for a 2 metre wide channel. However, the cost and availability of: materials; machinery; labour/ project management; design and feasibility studies, easements and/or the acquisition of land, flood offsets and legislative costs can skew this typical cost per linear metre significantly.
A concrete lined channel has minor ongoing costs to council post implementation. Why? Maintenance only generally involves implementing further works to reduce bed and bank stability issues and removing sediment or pollutants. However, failure can result in large ongoing costs to Council.
A concrete lined channel generally reduces flood related annual average damages to the community. Why? As mentioned above a concrete lined channel can reduce flood levels in critical locations particularly for small events but can significantly increases the velocity, timing and severity of flooding downstream. If flood modelling indicates there will be is no adverse impact downstream as a result of concrete lining the channel, then this option can reduce annual average flood damages to the community.
A concrete lined channel can potentially cause significant adverse flood impacts to other areas. Why? As mentioned previously a concrete lined channel is designed to increase water flow through the stream channel. This amplified conveyance therefore moves flood water downstream quicker and with more energy, causing possible damage to downstream assets. As a result feasibility and detailed design studies for a full range of flood events (from regular to extremely rare floods) are required to assess the upstream and downstream impacts of concrete lining a channel.