What is a channel realignment: As the title implies it is the realignment of the existing channel. Realigning a channel generally involves straightening and widening or redirecting water flow. This is typically done to increase the discharge capacity of the channel or build a bypass floodway that redirects excess water away from the main stream before it rejoins at a later time reducing the flood height and potential damages. The negatives can include increasing the velocity, timing and severity of flooding downstream, causing flooding to areas that did not previously flood and potentially causing stream bed and bank stability issues.
Channel realignments are generally neutral in improving community access and recreational use. Why? Channel realignment can increase public space that may be suitable for walking, cycle paths and sporting fields. However, channel realignment may also occupy previously accessible public land.
Channel realignments can disadvantage individual members of the community. Why? Although channel realignments typically occur within the footprint of the existing channel, there are some instances where private land will need to be purchased to accommodate the streams increased width or new flow direction. A redirected channel may also isolate properties during flood events that had previously not been isolated.
Channel realignments are generally neutral in providing safety to the community during flooding. Why? The realignment can re-divert flow and reduce flood heights in one location, but it can also increase the velocity, timing and severity of flooding downstream. As stated above, this can also isolate properties placing those residents and emergency management personnel at increased risk.
Channel realignments do not typically raise community awareness and understanding of the local flood risk. Why? Channel realignments are usually perceived as a local drainage measure and not as a flood reminder.
Channel realignments can have negative environmental and ecological impacts. Why? Channel realignments are generally undertaken to quicken the flow of water down a stream or redirect water to another area. This increased conveyance of water can have the negative environmental and ecological impact of pushing the flood peak downstream, increased erosion and scour leading to bed and bank instability both upstream and downstream, and destroying valuable plant and animal habitat. On the other hand a secondary channel may create new habitat for plants and animals and restore the channels flow regime offsetting the non-permeable urban surfaces.
Channel realignments can have negative water quality impacts. Why? As noted above channel realignments change the flow regime of streams both upstream and downstream usually by moving water downstream quicker. The impacts of this include: diminishing the capacity of the river to regulate its flows causing scour and erosion, changing temperature and oxygen gradients and increasing turbidity.
Channel realignments generally have moderate initial costs to council Why? Although they are relatively simple to construct, feasibility studies and detailed design studies are vital to insure their placement and function does not cause adverse impacts both upstream and downstream. The costs for channel realignment can vary significantly from $60,000 for local channel straightening works to millions for by-pass channels. The cost of the works can also significantly vary if easements and/or the acquisition of private land is required.
Channel realignments have minor to moderate ongoing costs to council post implementation. Why? Maintenance generally involves works to reduce bed and bank stability issues and riparian vegetation management.
Channel realignments generally reduces flood related annual average damages to the community. Why? Channel realignment can reduce flood levels and damages in critical locations but may increase the velocity, timing and severity of flooding downstream or to other areas. If flood modelling indicates there will be no adverse impact downstream or to other areas as a result of channel realignments, then this can reduce annual average flood damages to the community.
Channel realignments have the potential to cause adverse flood impacts to other areas. Why? As mentioned previously channel realignments are generally undertaken to quicken the flow of water down a stream or redirect water to another area. This amplified conveyance therefore moves flood water quicker and with more energy, causing possible damage to assets. On the other hand channel realignment could reduce flood heights at critical locations through redirecting and slowing down the flow through other areas. 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 channel realignment.