The proposed channel would be subject to sufficiently high velocities during flooding to scour its banks. Therefore, it is assumed that the banks will be protected with rock and be sloped.
Sweeneys and Saltwater Creek floodgate levees are raised Based on experience elsewhere, the environmental impacts of the excavation on a wide range of environmental, social and cultural issues would need to be carefully assessed prior to approval being granted. The benefits of the channel would also need to be fully justified.
The analysis indicates that an acceptable return on the initial outlay would be achieved if the channel scoured within 20 to 30 years and no rock protection is required. Without having undertaken specific geotechnical or hydraulic investigations into the likelihood of a similar channel forming naturally, it is difficult to make a definitive statement on the viability of such a proposal. However, based on anecdotal evidence of scouring both prior to and after the construction of Carello's levee, it is considered unlikely that a channel of this size would form naturally over a 20 to 30 year period.
The raising of the levees is not expected to have any environmental impacts given that the floodgates are already in place. The increase in floodgate levee height would bring a positive social benefit by increasing the warning time to residents and business protected by the levees. However, it is likely that the fall of the floodwaters in the town area will be slightly retarded by the increased height on the levees.
It was assumed that the levee would be concrete in front of waterfront properties and earth in other areas.
1. dredge to stockpile and sell in Cairns;
2. dredge to stockpile and sell locally;
3. dredge to spoil
However, anecdotal evidence obtained during discussions with long-term observers of the river would suggest that the sediment load in the river is relatively high. If this is the case, then there may be significant costs in maintaining the dredged river profile. It is possible that in a larger flood, the dredged channel may be filled which would then require that the full dredging be undertaken to maintain the flood benefits.
|Constructed Carello's Channel||0.29|| Provides widespread minor reductions
in flood levels.
BCR does not allow for maintenance, acid sulphate soils or cartage and does not use revised floor level data.
High capital cost.
Environmental considerations relating to the clearing of mangroves, excavation and disposal of soil.
|Scoured Carello's Channel||0.1 to 2.8|| BCR strongly dependent on period
taken for channel to scour.
BCR does not allow for rock protection. BCR used revised floor levels. BCR will be lower if Sweeneys and Saltwater creek floodgate levees are raised.
Environmental considerations relating to the clearing of mangroves, excavation and disposal of soil and deposition of scoured material in river system.
Further hydraulic and geotechnical investigation recommended to assess the likelihood of success.
|Raising of Sweeneys & Saltwater Floodgates||2.6|| Significant benefits in areas
protected by levees.
Some minor increases in flood levels 'outside' of the levee.
BCR uses revised floor level data.
Additional levee may be required at Scullen Avenue, although it is a high capital cost item with minimal benefit. Further investigation into the significance of a 30 mm increase in the peak 20 year ARI flood levels in the Jones Street area is recommended before including the levees in the scheme.
No environmental issues.