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flood mitigation

Some Possible Flood Mitigation Strategies

Case Study - Johnstone River Flooding

Constructed Carello's Channel
Webb Levee
River Dredging
Removal of Carello's Levee
Removal of Fill In Town Swamp
Lowering the Height of the Bruce Highway
Modifying the Shape of Carello's Levee
Dredging the River Channel
Impact of Carello's Levee on Once in Two Year Flood
Impact of Carello's levee on Once in Ten Year Flood

 

Constructed Carello’s Channel

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.

Webb Levee

It was assumed that the levee would be concrete in front of waterfront properties and earth in other areas.
 

River Dredging

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.

Removal of Carello's Levee

Removing Carello's levee would giv ethe river a wider channel through Innisfail

 

Removal of Fill In Town Swamp

By removing rubbish and fill that has accumulate in ther /town /swamp over the years river flow should be improved and there is a greater low lying area in which to hold surplus water during flooding events.

 
 

 

Lowering the Height of the Bruce Highway

The Bruce Highway is the major and only coastal main north south highway in the region.In order to prevent road transport from being stopped by minor flooding across the highway, it was raised sometime in the past. During major flooding the highwayacts as a giant low dam , backing up huge volumes of water and making it harder for the area to quicklydrain of flood waters. In backing up ther is potential for the waters to flood numerous home and businesses.

 

 

Modifying the Shape of Carello's Levee

This levee protects a residentail suburb but also reduces the flow of the main river channel

 

 

Dredging the River Channel

By removing accumulated river silt from the main channel, river flow is increased. But dredging lasts only a short time and is expensive. Flooding rivers carry huge volumes of sediment..Sediment load can only be reduced by revegitating river banks upstream.

 

Impact of Carello's Levee on Once in Two Year Flood

The levee protects those on the left bank but puts those on the right bank at greater risk.

Impact of Carello's levee on Once in Ten Year Flood

In a once in ten year flood a considerable area od the right bank is innundated to protect a small but populated area on the left bank.