Wilson Inlet 6
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Managing the bar and the Inlet
Management options
| Management option | Marine exchange | Potential for stratification | Initial capital cost | Annual maintenance cost | Prawn Rock Channel | Ocean Beach | Action Plan recommendation |
|---|---|---|---|---|---|---|---|
| Western opening 0-100 m | Status quo | Status quo | Nil | $5,000 | Status quo | Status quo | Recommended option. Combined with implementation of nutrient reduction activities. |
| Opening at higher water levels | Slight increase | Similar | Not known | $5,000 | Similar | Similar | Possible option. Detailed analysis of flooding and extent of flooding required. |
| Opening at lower water levels | Slight decrease | Similar | Nil | $5,000 | Similar | Similar | Not recommended. With consistent river flow the reduced residence time of nutrients in the inlet would only be a few days whilst scour of the bar would be reduced. |
| Eastern opening | Possible slight increase | Similar | Nil | $20,000 | Link channel required | Not known | Not recommended. Similar effiencies to western openings given the range of natural variability. |
| Existing channel extended by 375 m through sand island and deepened to ~0.7 m AHD | Increase (~60%) | Slight increase | $50,000 removal of 9,000 m3 sand | >$10,000 | Similar | Similar | Not recommended. Modest increase in ocean inflow. Better value for money by dredging the delta restriction. |
| As above dee[ened tp ~1.5 m AHD | Increase (~90%) | Slight increase | $100,000 removal of 24,000 m3 sand | > $10,000 | Similar | Similar | Not recommended. Better value for money by dredging the delta restriction. |
| Existing channel deepened and extended by 825 m through the sand island to the deeper natural channel in Inlet | Increase (~200%) | Increase | $500,000 removal of 59,000 m3 sand | > $10,000 | Higher velocities | Not known | Not recommended. Risk of increasing nutrient from sediment greater than benefit from comparatively small increase in nutrient export. |
| Deepening of natural channel restriction | Slight increase (~40% increase) | Slight increase | $20,000 removal of 3,500 m3 sand | > $10,000 | Similar | Similar | Possible ioption. Implementation may slow assessment of stratification risk of increased marine exchange and evaluation of potential to improve. Located away from sand deposition area. |
| Channel through delta (as proposed by WIRG, 1994) | Similar (~5% increase) | Similar | $500,000 removal of 123,000 m3 sand | > $10,000 | Similar | Similar | Not recommended. High cost, high disturbance and little potential for improvement. |
| Proposed WIRG (1994) channel extended through the bar | Increase (~230%) | Increase | $1 million removal of 163,000 m3 sand | > $10,000 | Higher velocities | Not known | Not recommended. Risk of increased release from sediment greater than benefit from comparatively small increase in nutrient export. |
| Permanent opening | Large increase depends on dimensions and length | Increase | Up to $10 million. Depends on depth, length and groyne type, disposan costs | > $10,000 | Higher velocities or cut off depending on design or groynes | Not known | Not recommended. Benefits exceeded by the impact. Unlikely to have much effect on nutrient concentrations or plant growth but would reduce periods of anoxia, Potential impact on Ocean Beach from engineering structures, more marine water would change the fishery, increase tidal fluctuations, and may effect riparian vegetation. |
| Controlled closure | Similar | Similar | Nil | $10,000 | Similar | Similar | Possible option. Would reduce sand over wash and build up of sand banks in the delta in the period from January on. Depends on effect of summer rain. |
Figure 16: Comparison of Management Options. Changes are in reference to a western opening 0-100 m from the cliffs.
A range of management options for the bar opening have been evaluated using the current understanding of the hydrodynamics and coastal processes as outlined and our scientific understanding from the NEMP studies. Because of the complexity of the natural system and the limits of current understanding, some options will involve a degree of risk. The options examined include:
- Western opening (status quo)
- Opening at higher Inlet water levels
- Opening at lower Inlet water levels
- Eastern opening
- Existing channel deepened and extended through to the bar
- Existing channel deepened to -1.5 m AHD
- Existing channel deepened to -0.7 m AHD
- Channel through delta restriction
- Channel through delta (as proposed by WIRG 1994)
- Proposed WIRG 1994 channel extended through to the bar
- Permanent opening
- Controlled closure
The main features of each option are outlined in summary in Figure 16. The table contains an assessment of each option relative to the present management (western openings). The assessments are based on the investigations and modelling completed and the expert knowledge of the reviewers.
The main focus of these management options is, by and large, to increase the amount of marine exchange with the aim of removing more nutrients from the Inlet. Since flushing occurs as a result of rainfall and runoff these management options can only effect marine exchange as they have no influence on the rainfall and runoff from the catchment.
It has also been noted that there is a trade off between marine exchange and stratification; if marine exchange is increased, stratification is likely to increase also. Unless the volume of marine exchange is very large, the stratified layer will deoxygenate causing the release of nutrients from the sediment. So, while increased marine exchange will remove nutrients from the water column, it may also cause the release of further nutrients from the sediment.
The options were assessed in terms of their potential to improve flushing and exchange, their potential for increasing stratification, their costs and their impacts on Prawn Rock Channel and Ocean Beach.
Location of opening
In terms of marine exchange it is clear that the difference between an east or west opening is liable to be much smaller than the interannual variation that already occurs between years. In terms of costs, historically little has been spent on bar openings. A major difference between east and west openings is that the bar channel for a western opening connects with Prawn Rock Channel. To maintain the amenity of Prawn Rock Channel with eastern openings would need regular dredging of an east-west channel behind the bar to connect with Prawn Rock Channel. This would add to the annual cost of using an eastern opening compared to a western opening.Timing and water level of opening
It has been suggested that openings earlier in the winter would reduce the time that nutrients from runoff remained in the Inlet. The drawback to this being that earlier openings would be at lower Inlet water levels, resulting in less scour through the bar and delta. Computer modelling suggests that even for high river flow rates it takes at least 5 days for river flow entering at the eastern end of the Inlet to reach the mouth. This period of time is more than sufficient for plants to absorb as much of the nutrients as they need. Therefore the only effect of an earlier, lower opening would be to reduce the scour of material from the bar and delta.| There is no advantage in earlier, lower openings. |
In contrast opening at higher Inlet water levels would mean greater potential energy available to scour sand from the bar and delta into Ratcliffe Bay, however the flooding of adjacent lands (which has not been investigated here) then becomes an issue.
| It should also be noted that opening much later than the present situation risks increasing stratification events as the winds required for mixing are at a minimum in October. |
Controlled closure of the opening is seen as a possible option, when the bar is close to closing naturally, so as to prevent sand washed in from the ocean building up in the delta.
Figure 17: Bar opening. Photo: N. Boughton
Dredging
Numerical and hydraulic modelling have been used to investigate different dredged channel configurations. The configurations investigated represented both major and minor dredged channels reaching back from the bar, in some cases as far as Poddyshot Point. The results indicate that there would be a substantial increase in the channel flow rate by creating better flow paths through the shallow delta area and comparatively little would be gained by dredging further back in the Inlet.| In other words the most value for money in terms of increased marine exchange would be achieved by dredging the area immediately behind the bar as this is the main hydraulic control. |
It should be remembered that dredging to increase nutrient removal by marine exchange might also increase nutrient release by stratification. Even with increased marine exchange nutrients released from the sediment in stratified conditions are not removed fast enough compared to the rate that plants can take them up. Therefore the management of stratification must be considered, especially given that marine exchange is not an efficient nutrient removal mechanism in Wilson Inlet (compared to say the Peel-Harvey where nutrient concentrations in the water when the Dawesville Channel was proposed were 4-5 times higher than in Wilson Inlet), and a balance between the processes must be struck. The balance will vary from year to year depending on factors such as winds.
