Tamar Lake Benefits
For an infrastructure investment of around $320m, Tamar Lake Inc has demonstrated the following benefits, over and above the silt removal benefits, to the State, Launceston and the whole of the Tamar Valley:
Economic
Economic studies carried out for Tamar Lake assumed a start date for construction of the barrage in 2019, a construction period of 3 years, and then modeled the economic impact over the subsequent 15 years of operations.
The KPMG analysis found that over the 18 year period of the modelling:
o Over the 3 years of construction, the combined capital works from barrage construction and construction of the irrigation scheme, would contribute approximately $315.5 Million in net additions to Gross State Product (value added) and support the employment of 856 jobs
o Over the subsequent 15 years, the capital works relating to irrigation scheme connections by the users, combined with the expenditure from operations of the barrage, suppliers and users of the irrigation schemes would initially contribute approximately $10.28 Million per annum, rising to $19.64 Million per annum in net additions to Gross State Product (value added) and support the initial employment of 67 jobs rising to 128 jobs per annum, as the irrigation scheme becomes fully subscribed and operational, and
o The favourable impact on tourism would more than offset the adverse impact on the existing fisheries, and in net terms, will contribute $112.5 Million per annum in net additions to Gross State Product (value added) and support the employment of an additional 716 jobs.
In summary, over the 3 years of construction of the barrage and the following 15 years of operations in agriculture and tourism, the State will benefit from the support of 856 jobs in the construction phase and a further 844 jobs per annum in direct and dependent industries at the end of the 15 year modelling period.
It should be noted that these economic benefits result only from the construction of the barrage, and subsequent operations in the agriculture and tourism sectors over the 15 years.
The NERA economic study carried out for Tamar Lake Inc forecast a net increase in residential and commercial property values in Launceston and the upper Tamar Valley at $333 Million over the same period due to the formation of the Tamar Lake. The substantial boost to construction and the service industries resulting from this perceived increase in household net asset values, has not been included in the KPMG results.
Siltation
Silt Accumulation in the Tamar almost completely eliminated!
Sedimentation transport studies carried out for Tamar Lake Inc by consultants BMT WBM using the recently completed 3D hydrodynamic model of the Tamar have shown that the construction of a barrage at Long Reach will almost completely eliminate new silt accumulation in the Tamar and deliver a net export of residual silt over time with each significant flood event.
Specifically, the study focused on the critical areas of Home Reach/Yacht Basin and between the barrage and the estuary entrance at Low Head.
Home Reach/Yacht Basin
The studies showed that with the removal of the current flocculation zone from Tamar Island/Freshwater Point to downstream the barrage, and the complete removal of the asymmetrical tidal action, the incoming new silt from the catchment travels in suspension one way through the lake and beyond the barrage gates.
The net result in this area is an almost clear freshwater lake with a constant level of 0.3m below the current high tide.
The studies also show that over time with each flood event that tops the Trevallyn Dam, residual silt on the bed of Home Reach/Yacht Basin will be eroded downstream, never to return.
Mechanical agitation in the form of raking could be used to increase the rate of erosion of this residual silt.
12 months Sedimentation – Maximum inflows
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Home Reach – Tidal |
Home Reach – Tamar Lake |
Barrage Downstream
As new silt from the catchment enters the Tamar principally in times of heavy rainfall, this silt suspended freshwater travels through the barrage gates as they are opened on an ebb tide, and are carried out to Bass Strait with limited mixing (and hence flocculation) due to the greatly reduced tidal prism in this area.
The net result is no reduction in water clarity of this pristine marine environment, and even less silt deposition than in the current tidal environment.
12 months Sedimentation – Maximum inflows
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Tidal Estuary – Tidal Env |
Tidal Estuary – Tamar Lake Env |
3 Months Sedimentation – Summer Inflows
The 3D sedimentation studies confirm that over a 3 month period of normal to dry inflows, the asymmetric tidal flows in a tidal environment transports sediment upstream to the Home Reach/Yacht Basin area, while in a Tamar Lake environment there is no new silt deposition in this area.
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Home Reach – Tidal Env |
Home Reach – Tamar Lake Env |
Water Quality and Turbidity
Provided the TasWater Launceston Sewage Improvement Program (LSIP) is successfully completed in the 10 year time frame, a water quality of B+ in Tamar Lake should be achievable once the flushing of the lake has been carried out.
This also applies to the turbidity of the water in the lake. With no flocculation in the freshwater, there is no reason why the turbidity should not be as clear as the sample taken from Lake Tevallyn shown in the jar with the green lid shown in the photo below. The jar with the red lid is a sample from Royal Park.
The 3D sedimentation studies, which looked at the Total Suspended Sediment (a measure of the turbidity or clarity of the water) in the Tamar From the Cataract Gorge to Low Head, confirmed that in a Tamar Lake environment, the water is clear with minimum turbidity.
Water Turbidity – Cataract Gorge to Low Head
Base Case is current tidal estuary, developed case is the formation of Tamar Lake.
The following photos of the Gorge and Home Point with blue, clean, freshwater, show how this area of Tamar Lake will present to tourists and residents after the formation of Tamar Lake, instead of the current brown, muddy tidal river.
These photos were taken by Geoff Smedley and Jim Collier after Lake Trevallyn was emptied for maintenance in 2003, and the clean freshwater from the lake temporarily displaced the tidal flows.
Flood Mitigation
Tamar Lake flood modelling, which compares the effect on flood levels with and without the barrage installed, has been carried out by BMT WBM.
The study shows that with normal operation of the flood gates in the barrage, there is no detrimental effect on flood levels in Launceston for either the current sea level conditions, or assuming a future rise in sea level of 0.8m
With the ability to lower the level of the lake to mid tide level to create a buffer of more than 45,000 ML of flood waters, coupled with the removal of any tidal effects upstream of the barrage, there is up to a 1.0m reduction in flood levels along the length of the lake.
Sea Level Rise
The barrage provides a protection against a sea level rise of up to 0.8m for the whole of the Valley upstream of the barrage.
This is particularly important for the low lying areas of Invermay.
Water Table Management
The level of the lake may be adjusted to maintain a constant water table in the Invermay/North Esk area.
With all the new commercial construction taking place in Invermay, this is even more important than the protection against sea level rise.
Fresh Water Supply
An almost unlimited supply of that very valuable commodity, freshwater, at any point on the lake for agricultural, industrial, commercial and residential applications.
Marine Navigation
With a lock in the barrage able to accommodate vessels as large as the Wyuna or the Queenscliff ferry, and water level in the lake at 0.3m below current high tide, 24 hour a day navigation along the whole of the current watercourse will be achievable with a higher margin of safety than at present.
With a reduction in the very large tidal flows at the Bell Bay port, the risk of grounding of vessels berthing at the wharves is greatly reduced.
Also, with the reduced tidal flows, the passage and mooring of larger cruise liners in the Port Dalrymple area is facilitated.