Tamar Lake Feasibility Reporttamarlake.com.au/wordpress/wp-content/uploads/2020/... · construction challenges of the Marina Barrage in Singapore which has a similar channel width

Tamar Lake Inc. Incorporation Number IA 10501

Email: [email protected]

Web: www.tamarlake.com.au

Ross Ambrose Scott Anthony Charles Booth Errol Stewart Kevin French

Peter Thyne Ralph Norton Ted Pedley Mike Steele David Vautin

David Youngman Jack Bain Tim Dowling Tony Gray Andrew Lovitt

Bob Ruddick Alec Purves Stu Cottrell Denis Tucker Bill Woolcock

Phil Frith Craig Yates Richard Matson Marcos Ambrose Ross Peck

Robin Frith Barry Larter Martin Rees Tim Lack Peter Keam

Scott Bell

Rob Dowling

Robert Dutton

Sam Tucker

John Scott

Jim Dennis

Ian Goninin

Peter Watt

John Ferrall

John Dingemanse

Anna Lovitt Ben Lovitt Brian Faulkner Jan Pesl Joanne Larter

Nicolas d’Emden

Tamar Lake Feasibility Report Small Lake Concept

March 2020

Key Finding:

The Tamar Lake project is a transformative project that provides the

community with a substantial reduction in sediment deposits in the

Upper Reaches and protection against sea level rise in the Launceston

area, while also providing the stimulus for huge economic benefits

through greatly improved aesthetic presentation and aquatic amenity of

the Tamar.

Home Point with freshwater flowing one way downstream This is what a Tamar Lake would look like

Founding Members

mailto:[email protected]

Page 2 of 14 Tamar Lake Feasibility – Small lake option concept - January 2020

Introduction

After 9 years of intensive research, the private investment of over $500K into consultant studies, and

hundreds of man-days of in-kind contributions by members and consultants, the not-for-profit,

member funded Tamar Lake Inc. in February 2019 delivered to the community a complete report

into the Feasibility of the Tamar Lake project assuming a barrage forming a large freshwater lake

enclosing 70% of the Tamar Estuary was constructed near Point Rapid at the South end of Long

Reach.

While Tamar Lake Inc. still considers this the ideal solution to sediment accumulation in the Upper

Reaches with new sediment passing through the lake and into Bass Strait, feedback from the

community suggests we consider a smaller lake option with a barrage located Just South of

Freshwater Point.

The purpose of this document is to explore this small lake concept using the experience and results

obtained from all the modelling done for the large lake project.

Principal Goals of the Tamar Lake project While this transformative project will provide a range of major benefits to Northern Tasmania, the

principal goals of this study are to show the feasibility of the installation of a barrage:

1. Substantially reducing sediment deposition in the upper reaches of the Tamar –

short term goal

2. Provide protection of the low-lying areas of the greater Launceston Municipality

from sea level rise

Sediment Accumulation in the Upper Reaches 1 – cause and effect

Cause Previous studies, carried out for the local Authorities over the last 50 years, had identified that the

accumulation of silt flowing into the Tamar from the South and North Esk catchments, is deposited

in the flocculation zone between Tamar Island and Freshwater Point, then is “pumped” upstream to

the Home Reach/Yacht Basin area by the asymmetrical tidal action, particularly in the summer

periods of low flows down the South Esk.

All the studies carried out for the local Authorities have confirmed that new silt from the catchment

is entering the Tamar at the average rate of 152,000 cubic metres per annum from the North and

South Esk rivers, of which 46% deposits in the Yacht Basin and Home Reach, and the balance further

downstream.

With the entrance to the Trevallyn power station penstock situated 2 metres below the top of the

33-metre-high Trevallyn Dam, except in times of flood topping the dam the only sediment entering

the Tamar from the South Esk catchments is the fine particle silt in suspension in the fresh water,

with the coarser sand and gravels trapped behind the dam in the bottom of Trevallyn Lake.

They would cause great damage to the turbines if sand and gravel passed through the Power

Station.

1 The Upper Reaches of the Tamar include the Yacht Basin, Home Reach and the lower reaches of the North Esk river


Installing a barrage (special type of weir with gates to release the water; not a dam) moves the

flocculation zone downstream the barrage and removes any asymmetrical tidal action from the

upstream zone.

Effect

The effect of the barrage will be to form a freshwater lake behind the barrage with water flowing in

one direction from the entrance to the Tamar of the Tailrace, Cataract Gorge and North Esk.

The volume of freshwater from these sources has been measured at between 1500 and 4000 GL

(1.5x109 to 4.0x109 cubic metres) per annum depending on rainfall in the catchments. This averages

out at between 50 and 126 cubic metres per second, most of which flows out through the Trevallyn

Power tailrace.

The effect of this one-way flow of the freshwater is to carry the fine particle sediment in suspension in the freshwater to the barrage and through the flood gates. Downstream the barrage the freshwater has the potential to mix with the salt laden tidal water and cause the sediment to flocculate and deposit on the bed of the river in this new flocculation zone. Studies carried out for Tamar Lake Inc, has confirmed that this change will greatly reduce the deposition of new silt in the Upper Reaches, and with the long term scouring of residual silt by flood waters, there will be a net export of silt beyond the barrage with each major flood event.

The studies have confirmed that provided the freshwater is released on an ebb tide, the flocculation zone will be moved further downstream. How much further downstream before the sediment accumulates on the floor of the estuary will depend on the strength of the freshwater flows. Tamar Lake modelling assumed a water release twice per day on each ebb tide; but there is scope to model the effect of higher rates of flow with water releases far less frequently.

The studies also show that over time, with each major flood event that tops the Trevallyn Dam, residual silt on the bed of Home Reach/Yacht Basin will be eroded downstream, never to return because of the removal of the asymmetric tide. Mechanical agitation in the form of raking could be used to increase the rate of erosion of this residual silt.

Alternative Barrage Locations

This study initially considered two locations on the Tamar River for the installation of the barrage: Point Rapid at the South end of Long Reach near Bell Bay Port, and Freshwater Point at Legana. Let’s call the lakes formed behind the barrages as the small lake and the large lake, with the freshwater storage volume behind the small lake approximately 40 GL and the large lake 450 GL.

2. Barrage near Point Rapid

Freshwater Point initially appeared to be the logical location from the point of view of the ecology

and the fact that the river is substantially fresh from Freshwater Point South to the Yacht Basin as

shown in the salinity profile in Figure 3. below, limiting ecological change.

1. Barrage near Freshwater Point


3. Salinity profile of the Tamar Estuary from Low Head to the Yacht Basin

However, our barrage designers expressed concern that the geology of this area (mostly a 16m thick

mud base) and the width of the river may make the construction of the barrage gates and the ship

lock too expensive and exacerbate flood levels in Launceston.

In addition, the community at that stage were concerned that installing a barrage in this location

would simply move the sediment accumulation down river beyond the barrage (Not necessarily a

bad thing if the sediment accumulates where it does little harm and has a much larger area in which

to spread).

Small Lake Barrage – Construction considerations

For the review of the site for a barrage for the small lake option, we were fortunate to obtain, pro

bono, a brief review of the possible construction issues of this site by a well credentialed Civil

Engineer, John Bower, who is normally resident in Queensland but has strong Tasmanian

connections and has been a supporter of the Tamar Lake concept for many years.

John’s notes on the review are shown in Appendix A in which he also draws a strong parallel with the

construction challenges of the Marina Barrage in Singapore which has a similar channel width of 350

metres and depth as the Freshwater Point site shown in Figure 1. above.

An article on the construction of the Marina Barrage may be viewed at

https://www.scribd.com/doc/66032092/Marina-Barrage-Unique-3-in-1-Project.

https://www.scribd.com/doc/66032092/Marina-Barrage-Unique-3-in-1-Project


The barrage design carried out by C D M Smith for the barrage at Point Rapid to accommodate a

one in 200 year flood event of 4000 m3 /s without any detrimental flood effects in Launceston,

provided for 10 flood gates each 35m wide and 5 metres deep and a ship lock 8m wide giving a total

width of around 400 metres.

Conceptually, it would appear that this design could be accommodated across the width at the

location shown above using the same construction techniques used for the Marina Barrage, but this

will need to be confirmed by an engineering firm for construction design and the same flood

modelling as carried out for the large lake.

Sedimentation Summary

Both barrage locations will substantially reduce new sediment accumulations in the Upper Reaches,

with the accumulation zone moving to the Rosevears/Dilston area with the barrage at Freshwater

Point, and out to Bass Strait with the barrage at Point Rapid.

Full sediment transport and accumulation modelling studies will need to be carried out for the full

length of the Tamar to show the change in locations and accumulation rates compared with the

current tidal situation.

Sea Level Rise – Protection of the low-lying areas of Launceston

Both barrage locations will protect the low-lying areas of Launceston against a Sea Level rise of up to

0.8 metre.


Which Location? See table 4. below for a summary of the benefits and disadvantages of each location.

It was decided that because of cost; the potential for a substantial percentage of the sediment

entering the Tamar being transported in suspension directly for deposit in Bass Strait and the

potential for flood mitigation instead of exacerbation, that Tamar Lake Inc. would carry out detailed

studies on the technical, environmental, economic and funding feasibility with the barrage at Point

Rapid.

Feature Freshwater Point location Long Reach location

Sediment

accumulation

Greatly reduced sediment accumulation

upstream in the Yacht Basin/Lower North

Esk and Home Reach areas

Sediment accumulation moved

downstream between barrage and

Whirlpool Reach; probable flocculation

and asymmetric tide in this area

Greatly reduced sediment

accumulation upstream

Very little new sediment accumulation

between barrage and Bass Strait;

elimination of flocculation and

asymmetric tide

Turbidity Low turbidity in the lake; some increased

turbidity downstream due to flocculation

Low turbidity between Yacht Basin and

Bass Strait

Navigation Assume single lock for leisure craft, 5

metres depth in lake: same as Long Reach

lake

5 metres depth from Seaport to Bass

Strait through a lock

Ecology Minimal change, particularly for migratory

birds and aquatic life

Very significant change but no listed

species threatened

Flooding This is the big question – can a barrage be

designed in this location that has no

detrimental effect on 200-year flood

events in Launceston

Flood studies show no detrimental

effect with a 45 GL buffer providing

the opportunity to reduce flood levels

Barrage cost 400 metres span, but unknown foundation

for construction, most likely a thick silt bed

800 metres span constructed on rock

foundations

Economic

benefits

Launceston would have the same aesthetic

and aquatic benefits as the larger lake, but

Tamar Valley irrigation benefits would be

reduced

Major irrigated agricultural benefits

Water

quality

This is the major benefit over Long Reach,

as the much larger rate of the lake

flushing, and shallow water depth would

limit algal bloom growth

There is seasonal potential for algal

blooms to form just upstream of the

barrage. This needs more study.

Freshwater Adequate freshwater upstream of the barrage,

but would need to be piped to the larger

irrigable areas downstream of the barrage

Almost unlimited freshwater availability

along the 60 km length of the lake, and

particularly adjacent to the Bell Bay

Industrial zone

Sea Level rise

protection

Will protect Launceston against sea level rise of

up to 0.8 metres

Will protect Launceston against sea level

rise of up to 0.8 metres

4. Table showing benefits and disadvantages of the alternative barrage locations


Forecast feasibility findings for the small lake construction

With a barrage installed at the Freshwater Point location shown in Figure 1. above, a medium sized,

constant level, freshwater reservoir of approximately 45,000 ML is formed behind the barrage.

Environmental Impact

Based on the modelling carried out for the large lake option, the following benefits are expected

when a full feasibility study is carried out on the small lake option.

• Ecology

The current ecology of the area south of the Freshwater Point barrage location is principally

a freshwater ecology and with the formation of the freshwater lake, it is expected that

change in the ecology will be minimal.

• Sediment Management

With the movement of the flocculation zone to downstream the barrage in the much wider

and deeper section of the Tamar between Rosevears and the Batman Bridge, and sediment

flowing in suspension in the freshwater flowing one way through the lake, it is expected

there will be very limited new sediment deposition in the lake occupying the Upper Reaches

of the Tamar, depending on the strength of water flows from the catchment.

With limited new sediment deposition in the lake, it is expected that results will show a net

migration of the sediment load out of the lake with each significant flood flow event. This

will be particularly noticeable in the Yacht Basin/Lower North Esk/Home Reach section of

the lake.

• Water Turbidity

The current muddy water appearance in the area from the St Leonards weir on the North

Esk to Freshwater Point, including the Yacht Basin and Home Reach, will transition within 12

months into the same clear water clarity as the Trevallyn Lake.

6. Fresh, low turbidity water entering the Tamar from the Tailrace

• Water Quality – catchment sourced and normal STP discharges

The water quality in the Yacht Basin/Home Reach area will, within 12 months, be greatly

improved with pollutants flowing one way downstream from the Tailrace instead of being

“pumped” upstream into the Home Reach/Yacht basin area.

With the removal of the asymmetrical tidal action from the Upper Reaches, 80% of all new

pollutants entering the Tamar down the Tailrace, or from the sewage treatment plants at Ti

Tree bend and Riverside flow one way downstream from the Tailrace.

5. What the Tamar Lake will look like with freshwater flowing from the catchment


• Water Quality – upstream the barrage As with any large body of constrained freshwater fed from a large rural and urban catchment area, Tamar Lake will have the potential for algal blooms to form on the surface of the lake during the warmer days of mid to late summer. Tamar Lake studies have shown that this has the potential to be a serious problem in the large and deep lake with the barrage at Point Rapid, but is unlikely to be a major problem in the much smaller and shallower lake with the barrage at Freshwater point.

Lake Burley Griffin in Canberra is a prominent example of this situation.

• Rice Grass Eradication

The pest rice grass, introduced in 1947, has spread in the area between Freshwater Point

and the Batman bridge to cover 415 ha of terraces and marsh islands that has reduced the

utility value and aesthetic appeal of the shoreline.

The small lake will have no effect on the rice grass, and it will continue to propagate.


Technical • Flood Mitigation

The effect on flood levels in Launceston in the small lake environment are unknown at this stage with the blocking of the incoming tide having a mitigating effect but there will be no significant buffer to manage levels and the ability to design and install barrage gates that offer no additional resistance to flood flows has not yet been determined.

• Freshwater Supply With a freshwater storage volume of 450 GL in the large lake and 40 GL in the small lake, and an annual input flow of between 1500 and 4000 GL from a catchment of approximately 20% of the area of Tasmania, the large lake opens up huge opportunities for commercial development in the industrial, agricultural, residential, and tourism sectors of the economy. The small lake greatly enhances the aesthetic presentation and aquatic amenity of the Tamar for the residents and tourists visiting Launceston and could also provide an additional freshwater source for the new Tamar Irrigation scheme shown below and potentially for the planned Hydrogen production plant at Bell Bay.

7. Tamar Valley Irrigation Scheme

• Marine Navigation – transition from Low Head to Launceston

With a ship lock in the barrage, and a permanent 24/7 high water level, a minimum

navigation depth of 5m will enable private and commercial vessels, including large tour

boats, to transition the Tamar from Low Head to the Launceston Seaport at any time of the

day or night.


Economic • Tourism

With clean, green, low pollution freshwater and a permanent high tide in the Upper Reaches, the potential exists for Launceston and the whole Tamar Valley to become a mecca for all forms of aquatic sports, and new tourism property developments on the banks of the reservoir. The greatly improved aesthetic presentation of Home Reach, the Yacht Basin and the North Esk can only greatly enhance the returns on the investments being made in the relocation of UTas to Inveresk, and any other new developments planned for the Launceston CBD.

• Recreational Fishing With the installation of the barrage, flathead and snapper fishing will be limited to the estuarine section of the Tamar, but the freshwater reservoir opens up opportunities for both freshwater recreational fishing and aquaculture developments.

• Economic Growth Very conservative economic studies carried out for Tamar Lake forecast an additional growth in Gross Regional Product of 10% in the 15 years post barrage construction due to the formation of Tamar Lake. This looked at only the growth in the tourism and agriculture sectors, but, in the case of the large lake, it is also expected that the industrial and commercial sectors, particularly in the Bell Bay/George Town area, will also gain a large economic benefit.

Funding

Value capture funding methods Acknowledging the significant level of funding required to implement the project, Tamar Lake Inc. commissioned AECOM in February 2017 to estimate the potential for value capture funding methods to contribute to the funding of the Tamar Lake project.

Value capture funding methods are a relatively new concept in Australia. Traditional approaches to funding methods for new or improved infrastructure have used funds entirely sourced out of general taxation revenue, meaning all taxpayers share the burden of paying for the infrastructure despite the fact that many of them will not use or directly benefit.

By identifying and quantifying the value created from the development of the new infrastructure, and connecting it with the costs of the infrastructure, value capture mechanisms can help deliver projects through a fairer business model.

• AECOM conclusions

The report prepared for Tamar Lake Inc. by AECOM concluded:

Given the current indicated construction cost of $320 million, the benefits to property values alone from the project greatly exceed its costs:

• The uplift in median residential property values in Launceston, within 1km of the Tamar Lake shoreline, following construction of the Project and stabilisation of values is conservatively estimated to be $1 billion.

• The uplift in median unimproved commercial land values in the Launceston CBD within 500m of the riverfront following construction of the Project and stabilisation of values is estimated to be $434m.

AECOM identified the following potential value capture funding methodologies to be applied to the Tamar Lake project:

• Selling development rights and / or density above existing zoning controls.

• Sale of government-owned land that is enhanced or made developable by the project.


• Captured through public taxation system such as land tax, council rates and stamp duty, and possibly captured through sharing value uplift with surrounding landowners.

The encouraging outputs from this AECOM study support further work being undertaken into the application of value capture funding to the Tamar Lake project. Tamar Lake Inc. would recommend this work be included in the broader socio-economic study that would be initiated as part of preparation of formal business case for the development of the Tamar Lake.

The Choices

The abandonment by the Launceston City Council of the raking program in the Yacht Basin/Lower

North Esk/Home Reach segments of the river, and the acknowledgment by the Council that

increased flow rates down the gorge does little to mobilise sediment deposits out of the Home

Reach/Yacht Basin area, has left the Council without a solution to the continued build-up of

sediment deposits in this area which have detrimental effects on flood levels and navigation in the

Upper Reaches.

While it is acknowledged that the Council are seeking specialist opinions on how to manage the

continued build-ups of sediments in this area, Tamar Lake Inc. believes it has only two options:

• Over the next 30 years spend many millions of dollars of taxpayer revenue, without any

substantial direct boost to the economy, on a dredging program to maintain navigation

depth and maintain the current level of flood protection.

• Implement the Tamar Lake plan in the short term for an investment of up to $500 million (of

which 30 to 50 % could be privately funded), and provide the foundation for a huge short

and long term boost to the economy in the tourism, agriculture, industrial and commercial

sectors, with the formation of a freshwater, low pollution, low turbidity lake, with limited

new silt deposition and the gradual erosion of residual silt over time.

Conclusion

In producing this report, the executive and members of Tamar Lake Inc. have invested their time and

cash over 7 years, with no expectation of any return, in pursuit of a solution to what has been

acknowledged by almost all residents of Launceston as a totally unacceptable situation.

This situation has been brought about by the initial siting of Launceston at the confluence of the

Tamar and South and North Esk rivers; poor land management since European settlement by

farmers and loggers in the catchments; watercourse width and depth reductions both by natural

sediment infilling and the community’s infilling and bank modification of areas around Home Point

and Royal Park; and to an antiquated sewage treatment and stormwater system that has not kept up

to modern standards for effluent discharge and urban runoff.

With the release of this report, the altruistic investment by the members of Tamar Lake Inc. has

been completed. If a State Authority is nominated or formed to manage all aspects of the Tamar

Valley health and amenity, and funding is provided to that Authority to carry out the planning and

approvals process, all the reports commissioned by Tamar Lake Inc. will be made available to the

Authority at no cost to that body.

With the environmental, lifestyle and economic benefits that would accrue to the Northern

Tasmania community with the implementation of the Tamar Lake plan, there is no reason why all

three major parties in the State Parliament would not support a proposal to fund the planning and

approvals stage.


What is Tamar Lake’s Position going forward?

Having completed these feasibility studies, the members of Tamar Lake Inc., while convinced that

this is the right solution for Northern Tasmania, would not expect unqualified support from the

community until the full business case and Environmental Impact Statement (EIS) have been

prepared and presented to stakeholders and the community.


Appendix A In your email of the 28th you mentioned that “The key issue is whether a barrage can be economically constructed at Freshwater Point that has no detrimental effect on flood levels in Launceston”. In your most recent Overview you have stated that “our barrage designers expressed concern that the geology of this area (mostly a 16m thick mud base) and the width of the river may make the construction of the barrage gates and the ship lock too expensive and exacerbate flood levels in Launceston”. In Figure 1 of the Overview, you have shown a barrage location at what looks like “Dog Point” on the River Tamar chart – see extract below. The width at this location scales at about 380 m and the maximum depth is in the order of about 10-12 m below chart datum which is stated as being approximately the level of the lowest astronomical tide (LAT). Based on the “Preliminary Estimates of Spillway Widths” contained in the BMT WBM report Tamar Lake Preliminary Technical Assessment, dated February 2012, the gates of the barrage would need to extend most if not all of the width of the river at this location. Hence, I assume the concern about the width of the river mentioned above.

Figure 1 - Extract Hydrographic Chart Aus 168 River Tamar

My view is that the barrage would need to be designed to pass the required flood event (200-year ARI?). I suspect that this criterion is likely to control the overall geometry and gate opening sizes (both height and width) of gates etc. Whilst this may not be that difficult to achieve for the full width of the barrage, since the BMT WBM spillway width of 300 m (Table 5-1 of their Feb. 2012 report) would fit at this location, the barrage would need to be constructed in stages which quite possibly would represent the greatest risk.


I believe the Marina Barrage in Singapore offers some quite good parallels. I have sent through a separate copy of an article from Structural Engineering International which provides some useful information. Similarities between Freshwater Point and Marina Barrage are:

• Similar overall width – Marina Barrage 350 m with 9 x 30 m wide gates (no lock).

• Similar tidal range although Launceston is about a 4 m variation compared with 3 m at Singapore.

• Possible broadly similar soft ground conditions – in Singapore the whole structure sits on bored piers down to a depth of 74 m. Hopefully a Freshwater Point location would not need piles this deep.

• Constructed in two stages as detailed in the Structural Engineering International article. I suspect any meaningful assessment of a Freshwater Point location would need to consider construction staging. This might mean that the overall width of the structure needs to increase from what is possible at the narrowest location. Assuming two stages I suspect the total construction period would be 2-3 years, so the risk of a major flood during this period would not be insignificant. Figure 2 shows a highly idealised concept as to how staging might be possible. This would involve moving the barrage slightly further upstream from the minimum width at Dog Point. This might involve building cofferdams similar to Singapore and then determining the width and depth of channel required to pass the required flood flow past this with the required channel maintained by dredging during the construction period. Stage 2 would be a mirror of this with likely associated dredging as well. Moving the location slightly also provides more “space” for the lock. This is very conceptual and should not be taken too literally but I have prepared this to simply demonstrate a concept.

Figure 2 - Possible Stage 1 Construction Concept Any hydraulic modelling, in my opinion, would need to consider:

i. Capacity to pass required flood event with final configuration of the barrage + lock. ii. Capacity to pass required flood event(s) during construction staging with associated temporary channel dredging to maintain required waterway area and depth.

Prepared by: John Bower Brisbane 2 January 2020

Documents

Tamar Lake Feasibility Reporttamarlake.com.au/wordpress/wp-content/uploads/2020/... · construction challenges of the Marina Barrage in Singapore which has a similar channel width