ON MONDAY this week, the aldermen of Launceston City Council voted unanimously against any further pursuit of the ‘Tamar Barrage’ — a proposal to build a loch system across the Tamar River at Stephenson’s Bend in an effort to solve the siltation problem in the Upper reaches of the Tamar Estuary.
How is it that the health of the Tamar has degraded to such a degree that city elders are now contemplating such a radical engineering plan?
Only 4 months earlier (31st January) fifty people packed into the small auditorium of Henty House to listen to Professor Brian Jones, a sedimentologist from the University of Wollongong. The crowd included members of all media outlets, Mayors and Aldermen from a number of councils, Liberal, Green and Independent MHAs and MLCs — but no State Government members. It should be noted that the Legislative Council President and MLC for the local seat of Paterson, Don Wing, generously provided his own parliamentary research funds to support Professor’s study.
All had arrived to hear what the public have known for years and what the powers-that-be have been at pains to refute for decades — that the ever increasing disaster of siltation in the Tamar Estuary is both out of control and the product of poor land management practices in the North Esk catchment and hinterland.
Although the Tamar does not suffer from the abysmal heavy metal pollution which afflicts the Derwent sediments, it is rapidly being choked to death with silt. The siltation problem in the Tamar has been documented for many years and was traditionally managed via dredging programs from 1890 to 1966 to widen channels for cargo ships. Today, with siltation escalating, dredging remains the principle method of remediation, although on a much reduced frequency and with a small budget. Professor Jones points out that the dredging approach is only a short-term solution, addressing the symptom and not the cause.
Despite the long history of dredging, successive State Governments and others have spent decades in a State of Denial as to the cause of the problem. The official line has always been that siltation in the estuary is purely a ‘natural’ process that has been part of the river environment since before European settlement. Of the 54 studies — yes 54 — conducted into the problem since 1950 almost all have concentrated on the engineering aspects of dredging and none (until now) have looked at the contemporary science of sediment provenance (where the stuff comes from source) or deposition. Hence the prevailing view has effectively allowed years of abrogated responsibility. This is despite the fact that lead (Pb) isotope geochemistry, as used in this study, has been available and used for similar studies around the world for 20 years! You could say there has been a culture of obfuscation to any attempts to scientifically and independently study the cause of this significant environmental problem.
Consequently it must have come as a shock for some when they discovered that this good Professor had arrived in town to present the findings of his year-long study into the problem. Unlike any other previous study into Tamar siltation, this work was pure independent research — largely funded by a federal scientific body based in Sydney and with no encumbrances of state authorities or private companies. More to the point, Jones is not your average-bear scientist. He is in fact a world-leader in his field (sedimentology and geochemistry) who has studied river and estuary systems throughout the world and is the editor of the prestigious scientific journal, the International Journal of Sedimentology. Never has such an internationally renowned scientific big-gun been drawn into the Tamar debate.
The amazing vanishing yacht basin
The findings of the study were alarming. The Home Reach Basin (Old Launceston Sea Port) is silting-up at a phenomenal rate — 33 mm/year (or more than 1 foot every 10 years). When one considers how shallow the basin and main channel are now (the deepest part of the channel being about 4.5m at high tide), this rate is more than worrying. On an international scale it is typical to find sedimentation rates measurable by only a few millimeters per year (using the same lead isotope technique) — even muddy estuaries such as Chesapeake Bay USA have sedimentation rates
<10mm/year. The heavily industrialised Lake Illawarra in NSW (Wollongong has a population 5 times that of Launceston) has a sedimentation rate of a mere 1-2mm/year. The excessive rate in Launceston means that (without massive and constant dredging) in 100-200 years the Home Reach Basin will comprise a single narrow, shallow channel and in 300 years will be a very small stream. If this process where purely ‘natural’ — as some like to assert — it would normally take 5000-10,000 years to reach this stage. Clearly the rate of sedimentation is not representative of a natural system and has been severely elevated (by almost 20-30 times above background level) due to human activities.
Jones’ study highlighted that the North Esk River was a major contributor of silt. This has previously been disregarded owing to the fact that the North Esk catchment is only 1/10th the size of the South Esk catchment. In addition to this, recent increases in zinc concentrations in core samples from the basin (a method of measuring changes in sedimentation rates) suggest a rapid increase in siltation rates over the past 15 years (since about 1990). This information indicates that activities in the North Esk catchment since that time have resulted in huge amounts of soil erosion, culminating as excessive sedimentation in the Home Reach Basin.
The long-promulgated local urban myth that siltation in the Tamar is caused by tides bringing silt up the estuary was also dispelled by the study. Silt and clay distributions and sedimentation rates measured from a number of cores along the river demonstrated that this tide theory is not valid. Siltation is largely a function of the excessive amount of sediment entering the estuary via the terrestrial river systems and their catchments and then flocculating in the brine of the Home Reach area. This suggests that the concept of regulating Tail Race flows through the Trevallyn power station (the subject of a $200,000 Hydro/LCC study in 2004) may have limited if any impact on siltation as the physical evidence presented in the Jones Study suggests that the influence of tides on silt distribution is not significant.
It should come as no surprise that the study’s principle recommendation (namely to offer a long term solution to the ongoing siltation in the estuary) was to vastly improve catchment management practices — particularly the implementation of vegetation buffer zones (up to 100m wide — yes, you read it — 100 metres wide), tree planting, cattle control and better regulation of agricultural and forestry practices.
Spinning the mud into the pork barrel
Sadly, and predictably, the Professor’s pearls of wisdom are likely to quickly sink into the muddy quagmire of Tamar politics. Clearly caught on the back foot by the Professor’s impromptu visit, there was a rapid flurry of activity to put the appropriate spin on his comments. By the next morning the Examiner was concentrating on estimates of hefty costs for additional remedial dredging — despite the fact that the Professor clearly stated that dredging would only ever be a short-term solution and its cost was likely to be prohibitive. By Friday the Exaggerator editorial was telling its readers that Jones’ study actually said nothing new and only emphasised the need to build new flood levee banks around the City. Whilst levees certainly help protect low-lying Launceston suburbs from floods, it is difficult to see how they will stop soil erosion in the catchment!
On Tuesday 14th Feb (almost 3 weeks after Brian Jones left Tasmania), the State Government suddenly announced $32 million to expand the Launceston flood levee network and to send two council officers and two Government experts on a “fact finding” tour of New Orleans. Even if this money was sufficient to raise the present levee banks in Launceston by an extra foot or two, this gain would be negated by the build-up of silt in the basin within 10-20 years, based on the data presented in the Jones study.
Top-down or Bottom-up?
It may be more likely that the greatest relative movement on the Tamar is not the downward ‘sinking’ of the levees, but the upward movement of the river bed and banks from silt deposition! The inflowing sediments from both the North and South Esk Rivers will fill the available accommodation space within this narrow basin. The geography of the river bed and flood banks in this area is such that there is less space to hold peak flood waters within the existing levees. Any alleged subsidence of the levees may be eclipsed by the infill of the basin. And we now know from the Jones study that this rate of sediment infill in the basin is an average of 33mm/year. This suggests that since the initial Launceston Flood Works were completed in the mid-1950’s, the basin will have in-filled with 1.65m of mud in areas not affected by dredging. So any alleged subsidence of the levee walls would be a fraction of this in-fill.
Meanwhile, in the Deep South …
Part of the public justification for the expenditure on levee banks and trips to the USA has been the media frenzy beat-up on the theme that the present levee banks are ‘sinking’ and are direct analogous with the failure of the New Orleans levee banks during Hurricane Katrina. However, no-one has yet defined this ‘sinking’ of the Launceston levee. This is quite inexplicable, given that the old survey marker on the steps on the Launceston Customs House is a mere 20 metres from the main levee wall. Likewise no scientific opinion on the suitability of New Orleans as a compelling analogue model has been presented. Indeed Launceston City alderman now seem divided on where to send their ‘expert’ panel with the trip now likely to include a saunter to the polder and dykes of the Netherlands. At least the Dutch have had over a millennia of experience in flood mitigation of the mighty Rhine delta and the conversion of the Zuider Zee from a salty inland sea into a freshwater lake. After the disastrous floods of 1953 that killed 1800 people and made 200,000 homeless in one night, there was no complacency, the Dutch rose to the challenge. They worked methodically to understand the nature of the Rhine before building a range of man-made barrages, lochs and levees to control that river.
Those with a basic knowledge of the Mississippi Delta will know (without flying to New Orleans) that the main problem with New Orleans is that the city in on average 2m below sea level — due to regional subsidence driven by the massive weight of sediments (2.4 billion kg per annum) deposited at the mouth of the Mississippi. The last time we checked, Launceston was wholly above sea level! Large-scale regional subsidence is typical of large and active delta systems such as the Mississippi and the Ganges — hence flooding in these areas is largely enhanced by their negative topographies. However, as Jones states, the Tamar is distinctly different in that it is a tidal estuary system which is prone to sediment in-fill. Whilst there the engineering remedies may be similar, it is highly questionable that the geomorphology of New Orleans is a suitable analogy for Launceston.
However one similarity between Launceston and New Orleans which may be worth investigating can be found in a recent study conducted at the Delft University of Technology. The Delft study entitled: Strategies to reduce the maintenance dredging cost of the Mississippi River Gulf Outlet Channel — was conducted in conjunction with the US Army Corps of Engineers and concluded that the best way to reduce siltation (dredging costs) and mitigate flood risks in the upper delta was to promote the development of wetlands to reduce soil erosion. Since the 1930s about 70% of Louisiana’s upper wetlands have been eradicated by industrial and agricultural development leading to increased sediment load in the Mississippi River. Does this finding sound familiar?
Whilst building higher fences (or flood levees) to keep the watery enemy at bay is a nice psychological fix and obvious physical remedy, it may be more beneficial (in the long term) for Launceston to pay a small sum for a geomorphologist or sedimentologist to conduct some basic research rather than paying for four round-the-world air fares.
The concept of a barrage to create Launceston Lake
The concept of a loch system across the upper Tamar was originally suggested in 1903 and has had many reinventions since, most recently by local engineer, Geoff Smedley. The notion is that by damming the upper Tamar, a fresh water lake would be created thereby removing the salt wedge which causes silt to flocculate. Similar systems exist in the UK along the tributaries of the Thames.
A loch system will not, however, stop siltation — it will only slow the rate of sedimentation in a fresh water lake. Even in the UK analogue, dredging is still required. Whilst high sediment loads continue to be provided from the catchments via the Esk rivers, silt will ultimately arrive in the Tamar Basin and settle over time, but will however not be as noticeable initially. Despite this, and given that governments are unlikely to enforce catchment management, the Barrage proposal may at least be worthy of a scoping study. It was clear however that the motion put forward to Launceston City Council was pre-conceived to kill the concept off under the camouflage of ‘cost’.
Another cat out of the bag!
The recommendations of the Jones study are very much common sense, yet in the long history of the Tamar silt problem, no other study had ever publicly reported these findings and certainly no one had ever collected the modern scientific data to support them. This study has let the cat out of the bag — and now the public is starting to ask some uncomfortable questions. After years of muted public discourse on the matter, siltation, and its relationship with catchment management, was the talk of the town. It is bad enough that an internationally-renowned expert is needed to tell our leaders what the rest of us can see as plainly as day.
Mahatma Gandhi spoke of the four stages of change: ‘First there is Denial, then there is Ridicule, then there is Anger and finally there is Acceptance.’ Change takes time. The timeline in Tamar Siltation issue is a good example of those stages. We hope that we are approaching the last stage.
Jim Collier is a ‘Lorax’ of the Tamar; he and his wife Linda live on the River in their boat. Jim stood as a candidate in the recent State election as an independent for Bass. His involvement with the nature of River and its politics stretches back over nearly 4 decades.
First published May 24