Sir – It is remarkable how little comment there has been to mark the recent expiry of the 20 year moratorium that had prevented the construction of a flood mitigation dam on the Todd River north of Alice Springs.
Readers may be interested in a letter dated 13 August 1990 that I received from Barry Coulter, NT Minister for Mines and Energy, in relation to some aspects of flooding of the Todd River, which I now quote: “Your letter of 28 January 1990 to Hon. Roger Vale concerning the level of flooding in Alice Springs was referred to me some time ago for response. I would have replied earlier but the officers concerned in the Power and Water Authority have been fully engaged in preparing the Draft Environmental Impact Statement of which no doubt you are well aware by now.
“Authority records indicate that the largest flood since observations were commenced was in 1910. This is regarded as an 80 year flood. The flood of 1921 is classed as a 40 year flood. For comparison the 1983 flood was a 20 year event and the 1988 flood a 50 year event.
“A 100 year flood could be expected to run generally about 0.5 metres above the 50 year flood of 1988.
“Since the release of the Draft Environmental Impact Statement a revised assessment of the 100 year flood levels has been undertaken. For the property you refer to at the south-west corner of Stott and Leichardt Terraces [a two-storey house now replaced by the new Imparja headquarters] it is estimated that the 100 year flood would run at a depth of 1.1 metres over the ground level. This is some 1.3 metres below the elevated floor level. Since the floor level is well above the 100 year flood level it would appear that if the designer considered flooding, a substantial margin was provided.
“Many comments have been received over the years regarding the impact on flooding levels of the crossings, particularly the Casino Causeway [since this letter renamed Taffy Pick Crossing] which is seen to have the greatest effect. There is no doubt that even the bed level causeways have a constricting effect during major floods because the river bed is known to become mobile to a depth of at least 3 metres.
“However, because the river bed has a fall of some 10 metres from the Charles River junction to Heavitree Gap the backing up effect of crossings and other constrictions is very localised.
“For instance the Casino Causeway is estimated to back up only 400 metres in a major flood such as in 1988. The rise at the crossing is of the order of 0.5 to 1.0 metres depending on the flow and it tapers to zero 400 metres upstream. This is some 1000 metres from the Central Business District. However, the Casino Causeway is known to increase the flooding levels in 8 Housing Commission flats just upstream in South Terrace.
“I can see from your comments you will understand that with the dam in place flood peaks will be reduced and the backing up will not occur except for floods of severity well in excess of 100 year magnitude.
“Your support for this important project which is critical to the safety and quality of life in Alice Springs is appreciated”.
It’s interesting to note these days the 1988 flood is now generally referred to as a 20 year event, rather than a 50 year event as it was being described two decades ago.
In my original letter to Roger Vale, I had drawn attention to the old two-storey house located at the corner of Stott Terrace Bridge which had been built by the Driver family of Elkedra Station in the 1950s. I had been informed some time earlier that it had been designed to take account of flood levels recorded at this site earlier last century, well before the construction of causeways and drainage channels that are now recognised to have localised impacts on water flow in the Todd River within the urban area of Alice Springs.
It’s also worth noting the Imparja headquarters (pictured above), which would be inundated by more than a metre of water above ground level in a 100 year flood, is located just downstream from the Civic Centre and the Alice Springs Public Library. These public assets are equally at risk of inundation.
Of most concern to me is the Alice Collection at the Public Library, which contains a lot of material of great historical value; much of which would be difficult (and perhaps some of it impossible) to replace in the event of flood damage.
Given the history of rancorous debate about flood mitigation and damming the Todd, it’s no surprise there is a marked reluctance on the part of just about everyone to discuss this matter now, especially in the lead-up to the impending Territory elections.
So far we’ve gotten away with avoiding tackling this problem but eventually we’ll discover that sticking our heads in the sandy bed of the Todd River will be an inadequate defense and inexcusable failure to deal with it.
We have the capacity to resolve this matter but a complete lack of will to act on it.