Ian Rowan 26 May 2011
Introduction
In the past there have been numerous local stories about sea water intrusion into the Goolwa Channel and at one time the local irrigators complained to their local member of parliament regarding the operation of the Goolwa Barrage after such an event.
In 1973 salinity measurements were taken and confirmed such an intrusion and during the 1974 flood Radok proved that flow reversals did occur as a result of tidal influence despite the then flood conditions.
Since Oct, 2010, the barrages have been gradually opened to enable the release of flood waters which reached a maximum of 78 GL/d over Lock 1 in March 2011. During this period it was observed that flow reversals regularly occurred causing increased salinities to at least 12 km upstream of the Mouth in the Goolwa Channel despite the flood flow. Figure 1 shows the spikes in salinities (EC) at Reedy Island, 6.5 km upstream of the Murray Mouth, since October, 2010. These seawater intrusions were as a result of one or a combination of spring tides, barometric pressure changes and/or strong southerly to south-westerly winds.
In the period May 16- 27 all three of the above causes combined to produce saline intrusion to and past Point Sturt despite the flood conditions. From May 1-27 the flow over Lock 1 declined from 65 to 22 GL/d and the MDBA estimated that the flow through the barrages was 51 GL/d from May 12-18.
May 16-27 2011
From May 16 to 21 the spring tides caused flow reversals at every high tide with corresponding salinity increases to about 12 km upstream. These salinity increases were short lived and only lasted about 12 hours (see Figure 2). This spring tide effect can be observed to a greater or lesser extent every 2 weeks. During this period there was no input from either barometric pressure changes or wind.
On May 22 an intense low pressure system of 994 hPa with associated west to southerly winds of up to 40 knots crossed the area. This system increased the tide levels in Encounter Bay by about 0.66m above the predicted astronomical tide to 1.4m AHD or 0.65m above the designated FSL of 0.75m in the Lower Lakes.
This event reversed the river flow and sea water intrusion caused salinities to rise to over 40,000 EC in the Goolwa Channel to Narnu Bay. At Clayton a much smaller but measurable increase of salinity to 2800EC (from about 400 EC) was observed. In Currency Creek the back flow of water over the spillway at the northern end of the regulator increased salinities from about 2000 to 9000 EC in this area.
Table 1 is a summary of the data observed for various locations. The bold italic values in Table 1 indicate continuously high conditions rather than short term conditions.
To the southeast salinities upstream of the Mundoo, Boundary Creek, Ewe Island and Tauwitcherie barrages all increased to over 35,000 EC and salinities at Stony Point and Point Sturt rose to 2500 and 4500 respectively. A slight rise was also recorded at the automatic recorder 7 km SE from Milang. The recorder near Raukkan did not show any rise but unfortunately results from this recorder were fluctuating from 400 to over 1200 prior to this event possibly due to instrumental instability.
The duration and timing of the salinity increases depended on distance from the Mouth. At Reedy Island in the Goolwa Channel 6.5 km from the Mouth the event occurred about 1 hour(within the limits of the hourly recordings) after the influx of seawater past Barker Knoll close to but on the opposite side of the Mouth and maximum salinities were maintained for about 63 hours before salinities began to decline. At the HI Bridge and about 12 kms from the Mouth salinities began to rise 3 hours after Barker Knoll and maximum salinities lasted 56 hours. At point Sturt the first rise in salinity was noted 32 hours after Barker Knoll and the event lasted 22 hours.
Associated Impacts
The sudden increase in salinity caused a fish kill in the Goolwa Channel of fresh water fish and presumably other freshwater organisms. Redfin were the most commonly observed dead or near dead fish and others were carp, sandy sprat and bridled Goby (Ken Jury).
In earlier years during droughts and low flows in the River Murray it was not uncommon to have reports of dead freshwater fish floating on the lake and river surfaces during saltwater incursions. (Olsen 1991 p17)
The high tides and flow reversals of course meant that the flood flows could not be expelled and as a consequence levels in Lake Alexandrina and Albert rose from 0.1 to 0.25m. This rise was also recorded at Murray Bridge but here most probably due to wind seiche over the Lake. It is obvious that any increase in sea level due to whatever cause will make it increasingly difficult to expel any future flood water flows.
Conclusions
The impact of the flow reversals due to tidal and wind conditions was remarkable, not because it occurred but because of the extent of saline intrusion against what is considered flood conditions.
After a similar but less intense event earlier in the month, this event was predictable as the BoM forecast the passage the intense low pressure system 4 days previously.
It is now apparent that rising sea levels will influence the Lower Lakes well before the predicted 1m rise in 2100. This will produce flooding due to the inability to expel flood waters and rising salinities.
In the past, before the barrages were constructed, the area must have been subjected to numerous more severe saline intrusions at “average” or low flow conditions. In 1830 when Capt Sturt surveyed the Murray, he reported unpalatable water at Pomanda Point and salty conditions further south. This was not during a drought period.
Very recent political statements would suggest that sufficient water to maintain the Lower Lakes as a freshwater system will not be available for at least 8 years and possibly never if politics, climate change and rising sea levels are also taken into account. All the reports to date have assumed that sufficient water would be available. It is therefore recommended that these reports be reviewed and alternative strategies be developed.
Ian Rowan, Goolwa.
Read more about seawater incursions at Ian's Corner.
Notes:
drinking water - 100 ppm
restriction on drinking water - 500 ppm
limit drinking water - 1000 ppm
limit agriculture irrigation - 2000 ppm
brackish water - 500 - 30,000 ppm
sea water - 30,000 - 50,000 ppm
brine > 50.000 ppm
Flow reversal is just the reversal of the normal flow of a river. In this case saline incursions or intrusions refers to sea water flowing upstream as a result of reversed flow.