Lower Lakes and Sea Levels

Water levels in the Lower Lakes and Encounter Bay

In 2000 the then SA Dept for Water Resources in their report for the MDBC, wrote:

"The land surrounding the Lower Lakes and the eastern shore of the Coorong would generally have been heavily vegetated with low scrub and saltmarsh. The lakes would have fluctuated in level and salinity on a seasonal basis, probably over a range of EL (AHD) 0 m to EL 0.5 m during most seasons, giving water depths of 1–2 m. Severe drought and flood would have increased this range. The primary change for the lower River Murray, since the construction of the barrages and the dramatic reduction in flows, has been the loss of the vast estuarine system and the consequent loss of biodiversity and ecological resilience. The ecosystems are at risk of degrading into a uniform condition, having low diversity and interfering with the natural productive capacity of the region, particularly from a fishery perspective."

SA Dept Water Resources for MDBC, 2000

Barrage Gates Open January to May in 2011

Between January and May, 2011, the majority of the gates of the Lower Lakes barrages were open to allow River Murray flood waters flow out to sea. During this period flows over Lock 1 ranged from 50 GL/d, to 78 GL/d in mid March, before declining to 21 GL/d at the end of May when the number of open gates was reduced from 402 to 47 then 25 to prevent further sea water intrusions. The period whilst most of the gates were open was an ideal opportunity to observe the interaction between sea and river levels.

The results of recent water level observations from the Murray Mouth along the Goolwa Channel and into Lake Alexandrina are summarised in figure 1 below. The readings were mainly taken from the automatic recorders installed during the drought.

Relationship between Lower Lake water levels, tide and storm influences and seawater intrusion

Figure 1.

The plots A1 and A2 show River water levels from Murray Bridge to the Mouth at flows of 50 to 78 GL/d over Lock 1. At lesser more normal flows the slope would be shallower and approaching mean sea level (0m AHD) in the absence of the barrages.

Plot B shows that at normal maximum spring tides the flow of water was reversed and salinity recordings show sea water intrusion extended to just beyond the Hindmarsh Island Bridge.

Plot C occurred on May 22 when spring tides coincided with an intense low pressure system and storm winds to 40 knots. This event caused saline intrusion up past Point Sturt and to Milang and Mulgundawa in Lake Alexandrina. From May22 to July 31 there were 4 similar events but from June 1 the majority of the barrage gates were closed limiting the amount of sea water inflow.

On July 4 there was a “king tide” in Encounter Bay due to a combination of tide, ocean swell and storms and the water level at Reedy Island reached 1.42m AHD and in Encounter Bay sea level would have reached close to the maximum recorded (see plot D). Again only a total of 23 gates were open limiting sea water intrusion further upstream.

Conclusion

The above figure and observed effects clearly demonstrates just how vulnerable the Lower Lakes area is to sea levels in Encounter Bay. It can be concluded that:

- in pre barrage days sea water intrusion would have been common
- even with more active barrage management, it may not be possible to expel flood waters efficiently depending on the timing of river flows and sea levels, and
- even a small rise in sea level in the future will have dramatic consequences let alone a sea level rise of 0.5 to 1.0m.