Summary of Sea Water Intrusions into the Lower Lakes

October 2010 to October 2011

In September, 2010 the barrages, which had been sealed closed during the preceding drought years, were gradually opened to allow flood waters to flow out to sea. These flows increased from 20-30GL/d in October, 2010 to 60-80GL/d in Feb. to March, 2011 before decreasing to 20 GL/d by the end of June and 15GL/d now.

On Oct, 15, 2010 there was a severe storm and the combined effects of tide and wind reversed river flows and saline sea water flowed upstream past the various barrages. (This storm incidentally caused the death of many mutton birds, which were migrating south to their breeding grounds along the south-eastern coast of Australia including Nuyts Archipelago in SA.)

Since then numerous sea water intrusions have been monitored by the automatic recorders installed during the drought. These intrusions are caused by one of or a combination of tide, ocean swells and wind. The figure below shows EC water variations over the last 12 months at a monitoring location about 1.5km upstream of the Ewe Island Barrage.

These recordings show that there have been about 12 major intrusions over the past year and a similar amount of smaller intrusions. These events can happen at any time of the year but as expected they are more frequent during the winter months because of higher swells and a greater number of storm events.

During the storm and subsequent period from May, 22, 2011 to early June the majority of the barrage gates were open and as a consequence the volume of saline water intruding was such that it extended past Point Sturt into Lake Alexandrina even though River flows were still high. After this event the number of open gates was reduced to restrict the ingress of sea water during subsequent events.

It should be noted that during storm events the transmission of saline water, once it has intruded into the Lower Lakes, is aided by the effect of the wind over the water (wind seiches). Similar movement of water has been observed into and out of Lake Albert depending on the wind direction.

The relative water levels recorded over the period when the majority of the various barrage gates were open, are shown below.

Plot B shows that even normal diurnal or semi-diurnal spring tides will for a short period, reverse even moderately high river flows. C and D are water level plots during storm events in May and July where sea water intrusion penetrated further into the Lakes. The July event also flooded the road immediately downstream of the Goolwa Barrage. During lower flow conditions the slope of A1 and A2 would be shallower and under natural, pre barrage, conditions would approach sea level.

It is obvious that under natural, pre barrage conditions, sea water inflow into the Lower Lakes would have been common during low flow or even minor flood (up to approx 80 GL/d) flow conditions. It is also obvious that even a small rise in sea level will have a dramatic impact on the Lakes. The barrages could be kept shut and levee banks constructed on the lower islands but at some time the barrages will have to be opened to allow the River to flow out to sea and at these times the Lower Lakes will be vulnerable to high sea water levels. And irrespective of any surface physical barriers, there will be a net groundwater flow from the ocean to the Lakes as sea levels rise until water levels stabilise.  


Ian Rowan, Goolwa

Read more about sea water intrusions at Ian's Corner.