Twenty-five years ago in late April 1988, the Illawarra was drenched with 600mm of rain. This was a big event even for the Illawarra.
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On Saturday, April 30, Terry Hagan of Coledale had woken and went outside at about 3.20am to move his car.
At 3.23am, he heard a loud crack and saw Coledale railway embankment coming towards him at speed. Instantly, his house and family were swept away and his wife and son died.
Hours later, another house was swept off its foundations at Otford, north of Coledale. Landslides were happening all over the place.
What was happening just before the failure?
* Just after midnight: The Sydney to Wollongong railway track 100 metres north of the Coledale platform had dropped a little and had moved half a metre out of line. The landslide was starting to move.
* 3.20am: Railway personnel saw water flowing under the lines and sleepers. Was this flood water that couldn’t get through a culvert, as supposed by some after the collapse? Or was it simply water flowing west to east because the embankment had dropped enough and no longer acted as a low dam?
Three days later, engineers used chemical dye to show that the few sleepers and some minor junk did not stop the culvert passing water.
So we have some evidence and opinion that there was a culvert blockage and some evidence and opinion that there was none.
Later, a State Rail Authority investigation put the cause of failure down to three things:
1. “The long term instability…of the embankment [since its construction in the 1880s]…
2. the way in which the embankment was widened [in the 1980s]…
3. the blocked culvert [on the night of 29-30 April 1988].”
In my opinion as a long-time flood engineer, I think the SRA took the wrong option. The embankment was moving hours before water was running under the rails. The Coledale collapse had all the hallmarks of a conventional landslide.
Engineers have been paying some attention to the 1988 incident and the extraordinary flooding which occurred 10 years later in Wollongong – understanding the cause or causes could save lives and property when, not if, the next wild weather event occurs.
There was a lot of debris lying around after the 1998 flood, but was every report of blockage actually about water not getting through a culvert? Or was it shorthand for “I didn’t see debris caught in the culvert on that dark and stormy night, but the water came over the top so it must have been blocked”?
A few engineers’ opinions are important to you because, if they unnecessarily add blockage to heavy rainfall as one of hazards of living in Wollongong, then 250,000 of you will pay for it, one way or the other.
Robert French is a water engineer who has carried out extensive research into the 1988 landslide. He would like to hear from anyone who has evidence of culvert blockage west of Coledale train station in April-May 1988 or photographs of heavy debris masses in the mouths of culverts or bridges around August 17, 1998, contact him at robert.french@optusnet.com.au.
