Shannon Creek Dam Landslip

May 2010

The fragility of the sandstone derived soils at the Shannon Creek dam site south west

of Grafton was something we identified as far back as 1999, so to see an estimated

100,000 cubic metres of hillside slide gently into the dam came as no real surprise.

What did surprise us a little was North Coast Water's response when questioned

about the slip by Grafton's Daily Examiner, which stated:

“

The photo shows a landslip that occurred in one of the areas from which earth

fill was extracted for construction of the main embankment. The slip occurred

along a downward sloping layer of weak material that had been identified in

earlier geological studies. As the water level in the dam rose the weak material

became saturated and subject to water uplift pressure. Further uplift pressure

from the rising water induced downward sliding. The slippage continued over a

number of months, but appears to have stabilised over the last ten months.

The slippage has very little effect on the dam. The exposed soil areas are likely to

erode a little, but the eroded material soon settles out and has little or no effect on

the water quality. Council will continue to monitor the slip but will not take any

action unless a much larger slip occurs in the future. This is the only significant

slip that has occurred around the dam.

Part of the hillside in the process of sliding into the Shannon Creek dam