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Piha Stream erosion

Posted by SC Comments Off on Piha Stream erosion

Local and Sports Parks West advise that it has employed a consultant to undertake investigations on the erosion currently occurring along Glen Esk Stream near the Piha West Coast Gallery.

Water from the Piha Stream has been eroding the bank behind the gallery which occurs at a bend of the stream. The bank has been falling away, bringing the stream closer to the West Coast Gallery.

Investigation works will start late next week and be completed in mid July 2013.  Investigations will include site inspections, bore hole sampling and some stream surveys.  You may see people around this area of the stream, they should be visible as they should be wearing high visibility safety gear.

It has to be remembered that the Piha Stream has been profoundly altered by timber milling in the early 20th century.

Historic reports of the stream were that it was wide, shallow and stony and could be stepped into off the bank.

Timber milling involved damming water behind timber constructions in the hills, then releasing them to create a huge force of water to push logs down to the mill. Dams were tripped sequentially to coordinate the rush of water from several dams.

Piha Stream below Mill about 1917

Piha Stream below Mill about 1917

As well, the banks of streams were “snibbed”, that is clear of vegetation and outcrops to enable the smooth passage of logs. Around the mill site, the stream was straightened to remove natural meandering, which also improved the flow of water from a milling point of view.

Below the mill, considerable amounts of debris and sawdust from the mill flowed down the Piha Stream to the sea.

All these aspects of milling had the effect of damaging the stream banks and lowering the stream bed to what we see today.

At the same time, hardening up of the surrounding land for development, by importing soil or sand, has damaged the natural ability of the flood plain of the Piha valley to absorb and slow water during storm events. Much of the wetland has been lost, so that during storms events water tends to rush along a deep narrow passage, creating perfect conditions for erosion.

Revegetation could have some benefits in slowing stream flow and velocity but will be hard to establish on such steep, vertical stream banks.  The Piha Steam catchment is steep with high rainfall, so the volume of water it handles is large. No doubt the proposed solution will be some form of protection or armouring of the eroding bank. Left to itself it could continue to carve a new path to the sea.

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