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Fraser
Thomas Ltd was originally engaged by the Auckland/Waikato Fish & Game Council to
design remedial works for the Whangamarino Weir, following the failure of the original
weir in April 1995.
Prior
to the remedial works being undertaken on the weir, a scour hole was identified downstream
of the failed weir, due to the concentration of turbulent water flow.
Fraser
Thomas were subsequently engaged by the Department of Conservation to appraise the risk of
piping and uplift occurring adjacent to the stopbank, running along the Whangamarino
River, as a result of the construction of the weir and the creation of the scour hole.
The stopbank is
inferred to be underlain by a surficial layer of low permeability silts and clays, which
are in turn underlain by pumiceous sands. It was thought that the construction of the weir
and development of the scour hole may have created a hydraulic connection between the
pumiceous sands and the Whangamarino River, thus increasing the risk of piping occurring
and failure of the stopbank.
We concluded in our appraisal
that, in fact, prior to the construction of the original weir and subsequent development
of the scour hole, the river bed was founded within the pumiceous sands and therefore a
hydraulic connection existed before the development of the scour hole.
The results of the appraisal indicated that the risk of piping
failure occurring has not been changed by the construction of the weir or the development
of the scour hole downstream of the weir.
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