‘Huge amounts’ of gravel removed from Maitai
Diggers working in the Maitai River have cleared ‘‘huge amounts’’ of gravel washed down by the floods that saw the river dramatically burst its banks and flood nearby streets and homes.
Nelson City Council group manager of infrastructure Alec Louverdis said the gravel had settled near the Collingwood St bridge. It was a ‘‘substantially higher level of gravel than we would usually see on the Maitai riverbed’’.
Gravel had also been removed from the river after the floods of
December 2011.
Louverdis said there was no indication that gravel should have been removed pre-emptively before the rain event, based on hydraulic modelling and surveys measuring riverbed elevations.
‘‘Our modelling of the Maitai River . . . showed the distribution of gravel throughout the riverbed was normal and there were no areas where we urgently needed to remove gravel.’’
He said the environmental impact of disturbing the riverbed had to be taken into account when considering removing gravel.
‘‘While clearing gravel can have a moderate bearing on river levels, it does not prevent the sort of flooding that occurs due to the extreme rain we just saw here in Nelson.’’
Louverdis said the heavy rain in the Maitai catchment pushed Maitai River flows up by more than 100 times the usual, from a normal rate of about two cubic metres per second (cumecs) to 340-400 cumecs.
An Olympic swimming pool can hold 2500m3 of water, with 250m3 of water per lane in a pool divided into 10 lanes. At its peak of about 400 cumecs, the Maitai had just under two Olympic swimming lanes worth of water flowing past a given point every second.