Woody debris is cleared from Gladstone Rd Bridge after the build-up from flooding in Cyclone Gabrielle. Photo / Paul Rickard
COMMENT
Paul Mullooly and the Marion Drive/Graham Road group are right on target with their drive for early intervention in removing woody debris from the Gladstone Road bridge in heavy rainfall events.
This must be the first lesson learned from Cyclone Gabrielle.
It has long been acknowledged that flooding levels up the Taruheru River are controlled by the water level in the Waimata at the confluence of the two rivers just upstream of the Gladstone Road bridge.
This water level in turn is more and more controlled by the extent of accumulation of woody debris on the piers of the Gladstone Road bridge.
In Gabrielle, the peak flood level of 2.8m against NZVD datum upstream of the bridge at high tide would compare to sea level of approximately 0.8m in the inner harbour just 100m away, which would also be a rough indication of the water level in the Turanganui River just downstream of the bridges – ie, the woody debris was creating a back-up of around 1.5 to 2m at the Gladstone Road bridge.
The Taruheru River through Gisborne City is a 7 or 8km long bath-tub of seawater sloshing up and down with the tide nearly as far as Tucker Road. The Waimata is also tidal upstream, to past the city limits, and the elevated water level from the Gladstone Road bridge backs up through the whole length of both rivers through the city.
Early intervention on the Gladstone Road bridge with port company log handlers removing the larger logs, some complete with root bowls, from the river, is eminently feasible and would have a huge beneficial effect against flooding of riverside properties in future events.
The council, in conjunction with Waka Kotahi, owes it to the affected property owners and the community in general to set up an enduring agreement to instigate this intervention through CDEM (Civil Defence Emergency Management) as soon as a build-up of woody debris is seen to be occurring, and well before it becomes a risk to the survival of the bridge itself or to the health and safety of those working on it.
This would be a “win-win” for the council, Waka Kotahi and Gisborne City residents as a whole.
I believe the council also owes it to affected property owners to review the FOSAL categorisation of those properties facing the extreme financial stress imposed by the 2P and other categorisations that could be lifted by the reduction in flood levels resulting from the above early intervention in future flood events.
Bob Whinray Garnett visited New Zealand for the first time to learn about his ancestor.