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Let’s bring back flow for the Taruheru


In April 2017 I met with council staff over the September, 2015 flooding. Living on the Taruheru I realised that as charmed as our city may be, we had dodged a bullet. Accordingly I was fully-armed with information about the impediment to flow presented by the three quick-fix earth berms. That was my “push” strategy; herein my “pull”.

The river at Hall Street Bridge measures 91 metres across. The channel is 52m and the berm 39m. It is 2.2m high at the bridge and 1.4 at the bank. In 2015 two metres of water completely covered two-thirds of the berm. As an impediment it measures (an averaged) 1.7 x 39 = 66.3 square metres. The total cross-sectional area measures (91 x 1.7) 155 square metres; ie only 57 percent of the waterway is available. At a flow rate of 2.7m per second (standard number for floodwater) that means 180 cumecs short of its 418 potential. With water meaningfully eyeing the floor joists of one upstream property, removing that barrier would, significantly, release 76 percent more water (ie 418-180=238 =>180/238 as a percentage).

Derby Street: 108m total, channel 47m, berm 61m and three metres high throughout. At two metres depth, flow is 254 cumecs of a potential 535.

Roebuck Road: 86m total, channel 41m, berm 45m (+/- five metres as edge undefined). At two metres depth, flow is 220 cumecs of a potential 464. How much water adds to the system post Roebuck Road is unclear but here would be the obvious bottleneck.

In terms of Ian Ruru and friends' “mauri compass”, the Taruheru is not the “jewel in our crown” it could be and far from the “happy Huckleberry hunting ground” it was. Clearly the key to physical and energic rejuvenation is bringing back the “flow”. A skybridge has already been mooted and I am advocating three, as “pileless” replacements. In 2017 I obtained an estimate of $200,000 to build a suspension footbridge. Hall Street is the smallest and easiest; the logical place to begin.

Mark Bird

  1. Chris says:

    Log jambs blocking main bridges seemed to be stopping flow from the Taraheru in that flood.