My discussion with Murray prompted me to research the longitudinal profile of the Taruheru and I found it has just one metre fall from Makaraka through 7km to its confluence with the Waimata/Turanganui system at the Marina. That is a gradient of around 1:7000, which is minimal and can never generate a velocity anywhere near enough to mobilise and clean out silt from its channel. The rise and fall of the tide generates more than that, but is still far from a self-cleaning velocity.
Upstream of Makaraka the gradient improves slightly to around 1:1000 over another 7km to upstream of Harper Road (Murray’s picture) and a further 4-5km to Waihirere and Ormond at a similar gradient — still nowhere near enough for self-cleaning. There is about a 2m bank height above river level throughout to general land level adjacent to the river — not a lot for high tides, plus flood flows, plus back-up from the Waimata/Turanganui system behind the clogged Gladstone Road bridge.
Upstream of the A&P Showgrounds (opposite Harper Road on the north side) the Taruheru can hardly be called a “river”, more a “canal” or “drain”, and there are many tributary drains running into it. These all need constant maintenance for effective land drainage.
My contention is that the build-up of the berms of the Taruheru with the help of the Spartina grass in no way contributes to the flooding through the city or out on to our fertile flats further upstream. The cause is the blockage of the Turanganui River bridges with woody debris, coupled with the greatly increased flows down the Waimata River due to climate change.
Rather than destroying the wetland habitat up the Taruheru, my observation over many decades is that the Spartina grass has improved the numbers and variety of bird species visiting this waterway; changed, yes, for the better — definitely not destroyed. So, leave it alone!
Having said that, we indeed have a land-drainage problem which will only get worse with ongoing climate change. I, too, saw the flooding out on the flats and have explored inland hill country around our region and seen the saturated, sliding hillsides bringing down whole trees complete with root bowls — both exotic and indigenous — and I have seen the resulting silt and debris on the valley floors and river banks.
Clearly this is where we need to focus our efforts long term on improved land management. In the short term we need to ensure immediate clearance of woody debris from our bridges in heavy rainfall events, or mother nature will fix the problem for us — by taking out all three Waimata/Turanganui River bridges. Think about it — what a picnic that would be!!!
