Most people don’t realise that of the three rivers crossing the Heretaunga plains only one has transported gravels to the coast probably since the last Ice Age. That river is the Tukituki which flows around the southern boundary of the plains to the coast at Haumoana.
The Ngaruroro River has always dropped its gravel load on the western side of the plains (Roy’s Hill) with only silts and mud finding their way across the original swamps to the coast.
In the early 1800s this river flowed round the northern side of Roy’s Hill, then east across Longlands to Havelock North, along the Karamu, crossing Pakowhai and on down to the sea. In the 1967 flood it shortened its course by 14.5km, bypassing Havelock North and taking possession of the Waitio stream bed. The Karamu River has a very different cross section to that of a braided river, indicating it did not transport gravel.
The Tutaekuri River, which at one stage flowed down the Swamp Rd valley to Fernhill before breaking through the hills at Puketapu on to the northern side of the plain, flowed along Riverbend Rd into the Ahuriri lagoon (and was in the process of filling that with fine sediment prior to the 1931 earthquake). The shape of previous courses, now abandoned, indicate no braiding and therefore no gravel transport.
All along the coast, behind the gravel boulder bank the soils are peaty and heavy textured with no gravels (except adjacent to the Tukituki). Before development, the plain was a large swamp into which the flooding rivers deposited their sediment leaving the rivers to carry the low flow waters. Water courses created a frequently shifting mosaic pattern across the plain.
It was not until the Heretaunga Plains flood control scheme was completed in the 1960s that the Ngaruroro and Tutaekuri rivers were placed in their current positions. Before the scheme, the Ngaruroro River gravels had been deposited around Omahu Rd area. Now, because the stopbanks concentrate gravel movement, the gravel is moving down the rivers but is yet to reach the coast.
In reality, the Ngaruroro and Tutaekuri crossing the plain are not rivers in the sense that they have formed naturally, rather they are canals designed to carry floodwaters across the plains in the fastest and safest manner to protect both the largest food bowl in New Zealand and its urban populations.
In past years it has been fascinating standing on the Waitangi bridges watching large floods rush past and knowing the people had no idea a major flood event was happening. It’s only when an ‘above design capacity event’ happens that the community becomes aware.
Upstream of the Roy’s Hill area and upstream of Puketapu the rivers are natural and braided, carrying loads of gravels. These lengths need to be protected because of their intrinsic value, their rarity in the North Island and their ability to transfer surface water into the aquifers.
Downstream of these areas the main function of the waterways is to transport water safely to the sea with minimum impact on the plains. The flood control scheme has been very effective in achieving this. But increased protection is obviously needed. This is best done by raising the flood protection levels of the rivers to a one in 500 year flood level. To achieve this the stopbanks will need to be raised and the berms cleared to provide a free and fast path for major events to flow safely to the sea.
A more important issue that should be addressed is where is all the gravel coming from?
Is the gravel in the upper catchments just being washed downstream or are there increased supplies of gravel pouring into the rivers from erosion in the mountains? For nigh on 50 years nothing has been done to protect the ranges from erosion nor has the environment been monitored to identify the effects of feral animals overgrazing the essential protective native covers. Next the deer herds will be protected.
Are we now seeing the results of this lack of management with increased volumes of gravel? In the long term this lack of overall management can only result in increased damage to the plains. Long term it is much more important to manage the ranges to minimise the gravel output and flooding than to give the rivers more room to breathe.
