The Horseshoe Wetland of the lower Waitangi Reserve in October 2019 with 20-year-old native trees and shrubs inside the stopbank.
The Horseshoe Wetland of the lower Waitangi Reserve in October 2019 with 20-year-old native trees and shrubs inside the stopbank.
OPINION
In February the newly upgraded stopbank along the Tutaekuri River between EIT and Guppy Rd nearly over-topped.
Taradale and Napier were saved from flooding by this new stopbank. Congratulations to Hawke’s Bay Regional Council for a job well done. Now, six months later, the regional council is planting a native shrubbery/plantation/forest on the berm inside this new stopbank.
Is this a problem? In Hawke’s Bay Today (September 7) Jon Kingsford is quoted as saying the purpose of the plantings is to enhance the level of stopbank protection while offering additional cultural, biodiversity and recreational outcomes.
Based on the evidence of Cyclone Gabrielle, the purpose of these plantings must be questioned.
The plantings are on the berm. The berm is the area between the riverbank and the stopbank. Willows protect the riverbank from erosion but once a flood level gets higher than the riverbank it flows over the berm until it reaches the stopbank.
Therefore, the berm carries very significant volumes of floodwater during major events. The bigger the flood the more critically important it is that this flow is not impeded.
When flood water on the berm slows, sediment is deposited. So, with each major flood, the berm level rises. This increases the flood risk as the capacity of the flood control scheme is reduced, effectively lowering the stopbank!
So, will the native vegetation on berms cause deposition? A drive up the Esk Valley clearly shows the effects of trees, shrubs, vines and anything that slows the flows.
Any blockage results in silt deposition. In any major flood, shrub and tree vegetation on the berm will be ripped out, increasing the amount of debris and trash and potentially causing disastrous damage further downstream.
The same area in July 2023. Cyclone Gabrielle completely destroyed the native vegetation inside the stopbank.
Are we willing to take this risk? This destruction is illustrated in the Waitangi Regional Park where professionally maintained native 20-year-old plantings have been totally destroyed by Cyclone Gabrielle’s floods. This is shown graphically in the two photos.
It is difficult to see how these berm plantings can help protect the stopbank. Plantings close to the stopbank are more likely to cause scouring by water rushing around behind the trees which are generally shallow rooting.
Native plants are not designed to do this job. The trees are likely to cause blockages resulting in the river level rising and overtopping the stopbank, putting Taradale and Napier at risk; a result only just avoided during Gabrielle.
We have this almost mystical belief that native vegetation will solve all problems. It will not. The best berm cover would be an open, smooth grass surface, mowed and available as limited recreational areas. The model aeroplane airfield near Awatoto is a good example. The berms along all the stopbanks need to be kept clear of trees.
As residents we need a guarantee that the stopbanks are allowed to function effectively and not be compromised.
We must learn from Cyclone Gabrielle and maximise not minimise the flood capacity of the river so that we can live in safety.
Garth Eyles is Taradale-based and has worked in land management and written books about Hawke’s Bay landscapes. He was involved in the development of Tutira Country Park from its inception in 1998 until 2008.
”The science says live edge protection is most effective for the most frequent floods in our river systems (those that occur annually or all the way up to a 1-in-50-year event) and Hawke’s Bay Regional Council has followed the science in the design of the upgrade project.”
