Balancing act with rivers

COMPARE: The same area south of Gladstone Bridge shows significant bank-edge erosion ripping had softened the opposite bank. Had the trees at A been taken out prior to high river levels the erosion may not have happened. A combination of techniques can often prove successful. A shows trees planned for removal. B shows bank erosion. C shows beach ripped. P shows poplar tree marked for reference with above photo.

SOME of the Greater Wellington Flood Protection Department's work in Wairarapa is focussed on the needs of groups of ratepayers, communities who historically looked to council for localised solutions.

Landowner-funded river and drainage schemes are managed through 14 community committees who agree an annual programme of works. GWRC river managers often discuss the needs of individual property owners on site.

Property owners will often ask us to strengthen a river bank with rock, a more permanent fix for bank erosion that comes with a high cost. This might be an option, however in times of high water flow the natural energy of the river could then focus on an opposite, softer bank and cause erosion for a neighbour.

The river energy may focus downwards and scour out gravel from the river bed. This gravel load will eventually be dumped elsewhere in the river. This erosion, build up and dispersal of soil and rock is part of a natural process of any river. We can't prevent it.

The upper sections of the Ruamahanga River are down to bed rock and these areas have had little human intervention.

We can choose to use other management options, possibly creating a "soft" edge with trees, which will absorb some energy and reduce erosion. The land owner might not get the immediate solution hoped for but it is our responsibility to take a strategic overview.

The GWRC Flood Protection Department is often criticised for "cross blading" and "ripping" the river bed. River users believe these techniques cause unnecessary sediment movement, creating an unnatural looking environment and loss of fish habitat.

We try to avoid working in the wet river channel to limit sediment movement. We contract experienced machinery operators to rip or loosen the river beach where it has become compacted. This will mean that the hard beach edge will gradually erode instead of the river pushing over to the opposite bank and creating new erosion problems.

I accept that this technique does leave an unnatural look to the beach and makes driving or walking on the stones less pleasant for those who like to access the river this way.

Our priority is to provide a river channel that is well prepared for the next big flood and to prevent land loss that results from erosion.

We take every opportunity to limit the amount of sediment entering the river system and maintain or enhance the river environment, for example by preserving or creating small pools for aquatic life or avoiding disturbance to backwaters. There are sometimes compromises and cost can be a factor. Ripping part of a beach can cost about $750, to repair the damage can be more in the region of $30,000.

It's easy to point the finger of blame at a piece of machinery working on the riverbank. Bulldozers are easy to see. In truth it is harder for people to see and understand the complex geologic processes involved in changing river conditions. The change to the river bed that led to the recent closure of the Te Ore Ore Water Race is a good example. Lack of gravel entering the river system through natural erosion higher up in the catchment is causing the whole river bed to drop. As a result water can no longer enter the man-made diversion system. There is an engineering solution to this - but probably not one that ratepayers would be prepared to pay for.

A set of consents, or rules, guide the type of work the department can undertake in and around the river and when they can do it. These consents will soon need to be renewed. Part of this renewal process allows for public to put forward their concerns or ideas for river management. This year a new code of practice will support the Flood Protection Department's consent applications.

We are excited by the opportunity our code of practice presents for ongoing improvement in the work we do. Traditionally consents had long lifetimes and the process to change the way we worked when a new best practice option was developed could be drawn out.

Across the Wellington region Floodplain Management Plans are being developed for all major river systems. These plans reflect community priorities and guide the work of river managers. Members of the public can add their ideas or express their concerns during the development of these plans.