At the WBRA Committee meeting on Saturday, 13 April 2019 Ramon Strong from the Horizons Regional Council presented the findings from the Tonkin and Taylor report on the river mouth and inlet.
Summary: it’s a complex situation. Ramon Strong aims to hold a public meeting, probably in the first week of June 2019. Watch for further details.
Dated March 2019, the Tonkin and Taylor report is called
Coastal Geomorphological Assessment And Management Options. As an engineering report it’s highly technical, but some key findings that stood out are:
- our inlet and river mouth area is complex and very dynamic, so it changes a lot
- climate change makes this issue complex and climate change effects are uncertain
- the open coast north of the river is ‘prograding’ (building up sand) at about 2.5 metres per year while the open coast south of the river is prograding at about 1.4 metres per year
- there has been around 25 metres of shoreline erosion at the north end of the inlet in the past 10 years. (A large part of that was probably during the big storms early in 2018.)
- the Waikawa Stream is fairly ‘flat’ (not steep) and at the throat it’s flat, shallow and broad, so it doesn’t have much energy
- the river doesn’t flush out very well so sediment builds up
- a 120 metre groyne was installed around 1991. It runs roughly in line with the river. By 2000 it had lost 25 metres in length, and the shoreline has moved 40 metres towards the sea
- by 2012, the originally 120 metre long groyne seems to have become lower and broader
- in 2012 a 30 metre high-angle groyne was built — it juts out across the river. After this groyne was built erosion seems to have increased.
The report also mentioned key factors contributing to ongoing erosion:
- the river throat flushes less efficiently because:
- the groyne is at a high angle. Since the high-angle groyne was built (2012) the river more often comes closer to shore in the inlet.
- the throat is misaligned (there’s a kind of bump in its course). This seems to make the channel wider, reducing flow, and making it harder for the river to push sand along and out of the way.
- the open coast shoreline north and south of the inlet has ongoing accretion.
Another complexity is that the river throat and mouth don’t currently really come under the control of either Horizons Regional Council or Horowhenua District Council.
In order to
mitigate erosion along the shoreline of the inlet the report offers recommendations at various indicative costs from free to more than a million dollars:
- Do nothing
- River training: cutting, realign the groyne
- Protect the banks: construct dunes, additional groynes, use rocks to protect shoreline
- Combination of the above.
Any work is liable to add to our rates bills.
There may also be a case for work on the north side of the river, but a separate report would be needed on that.
One point Ramon Strong made is that rocks are in high demand, in part because of the Expressway building. That pushes the price of rock up.
Final decisions on work to be done will be made by Horizons Regional Councillors. ‘Our’ Councillors are Colleen Sheldon and Lindsay Burnell QSM.
What to do now —
- read the full Tonkin and Taylor report (13.5 MB, PDF)
- optionally, read Miraz’s plain language notes (42 KB, PDF)
- tell friends and neighbours about the report and encourage them to read it.
- discuss the report with others (for example, in the Comments area for this blog post).
- watch out for the public meeting. We’ll share the date and time as widely as we can. It’s being organised by Horizons Regional Council.
It’s great to know that we now have a technical report to inform future discussions about the erosion at the river mouth and inlet. It seems likely that some action will be taken to mitigate the erosion that’s caused so many problems. Now we just need to hope that gales and raging seas don’t create more damage before any work is done.