Review Group presents flood report

Published on 31 May 2016

31 May 2016

The Whanganui Flood Management Review Group presented some preliminary findings around the June 2015 Whanganui River flood event to both Whanganui District and Horizons Regional Councils today.

The report, produced by Horizons river management staff, looks at the size of the June 2015 flood, updates estimated flood levels for Whanganui and looks at what, if any, influence river bed sedimentation had on flood levels.

Horizons group manager river management Ramon Strong says one of the findings of the report is that the June flood is estimated to have a return period of 1 in 130 years, with a peak flow of 5,150 cubic metres per second. 

“Determining the peak flow for the June 2015 event is critical to further developing our understanding of flood risk for Whanganui as it is a key input to estimating future flood levels and protection standards,” says Mr Strong.

“Much of the rain fell close to the coast which meant that the flood peak increased in size considerably downstream of our Te Rewa recorder near Parikino. Estimating the peak flow at Whanganui required detailed and complex analysis to account for both the flow at Te Rewa and the significant rainfall and tributary flows in the lower catchment.”

With the peak flow for Town Bridge determined, and combined with an updated cross-section survey of the river channel, the model used to estimate Whanganui River flood levels has been updated.

“As the June event was the biggest on record, including it in calculations has impacted on the size of predicted flood levels for Whanganui urban area,” says Mr Strong.

“This means that design flood levels within the urban area are now marginally higher than previously assessed. These findings may require consideration to be given to the current standard of flood protection for Whanganui.

“That is a complex and involved conversation that will need to be had with the community early next year. However, we wish the community to know that we will provide guidance as requested and consider protecting people’s safety and livelihoods a priority.”

Mr Strong says the re-calibrated model not only allows for a greater understanding of the height of future flood events, but also how different events can behave.

“Prior to  June 2015, extreme flood events had mostly occurred in the upper catchment; by the time river flows wound downstream, flow from the lower catchment would largely be out to sea. As this event consisted of significant rainfall in the lower catchment instead, it has caused us to reassess how we account for this phenomenon in looking at flood protection for Whanganui.

“While the reality is that no two floods are the same, events such as June present an opportunity to evaluate existing systems and provide the community with more accurate information to make informed decisions.”

Mr Strong says the cross section survey results used to re-calibrate the flood prediction model will be of particular interest to the community. The river cross-section survey lines were re-surveyed late last year to determine if the river bed profile had changed when compared with survey data captured in 1995.

“The Review Group understands there may be a perception in the community that an accumulation of sediment in the river intensified the flooding last year. While that may appear to be the case, particularly after the event when there was some visible localised sedimentation, the community can be assured the 2015 survey found there is no reduction in the flood carrying capacity of the river.

“With this in mind, there is no need for measures such as dredging, which would provide a small increase in the river’s flood carrying capacity but is extremely expensive. It is also highly possible that the first flood would reverse most, if not all, benefit that the dredging provided,” he says.

Now that the Review Group has presented the report to both Councils, it is available to the public on the Flood Management Review page of our website and at http://www.horizons.govt.nz/. There are also some flood report FAQs available online.

“Whanganui floods and flood risk are complex issues and the report findings are very technical and may be hard to understand. We’d like to invite any members of the community that have further questions to contact us for more information via free phone 0508 800 800,” says Mr Strong. 

The Review Group will meet again in June to consider its next steps and progress on other priority areas.

“This report will be hugely beneficial to our group to help determine what those next steps may look like,” says Mr Strong.