Natural Hazards Update - No.9 2004

Flood forecasting trialled

When it comes to predicting flooding, it’s not enough to forecast how much it will rain. People need to know whether the rivers can cope with that rainfall, or will the water sweep through their homes and farms, wash away roads and bridges, and create the havoc which residents in Bay of Plenty, Taranaki, Rangitikei, Manawatu, Wanganui, Wairarapa, Hutt Valley, and Picton have seen in the past year.

NIWA is operating a full-scale pilot flood forecasting system in Otago, Bay of Plenty, and Gisborne, in collaboration with the regional and district councils.

The computer-based flood forecasting system uses the Topnet catchment model to predict the effect of rain on river flows. The detailed models of water movement in catchments and rivers are linked to weather model predictions, and to real-time measurements of river flow, in order to predict the flood flows in rivers up to two days ahead.

NIWA is also testing the model for hundreds of locations in nine other catchments. We are developing a web-based interface so impending floods can be detected and warnings issued automatically to organisations responsible for flood management.

Safe ports: swell monitoring

Jody F Millennium (R. Saxby, Gisborne).

NIWA has been advising the Maritime Safety Authority’s Environmental Assessment Task Force on environmental factors that affect safe access to and from New Zealand ports and harbours, how these factors should be monitored or forecast, and how maritime risks can be reduced.

One example is swell – waves generated by a distant wind event or storm. At some ports, the height of the swell can crucially affect the clearance under a ship’s keel as it makes passage through any dredged shipping channel. Several grounding incidents in the last two years, including Jody F Millennium (Gisborne) and Capella Voyager (Marsden Point), have highlighted concerns about monitoring and forecasting the sea state.

The best way to get information on wave conditions is to have one or more wave measurement devices, relaying relevant data reports to maritime operations staff.

At present, a wave-rider buoy is the most suitable device for water depths greater than 20 to 30 m. For instance, NIWA currently relays information from a wave buoy in Cook Strait via Greater Wellington Regional Council to assist Interislander ferry operations. For shallower environments typical of areas around shipping channels, NIWA has a range of other instruments such as Acoustic Doppler Current Profiler meters, which are more appropriate. Remote video or digital camera systems, such as Cam-Era, and portable microwave Doppler radar systems also have their uses, especially for monitoring waves either at river-bar entrances or off headlands.

To get a better understanding of wave conditions over a broader area, and to forecast conditions, NIWA operates a suite of computer wave models. For example, NIWA is currently trialling a wave/swell forecasting system that forecasts sea state up to 126 hours ahead once a day. These can be tailored to give forecasts for any site off the coast.

New Zealand technology for English beaches

Cam-Era images show changes in the width of Tairua Beach, which can be matched with changes in beach volume.
www.niwa.co.nz/services/cam-eraJuly

The University of East Anglia has contracted NIWA to provide a Cam-Era system for studying beach change in the UK.

Cam-Era is NIWA’s state-of-the-art automated video system which currently collects hourly images from eight New Zealand beaches, rivers, and tidal inlets.

Before such technology, it was nearly impossible to collect meaningful information on beach dynamics. Survey teams cannot be there often enough, especially as the most significant changes occur during and immediately after storms, and land-based surveys usually miss sandbar movements, which drive shoreline erosion and build-up. Cam-Era, by contrast, operates year round regardless of the weather, and captures information along the entire beach, including sandbars.

The processes we can use Cam-Era to monitor include: the movement, splitting, and merging of sandbars; the occurrence of rip currents; the formation of erosion ‘hotspots’; the speed and direction of longshore currents; and longshore sediment transport rates. We are also working on techniques to infer nearshore bathymetry from the video images, and other new applications for coastal managers and planners.

Undersea fault's secrets revealed

An EM300 multibeam image of the seabed off southern Fiordland, showing active traces of the Alpine Fault.

New maps of the offshore portion of the Alpine Fault, extending some 230 km south of Milford Sound, show bigger earthquakes could originate there than previously thought.

The unprecedented, high resolution images were produced using Simrad EM300 multibeam acoustic mapping technology on NIWA’s deepwater research vessel, Tangaroa.

‘We have combined the new images with dating of sediment samples collected from the seabed, improving our understanding of the detailed structure of the fault,’ says NIWA marine geologist Dr Phil Barnes.

The data suggest an earthquake centred on the offshore part of the fault could rupture the entire offshore section and propagate onshore, reaching magnitude 8.1. The estimated earthquake recurrence rate is less than 300 years.

NIWA is working with the Institute of Geological & Nuclear Sciences to incorporate this new information into hazard models which can be used to estimate the likely impact of earthquakes on South Island towns.

Aerial laser scanning: a tool for hazard mapping

Topography of a 4-km long reach of the braided Waimakariri River near Christchurch, surveyed by LIDAR in May 2000 (top) and again in July 2003 (middle). The map of elevation change (bottom) shows in-filled channels (green) and new channels cut (red). On average, the river bed rose by 13 cm over the 3 years between surveys.

More and more organisations are using aerial laser scanning (ALS) to survey large areas rapidly and accurately. Surveys using ALS, more commonly known as LIDAR (Light Detection And Ranging), appear expensive up-front, but the cost per square kilometre can be remarkably low.

NIWA has developed techniques for using ALS data to help map some natural hazards. For example, we can use the data with computer models to predict floodwater levels and also to monitor the build-up of sediment in river beds prone to flooding. We recently repeated an ALS survey of part of the South Canterbury coast, which showed the retreat of some cliffs and changes in gravel storage on beaches, indicating erosion by storm waves.