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Feature

Capturing Coastal Change

Watching how river mouths change over time can tell us a lot about coastal changes, but who has the time to do this?

Vincent Maire

Canterbury's 800km of coastline is the last barrier snow-fed rivers must pass before melting into the sea. This zone, where the land meets the sea, is fragile, dynamic and ever-changing. Understand the river mouths and we begin to understand how beaches, bars and sand dunes build, shift and erode over time.

Since July 1998, the Canterbury Regional Council has had two Cam-Era videos monitoring the mouth of the Ashburton River. The Cam-Era system is based around computer controlled cameras collecting images on an hourly schedule.

Capturing Coastal Change Figure E (26KB)
Cam-Era video monitoring at the mouth of the Ashburton River

"Cam-Era is an excellent system for obtaining information normally difficult to get," says Brodie Young, a Coastal Resource Scientist with the CRC. "The Ashburton River mouth migrates two kilometres along the beach and this alone makes surveying extremely difficult."

Like many Canterbury rivers, the Ashburton is an important source of irrigation. As flow rates change with irrigation use, so too does the quality of water which, in turn, affects migratory fish like trout and salmon.

The information obtained over eighteen months of video monitoring has enabled the CRC to understand how river mouth change is influenced by river flows and waves, and to predict with far greater accuracy the impact changes in the river mouth will have on the wider aquatic system.

Further north, Environment Waikato has two Cam-Era sites: at Tairua on the Coromandel Coast and at Mokau south of Raglan.

"Before Cam-Era we visited Tairua three or four times a year to survey cross sections of the beach," says Jim Dahm of Environment Waikato. "It was very costly in man-hours and we were never able to get a true picture of how the beach was behaving. We now have one camera that makes survey measurements hourly!"

At Mokau, two video cameras monitor an area affected by severe coastal erosion. One camera is directed at the bar and the other at baches near the river. Images from both the Mokau and the Tairua sites are posted to the Net which means bach owners can keep an eye on their properties without having to travel to Mokau.

In Tairua, surfers now visit the Web site to check conditions before heading to the beach!

Capturing Coastal Change Figure D (52KB)
George Payne servicing video camera at Tairua

Dr Terry Hume, a Coastal Resource Scientist at NIWA (the National Institute for Water and Atmospheric Research), is responsible for the co-ordination of management applications of the images.

"Our clients want an installation tailored to their specific application," explains Hume. "They can do much of the installation, routine maintenance and ground control survey work themselves. We supply the state-of-the-art technology, the data analysis skills and the image archival system."

The seven Cam-Era sites around New Zealand are collecting more than 150,000 images a year. These images are helping resource managers to better understand a range of issues such as beach erosion and accretions, river mouth migration, rip location and hazards, and to compile statistics on wave climate. Cam-Era is a complete package from data capture, to image processing and archiving. Because it is computer controlled, sampling strategies can be changed from the office. It is also highly effective for public education.

"Stringing together a series of hourly images creating a fast-forward' movie provides an excellent way to educate the public about how dynamic the coast can be" says Hume. "Census data on bird populations and how people make use of access ways are another use."

Cam-Era has been in use for nearly two years, and with each new site, new uses are emerging for this cost-effective medium of gathering what used to be data that were almost too hard to collect.

Capturing Coastal Change Figure A (32KB)

Cam-Era development was principally funded by the Sustainable Management Fund, Ministry for the Environment. Development partners were NIWA, University of Waikato, Massey University, and Environment Waikato. Funding to purchase the hardware at each of the sites has been provided by the management agencies.

Watching the Waimakariri

Vicki Hyde

Every day, the Waimakariri River, near Christchurch, is watched as part of the Cam-Era scheme. Two sets of images are updated daily at noon -- Image A looks upstream and Image B checks on the situation downstream.

The camera images provide NIWA scientists with a consistent record of changes in ever-moving patterns and positions of braids and bar which form in the lower part of the river. It is this river, and others like it, which have formed the Canterbury Plains, carrying millions of tonnes of material from the mountains to the sea annually. Where the camera sits, the Waimakariri flows by at a mean rate of 120 m3/s, carrying an annual suspended load of 3.1 million tonnes.

The river's one-kilometre width sees many braids form in the gravel that lies across the river area. The broad nature of the river and the instability of the channel locations can cause major headaches, hence the need for monitoring. At stake are issues of flood protection and gravel management.

The latter has implications for the former -- gravel needs to be taken from the river as it flattens out on the plain to reduce the likelihood of flooding, but the level of extraction can exacerbate problems. Take too little and the river bed level can rise, putting pressure on the stopbanks which protect the city. Take too much and this can lead to stress on bridge foundations and the undermining of banks.

Capturing Coastal Change Figure B (33KB)

Capturing Coastal Change Figure C (30KB)
The two photographs show the view looking downstream over the braided Waimakariri River for a 5 day period during which the flow is receding from 160 m3/s to 110 m3/s.
Over the 5 day period, an advancing submerged gravel bar, which is shown clearly in the foreground of the 25/6/99 photograph, causes the left bank of the foreground channel to erode by some 27 metres.
Eroded sediment is redeposited at the downstream end of the gravel bar on the left-side of the channel.

NIWA scientists Dr Murray Hicks and Jeremy Walsh have a FRST-funded program to look at the action of braided rivers and associated transport of gravel. Collaborative work between NIWA, the Canterbury Regional Council and the University of Cambridge is using the images in time-lapse mode to observe and measure bank erosion and the development and migration of gravel bars during floods.

They note that "although the main aim of the cameras is for river research, we expect that the images will also be useful for recreational users of the riverbed such as fishermen, jet-boaters, and four-wheel drive enthusiasts".

Further information can be seen at:
http://www.niwa.cri.nz/cam-era/sites/waimaka/waimakariri-a.htm

All images supplied courtesy of NIWA.

Vicki Hyde is the editor of New Zealand Science Monthly.
Vincent Maire, of Straticom, writes for the NIWA publication Aniwaniwa.