Natural, Physical Characteristics of Te Whaanga, Chatham Island

Te Whaanga is a 185 km², shallow lagoon that occupies about 20% of the landmass of Chatham Island. It has four compartments: a northern basin that is separated from the remainder of the lagoon by a shallow region that was used as a ford at one time; central and southern basins that are separated by a shallow region; and a mouth region located on the coast between the central and southern basins. There is one major inflow, from the Te Awainanga River, which flows into the southern basin. The outlet is the Hikurangi Channel, which is open for only some of the time. Te Whaanga is generally shallow (a few metres in depth at most), with extensive regions that are dry when the water level is low.

In 2003, a study was undertaken to determine from a physical point of view whether Te Whaanga was either an arm of the sea or a terrestrial lake. The idea was based on “the tide test” from ancient English and Scottish law. Basically, if the body of water receives tidal flow, it is part of the sea, otherwise it is terrestrial. Subsequently, the Foreshore and Seabed Act has come into force which designates Te Whaanga as part of the sea by statute.

In the study, three recording stations were established, shown as black dots in the map, in the North Basin, the South Basin and near the mouth. Each recording station contained a waterlevel recorder and a conductivity probe (for measuring salinity). The station shown to the right was at the mouth. The data were sampled every 15 minutes and recorded in a datalogger that was downloaded manually every three months.

Over the 9-month period of record from March 2003 to January 2004, we were fortunate to have a complete range of water levels from some of the highest to some of the lowest. At the start of the period, the mouth was closed and the level water steadily built up over the autumn and winter. By October the high levels were causing flooding around the edges, so the Chatham Islands Council opened the mouth manually using an hydraulic excavator.

Over the 187 days when the mouth was closed, the water balance went like this:

You can see that the inflow and change in volume have a very good match, in spite of several assumptions:

• seepage and groundwater flow completely neglected;

• rainfall assumed uniform across the lagoon;

• lagoon area assumed constant with water level;

• river flows other than Te Awainanga ignored.

After the mouth was opened, the regime in the vicinity of the mouth changed completely from a lake environment to that of a tidal inlet, with twice daily cycles in water level and salinity.

In January, the outlet had established itself as an 80 m wide channel. We gauged the flow at low tide on 6 January and found the flow to be 60 cumecs. Using this, and the hydraulics of the outlet, we estimated that the tidal flow varies between -200 cumecs (flood tide) and 120 cumecs (ebb tide). These flows are very large compared with the inflow from the Te Awainanga River, which is usually around 1 cumec and has an historical peak flow of 77 cumecs.

The influence of the tide extends only to between 5 and 25% of the total lagoon area. For the majority of the lagoon, whether the mouth is open or not has no effect on either salinity or water level, apart from an initial drop in water level when the mouth is first opened.

Acknowledgments

The assistance of many Chatham Islanders is gratefully acknowledged. Without their help, this project could never have proceeded. My particular thanks go to Pete Mason, NIWA Field Party Leader, Christchurch who attacked this project with his usual combination of gusto and professionalism. He carried out all of my requests, no matter how unreasonable, with a positive, responsible attitude. His staff Dave Paull and Marty Flanagan also contributed. Working in a remote, muddy, unfamiliar environment like Te Whaanga is not easy, and successfully acquiring all those excellent data represents an outstanding effort.

The full report of this work is available here (2MB).