Feature

RALPH and ALICE At Sea

It's neat to dip your feet in the Manukau mud -- but what does this tell us?

Malcolm Green

A collaborative experiment between NIWA and the Geological Survey of Canada (Atlantic) is leading to improved control of a major pollutant and to development of a new generation of oceanographic instrumentation for New Zealand.

The research is motivated by the recognition that suspended sediment is a ubiquitous pollutant in New Zealand's estuarine waters. By clouding the water, suspended sediment can have deleterious effects on plant life, benthic habitats and recreational amenity. In addition, chemical contaminants that are attached to highly mobile fine-sediment particles may become widely dispersed from their point of introduction into the estuary.

The FRST-funded research programme aims to provide improved understanding and predictive numerical models of fine-sediment transport in estuaries. Such models have application in identifying causes of sediment pollution, predicting environmental consequences of proposed developments, and assessing the efficacy of schemes for mitigating sediment pollution.

The objective of the collaborative experiment was to identify the "forcing functions" that mobilise bed sediment and cloud estuarine waters. There are two main candidates in that respect: tidal currents, which act slowly but methodically, and wind-generated waves, which can suspend vast amounts of sediment but only for short periods of time.

Tidal currents are more amenable to study than storm waves, for obvious reasons, but previous work indicates that in large estuaries the water clarity, which is a good indicator of suspended-sediment load, is directly linked to wave activity. Thus, fair-weather measurements would tell only part of the story, and possibly the insignificant part at that. Not only do measurements of waves, currents and suspended-sediment concentration (SSC) need to be made over several weeks to cover a range of environmental conditions, but measurements also need to be made in the bottom metre of the water column (within the "benthic boundary layer") as it is here that the energy of the moving water is imparted to the bed.

At the time of the experiment, NIWA did not possess suitable instruments for studying boundary-layer and sediment dynamics in the field. Thus, collaboration with the GSC was arranged to bring "RALPH" to New Zealand for the experiment.

RALPH is a 2.5-metre high tripod that holds a suite of instruments for measuring waves, SSC and boundary-layer flows and turbulence. The device is self-contained with pressure-resistant canisters housing a power supply, a hard-disk drive for storing data, and a powerful microprocessor for controlling sensors, acquiring data, monitoring system status, and communicating with the user.

RALPH has an endurance of up to eight weeks, and has been used by the GSC in a range of environments including the Bay of Fundy (famous for its huge tides) and the stormy waters of the Nova Scotian continental shelf.

For six weeks last year, RALPH was deployed on an intertidal flat just above the spring low tide level near Auckland International Airport in the eastern Manukau Harbour. Water depth at the site varied between zero and four metres.

In addition to RALPH, a number of soccer-ball-sized InterOcean S4 current meters fitted with infrared backscatter sensors for measuring SSC were used at various locations elsewhere on the intertidal flat and in nearby channels.

Analysis of the dataset is revealing fundamental differences between sediment transport in channels and on intertidal flats. In an eight-meter-deep channel 500 metres from the RALPH site, the water was always turbid and fluctuations in SSC were related to fluctuations in the strength of the tidal current. In that case, it is fairly straightforward to predict sediment movement.

In strong contrast to this, bed sediment on the intertidal flat was mobilised by waves. Wave activity and turbidity varied markedly over the tidal cycle. Both the wind speed and the wave period remained more-or-less constant over the tidal cycle, but the "significant" wave height (the average of the highest third of the waves) varied in phase with the water depth. This reflects the change in fetch -- the stretch of water over which wind blows to raise waves -- associated with the harbour-wide submergence and emergence of sandbanks that accompanies the rise and fall of the tide -- the longer the fetch, the bigger the waves.

In contrast, the peak wave-orbital speed -- the maximum back-and-forth current induced by the waves -- at the bed varied out of phase with the water depth, with it smallest at high tide. This is a consequence of the fact that under these waves orbital motions decay exponentially with distance below the mean water surface. Turbidity followed the orbital speed: the water was clearest at high tide.

It is clear that models will need to account for wave stirring of bed sediments if they are to accurately simulate sediment flux, water quality and bed stirring on the intertidal flat and, by extension, throughout the estuary as a whole. The modelling task is complicated by two factors: unlike tides, it is not easy to forecast waves; and, when they are present, waves vary greatly over the tidal cycle. Data from the RALPH experiment is guiding the model development and is being used to calibrate and verify model predictions.

In addition to the results concerning waves, the data shows unusual large, long-period (3-4 minute) oscillations in both current speed and SSC. These appear to be related to the way currents flow over subtle straight sandbars, some 50 cm high by hundreds of metres long, that are common in the eastern Manukau. Further analysis is expected to shed light on how these long "waves" are generated by mutual interactions between the seabed and flowing water, and what the associated implications are for the estuary-wide hydrodynamics.

The RALPH experiment has demonstrated that high-quality basic and applied oceanographic science is possible with this kind of instrumentation, which is presently possessed by rather few overseas universities and oceanographic institutes. As a result, and with the RALPH experience in the bank, NIWA is acquiring its own self-contained instrument package ("ALICE") for use in New Zealand's estuarine and marine environments.

Components of the package are being purchased from overseas suppliers and interconnected at NIWA. Notable components include a high-resolution acoustic current meter for measuring three-dimensional turbulence within one centimeter of the seabed, an acoustic backscatter sensor for measuring high-frequency fluctuations in SSC over the bottom metre of the water column, and a programmable water sampler for obtaining samples of suspended sediment. ALICE will look odd: the instruments are all mounted on a single vertical pole that hangs from a cantilevered beam attached to the apex of an asymmetrical "tripod". The unique design is meant to reduce eddy-shedding from the frame, which would otherwise obscure the detailed measurements of natural turbulence, and which is the bane of boundary-level oceanographers.

ALICE is undertaking its first role in Mahurangi Harbour, in a study of the ecology of the horse mussel Atrina. That experiment is being followed shortly by a two-month deployment on the inner continental shelf seaward of Mangawhai Beach (north of Auckland) in a study aimed at identifying the natural mechanisms that cause sand to accumulate in the nearshore "sand prism". This has implications for assessing the sustainability of sand extraction by the building industry and for predicting beach erosion. Later this year, we will return to the Manukau Harbour to obtain more detailed measurements of the way waves evolve over the tidal cycle in order to further improve the "forcing component" of the estuarine sediment-transport model.

Malcolm Green is with NIWA in Hamilton.