Small waves in estuaries
We know that waves cleanse estuaries of silts and clays, keeping intertidal flats sandy and healthy. But how big do waves have to be to be effective in this way? New research shows that very small waves can be just as effective as big waves.
When it's windy, estuaries tend to turn brown and dirty-looking.
The discolouration is due to silts and clays being eroded from intertidal flats by waves and placed in suspension in the water column. Although waves make the water look dirty, they actually tend to cleanse intertidal flats of silts and clays, leaving behind the sandy sediments that we associate with a clean estuary.
That's because the sediments resuspended by the waves settle back to the bed quite slowly, and while they are settling they either get carried out to sea by tidal currents, or they are carried up to the headwaters of tidal creeks, where they settle amongst mangroves and saltmarshes. Either way, these sediments are effectively lost from the open-water part of the estuary.
Knowing this, we can account for the effects of waves in the numerical models that we use to predict estuary sedimentation, the dispersal of contaminants such as heavy metals (which are typically attached to fine-sediment particles), and the ecological effects of sediments that get washed in from the land when it rains.
By accounting for the waves, we can get much better predictions of things like the likelihood of heavy metals building up to toxic levels in urban estuaries, how sedimentation rates are likely to change as a result of sediment runoff from catchment development, and how the estuary ecology will fare in the future as, for example, agriculture becomes more intense.
Like many things, we tend to assume that bigger is better, and therefore that the biggest waves associated with the strongest winds are most effective at cleansing estuaries of fine sediments (and any contaminants that are piggybacking on the fine sediments).
However, you might have noticed that, even in very slight breezes, estuaries can become quite turbid (less clear) around the edges in very shallow water (less than 10 to 20 cm deep), and that this turbidity is linked to the very small waves that lap the shore under such conditions.
This raises the question: is it the big waves, which occur only infrequently under strong winds, or is it the small waves, which occur often under mild breezes, that really keep our estuaries clean?
The answer to this question is relevant to the way we run our predictive numerical models.
We conducted an experiment in the Tamaki estuary, Auckland, to measure waves and sediment resuspension.
We designed the experiment to measure the 'orbital' (back-and-forth) currents underneath the waves, and to determine how these changed over the tidal cycle as the water depth changed. We used a small instrument package that contained a current meter for measuring orbital currents, a pressure sensor for measuring wave height, and an optical backscatter sensor for measuring the amount of sediment suspended in the water column. The latter works by firing out short pulses of infra-red radiation (heat, essentially) and recording the amount of radiation that gets reflected back by (backscattered from) particles in suspension. The greater the backscattered radiation, the more sediment suspended in the water column.
Measurements were made on an intertidal flat that was submerged to a depth of around 1.2 m at high tide.
During one particular tidal cycle, a sea breeze from the north blew at about 5 m/s (10 knots), generating waves about 10 cm high on the intertidal flat. This provided us with the opportunity to observe and study the way very small waves interact with the bed sediments on this intertidal flat.
We discovered that even these very small waves generate significant orbital currents that are quite capable of resuspending bed sediments.
The orbital currents underneath the small waves reduce quite rapidly down through the water column: more than about 30 or 40 cm below the water surface the orbital currents diminish to virtually nothing, but within about 20 cm of the water surface the orbital currents reach 20 to 30 cm/s. This is around half a knot, or 0.9 km/hr: if the whole water column were moving at this speed you would find it quite an effort to swim against.
As the tide rises, the top 20 cm or so of the water column that contains the strongest orbital currents gets lifted above the seabed, and no seabed sediment is disturbed.
However, as the tide falls, the strongest orbital currents get lowered onto the seabed: right before low tide, when the water is less than about 20 cm deep, the half-knot orbital currents touch down on the seabed and sediment is easily resuspended, creating the narrow band of muddy water that we see around the edges of the estuary when it is breezy.
When the tide returns to the site, the waves are brought along too, and early in the rising tide when the water is still less than about 20 cm deep, more sediment is resuspended.
We ran a model simulation based on 23 years of Auckland wind data to work out which waves do the most work on the seabed at the Tamaki site. The model simulated the generation of waves by wind, the orbital currents beneath the waves, the changing water depth due to the tide, and the resuspension of sediments by orbital currents that occurs when the currents are within range of the seabed.
We ran the model for a 23-year period for which we have Auckland wind data, and identified all of the times in that period when waves resuspended sediments. We also identified which parts of the intertidal flats were affected by waves.
We discovered that down at the base of the intertidal flat, where the water is greater than 2 m deep at high tide and the seabed is under water for nearly all of the tidal cycle, the largest waves associated with the strongest, least frequently-occurring winds do most of the work.
However, at the top of the flat, where the water is shallower and the seabed is submerged for only a small fraction of the tidal cycle, very small waves associated with frequently-occurring breezes do the most work.
This means that our models have to account for all waves – not just the largest waves – to make accurate predictions of sediments, contaminants and ecology in estuaries.
Green, M.O., 2011. Dynamics of very small waves and associated sediment resuspension on an estuarine intertidal flat. Estuarine, Coastal and Shelf Science, 93(4): 449–459.