Closed dry (typically January or February to May or June): Bar is closed so there is no marine water input, catchment is dry so there is little freshwater input.

Closed wet (typically June or July to August): Bar is closed so there is no marine water input, catchment is wet so there is freshwater input.

Open wet (typically August to October or November): Bar is open so there is marine water input, catchment is wet so there is also freshwater inflow

Open dry (typically October or November to December or January): Bar is open so there is marine water input, catchment is dry so there is little freshwater inflow.

Depending on which state the Inlet is in, a number of different processes may be occurring.

Freshwater flow and circulation

Initial outflow and marine intrusion

Tidal effects

Astronomic tides are due to the relative positions of the sun, the moon and the earth. Off the south coast of WA the astronomic tidal cycle is largely diurnal (one high and one low per day). The daily tidal range varies from about 1 m down to about 30 cm then back up to 1 m again on a roughly fortnightly basis.

Barometric tides occur as a result of changes in the air pressure. Changes in air pressure can force the water level up and down by up to 30 cm on cycles that are typically 5 to 7 days long.

Long term components are related to global scale ocean-atmosphere interactions, such as the El Niño Southern Oscillation, the effects of which last for many months to a year or more and shift the mean sea level up or down by approx 10 cm or so.

The astronomic tide is attenuated (reduced) by the restriction on flow caused by the bar and delta. By the time that the rising tide is starting to become noticeable in the Inlet it is already starting to fall again in the ocean. As a result the maximum astronomical tidal range in the Inlet is less than 10 cm, even when it is up to 1 m in the ocean.

On the other hand with its much longer period of 5 to 7 days the full displacement of barometric tides can be observed in the Inlet (up to 30 cm).

Barometric tides can pump large volumes of water into or out of the Inlet and therefore the passage of low or high air pressure systems has a much larger influence on exchange than the astronomic tide.

Flushing

Figure 12: Monthly volumes (in GL) of flushing (blue) and marine exchange (red) averaged over past 5 years.

In the past 5 years flushing has displaced on average about 40% more water from the Inlet each year than marine exchange (the two are linked -

poor river flow means a less effective bar opening which means poor marine exchange).

However flushing removes more than twice as much nutrient as marine exchange. This is because flushing occurs earlier in the season when nutrient concentrations are higher and therefore is more effective at removing nutrients than marine exchange (Figure 12).

Marine exchange

While marine exchange does remove some nutrient it is much less effective at removing nutrients from the Inlet than flushing. This is because plants have already taken up much of the nutrient before marine exchange is able to remove it and concentrations in the water are therefore lower. Marine exchange also leads to the problem of stratification.

Stratification

Figure 13: Cross-section of salinity contours through Wilson Inlet from the Inlet mouth to the Hay River during spring. The plot illustrates freshwater flow (brown) entering from the Hay River on the right and being mixed into the Inlet, and marine water (blue) intruding from the ocean on the left and forming a strongly stratified layer in the bottom of the Inlet..

The stratification resists mixing (mixing in the Inlet is largely driven by wind) because it takes more energy to mix the layers together than it would if they were the same density. As discussed in other Wilson Inlet Reports to the Community,

stratification can have profound negative effects on the Inlet's water quality by causing nutrient releases and algal blooms.

Stratification starts in the western basin. Within 4 weeks after opening there can be a saline bottom layer up to 1 m deep in the western basin and within a further 2 weeks a saline bottom layer up to 50 cm deep in the eastern basin. Moderate wind mixing is generally insufficient to destroy the salinity stratification. Only strong, continuous winds are able to completely mix the water column.

* Note: ppt stands for parts per thousand and is a measure of the salt content.

For example a 10 knot wind (the annual average) with 14 knot gusts would take 2 weeks to mix a 5 ppt.^* stratification in 3 m of water and 6 weeks to mix a 15 ppt stratification. As the water column is mixed the overall salinity of the Inlet increases. The well mixed water column does not persist for long after strong winds cease, as the stratification is re-established by successive tidal inflows bringing new marine water into the Inlet.

Although calms can occur at any time of the year, wind speeds (and therefore mixing) are usually at their minimum in October as winds shift from the predominant winter pattern of NW and NE to the summer pattern of SW and SE.

Figure 14: Wind waves in Wilson Inlet. Photo: W. Hosja

Winds blowing along the Inlet can cause wind waves up to 1.2 m high (Figure 12). Winds can also push the water into one end of the Inlet causing the water level to rise and fall up to 0.2 m at opposite ends of the Inlet; like water sloshing back and forth in a giant bathtub (known as seiching).

Stratification ceases to occur once either the bar has shoaled and marine water may no longer enter the Inlet, when the Inlet salinity itself is close to marine, or when the weather is consistently stormy enough to keep mixing the Inlet.

The amount of stratification that occurs in a year shows a reasonable correlation to the amount of marine exchange that occurs; the more marine exchange the more stratification. The main drivers of stratification are Inlet salinity, wind mixing, and the amount of marine exchange (which in turn is related to rainfall and run off and channel dimensions).