Wilson Inlet Hydrodynamic Modelling

Key points

Our computational modelling simulates how water moves in Wilson Inlet. Results suggest:

Denser salty seawater can sit under fresher river water for long periods, especially in high-rainfall years with long sandbar openings, creating layers that do not mix easily.
Opening the sandbar does not ‘flush’ the whole inlet. Some water reaches the ocean soon after the opening, but large amounts of water stay in the inlet for long periods. How much water exits to the ocean depends on things like river flows, tides, ocean levels and how wide and deep the channel opening is.
High-rainfall years wash much larger amounts of nutrients into the inlet from the surrounding catchment than drier years. This means that more nutrients remain in the inlet for longer in wet years, even when more water exits the inlet into the ocean

To better understand and simulate how water moves within Wilson Inlet, a hydrodynamic model of Wilson Inlet was implemented (Delft3D-FM, developed by Deltares). The model allows us to explore beyond what can be observed through monitoring and allows us to simulate how the inlet behaves throughout the entire water column over time.

Hydrodynamic model results, together with field measurements, are used to help make decisions about sandbar management in Wilson Inlet. The model is especially useful for understanding how the inlet works, comparing how different factors affect water movement, showing how conditions change over time and place and testing “what if” scenarios to see how management actions might affect the inlet before they are carried out.

To show how environmental conditions, inlet characteristics and processes can vary, we have compared model results from two contrasting years:

2017 – a high-rainfall (or “wet”) year with a large channel and long sandbar opening
2018 – a low-rainfall (or “dry”) year with a smaller channel and short opening

Wilson Inlet (Nullaki) is an intermittently open and closed estuary. The sandbar is typically closed during part of summer and through autumn. When the sandbar is closed, winter rainfall increases the inlet water level, and a channel is cut through the sandbar most years to allow estuarine water to exit towards the ocean and relieve the flooding of surrounding land. Soon after the opening, seawater starts intruding into the inlet.

The model results presented below provide insight into:

1. Salinity vertical profiles

When the sandbar is open, ocean water enters the inlet when offshore sea levels are higher than inlet water levels.

The modelled and measured water salinity in the inlet at the time of the sandbar opening, is often lower and fresher than that of seawater. When the sandbar is opened, the denser seawater (heavier than fresh water) intrudes into the inlet near the bottom. Meanwhile, the less dense river water (lighter than salt water) enters the inlet, sitting near the surface. These different characteristics separate surface and bottom waters, creating ‘stratified’ conditions.

Understanding stratification is important because of its influence on inlet ecology and water quality – stratification can lead to low-oxygen conditions near the sediment which is harmful to aquatic fauna and can lead to the release of nutrients from the sediment.

By modelling salinity and stratification under a wide range of environmental conditions, we can better understand and anticipate how climate and sandbar management decisions may affect water quality.

More information about stratification

2017
Wet year

2018
Dry year

The models above show that in the wet year of 2017, the larger sandbar channel allowed greater volumes of seawater to pass through and intrude farther into the inlet, reaching the eastern end more frequently. Lots of rain (resulting in high freshwater input) one month after the opening further strengthened the salinity stratification.

In contrast, in 2018, the saltwater intrusion was largely confined to the region near the mouth and rarely extended to the middle part of the inlet due to the shallower channel and reduced seawater exchange.

Before the 2017 opening, inlet salinities ranged from 15 to 20 parts per thousand (ppt) (about half the salinity of seawater). After persistent stratification in spring, the fresher and saltier waters were gradually mixed by winds. This was aided by the lack of river flow and the smaller channel in summer reducing density differences. Subsequent seawater inflow and continued mixing raised inlet salinities to near-marine levels (exceeding 32 ppt) by the time the inlet closed in March 2018.

The inlet salinity was higher at the time of the 2018 opening (~22-25 ppt). However, inlet salinity in 2018 varied less than in 2017, staying mostly between 21 and 25 ppt after the sandbar was opened. This was influenced by:

a smaller channel through the sandbar, restricting exchange with ocean water.
less river inflow – approximately one-third the amount compared to 2017
a shorter opening time (which was also influenced by less river flow and a smaller channel).

As a result, seawater had limited influence on inlet salinity in 2018 compared to 2017.

2. Water movement in the estuary

In the below model animation, virtual passive particles (which can be envisioned as imaginary water droplets) were released near the mouths of the five main rivers to simulate their paths through the inlet. Their positions were tracked over time to estimate the time required for them to move from the rivers and potentially exit to the ocean (known as water residence times).

The model suggests that water in the inlet does not simply “flush out” when the sandbar opens – much of it stays in the system for weeks or months. The model also indicates that during wet years, a large and long lasting sandbar opening allows much more inlet water to escape to the ocean (shorter residence times), leading to faster and greater renewal of inlet water compared to dry years.

Water residence times

Virtual passive particles

In both years, the tracks show extensive spreading and mixing of particles originating from different rivers after a couple of weeks. In 2017 (a wet year), most particles (94%) exited the inlet before the sandbar closed, whereas in 2018 (a dry year), 45% exited the inlet before the sandbar closed. This is due to the reduced connectivity between the inlet and the ocean.

Particles released near the Little River mouth (light blue), which is close to the sandbar, exit the inlet quickly through the channel. In contrast, particles released near the eastern river mouths (Hay River, Sleeman River, and Cuppup Creek) take longer to leave the inlet, suggesting that water originating on the eastern side of the inlet requires more time to reach the ocean. This is relevant to nutrients or other pollutants that enter the estuary via the eastern waterways, as they are likely to spend more time circulating in the estuary body than those entering from Little River or Denmark River.

The model also suggests that some of the water that moves through the channel into the ocean during outflowing conditions (e.g. high river flow and ebbing tides) is pushed back into the inlet again once ocean conditions change, such as with a flooding tide or high sea levels.

3. Understanding nutrient movement in the inlet

In model output 2, we saw that more particles left the inlet through the sandbar in the wet year (2017) than in the dry year (2018). However, this does not necessarily mean that water quality is better in a wetter year or that substances dissolved in water (like nutrients) are completely flushed out when the sandbar opens.

Wet years typically also bring higher river flows, which deliver more nutrients into the inlet compared to dry years. To explore this, we used the model to visualise a conservative tracer – a simple tracking signal that represents the changes of a real water quality component over time and space - in this case, total phosphorus.

As can be seen in the animations below, results suggest that more phosphorus remained in the inlet by the time the sandbar closed in the wet year (2017) than in the dry year (2018). Overall, this indicates that although wet years can increase flushing, they may also deliver and retain higher nutrient loads within the inlet.

Conservative tracers

In the wet spring of 2017, phosphorus tracer concentrations increased considerably at the eastern end of the inlet during peak river flow and remained high for a couple of weeks. Seawater inflows through the sandbar channel gradually reduced concentrations within the inlet, especially near the inlet mouth. Over time, the tracer entering from the rivers mixed and spread throughout the estuary and was partially exported to the ocean. However, some tracer remained in the inlet when the sandbar closed.

In 2018 (dry year), riverine phosphorus tracer input following the opening was lower than in 2017 (wet year) but still resulted in elevated phosphorus tracer concentrations at the eastern end of the inlet for a couple of weeks. Afterwards, the tracer quickly spread throughout the inlet, and overall concentrations at the time of sandbar closure were lower than those modelled for the wet year of 2017.

More information

Insights from the hydrodynamic model are used to inform our on-ground actions in partnership with Wilson Inlet Catchment Committee.

The Wilson Inlet sandbar is managed by the Nullaki (Wilson Inlet) Sandbar Management Working Group, consisting of the Department of Water and Environmental Regulation, Water Corporation, Shire of Denmark and City of Albany. Find out more about the opening of the Wilson Inlet sandbar and the framework and protocol that it is managed through.

A detailed report is currently being prepared and will provide technical information about the model and the science behind it. This report is intended for scientific readers seeking a deeper technical understanding of the modelling work and its outcomes. A link will be provided here once the report is published or contact estuary@dwer.wa.gov.au to have the report emailed to you upon publication.

Wilson Inlet Hydrodynamic Modelling

Key points

Salinity vertical profiles

Water movement in the estuary

Understanding nutrient movement in the inlet