3. Tides and Currents

• Introduction
• How tides affect water levels
• Ocean tides
• Tides in estuaries
• Tidal currents
• Intermittent entrances
• Tides and currents in Lake Illawarra
• Unique estuary
• Entrance
• Winds

Introduction

This information sheet examines tides and currents in Lake Illawarra, and the controlling influence of the ocean entrance.

How tides affect water levels

Many coastal lakes, lagoons and parts of rivers are tidal because sea levels have risen since the last ice age some 20,000 years ago. Low lying lakes and lagoons and lower reaches of rivers were inundated by the sea forming estuaries.

Freshwater floods, mixing of the water caused by wind, wave overtopping of sand dunes across an estuary mouth, evaporation and groundwater seepage also have an influence on water levels and flows.

The movement of water between an estuary, its tributaries and the sea is central in determining the nature of the estuary and its ecosystem. It affects the estuary's salinity levels, where sediments settle in the estuary, the water quality, and the type and distribution of estuary plants and animals.

Ocean tides

Ocean tides are the rhythmic rise and fall of ocean water levels over a period of hours. Along the open coast of NSW one high tide and one low tide (a tidal cycle) occur around every 12.5 hours. The rising tide is referred to as the flood tide whereas the falling tide is called the ebb tide. On the lower NSW coast, the average tidal range or difference between high and low tides is around 1.0m. However, this difference can be as small as 0.3m or up to 2.0m for king tides.

Tides are driven primarily by the changes in gravitational pull of the moon and sun on the oceans of the earth as it spins on its axis. That is why changes in tidal range are related to the phases of the moon. Large ranges (spring tides) occur every 2 weeks near full and new moon phases. Small (neap) tides occur in the week between spring tides near half moon phases. Barometric pressure, winds, waves, currents, etc., can also change the ocean level, but these are irregular, difficult to predict and generally have a much smaller influence.

Tides in estuaries

How fast the tide travels along an estuary depends on the water depth. Towards the top of high tide, water is generally flowing faster into the estuary than towards the bottom of low tide when it is flowing out at a shallower depth. As a result, there is a relatively long time interval between high water and the succeeding low water (6-7 hours), and a shorter interval between low water and the next high tide (5-6 hours).

As water moves from the ocean through an estuary entrance the tide loses energy. The tidal range therefore generally reduces progressively up the estuary.

Tidal currents

The ebb and flood of the tide results in tidal currents which can be quite strong in narrow, shallow entrances such as Lake Illawarra.

Estuaries with wide, deep entrances such as Port Jackson (Sydney Harbour) have relatively slow tidal currents. The entrance therefore has a major influence on estuary tidal behaviour.

Intermittent entrances

For some estuaries, tidal flows are not strong enough to stop ocean waves and wind building sand across the entrance. These estuaries are usually small but even large lakes with exposed entrances such as Tuggerah Lakes or Lake Illawarra can regularly close.

Once the entrance is closed, an estuary is sealed off from the effects of ocean tides until human intervention, floods or high seas breach the entrance sand dune. Where lakes and lagoons are closed to the ocean, small currents generated by wind blowing across the water surface help circulate the water body in the absence of ocean tides. Catchment runoff and changes in water density due to temperature and salinity differences can also create currents. Over the past 50 to 100 years Lake Illawarra has been recorded to have closed off some ten (10) times.

Tides and currents in Lake Illawarra

Unique estuary

Lake Illawarra is a large estuary compared to others on the NSW coast. It is also unusual in that despite a surface area of 35 square kilometres, it has a maximum depth of only 3.5m with a significant area of the lake being less than 1m in depth. The average lake depth is 1.9m. The Lake has a relatively long and shallow entrance that is periodically blocked off. Due to the nature of the sea entrance, very little tidal variation occurs in the lake.

Entrance

The channel linking Lake Illawarra to the sea is approximately 3.6km long. It winds through the lake opening which is approximately 1.8km long and 600m wide. The channel is shallow and sea water normally runs in on the northern side of Windang Island but occasionally enters from the southern side. At other times access to the channel may be cut off altogether by sand build up.

Figure 1: Variation in Tides with level of Entrance Opening

The long shallow channel connecting the Lake to the ocean acts as a friction device that slows or limits tidal exchange. The tidal level decreases with distance along the channel to the main body of the Lake, where the tidal rise is relatively small and uniform. The tidal range varies by 0.03m when the entrance is heavily shoaled to 0.1m when the entrance is scoured. The constrictive nature of the entrance channel causes the mean lake level to be higher than the mean sea level.

The falling tides have a longer duration than the rising ones (as previously described) and results in variations in the flow velocity of the water. The inflowing water, as a consequence, is faster moving which means more sand is moved into and upstream along the entrance than moves out.

The instability of the Lake's entrance is a significant factor affecting the water flow into and out of the Lake, which in turn affects water quality, flora and fauna, their habitats, sedimentation levels, nutrient and pollution levels.

Winds

Although tides and currents are not prevalent in Lake Illawarra, disturbances to the lake do occur with strong winds and storms.

High strength winds blowing over the lake are able to generate changes in lake levels, effectively tilting the surface. The variation in surface level can be as great as 0.02m to 0.06m for wind speeds of 18 to 36 km/hour respectively. This tilt in the lakes surface depends also on wind direction, fetch (length of the water body that the wind crosses) and the depth of the water.

As well as being able to impact on the water levels in the lake, the main contribution of winds to the lake is their impact on water circulation around the lake. Water movement through the lake has a retention time from between 60 to 250 days. A more open entrance resulting in greater tidal variations would reduce this time and alleviate some of the lake's problems.

Stabilisation works at the Lake's entrance would assist in restricting the movement of sand into the channel, improve current circulations, reduce flood levels and improve fish and prawn migration.

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