6. Sediments

• Introduction
• Effects of sedimentation
• Sediment sources
• Sediment budget
• Sedimentation in Lake Illawarra
• Reducing sediment loads
• Planning controls such as:
• Physical works such as:
• Community education such as:

Introduction

Many NSW estuaries are being adversely affected by sediments, nutrients and various pollutants. The previous information sheet on water quality discussed how catchment clearing and development increased the quantity of sediments and nutrients in runoff water from the catchment. This information sheet examines sediments in more detail, while the next newsletter will examine the problem of excess nutrients.

Effects of sedimentation

Inflow of excessive quantities of sediment can affect Lake Illawarra in a number of ways including:

• reducing light penetration (water clarity) and hence the water depth in which aquatic plants with roots in the bed can grow (eg. seagrass);
• burying important habitat areas such as seagrasses or mangroves;
• smothering animals living in the bed;
• nutrients and various pollutants are often attached to the very fine sediment which settles out in quieter or deeper areas of the lake. These areas can therefore have higher concentrations of pollutants which can be released back to the water under certain environmental conditions such as low oxygen levels;
• shallower areas can result in sediment being more easily resuspended from the bed by wind wave action reducing water clarity and ecosystem health;
• human usage of the waterway can be inhibited by lesser water depths and diminished aesthetic value.

Sediment sources

The most commonly recognised sources of sediment in the lake are catchment soil erosion and stream bank erosion. When catchments are cleared for human usage, the quantity and speed of water flowing off the catchment during rain increases dramatically. Catchment clearing of Lake Illawarra commenced in the mid 1800s. Only 35% remains forested and undeveloped today. Soil disturbed by construction activity, farming, bushfires, etc., is easily washed into watercourses. As water flows increase and watercourses become shallower, bank erosion occurs and is often made worse by clearing of bank vegetation, trampling by animals and wash from boats. Many of these sediments then find their way down the creeks into the estuary.

Less commonly known is that sediments also enter the lake from the ocean. These sediments are beach sands washed into the lake by waves and tidal currents. When the lake entrance is scoured water velocities are often high enough to stir sand from the bed. The inflowing tidal currents are generally stronger than outflowing currents (see Information Sheet No. 3). For most NSW estuaries the beach sands gradually move up the entrance channel.

Sediment budget

Sediments settle when the water flow is no longer fast enough to move them. Larger and heavier particles such as gravels and coarse sands are called bedload sediments because they are rolled and bounce slowly along the creek bed. Lighter particles, such as silts and clays, are called suspended sediments because they are mixed and carried along with the water.

Some bed load sediments, mainly sand size particles, eventually reach the lake where they deposit to form deltas. Suspended sediments are generally carried into deeper, quieter waters where they gradually settle. Some are carried through the lake to the sea, particularly during floods.

The settling of both bedload and suspended sediments in the lake gradually makes it shallower. The rate of infilling varies within and between estuaries. Average infill rates can vary from less than 0.1mm per year up to more than 5mm per year. Local rates over 100mm per year are possible, particularly on deltas. The average rate for Lake Illawarra is 2-3mm per year.

Sedimentation in Lake Illawarra

Ever since Lake Illawarra came into existence it has been slowly filling with sediments from the catchment. Lake Illawarra has an average sediment increase of around 100 000 cubic metres per year, more in flood years. This is up to 50 times greater than the pre-development period.

Most of these sediments are deposited around the edges of the lake where creeks and stormwater enter it. Land clearing during early settlement for timber and agriculture led to large amounts of soil loss from the open land areas and creek banks. This resulted in the formation of deltas at the mouths of streams, mudflat development, then widening and shallowing of bays.

In more recent times, sediment increases have been due to urban and industry development. More cleared land plus stormwater containing waste products end up in the lake, polluting as well as filling the lake.

Increased sediments in the lake have had the impacts of decreasing water circulation, water quality and water usage and covering aquatic flora causing its death along with the fauna for which it provides a habitat.

Figure 1: Movement of water, sediments and nutrients into and out of Lake Illawarra.

Some infilling of the lake is resulting from sands entering through the entrance. The volume is minimal compared with sediment runoff from land surrounding the lake, with only approximately 1000 cubic metres remaining in the channel entrance following a cycle of sand exchange.

The low energy conditions which generally exist in the Lake are partially responsible for the large volume of sediment deposition. Deposits of estuarine shell filled mud vary in depth from 5m to 14m.

Reducing sediment loads

The rate at which sediments enter Lake Illawarra can be reduced in a number of ways.

Planning controls such as:

• restricting land clearing to the minimum necessary, ie. roads and serviceways only for residential subdivision;
• prohibiting clearing along watercourses to prevent both an increase in channel flows and bank instability.

Physical works such as:

• constructing sediment traps and artificial wetlands on urban drainage lines to collect coarse sediment and gross pollutants, and reduce suspended solids and nutrients;
• constructing sediment settling ponds for major new developments and for existing unsealed roads, etc;
• undertaking bank revegetation and protection work where bank erosion is identified;
• dredging sediments from bays and deltas.

Community education such as:

• making rural property owners aware of land use practices that would limit soil erosion;
• notifying property developers of sediment control methods and the benefits of sediment control plans;
• making owners of riparian lands aware of the benefits from revegetating creek-side buffer strips.

Home | About us | MAP | Contact us | Lake facts | Links | Life in the Lake |