Processes Impacting on Lake Macquarie

Seawalls
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Why are seawalls so harmful to water quality and marine life?
The recent Land & Environment court judgement on the construction of seawalls in Lake Macquarie has gained coverage by some local media. I am often asked about how harmful they can really be to Lake water quality and marine life. The answer may come as a surprise to many.

Dr Dan Roberts, a highly respected marine biologist with over 20 years experience in marine ecology, has recently completed detailed research into the effects of seawalls.

Dr Roberts believes that there are two main ways that seawalls cause harm to marine life and water quality.

The first way is that the seawall increases the "bounce back" impacts of wave action, which would otherwise be largely absorbed by a naturally sloping beach. This action increases near shore water turbulence and can disturb seagrasses in shallow areas, unlocking sediments that are bound in the Lake-bed.

The second way that seawalls can harm water quality and marine life is by preventing the natural removal of dead seagrass (or wrack) from the water.
Dr Roberts' research in the southern areas of Lake Macquarie found that more wrack had accumulated in front of seawalls than where seawalls were not present.

Because this material cannot be removed, it rots in the shallow near shore areas and eventually forms a build-up of the smelly black ooze that people sometimes associate with swamp areas. This prevents the growth of new seagrass, because it prevents light penetration that is critical to plant life.

This affects the basis of the food chain and young fish also leave the area.
Some residents in Lake Macquarie have shown a great awareness of the effects of seawalls and replaced them, or used an alternative strategy on their waterfront properties.

In one recent example a property owner in west Lake Macquarie contracted Council's Civilake to construct a natural sloping beach using rock pebbles and foreshore vegetation. The results have been similar to other areas where this technique has been used with good water quality and a healthy ecosystem in front of these sites.

This illustrates how a return to natural systems, or "soft" engineering techniques based on natural models, can be more effective in balancing our desire to live on the water with the environmental health of Lake Macquarie.

 

Editiorials
Channel Challenges
Channel & Water Quality
Constant Change
Seagrass
Seawalls
Threats to the Lake
Vegetated Swales
Water Sensitive Urban Design
Wetlands
Wrack & Ruin
Introduction