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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
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
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