The proposed route for the Transmission Gully motorway is almost all
in the headwaters of Porirua Harbour catchments, and three quarters
is in the Pauatahanui Inlet catchment. During and after construction,
bulldosed cut and fill, and removal of protective vegetation cover along
the road line, could do irreparable harm to the harbour. Erosion of
clay and silt frombare exposed surfaces will pass down streams, cover
shellfish beds on the mudflats, and make the shore unpleasant for human
activities. Long term, increased vehicle numbers on roads will bring
increases in chemical pollution of waterways of the catchments and the
harbour itself, affecting particularly the microscopic life that processes
food chain nutrients.
The Transmission Gully road line has already been designated. Where
feasible the plans are that it will be revegetated before construction
starts. There is therefore an opportunity to make the road line a biological
corridor linking bush and pine forests that the road traverses. This
would be an environmental plus. It would be praiseworthy, but it would
not be enough.
Importance of cockles in the harbour ecosystem
Studies in Pauatahanui Inlet show that cockles are the most important
mud flat shellfish and the major source of food for fish and shore birdlife.
Cockles have decreased in numbers and total volume by more than a half
in twenty years. If this rate of decline continues other organisms will
follow suit, either because they are affected by the same factors that
are causing the decline of the cockles, or because depletion of cockles
is in itself a cause of environmental change. Cockles feed solely by
filtering food material from sea water. Their absence will remove a
major cleaning and clarifying mechanism from the Inlet waters. It is
possible that the outcome would be increasing concentrations of harmful
microscopic organisms in the water - perhaps algal blooms. Possibly
major increases in sea lettuce carpeting the mud flats is connected
with this increase in organic matter, which on decay produces an overload
of nutrient material.
Reasons for shellfish decline
Analysis of where the cockle decline is worst suggests that a primary
cause is silt and clay accumulation downstream of subdivision and road
construction. Another possible cause of some of the decline is some
form of chemical pollution. A study by officers of the Regional Council
in 1997 showed there was evidence of some types of chemical pollution
which usually comes from vehicle exhausts or from materials in brake
drums or tyres. More research is needed, but this vehicle pollution
could be contributing to the cockle decline. While this type of pollution
is not at alarming levels at present, it will inevitably increase as
vehicle numbers and speeds increase in the Inlet catchment. Other chemicals
such as those released into the wastewater drains horticultural sprays,
paint, and household chemicals have occasionally caused problems.
How can effects of run-off and pollution be reduced?
Fortunately, enlightened planning could anticipate possible harmful
consequences of the building of the Transmission Gully Highway, and
go some way towards ameliorating its affects. The decision to designate
the road line well in advance of construction work, allowing it to be
fenced and where possible planted is a hopeful indicator. Perhaps though
the planning should go further, to make the motorway construction an
environmental plus - an opportunity to not merely mitigate the effects
of the road construction earthworks, but to create ponds and wetlands
that could act as settling areas and reduce rates of run-off and storm
peaks in the streams.
Artificial wetlands and riparian planting
There would need to be prior construction of settling ponds down stream
of the roadworks, and planting of the ponds to form permanent wetlands.
Wetland swamps can act as both physical and chemical filters, reducing
the passage of silt and clay, and absorbing chemical pollutants from
motor vehicles. Wetland construction could be accompanied by encouragement
to farmers to fence stream banks and undertake riparian planting. The
value of riparian planting is most apparent at times of storms. Studies
of Pauatahanui catchment streams in the seventies showed that floods
carried twenty times as much volume of water as the average flows, and
the resulting discoloured water and yellow mud on the Inlet shore are
consequences seen frequently after rain storms. Planting along stream
banks and on flats prone to flooding can provide a vegetation cover
that has several environmental advantages - most obviously stream bank
erosion is reduced by the binding of plant roots. More significantly,
the flow into the stream is impeded: water slows down and drops its
load of silt and is filtered through the vegetation before reaching
the stream. A greater proportion of the water takes a slow path downwards
into the water table, instead of quickly running off the surface and
contributing to a flood peak. When the water does reach the stream the
volume is reduced, the flood peak is reduced, and it is spread over
a longer period of time.
Public education is needed so we understand and accept the social and environmental costs
The measures suggested above would not be cheap. Farmers would need
financial inducement to retire apparently productive land on stream
flats, and would need assistance to cover the costs of fencing and planting.
A comprehensive public education programme would be needed to bring
this vision to fulfilment.
Neil Bellingham, 9/6/98
