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Glenside Streamacare Site at Stebbings

Glenside Streamcare planting site at Stebbings stream


The submission of the Glenside Streamcare Group with regard to the Westchester Link Road extension.

I am Zena Kavas and I am representing the Glenside Streamcare Group. Our aim is to replant the local stream areas with native, eco-sourced plants, so that the environment in and around the streams improves. We are a voluntary organisation and have been working together for about 9 years. We have worked on three sites now;

Stebbings Dam was our first site, and we still return there each year to continue planting and getting rid of gorse, blackberry etc

The Debris Arrestor was our next site, a 300m strip between Middleton Road and the Porirua Stream

Our present site is in the vicinity of the Rowells Road lay-by, and this is our last season on this site.


Our whole aim is to improve the health of the stream environment, and riparian vegetation is one way to do this. The benefits of riparian planting include;

Shade to keep the water temperature lower

Providing habitat and food source for native fish

Places for spawning of native fish

Habitat for insects which provide food for native fish

As the plants grow larger, they provide habitats for birds and other wildlife, giving a more diverse stream ecosystem.

It is our concern that years of work to improve the stream ecosystem, including downstream from the proposed road construction, will be negated and more by the adverse effects of the construction of the proposed Westchester link road.

Effect on Immediate area

There are a number of small tributaries which run into Stebbings Stream through the area of the proposed road. There is no mention of these in either the application or the Greater Wellington or Wellington City Council reports. I am hoping that it is because they will be unaffected by this proposed construction, and that they will be left in their natural and pristine state. However, I suspect that it is because they are not deemed to be significant enough to be worthy of mention in your reports. Although these small streams are unnamed and do not appear on maps, the health of them is critical to the health of the catchment. They mitigate flooding, maintain water quality, recycle nutrients and create habitat for plants and animals.

The Glenside Streamcare Group is very concerned that provisions have not been made for these tributaries in your plans. (Or have you just forgotten to mention them?)                     

We are also concerned about the amount of rip rap erosion control proposed for the stream in the vicinity of this proposed road. (20m around the eastern bridge and 30m around the new channel). This will considerably reduce habitats for native fish and invertebrates. There is no riparian planting proposed amongst the rocks to help mitigate the significant impact that this rip rap will have on the stream function and the biodiversity in these areas. At present most of the area is overgrown and pest plants, however even these plants provide shade to prevent over-heating of the water in the summer, and habitats and shelter for the fauna of the stream.

An additional area of concern for the Streamcare group is the maintenance of the rip rap erosion control. Gorse, broom, blackberry and other plants will grow between the rocks. The rip rap at Stebbings Dam is currently maintained by spraying. This has the potential to cause problems in the stream and with spray drift, poison other plants in the vicinity of the rip rap. We have lost well-established plants at our Stebbings Dam site due to spraying by GW Flood control of gorse and broom nearby. 

We are also concerned about the retaining wall to be built in the bed of the stream. The construction of this will significantly disturb the stream bed, and the fish and invertebrates there, and cause a considerable amount of sediment. The area of the stream with the retaining wall will have no variation in it along the bank and no riparian planting to provide a suitable environment for fish or invertebrate. This will cause a reduction in stream function.

We are concerned about the construction of the bridges for the proposed road. Extensive work will be required very close to the stream bed, and this will adversely affect the stream life in the immediate area of the construction. The western bridge will require a large drilling rig to bore a 2.4 meter diameter hole to the footing for the central pier of this bridge in the bed of the stream. This will significantly disturb the stream bed, and with machinery working in the stream there is always a risk that there will be discharges of sediment from the work, or accidental discharge from the machinery.

There is a very real risk with use of concrete in and near the stream. The lime in the concrete is quite soluble and when it dissolves in water it becomes very alkaline. This is extremely toxic to aquatic life. Results from Auckland Regional Council indicate that 30% of all fish deaths reported in Auckland streams are due to cement and concrete wastewater. The statistics are likely to be similar for Wellington. This indicates that contractors are largely unaware of the consequences of concrete not being contained or being rinsed into the stream.

Has consideration been given to placing the central pier on the bank of the stream (and not calling it a central  pier)? This would at least reduce the chances of concrete or lime finding its way into the stream. The construction of this central pier, whether in the stream or near it would have a detrimental affect of the stream flora and fauna during construction, and it would alter the flow of the stream permanently.

The major concern of the Streamcare group, one which has extremely adverse effects which go far beyond Glenside and our planting sites is sedimentation. Although sedimentation and its mitigation are mentioned in the GW and WCC reports we suggest that it hasn’t been taken seriously enough. The reports euphemistically note that Stebbings Stream has been subject to modification as a result of subdivision works, and that the stream has undergone sedimentation pressures for the past three years.  What this means is that the stream has already been degraded by regular and seemingly uncontrolled discharges of sediment into the stream. As far as we are concerned, there is NO acceptable amount of sediment that can be discharged into the stream. GWs own report lists the major adverse effects of sediment on aquatic life;

Smothering of aquatic life by a build-up of sediment

Reduction of habitat quality and diversity due to deposition of sediment

Disruption of juvenile upstream migration

Smothering of eggs

Avoidance of waters with high suspended solids by invertebrates, fish and birds

Clogging of gills and loss of function of fish and invertebrates

Destruction of spawning grounds by smothering with sediment

Reduced feeding rates and growth rates

Abrasive action against aquatic life increasing susceptibility to disease

Scouring of algae which is a major food source

Increased turbidity reduces feeding because organisms cannot see their prey

Changes in temperature as increased turbidity affects heat absorption

Reduced photosynthesis in aquatic plants due to reduced light penetration.


This range of adverse affects is not limited to the immediate area of the construction of the proposed road, but will be felt to varying degrees all the way downstream. The sediment will eventually end up in Porirua Harbour, which is suffering considerably from the cumulative effects of sedimentation.

This proposed road and it’s planning seems to lend itself to extra risk of sedimentation. The rock which makes up the hillside where the road is proposed is greywacke with a high percentage of clay. This means that any disturbance of this rock will result in it breaking down into a very fine clay. This clay will be very difficult to manage and much of it will eventually end up in the stream. Factors contributing to excessive and unacceptable risk of significant sediment discharge into the Stebbings Stream are;

The composition of rock

Steep terrain

Construction of the proposed road close to stream

The amount of fill to be moved

Site of fill dumping upstream in the same catchment

Efficiency (or lack of) sediment control measures

The terrain in which the road is proposed is quite steep, and it will be quite difficult to maintain adequate sediment control along the length of the construction. The report states that the sediment retention ponds will be built within the road alignment and will require constant moving to keep up with the progress of the road. However, it does not say how this will be done. The Streamcare group suggests that it will in fact be impossible to control the amount of sediment entering the stream, and that the applicant does not fully understand the difficulty in maintaining sediment control along the length of the proposed road.

The proposed road is in close proximity to Stebbings stream. This greatly increases the chances of sediment being discharged into or accidentally entering the stream. Some parts of the proposed road are very steep and close to the stream, making sediment control measures extremely difficult, and probably quite inadequate.

The considerable amount of cut is another issue adding to the high risk of sediment discharge. The GW report notes that the earthworks for this proposed road are intensive. It notes that 35,000m3 of cut per kilometre of road is common on most state highway projects, whereas this proposed project requires more than double that at 80,000m3 per kilometre of road. The amount of approximately 55,000m3  of cut, with 8,000m3 of fill will generate substantial runoff resulting in a large amount of sediment entering the stream. The GW report notes that this will ‘discolour the stream’.

About 47,000m3 of excess fill will be trucked to a cleanfill site on Ohariu Valley Road. Unfortunately, this site is in the same catchment as Stebbings Stream, upstream from the proposed road. So the fill will be removed from the proposed road site, causing sediment to enter the stream as runoff. It will then be transported upstream where any runoff from this site will eventually enter Stebbings Stream. So it is double jeopardy for the stream with two opportunities for sediment to enter the stream from one activity.

The GW report suggests that because the cleanfill site is several kilometres away form the proposed construction site, and that there are multiple stormwater input between the cleanfill site and the proposed construction site, that this sediment should have negligible effect on Stebbings stream. This only means that the sediment will be more dilute, however, the amount of sediment will be the same. ALL of the sediment that enters the catchment from the cleanfill site WILL end up in Stebbings Stream and eventually WILL end up in Porirua Harbour.  This appears to us to be exceedingly poor planning, or total lack of attention to detail. We state that this poses an unacceptable risk of sedimentation for Stebbings Stream, and ultimately, sedimentation of Porirua Harbour.

The Glenside Streamcare Group is also very concerned with the efficiency of the sediment control measures. The Auckland Regional Council notes that the potential maximum efficiency of sediment retention ponds is 30% without chemical treatment, which is what the applicant proposes. This translates to 70% inefficient. However, with chemical treatment, or flocculation, the efficiency of a ponds performance increases to about 75%. This still translates to a 25% inefficient system, and this is without all the contributing risk factors of this site;

The steep terrain

The close proximity of the proposed road to the stream

The greywacke rock with a high percentage of clayT

The significant amount of fill to be moved.


This also excludes rain events, where the sediment ponds fail due to the rain being heavier than a one in two year event, over a period of 6 hours. It is internationally recognised (except for our National Government) that climate change is occurring, and that extreme weather conditions are becoming more frequent. Perhaps the rain event threshold needs to be increased. For an issue like sedimentation, with so many adverse consequences on the environment, this level of efficiency cannot be accepted.

Not withstanding the amazing job that many individuals within GW do, the system of compliance and monitoring is not adequate. Some complaints about sedimentation are followed up. Others appear not to be. I have been told by the operator that if the complaint is an on-going issue, it will not be looked into immediately, but followed up at a later date. Recent data shows that this year to date there has been only one complaint about Stebbings Stream. I have made more than one myself, and I am sure that I am not the only one complaining.

The issue of sediment in our waterways seems to becoming normalised. It is becoming acceptable to have our streams and rivers a dirty brown or yellow colour after rainfall. But this is not acceptable. In a healthy natural environment, it is not normal to large amounts of sediment in our waterways after rain, and this is what we should be aiming for.

The GW report suggests that the proposed construction is stabilised over the winter months of June, July and August, as this is when the highest risks of sedimentation are. This means that any work undertaken during this period must be approved by GW. The report also notes that native fish migration is adversely affected by sediment in the stream, and this may reduce the number of fish reaching the adult habitat. However, banded kokopu are very sensitive to turbidity in the stream, and their migration time is between September and November. There are no measures to take this into account.

Porirua Harbour

A major concern of the Streamcare Group is the sediment that reduces the stream function and biodiversity of Stebbings Stream eventually ends up in Porirua Harbour. There seems to be no consideration for this in the application or in the GW or WCC reports. There is an increasing community awareness and recent publicity, for example articles in the Dominion Post on 21 and 22 of this month.

These articles state that Porirua Harbour could be damaged beyond repair if immediate action is not taken to begin restoring it. The articles draw attention to development sites (including Stebbings Valley) that are causing concern. Keith Calder, Porirua Harbour Strategy Co-ordinator, says that before the harbour cleanup can begin, silt must be prevented from entering the stream. Yet the applicant is asking for consent [27481] ‘to discharge sediment laden water to land and to water from areas of bulk earthworks to the Stebbings Stream and Porirua Stream associated with the construction of a road.’  There is a growing body of research investigating the detrimental effects of excessive sediment in estuaries and harbours.

Sedimentation significantly reduces the health and biodiversity of estuaries and harbours. Guardians of Pauatahanui Inlet have carried out surveys in the Pauatahanui Inlet arm of Porirua Harbour. Results from their surveys can be assumed to be applicable to the Onepoto Arm (where the sediment from Stebbings Stream ends up.) Cockles and other filter feeders live just below the surface of sediment and draw water through their gills to get the food and oxygen that they need. In doing this they clarify the water by removing plankton and tiny particles of plant and animal matter.

In 1976 cockles made up 80% of the intertidal biomass of the Pauatahanui, and it was estimated that the cockles filtered one third of the volume of the incoming tides each day. This makes them a useful indicator of the health of the inlet. Cockles are also a major food source for fish and shore birds. By 1992, the density had dropped to about 40% of the 1976 density. The density has remained at about 40-50% of the 1976 density since then. This indicates that the Inlet is capable of supporting only about half of the 1976 population.

It appears that silt washed into the Inlet from subdivisions and road works in the catchment is a major factor in the decrease in cockle numbers. Data from 2004 shows that significant numbers of cockles are smothered by silt at the outlets of major streams in the Inlet. Very fine clay particles do not settle out on the bottom quickly and this reduces the clarity of the water. The increase in inedible particles reduces the feeding and ability to obtain oxygen, and can clog the gills of the cockles so badly that it kills them. The loss or reduction of eelgrass beds reduces the amounts of small plant particles that the cockles feed on. Although this survey is from the Pauatahanui Inlet rather than the Onepoto Arm of the Porirua Harbour, the results are likely to be similar, or worse, since there is a worse sedimentation problem in the Onepoto Arm.

There is a Maori saying, te maunga ki te moana, meaning that what comes down our streams and rivers affects our coastline and fisheries. The North Islands west coast snapper industry now depends entirely on the nurseries in the Kaipara Harbour. The most important habitat for baby snapper and other fish are sub-tidal sea grass beds and horse mussel/sponge communities. These have now mostly disappeared from west coast harbours as a result of sedimentation. These aquatic plants are susceptible to smothering by sedimentation, and the increased turbidity caused by fine particles reduces the light reaching the plants, so reducing photosynthesis. So when the GW report says that “This will discolour the stream” when referring to the sediment entering the stream, it demonstrates a lack of understanding of the scope of the consequences of sediment entering the stream.

The Glenside Streamcare Group opposes the designation of the proposed road because the construction of such a road is just too costly to our environment, and not just the stream environment in the vicinity of the proposed road, but Porirua Harbour too. The lack of consideration of the tributaries in the proposed construction area, the use of rip rap erosion control, the construction of a retaining wall in the stream bed, the diversion of part of the stream and the construction of bridges in and close to the stream are enough reason for the application to be declined. However, when the issue of sedimentation and the scope of the damage that it causes, is considered, the construction of this proposed road should not even be contemplated.

If we know the consequences of what we are doing, we cannot plead ignorance. It is not enough to do a bit better than we have been doing. We must do what is right and the right thing to do is to not allow this road to be built.