RFI - Attachment F - Engineering Report

Not considered in the Hughes Truman’s export rates is the bed load. It was observed that
176 kg/ha of the exported load was bed load. This is a considerable mass not to be
considered. This has great implications when determining maintenance frequency of
treatment devices


Conclusion
It is apparent from this study that land use and topography have a large effect on
contaminant loading and composition. It was also found that climatic conditions also
influence loading and composition. Wind and rainfall have the ability to move and break
down contaminants making them more difficult to capture.

It was generally found that the highest rates for contaminants were found in the
commercial area. High loading rates were also encountered at the base of the hill on
which the sub-catchments were located.

Loads and volumes were related to rainfall and it is suggested that all Enviropods in the
catchment should be cleaned after 100mm of rainfall

Composition of the retained material was found to get finer as you travel down the
catchment. High overland flow velocities and wind is thought to be transporting particles
from the top of the catchment to the base of the catchment, down the steep slope. These
climatic forces could also be eroding the particles

The composition of the retained material showed a seasonal variation. The retained
material was coarser in spring and in early summer months. It is suggested this is from
wind as these months historically have stronger winds.

It was found that the concentration of nitrogen in retained material was related to the
organic content. It was also found that nitrogen and organic loadings were highest in
windy months when leaves could be ripped from trees

Phosphorous concentration was not related to organic content of the retained material. It
was therefore suggested that a possible source of phosphorous was erosion of soil and
rock from neighbouring properties. Higher phosphorus loads were encountered in large
particles than finer particles; this is contrary to previous research which has sampled
stormwater at the end of a reticulation system. It is thought that eroded soil and rock
particles are breaking down as they travel entrained in reticulation system.

The composition of material entering the enviropod is mainly made up of suspended solid
size particles. The Blackbutt creek catchment had a low gross pollutant load. This is
thought to be associated with the low organic load from the site. Approximately 32% of
the retained load (196 kg/ha/yr) was bed load. Bed load should be considered in export



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