2. Mangatawa catchment consents for earthworks, storm water ...

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
Treatment Wetland) that has been extensively planted with native vegetation. The pond discharges to
Rangataua Bay through an area that has been identified by Environment Bay of Plenty as having significant
ecological value - Special Ecological Site - SES (Environment Bay of Plenty, 2003).
5.13.3 Stormwater Treatment Wetland
The stormwater treatment pond is sited within a rectangular block of almost level land between SH2 and the
Rangataua Bay shoreline and sewage ponds. In its irregular outline this pond is an enlargement of a former
natural depression. The pond is a specifically designed and constructed artificial pond developed by the
Tauranga City Council. The water-level of the pond was about a metre below the ground surface at the time of
the assessment. The depth of the pond is unknown. No sign of prolonged flooding up to the bank tops or
beyond was apparent.
The soil-forming substrate comprised mainly sand, but thin layers and patches of peat were also present. No
buried wood was seen in the pond’s banks or in the sides of the Maranui Drain. The remainder of this
rectangular block is vegetated to its boundaries, as discussed below.
a) Vegetation
The water of the pond was dark brown-stained by the peat. No floating aquatics, such as duckweed (Lemna,
etc) were recorded in the pond. A covering of parrot’s feather (Myriophyllum brasiliense) was evident for 1-2m
out from the bank of the pond. A much smaller amount of Potamogeton suboblongus was present in places.
The pond’s banks are a metre or so high. They are stable despite their sandy nature, and are mostly thickly
vegetated. In places their cover is predominantly of jointed rush (Apodasmia similis) and searush (Juncus
kraussii), along with a lesser amount of bachelor’s button (Cotula coronopifolia) and Triglochin striatum. In
other places, perhaps particularly where flax (Phormium tenax) comes right to the upper edge of the bank,
there are dense growths of pampas grass (Cortaderia selloana). (It appears that herbicide may be being used to
control this weed).
The remainder of the block is a managed cover, partly of mown grass paths and clearings and partly of a tall
scrub dominated by cabbage trees (Cordyline australis). The largest cabbage trees (up to a dozen) reach 13 m
or more tall and 1 m or more in basal diameter. The majority of these trees are multi-trunked. It appears that
at least the largest individuals, (and possibly also several of the lesser stature ones), have persisted from a
former situation as relics in pasture. (A few such veterans persist in the pasture adjoining to the east). Only a
small amount of shrinkage of any underlying peat seems to have occurred around these trees.
Under the broken upper layer of these 5-13 m tall cabbage trees there was a dense cover of native shrubs and
small trees, particularly Coprosma repens and C. robusta, and numerous (very vigorous) bushes of flax and the
surface of the ground was covered by leaf litter.
Status: Final Page 52 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE