1. The total of 33 samples from 11 locations was collected from Orakei Basin and 16 more cores collected by Worley Consultants Ltd. were studied and sub-sampled.
2. Mechanical analysis of the samples using the Galai machine, X-ray diffraction, X-ray radiography, total TOC, shear strength with field mapping and petrographic studies were carried out on the collected samples.
3. Sediments were found to consist predominantly of silt (20-60 microns) and very fine sand (60-125 micron). The finer materials are concentrated at the southwestern and western corners while the more sandy materials are found at the shallower parts of the northwestern and the eastern sides.
4. The sediment mode (the most common grain size) is between 16-20 micrometre in the southwestern and western parts of the basin. These fine materials are found mostly in the surface layer and then become coarser with depth.
5. Accumulation of mud (silt size) is occurring mainly in the southwestern and western parts of the Basin. This means that the old mangrove mud flat in the southwestern side of the basin still has the same old sedimentation regime, although surface sediments are finer than the normal mangrove mud. The shallower parts of the northwestern side of the basin underwent a considerable erosion by stormwater. Maximum erosion happens mostly during heavy rainfall periods when the basin is flushed out. No major silt deposition is happening in the eastern side of the basin where sediments are dominantly sandy.
6. Sediments are formed of 45% clay minerals (kaolinite and Illite), 40% quartz, 15% feldspar (orthoclase, plagioclase and anorthite) with traces of calcite and pyrite. Clay minerals are derived from the weathering (chemical decay) of feldspar, quartz is the component of Auckland sands and derived either from the Waitemata sandstone or of marine source, the feldspar is derived mainly from the Auckland Volcanic Field or marine source also. Calcite may be extracted by algae from the seawater or coming from the catchment area.
7. The amount of Total Organic Carbon (TOC) in Orakei Basin sediment (3-20%) is relatively high compared to Recent sediments of comparable sedimentation conditions. This is probably due to the algal contents of these sediments plus the organic materials derived from the vegetation in the nearby catchment. The highest levels of TOC (more than 20%) are found in the western parts of the basin, opposite the shell bank. The lowest levels are found at the eastern parts of the basin. This is in alignment with the depositional regime of the black mud also. The highest levels of TOC are associated with the highest level of mud deposition. In most of the cases, the amount of TOC increases with water depth, i.e. samples collected near the highest spring tidal mark contain lower TOC levels. This is due to the low mud deposition, erosion rates and oxidation. With few exceptions, the levels of TOC decrease with depth. TOC are half or less the surface values at 40 cm, but still the preservation rate of the TOC is high, which means deeper parts of the sediment column still contain high amount of organic materials.
8. The odour of the basin is probably generated from the diagenesis of the organic matter where the sulphate ions may react with the organic matter to produce hydrogen sulphide gas. Aligned pores in the form of Bird's- eye texture were found in cores recovered from the southwestern parts of the basin. This texture is widely reported from Recent sediments world-wide and normally indicates gas seepage from sediments
9. The most dominant wind direction
is the south-west and west with secondary maximum from north-east.
The topography of the Basin provides enough shelter from the prevailing
wind, particularly in the southwestern part, where the mangrove
belt was established. The water depth of the basin as estimated
by Turnbull (1954) does not exceed 2.40m in most of its parts.
The approximate tidal speed in Purewa Creek, calculated from data
published by Bowman and Chiswell (1982), ranges probably between
2-3cm per second.
10 Three mechanisms were suggested for the sediment accumulation
in Orakei Basin and Creek. Deposition from stormwater discharging
from the neighbouring catchment, direct deposition of finer materials
either due to sediment flocculation at the mixing zone of freshwater
with marine water or by the baffling action of algae, where they
act as sponges to which sediments are adsorbed gradually until
they become heavy enough to settle down. Sedimentation may also
deposit from the normal marine water that enters the basin.
11 According to the shear strength measurements, the sediments are divided into an upper incompetent bed and lower competent bed. The shear strength of the surface sediment is low (1-3 Kpascal) but increases with depth.
12 More frequent draining of the basin water is recommended. This will improve the water quality by reducing the nutrient levels and thus preventing algal blooming. It will also transport the fine sediments which otherwise will settle down. More frequent draining will prevent water stratification and improve the oxygen levels and consequently accelerate oxidation of the black mud and its organic content through the exposure to the weather elements.
13 The most critical stage regarding the sediment erosion is when the basin is empty. At such times, stormwater particularly during heavy rain periods will erode and lift fine sediments at the margins of the basin. It is recommended that releasing the water from the basin should be programmed during fair weather periods as much as possible.
14 The release of water from the basin should be carried out around high tide. The slow water at Purewa Creek and Hobson Bay will slow down the retreating water from the basin and reduce its erosive power. What is recommended here is a slow drainage of the basin instead of the present regime, which may have a strong release if the tide is low.
15 Under more frequent draining or full tidal regime, some of the fine sediments at the southwestern and western corners of the basin (2-50 micrometre in size) will probably move. To avoid this, these two corners may be covered with a thin layer of coarse, quartzic sand. This sand layer will hinder the re-colonisation of these areas by mangroves. A similar case
