Renagi, Ora (2009) Dispersal patterns and quantities of sediment discharged from the Markham River, Papua New Guinea. PhD thesis, James Cook University.

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Abstract

The Markham River is a small river draining a tropical mountain range with altitudes between 1000 to 3000 m. The river has an estimated sediment load of 12 Mtyr-1 and discharges directly into a submarine canyon. The estuary is located offshore, with a mixing zone outside the river channel. Profiles of salinity and suspended sediment concentrations (SSC) show that sediment is dispersed via a plume with components at both the surface, intermediate depth along isopycnal surfaces and along the steep seabed slopes in the Markham canyon. The dispersal pattern of the surface freshwater plume is largely determined by the buoyancy force. However strong northwesterly winds create upwelling along the fringes of the northern coast. During a high discharge event, the discharge was 1600 kgs-1 which amount to a daily load of 0.14 Mtons. Estimates of the horizontal sediment flux gradient of the surface plume along the estuary axis suggested that about 80% of the sediment discharge was lost to subsurface waters within a radial distance of 2 km from the river mouth. The layers of sediment within the water column contained about 0.03 Mtons which is about 30% of the daily discharge. The amount of suspended sediment accumulating in thick layers over the seabed is estimated to be about 0.4 Mtons which is about 4 times the daily discharge. Particle fall velocities estimated from the vertical flux indicate values less than those of highly flocculated material but particles will settle up to 100 m in a day. High SSCs observed near the seabed indicated sediment falling through the water column into a bottom boundary nepheloid layer. SSCs over the seabed did not exceed 1000 mgl-1 indicating continuous downslope flow of sediment. Sediment at this layer was either directly transported from the river mouth or from the seabed slopes where previously deposited fine material have been resuspended and transported along the seabed. Some of the sediment is transported by advection to form layers of high SSC at isopycnal surfaces. SSCs near the seabed of between 250 and 1000 mgl-1 suggest that layers of significantly elevated density existed near the seabed, moving under the influence of gravity down steep seabed slopes of the Markham canyon. The SSC data confirms that Markham River is a high yield river suitable for studying sediment transport mechanisms analogous to systems during low stand sea levels. The application of the modified hydrodynamic Princeton Ocean Model (POM) has been successful for the Markham estuary. The model simulates surface plume distribution patterns under varying discharge and wind conditions which are comparable with measured data. The model clearly shows the behavior of the surface plume that tends to mix slowly along the fringes of the coast compared to the mixing of plume direct to open sea. The effect of the strong northwesterly wind that caused upwelling, have been well represented by the plume. The model proves to be suitable for predicting plume behavior for further studies on e.g. influence of changing river mouth features caused by weather patterns e.g. the El Nino cycle and the effects of the new tidal basin (700 m x 400 m x 14 m depth) development for the Lae Port Authority which will be dredged out from the swampy land adjacent to the Milford Haven Bay. A new instrument called the Triple differential pressure sensor (TDPS) for measuring small (<100 mgl-1) amounts of depth averaged suspended sediment concentrations (SSC) has been designed and a prototype built. The new instrument measures SSC directly without the calibration problems that can be associated with optical instruments. Although the specification of the differential pressure sensor indicate that resolutions of up to 10 mgl-1 can be reached, the prototype TDPS had resolutions of 100 mgl-1. The low resolution can be attributed to the fabrication process where air pockets may have been trapped within the tubing channels of the device. Nevertheless, there is potential to improve the sensitivity of the instrument. The instrument can be used in coastal waters, rivers and lakes.

Item ID:	11679
Item Type:	Thesis (PhD)
Keywords:	Markham River, Papua New Guinea, small rivers, estuaries, sediments, sediment transport, processes, dispersal patterns, discharge, plumes, plume behaviour, instruments, Triple differential pressure sensor, TDPS, hydrodynamic models
Date Deposited:	21 Jul 2010 22:20
FoR Codes:	09 ENGINEERING > 0915 Interdisciplinary Engineering > 091508 Turbulent Flows @ 60% 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 40%
SEO Codes:	96 ENVIRONMENT > 9609 Land and Water Management > 960903 Coastal and Estuarine Water Management @ 100%
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