Year

2014

Degree Name

International Bachelor of Science

ANZSRC / FoR Code

040608 Surfacewater Hydrology

Department

School of Earth & Environmental Science

Advisor(s)

Brian Jones

Abstract

A project aimed at determining the geomorphic impacts of the weirs at Mullet Creek and Macquarie Rivulet was conducted in the Illawarra Region of New South Wales, Australia. Dams are known to cause disruptions to sediment and water flow, with the effects of large impoundment dams on fluvial geomorphology well researched. It has also been previously hypothesised that the weirs in this study may be acting as sediment traps and limiting the sedimentation rate of the deltas where both streams flow in to Lake Illawarra. The removal of small run-of-river dams and weirs has been recently emphasised both locally and internationally. However, there is limited understanding of the morphological impacts these smaller structures have on their fluvial systems.

The project involved; ascertaining the length of time the weirs had been established through aerial and historical photographs and literature, a GIS analyses of channel surface area change through time, surface sediment sampling and vibracoring. These gave a detailed assessment of the geomorphic effects of the weirs on their fluvial systems.

Channel surface areas and rates of change through time were found to occur independently of the weirs at Mullet Creek and Macquarie Rivulet. It follows that weirs in both systems do not act as focal points for geomorphic change. Coarse material is transported from areas upstream of the weirs to areas downstream and does not accumulate in considerable amounts upstream. Due to the hydraulic effects of Lake Illawarra, coarse material is not represented in the deltas of both systems. In addition, the weirs had no effect on flood risk to their respective catchment areas. In summary, the weirs have no negative geomorphic impacts on the fluvial systems of Mullet Creek and Macquarie Rivulet. These findings oppose the previous hypothesis that weirs may act as sediment traps. It is recommended that the weirs be left in place due to the benefits to the stakeholders for continued access to freshwater.

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