Sedimentation effects of salinity-control weirs in Illawarra streams

Macquarie Rivulet and Mullet Creek are the two main rivers draining the Illawarra escarpment west of

Lake Illawarra, NSW. Both rivers have low run-of-river salinity-control weirs to prevent brackish water

from Lake Illawarra penetrating upstream into the main agricultural areas.

The weir on Mullet Creek has been in place since the late 1800s associated with a river crossing

whereas the Macquarie Rivulet weir was only emplaced in 1963. Detailed GIS analysis of aerial

photographs shows no signs of sediment build-up behind the weirs and no significant scouring on the

downstream side of the weirs. This is especially apparent on Macquarie Rivulet when pre- and postweir

construction could be analysed. The results show that channel surface areas and rates of

change through time occur independently of the weirs with most change occurring after major floods,

e.g. the 1984 flood on Mullet Creek.

Surface sediment samples and vibracores were collected both upstream and downstream from the

weirs in both rivers. Upstream from both weirs showed significant scouring with almost no sediment

accumulation in the middle of the rivers and only minor accumulations of both coarse and fine

material near the banks. During flood conditions when the river level is 2-3 m above the weir both

coarse and fine material are entrained by the turbulence upstream of the weir and transported over

the weir. Immediately downstream of the weir a scour pool is present with sediment deposition

occurring 30-50 m below the weir. Coarse layers, up to 50 cm thick in bank-attached bars, represent

flood deposits that may be separated by finer grained low flow deposits. The progressive fining of

sediment downstream towards the mouth of the river is a function of the backwater effect caused by

the lake. No anthropogenic pollution is being stored in fine material upstream of the weirs.