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The West Dapto Flood of February 1984: Rainfall Characteristics and Channel Changes

Journal Article


Abstract


  • This paper describes the distribution, magnitude and areal pattern of recurrence intervals for a very severe rainstorm centred over the West Dapto area of Wollongong in February 1984. It also examines the impact of this storm on the channel geometry of streams draining the Illawarra escarpment. The storm was the largest 24 hour rainfall ever recorded in temperate Australia, and was very likely in excess of the 200 year event for the Dapto area. However, the localised nature of such storms in the region Suggests that Wollongong can expect a major flood (in excess of the 100 year event) somewhere within the city every 25���50 years���. Similarly, because the catchment of Lake Illawarra is usually larger than the areal distribution of these very high-intensity storm centres, the frequency of severe flooding in the lake is bound to be greater than the frequency of any individual storm event. The most pronounced channel erosion (up to a fourfold increase in channel size) resulting from the 1984 storm occurred along the steep-gradient alluvial channels in the escarpment foothills, for here almost all the flow was retained by the actual channel with relatively little spilled across the adjacent floodplains in the form of overbank flow. In contrast, the downstream low-gradient reaches showed very little alteration because their constricted geometry displaced most of their flood discharge over well-grassed floodplains. There is a strong positive correlation between bankfull stream-power and channel erosion. The former is a direct function of channel size and slope, both of which are greatest in the upstream reaches. �� 1985, Taylor & Francis Group, LLC. All rights reserved.

Publication Date


  • 1985

Citation


  • Nanson, G. C., & Hean, D. (1985). The West Dapto Flood of February 1984: Rainfall Characteristics and Channel Changes. Australian Geographer, 16(4), 249-258. doi:10.1080/00049188508702880

Scopus Eid


  • 2-s2.0-0006514446

Web Of Science Accession Number


Start Page


  • 249

End Page


  • 258

Volume


  • 16

Issue


  • 4

Place Of Publication


Abstract


  • This paper describes the distribution, magnitude and areal pattern of recurrence intervals for a very severe rainstorm centred over the West Dapto area of Wollongong in February 1984. It also examines the impact of this storm on the channel geometry of streams draining the Illawarra escarpment. The storm was the largest 24 hour rainfall ever recorded in temperate Australia, and was very likely in excess of the 200 year event for the Dapto area. However, the localised nature of such storms in the region Suggests that Wollongong can expect a major flood (in excess of the 100 year event) somewhere within the city every 25���50 years���. Similarly, because the catchment of Lake Illawarra is usually larger than the areal distribution of these very high-intensity storm centres, the frequency of severe flooding in the lake is bound to be greater than the frequency of any individual storm event. The most pronounced channel erosion (up to a fourfold increase in channel size) resulting from the 1984 storm occurred along the steep-gradient alluvial channels in the escarpment foothills, for here almost all the flow was retained by the actual channel with relatively little spilled across the adjacent floodplains in the form of overbank flow. In contrast, the downstream low-gradient reaches showed very little alteration because their constricted geometry displaced most of their flood discharge over well-grassed floodplains. There is a strong positive correlation between bankfull stream-power and channel erosion. The former is a direct function of channel size and slope, both of which are greatest in the upstream reaches. �� 1985, Taylor & Francis Group, LLC. All rights reserved.

Publication Date


  • 1985

Citation


  • Nanson, G. C., & Hean, D. (1985). The West Dapto Flood of February 1984: Rainfall Characteristics and Channel Changes. Australian Geographer, 16(4), 249-258. doi:10.1080/00049188508702880

Scopus Eid


  • 2-s2.0-0006514446

Web Of Science Accession Number


Start Page


  • 249

End Page


  • 258

Volume


  • 16

Issue


  • 4

Place Of Publication