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Modifications to Stream Channels in the Brisbane Metropolitan Area, Australia

Knowledge is needed of the geomorphological characteristics of stream-channels in urban areas in order to manage the streams for recreation, biota conservation, environment enhancement, erosion control, and the discharge of stormwater runoff. This study is concerned with how urban streams in Brisbane have been substantially modified by many different human activities, and how the future behaviour of the channels is likely to be very complex because of the diversity and magnitude of these modifications. Major water-supply dams have been constructed on three of the streams, and some flood-detention storages are soon to be constructed on another. The earth embankment of the South-east Freeway fills the upper reaches of Norman Creek, and stormwater runoff is now carried in a man-made channel. Elsewhere in this catchment, flood-flows have been modified by culverts—where the Freeway and access roads cross the drainage channels, and where the Freeway embankment occupies a substantial part of the floodplain of the Creek. The construction of Brisbane Airport has already modified the tidal reaches of Kedron Brook, and the proposed redevelopment of the Airport will replace the existing modification with a major new floodway. Several of the stream-channels have been deepened, widened, and straightened, for flood-mitigation purposes, with 465,000 m3 of material removed from Breakfast Creek alone. Three of the stream-channels have been used as major sources of sand and gravel for building activities in the City. Flood characteristics of the streams appear to have been changed by increased runoff from impervious surfaces when these reached a critical threshold in the urban areas, and by increased velocities of flow in piped, lined, and other, artificial stormwater channels. Construction of new artificial channels amounts to some 15 km per year. The modifications which have occurred differ among the urban streams in Brisbane, and each stream requires individual study to assess the long-term effects of the changes. All of the streams reported on in this study have been substantially modified by human activities.