Use of regime relationships in natural urban waterway design

Urban developments directly modify significant areas of the earth's surface. This modification greatly affects the components of the hydrological cycle and consequently the process of erosion and sedimentation. The removal of vegetation and the increase in impervious cover leads to a decrease in infiltration and hence an increase in surface runoff The urban population acts as a driving force which changes the landscape and the hydrological cycle of the area, while increasing the demand for natural and water related environments. For the population living in the immediate vicinity, these environments provide a recreational facility, a refuge for wild life and also open space and vegetation to mitigate air and noise pollution. Therefore planners and engineers should adopt less artificial measures to arrest the deterioration of urban streams and their corridors. In this context natural urban waterway design is finding increasing favour with the engineering profession, the public, and planners. The Natural Urban Waterway Design method aims to design a stable channel which looks natural, consisting of meanders, pools and riffles ·with bank stability provided by suitable riparian vegetation which also provides diverse habitats for flora and fauna. The regime relationships originally developed for irrigation channels in India have been shown to be suitable for application to natural streams. These empirical relationships are not universal, because their applicability depends on the climatic, geologic and vegetation characteristics of the catchment, and must be calibrated so as to account for the catchment geological and hydrological characteristics before using them in a design method. Two creeks within the Brisbane City Council area, Australia, were selected as part of the study. Both streams have fairly stable natural cross-sections and are located within catchments whose degree of urbanisation is quite low. Historical information was available for these creeks in the form of aerial photographs. The detailed crosssections and flood frequency information which was used were the result of recent flood studies undertaken by the Brisbane City Council, Australia. In this study regime type relationships for mobile sand bed and gravel bed streams in a sub-tropical humid type climate are derived. The variations of hydraulic characteristics such as width, depth, area, slope and velocity with bankfull discharge are compared with the regime relationships presented in the published literature. The gravel bed stream data showed good agreement with the published literature for gravel bed streams. The sand bed stream, on the otherhand, showed similar slopes to the trend lines for sand bed streams, but the data occupies a much higher position than the published studies for such beds. The difference in position is discussed. Bankfull discharges and their probabilities of occurrence were derived for these creeks. It is found that the sand bed stream has a lower average recurrence interval of bankfull discharge compared to the gravel bed stream. The effect of bank vegetation on the channel parameters is also discussed. In this study regime equations were calibrated to account for the geological, hydrological and vegetational characteristics of the streams. The results demonstrates the viability of using the regime relationships for the prediction of stable channel properties for natural urban waterway design. Useful recommendations that can be used in the design of such streams are also provided in the conclusions