Quasi-equilibrium conditions of urban gravel-bed stream channels in southern Ontario, Canada, and their implications for urban-stream restoration .

摘要

Urban gravel-bed stream channels in southern Ontario, Canada, identified to be in a state of quasi-equilibrium have been studied over the past 15 years and compared against rural gravel-bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1-year return period with many sites ranging between two to eighteen bankfull events per year with higher intensity and shorter duration urban flood responses.;The cumulative volume of bankfull and larger flood events from the urban-stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed-material supply was found to decrease with increasing urbanization and the reduction in bed-material supply appears to be offset by the smaller bankfull channel width, depth, and access to floodplains during large flood events. Field evidence may also suggest an even greater reduction in channel width trajectory, relative to the rural setting, with floodplains to maintain quasi-equilibrium conditions as bed-material supply continues to decrease with increased anthropogenic activity.;Compared to surrounding rural watersheds, urban belt widths were found to decrease, while meander wave lengths and radii of curvature were found to increase as a function of bankfull width. The stream-wise elongation of meander wave lengths (and thus increase in radii of curvature) are a result of increased flood flow frequency and volume in the urban environments combined with reductions in bed-material supply.;An increased frequency in riffles and pools was also observed along each reach. Additional pools appeared along straight sections between bends, although they were shallower than pools on bends. The changes in bedforms result from brief but frequent discharge events that exceed critical shear values, resulting in sediment pulsing and the frequent placement of keystone clasts that create frequent riffle (and pool) development. Field observations of standing wave patterns in flood flows also support the role of 'dune-like' formations as a means of maximizing flow resistance.;Several methods of estimating channel-forming discharge were also evaluated to test their applicability in the urban condition. Bankfull stage was identified at a series of locations along each study reach and it was found that the most consistent observations of bankfull discharge occurred during flood conditions where bankfull stage was identified at the top of point bars along the convex arc of bends. The largest errors in estimation occurred at gauge stations where cross-sectional geometry had been altered to conform to bridges or culverts rather than the channel morphology. Independent evaluations of channel-forming discharge were conducted by eleven practitioners ranging from 10 to 43 years of experience with similar findings and errors.;Various methods of relating frequency return periods were evaluated using annual peak series discharge observations and continuous 15-minute systematic discharge records using partial duration series analysis. No specific correlations were identified between frequency return periods and land-use change. However, based upon the findings of this study, the applicability of employing annual series peak discharge data to evaluate bankfull frequency return in urban-stream channels is highly discouraged.