Search

The concept of channel-forming ([Q.sub.cf]) or dominant discharge is now a cornerstone of river channel restoration design. Three measures of channel-forming discharge are most commonly applied: effective discharge ([Q.sub.eff]), bankfull discharge ([Q.sub.bf]), and a discharge of a certain recurrence interval ([Q.sub.ri]), which theoretically are similar in geomorphically stable channels. The latter two measures have become particularly widely applied in some channel restoration design procedures, often to the exclusion of [Q.sub.eff] analyses, despite the additional utility of [Q.sub.eff] analysis for most channel design problems. We quantify the three measures of [Q.sub.cf] for four case studies and then follow this with a synthesis of previously published studies to illustrate sources of variability. This synthesis suggests that agreement among the three measures of [Q.sub.cf] is best for snowmelt-hydrology, nonincised channels with coarse substrate. Departures from these conditions result in greater discrepancy between the measures. Channel incision produces [Q.sub.bf] far greater than [Q.sub.eff], and flashy hydrology is associated with generally larger, briefer, and more frequent [Q.sub.eff]. Regional mean or median values for the relative magnitudes of the three measures can be tightly constrained, but site to site variation is quite large. The construction of a cumulative sediment discharge curve and associated determination of [Q.sub.eff] allows quantification of the sediment budget of a channel for a given hydrologic regime, which provides process-based insight of drivers of current and future trajectories of channel stability, and is thus the recommended measure of channel-forming discharge. Reliance on only return-interval or bankfull discharge for channel design is not recommended for channel design activities. DOI: 10.1061/(ASCE)0733-9429(2007)133:7(831) CE Database subject headings: Channel design; Geomorphology; Stream improvement; Rivers; Restoration.

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects. DOI: 10.1061/(ASCE)0733-9429(2007)133:6(596) CE Database subject headings: Hydraulic structures; Weirs; Rivers; Restoration; Scout: Turbulence.

Aquatic habitats in a deeply incised sand-bed channel were modified by adding 1,380 t of stone and planting dormant willow posts. Restoration structures (groin extensions and longitudinal toe protection) were designed as complements to existing channel stabilization works. Prior to restoration, base-flow aquatic habitats were characterized by uniform conditions, little woody debris or riparian vegetation, shallow depths, and sandy bed material. The stage-discharge relationship, channel geometry, and bed material size were unaffected by restoration, but the average depth of scour holes adjacent to extended groins increased from 32 cm to 72 cm, and pool habitat in the lower half of the study reach increased from 2.9% to 14% of water surface area. Median water depth at base flow increased from 9 cm to 15 cm. Woody vegetation cover on one side of the channel increased from 38% to 78%. Fish numbers tripled, median fish size increased by 50%, and the number of species increased from 14 to 19. Groin extensions experienced partial failure due to erosion of sand from underneath stones.