Modelling channel profile response to weir removal: South saskatchewan river at saskatoon

Abstract

A weir across the South Saskatchewan River at Saskatoon was constructed in 1939 to provide higher stable water levels during low flow periods. Regulation of the South Saskatchewan River by the Gardiner Dam, located approximately 120 km upstream of Saskatoon, currently provides a consistent flow throughout the year, with slightly higher flows during the winter. This has reduced the need to stabilize water levels locally. As the weir structure ages and deteriorates, several options can be considered, including repair, replacement or removal. The present paper examines the geophysical implications of the last option. A mobile boundary flow model (MOBED) was applied to the river to assess possible changes to the longitudinal bed and water surface profiles that would result if the weir was removed. The model utilizes information about channel geometry, sediment characteristics and hydraulic conditions to predict these changes. The model output provides information about the potential impact of weir removal: changes in bed elevation, flow depth, flow velocity and channel slope. Results show that, under mean post-regulation flow regimes and weir removal, degradation in the channel would extend 3000 m upstream of the weir site. The amount of degradation would increase downstream of this point to a maximum of 0.5 m immediately upstream of the weir. A water surface profile upstream of the weir site mirrors the new channel bed. The model indicates that the channel downstream will aggrade from 0.5 m immediately below the weir, eventually grading to the existing channel profile within 2000 m. © 1998 Taylor & Francis Group, LLC.