Open cut has traditionally been the preferred method of pipeline installation traversing watercourses. It is well understood and accepted that open cut excavation of the channel bed and banks during construction causes temporary disturbance to watercourse and aquatic habitat. Horizontal directional drilling, direct push and other subsurface installation methods can potentially avoid channel bed and bank disturbance but may have unique environmental effects such as frac-outs of drilling fluid. Although highly dependent on site conditions, open cut crossings are generally less costly than comparable subsurface installation methods.
When a pipeline is installed in an open cut, the pipe is typically installed on a gravel or sand bed, laid in place, surrounded by a sand pack and surrounding soils placed back in the cut in a manner that attempts to replicate the soil lithology, horizons and native compaction of the cut. It has long been thought that this sediment and soil disturbance and backfilling has the potential create a zone of geomorphological weakness at the cut where soil and sediment become dissimilar to the surrounding channel bed and banks and can result in the acceleration of bed scour, bank erosion, widening and slope instability. In this paper we examine the longer-term effects of open cut pipeline installations on the geomorphic characteristics of watercourses.
Over the course of four years of field investigation, nearly 750 pipeline watercrossings throughout Ontario were visited and assessed for geomorphic stability and depth of cover. The fluvial geomorphology of Ontario is diverse and ranging from alluvial, sinuous, unconfined, low gradient watercourses in the southwest to karst-influenced morphologies in eastern Ontario and often greater slope, confined and bedrock dominated watercourses in the north. By examining the field-based geomorphological characteristics of pipeline watercourse crossings in Ontario installed by open cut and crossing a wide range of fluvial geomorphological types we will explore and draw empirically-based conclusions on whether open cuts do in fact affect the long term geomorphological conditions of the watercourse.