A Geomorphic Approach to the Design of Pipeline Crossings of Mountain Streams

Abstract

Several hydrological methods are available to determine flood discharge and scour of streams at pipeline crossings. These methods are appropriate for streams dominated by purely hydrological processes, but fail where other, more hazardous processes occur within the design recurrence interval. Several investigations have shown that scour, impact and aggradation associated with debris flows, outburst floods or related phenomena may fundamentally change the parameters needed for proper pipeline crossing design. Depending on the process type, the peak discharge of the hazardous process can exceed that of the design flood (typically 50 to 200 year return period) by a factor of 2 to 50. Similarly, scour or aggradation by a non-fluvial process can exceed the hydrologically-derived estimates by several factors. It is therefore recommended that a geomorphic approach be taken in recognizing and quantifying the potential for non-fluvial processes and that the findings be integrated in the design of pipeline crossings.

Regional prioritisation of flood risk in mountainous areas

“…Flash floods in such small, steep watersheds can further be conceptualised to occur as debris flows, hyperconcentrated flows or clear water flows (Hyndman and Hyndman, 2008;Jakob et al, 2004;Costa, 1988). Costa (1988) differentiates ( The most severe events include the following.…”

Late Holocene catastrophic slope collapse affected by deep-seated gravitational deformation in flysch: Ropice Mountain, Czech Republic

“…Hungr and Evans, 1996;Pirulli, 2006). Lack of success with other models excludes hyperconcentrated flows as a cause of the deposits; their creation from debris flows through dilution is also less probable, as suggested by Jakob et al (2004).…”

Entrainment of material by debris flows

“…The corresponding term in sediment hydraulics is a``supply-limited'' channel (cf. Bovis and Jakob, 1999;Jakob et al, 2004). It is possible to estimate the amount of debris available in ®rm-base channels by a direct visual inspection (Thurber, 1983;Hungr et al, 1984;VanDine, 1985).…”

“…This process of bed destabilization can be quanti®ed to some degree as shown in the following section, although the necessary data regarding bed stratigraphy, bed material, substrate strength, and drainage characteristics is dicult to obtain. Knowledge of erosion depth is also useful for practical purposes, such as the protection of pipelines crossing a debris-¯ow channel (Jakob et al, 2004b).…”