Sediment entrainment in rivers caused by pipeline watercourse crossing construction may represent a constraint on pipeline route selection and construction methods as designers attempt to develop a sediment and erosion control plan which meets regulatory approval without risk of costly delays. To avoid the risk of significant sediment entrainment, conventional open-cut crossing techniques may be replaced by more costly directional drilling methods. However, the concern over suspended sediment is greatest in high velocity rivers where the bed material includes a large fraction of fine sand, and in rivers with a large fraction of fine grained bed material which becomes suspended upon disturbance by construction activities. According to the current understanding of aquatic impacts due to elevated suspended sediment levels, the occurrence of suspended sediment may not be excessive at open-cut excavation in certain types of streams depending on the material consistency, fine sand content and river flow velocity (Anderson et al, 1996). Control of sand entrainment can normally be achieved by low cost sediment control systems during construction. Methods of prediction, impact assessment, and control of sediment entrainment have been developed so that high risk crossings can be identified and impacts minimized. The application of the study findings and best management practices (BMPs) for sediment control will allow developers to choose the most appropriate crossing method while avoiding potentially adverse impacts, based on a sound understanding of river sediment transport, bed material conditions and downstream aquatic resources.

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Erosion is a common problem at pipeline watercourse crossings. Watercourses are naturally vulnerable to erosion but the risk is particularly acute after sub-soil and armour materials have been disturbed by trenching and backfilling during pipeline construction. The process of pipeline exposure at watercourse crossings can be grouped into these types. One common type of erosion phenomenon is episodic exposure resulting from general and local scour. This is associated with temporary river scour during flood events. It includes general scour involving temporary lowering or the entire river bed during high flows, as well as local scour which involves development of a scour hole during high flow events at a predictable location on the river bed. River engineers have adopted various design methods to ensure sufficient pipeline burial depth which minimizes pipeline exposure due to such periodic occurrences. A second type of erosion phenomenon causing pipeline exposure is progressive river channel bed and bank erosion. This is not a function of a single event but occurs periodically, resulting in progressive removal of pipeline cover. Progressive erosion at pipeline crossings includes riverbed degradation, bank erosion and growth of gullies. River bed degradation (progressive river bed lowering) is a complex phenomenon associated with the stage of geomorphic development of the drainage basin. Its prediction is based on a sound understanding of sediment supply, river hydraulics and river outlet conditions. Bank erosion is a common occurrence and is readily observed. It may be a continuous or episodic occurrence and is often related to the river’s tendency to change its meander pattern, cross sectional shape or bed level. Growth of gullies is a very common cause of erosion at pipeline crossings and results from changes in land use, soil composition, and landscape drainage networks. Techniques for predicting progressive erosion are not well developed and widely understood. As a result, progressive erosion is a common cause of erosion and even pipeline exposure at pipeline crossings of rivers, streams, and gullies. A third mechanism of pipeline exposure is river avulsion. This is often associated with a tortuously meandering channel, a steep braided channel with a wide flood plain or an immature channel on a delta. Methods of avoiding or controlling erosion are based on a sound understanding of causal factors. Each river crossing location is unique and the local risks of pipeline exposure must address specific local conditions. Methods of estimating the risk of local and general scour, progressive erosion, and river avulsions are discussed. Methods of mitigating erosion at pipeline crossings include proper siting of pipeline crossings, deep burial, conventional armouring and a combination of bank toe protection and upper bank vegetation cover.