A Rating Method for Assessment Risk at River Crossings

Detailed studies performed to design works at river crossings provide all the information needed to asses hydro technical hazards. TGN performed such studies for critical rivers as part of its Integrity Management Program. By monitoring performance of remediation works, experience was built throughout 10 years comprising different hydrological years. This was the base for the development of a simpler methodology aimed at assessing risk using information that can be readily available in regions such as Latin America where the existence of gaging stations and historic records at most rivers are not common. The method is based in river geomorphology, summarized with two parameters: area of drainage basin area and river slope at the crossing. They characterized the type of problems that can be expected and they can be estimated from topographic maps or digital terrain models available from the internet. The rating method follows a basic structure consisting in the product of two factors: causes and consequences. Causes include: bank erosion, river bed scour, meanders and river diversion along the right of way. Then, increasing factors are applied accounting for deforestation, land use and the occurrence of debris flows; decreasing factors consider proper remediation works, design and construction aspects implemented during construction. Finally, consequences are focused at loss of human life, impact on the environment, and interruption of fluid transport. This method is aimed to be performed by a pipeline operator that can have a good feeling of problems related to rivers, without having the technical knowledge of a specialized consultant. While rating, subjective judgment still plays an important role. However, this methodology provides a systematic approach that includes all aspects affecting river crossings, allows for prioritizing works based on rates and, as new rivers are included from new watersheds, it can be improved as prediction and characterization tool.