RIVER MIGRATION, DESIGN FLOOD AND SCOUR DEPTH COMPUTATIONS AT PIPELINE CROSSINGS

Abstract

Pipeline are the safest and most efficient means of transporting crude oil and natural gas from producing fields to refineries and processing plants. The pipelines in its route have to cross a number of streams and rivers, which may be at risk due to bed and bank erosion. Buried pipelines may be exposed in the channel bottom due to scour of the bed material as well as in the floodplain because of migration of the channel banks. Estimation of peak discharge that to pass through the river cross section at the site where the pipeline is crossing is crucial for the safety of the pipeline. The scour depth depends upon the peak discharge, channel geometry and the type of soil. Thus an over estimation of design discharge will result in an over estimation of scour depth. This will increase the burial depth of the pipeline and will push up the cost of excavation. An underestimation may result in the exposure of the pipeline during the peak flooding and hence the safety of the pipeline may be at risk. This dissertation report concentrates on the river migration, design flood studies and scour depth studies at Rupen river crossing, which is required in order to avoid the risk involved in case of pipelines due to the meandering behaviour or the bed and bank erosion nature of the river. The Barmer- Salaya pipeline project passes through Rupen river. Rupen river originates from the foothills of the Aravalli range and empty in to the Gulf of Kutch. Rupen river descends in a south-westernly direction through the province of Gujarat state. The Rupen river basin is the northern part of the basin and is bounded between 23°00' to 24°15' North latitudes and 71015 'to 73000'East longitudes (approximately). The pipeline crosses Rupen river at 71°55'6.516"E longitude and 23°29'29.918"N latitude. In order to carry out the river migration studies all the satellite imageries were first geo-referenced and converted to same projection system. Within this area, riverbeds from each of the satellite imageries were captured and overlaid to determine the changes in riverbeds and to determine the variations in the length of river crossing. The next stage in this process is the design flood estimation. Because of the absence of the short interval rainfall and discharge data at Rupen river crossing, regional unit hydrograph approach, as recommended by Central Water Commission of India (CWC) is adopted. The catchment of Rupen river falls in subzone 3(a) as identified by CWC. Thus the relationships adopted by Central Water Commission have been used to identify the design parameters of the unit hydrograph. And finally the scour depth is calculated. Scour is the removal of soil particles by the flowing water. Scour on river systems generally refers to the removal of material from the bed and banks of the river from streamflow. The objective of the study is to check the adequacy of scour depth for the pipeline crossing in the design, so as to minimize the risks of pipeline exposure or damage to the pipeline for the estimated design of flood during the economic life of the pipeline which in this case is adopted as 25 years. The information about the river cross-sections and the silt factors from the relevant sources have been used in the computation of the scour depth. For the purpose of scour depth computation Lacey-Inglis method has been used which is one of the prescribed method in Indian Road Congress (IRC 1998 & 2000) for the natural channels flowing in alluvium. Then the corresponding maximum scour depth is computed using suitable multiplication factor based on the sediment size gradation, maximum flow rate and duration of interval between maximum flow rates and the stream condition i.e., straight or bent. The scour depth was then cross checked with another approach given by Farraday and Chariton (1983) in Smart and Herbertson (1992). iv