Natural Backfilling of Pipeline Trenches

Abstract

Fredsoe, Jorgen, Inst. of Hydrodynamics and Hydraulic Engineering, Technical U. of Denmark* A theory, is developed for predicting the rate of natural backfilling of pipeline trenches through the action of waves and currents. The pipeline trenches through the action of waves and currents. The sedimentation originating from bed and suspended loads is studied separately. Four numerical examples demonstrate how to use the theory, which is verified by experiments. Introduction The aim of the studies presented here is to provide mathematical models for calculation of natural backfilling in pipeline trenches under the assumption that the seabed consists of noncohesive sediments and that the depth of the trench is small compared with the water depth (Fig. 1). The transportation of noncohesive sediment normally is separated into the transportation of bed and suspended loads. In many contexts, such a distinction of minor importance, but for our purpose it will be demonstrated that such a distinction is necessary. For example. the especially simple case where the alignment of the dredged trench is parallel to the direction of the current is considered. In this case, no suspended sediment is carried from the undredged regions to the trench by the current, if secondary currents are neglected. As long as the variations in water depth are small, the magnitude of secondary currents is negligible. However, while the suspended load is carried in the current direction, the direction of the transport of bed load deviates from the current direction because of gravity effect on the bed load moving on the slopes of the dredged trench. As a result. sedimentation will occur in the trench because of gravity infill of bed load.If the current crosses the trench under a certain angle, some suspended sediment also may settle because of decreasing current velocity and wave action at the bed in the dredged trench.From these considerations, one can see that the sedimentation caused by the bed and suspended loads must be treated separately because the physical background is quite different for the two sedimentation mechanisms.These two different physical processes are described for the case of current only and for the case of combined currents and waves. Numerical examples are given, and the magnitude of backfilling from the various processes is compared. A model test that supports the theoretical findings is described. Sedimentation of Bed Load Fig. 1 defines the velocity and concentration profiles and the bed geometry. Current The simplest case, where the current is parallel to the alignment of the trench, was treated in detail by Fredsoe . The continuity equation for the bed-load transport is given by (1) where Dt is the local depth of the trench, is the porosity of sediments, x1 is a coordinate in the transverse porosity of sediments, x1 is a coordinate in the transverse direction of the trench, and qx is the bed-load transport in the x1-direction, given by (2) JPT P. 1223