ABSTRACT The mode and rate with which subsea trenches evolve and infill has been studied with a numerical model of sediment transport in the marine environment. The model had been developed previously and calibrated 'with field data. Annual 'wave and bottom current climatologies with a minimum number of data points have been created for locations on the northeast North American shelf and for an example Gulf Coast estuary. These elements have been used to study the time-history of the infilling of various types of subsea trenches in environments representative of an open continental shelf, an open ocean shoreface and an estuary. The results show that small trenches, such as those commonly used to shelter pipelines, have high initial infilling rates if adequate bed load material is available. This conveniently provides enhanced shielding of the pipeline against hydrodynamic loads. The results also show that there is a marked reduction in the sedimentation rate as infilling continues. This is caused by the progressive reduction of the side slopes and appears to explain observations that subdued remnants of subsea trenches are preserved for long time periods, even in areas of vigorous bed load movement. INTRODUCTION Sea floor trenches have many applications in offshore engineering. These include:Pipeline trenches used to shield against hydrodynamic loads, fishing trawls and ice keels.Trenches which are backfilled to protect outfalls and pipelines against anchor damage in navigation fairways or long term erosion of shore-crossings.Navigation channels can be considered subsea trenches.Glory holes which protect subsea 'well heads and templates against ice damage in cold environments.Subsea trenches which result from beach nourishment programs.Borrow winning sites for gravel island construction. Infilling of these subsea trenches is of varying concern. When there is a significant duration between the time of trenching an the tie at which an outfall or pipeline section can be located in the trench is often necessary to establish over dredging requirements to allow for natural sedimentation. Infilling of pipeline trenches is generally beneficial as it enhances the shielding against hydrodynamic loads and trawl boards. However the infilling is of concern when pipeline repairs are contemplated. Infilling of glory holes can cause interference 'with operation and maintenance. Provisions may be necessary to permit an appropriate build-up of sediment or to allow for its periodic removal. The infilling of borrow and beach nourishment sites is of concern only insofar as the time span required for a return to natural conditions needs to be known. Considering this range of problems related to the infilling of subsea trenches it is notable that little attention has been given to this subject. The purpose of the resent study is to develop practical information about the typical style and rate of subsea trench infill as a guide to engineering evaluations. The results of his study are to provide sufficient information to allow preliminary judgement- based assessment of the detrimental or beneficial effects of infilling in particular