Application of ‘Structure from Motion’ (SfM) technique in physical hydraulic modelling

Abstract

There are many methods available for measurement of bed morphology in physical hydraulic model studies considering mobile bed sediment. Among which, there are sophisticated instrumentations which provide quality results in shorter time but are vastly expensive and requires special training for execution. Whereas the conventional surveying methodology, which is simple and inexpensive, requires plenty of time for the measurement and processing of the data. That is why the recent developments in ‘Structure from Motion’ (SfM) technique have made it a potential candidate for an inexpensive and efficient tool for measurement of bed morphology in physical hydraulic model studies. SfM method allows to simultaneously determine both the parameters of the camera and the 3D structure of a scene by combining 2D images taken from multiple viewpoints. SfM tools can create a dense point cloud out of a set of partially overlapping photographs taken even by a budget friendly digital camera. The SfM method have already been used as an alternative for topographic surveying to create high-resolution digital elevation models (DEM). Some researchers had also used it for measurement of bed morphology in laboratory experiments. In this study, different freely available SfM tools were used to create a dense point cloud from a set of photographs representing a short reach in a river model in the hydraulic laboratory at Hydro Lab. The selected tools were compared with each other and against a commercial software, based on the methodologies used, processing time and quality of the output. Then the results from SfM method were compared with actual measurements in the physical model done with a conventional surveying technique using a theodolite and a level machine. The results showed that free SfM tools can also produce efficient results compared to commercial tools and SfM method can be used as an inexpensive and efficient alternative for bed morphology measurements in physical hydraulic models.