Application of automated grain sizing technique (AGS) for bed load samples at Rasil River: A case study for supply limited channel

Obtaining grain size distribution (GSD) is necessary in fluvial research. Conventional sieving and manual picking of particles are widely practiced to obtain representative sizes. Sieving method emulates the use of frequency by mass whereas manual picking method entails the use of frequency by count technique. However, in some instances, bed load transport rates can be quite low, making it difficult to obtain sufficient sample mass for sieve analysis. Inadequate sample size may hinder the accuracy of GSD if the frequency by mass method is applied. Manual hand picking method is almost impossible for supply limited channel where majority of particles in motion are less than 8 mm. This paper tests the use of automated image processing techniques for obtaining size statistics of small sample mass of bed load samples. Comparison of GSD between the bed material and bed load successfully elucidate the applicability of automated grain sizing (AGS) technique for small size of bed load sample. Bed load GSD at supply limited channel exhibit the size selective pattern where GSD for bed load is much finer than GSD from surface material and almost overlay with GSD from subsurface material.