Particle friction angles ?? represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows. We determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force F(d) required to pivot or slide a particle out of its natural resting place. Within each site, median ?? values were very similar to previously reported relations, yet different enough between sites that a location-general predictive empirical relation would produce errors in ?? of ??10 degrees for D(i)/K(s) > 1. Furthermore, within a D(i)/K(s) class at a given site the range in ?? was as large as 80??, much greater than the range of median values between classes for natural sediment mixtures. Using estimates of ??(c)/* from extensive bed load measurements made by Andrews and Erman  at Sagehen Creek and the in situ ?? measurements made in this study together with, a theoretical model developed by Wiberg and Smith , we show that ?? measurements made with this new method can be used to accurately predict ??(c)/* for natural, water-worked sediments. Additionally, these results confirm that a ?? value <?50 is more appropriate for predicting ??(c)/* of a given size class.
Additional publication details
In situ determination of particle friction angles of fluvial gravels