Organic detritus particles: Initiation of motion criteria on sand and gravel beds

Laboratory experiments were used to extend the application of the Shields entrainment function to both organic sediments and inorganic sediments overpassing a bed composed of particles of a different size. A total of 89 flume experiments were conducted to simulate organic sediment motion over sand and gravel stream beds. Although the study emphasis was on organic particles, some of the test included inorganic sediments. The bed particle diameters ranged from 0.7 × 10−3m to 22 × 10−3m, while the organic particle diameters ranged from 0.56 × 10−3m to 7.45 × 10−3m. A visual technique was used to determine the initiation of motion. The laboratory results were used to derive a modified Shields relationship which includes both the transported and bed particle characteristics. This modified Shields function predicts the initiation of motion of both organic and inorganic particles overpassing beds whose composition is significantly different than these particles. These results have direct application to the prediction of incipient motion of organic and inorganic detritus particles in streams and estuaries.