Model experiments on mobile, paved gravel bed streams

Laboratory experiments which model streams with poorly sorted gravel beds at reduced scale reveal that pavement is a mobile bed phenomenon. It acts to decrease the inherent difference in mobility between large and small grains by overrepresenting the percentage of large grains exposed to the flow. Pavement can coexist with the motion of all available grain sizes because motion is sporadic, so that at any given time only a small percentage of surface grains are actually in motion. Particles in the immediate subpavement are only occasionally moved, and deeper particles are hardly ever moved. The experiments were conducted at constant discharges. They provide accurate quantitative models of two field streams in that they obey the same bedload law as field data. Both sediment feed and recirculating configurations were utilized. The only systematic difference between the two is in the behavior of the finer grains.