Sediment Transport Stream Capacity - The capacity of a stream or river is the total amount of sediment a stream is able to transport comprised of three types: Bed Loads, Dissolved Loads, Suspended Loads. Capacity = c V 5 Stream Competency The size (mass) of the particles that the stream can transport. Competence = c V 6 Stream Capacity Stream capacity varies with: Gradient - elevation change over horizontal distance, or slope of the stream channel. Steeper gradient --> greater capacity. In the short term, in the absence of dramatic erosion or deposition, gradient remains constant. Velocity - rate of flow of water (m/sec) Higher velocity --> greater capacity. If v is doubled, capacity increases 8-16 times! Related to Discharge. Varies with discharge. When streams flood, velocities increase substantially, so the stream can carry much more material. Channel size and shape Wetted perimeter is the cross-sectional edge of the stream channel, where the stream makes contact with its channel. This produces friction. Shapes that minimize perimeter-to-area ratio have higher capacity If shape is held constant, larger channels have higher capacity. Like gradient, channel shape and size do not vary in the short-term, as long as erosion and deposition are not dramatic. Stream Capacity vs. Load If Load >= Capacity, sediment will be deposited. If capacity exceeds load, the stream has excess energy (gravitational, potential energy) so it can erode more sediments. It is sediment size depend. A stream can have excess capacity but may still not be able to transport the sediment available. Transport Capacity vs. Sediment Source Sediment Transport Profile Suspended Sediment Load Bed Load Sediment Graph Hysteresis Hysteresis in Sediment Observations Hysteresis - a system with hysteresis exhibits path-dependence, or "rate- independent memory. Sediment rating curve, if channel gradient and shape do not change over the short term discharge can be to describe capacity. A sediment rating curve must be redeveloped if that are significant physical changes to the system. Seen in other systems Floods The higher the discharge the higher the velocity the greater the capacity and the potential for change. Rare floods --> low frequency events --> accomplish much work (redistribution of sediments) per event --> moderate amount of work performed in aggregate over long time period. But large events can move the location of a channel and build terraces. Annual floods --> high frequency event --> moderate work accomplished per event --> many events add up to lots of geomorphic work performed, in aggregate over long time period. These events are the primary shaper of fluvial landforms. The basic channel form is the function of events ranging between 1 10 year return periods. Normal flow --> accomplishes little work, even accumulated over long time periods Result is that geomorphic work occurs in pulses associated with floods. The big floods look most dramatic (greatest short-term sediment redistribution), but annual floods do most work over the long haul in defining the Stream Equilibrium Streams switch back and forth from depositional to erosional agents, depending on load vs. capacity. Streams can erode along one stretch and deposit along another, since gradient and channel shape/size vary along the stream's course. Streams can erode during periods of higher velocity or discharge (floods), and deposit during periods of lower velocity or discharge. Streams can be thought of as a sediment transport system that attempts to maintain an equilibrium, a balance between load and capacity. Anything that alters the sediment load delivered to the channel, or that alters the stream's capacity to carry that load, will cause the stream to adjust its gradient or channel geometry (by erosion or deposition) to achieve a new equilibrium, a new balance between load and capacity. Lanes Equation: (SL * SS) (Slope * Q) Use of Lanes Equation What would happen downstream if a small reservoir trapped all the bed load but did not influence peak discharge for large events? What would happen if sediment was delivered into channel from an eroding mine tailing? Channel Stability Aggradation The process by which a stream's gradient steepens due to increased deposition of sediment. Degradation The process by which a stream's gradient becomes less steep, due to the erosion of sediment from the stream bed. Generally follows a sharp reduction in the amount of sediment entering the stream or increase in energy.