For small particles (clay and silt) considerably higer velocities are required for erosion than for transportation because these finer particles have cohesion resulting from electrostatic attractions. Notice that for coarser sediments (sand and gravel) it takes just a little higher velocity to initially erode particles than it takes to continue to transport them. Hjulstrom's Diagram plots two curves representing 1) the minimum stream velocity required to erode sediments of varying sizes from the stream bed, and 2) the minimum velocity required to continue to transport sediments of varying sizes. In laminar flow, suspended particles will slowly settle to the bed. Fine particles will only remain suspended if flow is turbulent. Streams carry dissolved ions as dissolved load, fine clay and silt particles as suspended load, and coarse sands and gravels as bed load. The water molecules don't follow parallel paths. At higher velocities turbulence is introduced into the flow (turbulent flow). Units of discharge are volume per time (e.g., m 3/sec or million gallons per day, mgpd).Īt low velocity, especially if the stream bed is smooth, streams may exhibit laminar flow in which all of the water molecules flow in parallel paths. It is calculated as Q = V * A, where V is the stream velocity and A is the stream's cross-sectional area. Stream discharge is the quantity (volume) of water passing by a given point in a certain amount of time. So as hydraulic radius increases so will velocity (all other factors being equal). The greater the cross-sectional area in comparison to the wetted perimeter, the more freely flowing will the stream be because less of the water in the stream is in proximity to the frictional bed. For the same hypothetical stream the wetted perimeter would be the depth plus the width plus the depth (W + 2D). For a hypothetical stream with a rectangular cross sectional shape (a stream with a flat bottom and vertical sides) the cross-sectional area is simply the width multiplied by the depth (W * D). Hydraulic radius (HR or just R) is the ratio of the cross-sectional area divided by the wetted perimeter. Stream velocity is greatest in midstream near the surface and is slowest along the stream bed and banks due to friction. Units are distance per time (e.g., meters per second or feet per second). The natural processes of periodic flooding, accompanied by erosion and deposition, bring changes to the topography, soils, vegetation, and physical features (such as meanders, braided channels and oxbow lakes) within these areas over time.Stream velocity is the speed of the water in the stream. The movement of water between these stores is called transfers. The major stores of water are the ocean, ice caps, land and the atmosphere. Some water percolates deeper into the ground and is slowly transferred back to the river or sea.This through flow moves more slowly back to the river than surface run-off. Surface run-off is more likely to occur if the ground is saturated with water or when the rock is impermeable. This happens when the surface doesn’t allow water to penetrate.
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