River and Stream Sediments

River and Stream Sediments

Sediment in streams is defined somewhat differently than in lakes. The similarity comes from examination of stream or river bed strata. The dissimilarity comes from determining the amount of sediment that is moving as a result of stream discharge. The total sediment or total load is all the sediment in transport. It is usually expressed as tons/day or tons/year.

The sediment moving as suspended load plus that moving as bedload. Suspended load or suspended sediment is defined as the sediment that is supported by the upward components of turbulent currents and that stays in suspension for appreciable lengths of time. Suspended sediment is usually expressed as mg/liter.

Bedload is the sediment that moves by saltation, rolling or sliding on of’ near the streambed, usually expressed as tons/day or tons/year.

Basic Concepts of Sediment Transport
Brief qualitative review of sediment movement processes. The importance of turbulence in sediment transport processes been recognized.

In turbulent flow, water moves in a prevailing horizontal downstream direction, but at any given point the magnitude of horizontal velocity component fluctuates about some mean value. Velocity components also exist in the vertical and lateral directions, and their magnitudes vary in a generally random fashion around respective mean values.

Two forces act on any given particle, drag and gravity. The drag force is caused by the relative motion between the sediment particle. Fluid may act either upward or downward, while gravity always exerts a downward force. The resultant direction of a particle in motion depends upon the relative magnitude of these two forces.

For example, a particle caught in a falling current will move downward because both gravity and the currents are acting in the same direction. However, a particle in a rising current can fall, rise, or remain stationary depending upon the vector sum of the forces.

For any small reach of stream, a certain number of suspended particles will enter the reach per unit time. However, over the same unit of time a nearly equal number will leave the reach. The final result is a density of suspended par titles that, in time, fluctuates in a random fashion about some mean concentration value.

For any given size of particle, the concentration increases from the water surface to bed surface. Also, the range of concentration from water surface to bed surface increases with increased particle size.

A general expression for the concentration of sand-size particles in two dimensional flow is:
C=(d-y/y)*2 when the concentration of a given size of sediment is known at one point in the vertical, concentrations at other points can be expressed as:
Cy/Ca=(d-y/y x a/d-a)*2 where C is the sediment concentration at elevation y above the streambed, d is the stream depth, Cy is the concentration of a specified sediment size at y, Ca is the concentration of the same size of sediment at reference level, y=a.
The exponent z is empirical, and for flow over a flat bed is:
z=W/(ku*) where W is the settling velocity of a given particle size, k is the coefficient of turbulent exchange, and u* is the shear velocity, g is the gravity constant, s is the energy gradient slope, and d is the stream depth.

For a particle to be moved by streamflow, the force of the discharge must be greater than the shear stress. The shear stress on the bed, which can be thought of as a force tending to roll particles on the substratum, is correlated with the square of the velocity of flow, but the relationship is complex. Increasing velocity picks up or rolls particles of increasing size along the bed. They are then carried downstream, but at a rate which is less than that of the mean velocity of the water.

The presence of suspended particles in the water cuts down the turbulence. This reduces the resistance to flow offered by the streambed. Thus, silty or sandy water picks up more material only if it flows more rapidly. Individual particles are constantly exchanged between the water and the bed.

A further complication is introduced by the packing coefficient of the particles making up the sediment. Compact clay is less readily carried up into the water than is and, even though clay particles are much smaller.

Tables that show the mean rates of flow that will just begin to move particles of given sizes are available.
Critical mean current velocity
Cm/sec.
Type of bed Clear water Muddy water
Fine-grained clay 30 50
Sandy clay 30 50
Hard clay 60 100
Fine sand 20 30
Coarse Band 30-50 45-70
Fine gravel 60 80
Medium gravel 60-80 80-100
Coarse gravel 100-140 140-190
Angular stones 170 180

The mean current velocity of clear and muddy water required to initiate movement along a stream bed of various types of bottom deposit.

Speed of flow Diameter of mineral
cm/sec. particles moved, cm.
10 0.2.
25 1.3
50 5.0
75 11.0
100 20.0
150 45.0
200 80.0
300 180.0

The speeds of flow required to move mineral particles of different sizes.

It should also be noted that the minimum current speed that will erode a material, for example compacted clay, is often different from that at which the resulting suspended particles will begin to settle.

In general, however, it can be said that fine sediment (silt and mud) will settle out at mean current speeds up to about 20 cm/sec. Where stream velocity does not normally exceed the range 20-40 cm/sec, the bed is likely to be composed of sand type materials.

Larger particles are likely to persist for some time. It should be clear from our consideration of the complexities of flow on rough stony bottoms that large stones provide shelter for considerable quantities of sand and gravel. Therefore, sand and gravel can persist among larger stones.

The larger the current velocity means the larger the suspended particle that can be moved. Large particles can protect smaller ones from being entrained because of the shelter they provide. Nevertheless, the mean particle size of most rivers decreases in a downstream direction.

Also, there is a correlation between the reduction of sediment particle size and the reduced slope of the channel. Part of the explanation for this probably lies in the fact that the shear stress on the stream bottom decreases with increasing discharge.

Many factors combine to make it probable that the general size of the particles forming the streambed will be smaller the further downstream it will go. The distance downstream that large particles persist depends upon:
a) The composition of the rock, for example granite breaks up more readily than basalt, and limestone more readily than quartzite.
b) The degree of weathering, which depends upon climate and determines how often the rock particle is wet and dry.
c) The length of time the particle is stored in the flood plain before it reenters the stream system.
These factors tend to increase the uniformity of the particles making up the streambed as you go downstream.

However, curves and meanders plus banks that collapse and slough and other important factors continuously add particles of variable sizes. Combining all these factors produces the high variability observed in sediment measurement.

Sediment movement is affected by constantly changing environment. Sediment quantity and characteristics can be correlated with environmental changes over time.

Next Topics…
Sediment Significance to Fish and Aquatic Life

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