Laserfiche WebLink
<br />Fluvllll Proc:es_ . <br /> <br />-- <br /> <br />2.2.4 Bank Materlal- <br /> <br />The origin and composition of the materials canprising stream banks have a significant <br />infiuence upon channel morphology and bank erosion. Generally speaking. banks can <br />be classified as either cohesive, oon-cohesive, or composite. From a practical <br />standpoint, cohesive banks can be described as those containing appreciable amounts of <br />clay. A beller technical description might be that a cohesive sediment mixture is one in <br />which the internal shear strength is increased over that expected from the simple <br />discrete particle properties of shape, size, density, and relative position. The increase is <br />usually due 10 molecular atlraCtion (as is the case with most clays), but could also be <br />caused by such things as the binding strength generated by plant root masses (Chang <br />1988). <br /> <br />Non-eohesive materials, as the name implies, do not possess this added Slrenglh. Sands <br />and gravels are oon-eohesive sediments. Silts usually are. Composite banks have a <br />layered structure containing both cohesive and oon-cohesive materials. They are <br />commonly found in Colorado, particularly on rivers with a high bed load. Lower <br />portions of the bank are historic bar deposits of non- cohesive bed material, while the <br />cohesive upper banks were fonned by fine sediment deposition during flood recession. <br />Chapter 3 presents a discussion of the influence of bank material type upon bank failure <br />and erosion rates. <br /> <br />2.3 Fluvial Processes - <br /> <br />An alluvial river is generally and continually changing its shape, position. and other <br />characteristics as a consequence of the bees imposed upon it The fluvial processes <br />involved are exlremely complicated and the variables of importance are often difficult 10 <br />isolate. Many laboratory and field studies have been carried out in an attempt 10 <br />detennine relationships among these and other variables. <br /> <br />The major factors affecting alluvial slream channel foons are: (1) slream discharge, (2) <br />sediment load, (3) longitudinal slope, (4) resistance to flow, (5) vegetation, (6) geology, <br />(7) sediment composition, and (8) works of man. This section discusses some of these <br />factors and their interrelating variables. <br /> <br />2.3.1 Basin Dynamics - <br /> <br />A commonly accepted concept in the study of fluvial geomorphology is that of time <br />cycles. The primary caIegories are cyclic, graded, and steady time. Cyclic time is <br />comparable 10 geologic time, and is generally considered 10 be on the order of one <br />million years or more. This is the time over which significant earth-shaping changes <br />occur. Graded time covers periods of 100 to 1,000 years, the time necessary for drainage <br />basins to reach maturity and for most slreamS 10 reach equilibrium. Steady time, on the <br />other hand, may be as short as one day, and can be considered that time necessary for a <br />single event to occur (Schumm, 1977). <br /> <br />e <br /> <br />Colorado Erosion COntrol Manual <br /> <br />7 <br />