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I I il I I I I I I I I I I I I I I II I STREAM CLASSIFICA nON There are three general panerns of streams in nature: straight, meandering, and braided. These pattern names are hydrologic shorthand used to describe what is in reality a continuum of channel form. s.,there~y variations on each pattern, and panerns that are in between. What is known is that meandering streams occur as part of landscapes that are low gradient with low bed-load transport, and that braided streams occur in steep gradient, high bedooload transport landscapes. Schumm and Meyer (1979) used the relationship of grade and bed-load transport to identify five types of stream forms (Figure 1). Streams become less predictable as they transition from straight to meandering to a braided panern. The bed-load transport is a function of channel velocity and bed material size. The mode of bed material transport changes from mostly bed material transport to mostly suspended material transport as velocity increases. Laursen (1960) identifies the limits of each mode of transport as follows: Table 1. Modes of Bed Material Transport U./w Transport Mode < 0.50 mostly contact bed.load transport 0.50 to 2.0 mix of suspended and contact bed.load transport > 2.0 mostly suspended bed.load transport where U. is the shear velocity (= (g d st where g is the acceleration of gravity, 32.2 fT./s'; d is the mean depth of flow, ft; and S isthe slope of the energy grade), and w is the fall velocity, ft/s. The shear velocity can be thoUght of as the near bed velocity in a channel and is logarithmically proportional to the mean velocity of the flow, The fall velocity of bed material in the range of fine sand to gravel is given in Appendix 1. For stream classification, the shear velocity is based on estimation of a dominant discharge. Methods for determination of dominant discharge are given in Appendix 2. '-../ "/~~ \ii. R - ,.. ~ 5 1/1 3