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<br />Proper values of nb' nl to n4, and m for various types of channels <br />will be presented in detail in following sections. <br /> <br />Selection of Base n Values (nb) <br /> <br />In the selection of a base n value for channel subsections, the <br />channel must by classified as a stable channel or as a sand channel. <br /> <br />A stable channel is defined as a channel in which the bed is composed <br />of firm soil, gravel, cobbles, boulders, or bedrock and which remains <br />relatively unchanged through most of the range in flow. Table 1 (Aldridge <br />and Garrett, 1973) lists base nb values for stable channels and sand <br />channels. The base values of Benson and Dalrymple (1967) generally apply <br />to conditions that are close to average; whereas, Chow's (1959) base <br />values are for the smoothest reach attainable for a given bed material. <br /> <br />Table l.--Base values of Manning's n <br /> <br />[Modified from Aldridge and Garrett, 1973, table 1] <br /> <br />Median size of bed material <br /> <br />Base n value <br /> <br />Channel or <br />flood-plain <br />type <br /> <br />Millimeters <br /> <br />Inches <br /> <br />Benson and <br />Dalrymple <br />(1967)Y <br /> <br />Chow <br />(1959)Y <br /> <br />Sand channels <br /> <br />(CXlly for upper <br />regime flow where <br />grain roughness <br />is predominant.) <br /> <br />0.2 --------- 0.012 <br />.3 --------- .017 <br />.4 --------- .020 <br />.5 --------- .022 <br />.6 --------- .023 <br />.8 --------- .025 <br />l.O --------- .026 <br /> <br />Stable channels and flood plains <br /> <br />COncrete---------- ------ --------- 0.012-0.018 0.011 <br />Rock cu t---------- ------ --------- ----------- .025 <br />Firm soil--------- - ----- --------- .025- .032 .020 <br />Coarse sand------- 1- 2 --------- .026- .035 <br />Fine gravel------- ------ --------- ----------- .024 <br />Gravel------------ 2- 64 0.08- 2.5 .028- .035 <br />Coarse gravel----- - ----- --------- ----------- .026 <br />Cobble------------ 64-256 2.5 -10.1 .030- .050 <br />Boulder----------- >256 >10.1 .040- .070 <br /> <br />YStraight uniform channel. <br />YSmoothest channel attainable in indicated material. <br /> <br />4 <br />