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<br />e <br /> <br />" <br /> <br />If the designer has no knowledge of the erodibility of the soil <br />at a particular channel site, a reasonable estimate of d <br />max <br />may be obtained by interpolating half-way between the '~rosion <br />resistant" and "erodible" lines of the maximum permissible <br />depth charts (except Chart 27 for rock riprap, where no range <br />is given because the underlying soil has no influence on the <br />erosion resistance of the riprap lining). <br /> <br />i <br /> <br />Hydraulic Resistance <br /> <br />The flow velocity charts were developed to define the relation- <br />ship between the hydraulic radius of the channel, R, longitudinal <br />slope of the channel, So' and mean channel velocity, V, for a <br />given channel lining. For some linings, such as rock riprap <br />of a given size and fiber glass roving tacked with asphalt, the <br />Manning equation may be used since the n value is essentially <br />constant. For rock riprap, the Manning n value varies with <br />mean stone size, as follows (6): <br /> <br />n . 0.0395 D501/6 <br /> <br />Thus, the following n values apply for common stone sizes: <br /> <br />e <br /> <br />D50 (ft.) <br /> <br />n <br /> <br />0.25 <br />0.50 <br />0.75 <br />1.00 <br />1.50 <br /> <br />0.0314 <br />0.0352 <br />0.0377 <br />0.0395 <br />0.0423 <br /> <br />For fiber glass roving tacked with asphalt, Cox (4) found that <br />the Manning n value was approximately a constant: <br /> <br />. <br /> <br />Smooth Rolled Channels <br /> <br />Channels with Clods <br />and Tracks <br /> <br />.., <br /> <br />Single layer <br />Double layer <br /> <br />0.030 <br />0.020 <br /> <br />0.035 <br />0.025 <br /> <br />The higher values of n were used in the development of Charts 5 <br />and 6, assuming that most highway channels will be rather rough <br />after seeding and mulching. <br /> <br />e <br /> <br />9 <br />