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<br />~~:":~;4: ~"* <br /> <br /> <br />This supplement describes a method for estimating the roughness <br />coefficient n for use in hydraulic computations associated with natural <br />streams, floodways and similar streams. The procedure proposed applies <br />to the estimation of n in Manning's formula, This formula is now widely <br />used, it is simpler to appl"' than other widely recognized formulas and <br />has been shown to be reliable, <br /> <br />Manning's f<:>rmula is empirical, The roughness coefficient n is <br />used to quantitatively express the degree of retardation of flow. The <br />value of n indicates not only the roughness of the sides and bottom of <br />the channel, but also all other types of irregularities of the channel <br />and profile, In short, n is used to indicate the net effect of all: <br />factors causing retardation of flow in a reach of channel under considera- <br />tion. <br /> <br />There seems to have developed a tendency to regard the selection of <br />n for natural channels as either an'arbitrary or an intuitive process, <br />This probably results from the rather cursory treatment of the roughness <br />coefficient in most of the more widely used hydraulic textbooks and hand- <br />books, The fact is that. the estimation of'n requires the~e~ci~~.::,~ <br />cri tical judgment in the evaluation of the primary factor~f~~. <br />These factors are: irregulari ty of the surfaces of the channel sides. <br />and bottom) variations in shape and size of cross sections) obstructions) <br />vegetation) meandering of the channel. <br /> <br />The need for realistic estimates of n justifies the adoption of a <br />systematic procedure for making the estimates, <br /> <br />Procedure for estimating n, The general procedure for estimating <br />n involves) fi~st, the selection of a basic value of n for a straight, <br />uniform, smooth channel in the natural materials involved) then, thrOugh <br />critical consideration of the factors listed above, the selection of a . <br />modifying value associated with each factor, The modifying values are <br />added to the basic value to obtain n for the channel under consideration, <br /> <br />, <br /> <br />In the selection of the modifying values associated with the 5 <br />primary factors it is important that each factor be examined and consider- <br />ed independently. In considering each factor, it should be kept in mind <br />that n represents a quantitative expression of retardation of flow. <br />Turbulence of flow can, in a sense, be visualized as a measure or indicatoI <br />of retardance. Therefore, in each case, more critical judgment may be ex- <br />ercised if it is recognized that as conditions associated with any factor <br />change 80 as to induce greater turbulence, there should be an increase in <br />the modifying value, A discUBsion and tabulated guide to the selection <br />of modifying values for each factor is given under the following procedur- <br />al steps. <br /> <br />.' <br /> <br />, <br />