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; ... ~ ~ " .. The purpose of this group of pictures, selected from Technical Bulletin No. 129, "Flow of Water in Drainage Channels" by C, E. Ramser, is to illustrate the wide range of the roughness coefficient "n" of Manning's formula for chan- nel valocities related to actual channel conditions, Study of the pictures and information shown should assist in selecting realistic values of "n" for both present and future constructed channels. The pictures usually were taken when the channels were at low flows so that the channel condition could be seen. The tables show several measured values for the channels at different depths of flow. In most cases the pictures were not taken at the same time of the measurements. The "n" values shown in the tables under the pictures were calculated by using the measured values of slope, hydraulic radius, and discharge in the Kutter formula for velocity. The calculated "n" values would have been slightly less had the measured values been substituted in the Manning formula, Many engi- neers use the same "n" value of either formula, When the hydraulic radius "r" is 3,28, the same "n" value used in the two formulae will give the same value for the velocity, When "r" is less than 3,28, the velocity calculated by the Manning formula will be slightly I;l:reater than if calculated by the Kutter formula. When "r" is greater than 3.28, the velocity calculated by the Manning formula will be slightly less than if calcu- lated by the Kutter formula. For all practical purposes, the "n" values calculated by Kutter's formula may be considered to be the same for Manning's formula. The velocity determined by Manning's formula varies inversely as the value of "n", This. factor affects the velocity more than any other factor and its value depends entirely upon channel characteristics which are evaluated generally only by observation, Therefore, it is important that the observers who select these factors have some basis for evaluation, These pictures should aid in making a selection of the "n" value, The following should be considered carefully in estimating the value of "n" for a channel: 1. The material through which the channel will be constructed, such as earth, rock, gravel, etc, 2, Surface irregularity of the sides and bottom of the channel. 3, Variations of successive cross sections in size and shape, - 3 -