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B.3 lIIIIIIIIIIIselection of modifying value for variations in shape and s~s sections. In considering changes in size of cross sections judge the approximate magnitude of increase and decrease in successive cross sections as compared to the average. Changes of con- siderable magnitude, if they are gradual and uniform, do not cause significant turbulence. The greater turbulence is associated with alternating large and small sections where the changes are abrupt. The degree of effect of size changes may be best visualized by considering it as depending primarily on the freQuency with which large and small sections alternate and secondarily on the magnitude of the changes, In the case of shape variations, consider the degree to which the changes cause the greatest depth of flow to move from side to side of the channel. Shape changes causing the greatest turbulence are those for which shifts of the main flow from side to side occur in distances short enough to produce eddies and upstrea.m currents in the shallower portions of those sections where the maximum depth of flow is near either side, Selection of modifying values may be based on the follow- ing guide: Character of variations in size and shape of cross sections Modifying value Changes in size or shape occurring gradually 0.000 Large and small sections alternating occasionally or shape changes causing occasional shifting of main flow from side tlo side 0.005 Large and small sections alternating frequently or shape changes causing freQuent shifting of main flow from side to. side 0,010 to 0.015 -.... Selection of modifying value for obstruction~, The sel~ be based on the presence and characteristics of ob- structions such as debris deposits, stumps, exposed roots, boulders, fallen and lodged logs, Care should be taken that conditions consider- ed in other steps are not re-evaluated or double-counted by this step, In judging the relative effect of obstructions, consider: the degree to which the obstructions occupy or reduce the average cross sectional area at various stages; the character of obstructions, (sharp-edged or angular objects induce greater turbulence than curved, smooth-surfaced Objects); the position and spacing of obstructions transversely and longitudinally in the reach under consideration. The following tabie may be used as a guide to the selection.