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I' Natural Overbanks In general, the most significant factors affecting flow resistance in natural overbanks are the amount, type, and density of vegetation, Although other factors may have relatively little effect on roughness, they need to be evaluated as well. Because very little verified n value data are available for overbank flow, considerable personal judgment is involved. General guidelines for estimating values of n for overbanks are given in table 6. The factors to be considered in evaluating roughness for minimum, normal, and maximum conditions on overbanks are discussed below. Surface roughness and bed-material size generally are finer on the overbanks than in channels. Bed materials generally are sand and silt except along mountain streams where deposits may be similar to streambed deposits. If there is considerable vegetal cover, the bed materials may have little effect on the roughness values, If particle size seems to dominate overbank roughness, the procedure for determining the n values of natural channels needs to be used. Most overbank surface irregularities are due to sediment ridges, old meander scars, and potholes. These irregularities increase turbulence and roughness. Flood-plain widths generally do not change rapidly within a reach, and expansion and contraction losses are not significant. Obstructions may be due to roads, fences, irrigation ditches, brush, debris, and downed trees. These obstructions increase turbulence and roughness. Generally, the type, amount, and distribution of vegetation have the greatest effect on overbank roughness, A dense cover of grass can completely obscure the effects of surface materials; dense brush and trees can obstruct flows significantly, reducing overbank conveyance to a minimum. Seasonal changes in vegetation density and cover need to be considered. In general, Colorado streams flood in the spring and summer 'when vegetation growth and density are at their maximum, No adjustments for meandering are taken for overbank flow, because flow generally is directly downvalley, The most significant factor to consider is whether the depth of flow and flow velocity are sufficient to bend the vegeta- tion over. Generally, flows exceeding 1.0 ft in depth are capable of bending grass, Grass and brush that can be bent over offer greatly reduced resistance to flow. The velocity of flow will depend on the overbank roughness and channel gradient. Values of n may exceed the maximum values shown in table 6. Petryk and Bosmajian (1977) reports that the n value may be as large as 0.40 in heavily vegetated flood plains. Arcement and Schneider (1984) have developed a guide, consisting of quantitative procedures and photographs, to help determine roughness coefficients for densely vegetated overbanks in tranquil flowing streams. 39