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I 11 I. significantly affect peak-flow conveyance computations. Vegetation such as grass flattened over the bed material in a gravel-bed channel may actually result in less resistance to flow than if the vegetation was not present. For most conditions, however, if the vegetation is not removed but is laid over, the degree to which flow resistance is affected is a factor that must be considered and evaluated. Under these conditions, a value for n4 must be assigned. For example, figure 9 is used to roughly estimate whether or not the forces of flow will layover (angle exceeding 450 from the vertical) vegetation. The resistance to flow attributed to vegetation at 450 from vertical can still be substantial under certain conditions such as low-flow depths. The streamlining of vegetation when it is laid over also may require consideration and would make the assessment of n4 under these conditions very difficult. Unfortunately, very few studies have actually isolated and verified n4 under controlled conditions. Phillips and Ingersoll (1998) developed an equation that relates percentage of flow blocked by vegetation to the corresponding magnitude of n4' This equation is as follows: r I n4 = 0.00088-0.0007, (9) where B = percentage of flow blocked by vegetation. , Values of B used in the derivation of equation 9 ranged from 3 to 25 percent. Results from the equation are questionable if the equation is used for sites where vegetation conditions are substantially beyond the range of data used in its derivation (Phillips and Ingersoll, 1998). Values of n4 were determined for each site for preflow- and postflow-vegetation conditions using available guidelines and equation 9. These values were added to the estimates of nb to obtain the total roughness coefficient. The complex dynamics and highly variable nature of flow-induced changes in vegetation conditions, however, serve to maintain a certain degree of uncertainty and subjectivity in selections of n4 and subsequent conveyance computations. r i I Channel-Conveyance Computations Channel conveyance was calculated for preflow- and postftow-channel conditions at each of the sites to illustrate the potential differences in conveyance values attributed to inaccurate assessment of vegetation conditions for peak-flow computations. Channel conveyance was calculated using standard procedures (Dalrymple and Benson, 1967). Channel conveyance, K, is defined as K = (I.486/n)AR2/3, (10) where A = cross-section area, in square feet. The changes in water-surface elevations for preflow and postftow conditions also were computed (table 9). Discharges were held constant for these computations, and water-surface elevations were determined by an iterative procedure. Potential errors in n-value estimates can result in substantial differences in calculated water-surface elevations (table 9). Recurrence intervals for most of the flows in this study ranged from I to 20 years. The difference in water-surface elevations when n4 is incorrectly assessed, however, can be much greater for larger discharges such as those with recurrence intervals of 50 and 100 years. Discussion and Example Case Preflow assessment of roughness coefficients must be made for a variety of hydraulic studies that include delineation of floodways and design of hydraulic structures. Inaccurate assessment of peak-flow roughness conditions made before flow in densely vegetated channels can result in substantial errors. For example, the Hassayampa River below Old U.S. 80 Bridge site has dense growths of saltcedar and willow from time to time. The almost continuous irrigation return flow at this site contributes to accelerated vegetation growth, and subsequent to either artificial or natural removal, the vegetation is often near maturity after several growing seasons. Estimates of n4 can be Effects 01 Flow-Induced Vegetetion Changes on Channel Conveyances 21