FLOOD01296 - Laserfiche WebLink

Home Browse Search

the basis of collected field data and descriptions of the bed material. The flow duration is important because long-duration flows have a greater potential for significant movement of substrate material (table 8). Finally, estimating the size and density of vegetation-root systems (thus its strength) proved to be unattainable for this study. The complex nature of vegetation-root systems and the difficulty in determining their strength and size made developing a relation for vegetation removal impractical until more data become available. Critical Shear Stress Sediment size is an important factor in determining the ability of shear stresses to degrade substrate material and subsequently remove vegetation. In general, as particle size increases, the amount of shear stress required to initiate movement also increases (Shields, 1936). The magnitude of shearing forces required to initiate movement of bed material is called the tractive or critical shear stress (Vanoni, 1977). Several investigators have presented empirical relations used to determine the critical shear stress for specific sizes of bed material (Vanoni, 1975). For stream channels presented in this report that contain substrate material with a median diameter (dso) less than 10 mm, an equation presented by Vanoni (1977) is used to determine critical shear stress. This equation is tc = 0.016exp [0.1269 (1ndso/O.l)l (5) where tc = critical shear stress, in pounds per square foot, and In = logarithm to the base e. For channels containing substrate material with a dso larger than 10 mm, an equation presented by Carson and Griffiths (1985) is used to determine critical shear stress. This is equation is tc = O.Ol27dso . The values of shear stress were divided by the critical shear stress (table I and table 8). For sites where the shear stress (eq. 2) during peak flow is less than the critical shear stress, channel boundaries are considered stable and vegetation removal is unlikely. Flow Duration (6) One important factor in sediment transport not considered by the critical shear-stress equations is the duration that the bed material is subjected to shear stresses greater than the critical shear stress (tjtc > 1.0). Peak flows in Arizona can be either short-duration flows that are characterized by sharp rises and falls in stage or long-duration flows that are characterized by gradual changes in stage. Short-duration peak flows typically occur in small basins (<50 mi') as a result of intense and short-duration thunderstorms. Long-duration peak flows generally result from winter storms generated by frontal systems and generally cover large areas (>50 mil). The long-duration flows also occur as a result of reservoir operations. Reservoir releases can Jast for many hours, days, or even weeks. For each flow, the approximate duration (in seconds) that the substrate material was subjected to shear forces greater than the critical shear stress was estimated on the basis of stage or hydrograph data where available (table 8). A predictive method for vegetation removal or obliteration is not presented; however, it is hoped that the information and photographs presented in this section and the section entitled "Basic Data" at the end of the report will aid in the evaluation of substantial changes in roughness characteristics that occur when vegetation is removed from channel boundaries. At the three sites where vegetation removal occurred, the channel substrate generally was composed of sand-sized material (table 8). For the three sites where the vegetation was obliterated, the channel substrate was composed of cobbles (table 8). When vegetation is completely removed by flow, a dramatic decrease in flow resistance can result, which increases channel conveyance and subsequently lowers water-surface elevations. As the uprooted vegetation is transported downstream, however, it may accumulate on more resistant vegetation (figs. 5 and 7) and manmade structures, such as bridge piers and culverts, which can dramatically decrease channel conveyance and 18 Method to Estimate Effects ot Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Cantral Arizona