WSP03817 - Laserfiche WebLink

Home Browse Search

I I r I t I I I I I I I 1 I t OC1830 y Vi = unit weight of water (62.4 Ibs/ftJ) R = hydraulic radius (AlP) (ft) (Eqn 7,6) AlP = cross-sectional area I wetted perimeter (ft) S[ = friction slope (filft) If vegetation, meandering, or other large form features significantly influence roughness, then particle shear is a better representation of the forces acting upon the bed material. In the case of the Arkansas River study area, total roughness is primarily a function of particle size; therefore, application of Shield's equation is appropriate. 7.4.1. Determination of the Dimensionless Shield's Parameter Various references, including Gessler (1971), report the dimensionless Shield's parameter, ranging between 0.030 and 0.047, for all types of bed material. For this study, Shield's parameter determination was achieved through calibration using bedload sampling at cross-section AR-O. Collected data included cross-section surveys, water surface slope measurements, discharge calculations, and bedload sampling using a Helly- Smith sampler. Cross-section area and wetted perimeter were used to determine the hydraulic radius. Bed shear was then solved for using Equation 7.5, where the friction slope (Sr) is assumed to equal the measured water surface slope. Several calculations were solved to determine values for the dimensionless Shield's parameter (F*), using the following equation: F* = 't/(ys Ds) (Eqn 7.7) Sediment diameter (Ds) was taken as the maximum particle size found in the bedload sample. Additional calculations for the Shield's parameter were determined by finding shear values for the entire cross-section and the primary cross-section area, where the latter leaves out shallower areas crossing over finer gravel bars where larger material was not in motion (Table 7,7). Bedload sampling and gradation curves were produced from separate data collection efforts at cross-sections AR-O, AR-I0, and AR-20. These data were used to further verify Shield's parameter selection by examining calculated sediment sizes resulting from the use of/ow and high parameter values (0.030 and 0.047), and comparing them to actual bedload size distributions. Table 7.8 lists various incipient motion sediment sizes calculated for varying Shield's parameter selection. Results indicate that all calculated particle diameters were larger than those found in the bedload samples. Using a Shield's parameter of 0,047 generates smaller calculated particle diameters, producing results closer to actual sampled gradations than those determined using the smaller Shield's parameter of 0.030. The Shield's parameter of 0.046, calculated from cross-section AR-O and discharge data, was taken to be the most representative value available from current data. May 7, 1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 66