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.. 148 HYDRAULIC ENGINEERING '94 Flume and numerical model investigations have been conducted to evaluate the effect of bed load transport on hydraulic roughness. and the characteristics of the bed load transport itself. The flume study was conducted at the US Army Engineer Waterways Experiment Station (WES). In conjunction with the flume study, a numerical sedimentation model study was conducted by the US Army Engineer District. Los Angeles, Description of the Study Area The Mission Creek watershed comprises about 29.8 square kilometers and is located in a narrow coastal area which extends from the Santa Ynez mountains on the north to the Pacific Ocean on the South. Mission Creek rises about 1140 meters in elevation and flows about 13 kilometers to empty into the Pacific Ocean. In the headwater areas. stream gradients are as steep as 492 meters per kilometer and average 189 meters per kilometer. In the lower reaches. on the alluvial plain below the foothills. everage slopes are about 28 meters per kilometer. Gredetion Analysis Sediment samples were collected along the creek and a gradation analysis was performed to determine the grain size distribution and the maximum particle size. Along the channel, a backhoe was utilized to collect the sediment samples since the bed material consisted primarily of boulders and cobbles. The maximum grain size of the bed material collected was about 305 mm with an average d50 of about 50 Mm. Inflowlng Load General ADDroach. The amount of sediment inflowing to the proposed channel wes estimated by routing e 1 OO-year balanced hydrogreph through the natural channel upstream of the project reach. The sediment routing was performed with e special version of TABS-l (i,e, HEC-61. a one-dimensional numerical model developed at WES. which ellows for transport of individual grain sizes larger than 64 mm along with sand and gravel sizes. Equilibrium transport was assumed at the upstream boundary of a 2.9-kilometer study reach. In this analysis, it was expected that the bed load material will consist primarily of gravel. cobbles. and boulders, Sand and silt sized material should be carried as suspended~load given the high velocities of the relatively steep stream system. Sensitivity Analysis. A sensitivity analysis was conducted since prototype data did not exist for model adjustment. The sensitivity analysis consisted of testing various sediment transport functions and then varying the inflowing sediment load for each function. The transport functions included Yang's unit stream power, a combination of the Toffaleti and Schoklitsch functions. a combination of the Toffaleti and Meyer~Peter and Muller functions, . . BED LOAD ROUGHNESS 149 and the Meyer-Peter and MOiler function by itself. The inflowing sediment load ratio was halved and doubled at the upstream end of the numerical model during the simulations as an additional sensitivity test. ~. Results for the sediment routing for the peak discharge are shown in Table 1 and indicate that the concentrations flowing into the concrete channel may vary between 1450 and 17.900 ppm for the total load, in which the gravel. cobble and boulder concentrations vary from 61 to 1030 ppm. These results are for an inflow sediment load ratio of 1.0. Additionally. it appears that the combination of the Toffa1eti and Meyer-Peter and Maller transport functions gives the highest bed load concentration. Purpose of the Flume Study As eerly as 1946. Vanoni (19461 demonstrated thatsuspended sand. et concentrations between 1200 and 3300 ppm, caused a reduction of up to 10 percent in effectiye bed roughness over a flat bed. It has also been demonstrated by many investigators that both sand and gravel bed forms moving along the bottom of a flume or river at a velocity much slower than the flow can significantly increase effective roughness. It was anticipated that flow energy in Mission Creek would be sufficient to prevent the establishment of bed forms at expected rates of bed-load transport. However. it was expected that bad load moving along the bed of the channel at a velocity slightly less than the velocity of the water would introduce some drag and would therefore tend to increase the effective hydraulic roughness. The purpose of the flume study was to quantify this increase and to determine at what concentration bed forms begin to appear. Flume Studies The tilting steel flume used in this study was about 24,4 meters long and 0.9 meters wide. The flume tests were conducted using steady uniform Table 1, Totel and gravel concentretions at peak discharge for different transport functions Transport Total Total Gravel Gravel Function Load Concentration load. Concentration. (t/dl (ppml (lIdl (ppml Yang 323.700 17.900 1100 61 Toff-Schoklitsch 248.800 13.800 10,500 580 Toff,MPM 160.600 8880 18.600 1030 MPM 26.300 1450 17.200 950 . Includes graver. cobb es. and boulders ~