Head Loss Characteristics and Surface Roughness of Overtopped Rockfill

Abstract

During flood overtopping on the rockfill embankment, there appear a flow seeping through the rockfill (throughflow) and a flow running over a top surface of the rockfill (overflow). Seepage analysis to calculate the throughflow requires the coefficient values which describes a nonlinear head loss equation of flow through the rockfill. In an integration method or a step method for the varied overflow calculation, a roughness coefficient which determines a magnitude of friction slope is required to be known. In order to design and construct a mechanically stable rockfill structure during flood overtopping, the hydraulic conditions of the throughflow and overflow should be properly calculated using reliably-determined hydraulic parameters such as the coefficients values of the head loss equation and the roughness coefficient mentioned above. Employing coarse gravels and cobbles 50 to 200 mm in diameter, two types of the laboratory water flume test are conducted to determine these hydraulic parameters. The coefficient values of the head loss equation are reported to show the highly significant relationship with the hydraulic mean radius of rockfill voids. The roughness coefficients of rockfill surface are found to increase with the rock particle diameter described by the predominant wave length of the rockfill surface configuration, and be 0.04∼0.06 for the rock particle size tested.