New and improved hydraulic radius for channels of the second kind

The Manning’s equation is commonly used to calculate discharge and mean velocity of the uniform flows. According to experimental data, Manning’s equation with constant Manning’s coefficient overestimates the discharge of second-kind channels (channels with a closing top-width), under the partially full flow. This problem can be solved by altering the Manning’s coefficient depending on the relative flow depth or changing the definition of the conventional hydraulic radius, that is, flow area divided by the wetted perimeter. Since, Manning’s coefficient theoretically depends only on the materials of the wall, so it seems that the second method is preferable.In current research a new and improved definition of hydraulic radius for closed conduits flowing partially full, is presented. This definition is efficient enough and provides powerful tool to determine the channel discharge and friction slope of uniform flow via Manning’s equation.