Hydraulic calculation of field drain pipe diameter: Using the theory of spatially-varied flow with increasing discharge

The flow in field drain pipes is classified as a spatially-varied flow (SVF) with increasing discharge. In this study, the parameter of “critical slope” was defined for SVF with increasing discharge. If the drain pipes have free outlet and the drain line slope is equal or less than the critical slope, which mostly is the case for the drain pipes, then, the critical section occurs towards the end of drain pipes. At the critical section, the Froude number is unity, and the flow depth at the end of drain pipes can be determined using the critical depth computation for the circular section. This method was used to calculate drain pipe diameter and the results were compared with the standard method of drain pipe computations based on Manning's equation. The results showed that the standard equations and design charts, based on Manning's equation, over estimate the drain pipe diameter.