Functional relationship to describe drains with entrance resistance

Numerical flow models usually represent drains as a system dependent bounda ry condition. If soil is saturated, drains act as the Dirichlet boundary co ndition with pressure head set equal to zero, and if soil is unsaturated, d rains act as the Neumann boundary condition with flow set equal to zero. Th e underlying assumption is that drains exhibit ideal behavior. In reality, however. this is generally not so, and the flow encounters additional resis tances due to pipe slotting and clogging of the envelope material around th e drains. To account for the resulting resistance, a Hooghoudt-type boundar y condition was developed that prescribes drain flow in relation to the gro undwater level at a reference position. The measured drain discharge in an old drainage system was compared with calculated discharge assuming an idea l drain. It was found that the ideal drain assumption led to large errors i n simulated discharge. With a correctly formulated and calibrated Hooghoudt boundary condition, however, more accurate drain discharges were obtained.