A French drain is constructed to intercept an underground aquifer. It should never be used as a drainage collection system for surface water flow. Contractors who sell a French Drain as a solution for correcting surface water flow do not understand the problems caused by their recommendations, or, at the least, are just interested in cash flow, not water flow.
        Usually finding the source of an aquifer is difficult, at best. Once the source is determined and the flow rate established, a French Drain can be designed & constructed. A typical French Drain is shown below. The hydraulics of a drainage system determines its effectiveness. One should never recommend a French Drain as a solution for intercepting underground water unless the source can be verified and the flow rate established.