The reason for this blog: The following is an excerpt from the EPA manual Storm Water Management Model Reference Manual Volume III – Water Quality (PDF) which can be found here

Underdrains are used in the following LID Controls

6.5.1 Underdrains

Underdrains are either recommended or required when the natural soil infiltration rate is insufficient to prevent the LID unit from flooding. There are three user-supplied parameters that describe underdrain flow: a discharge coefficient (C3D), a discharge exponent (h3D), and a drain offset height (DD3). While the drain offset is part of the cell’s physical design, the discharge coefficient and exponent must be inferred from the hydraulics of underdrain flow. There are several approaches that can be used for this:

1. Assume the flow rate is limited by the flow capacity of the slotted pipe used as the underdrain.

2. Assume the flow rate is limited by the rate at which water can enter the slots in the drain pipes.

3. Assume the flow rate is limited by a flow restriction (such as a throttling valve or cap orifice) on the drain’s discharge line.

To use option 1, the full flow capacity of the drain pipe can be computed from the Manning equation as follows: