The Rational Method is implemented as described in the National Transport Commission Road Drainage Manual, with an additional option to specify specific point rainfall for each subcatchment.
The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period comprised of multiple time steps.
The detention storage facility may be applied in two ways. Firstly, it can calculate the required storage volume and keep all the excess flow in detention storage. In the second instance the designer may specify the volume of available detention storage at a node. Should the calculated storage requirement be more than the available volume the excess flow will be diverted to an overflow node.