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- Created by Unknown User (juanir) on Jan 07, 2016

**xpdrainage uses a Muskingum-Cunge algorithm** to calculate the Time of Travel and Retention Coefficient for all Stormwater Controls. In the case of a Stormwater Control with two subsystems, i.e. Bioretention has a Ponding area and Filter layers, the software will perform a set of calculations for each individual subsystem.

The Muskingum-Cunge algorithm is a variation of the Muskingum routing method. The algorithm calculates basic hydraulic parameters in a modified manner, using Manning's velocity equation and modified flow wave celerity equation.

The Manning’s velocity equation :

Hydraulic radius:

And reference depth, we assume the reference flow:

Flow wave celerity:

K coefficient is defined as

And eventually Retention Coefficient:

Where

A – wetted area

W – structure width

S_{0} – slope of the structure

L – length of the structure

*Note:* The the wetted area and width are determined at a set height up the stormwater control, or subsystem. This is based on the Reference Height Fraction that is specified on the **Analysis Criteria** form.

With the hydraulic radius, wetted area and reference width calculated the engine will calculate the velocity and K, and will perform further calculation for the Muskingum method.