## Rational Routing in InfoWorks ICM

In addition to various unit hydrograph transform methods and SWMM nonlinear reservoir runoff method, InfoWorks ICM also has a built-in rational routing model for rainfall-runoff process. To utilize the Modified Rational Method hydrograph concept (** Figure 1**), the InfoWorks rational routing model first converts a continuous rainfall hyetograph into discrete ones with the same rainfall duration of TOC. For each discretized rainfall duration of TOC, the sub-hydrograph will be generated according to the procedures of Modified Rational Method and the final total hydrograph is the sum of all the sub-hydrographs (

**).**

*Figure 2*The rational routing model only needs two parameters of a subcatchment – time of concentration (TOC) and runoff coefficient (C), which are relatively easy to estimate given a subcatchment. The rational routing model implementation in InfoWorks ICM is explained below.

First, define a rational routing runoff surface as shown in ** Figure 3**. For this surface, set the routing model as “Rational” and the runoff volume type as “Fixed”. Enter the Fixed runoff coefficient as the rational method runoff coefficient C (

**) or 1.0 (**

*Figure 3***).**

*Figure 7*Second, create a land use using the runoff surface created above and set its area percentage as 100% (** Figure 4**).

Third, in subcatchment property editor, choose “Rational” as routing method and enter time of concentration in minutes (** Figure 5**). Make sure the land use is selected as the one created above (

**).**

*Figure 6*The rational method coefficient is entered as “Fixed runoff coefficient” in ** Figure 3**. Alternatively, calculate the subcatchment contributing area as C x A to account for rational method runoff coefficient C (

**), and at the same time set the Fixed runoff coefficient as 1.0 for the rational routing runoff surface. This is useful if different subcatchments have different runoff coefficient C and you donâ€™t want to create a runoff surface for each subcatchment individually.**

*Figure 7*An example project was set up using Rational Routing Model with TOC of 60 min and its runoff hydrograph is shown in ** Figure 8** under a 24hr SCS storm event.

Like the traditional rational method, the rational routing model is applicable for a small subcatchment area not larger than 200 acres (some regulating agencies allow this upper limit up to 640 acres).

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