HydroCAD Stormwater Modeling - Since 1986


Finer Routing

By default, all runoff and routing calculations are performed using a common time step, commonly called the "dt". Under certain circumstances, the Finer Routing parameter can be used to perform calculations more frequently, but without reducing the project-wide time step. This is sometimes useful for resolving stability problems such as oscillations or overshoot while using fewer system resources than a smaller dt.

Important:  Before using Finer Routing values above 1, review any other warning messages in order to determine the cause of the instability.  Correcting the root cause (such as overfilled storage) is always preferable to "masking" the issue with Finer Routing.

Background

The availability of the finer routing increment depends on the routing method selected on the Settings|Calculation screen:

bulletWhen using a sequential routing method (such as Stor-Ind), the finer routing is set on the Advanced tab for each node. Values greater than 1 will reduce the time step for that node to dt/2, dt/3, dt/4, etc.
 
bulletWhen using a dynamic routing method (such as Dyn-Stor-Ind), the finer routing is set on the Time Span tab of the calculation settings, and applies to all nodes in the project. Values greater than 1 will cause the routing calculations to be repeated at each time step. Although this does not actually reduce the time step, it gives the model time to stabilize at each time step. This is especially useful when there is a tailwater dependency, which can cause an interaction between adjacent nodes.
 
bulletThe finer routing parameter is not available for a simultaneous routing procedure. If a smaller time step is required in these cases, the common dt must be directly reduced on the Settings|Calculation screen.

The default value of 1 causes each node to use the exact dt specified on the calculation settings screen. Values greater than 1 cause multiple calculations to be performed as described above.

When to use Finer Routing

The finer routing parameter should be used only in response to a specific warning message, such as an outflow overshoot or oscillation. Setting the parameter to 2 or 3 will often resolve these situation and eliminate the warning. Higher values are often no more effective, and may decrease the routing accuracy if they are not specifically required. The default value of 1 is recommended for all other cases.

 

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