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Modeling a Flow Diversion

HydroCAD provides two basic mechanisms for modeling a flow diversion, or flow split.  If the flows will be split according to a known ratio or threshold, a link provides the most direct solution.  But if the flow is being split with a configuration of hydraulic structures (such as a weir and a culvert) the best approach is to use a pond with the appropriate outlet devices.

Splitting a flow with a link

To divert all the flow exceeding a certain value, create a link and set the flow threshold.  All flows up to this value will appear as the primary outflow, and any excess will appear as the secondary outflow.  This will produce a primary outflow with a "flat top", and a secondary outflow containing the "clipped" portion.

To divert the inflow according to a pre-determined ratio, set the discharge multiplier according to the fraction of flow to appear in the primary outflow.  The remainder of the flow will appear as the secondary outflow.

Splitting a flow with a pond

A pond can be used to split a hydrograph according to the specific hydraulic characteristics of two (or more) outlet devices.  Common examples would include a splitter box with two outlet pipes, or a weir diversion structure.

If you wish to consider the detention effects in the diversion structure, select "Detention Pond" and enter the storage characteristics.  Otherwise, use the "zero storage" option to neglect the storage effects.

On the Outlets tab, enter each of the outlet devices, setting the appropriate device routing.  For example, a splitter box feeding two pipes would have two culvert outlets.  (Note that the pipe sizes do not have to be the same.)  One pipe would be routed to primary, and the other to secondary.

If the flow is being split between a pipe and a weir, enter a culvert and a weir outlet, with the weir routing set to secondary.  If the weir flow is exiting the structure through another pipe, you could model the system with these outlets:

Device#1 = Culvert, Routing=Primary      (the low-flow outlet)
Device#2 = Culvert, Routing=Secondary  (the high-flow-outlet)
Device#3 = Weir, Routing=Device#2        (the diversion weir, discharging to the second pipe) 

This configuration uses a compound outlet, where one device is routed through another, rather than flowing directly to the secondary outflow.  For details on compound outlet setup see this pond example.

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