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HydroCAD Stormwater Modeling - Since 1986

DSI Routing

Using the Dynamic Storage-Indication Method

What is the Dynamic Storage-Indication method?

Click for complete self-training materialsHydroCAD-7 introduces a new Dynamic Storage-Indication routing procedure, abbreviated as DSI.  By calculating all nodes at each time step (dynamically), this procedure allows each node to respond to upstream and downstream conditions, such as tailwater.  This contrasts with the traditional Storage-Indication method, which must be applied to one node at a time (sequentially), and therefore cannot account for variable downstream conditions.

When should I use the DSI procedure?

We recommend that initial modeling efforts employ the default Storage-Indication method.  If tailwater situations occur that cannot be handled with this routing procedure, HydroCAD will generate an appropriate warning message, suggesting that the DSI procedure may be used.

When is DSI not applicable?

The DSI procedure should be used only when required.  HydroCAD will issue a warning message in these cases.  Although DSI could be used in all cases where the traditional SI procedure is used (the results should be the same), it would use more computing resources without any real benefits.

Systems with a reversing flow may require use of the Sim-Route procedure.  (Sim-Route is required only when the flow actually changes direction at one or more nodes.  DSI is recommended for most tailwater situations in which the flow is always in the same direction.)

How do I set a fixed tailwater elevation with DSI?

When the DSI procedure is selected, the options for manual tailwater settings are disabled.  Instead, each pond determines it's tailwater conditions by examining the downstream nodes(s) at each time step.  However, you can use a link in order to enter a preset tailwater elevation.

How is the tailwater determined?

DSI calculations always start at the top of the watershed and progress downstream.  This ensures each node's inflow values are always up-to-date at each time step.  But since the tailwater is determined by downstream node(s) that have not yet been updated for the current time step, the tailwater must be determined by using the water surface elevation at the previous time step.  This causes an inherent one-step delay in the tailwater response, which can lead to oscillations if there are abrupt changes in the relative elevations of adjacent nodes.  This effect can be minimized by using a smaller time step, as described below.

What about initial water levels?

A pond with a pre-set starting elevation that exceeds its outlet device(s) will start to discharge immediately, unless the flow is reduced or prevented by a downstream tailwater elevation.  However, the inherent one-step tailwater delay (see above) may cause an initial increase (spike) in the discharge, until the routing has a chance to "catch up" with the tailwater.

Although most ponds are able to catch-up within a few time steps, smaller ponds may discharge a significant part of their volume before they stabilize.  This effect can be reduced by using a smaller time step, or eliminated by using the option to Anticipate initial tailwater on the Advanced tab of the Settings|Calculation screen.  This option was added in HydroCAD-8.5, and is automatically enabled whenever a tailwater-sensitive routing procedure is selected.  The option may be de-selected for backward compatibility with earlier versions.

What are "oscillations"?

Oscillations are rapid outflow fluctuations that can occur when the routing increment (dt) is too large for a particular reach or pond.  HydroCAD automatically checks each routing calculation to see if oscillations are present, and generates a warning message when required.  The presence of oscillations generally indicates that the routing results are not valid and should not be relied upon.

Why am I getting so many warning messages?

With a dynamic routing, an oscillation in one node will propagate downstream and upstream throughout the entire drainage network, often triggering additional warnings in other nodes.  Therefore you should resolve all oscillation warnings before attending to other warnings or results.  Correcting the source of the oscillations will often resolve many other warnings.

How do I correct oscillations?

Click here for details.

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