HydroCAD Stormwater Modeling - Since 1986

Frequently Asked Questions

    Also see the HydroCAD modeling tips and how-to page.
    For all printing issues, please see the Windows Printing Tips.
    If you have trouble starting HydroCAD, see the startup notes.
    Also read about subcatchments, reaches, ponds, pipes, and storm sewers.

How do I expand the capacity of my program?

The capacity of your HydroCAD program can be enlarged up to a maximum of 1000 nodes per diagram. You can place you upgrade order online or by phone, and generally have your expanded system within the hour. It's also possible to use a link to model a large project in several sections.

Note: If you exceed your node capacity and are unable to save your file, you can leave the file open while you upgrade your node capacity, so you won't lose your work!

The edit screen disappears when I click Apply!

The edit screen was probably hidden by the updated message window.  If the active edit window is hidden, the program may appear to be locked-up.  Press Alt-Tab to switch back to the edit screen.  To prevent this from reoccurring, move or resize the message window so it will not completely hide the edit windows.

Why can't I open my project?

First make sure the project is not in use by another program, or already open by HydroCAD.  You must also have full read/write access to the file.  (This means it cannot be on a read-only medium like a CD ROM.)   To check the file status, right-click the file and select "Properties."  Make sure the "read-only" box is not selected.

How do I model my site?

It all depends on the objectives of your HydroCAD model, and your local regulations.  However, we can offer some general how-to recommendations to keep you on the right track.

Can I do TR-55 studies with HydroCAD?

Yes.  Although HydroCAD does not have a specific TR-55 option, it is appropriate for any study that calls for TR-55.  When calculating the Tc, HydroCAD offers all the standard TR-55 procedures, plus several others.  For runoff, use the TR-20 runoff option, which provides greater accuracy than the TR-55 "Tabular Method."  (The Tabular Method is designed to approximate TR-20 results, upon which it is based.  For details see Understanding Hydrology and the TR-55 background information.

What about TR-20 studies?

HydroCAD implements most of the calculations employed in TR-20, and will give extremely close results (within 1%) given the same input data.  This is demonstrated by the "TR-20 Sample Project" which is installed with HydroCAD.  Details here.

Note that HydroCAD also provides a wide range of features that are not present in TR-20, such as Tc and hydraulics calculations.  For further details see the TR-20 background information.

Can I use the Modified Rational method with HydroCAD?

Yes.  Just select the "Rational" runoff method and set the desired rise and fall rates on the "Rainfall" tab.   Modified Rational method often uses a fall rate that is twice the Tc, although other rise/fall rates are also used.  Details here.

Why can't I route an upstream node through a subcatchment?

A subcatchment contains only the information needed to perform a runoff calculation. Use a reach if you want to route another hydrograph through this land area. For shallow overland flow, use a wide channel with a suitable Manning's number.  For details read about overland flow.

How do I add two hydrographs?

Hydrographs are automatically added whenever two or more outflows are routed to the same node.  If you want to sum two hydrographs without doing any further routing, create a reach, pond, or link with no description.  When the node is calculated the inflows will be added and passed through unchanged.

Click for complete self-training materialsWhy don't my peaks add up?

When hydrographs are added, HydroCAD adds the corresponding flow at each time step. The peak flows will add directly only if they occur at the exact same time. If the peaks occur at different times, the peak value will be somewhat less than the sum of the individual peaks. To verify that hydrographs are being added correctly you can compare the total inflow volume, which should always be the exact sum of the individual hydrograph volumes. To review the hydrograph summation, right-click the tabular hydrograph and select the inflow detail.  More info.

Why isn't the entire outflow of my pond being routed?

Make sure there isn't an un-routed primary or secondary discharge, which will appear as an outflow handle on the node. If so, you must route the outflow to another node or reconfigure the outlet devices to eliminate the outflow.

Can HydroCAD calculate the Hydraulic Grade Line (HGL) ?

Determining the HGL traditionally involves a steady-state analysis, with the entire drainage system at equilibrium. Since hydrograph routing models are handling a time-varying flow, there is no single HGL for the system. However, the peak elevation calculated at each node can be used as the effective HGL.  For details read about storm sewers.

How do I compare the existing and proposed conditions for my site?

First create and save the model for the existing conditions. Then use Project|SaveAs to save a copy of the project under a new name for modeling the proposed conditions.  To compare the existing and proposed conditions, you can use the node comparison report.

If your ultimate goal is to design a pond to maintain the pre-development discharge rate, record the pre-development peak discharge for later use when sizing the new pond.

How do I show the existing and proposed hydrographs on the same plot?

Open the model for the existing conditions (see above) and select the desired node.  Then open the model for the proposed conditions and select the node to be compared.  Click the "Comparison Report" icon on the toolbar to view the report.  You can also use this report to compare two or more nodes in the same project.  Just select the desired nodes and click the toolbar button.  Details here(The feature was added in HydroCAD 8.5)

How do I make a copy of an existing node?

To make an exact clone of any node, drag the node while holding down the Ctrl key.  With HydroCAD-8 (or later) you can also Copy the node to the clipboard and Paste it back to the routing diagram.  Both procedures will make an exact copy of the node, but with a new (unique) node number.

How do I define my rainfall events?

Open the Settings|Calculation screen and select the Rainfall tab.  Enter the appropriate rainfall values, including the rainfall type, storm depth, etc.  Then type the corresponding name (such as 25-year) in the "Rainfall Event" box and click the "Save" button.  Repeat to define each required event.  Once a given set of events are defined, you can use the "Import" button to automatically load them into another project.

How do I compare different rainfall events?

Due to the large volume of information associated with any given rainfall, HydroCAD retains only one set of results at a time. However, by defining a set of rainfall events, you can instantly select any event, and even print automatic reports for multiple events. (Using a separate "project" for each event is not recommended, since each project will have to be manually updated whenever changes are made.)

What is the best way to model a pipe?

A pipe can be modeled in four ways. 1) If the pipe always operates under open channel flow, you can model it as a separate pipe reach. 2) Some open-channel pipes are most easily modeled as a flow segment within a subcatchment. 3) The most complete solution for pipe flow is to model the pipe as a culvert outlet on a pond, even if the "pond" is simply a roadway impoundment, approach channel, or drop inlet.  4) If the pipe is used to store water, with the discharge controlled by another outlet device, it can be modeled as pipe storage or chamber storage within a pond.  For additional details see the full discussion of pipe modeling.

How do I model a catch basin?

A catch-basin is best modeled as a pond with a culvert outlet. If the catch-basin provides negligible storage, it can be modeled as a "zero-storage" pond. Or you can evaluate the detention effects by defining the pond storage, including any above-ground storage that is used when the basin overflows. (Always enter enough stage-storage data to prevent a "data exceeded" warning.) This will allow a more accurate culvert analysis to be performed on the pipe, including the effects of headwater and inlet losses. In some cases the Dynamic Storage-Indication routing procedure may be used to handle varying tailwater conditions.  More info.

What about a closed storm sewer?

A storm sewer can sometimes be modeled as a series of ponds with culvert outlets as described above, subject to the same tailwater considerations. Due to the computational problems of routing hydrographs through a closed storm sewer, these systems may have to be modeled with two separate tools, first performing a steady-state analysis of the closed drainage system, followed by a hydrograph routing to consider detention effects. However, the dynamic Storage-Indication method is now able to handle many of these situations.  More info.

What about gutter flow?

You can use a reach to model flow in a gutter.  The reach summary will show the maximum width and depth of flow.

What about grate bypass?

To model gutter flow that is (partially) intercepted by a grate, you can use a zero-storage pond with a grate outlet, plus a weir or channel outlet to model the bypass.  If you prefer to specify the grate capacity directly, you can use a link with an appropriate flow threshold and/or flow multiplier.

When should I use the Dynamic Storage-Indication Method?

The Dynamic Storage-Indication method is intended primarily for "coupled ponds," where one pond creates a tailwater that influences an upstream pond. Common applications include a set of catch basins, or a culverted road crossing with ponding occurring on both sides of the road. If there is no tailwater effect, or the tailwater is constant, the traditional storage-indication method is still recommended.

When should I use the "simultaneous" routing procedures?

The Sim-Route procedure is intended specifically for ponds with reversing flows. That is, flows that actually change direction during the course of the routing and require a reverse flow connection. For normal tailwater conditions where the flow does not reverse, use the Dynamic Storage-Indication method instead. The Sim-Route procedure may also be precluded for very small ponds (such as catch basins) that will not "track" properly, even with the smallest dt. Although the Sim-Route procedure may work with some catch basins, it is intended for larger ponds that have a significant storage volume in relation to the inflow hydrograph.

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