HydroCAD® Stormwater Modeling - Since 1986
This document is an ongoing archive of the "Tip of the month" as emailed to all HydroCAD customers.
Understanding the Time Step
The time step or "dt" is the time increment at which hydrographs are generated and routed. HydroCAD uses a default time step of 0.05 hours, which provides a good balance for most applications. However, this value can be adjusted to address special situations. Details here.
Modeling the Water Quality Volume
Many stormwater regulations include special requirements for handling of the Water Quality Volume, or WQV. Although the specific definition will vary, the WQV is commonly considered to be the runoff volume that includes 90% of all rainfall events in a given year. Since the majority of all rainfall typically occurs in relatively small events, managing the discharge of the WQV is considered to be a cost-effective standard for minimizing overall pollutant discharge. Details here.
Modeling Flow Diversions
If you want to divert a specific fraction of a hydrograph, or separate the flow that exceeds a certain threshold, you can use a link with the corresponding values on the "Advanced" tab. But most real-world diversions don't adhere to these simple mathematical rules. For more accurate results you should use a "pond" with separate outlet devices routed to Primary and Secondary. This will allow HydroCAD to accurately simulate the flow split based on the actual head experienced by each device. Details here.
The node comparison report is a frequently overlooked feature that makes it easy to compare the results for two or more nodes. Starting with HydroCAD 8.5, just select the nodes you want to compare and select View|Comparison Report. Use the drop-down list in the upper-right to select the specific parameter (inflow, outflow, etc) you want to compare. Starting with HydroCAD-10 build 7 you can also compare water surface elevations and pond volumes. The report works with any number of nodes. You can even compare nodes in separate files, such as existing and proposed conditions. Details here.
Using the Rainfall Lookup Tables
Starting with HydroCAD-10 build 14, separate lookup tables are provided for TP-40 and Atlas-14 rainfall data. These tables can be used to automatically define the rainfall events for most counties in the US. While the TP-40 table is limited to the earlier Type I, IA, II, and III rainfall distributions, the Atlas-14 table also supports the newer NOAA, NRCC, Nevada, and MSE rainfall distributions added in HydroCAD-10 build 14. This table makes it easy to use the latest Atlas-14 data without having to download site-specific data from NOAA or NRCC. To use the lookup tables, select the Rainfall tab of the Settings|Calculation screen and click "Import Events From...Lookup Table". For further details read about rainfall event setup.
Have you ever needed more decimal places for a certain parameter, such as the orifice diameter? HydroCAD makes it easy to adjust the precision of any of the data entry parameters. Just open the project you want to modify and select Settings|Units from the main menu. Click the Custom button to see a complete list of parameters. Scroll down to the parameter you want to adjust (e.g. "Diam") and select the check-box to open the editing screen. Now you can adjust the number of decimal places, or even change the maximum allowable value. Your changes apply only to the current project, and will stick with the project regardless of who uses it. For further details click the Help button on the units screen and read about HydroCAD units.
HydroCAD webinar recordings now available on YouTube!
Most of the 60-minute sessions focus on a specific topic, such as runoff calculations, outlet devices, dam breach, and underground storage. But there are also general sessions, such as "Getting Started with HydroCAD", which are among the most popular. Beginner or expert, there's something for everyone in each session. For details visit our webinars page or go straight to the Stormwater Modeling channel on YouTube.
Using Multiple Curve Numbers
Although it's common practice to combine multiple CN values and calculate the runoff based on the weighted average, this approach can produce incorrect results, especially when combining pervious and impervious surfaces. But HydroCAD offers several other procedures that can improve runoff accuracy, including an adjustment for unconnected impervious surfaces, and even a "weighted-Q" approach that calculates the runoff separately for each CN value and then combines the final flows. You'll find these options on the Advanced tab of the Settings|Calculation screen. For further information read about Curve Number issues.
Understanding the License Agreement
HydroCAD provides license pooling, a mechanism that allows you to share your license(s) between multiple computers at the registered location. There are also special provisions for off-network use, such as a laptop or portable computer. But each of these options works by the honor system, so it's your responsibility to read the license agreement and understand its terms before you deploy the software in your office or organization. Your cooperation allows us to avoid the use of a restrictive activation process, as employed by most programs.
Using local Precipitation Frequency Data with HydroCAD
Although most HydroCAD studies will continue to be performed with one of the standard rainfall distributions that are pre-installed with HydroCAD, you also have the option to generate a custom mass curve using local rainfall, which may be downloaded from NOAA or the new NRCC web site. Details here.
Modeling an elliptical or arch pipe
Starting with HydroCAD 9.0 you can use an arch or elliptical pipe for any culvert, pipe reach, Tc segment, or pipe storage. Consult the pipe manufacturer for the exact pipe width and height (span and rise). For an arch pipe you'll also need the top and bottom radii. For an elliptical pipe, if the top/bottom radius isn't available, use the up/down buttons to adjust the radius until you achieve the manufacturer's cross-sectional area. With HydroCAD-10 just select from the list of standard pipe sizes and all the dimensions are provided automatically. Details here.
Sharing your license over a network
The HydroCAD license agreement gives you the option to share each HydroCAD license between multiple PC's at the same location - as long as each license is only being used on one computer at a time. Just do a standard installation on each PC, open the Settings|Serial Number screen, and select a shared network folder to store the license information. For complete details read about HydroCAD installation.
Defining Rainfall Events
HydroCAD provides multiple options for defining your rainfall events, including a county-level lookup table that was added in HydroCAD 9.1. You can even download NOAA rainfall data and create local rainfall distributions with HydroCAD 10.0. For more information see the summary of rainfall options.
Modeling Float-Activated Flow Devices
Float-activated devices, such as gates and self-priming siphons, are sometimes modeled using a "pump" outlet. However, the preferred solution is to use the new Tube/Siphon/Float Valve option that was added in HydroCAD-10. This outlet option makes it easy to set the start and stop elevation for a siphon or float valve.
Changing the Storm Duration
Although most watershed studies are performed with a 24-hour rainfall distribution, some agencies require the analysis of low-depth events using a 6 or 12 hour event. Although HydroCAD has the ability to re-scale any rainfall to a different duration, it does this by stretching or compressing the entire storm, thus changing the peak intensity. While this is appropriate with some rainfall distributions (e.g. Huff distribution), the most commonly used Type I, IA, II, and III rainfalls should not be compressed. In most cases the correct approach is to extract the center portion of the 24-hour curve, thus maintaining the exact shape and peak intensity. To address this need we've developed 6 and 12 hour versions of the Type II and III rainfalls which can be downloaded here.
One of the most significant new features in HydroCAD-9 is the ability to perform detailed land-use analysis. This includes the option to estimate annual pollutant-loading based on the "Simple Method", or calculate single-event loading based on the actual runoff volume. Click here for details.
HydroCAD-9 provides a new pump capability that makes it easy to model lift stations and other pump applications. Just create a pond with a pump outlet, and enter the pump and discharge parameters. You can do limited pump modeling with earlier versions by using a special outlet device, but you'll need version 9 to model pump cycling and tailwater effects. Click here for details.
Diverging Device Flows
Compound outlet devices, such as a riser structure, generally consist of two or more inflows that converge and exit through a common culvert. This is easily modeled by setting the appropriate device routing for each device, as shown in this example. But what do you do if the flow splits inside the riser, such as a weir plate that also contains a low-flow opening? One solution is to use a compound outlet, which can be used to model any arbitrary weir or outlet shape, including a "device" with multiple openings. For details and examples look-up "custom weir/orifice" in HydroCAD help.
Importing Data Into HydroCAD
There are numerous options for importing data into HydroCAD, ranging from basic hydrograph information to complete project data. And we're working with our partners to provide better communication with site design software. Details here.
Modeling Porous Pavement
Porous pavement can provide substantial benefits for stormwater management, but how do you model it in HydroCAD? There are several issues and options you need to consider, including runoff effects, infiltration, and even detention in the base material. Details here.
Modeling Rain Gardens
Rain gardens, or bioretention cells, can be modeled in HydroCAD as a small "pond". The key is to properly identify the storage volume(s) and outlet control(s) that will be included in the model. Details here.
Modeling embedded storage chambers is easy with the HydroCAD chamber wizard. Just select the chamber model and the number of chambers, and HydroCAD will automatically size the field with the required stone envelope. Or you can define the overall drainage field by hand, and then embedded chambers in the stone. Complete details here.
Depending on the flow conditions, you can model check-dams by using a reach with a higher Manning's value, or with a series of ponds. Details here.
Compare existing vs. proposed conditions
With HydroCAD 8.5 or later you can get an instant graphical comparison of any two or more nodes. Start by selecting the first node (with a single click) in the existing analysis. Then switch to the proposed model and select the corresponding node to be compared. Now click the "Comparison Report" icon in the toolbar to get a graphical comparison of the nodes! The report can also be used with multiple nodes in the same project. For further details, click the Help button on the report. Details here.
Define your own Rainfall Events
Defining the rainfall events for each project makes it easy to switch between different rainfalls, such as the 10-year and 25-year events. It also lets you include multiple events in a single printed report. To define your events, open the Calculation Settings screen and select the Rainfall tab. For complete details click the Help button on that screen, or read more about rainfall and runoff issues.
Setting a custom report size
Report windows can be enlarged for better viewing by dragging the window border. To make all windows automatically appear at your preferred size, right-click the report and select Window Size | Save... from the context menu. You can set the same size for all node types (subcat, reach, etc.) or you can set a different size for each type of node.
In addition to the Export button on each report window, HydroCAD provides an auto-export capability that lets you automatically export data for an entire project. Just select the desired options on the Settings|Export screen to create a set of spreadsheets with full details on the entire project. More info.
How do I model my project?
Due to the wide variation in design standards and modeling objectives, it's difficult to offer simple answers or one-size-fits all examples. However, we do have some general guidelines that can help you determine the best way to model each project.
Warning 93, "Pond storage range exceeded", means that the program doesn't have enough storage information to perform an accurate routing. The problem is easily fixed by adding another stage to the typical stage-area table. You don't have to make the pond "bigger", you just need to describe what will actually happen as the water level rises. Details here.
Modeling a storm sewer
When modeling a storm sewer, each catch basin is generally modeled as a "pond", with a culvert outlet that is routed directly to the next catch basin. Don't use a reach, which is intended only for open-channel flow. For details read about pipe modeling and storm sewers.
Compound outlet devices
Complex outlet structures, like a riser, are easily modeled with a combination of basic devices, such as a weir, orifice, and culvert. Since this is the most common support question, we've prepared illustrated step-by-step instructions to help you through the process.
Complex pond storage
Starting with HydroCAD 7.0, each pond can contain multiple volume definitions. This makes it easier to define complex storage arrangements, such as a single pond with underground storage and an above-grade overflow area. HydroCAD can even handle calculations for storage chambers in a crushed stone bed. For details read about pond storage.
Share your work!
The best way to let other people explore your HydroCAD analysis is to send them a copy of the .HCP project file by email. If the recipient doesn't already own HydroCAD, they can open the file using a copy of the free HydroCAD Sampler.
Automatic project backup
HydroCAD 7.1 includes an automatic backup capability that helps to protect against power failures and other computer problems. To enable automatic backups, open the Settings|General screen and set the desired backup interval. For further details, click the Help button on the Settings|General screen.
Using the "Shrink" button
Tabular hydrographs are normally "shrunk" so they take less space, but you can pop up the "Shrink" button to see more values. Bold highlighting marks the maximum value, which will be somewhat lower than the interpolated peak shown on most reports.
Use Ctrl-Drag to clone a node.
If you hold down the control key while dragging a node, HydroCAD will make an exact copy of the node! This can be a great time-saver when creating similar nodes, such as subcatchments that share the same curve numbers. You'll find other handy shortcuts like this in the HydroCAD Tutorial. Or look under "Mouse" in the HydroCAD Help Index.
Use the Summary Report to verify your input!
Each node offers a summary report which includes all your input data, plus a summary of the results. You should always use the summary (rather than the input screens) to verify your entries. In addition to showing all your input in one place, the summary shows how your input has been employed in the calculations.
Document your reports!
Self-explanatory reports are more easily understood and approved. Node notes (added in HydroCAD 7.0) let you provide any additional details your reader may need. HydroCAD 7.1 also added general project notes which can be entered on the Project|Notes screen. You should also use descriptive names for individual outlet devices, flow segments, and nodes. Including this information within your HydroCAD project minimizes the need for supporting documents, and speeds up any review process.
Each HydroCAD summary and hydrograph plot lists the peak flow(s) attained during the routing. This is an interpolated value, and is generally somewhat higher than the maximum flow shown in the tabular hydrograph. Although interpolated peaks are reported for most parameters, some reports (such as the device flow breakdown) use non-interpolated maximum values, which may be slightly lower that the corresponding peak value. For details read about peak flow calculations.
Use the simplest routing method that does the job!
The default Storage-Indication method is recommended for most projects. You can switch to the Dynamic Storage-Indication method (added in HydroCAD-7) if you need to handle variable tailwater effects. But the Sim-Route procedure should be used only for the very few cases that involve flow loops or reversing flows. Otherwise, stick with SI or DSI, which are faster and more stable. In general, start with the default SI setting and change the method only if you encounter a warning message that specifically recommends a change.
Routing through a subcatchment.
Subcatchments are used only to generate runoff hydrographs. But you can use a reach if you need to route a separate hydrograph through that portion of the watershed. In some cases you may even have a chain of reaches, with subcatchments and other inflows being added at the junctions.
Sending HydroCAD reports by Email
To email a HydroCAD report, create a PDF file by "printing" to a special printer driver called a PDF Writer. Or you can send your actual HydroCAD project file, which the recipient can open with our free HydroCAD Sampler.
When adding multiple inflows, the hydrograph volumes will add directly, but the peak flows will add up only if they occur at the same time! When the peaks occur at different times, the resulting peak may be much less than the sum of the individual peaks. In some designs, peaks are deliberately sequenced in order to minimize the final peak outflow. Of course, this does nothing to reduce the discharge volume, and downstream flooding may still be induced by the broader peak of a post-development hydrograph. For details read about peak flow calculations.
Using the Apply button
Many HydroCAD forms have an Apply button, which lets you implement new values on that form, while keeping the form open for successive changes. To see this in action, open one (or several) node reports, edit any of the nodes, and use the Apply button to update everything! When making a single change, you don't need to use the Apply button at all - Just click OK.
Zoom in on any graph
To see greater detail on any graph, drag the mouse pointer over the region of interest. Start by clicking on the upper-left corner of the desired area. Now drag the pointer to the lower-right corner and release the button. The graph is instantly re-drawn! You can repeat the process several times to get an even closer view. To restore the original view, just drag in the opposite direction - from lower-right to upper left.
If you have trouble zooming to the exact area you want (or if you get no zoom at all), switch to a 2D view before you zoom. This makes it much easier to select the precise region of interest.
Click on any message for details!
Whenever calculations are performed, HydroCAD generates a number of calculation messages, which appear in the message window. For full details on any message, including links to related topics, just click on any line in the message window. Calculation messages (particularly warnings) contain important information and should not be ignored! Starting in HydroCAD 7.1, you can also click on any messages at the top of the summary report for each node.
How to edit a project file directly
In most situation it is easier (and safer) to edit your projects using HydroCAD itself. However, there may be situations (such as changing the n value for all your culverts) where it can be faster to edit the project file directly.
To edit a project directly, right-click the HydroCAD file and select "Edit" from the menu. This will open the file in Notepad. Before you make any changes, spend a few minutes browsing through the file to get familiar with the format. For changes to individual nodes, you'll want to locate the appropriate [node] section, and then the specific line, such as "n=0.0.12" After making the desired changes, save the file and close the editor. You can then click (or double-click) the modified file to open it in HydroCAD.
How to plot pond elevation vs. time
Many HydroCAD plots contain hidden data which can be selected by right-clicking the graph itself. For example, most hydrograph plots can also display elevation vs. time. Just right-click the graph to select the curves you want to view. You can also right-click most tables to view or hide specific columns.
Don't close that window!
You can change the rainfall settings or edit any node without closing your current report windows. Just leave the window(s) open as you make changes to the project, and the reports will be automatically updated. This feature lets you monitor any critical nodes (such as a detention pond) while you make changes to other parts of the system. If you work with several projects open at once, you'll also find that the report windows are automatically minimized and restored as you switch between projects.
Working with multiple nodes
Did you know you can perform many operations on several nodes at once? Use control-click to select each node, or drag a selection rectangle over several nodes. To move the entire group just drag any selected node. Or right-click any selected node for other options, such as opening multiple report windows! For further details see "Mouse" in HydroCAD help.
Email your HydroCAD projects to anyone!
Emailing most projects is just a matter of right-clicking the project and selecting "send to email recipient." If the recipient doesn't own a full HydroCAD program, they can still open the project by installing a free HydroCAD Sampler. For further information please read about sending and receiving HydroCAD projects.
Change the default report size
Each time you open a report window, HydroCAD sets the window to a specific size. If you consistently find the default size to be too small, you can adjust the default. For HydroCAD 7.1 and later, resize the window as desired, then right-click the window and select Window Size | Save Current Size. (For earlier versions, you can set the default size on the Settings|General screen. For a slightly larger report, try a setting of 500x300. To restore the default report size, set both dimensions to zero.)
Set your own project defaults
When you create a new project, HydroCAD uses preset default values for all
the calculation parameters, such as a Type II storm. If your projects tend to
differ from the factory defaults, now you can customize your defaults: Just
create a new project called "Default" and change the settings to suit
your needs. Now save and close the project. Whenever you create a new project in
the same folder, it will automatically inherit your "Default"
settings! For further details see Section 9 the HydroCAD Owner's Manual, or
see "Default Settings" in the HydroCAD help index.
Also see the Windows Printing Tips.
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