This document is an ongoing archive of the "Tip
of the month" as emailed to all HydroCAD customers.
Also see the HydroCAD FAQ's and
Looking for the latest rainfall data?
As states gradually migrate to the latest data, it can be
confusing to navigate the growing library of rainfall distributions that are
included with HydroCAD. Although you can always setup your events by hand, the
easiest solution is to click "Import Events From...Lookup Table". Make sure the
lookup file is set to NRCS-Rain. Then select your state and county to
automatically define the rainfall events for your location. This process
includes the latest NOAA, NRCC, and MSE distributions, which have superseded the
Type I, IA, II, and III distributions for most of the US. For further
details read about the event lookup tables.
Using a minimum Tc value
If your local regulations prescribe a minimum Tc value, you can
specify this value on the Calculation Settings screen. This allows you to
calculate the "true" Tc value for each subcatchment, and the value will be
automatically increased to the minimum as required. However, a minimum Tc value
is not required by HydroCAD itself, which will accept a Tc down to zero.
For details read about the time-of-concentration.
Increasing the node diameter or "symbol size"
The Symbol Size (on the Settings>Diagram
screen) sets the diameter of all the nodes in the routing diagram. Increasing
the symbol size reduces the space between nodes and makes the diagram more
readable. But if any of the nodes are too close together the symbol size will be
limited (and even reduced) to prevent them from overlapping. In order to
increase the symbol size beyond this limit you have to move the closest nodes
further apart. Starting with HydroCAD 10.0 build 24, you can click the
"Details" button (on the Settings>Diagram screen)
to automatically locate these closest nodes that are creating the limit.
For further details read about the symbol size.
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.
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 training videos 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 video
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.
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.
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
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, this is done 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. For other rainfall
distributions you should use the "trim" option
added in HydroCAD 10.1
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.
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.
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.
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
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.
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.
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
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.
What is the peak flow?
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
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
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.
Peaks don't add up?
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
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
Note: Since improper edits can damage a file,
you may want to make a backup copy first. Just right-drag the project
file (which has an .hcp extension) and select "Copy" from the menu.
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
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
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 "Default Settings" in the HydroCAD help index.
Also see the Windows Printing Tips.