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HydroCAD Stormwater Modeling - Since 1986

Outlet Devices
Weirs Culverts Vortex Valves Dam Breach Pumps Skimmers Exfiltration Special Outlet

Outlet Device Options

HydroCAD provides a wide range of basic pond outlet devices, including orifice flow, weirs, culverts, tubes, and siphons.  These may be used individually to model simple outlets, or combined in various arrangements to model more complex outlets, such as a riser structure or notched weir.  HydroCAD can also be used to model active outlet devices, such as pumps, or time-varying outlets such as a dam breach.  Each pond can have an unlimited number of outlet devices.

Many new outlet devices were added in HydroCAD-10, including tubes, siphons, asymmetrical weirs, constant-flow devices (such as floating weirs), and even a progressive dam breach.  Update details here.

For devices than cannot be modeled with one of the standard outlet devices listed above, a special outlet can be used to define a custom stage-discharge curve.  HydroCAD also includes a library of predefined rating curves for certain proprietary devices, allowing them to be easily modeled with a special outlet.  Details here.

Click for complete self-training materialsOutlet Calculations

Complete details of the hydraulics calculations are provided in the HydroCAD Owner's Manual, as well as the built-in Help system.  Just click the help button on the outlet selection screen, or on a specific outlet device.

Outlet Device Routing

The flow from each outlet device is routed to a specific node outflow.  By default, most devices are routed to primary.  This causes their flow to be directed to the primary outflow arrow on the routing diagram.  If desired, individual devices can also be routed to secondary or tertiary.  This causes their flow to be calculated and reported separately, and makes it available as a separate outflow on the routing diagram.

Any number of devices can be routed to each outflow.  When several devices are routed to the same outflow, their individual flows are combined and routed together.

Flows that are not subject to further routing (such as exfiltration), are usually routed to the discarded outflow.  The device routing can also be used to construct compound outlet structures, such as a riser.

Outlet Device Reporting

The summary report for each pond includes a graphical representation of the outlet devices, plus an analysis of the maximum flow through each device.

1) Each discharge (primary, secondary, discarded) is labeled with the following information:

Maximum flow rate (non-interpolated)
Time of maximum flow
Headwater elevation at time of maximum flow
Tailwater elevation at time of maximum flow
Tailwater settings

The maximum flow is calculated directly from the discharge equations at the listed HW and TW elevations.  This value may be slightly less than the interpolated peak flow that appears a the top of the summary.  This difference can often be reduced by using a smaller time step, but some difference is expected due to the different calculation procedure.  If desired, the devices flows may be removed from the report by using the summary report context menu.  (Requires HydroCAD-8.5 or later.)

2) Each device (culvert, weir, etc.) includes one of the following messages describing its role in determining the maximum flow through its designated discharge point:

Controls x.xx cfs @ x.x fps - The device is acting as a control point, and is restricting the discharge to the specified flow. This does not necessarily mean that the device is flowing full, but that under the current conditions, its flow is more restrictive than other devices to which it may be connected. The message also indicates the approximate discharge velocity corresponding to this flow, and the governing discharge equation.

Passes xx.xx cfs of xx.xx cfs potential flow - The device is passing a specified flow that is less than its potential flow for the headwater/tailwater conditions at that time. The device is not a control point. The maximum flow is being controlled by other device(s).

Passes < xx.xx cfs potential flow - The device is passing a flow that is less than its potential flow, but the flow could not be determined, since it is apportioned between two or more devices. The device is not a control point. The maximum flow is being controlled by another device to which this device contributes a portion of the total flow.

Since the flows (and control points) are recalculated at each time step, these values can be expected to vary throughout the routing.  The flow analysis shows only the conditions at the time of maximum discharge.  Furthermore, when using a dynamic tailwater procedure, the maximum flow may occur at a different time for the primary and secondary discharge.

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