How to Add Outflow Devices

This step completes the stage – storage – discharge relationship you’ll ultimately use to route hydrographs through detention ponds. Here, you’ll specify the outlets which in turn generate discharge values corresponding to the stage values entered in the previous step.

You’ll add outlet devices, one-by-one, and each will be evaluated starting from the bottom of the pond up to the top (“Top of Pond”). Make sure any overflow weirs (spillways) have their crest elevations set below the “Top of Pond” or they will be missed during the stage-discharge calculations.

Hydrology Studio has several unique outlet structures to choose from. They can be set up as independent structures or can be put in series for multi-stage configurations.

Structure Options

Below are descriptions of each type of outlet device you can add to your pond.

Click the Help radio button to view a diagram to assist in data entry

Primary Culvert
Can be circular or rectangular with multiple barrels and may include a restrictor plate.

Restrictor plates can be a rectangular plate at a specified position (invert) from the Culvert’s invert or a circular orifice with any diameter that is less than the culvert’s Rise.

Up to three, circular or rectangular. Can flow through the Primary Culvert (typical) or act independently.

Riser Structure
Used for multi-stage structures. Can act like a weir. Can be circular, rectangular or sloped. Is assumed to be connected to the Primary Culvert at its upstream end.

If you’re modeling a Riser that has a grate on top, use the Sloped shape and set the upper and lower elevations the same so that it’s flat on top. The software automatically takes into account the grate during calculations so you will not need to make any adjustments for bar widths. Please see Computational Methods for more information on sloped risers.

You do not need to add a Riser when working with underground detention ponds. One will automatically be added for you.

Up to three. Can be Rectangular, Cipoletti, V-notch, Broad Crested w/ side slopes or Compound. Can flow through the Primary Culvert or act independently as a spillway.

Perforated Riser
A stand-alone riser with perforations. Converts to an orifice plate when a Riser is present. Not to be confused as a restrictor plate for the culvert.

A.K.A. Infiltration. Water exiting the pond as a rate which is applied to the pond’s contours. Can automatically be extracted from routed outflow hydrograph.

Directly enter known discharges at each stage increment.

Doesn’t contribute to, but can affect, outflow. A single tailwater elevation can be specified. Note that this tailwater elevation does not carry over to the final pond routing back at the Basin Model on the Main Window. You will have an opportunity there to specify tailwater elevations for each return period.

In computing the outflows, Hydrology Studio treats the discharges from these outlet structures as a function of stage or water surface elevation. Partial and full flow conditions are computed as well as inlet and outlet control, and submergence.

About Multi-Stage Configurations

Unless the multi-stage option ( “Flows through Culvert”) is checked On, each outlet structure is treated independently. The multi-stage option puts the structure(s) in series directing the outflows from that device to flow through the Primary Culvert.

The structure with the least capacity at any given stage will control the final outflow. The Culvert is always the final outflow device so it does not have a multi-stage option. Additionally, Riser structures are automatically set to be a multi-stage device as they are assumed to be connected to the Culvert.

The structure shown below contains a Culvert with a Riser, an Orifice and a secondary Weir. The Weir and Circular Orifice (Rise and Span are the same) are set as multi-stage (“Flows through Culvert”).

When using the Multi-Stage option, Hydrology Studio checks the head elevation produced by the Culvert. This head is then used as a tailwater elevation against other multi-stage devices. As this head increases, the outflows from the orifice(s) and weir(s) decrease. When this head equals the current stage, the Culvert becomes the controlling structure and contributing flows from the orifice(s) and weir(s) diminish.

Notice the Orifice at bottom of Riser is totally submerged and no longer contributes to outflow.

Adding Outlet Devices

You may add any combination of outlet devices to your pond but you may have only one of each. Remember however that you can add up to three weirs, three, multi-holed, orifices and other devices in a single pond. Outlet devices can be added in any order you wish but we recommend adding them starting from the most downstream device and working upstream.

To add a device, click on the button corresponding to the device. For example, click the Culvert button to add a culvert. Then add the required data, and then click the [Add/Update] button. Once added, the header or device name will be shown in bold. For example, a Culvert, Riser, Orifice and a Weir have been added to a pond as shown in the screen below. Notice their header labels are in bold.

The output table and charts on the right will be updated with the new stage-discharge data. To see diagrams of each of these devices, click on the Help option button.

You can deactivate any structure by turning Off the Active check box. This preserves the structure’s input data but removes it from the stage-discharge calculations.

Viewing the Stage – Discharge Results

After adding your devices, you will be able to view the results in a variety of ways by selecting a radio button at the top of the Canvas.


The Table displays the Stage-Discharge results in a numeric format. Each Stage increment from Step 2, where you added your pond’s physical data, is divided into 10 intermediate increments for more detail. The outflows from each structure are shown in their individual columns. The Total Q is the sum of the individual structures. If, however, the secondary devices are multi-stage, then the Total Q will be either their sum total or the flow from the Primary Culvert, whichever is in control.

The Drain Time indicates the time it takes to drain the pond from the corresponding elevation in the pond down to empty (Stage zero).

The Culvert column also includes the culvert’s velocity

Structure Chart

Hydrology Studio will create a quasi-schematic detailed drawing of your structure(s) so that you can better visualize the final design. You’ll have the option to view this schematic in Front, Side and Plan views by clicking on the links at the bottom of the image. It’s interactive and you can click (or double-click) on any device to edit it.

In addition, you can click on the Expander at the bottom to pop up a table which shows the numerical results. Click on any of the tables rows to plot the corresponding water surface.

Frontal view of structure schematic showing riser with orifices.


Side view. Notice the elevation lines corresponding to the Target Q and Actual Q. Target Q was developed in Step 1. The Actual was developed here based on your outlet devices. Adjust your devices to where these two lines match in elevation.

Export the Structure to an image file

Save this drawing to an image file by right-clicking on it and selecting, “Export this chart..”.

Plan / Profile Drawings

This option will draw your entire detention pond including the outlet structures in Plan and Profile views. For the pond’s contours, it draws from the input data used back in Step 2 – Create Pond. This option is available for ponds using either Contours or Trapezoidal shape only. When using contours, it assumes the pond is a box shape.

Note that this plot is fully interactive in that you can click on a device to edit it. This drawing is also available back at the main window on the Charts tab.

This plan shows the computed 10-year routed water surface. Click the Profile text link at the bottom of the plot to view this in profile.

Stage-Discharge Chart

You can view your outlet data at any time by selecting the Stage vs. Q option button on the top of the output table. The outflows from the individual structures can be toggled on or off as well as the Structure HG, Total Q and Target Qs. The Stage-Discharge chart below shows this multi-stage device with all structures turned On.

When using the multi-stage option, Hydrology Studio checks the head elevation produced by the Culvert. This head is then used as a tailwater elevation against other multi-stage devices. As this head increases, the outflows from the orifice(s) and weir(s) decrease. When this head equals the current stage, the Culvert becomes the controlling structure and contributing flows from the orifice’s and weirs diminish. The chart above explains this in graphic detail.

Notice how the flows from the Orifice and Compound Weir diminish around elevation 107.5. This indicates that the Riser is filling up due to backwater head produced by the Culvert. The light blue line is the Structure HG and indicates the depth of water inside of the Riser. When this line merges with the Total Q line, the Culvert is the controlling structure. This occurs at approximately elevation 108.

Learn more about these charts by reading this blog post : Chart Tales, The stage-discharge curve explained

Stage vs. Drain Time Chart

This chart provides an excellent visual guide of your pond’s drain time. The drain time is the time it takes for the pond to completely empty itself from a given elevation. The chart below shows the Full Spectrum volumes inputted at Step 1. It takes 2.8 to 2.9 hours for the Channel Protection Volume (CPv) to drain out. Your goal is to make adjustments to your outlet devices until the desired drain time is achieved.

Editing Outlet Devices

Edit any existing outlet device by selecting it from the Outlet Device Selector. Alternatively, click on the device shown on the Schematic plot. You can permanently remove any device by selecting it and clicking the [Trash] button.

Using the Interactive Tool

This is a very productive method used to fine tune your outlet structures. This tool contains buttons as shown below and basically allows you to edit most of the input items without having to manually type and re-type their values. And with the [Auto] button checked On, the edits will be applied automatically. You’ll see instant results on your output table, schematic and stage-discharge curve.

Here’s how it works: Put your cursor on any input item, for example, Orifice Diameter. Then click on either the [Arrow Up] or [Arrow Down] buttons. This will increase or decrease the Orifice Rise.

The default increment is typically 0.01 ft. Holding the [Shift] key down increases the increment to 0.1 ft. Holding the [Ctrl] key down makes the increment 1.

Note that Culvert sizes will automatically use commercially available sizes. Certain inputs are not adjustable using the Interactive Tool. For example, Culvert Length or Weir Coefficients. In these cases, the Interactive tool will show to be disabled.


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