The DREN-URBA’S 2.0 Surface Drainage TAB

Our criterion, with regard to the urban drainage system design, is that a proper determination of the location and flow intercepted by drainage inlets is made.

To do this, in DREN-URBA’S first version, we created the “street” object, that acts as a channel with hydraulic characteristics, and that, under certain conditions and properties, allows the designer to know what the characteristics of the respective flood were (e.g., spread, water depth, etc.).

Thus, it was possible to generate a “streets network,” acting precisely as a system of channels ending towards one point of discharge. So the software user could establish, bearing in mind the above mentioned flood values at every street of this network, the Drainage Inlets location that guarantees it will not exceed allowable flood spread, generally associated with the street’s importance inside the sector under study.

In the new version of our stormwater drainage design software, we have further improved the concept of this street network, as we have now added the ability for each “street” (or channel) to have one of five cross-section types (in the first version the typical section was only one).

Additionally, it is now possible to have multiple street and channel networks, so that you can model the whole urban sector for which you are performing the drainage system design.

In this tutorial, you will learn about the tools included in the SURFACE DRAINAGE tab of DREN-URBA version 2.0, which will help you to create the surface system as well as to make the allocation of stormwater drainage inlets. These tools are everything you need for the proper design of the surface system in urban drainage.


The Surface Drainage tab gives you the tools to undertake the hydraulic parameters calculation of what we have called the “streets network” since the initial release of the program.

Remember that DREN-URBA’S approach to the design and calculation of an urban stormwater drainage system is to represent, under steady flow conditions, the flow in the streets and avenues as if they were water channels. This makes it possible, as part of the design, to estimate parameters such as the flood spread that occurs on them under certain rainfall.

The main difference from the first version, in this approach, is that we have now extended the street object to be a water channel, in the strict sense of the word, and which can have one of five cross sections as you will see in the Cross Sections Manager at the Settings tab.

Thus, to start the urban drainage system calculation in the software, and on the understanding that you are interested in studying the hydrologic and hydraulic behavior of the streets, avenues, or channels system in the surface part of that system, you must create a Surface System by pressing this button.

Here you must specify a unique name for this system. You also have the option to enter a description and, to automatically generate the labels of the endpoints of each stretch or channel to be created when you start drawing, specify a prefix and an initial counter.

Once created, you will see it in this list so that you can set it as active and be able to create and edit their characteristic components in addition to performing the respective calculation.

Here are several aspects to highlight:

  • Each system must have a unique discharge or outflow point, which you must set by selecting the system’s furthest downstream point and activating the Discharge Point Option in its properties dialog.
  • Note that here is the option to visualize only those objects that make up the current Surface System. This will avoid confusion while you’re creating several stormwater surface drainage systems in the same project.
  • When multiple surface systems exist in a project, you can only select street sections and endpoints for the one that appears as “Current” at the System panel. For example, in this case, since the Surface System 2 is the current one, I can only select the streets that have been created within it.

You can delete the current system by pressing this button, also eliminating, of course, all the objects that conform it.

And, if you want to change the name or prefix for the current surface system, you can use this button to display its Properties dialog.

Then, at the streets and channels panel, you’ll find the tools to create them as well as to arrange the street section position in the current system at the respective data table.

Similar to any hydraulic network, the surface drainage system is modeled via nodes, which in this case we named street or channel points, as well as sections that, as you know, we called streets or channels.

So you can, using this button, create in the drawing area street points that can then become the start and end points of the street sections to be created by clicking this other button.

In case the Create Streets section command does not find a street point in the drawing area, it will create it; thus, most often, this is the command you will use in order to create the topology of the surface urban drainage system.

In this case, the project is set to draw objects in an upstream direction, as you will see in the Settings tab, General button.

This means that, when creating a street or channel section in the drawing area, the first click that you do will create the section’s downstream point, and the second click is the upstream one. That is, you go from the discharge point to the surface drainage system’s endpoints.

In cases when the downstream drawing direction option is active, you can have the opposite effect, in which you go from the most upstream points towards the system’s or section´s discharge  point.

If you miss defining the flow direction of any drawn street section, you have the option, with this button, to swap its endpoints when you click on it.

Note that here we also have a selector, in this case for you to set the current cross section, from those you have previously created in the project, and which will be the one assigned to each new section you draw…

Also, in the Streets Point properties dialog, there is a button that lets you activate for it the automatic elevation assignment from the elevation points you have created in your project. Of course, it is also possible to globally allocate their elevations using the same option on the DRAWING tab.

Finally, in this panel are the commands that allow you to order, if you want, the rows in the surface drainage’s streets or channels table.

With these two buttons, it is possible to move the current row upwards or downwards.

And, with this button, you can sort the rows according to the respective street’s order; that is, the first order streets are those that have not affluent streets so they will be located in the first table’s rows; also its label column cell will be grayed and; in accordance street sections have one or more affluent, its order will be greater and will appear below the previous ones. Of course, under this scheme, the last street section of the table will be the one that is connected to the surface drainage system’s discharge point, as it is what receives, so to speak, all the other sections.

Also, you have the button that lets you create one of the most important components in any drainage system design project or calculation: the sub basin.

Remember that, in the case of flows generated from any rainfall, you need to have defined the tributary areas on which the transformation of rain into runoff takes place.

So, when you press the button, the option for you to create on the drawing area a closed polygon is activated, so as to model a hydrologic sub basin which will later be assigned to a street, channel or manhole, as we shall see in a moment.

Note that the properties to be specified at each sub basin, besides its area value, which in this case is disabled because it is calculated from the polygon’s vertices in the drawing area, are aimed at obtaining the necessary parameters to determine the runoff flow rate through the rational method.

That is, you must enter the sub basin’s runoff coefficient, for which you can use as a help the table with reference values that appears by clicking here.

You must also specify the sub basin’s time of concentration, and it is possible, in this new version, to choose from three options:

  • Directly enter the value of the time of concentration, in which case, click here and enter the value in the text box.
  • Let the software calculate it, using the Runoff velocity criterion, in which case you should activate this option and enter the flow length, the hillsides mean slope and select from three possible ground coverages.
  • You can also, with this last option, let Kirpich’s formula be used to calculate the time of concentration, which is used mainly in rural type basins. Here you must specify the length of the sub basin’s main channel and its mean slope in percent.

Note that, with the latter two options, the dialog shows the calculated value for each method.

By default, each area that you create in the project will be automatically assigned by the software to the nearest street or manhole, depending on the option you have specified in the project’s settings.

See that, after the calculation is performed, assignment lines are created in the drawing to tell you to which street’s section, in this case, the sub basin is associated.

Likewise, when you check the street section’s properties, you’ll see it appears as one of the assigned sub basins.

Since we are here, it is convenient for you to know that it is possible to create and assign to any street’s section or manhole a user-defined sub basin from this dialog box.

When you press the button, the known sub basin’s properties dialog is shown, but note that now the option for you to enter its area is enabled. This is a way to create sub basins without having to draw them, so keep that in mind.

Now, returning to the drawn sub basins, note that it is possible to override the automatic allocation from the Properties dialog.

Activating this box, it is possible that you pick the object to which this sub basin will be assigned. To do this, select from the list the surface system or sewer network to which it belongs, and then the label that identifies it.

When you click OK and perform the calculation, you will see that the assignment line changes …

This will remain so until you come back to the sub basin properties dialog and modify this condition.

Keep in mind that when creating polygons in the drawing area, the option of acquiring the other polygons’ vertices position is available by default, so as to make its drawing more precise.

If you do not want this option to be active when drawing, hold down the Control key before clicking on the desired vertex position, and you’ll see that the blue box does not appear.

Additionally, you will see that, when you select one of these polygons and click the mouse’s right button, the contextual menu appears, but with additional options to add, delete, or move the polygon’s vertices. This way you can adjust to your preferences within the drawing area in which you perform the drainage system design.

Note that, in any moment, messages are shown in the status bar so as to guide you through the process.

The next panel you’ll find in this tab is the one that allows you to select and add drainage inlets to the surface drainage system you are studying.

The idea of including inlets in the drawing area is to locate them near the downstream endpoint of any street or channel section so that, when performing the calculation, the software automatically assigns it to that section and thus calculates the respective intercepted flow based on the values of water height and spread for the runoff flow calculated for the street section in question.

In this list, the previously created drainage inlets on the respective administrator at the Settings tab will be shown.

From here, you choose the one to set as current, click on the add button, enter the rotation angle its symbol will have respecting the horizontal, and finally click the insertion point’s location in the drawing area.

Thus you will create the drainage inlets in the desired sites. When finished, you can press the escape key or click the right mouse button.

When you perform the calculation of the current surface system, as was referred to, the program will search the nearest street or channel section to each inlet so as to allocate it to that section. Each street section will have only one assigned drainage inlet.

You can see this when you go to the street section’s properties and verify that an inlet has been assigned, with the respective intercepted and bypassing flows.

Likewise, if you show the drainage inlet’s properties, you will see specific results for it and you can also change the actual type or model, if applicable.

Finally, you find the button to perform the calculation of the current surface system. When you click this button, DREN-URBA verifies that the data are consistent and, if appropriate, will show the detected errors to be corrected by the user.

If no errors are detected, the calculation will be performed, updating values in the respective table, allowing you to move on to analyze your surface drainage system design.