Using The Water Drop Utility To Help With Drainage Design

Introduction

When determining time of concentration for a drainage basin, it is helpful to be able to track a drop of water from the upstream end of a drainage basin to its most downstream point. Not only is the distance traveled useful, knowing the slope over that distance is necessary as well to accurately measure the time it takes for the entire basin to contribute to flow at the downstream point, or outlet. The time of concentration is then used in a variety of calculations which help the designer determine peak flow for a particular basin. The Water Drop Utility in Civil 3D is the perfect tool for this task.

Applying the Water Drop

Once basin lines have been determined for a surface and the discharge point has been identified, water drops can be applied to the surface to show the path water takes from its highest elevations to its lowest elevations. From the Surfaces pull-down menu and the Utilities option, begin the Water Drop command.

A dialog box will now pop up with a few settings to be applied. The user can decide whether or not to use a marker at the beginning of the path, what layer to put the path objects on, and whether the path object is a 2D or 3D Polyline.

So that the elevation and slope information can be used, 3D Polyline is the better option here. Then, the user can select a location at the upstream end of the basin and see the resulting path drawn across the basin.

It is important to keep in mind the object of the task: to determine the slowest path water takes across the basin. It may not necessarily be the longest path as the effects of slope must be taken into account. For large or oddly shaped basins, this may require several drops of water and several runs through the time of concentration calculations to identify the slowest path water takes across the basin.

Analysis

Now, to obtain full use of the path object, it is helpful here to turn this 3D Polyline into a Civil 3D Feature Line as these are far more versatile. This is done from the Grading pulldown menu and the �Create Feature Lines from Objects� command. Taking the default values on the subsequent Feature Line Creation dialog box is fine here because the path objects will not need to be shown on a set of plans.

To see the elevation effects on the water path, the user may now select the Feature Line, right-click and proceed to the Elevation Editor.

Examining our results, it is easy to see that there is a lot of unnecessary data here, mainly because of the triangulation in the surface itself and the relatively short segments on the Feature Line. The user needs a way to simplify the object for use in a time of concentration calculation and we will do this using a Quick Profile and a �Profile by Layout.�

Again, select the Feature Line, right-click and go to Quick Profile. Take the defaults on the Create Quick Profile dialog box. The resulting profile is simple and temporary but it is all that is needed right now.

Next, use the Profiles pulldown menu and the �Create Profile by Layout� option to bring up the profile creation toolbar. On the first button of the toolbar, use the Draw Tangent option and trace over the profile in 5 representative sections making sure the first segment is no longer than 100 feet.

We now have simplified distance and slope information for use in a time of concentration calculation. As an example, here is an image taken from the time of concentration calculator from WinTR-55, a USDA program used for hydrology in small watersheds:

This calculator gives the user the opportunity to enter five segments along the water�s path, each with its own slope, distance and stream characteristics. Even with some work to be done to determine flow area and wetted perimeter for the �Channel� segments, it is easy to see that the ease of the water drop utility and the power of the Feature Line made the whole process easier and made the user more efficient.

Conclusion

Whether you use a program like WinTR-55 or the Hydraflow solutions now included with a subscription to AutoCAD Civil 3D 2008, here are a couple of notes and things to keep in mind:

The reason the first segment should be limited is that �Sheet� flow cannot be more than 100 feet.
Manning�s �n� value for surface flow can be found in any good Open Channel Flow Handbook or Engineering Reference Manual.
This is just an approximation and the user will see that minor variations in these values will likely result in even smaller fluctuations in the ultimate time of concentration for the basin.
Become familiar with the tools you use for these calculations and know their limitations.
Always remember to use sound engineering judgment in any design endeavor you undertake. The software itself seems to grow by leaps and bounds with each release but, in the end, it is still just a tool in the overall realm of good practice and design.