Culvert Analysis in SSA

Autodesk Storm and Sanitary Analysis offers users the ability to perform runoff calculations and hydraulic routing calculations in a single package.  This combines the best of both worlds.

SSA’s hydraulic calculations are founded in the EPA SWMM calculations which performs the one-dimensional Saint Venant flow equations. The Saint Venant equations consist of the continuity and momentum equations for conduits and a volume continuity equation at nodes.  What this means is culvert entrance and exit losses are computed not on the nodes themselves, but rather on the pipe ends.

Let’s dissect how this happens.

1. Open the SSA Sample Culvert file.  (C:\Program Files (x86)\Autodesk\SSA 2013\Samples\Culvert\Culvert.spf)
2. Double Click on the Culvert Pipe shown in the image above.  The Shape definition is described as a “Culvert”.  In many cases, if you exported your Civil 3D Pipe network, the imported pipe may be just a “Pipe”.  
3. By selecting Culvert, the available properties of the conveyance link changes.
4. The HDS-5 Culvert type and Culvert Entrance properties become available.  Be sure to visit the Flow Properties section immediately below and properly set the Entrance and Entrance Losses.

The model is set for analysis. 

Here is one tip when exporting your Civil 3D pipe network.  If the Part Matching Default Settings on Export, set the upstream structure to Headwall, SSA imports that structure as a Junction, and sets the following downstream pipe to a Culvert.

One could conceivably use the Part Matching Customization Settings in SSA to match set a specific part family to a culvert, but that is a topic for another day.

Comments

[Toby]

There is something that doesn't quite seem right when modeling culverts using a storage node at the inlet of a culvert. What I mean by this is that the storage node seems to be limiting the flow that can pass through the culvert(s), and it is not easy to decipher how the storage node is truly connected to the culvert (ie when modeling a pond with a storage node, you are connecting weirs and orifices to model the outlet structures). To illustrate the "problem" if you take the culvert example straight from sample files and run the analysis, you get a max water elevation of 505.55 in the storage node (depth of 5.05 ft) and a flow of 6.95 cfs through the 18" culvert. Now if you increase the number of barrels to 2, you still get a water elevation of 505.55 and a flow of 6.83 cfs. I know the inlet or outlet control could be coming into play, but it logically would seem that two barrels in a flooded scenario would turn some different results. If you lower the outlet elevation to 499, you still get 6.83 cfs and an elevation of 505.55. In the case of changing the number of barrels or increasing the slope, the design capacity will change, but the actual flow numbers do not. Now if you change the storage node to a junction (I realize you lose the benefit of storage, and you need to delete the road overflow to run the analysis) you can get different flow numbers through the culverts. Any insight would be appreciated.

Thanks,
Toby