How to Model Broken-Back Culverts

You can use Stormwater Studio to model broken-back culverts or sometimes referred to as Inverted Siphons. These are cases where the pipes need to duck under and temporarily travel beneath an obstruction such as a water line, and then slope back up to a normal cover.

Since Stormwater Studio uses a true energy-based method for computing water surface profiles, you can input pipes on any slope, positive or negative. This allows you to go down in cover and then back up again. The basic procedure is to use the “None” junction type when connecting pipes together as shown in the plan below.

Broken-Back Culvert | Stormwater Studio
Plan view of inverted siphon

This plan has a total of 5 Lines. Lines 2 and 3 are pipes that use the “None” junction type. Line 2 has a negative invert slope and Line has a zero slope. Line 4 has a positive slope and uses a “Headwall” for its junction type. The first and last Lines are setup as Open Channels. A utility crossing was added to the model to check and verify clearances.

The profile looks like the following:

Broken back culvert
Profile of broken-back culvert

Once setup you can add flow data (in this case a single Known Q of 15 cfs was added at Line 5) and compute results just like any other storm sewer system.

Using HDS-5 Calculation Methods

Stormwater Studio will use HDS-5 methodology for checking inlet and outlet control on any Line that uses a Headwall. Be sure to have “Check Inlet Control” On in the Calculation Options as shown below. The software assumes a square edged, concrete headwall is used.

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