---
title: "How to add 1D conduit units"
slug: "how-to-add-1d-conduit-units"
updated: 2026-10-16T18:25:48Z
published: 2026-10-16T18:25:48Z
canonical: "help.floodmodeller.com/how-to-add-1d-conduit-units"
stale: true
---

> ## Documentation Index
> Fetch the complete documentation index at: https://help.floodmodeller.com/llms.txt
> Use this file to discover all available pages before exploring further.

# How to add 1D conduit units

Culverts within the watercourse can have a significant impact on water levels and flow conveyance. It is particularly important to account for hydraulic losses at the culvert inlet and outlet along with losses throughout the culvert barrel. Flood Modeller can be used to represent culverts using a combination of culvert inlets, outlets, bends and conduit units. The conduit unit models the cross-sectional area of a closed conduit (circular, arch, rectangular or symmetrical) based on the St Venant equations and Preissmann Slots to calculate pressurised flow.

To add a conduit unit to an existing reach:

1. Start with an existing **Culvert Inlet unit** at the location of the culvert inlet. See [How to add 1D culvert inlets](/v1/docs/how-to-add-1d-culvert-inlets-1) for details.
2. From the **1D River Build** tab, navigate to the **River** menu and from the **Conduits** dropdown select the required conduit shape. In this example, select **Circular**.
3. Manually place the **Circular Conduit** unit downstream of the culvert inlet unit.
4. Give the Circular Conduit unit an appropriate node label and copy this label into the **Downstream Label** of the Culvert Inlet unit.
5. Populate the **Geometry** attribute fields of the Conduit unit, including **Distance to Next Conduit** and **Elevation of Invert.** Note that for a circular conduit the culvert dimensions are defined using only the diameter. For other conduit types, additional attributes are required.
6. Populate the **Friction** attribute fields of the Conduit unit. Friction can be applied using either the Manning or Colebrook-White equations.
7. Repeat steps 1 to 6 for an additional conduit unit at the culvert outlet, instead linking to the **Culvert Outlet unit.** See How to add 1D culvert outlets for details. Note that the **Distance to Next Conduit** for the conduit at the outlet should be **set to zero**.
8. Repeat steps 1 to 6 for any additional conduit units needed along the culvert barrel. These additional conduit units can be used to represent changes in culvert elevation along the barrel, change in conduit shape or to add culvert bends and manholes. Alternatively, **Replicate units** can be used within long culvert barrels.
9. Plot a long-section of bed elevation to confirm your conduit units have been added correctly.
10. Your Conduit unit has now been added to your existing reach. Make sure you save the adjusted river network!

Download the sample data used in the video to try it yourself:

[Culvert.zip](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/Culvert.zip?sv=2026-02-06&spr=https&st=2026-11-10T16%3A13%3A28Z&se=2026-11-10T17%3A13%3A28Z&sr=c&sp=r&sig=copt%2B8HYlFXh8wmtaQ3AyMw%2Fzx69NKmhdgfh5eQvsK4%3D)