---
title: "Add Spills to Bridges"
slug: "how-to-add-spills-to-bridges"
description: "Learn how to add a Spill unit to allow flow to overtop a bridge deck."
updated: 2023-07-10T15:09:39Z
published: 2023-07-10T15:09:39Z
canonical: "help.floodmodeller.com/how-to-add-spills-to-bridges"
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.

# Add Spills to Bridges

Extreme flood events can often overtop bridges and culverts. If a spill is not included then water can artificially back up causing an unrealistically high water level upstream of the structure and an unrealistically low water level downstream.

A spill unit can be used to allow flow to bypass the structure via the deck of the bridge or culvert and return to the channel downstream.

![](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/add-a-bridge.PNG)

### Example dataset

Download the sample dataset:

[Bridge.zip](https://floodmodeller-knowledgebase.s3.eu-west-2.amazonaws.com/Bridge.zip)

Note:

The spill unit should be used where the user expects the flow to return to channel downstream and be conveyed elsewhere once overtopping the structure.

### Steps

1. Starting with an existing **Bridge** unit (Arch, USBPR or Pier-Loss) connected to upstream and downstream River Sections, open the Bridge unit and update the upstream and downstream node labels to become unique labels (i.e. different to the labels of the upstream and downstream River Sections). E.g. i

Tip:

If your upstream River Section label is **XS01**, your bridge upstream label might be **XS01b**, for instance.
2. In the **1D River Network** table, select the River Section upstream of the bridge unit.
3. From the **1D River Build** tab, navigate to the **Connectors** menu and from the **Junctions** dropdown menu, select an **Open Junction**. Manually place the Open Junction unit between the upstream River Section and the bridge unit. Copy the upstream River Section label and the bridge unit upstream label into the Open Junction unit. Repeat this to add a second **Open Junction** unit downstream of the bridge.
4. Within the bridge unit, copy the upstream and downstream River Section labels into the **Upstream Remote** and **Downstream Remote** label fields. The bridge unit should now be linked to the upstream and downstream River Sections via Open Junction units.
5. From the **1D River Build** tab, select a **Spill** unit. Manually place the spill unit on the deck of the structure adjacent to the bridge unit. Within the **Spill** unit, specify the upstream and downstream node labels. E.g. if your upstream River Section label is XS01, your spill upstream label might be XS01spu. Copy the upstream and downstream **Spill** unit labels into the upstream and downstream Open Junction unit labels respectively.
6. Populate the **Spill Section Data** of the Spill unit with the cross-section data for the structure deck, noting that the width of the spill section should be consistent with that of the upstream and downstream River Sections. Consider an appropriate weir coefficient (Cd) for calculating discharge over the deck of the structure.

Note:

Your spill unit has now been added to your existing bridge. Make sure you save the adjusted river network!