---
title: "Velocity Calculations | Technical Reference"
slug: "velocity-calculations"
description: "Learn how to calculate nodal velocities through the power law function. "
tags: ["1D river modelling", "1D urban modelling"]
updated: 2022-08-07T16:41:52Z
published: 2022-08-07T16:41:52Z
canonical: "help.floodmodeller.com/velocity-calculations"
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.

# Velocity Calculations

An approximate formula must be used to calculate nodal velocities if the flood routing model is to be used with the water quality model. These values are then used to determine flow areas which are saved for use in the water quality simulation.

Two options are available to specify velocity as a function of flow:

1. Power Law Function

|  | \| V = Vo + a( Q - Qo )b for Q > Qo \| (1) \| \| --- \| --- \| \| V = Vo for Q <font class="hcp5" face="Symbol" style="font-family: Symbol;">£</font> Qo \| (2) \| \| --- \| --- \| |
| --- | --- |
| V = Vo + a( Q - Qo )b for Q > Qo | (1) |
| V = Vo for Q <font class="hcp5" face="Symbol" style="font-family: Symbol;">£</font> Qo | (2) |

This formulation is introduced to avoid potential problems in the quality model when the flow dips initially, possibly attaining a negative value; a property associated with the Muskingum algorithm.
2. User Supplied Relationship  
In this case you provide velocity and flow data pairs and Flood Modeller linearly interpolates between these values.