How to set up outfall boundary using rating curve

source: github

Introduction

Rating curve defines the relationship between the depth and flow. It is often used to define the boundary condition of an outfall,

• Flow into a treatment plant
• Split a model into sub models, and model the flow condition at the split locations

In InfoWorks ICM, the easiest way to add a rating curve to an outfall is to convert it to a screw pump.

1. Make a copy of the outfall
2. Change the existing outfall to a break node
3. Add a pump link
4. Change it to screw pump and set the on/off base level the same as the break node
5. Update the head discharge table with the rating curve

Example Model

A simple model with rectangle outfall pipe is modeled in both InfoWorks and SWMM5.

The results are shown as a scatter plot of depth vs velocity as shown in the following figures,

• Orange is the normal depth calculated from Manning’s equation
• Green is critical depth calculated from the flow
• X is simulated results from the model

For free outfall, InfoWorks ICM and SWMM5 show similar results, it is the lower of the Yn and Yc.

Since InfoWorks doesn’t have a normal outfall condition, a screw pump with a rating curve is used, and it shows similar results as SWMM5.