Purpose

This article provides conceptual background and key information regarding the observed issue of incorrect spread calculations for combination inlets when using the "Curb Only" calculation option in OpenFlows StormCAD and related products. It aims to clarify why this issue occurs, its implications, and what users should be aware of when modeling such inlets.

Audience 

This article is intended for drainage engineers, hydraulic modelers, and technical users of OpenFlows StormCAD, CivilStorm, SewerGEMS or OpenRoads Designer who are responsible for modeling and analyzing stormwater inlets, particularly those using VDOT standards or similar workflows.

Background

The issue arises when modeling combination inlets (such as VDOT Standard DI-2) using the "Curb Only" option for the "Active Components for Combination Inlets In Sag" in the calculation options:

The expectation, based on hydraulic standards like the VDOT Drainage Manual, is that the "Curb Only" option should ignore the interception capacity of the grate and only account for the curb opening length. However, users have observed that, when using the "Curb Only" option, the spread is much less when compared to a DI-3 curb inlet, or a a DI-2 combination inlet with 100% clogging, when all other characteristics (inlet location, depression, gutter, and cross slopes) are identical. This discrepancy suggests that the software is not correctly ignoring the grate's capacity for combination inlets when "Curb Only" is selected. The issue has been present in multiple versions and may affect the sizing and performance evaluation of inlets in design workflows where the grate must be ignored, potentially leading to undersized infrastructure if not recognized.

Key Information 

• The "Curb Only" calculation option is intended to ignore the grate's interception capacity for combination inlets, per hydraulic standards (e.g., VDOT Drainage Manual).
• Observed behavior: when using the "Curb Only" option, the spread is different (and much less) when compared to a DI-3 curb inlet, or a a DI-2 combination inlet with 100% clogging, contrary to expectations.
• When the calculation option is changed to "Grate and Curb" and the clogging factor is set to 100%, the spread is as expected and matches that of a DI-3 curb inlet.
• The problem only appears to occur with in-sag inlets.
• The issue is also present in FlowMaster (as of version 2025) as well.
• The issue has been reported to development for investigation and resolution in a future release. Reference number 2081456
• Potential impact: If unaddressed, this issue could result in incorrect spread calculations and undersized combination inlets in design projects where the grate needs to be ignored.

Workaround

In OpenRoads Designer, a workaround is to use the "Grate and Curb" calculation option and set the clogging factor to 100% for combination inlets where the grate should be ignored, or use a curb inlet type if compatible with project standards. This can be used in Prototypes for feature definitions, in OpenRoads Designer.  Note that using a curb inlet may not be an option for OpenRoads Designer due to the way prototypes and feature definitions work.

Additional Notes