Understanding the Orifice Equation in Stormwater Management
Welcome, stormwater designers! In today’s hydrology education feature from Clear Creek Solutions, we're diving into the Orifice Equation—a key concept in both stormwater management and fluid mechanics. Whether you're developing stormwater models or brushing up for a fluid dynamics course, this guide will provide the fundamentals, real-world applications, and insights into using orifices in stormwater systems.
What Is an Orifice?
An orifice is defined as an opening in the side or bottom of a tank or reservoir, designed for discharging contained liquid—typically water in stormwater systems. Importantly, the liquid surface must be above the upper edge of the opening for it to be considered an orifice.
Orifices can vary based on:
Size
Shape (e.g., circular, rectangular, notched)
Discharge conditions (free-flowing vs. submerged)
These characteristics affect how the orifice behaves, how efficiently it discharges water, and how it is modeled in stormwater systems.
Visualizing Orifice Functionality
In practice, the orifice’s position within a tank or pond determines how and when water is discharged. Below is a simplified diagram showing:
Total water height above the orifice
Height of the orifice from the tank bottom
These heights are essential for calculating flow and pressure through the orifice.
Real-world orifice tanks used in labs often include multiple openings or adjustable outlets, allowing experimentation and performance measurement under varying conditions.
Two Types of Orifice Flow Conditions
There are two primary orifice flow scenarios:
Free Discharge into the Atmosphere
Water exits the orifice directly into the air.
Simpler flow conditions, fewer variables.
Governed by:
Discharge coefficient (C)
Cross-sectional area (A)
Acceleration due to gravity (g)
Height of the fluid above the orifice (H)
2. Submerged Orifice Discharge
Water exits into another body of water or submerged pipe.
More complex due to added variables and pressure conditions.
Common in steady-state flow systems like stormwater ponds.
Lab Testing: Falling Head Test
The orifice equation isn’t just theory—it can be tested and validated through lab experiments. A typical test is the falling head test, where you:
Fill a tank with water
Record the time it takes for water to drain through the orifice
Calculate the discharge coefficient based on time and head loss
This experiment helps engineers calibrate real-world designs and validate theoretical models.
Applying the Orifice Equation in Stormwater Management
In stormwater design, orifices are used in:
Detention and retention ponds
Outlet structures
Riser tanks
For example, in software like WWHM 2012 or WWHM-SWMM engineers simulate water flow through orifices within larger detention systems.
Key Components:
Riser: Determines when water enters the tank
Orifice: Controls discharge flow back into the system
Submerged Flow: Often the default condition, requiring the full orifice equation
Design variables include:
Orifice height
Diameter
Discharge coefficient (based on size and shape)
Tank and riser geometry
These variables interact to ensure that water levels remain controlled, preventing overflow and ensuring effective runoff management.
Why Use Modeling Software?
Manually calculating stormwater systems with orifice equations can be time-consuming and error-prone due to:
Numerous variables (up to 9 in complex setups)
Changing coefficients
Variability in system geometry
Solution: WWHM-SWMM
WWHM-SWMM is a continuous simulation hydrology model that simplifies this process. Its AutoPond tool allows engineers to:
Automatically size ponds
Insert risers and orifices
Simulate performance
Ensure compliance with design standards
This saves hours of manual calculation and improves model accuracy.
Free Resource: Orifice and Pond Template Pack
To help you get started, we’re offering a FREE template pack with 15 pre-built models for use in WWHM 2012 and WWHM-SWMM. These include:
Basin models
Bioretention templates
Tank and riser setups
✅ Just visit our website and submit your email to receive the free download.
Final Thoughts
The orifice equation is a cornerstone of hydraulic engineering and stormwater design. Understanding how orifices work—and how to model them effectively—can make your stormwater facilities more efficient, reliable, and compliant.
Stay tuned for more videos and articles from Clear Creek Solutions, and don’t forget to subscribe to our YouTube channel for future hydrology education content.
Thank you for reading—and happy modeling!
