Introduction
Outflow control structures are hydraulic arrangements that regulate the rate at which water discharges from drainage structures such as storage systems, manholes, or conduits into downstream pipes or channels. These structures are widely used in stormwater management, detention systems, and combined sewer systems to prevent flooding, reduce peak flows, and protect downstream infrastructure.
Types of Outflow Control Structures
GeoSTORM supports two types of outflow control structures:
- Orifice Control
- Vortex Control
Orifice Control
An orifice control regulates flow from a manhole into downstream pipes or channels. In GeoSTORM, it is represented as a flat plate with a circular or rectangular opening that limits the amount of water passing through. It can also include a flap gate to prevent reverse flow when downstream water surface elevations are higher. Orifice controls are some of the simplest and most cost-effective methods for managing outflow; they are commonly used in stormwater detention systems to slow down discharge rates, reduce peak flows, and protect downstream infrastructure.
Vortex Control
A vortex control regulates flow from a manhole into downstream pipes or channels using the hydraulic principle of vortex formation rather than a simple orifice opening. In GeoSTORM, it is defined by a head-discharge rating curve, which specifies the flow rate at various water surface elevations. It can also include a flap gate to prevent reverse flow when downstream water surface elevations are higher. Vortex controls are especially useful in stormwater detention systems because they help to reduce peak discharges while allowing larger openings that minimize the risk of clogging.
Defining Outflow Control Structures in GeoSTORM
In GeoSTORM, the user can define outflow control structures using the Outflow Control panel from the Manhole Data dialog box. Refer to this article in our knowledge base to learn more about the Manhole Data command.
Common Errors When Defining Outflow Control Structures
The following errors may be encountered when setting up orifice and vortex outflow controls in GeoSTORM:
Orifice Controls
- Incorrect Placement: Defining the orifice at the wrong elevation (e.g., too high above the base of the manhole) can cause the structure to release flow too late or not at all.
- Wrong Dimensions: Entering incorrect diameter or area values can cause the discharge rate to be overestimated or underestimated.
- Missing Flap Gate: Forgetting to add a flap gate in backflow-prone areas may result in reverse flows when downstream water surface elevations rise.
- Unit Mismatch: Confusing metric and imperial units when entering size parameters leads to unrealistic results.
Vortex Controls
- Incomplete Head-Discharge Curve: If the rating curve does not cover the full expected water depth range, GeoSTORM may produce errors or unrealistic outflow behavior.
- Non-Monotonic Data: Entering a head-discharge curve where flow decreases at higher heads (instead of remaining constant or increasing) can cause model instability.
- Improper Scaling: Using very small or very large flow values without proper calibration may exaggerate throttling effects or show no vortex formation at all.