Flow Control Manhole and Back Flows

In stormwater drainage systems, flow-control manholes (FCMs) are critical elements used to regulate flow between different segments of a network, especially storage units and downstream conduits (pipes or culverts). These flow control manholes are crucial for managing the volume and velocity of stormwater in a drainage system, ensuring its efficiency and stability. Improper handling of flow direction or backflows can lead to downstream flooding during periods of heavy rainfall.

Flow Control Manhole

A flow control manhole is a junction structure included in a stormwater drainage system that is designed to restrict, delay, or control the movement of water through a conduit or open channel. These controls may be achieved through use of the following elements in a drainage system:

• Orifices
• Weirs
• Flap gates
• Internal restrictions caused by specific pipe geometry

Flow control manholes are often placed between a storage unit and an outlet pipe to prevent overflow or backflow during peak storm events.

Understanding Backflow Conditions

Backflow in a drainage system occurs when water flows in a direction opposite of its intended flow. This happens due to:

• High tailwater elevations in the downstream system
• Flat or zero-sloped conduits
• Surcharged manholes or pipes
• Improper configuration of pipe flow directions

Design Considerations and Best Practices for Modeling Flow Control Manholes

• Check and verify pipe orientation to ensure expected flow behavior.
• Utilize invert elevations and storage depth limits to manage surcharge and reverse flow in a drainage system.
• Model orifices and flap gates correctly in the software to simulate the internal hydraulic behavior of flow control manholes.

Handling Outflows and Backflows in GeoSTORM

GeoSTORM simulates flow behavior using the dynamic wave routing method, including the reversal of flow when downstream hydraulic grades exceed upstream inverts. GeoSTORM uses the St. Venant equations, accounting for flow reversal based on head differentials between nodes.

Stormwater pipes are modeled with designated start and end nodes, and the flow direction is computed dynamically. If the downstream head (Hydraulic grade line) exceeds the upstream invert elevation, GeoSTORM allows flow reversal, and results may show negative flow rate values.

Note that a flap gate or control element (such as an orifice plate or vortex flow control device) must be correctly modeled for the software to prevent flow reversal.

In GeoSTORM, the user can navigate to the Outflow Control panel in the Manhole Data dialog box to specify how the manhole discharges flow from control devices to the downstream pipe.

Refer to this article in our knowledge base to learn more about the Outflow Control panel of the Manhole Data command.

Common Issues Affecting Flow Control and Backflows