Stormwater Outlet Control Structures: Mastering the Difference Between Outfalls and Outlets

Understanding Stormwater Outlet Control Structures

Stormwater outlet control structures are specialized devices designed to manage the discharge of collected surface and stormwater runoff. These structures are the gateway between stormwater conveyance systems, such as pipes, channels and detention facilities, and the ultimate receiving waters, like streams, rivers, or wetlands. Their primary functions include:

• Flow control: Outlet control structures regulate the rate and volume of stormwater discharge, preventing erosion, scouring and downstream flooding.
• Water quality treatment: Some outlet control structures incorporate sediment traps, oil/water separators, or filtration media to remove pollutants from the outflowing water.
• Energy dissipation: Outlet structures employ techniques like baffles, splash pads, or riprap to dissipate the kinetic energy of the discharged water, minimizing the potential for erosion and habitat disruption.
• Regulatory compliance: Properly designed outlet control structures help stormwater managers meet the requirements of the Clean Water Act, NPDES permits and other environmental regulations.

Differentiating Outfalls and Outlets

While the terms “outfall” and “outlet” are often used interchangeably, they have distinct meanings in the context of stormwater management:

Outfall

An outfall is when collected stormwater is discharged from a pipe, channel, or other conveyance system into a receiving water body, such as a stream, river, lake, or wetland. Outfalls are the terminal points of a stormwater drainage system and must be carefully designed to mitigate the environmental impacts of the discharged water.

Outlet

An outlet control structure is a broader term that refers to the discharge mechanism or control structure that regulates water flow out of a stormwater management facility, like a detention basin, retention pond, or underground stormwater storage tank. Outlets can take various forms, including risers, weirs, orifices, or valves and are responsible for metering the release of stored stormwater.

Bubble-Up Structures: A Specialized Outlet Control

One unique outlet control structure design type is the “bubble-up” design, a bubble-up catch basin or bubble-up chamber. These specialized structures are employed to manage the discharge from stormwater detention or retention facilities, particularly in areas with flat terrain or high groundwater tables.

The bubble-up drainage structure consists of a vertical riser pipe or chamber with a perforated bottom submerged in the stored stormwater. As the water level rises in the facility, it “bubbles up” through the perforations, creating a controlled release that prevents sudden, high-velocity discharges. This design helps to dissipate the energy of the outflowing water, reducing the potential for erosion and providing a more gradual release.

Bubble-up stormwater structures are often used with other outlet control structures, such as orifice plates or weirs, to fine-tune the discharge rate and meet specific regulatory requirements or watershed management goals.

Stormwater Outfall Structures

Outfall structures, the termination points of stormwater conveyance systems, require careful design to mitigate the environmental impacts of the discharged water. Standard components include:

• Headwalls: Vertical concrete or masonry structures that provide a stable, reinforced end to a pipe or channel.
• Flared end sections: Tapered, funnel-like structures that smoothly transition the flow from a pipe to the receiving water.
• Riprap or bioengineered splash pads: Coarse stone or vegetated surfaces dissipate the discharged water’s kinetic energy and prevent erosion.
• Level spreaders: Shallow, level channels or depressions designed to distribute the outflow in a sheet flow pattern, reducing concentrated discharges.
• Outfall aprons: Reinforced concrete or rock structures that provide a stable, erosion-resistant transition from the outfall to the receiving water.

Detention Basin Outlet Structures

Detention basins, also known as dry ponds or retention facilities, are a common type of stormwater management infrastructure. Outlet structures in these systems play a critical role in regulating the release of stored stormwater, ensuring that discharge rates and volumes do not exceed predevelopment conditions or cause downstream flooding. Trash rack outlet structures may also be used.

Typical outlet control structures include:

• Riser pipes: Vertical pipes with perforations or orifices that allow water to drain from the basin slowly.
• Weirs: Adjustable, overflow-type structures that control the water level and discharge rate.
• Orifices: Precisely sized openings in the outlet structure that meter the outflow based on design specifications.
• Multi-stage outlets: Combination structures with multiple control devices to manage both frequent, small storms and less frequent, more significant events.

Factors such as basin size, watershed characteristics, regulatory requirements and the desired level of flow and water quality control influence the specific design of the detention basin outlet control structure.

Pond Outlet Control Structures

Like detention basins, stormwater retention ponds or wet ponds also require carefully designed outlet control structures to manage the release of stored water. These structures often incorporate a combination of features, such as:

• Principal spillway: The primary outlet, typically a riser pipe or weir, regulates the average discharge from the pond.
• Emergency spillway: An overflow structure designed to safely convey more significant, less frequent storm events and prevent the pond from overtopping.
• Skimmer or baffle: Devices that trap floating debris, oil, or other pollutants before the water is discharged.
• Orifice plates or valves: Adjustable flow control mechanisms can be tuned to meet specific discharge rate requirements.

The design of pond outlet control structures must consider factors like the pond’s storage volume, watershed hydrology, water quality objectives and regulatory compliance needs.

Culvert Outlet Control

Culverts are pipe or box structures that convey water under roadways or embankments and require outlet control considerations. Culvert outlet control structures are designed to:

• Dissipate energy: Employing techniques like riprap, plunge pools, or energy dissipaters to reduce the kinetic energy of the discharged water and prevent erosion.
• Prevent clogging: Incorporating features like trash racks or debris barriers to keep the culvert inlet clear and functional.
• Provide stability: Reinforcing the culvert outlet with headwalls, wing walls, or other structural elements to maintain the integrity of the crossing.

Stormwater Structural Controls

In addition to the outlet control structures discussed, stormwater management systems may incorporate a variety of other structural controls to regulate flow, remove pollutants and protect the environment. These include:

• Detention/retention ponds: Facilities that temporarily store and slowly release stormwater, providing flow control and water quality benefits.
• Bioretention/bioswales: Vegetated depressions or channels that use soil and plants to infiltrate, filter and treat stormwater.
• Permeable pavement: Porous surfaces that allow stormwater to infiltrate, reducing runoff volumes and rates.
• Infiltration basins or trenches: Excavated areas designed to capture and infiltrate stormwater into the ground.
• Constructed wetlands: Engineered wetland systems that provide natural treatment and habitat benefits.

When integrated with outlet control structures, these structural controls form a comprehensive stormwater management strategy to mitigate development impacts and protect the health of local water resources.

Maintenance and Monitoring of Outlet Control Structures

Proper maintenance and monitoring for stormwater outlet protection are essential for ensuring their long-term functionality and performance. Key maintenance activities include:

1. Debris and Sediment Removal: Regularly inspect and clean out accumulated debris, trash, and sediment from the outlet structures to maintain flow capacity and prevent clogging.
2. Structural Integrity Checks: Inspect the condition of the outlet components, such as pipes, valves, weirs and energy dissipation features, to identify and address any signs of deterioration or damage.
3. Vegetation Management: Maintain the health and proper growth of any vegetation associated with the outlet structure, such as in bioswales or level spreaders, to ensure optimal water quality and flow control.
4. Performance Monitoring: Regularly measure the discharge rates, water quality and other performance metrics to verify that the outlet control structure is functioning as designed and meeting regulatory requirements.

By diligently maintaining and monitoring these critical stormwater infrastructure elements, communities can ensure the long-term effectiveness of their outlet control structures and protect the health of local waterways. Get in touch with AQUALIS today to learn more about stormwater management.

Questions About Structural Stormwater Controls

Some common questions that arise when discussing outlets vs outfalls include the following:

What Is the Outlet Control Structure in Stormwater?

An outlet control structure regulates stormwater flow out of a stormwater management system, such as a detention pond, retention basin, or other stormwater Best Management Practices (BMP).

How Do Stormwater Outlet Control Structures Work?

Outlet control structures regulate the rate and volume of stormwater discharge from a stormwater management facility. They achieve this by using various design elements, such as orifices, weirs, outlet pipes and valves.

What Is the Difference Between an Outfall and an Outlet?

The outlet is the point where stormwater leaves a specific stormwater management facility, such as a detention pond or a subcatchment, while the outfall is the final discharge point of the stormwater conveyance system, where the water is released into a receiving water body, such as a stream, river, or the ocean.

What Is a Bubble-Up Structure?

A bubble-up structure is designed to provide a controlled release of stormwater when the downstream conveyance system is at capacity or where the receiving water body is prone to backflow.

What Is the Outfall Control Structure?

In addition to the outlet control structures within the stormwater management system, outfall control structures are designed to manage the final discharge of stormwater into the receiving water body. This helps with energy dissipation, flow orientation, habitat protection and sediment removal.

What Are Stormwater Structural Controls?

An outlet is an example of one stormwater structural control. Others include detention and retention basins, permeable pavement, green roofs, underground detention systems and more.

By mastering the design, implementation, and maintenance of these specialized structures, stormwater professionals can help communities navigate the complexities of stormwater management with stormwater pond outlet structures, mitigate the risks of flooding and water pollution and promote the long-term sustainability of local water systems.