Planning

This standard illustrates the plan view, cross-section, and recommended design notes for a bioretention bulge, a type of curb extension bioretention system designed for use in constrained streetscape environments.
The bioretention bulge is implemented where the vegetated treatment area cannot extend into the existing boulevard or sidewalk zone due to space limitations, utilities, or pedestrian access requirements.

Purpose and Function:
The bioretention bulge provides a localized stormwater management feature within the roadway right-of-way by capturing, filtering, and detaining runoff from adjacent street surfaces. Installed as an inset bulge along the curb line, it allows runoff to enter through curb cuts or catch basins, where it is temporarily stored and filtered through engineered soil media and vegetation before infiltrating or discharging to an underdrain.

Key Features:

• Located entirely within the roadway footprint, projecting into the curb lane while maintaining pedestrian and utility clearances.
• Includes pretreatment measures such as sediment pads or curb cut inlets to capture debris and reduce clogging.
• Utilizes bioretention soil media, mulch, vegetation, and an underdrain system consistent with standard bioretention design specifications.
• Typically bounded by concrete curbs to define the treatment area and protect vegetation from traffic impacts.
• Designed to enhance streetscape aesthetics while improving stormwater quality and reducing runoff volume.

Design Notes (for drawing callout):

• Surface grading should promote positive inflow from adjacent roadway surfaces while maintaining maximum 3H:1V interior side slopes.
• Overflow structures shall safely convey flows exceeding design capacity to existing catch basins or storm drains.
• Vegetation should be tolerant to deicing salts, periodic inundation, and urban heat conditions.
• Total system depth (from existing road grade to bottom of aggregate or liner) should not exceed 1.2 m.
• Ensure setbacks from underground utilities and coordinate with service providers before construction.

The bioretention profile represents a template cross-sectional layout of a typical bioretention system. While actual dimensions, configurations, and materials may vary depending on site-specific conditions, contributing drainage area, and desired hydrologic performance, each system generally consists of the same core layers and functional components. These layers are designed to work together to filter pollutants, promote infiltration, and support vegetation growth while maintaining structural stability and water quality performance.

Purpose and Function:
Bioretention systems serve as an essential green infrastructure practice that manages stormwater through filtration, infiltration, evapotranspiration, and biological uptake. The layer composition and depth determine the system’s ability to:

• Capture and temporarily store runoff, reducing downstream peak flows.
• Filter sediments, nutrients, and metals through engineered media.
• Promote infiltration into subsoils where feasible.
• Support healthy plant growth that enhances pollutant removal and provides aesthetic and ecological value.
• Protect underlying structures through bedding and liner systems as required.

Typical Components and Layer Specifications:
The following table summarizes the standard layers that make up a bioretention system profile, including recommended thicknesses and applicable specifications.

Design Considerations:

• Depth and Sizing: Bioretention depth and footprint area should be based on hydrologic modeling of the contributing drainage area and target stormwater volume capture (commonly 25–100 mm of runoff).
• Media Composition: The soil media should typically consist of a mixture of sand, loam, and organic material, tested to ensure appropriate infiltration rates (generally 50–300 mm/hr).
• Vegetation: Plant selection should be based on tolerance to periodic inundation and drought, and should include a mix of native grasses, shrubs, and herbaceous perennials.
• Underdrain System: Where infiltration into native soil is limited, include a perforated underdrain embedded within the aggregate layer to convey filtered water to downstream storm infrastructure.
• Overflow Structure: An overflow or bypass inlet should be incorporated to manage flows that exceed the design storm.
• Maintenance Access: Ensure adequate access for inspection and media replacement when necessary.

Construction and Maintenance Notes:

• The bioretention system should be constructed in accordance with approved design drawings and specifications, ensuring that layer thicknesses and material gradations meet design intent.
• Erosion control measures must be implemented during construction to prevent sediment contamination of the bioretention media.
• Periodic maintenance should include mulch replenishment, vegetation management, and inspection of inlets and underdrains to maintain infiltration and filtration capacity.

Description:
Pretreatment sediment basins function as small settling areas or sumps designed to capture coarse sediments, debris, and floatables before stormwater enters the main bioretention system, rain garden, or tree trench. While they serve the same general purpose as sediment pads, they are specifically designed for larger drainage areas and higher sediment loads, providing greater storage capacity and sediment retention.

Purpose and Function:
The primary functions of pretreatment sediment basins are to:

• Capture and settle coarse sediments and floatable debris from runoff before it enters downstream treatment structures.
• Reduce flow velocity and energy, allowing sediment to settle out and minimizing erosion at the bioretention inflow.
• Prevent clogging and premature failure of the infiltration or filtration media by removing sediment at the source.
• Prolong the effective lifespan and efficiency of the downstream green infrastructure asset by reducing maintenance frequency and repair needs.

Typical Placement:
Sediment basins are typically installed:

• At the most upstream inlets of bioretention systems, where the runoff first enters from a larger contributing catchment.
• Downstream of curb cut inlets or storm drain discharges that collect flow from expansive paved or impervious areas.
• In locations subject to high sediment transport, such as roads with frequent vehicular traffic, construction areas, or unpaved shoulders.

Design Considerations:

• Configuration: Sediment basins may be designed as shallow depressions, concrete forebays, or subsurface sumps integrated with the inlet structure.
• Storage Volume: The basin should provide sufficient capacity to store accumulated sediment between maintenance intervals, typically sized to capture 0.1 to 0.25 inches of runoff from the contributing drainage area.
• Hydraulic Control: Flow spreaders or level spreader weirs can be used at the outlet to evenly distribute treated flow into the bioretention media.
• Access and Safety: Design should allow for safe and easy access for maintenance vehicles or crews to remove sediment and debris.
• Material: Basins can be constructed using reinforced concrete, stone-lined depressions, or stabilized earth, depending on site constraints and aesthetics.

Maintenance and Inspection:
Proper maintenance is critical to ensure consistent performance:

• Inspection Frequency: After significant storm events and at least two to four times per year.
• Sediment Removal: Sediment should be removed once it occupies approximately 50% of the basin’s storage depth or when inflow capacity becomes restricted.
• Debris Management: Remove accumulated litter, organic material, and floatables during routine maintenance to prevent clogging.
• Structural Repair: Inspect for erosion, undercutting, or damage to inlet/outlet structures, and repair promptly to maintain function.

Applications:
Pretreatment sediment basins are particularly effective for:

• Large drainage areas that convey substantial runoff volumes and sediment loads to a single bioretention or infiltration system.
• Upstream control points in multi-cell green infrastructure networks, providing staged pretreatment.
• Urban retrofit or high-traffic sites where surface dust, sediment, and roadway debris can rapidly accumulate.

By providing robust sediment capture and hydraulic moderation, sediment basins enhance the performance, durability, and maintenance efficiency of downstream green infrastructure systems.

Description:
Pretreatment sediment pads are small, engineered surface features placed downstream of curb cut inlets to protect green infrastructure (GI) systems from excessive sediment and erosion. They serve as a first line of defense by capturing coarse sediments and dissipating the energy of stormwater runoff before it enters sensitive treatment areas such as bioretention cells, rain gardens, or tree trenches.

Purpose and Function:
The primary objectives of pretreatment sediment pads are:

• To prevent erosion of the bioretention media and surrounding soils by reducing the velocity of incoming flow.
• To capture and retain sediments, debris, and other coarse materials that may otherwise accumulate within the green infrastructure system.
• To extend the functional lifespan of bioretention and infiltration facilities by reducing clogging and maintenance frequency.
• To enhance the overall water quality of stormwater entering the treatment area by providing a basic level of pretreatment.

Typical Placement:
Sediment pads are typically installed:

• Immediately downstream of curb cut openings, where runoff transitions from paved surfaces to vegetated or media-filled green infrastructure systems.
• In small drainage catchments or localized flow paths where stormwater from streets or parking areas enters a GI facility.
• At downstream inlets of bioretention systems or tree trenches, particularly where concentrated flow may cause erosion or scouring.

Design Considerations:

• Material: Pads may consist of coarse aggregate, permeable pavers, concrete blocks, or stabilized stone, designed to resist erosion and allow for easy maintenance.
• Size and Shape: The pad should be large enough to distribute inflow evenly and accommodate the expected volume of runoff without overtopping or causing backflow.
• Slope: A gentle slope is recommended to promote uniform flow dispersion and minimize erosion potential.
• Maintenance Access: Pads should be located where they are easily accessible for periodic cleaning and sediment removal.
• Integration with Vegetation: Where possible, vegetation adjacent to the pad should be selected for tolerance to intermittent inundation and sediment deposition.

Maintenance and Inspection:
Regular maintenance is critical for ensuring the long-term performance of pretreatment sediment pads.

• Inspection Frequency: At least twice per year and after major storm events.
• Sediment Removal: Accumulated material should be removed once sediment depth exceeds approximately 50% of the pad’s design capacity.
• Repair: Any damaged or displaced aggregate or paving should be reset or replaced promptly to maintain effectiveness.

Applications:
Pretreatment sediment pads are best suited for:

Retrofit projects where adding a simple, low-cost pretreatment measure can significantly improve system resilience and reduce lifecycle maintenance costs.

Small drainage areas contributing to bioretention systems, tree trenches, or infiltration planters.

Sites with moderate sediment loads or potential for debris accumulation at curb openings.