6.2 Quantify costs and effectiveness of stormwater best management practices

Key Message: Stormwater Best Management Practices (BMPs) can capture and treat nutrient-laden runoff before discharge to water bodies, some with greater efficiency than others. Where conventional methods, such as building wetlands or retention ponds, are not feasible or too costly, smaller green infrastructure solutions should be encouraged, especially in dense urban settings. Examples include green roofs, rain gardens, cisterns, pervious pavement and multi-use “stormwater parks.” Additional research is needed to quantify the costs and effectiveness of all stormwater BMPs.

Importance

Preventing nutrients from entering stormwater in the first place is the most efficient way to reduce nutrient runoff. Beyond that, stormwater best management practices (BMPs) that capture and treat runoff for nutrient reduction are essential to protect the water quality and designated uses (e.g., swimming, fishing) of receiving water bodies. A number of conventional and provisional stormwater BMPs can intercept and reduce nutrients through natural processes, such as settling of particulates, assimilation and uptake by soils and plants, and denitrification by aquatic and soil bacteria before stormwater is discharged to receiving water bodies.

Overview

Conventional stormwater BMPs can include creating wetlands, installing baffle boxes, building retention ponds, sweeping streets, and repairing or replacing broken, clogged, or undersized pipes, culverts, and other conveyances. They may require extensive planning, construction, or coordination and typically mitigate stormwater runoff across a large area.

Where large projects are impractical or too expensive, a series of smaller green infrastructure BMPs can be implemented (Figure 6.2.1). Green infrastructure practices (also known as Low Impact Development or Low Impact Design (LID)) reduce and treat stormwater at its source, minimizing the volume of water and pollution flowing from the built environment.

Examples of green infrastructure practices:

• Canopy trees and green roofs can intercept rainfall before it hits the ground.
• Rainwater harvesting systems, such as rain barrels and cisterns, can capture rainfall and store it for later use.
• Vegetative buffers and littoral zones around shorelines, ponds, and waterways can filter pollutants and litter from runoff before it enters a water body.
• Pervious surfaces for parking areas, walkways, and drives, such as pavers, bricks, or gravel, can reduce runoff after light rainfall, allowing gradual infiltration into underlying soils.
• Rain gardens, vegetated swales, and recessed tree islands can capture runoff and allow it to evaporate, percolate into the ground, or be used by vegetation.
• Stormwater parks are an innovative approach to stormwater management that combine recreational opportunities, public amenities, wildlife habitat, flood protection, and stormwater storage and treatment into one area.

The conventional BMPs are currently widely used and accepted by regulatory agencies under current Florida stormwater rules (see Chapter 6.3). Green infrastructure BMPs are considered provisional; they are accepted by regulatory agencies on a case-by-case basis and have not yet been widely adopted by the regulated community. Nonetheless, their efficacy is supported by recent and ongoing research, and it is likely that these newer techniques will be needed to attain nutrient reduction targets in the foreseeable future.

Builders have been slow to embrace some provisional or green infrastructure BMPs, particularly pervious pavement and green roof systems. These BMPs can potentially accommodate stormwater and infrastructure needs concurrently but have significant capital costs that may only be partially offset by life cycle considerations (Center for Neighborhood Technology, Undated).

Typically, suburban land values are not high enough to justify the premium cost of building green infrastructure versus setting land aside for conventional stormwater detention areas. However, urban land values may be high enough to justify such expenditures for new construction and redevelopment projects. Retrofitting older neighborhoods using green infrastructure should be prioritized (see Chapter 7.1).

Approach

Ongoing research quantifying the costs and effectiveness of stormwater BMPs in Florida should be supported. Research findings should be curated into a statewide database for cost and effectiveness of stormwater BMPs.

The nutrient removal efficiencies of stormwater BMPs vary widely (Table 6.2.1), and none used alone meet state standards for pollution reduction (see Chapter 6.3). Retention ponds that hold water without releasing to natural surface waters have the highest nitrogen reduction capacity; wet detention ponds are not particularly effective. Stormwater harvesting, bioswales, and use of nutrient-reducing media are relatively more efficient in many applications.

For any stormwater BMP, the return on investment (ROI) in dollars per pound of nutrient reduced varies depending on factors such as geographic location, surrounding land use, and volume treated. An initial review of case studies (Table 6.2.2) indicates that use of nutrient-reducing media like sawdust or wood chips has the greatest potential ROI (although some are not specifically intended for stormwater treatment), followed by pervious pavement and stormwater harvesting. Floating aquatic vegetation islands have the lowest ROI. Additional case studies should be developed to augment this analysis.