Best Management Practices (BMPs) & Design Flashcards

The water quality volume (WQv) is a key design parameter for many stormwater BMPs. It is typically defined as the runoff generated by:

Answer: A specified percentile of daily rainfall depths (e.g., 90th percentile)

WQv is commonly defined as the runoff from a rainfall depth representing the 85th or 90th percentile of all storm events, capturing the frequent small storms that contribute most to annual pollutant loads.

Which maintenance activity is most critical during the first two years after installation of a bioretention facility?

Answer: Irrigation and vegetation management to ensure plant establishment

Adequate irrigation and weed control during the plant establishment period (typically 1–2 years) is critical to achieve the designed vegetation cover that supports long-term BMP performance.

An engineer must design a BMP for a site with contaminated groundwater where infiltration is prohibited. Which BMP best provides water quality treatment while preventing infiltration?

Answer: Lined bioretention cell with underdrain

A lined bioretention cell with an underdrain treats stormwater through media filtration and plant uptake while the liner prevents any infiltration into the contaminated groundwater.

Constructed wetlands used for stormwater treatment should maintain a minimum permanent pool depth to support emergent vegetation. What is the typical maximum water depth for the emergent marsh zone?

Answer: 18 inches

Emergent marsh zones in constructed stormwater wetlands are typically designed with water depths of 6–18 inches to support common emergent species like cattails and bulrushes.

The 'first flush' phenomenon refers to which stormwater quality characteristic?

Answer: The disproportionately high pollutant concentrations carried in early runoff compared to later portions of a storm

The first flush describes elevated pollutant concentrations in the initial runoff from impervious surfaces as accumulated dry-weather deposits are mobilized by early rainfall.

A stormwater BMP is designed with a 40-hour drawdown time for the water quality volume. This design criterion is intended primarily to:

Answer: Ensure adequate hydraulic residence time for pollutant removal while preventing standing water mosquito breeding

A 24–72 hour drawdown window balances pollutant removal (longer residence time is better) against public health concerns, as mosquitoes can complete early larval development in standing water beyond 72 hours.

Which performance metric best evaluates a BMP's effectiveness at reducing nutrient loads to receiving waters?

Answer: Event mean concentration (EMC) reduction for total nitrogen and total phosphorus

Event mean concentration reductions for total nitrogen and total phosphorus directly measure the BMP's ability to remove the pollutants of concern for nutrient-impaired receiving waters.