Best Management Practices (BMPs) & Design Flashcards

Green roofs provide stormwater benefits primarily through which mechanism?

Answer: Retaining rainfall in growing media and releasing it through evapotranspiration

Green roofs absorb rainfall into the growing medium and return the majority of it to the atmosphere through evapotranspiration, reducing both runoff volume and peak flow.

When calculating the design infiltration rate for an infiltration BMP, a safety factor is commonly applied to measured field Ksat values. What is the typical safety factor used?

Answer: 0.50 (one-half of measured Ksat)

Designers typically apply a safety factor of 0.5, using half the measured Ksat to account for variability, clogging over time, and construction disturbance of native soils.

Which of the following represents a structural BMP rather than a non-structural (source control) BMP?

Answer: Detention pond

Detention ponds are engineered, constructed facilities (structural BMPs), while the other options are programmatic or regulatory measures (non-structural BMPs).

A stormwater management plan includes a rainwater harvesting cistern. Which design consideration is most important to ensure the cistern provides both water quality and volume reduction benefits?

Answer: Sizing the cistern with sufficient drawdown between storms to provide available storage capacity

A cistern only provides volume reduction if it has available capacity when the next storm arrives; adequate drawdown through planned reuse is essential for stormwater management benefits.

In a low impact development (LID) approach, which strategy best mimics the pre-development hydrograph by reducing both runoff volume and peak discharge?

Answer: Distributing multiple small-scale infiltration and retention BMPs throughout the site

Distributed small-scale LID practices intercept and treat runoff near its source, collectively mimicking pre-development conditions by promoting infiltration, evapotranspiration, and slow release.

Which channel lining material provides the highest Manning's n roughness value and therefore the slowest flow velocities for a given channel geometry and slope?

Answer: Dense grass

Dense grass has a Manning's n of approximately 0.03–0.10 or higher depending on flow depth, significantly higher than concrete (~0.013) or riprap (~0.033), resulting in slower velocities.

A designer is selecting a BMP for a site with a high seasonal water table at 18 inches below grade. Which BMP type would be LEAST appropriate?

Answer: Infiltration basin

Infiltration basins require adequate vertical separation between the basin bottom and the seasonal high water table (typically ≥2 feet) to function properly and avoid groundwater contamination risks.