LARE Stormwater Management Flashcards

A landscape architect is designing a stormwater management system for a commercial development. The design includes a large detention basin. What is the primary function of a forebay within this system?

Answer: To capture and settle coarse sediments before they enter the main basin.

A forebay is a small settling basin or pool located at the inlet of a larger stormwater BMP, like a detention basin. Its main purpose is to slow down incoming runoff and allow heavier, coarse sediments and pollutants to settle out before the water enters the main treatment area. This pre-treatment step makes maintenance easier by concentrating sediment in an accessible location and improves the overall pollutant removal efficiency of the larger basin.

Which of the following Low Impact Development (LID) techniques primarily functions to reduce stormwater runoff volume and mitigate the urban heat island effect by utilizing a building's roof surface?

Answer: Green Roof

A green roof, also known as a vegetated roof, is specifically designed to be partially or completely covered with vegetation over a waterproof membrane. This technique directly addresses stormwater by absorbing and evapotranspiring rainfall, which reduces runoff volume. Additionally, the vegetation and soil provide insulation and cool the air through evapotranspiration, which helps to mitigate the urban heat island effect.

In designing a grass-lined swale for a new park, the landscape architect specifies a series of check dams. What is the principal reason for including check dams in this context?

Answer: To reduce the velocity of water flow, preventing channel erosion and promoting infiltration.

Check dams are small barriers constructed across a swale or drainage ditch. Their primary function is to interrupt and slow the flow of water. By reducing the velocity, they flatten the hydraulic gradient, which minimizes the erosive force of the water on the channel itself and allows for some infiltration into the soil. While some sediment may settle behind them, their main purpose is erosion control through velocity reduction, not large-scale sediment capture.

A landscape architect is specifying the bioretention soil media (BSM) for a series of rain gardens. To ensure proper function, what is a critical performance requirement for this specialized soil mix?

Answer: A minimum long-term infiltration rate of at least 5 inches per hour.

Bioretention soil media must balance the ability to support plant life with the need to drain water effectively to treat runoff and prevent prolonged ponding. A key specification is achieving a long-term, in-place infiltration rate of at least 5 inches per hour. This ensures the facility can handle runoff from storm events without becoming clogged. High clay content or excessive compaction would inhibit infiltration, while an overly high compost ratio could lead to nutrient leaching.

When calculating peak stormwater runoff for a small watershed (less than 20 acres) using the Rational Method (Q=CiA), what does the runoff coefficient 'C' represent?

Answer: A dimensionless fraction representing the portion of rainfall that becomes direct runoff.

In the Rational Method formula, the runoff coefficient 'C' is a dimensionless value that represents the ratio of runoff to rainfall. It accounts for the integrated effects of infiltration, evaporation, and surface storage. The value of 'C' varies based on the characteristics of the surface (e.g., pavement has a high 'C' value close to 1.0, while a lawn has a much lower 'C' value).

A landscape architect needs to transition concentrated stormwater flow from a pipe outlet to gentle sheet flow across a stabilized vegetated slope to prevent erosion. Which stormwater management device is specifically designed for this purpose?

Answer: Level Spreader

A level spreader is a structure designed to take concentrated runoff and distribute it evenly as shallow sheet flow over a stabilized area. Its key feature is a perfectly level lip that ensures uniform discharge, reducing water velocity and preventing the erosion that would occur if the concentrated flow were discharged directly onto the slope.