CLARB - Council of Landscape Architectural Registration Boards Grading, Drainage, and Stormwater Questions and Answers — Questions and Answers
Question 1: A landscape architect is designing a stormwater management system for a commercial property. The primary goal is to manage peak runoff rates by temporarily holding water and releasing it slowly. The system should be dry between storm events. Which of the following is the most appropriate solution?
- A retention basin
- A detention basin (Correct answer)
- A French drain
- A bioswale
Correct answer: A detention basin
A detention basin is designed to temporarily store stormwater runoff and release it at a controlled rate, so it is typically dry between rain events. A retention basin, in contrast, maintains a permanent pool of water. A French drain primarily manages subsurface water, and a bioswale is a vegetated channel designed to slow and filter surface runoff, but a detention basin is specifically engineered for temporarily holding and slowly releasing large volumes of water to manage peak flow.
Question 2: When grading a site to direct surface water away from a building foundation, what is the generally accepted minimum slope?
- 0.5% (1/16 inch per foot)
- 1% (1/8 inch per foot)
- 2% (1/4 inch per foot) (Correct answer)
- 5% (5/8 inch per foot)
Correct answer: 2% (1/4 inch per foot)
A minimum slope of 2% is the standard guideline to ensure positive drainage away from structures, preventing water from pooling near the foundation. This equates to a drop of at least 2 inches for every 10 feet of horizontal distance. While steeper slopes can be used, 2% is considered the minimum for effective surface water management in most conditions.
Question 3: A client wishes to address soggy lawn conditions caused by a high water table and subsurface water movement. They want an underground solution that will intercept and redirect this water away from the problem area. Which drainage system should be recommended?
- A vegetated swale
- A catch basin
- Permeable pavers
- A French drain (Correct answer)
Correct answer: A French drain
A French drain is specifically designed to manage subsurface water. It consists of a perforated pipe within a gravel-filled trench that collects groundwater and channels it away. A swale and permeable pavers manage surface water, while a catch basin is designed to collect surface runoff and direct it into a pipe system.
Question 4: A path has a starting elevation of 102.5 feet. The path is 80 feet long and has a constant slope of 2.5%. What is the elevation at the end of the path?
- 100.5 feet (Correct answer)
- 102.0 feet
- 104.5 feet
- 105.0 feet
Correct answer: 100.5 feet
To calculate the change in elevation, multiply the length of the path by the slope percentage. Elevation change = 80 feet * 0.025 = 2.0 feet. Since the question does not specify if the path slopes up or down, but standard practice dictates paths drain away from starting points, we assume a downward slope. Therefore, the ending elevation is the starting elevation minus the change: 102.5 feet - 2.0 feet = 100.5 feet. If it were an upward slope, the answer would be 104.5 feet.
Question 5: Which of the following stormwater management techniques is a form of green infrastructure designed to slow, filter, and infiltrate surface runoff through a shallow, vegetated, and gently sloped channel?
- A dry well
- A bioswale (Correct answer)
- A trench drain
- An underground storage tank
Correct answer: A bioswale
A bioswale is a landscape element specifically designed as a shallow, vegetated channel to slow down water flow, promote infiltration, and filter pollutants from surface runoff. A dry well is an underground structure for infiltration, a trench drain is a linear surface drain, and an underground tank is for storage, not filtration and conveyance in a vegetated channel.
Question 6: During the design of a large parking lot, the landscape architect must incorporate a stormwater feature that not only manages runoff but also improves water quality and provides aesthetic and habitat benefits. The feature should maintain a permanent pool of water. What is the most suitable option?
- A detention pond
- An infiltration trench
- A retention pond (Correct answer)
- A channel drain
Correct answer: A retention pond
A retention pond, or wet pond, is designed to maintain a permanent pool of water. This permanent pool allows for significant pollutant removal through settling and biological processes and can be designed as an aesthetic amenity and wildlife habitat. A detention pond is dry between storms, an infiltration trench is primarily for subsurface infiltration, and a channel drain is for surface water collection.
