LARE Drainage System Design Questions and Answers — Questions and Answers
Question 1: A landscape architect is designing a storm drain line connecting two catch basins. Catch Basin 1 (CB-1) has an outlet invert elevation of 152.50 ft. The two catch basins are 120 feet apart, and the pipe connecting them is specified to have a minimum slope of 1.5%. What is the required invert elevation for the inlet of Catch Basin 2 (CB-2)?
- 154.30 ft
- 152.32 ft
- 150.70 ft (Correct answer)
- 151.00 ft
Correct answer: 150.70 ft
To calculate the downstream invert elevation, you must determine the total vertical drop over the length of the pipe. The drop is calculated by multiplying the pipe length by the slope (120 ft * 0.015 = 1.80 ft). This drop is then subtracted from the upstream invert elevation (152.50 ft - 1.80 ft = 150.70 ft) to find the required downstream invert elevation, ensuring positive flow. [22, 29]
Question 2: When laying out a storm sewer system, what is the fundamental principle for setting pipe elevations as they pass through a manhole or catch basin to ensure proper hydraulic function?
- The crown of the inlet pipe must align with the crown of the outlet pipe.
- The invert of the outlet pipe must be lower than the invert of all inlet pipes. [26, 27] (Correct answer)
- All pipes entering and exiting a structure must have the same invert elevation.
- The invert of the main trunk line should be at least 24 inches below any lateral lines.
Correct answer: The invert of the outlet pipe must be lower than the invert of all inlet pipes. [26, 27]
To ensure positive flow and to account for energy losses as water moves through the structure, the invert of the outlet pipe must be lower than the inverts of any inlet pipes. [26] This difference in elevation, often called a 'drop,' prevents backflow and keeps the system flowing efficiently. [27] Matching crowns can cause issues, especially with different pipe sizes, and having all inverts equal would impede flow.
Question 3: A landscape architect is designing a drainage solution for a persistently soggy lawn area at the base of a slope with heavy clay soils. Surface water is already managed by a shallow swale. Which of the following is the most appropriate system to intercept and remove the problematic subsurface water?
- A series of area drains connected to a solid pipe.
- A French drain system installed across the slope, upstream of the wet area. (Correct answer)
- Increasing the surface slope of the lawn to promote faster runoff.
- A dry well filled with coarse aggregate.
Correct answer: A French drain system installed across the slope, upstream of the wet area.
A French drain is specifically designed to manage subsurface water. It consists of a perforated pipe in a gravel-filled trench that intercepts, collects, and redirects groundwater away from an area. [2, 14] Area drains handle surface water, increasing the slope doesn't solve the subsurface issue, and a dry well is for infiltration, which is difficult in heavy clay soils.
Question 4: In storm drain pipe design, specifying a minimum gradient to achieve a 'self-cleansing velocity' is a critical requirement. What is the primary purpose of maintaining this minimum velocity?
- To maximize the hydraulic capacity of the pipe during peak flow.
- To reduce the structural load on the pipe from the weight of the water.
- To meet the manufacturer's warranty requirements for the pipe material.
- To prevent the deposition of sediment, sand, and debris within the pipeline. [7, 17] (Correct answer)
Correct answer: To prevent the deposition of sediment, sand, and debris within the pipeline. [7, 17]
Self-cleansing velocity, typically around 2-3 feet per second, is the minimum speed of flow required to keep suspended solids like sand, silt, and other debris moving through the pipe system. [7, 11, 18] If the velocity drops below this threshold, solids will settle out, leading to clogs, reduced pipe capacity, and increased maintenance needs. [17]
Question 5: A landscape architect is calculating the Time of Concentration (Tc) for a small watershed as part of a drainage study. Which of the following design changes would result in a *decrease* in the Time of Concentration?
- Increasing the average slope of the overland flow path. (Correct answer)
- Changing the primary ground cover from asphalt to a densely vegetated meadow.
- Increasing the length of the longest flow path by adding meanders to a channel.
- Routing the flow through a small detention pond within the watershed.
Correct answer: Increasing the average slope of the overland flow path.
Time of Concentration (Tc) is the time it takes for runoff from the most hydraulically distant point of a watershed to reach the outlet. [6] Increasing the slope of the flow path makes the water travel faster, thus reducing the travel time and decreasing the Tc. [9] Changing to vegetation (increased roughness), lengthening the flow path, or adding detention would all slow the water down and *increase* the Tc.
Question 6: What is the primary function of installing a perforated underdrain pipe with the perforations oriented downwards within a gravel-filled trench?
- To allow collected subsurface water to enter the pipe from below as the trench fills. (Correct answer)
- To distribute water out of the pipe for subsurface irrigation.
- To prevent debris from entering the pipe from the surface.
- To increase the structural load-bearing capacity of the trench.
Correct answer: To allow collected subsurface water to enter the pipe from below as the trench fills.
In a collection system like a French drain, the perforations are placed facing down. As groundwater seeps into the gravel trench, the water level rises from the bottom. The downward-facing holes allow this rising water to enter the pipe efficiently and be carried away. [1, 3, 5] If the holes were facing up, the trench would have to become nearly saturated before water could enter the pipe.
LARE Drainage System Design Questions and Answers
A landscape architect is designing a storm drain line connecting two catch basins.
Catch Basin 1 (CB-1) has an outlet invert elevation of 152.50 ft.
The two catch basins are 120 feet apart, and the pipe connecting them is specified to have a minimum slope of 1.5%.
What is the required invert elevation for the inlet of Catch Basin 2 (CB-2)?