CPESC Hydrology & Hydraulics 1 — Questions and Answers
Question 1: Which hydrologic method is most commonly used to estimate peak runoff rates from small urban and agricultural watersheds for erosion and sediment control design?
- Rational Method (Q = CiA) (Correct answer)
- SCS Curve Number Method
- Unit Hydrograph Method
- Manning's Equation
Correct answer: Rational Method (Q = CiA)
The Rational Method (Q = CiA) is the most widely used approach for estimating peak runoff from small watersheds, making it standard for erosion and sediment control sizing.
Question 2: In the Rational Method formula Q = CiA, what does the variable 'C' represent?
- Channel capacity coefficient
- Runoff coefficient reflecting land cover and soil type (Correct answer)
- Concentration time factor
- Curve number adjustment factor
Correct answer: Runoff coefficient reflecting land cover and soil type
The runoff coefficient 'C' represents the fraction of rainfall that becomes surface runoff, varying by land use, soil type, and slope.
Question 3: Time of concentration (Tc) in hydrology is defined as:
- The duration of a design storm event
- The time for runoff to travel from the hydraulically most remote point to the outlet (Correct answer)
- The lag time between peak rainfall and peak discharge
- The time required for sediment to settle in a basin
Correct answer: The time for runoff to travel from the hydraulically most remote point to the outlet
Time of concentration is the time it takes for water to travel from the farthest point of the watershed to the design point, which determines the peak runoff rate.
Question 4: Which NRCS (SCS) Hydrologic Soil Group has the highest runoff potential and lowest infiltration rate?
- Group A
- Group B
- Group C
- Group D (Correct answer)
Correct answer: Group D
Hydrologic Soil Group D soils (clays, shallow soils) have very low infiltration rates and produce the highest runoff, making them critical to identify for erosion risk.
Question 5: Manning's equation is primarily used in erosion and sediment control to:
- Calculate sediment transport capacity in runoff
- Estimate flow velocity and discharge in open channels and swales (Correct answer)
- Determine the SCS curve number for a site
- Calculate the required storage volume of a sediment basin
Correct answer: Estimate flow velocity and discharge in open channels and swales
Manning's equation (Q = (1/n)AR^(2/3)S^(1/2)) is the standard formula for computing flow velocity and discharge in open channels, ditches, and vegetated swales.
Question 6: A watershed with a high SCS Curve Number (CN) value indicates:
- High infiltration and low runoff potential
- Low infiltration and high runoff potential (Correct answer)
- Well-vegetated, undisturbed land conditions
- Sandy soils with high permeability
Correct answer: Low infiltration and high runoff potential
High CN values (near 100) indicate little infiltration and high runoff potential, typically associated with impervious surfaces or saturated soils.
Question 7: For a construction site, which factor most significantly increases peak runoff compared to pre-construction conditions?
- Increased vegetation density
- Removal of topsoil and vegetation reducing infiltration (Correct answer)
- Deeper soil profiles from grading
- Installation of perimeter silt fences
Correct answer: Removal of topsoil and vegetation reducing infiltration
Removing vegetation and topsoil eliminates infiltration capacity, dramatically increasing imperviousness and peak runoff rates during construction.
CPESC Hydrology & Hydraulics 1
Which hydrologic method is most commonly used to estimate peak runoff rates from small urban and agricultural watersheds for erosion and sediment control design?