CFS Hydrologic & Hydraulic Analysis 2 — Questions and Answers
Question 1: In the Rational Method, what does the runoff coefficient 'C' represent?
- The ratio of peak runoff rate to rainfall intensity (Correct answer)
- The total volume of runoff per unit area
- The time for runoff to travel from the hydraulically farthest point to the outlet
- The average annual rainfall depth for the watershed
Correct answer: The ratio of peak runoff rate to rainfall intensity
The runoff coefficient C is a dimensionless ratio representing the fraction of rainfall that becomes direct runoff, ranging from 0 to 1.
Question 2: Which hydraulic assumption underlies the HEC-RAS steady-state water surface profile computation?
- Flow depth varies rapidly over time
- The energy equation is applied between successive cross-sections assuming gradually varied flow (Correct answer)
- Supercritical flow is assumed throughout the reach
- Manning's roughness coefficient is zero for open channels
Correct answer: The energy equation is applied between successive cross-sections assuming gradually varied flow
HEC-RAS steady-state profiles solve the energy equation (standard step method) between cross-sections under the gradually varied flow assumption.
Question 3: What is the primary purpose of a stage-discharge rating curve?
- To determine soil infiltration rates at a gauging station
- To relate water surface elevation to flow rate at a specific channel location (Correct answer)
- To estimate evapotranspiration losses from a reservoir
- To calculate the Froude number at a bridge opening
Correct answer: To relate water surface elevation to flow rate at a specific channel location
A rating curve defines the relationship between stream stage (water surface elevation) and discharge at a gauging station, enabling flow estimates from stage readings.
Question 4: The TR-55 graphical peak discharge method is most appropriately applied to watersheds with drainage areas up to:
- 2,000 square miles
- 20 square miles (Correct answer)
- 200 square miles
- 2 square miles
Correct answer: 20 square miles
TR-55 is intended for small watersheds up to approximately 20 square miles where simplified hydrologic assumptions are reasonable.
Question 5: In open channel hydraulics, critical flow occurs when the Froude number equals:
- 0
- 0.5
- 1.0 (Correct answer)
- 2.0
Correct answer: 1.0
Critical flow is defined as the condition where the Froude number equals 1.0, representing the transition between subcritical and supercritical flow.
Question 6: When performing a backwater analysis in HEC-RAS for a subcritical flow regime, computations proceed in which direction?
- Downstream to upstream (Correct answer)
- Upstream to downstream
- From the midpoint outward in both directions
- From the highest elevation cross-section downward
Correct answer: Downstream to upstream
Subcritical flow profiles are computed from downstream to upstream because downstream boundary conditions control the water surface elevation.
Question 7: Which hydrologic loss method in HEC-HMS uses antecedent moisture condition (AMC) to adjust curve numbers?
- Green-Ampt infiltration
- SCS Curve Number method (Correct answer)
- Exponential loss method
- Smith-Parlange method
Correct answer: SCS Curve Number method
The SCS Curve Number method adjusts CN values based on AMC-I (dry), AMC-II (average), and AMC-III (wet) antecedent soil moisture conditions.
CFS Hydrologic & Hydraulic Analysis 2
In the Rational Method, what does the runoff coefficient 'C' represent?