PE Water Resources Hydraulics and Hydrology:
Practice Exam Questions, Correct Answers, and
Detailed Explanations for Students||Already Graded
A+
1. A rectangular open channel has a width of 3 m and a flow depth of 2 m. The
flow velocity is 1.5 m/s. What is the flow rate?
Answer: 9 m³/s
Rationale: Q = A × V; Area A = width × depth = 3 × 2 = 6 m²; Q = 6 × 1.5 = 9
m³/s.
2. Manning’s equation for open channel flow is used to estimate:
Answer: Flow velocity or discharge
Rationale: Manning’s equation relates channel slope, roughness, and hydraulic
radius to flow velocity or discharge.
3. A watershed produces 10 cm of runoff over a 5 km² area. The total runoff
volume is:
Answer: 500,000 m³
Rationale: Volume = depth × area; convert 10 cm = 0.1 m, area = 5,000,000 m²;
Volume = 0.1 × 5,000,000 = 500,000 m³.
4. The unit hydrograph is used to:
Answer: Transform rainfall excess into runoff
Rationale: It represents watershed response to a unit of effective rainfall.
5. For a circular pipe flowing half full, the hydraulic radius is:
Answer: Area / Wetted perimeter
Rationale: Hydraulic radius = A/P; depends on flow geometry, not just pipe
diameter.
6. The time of concentration for a watershed is defined as:
Answer: Time for runoff to travel from the most distant point to the outlet
Rationale: It affects peak discharge in hydrologic modeling.
7. In a rectangular channel, the critical flow condition occurs when:
Answer: Flow velocity = wave velocity (Froude number = 1)
Rationale: Critical flow occurs at Froude number Fr = 1.
, 8. A 25-year, 24-hour rainfall is 120 mm. What is the average rainfall intensity?
Answer: 5 mm/hr
Rationale: Intensity = total rainfall / duration = 120 mm / 24 hr = 5 mm/hr.
9. The Rational Method for peak discharge is best suited for watersheds:
Answer: Less than 200 acres (≈0.8 km²)
Rationale: Empirical method valid for small urban catchments.
10. The infiltration capacity of soil is exceeded when:
Answer: Rainfall intensity > infiltration rate
Rationale: Excess rainfall generates runoff when infiltration cannot keep up.
11. The Darcy-Weisbach equation is used to:
Answer: Calculate head loss due to friction
Rationale: Relates energy loss to flow velocity, pipe length, diameter, and
friction factor.
12. In hydrology, “antecedent moisture condition” (AMC) affects:
Answer: Runoff volume
Rationale: Wetter soils generate more runoff for the same rainfall.
13. The hydraulic radius of a trapezoidal channel with bottom width b, side
slopes z:1, and depth y is:
Answer: R = A / P = [y(b + zy)] / [b + 2y√(1+z²)]
Rationale: Standard formula for trapezoidal channels.
14. The stormwater detention time is primarily designed to:
Answer: Reduce peak discharge
Rationale: Detention basins temporarily store water, lowering flood peaks.
15. The Froude number is defined as:
Answer: Fr = V / √(gD)
Rationale: Ratio of flow velocity to wave celerity; determines subcritical or
supercritical flow.
16. In a groundwater aquifer, transmissivity is:
Answer: Hydraulic conductivity × aquifer thickness
Rationale: Measures aquifer’s ability to transmit water horizontally.
17. The runoff coefficient (C) in the Rational Method accounts for:
Answer: Land use, soil type, and imperviousness
Rationale: Determines fraction of rainfall contributing to runoff.
Practice Exam Questions, Correct Answers, and
Detailed Explanations for Students||Already Graded
A+
1. A rectangular open channel has a width of 3 m and a flow depth of 2 m. The
flow velocity is 1.5 m/s. What is the flow rate?
Answer: 9 m³/s
Rationale: Q = A × V; Area A = width × depth = 3 × 2 = 6 m²; Q = 6 × 1.5 = 9
m³/s.
2. Manning’s equation for open channel flow is used to estimate:
Answer: Flow velocity or discharge
Rationale: Manning’s equation relates channel slope, roughness, and hydraulic
radius to flow velocity or discharge.
3. A watershed produces 10 cm of runoff over a 5 km² area. The total runoff
volume is:
Answer: 500,000 m³
Rationale: Volume = depth × area; convert 10 cm = 0.1 m, area = 5,000,000 m²;
Volume = 0.1 × 5,000,000 = 500,000 m³.
4. The unit hydrograph is used to:
Answer: Transform rainfall excess into runoff
Rationale: It represents watershed response to a unit of effective rainfall.
5. For a circular pipe flowing half full, the hydraulic radius is:
Answer: Area / Wetted perimeter
Rationale: Hydraulic radius = A/P; depends on flow geometry, not just pipe
diameter.
6. The time of concentration for a watershed is defined as:
Answer: Time for runoff to travel from the most distant point to the outlet
Rationale: It affects peak discharge in hydrologic modeling.
7. In a rectangular channel, the critical flow condition occurs when:
Answer: Flow velocity = wave velocity (Froude number = 1)
Rationale: Critical flow occurs at Froude number Fr = 1.
, 8. A 25-year, 24-hour rainfall is 120 mm. What is the average rainfall intensity?
Answer: 5 mm/hr
Rationale: Intensity = total rainfall / duration = 120 mm / 24 hr = 5 mm/hr.
9. The Rational Method for peak discharge is best suited for watersheds:
Answer: Less than 200 acres (≈0.8 km²)
Rationale: Empirical method valid for small urban catchments.
10. The infiltration capacity of soil is exceeded when:
Answer: Rainfall intensity > infiltration rate
Rationale: Excess rainfall generates runoff when infiltration cannot keep up.
11. The Darcy-Weisbach equation is used to:
Answer: Calculate head loss due to friction
Rationale: Relates energy loss to flow velocity, pipe length, diameter, and
friction factor.
12. In hydrology, “antecedent moisture condition” (AMC) affects:
Answer: Runoff volume
Rationale: Wetter soils generate more runoff for the same rainfall.
13. The hydraulic radius of a trapezoidal channel with bottom width b, side
slopes z:1, and depth y is:
Answer: R = A / P = [y(b + zy)] / [b + 2y√(1+z²)]
Rationale: Standard formula for trapezoidal channels.
14. The stormwater detention time is primarily designed to:
Answer: Reduce peak discharge
Rationale: Detention basins temporarily store water, lowering flood peaks.
15. The Froude number is defined as:
Answer: Fr = V / √(gD)
Rationale: Ratio of flow velocity to wave celerity; determines subcritical or
supercritical flow.
16. In a groundwater aquifer, transmissivity is:
Answer: Hydraulic conductivity × aquifer thickness
Rationale: Measures aquifer’s ability to transmit water horizontally.
17. The runoff coefficient (C) in the Rational Method accounts for:
Answer: Land use, soil type, and imperviousness
Rationale: Determines fraction of rainfall contributing to runoff.
