CFS Hydrologic & Hydraulic Analysis 3 — Questions and Answers
Question 1: A floodplain surveyor finds that a stream cross-section has a main channel and two overbank areas with different Manning's n values. How should composite roughness be calculated for the entire section?
- Use the n value of the main channel only
- Average the three n values equally
- Use a conveyance-weighted composite n for each subsection (Correct answer)
- Use the highest n value to be conservative
Correct answer: Use a conveyance-weighted composite n for each subsection
Total discharge is computed by summing conveyance (K = (1.486/n)AR^(2/3)) for each subsection; composite roughness reflects the conveyance-weighted combination.
Question 2: The concept of 'equal conveyance reduction' is used in HEC-RAS to determine:
- The location of the 100-year floodplain boundary
- The effective flow area by excluding ineffective flow regions (Correct answer)
- The correct Manning's n for vegetated overbanks
- The critical depth at a culvert outlet
Correct answer: The effective flow area by excluding ineffective flow regions
HEC-RAS uses ineffective flow areas to exclude portions of a cross-section where water is ponded or not actively conveying flow in the downstream direction.
Question 3: Which FEMA-accepted hydrologic method is specifically designed for ungauged, rural watersheds using USGS regression equations?
- Rational Method
- TR-20 unit hydrograph
- Regional regression equations (StreamStats) (Correct answer)
- SWMM continuous simulation
Correct answer: Regional regression equations (StreamStats)
USGS StreamStats provides regional regression equations for estimating flood frequencies on ungauged rural watersheds based on basin characteristics.
Question 4: In a Log-Pearson Type III frequency analysis, what does a negative skew coefficient indicate about the flood data distribution?
- The distribution has a longer tail on the high-flow side
- The distribution has a longer tail on the low-flow side (Correct answer)
- The data are perfectly symmetrical
- Insufficient data to determine the skew
Correct answer: The distribution has a longer tail on the low-flow side
A negative skew coefficient means the data distribution is skewed toward lower values with the longer tail on the left (low-flow) side.
Question 5: When a culvert operates under inlet control, what factor primarily governs the flow capacity?
- The roughness and length of the culvert barrel
- The downstream tailwater elevation
- The geometry of the culvert entrance (headwater depth and inlet geometry) (Correct answer)
- The slope of the culvert barrel
Correct answer: The geometry of the culvert entrance (headwater depth and inlet geometry)
Under inlet control, the culvert capacity is controlled by the inlet geometry, headwater depth, and opening size, not the barrel characteristics or tailwater.
Question 6: The 'effective reach length' in a HEC-RAS model is the distance used to compute friction losses between cross-sections. For overbank areas, how is this typically determined?
- Always equal to the main channel reach length
- Measured along the overbank flow path, which may differ from the main channel path (Correct answer)
- Estimated as twice the main channel distance
- Set to zero when overbanks are inactive
Correct answer: Measured along the overbank flow path, which may differ from the main channel path
Overbank reach lengths are measured along the actual overbank flow path, which is often shorter or different than the main channel centerline distance.
Question 7: According to Bulletin 17C guidelines, what is the recommended approach when historical flood data contains low outliers?
- Delete the low outlier records entirely from the dataset
- Extend the period of record using regional data to better define the distribution
- Use the Conditional Probability Adjustment to handle potentially incomplete records (Correct answer)
- Replace low outliers with the mean annual flood value
Correct answer: Use the Conditional Probability Adjustment to handle potentially incomplete records
Bulletin 17C recommends the Conditional Probability Adjustment when records may be incomplete at the low end, accounting for years when flows did not exceed a threshold.
CFS Hydrologic & Hydraulic Analysis 3
A floodplain surveyor finds that a stream cross-section has a main channel and two overbank areas with different Manning's n values.
How should composite roughness be calculated for the entire section?