CSM Hydrology & Watershed Management 3 — Questions and Answers
Question 1: Which soil hydrologic group has the highest runoff potential and lowest infiltration rate when thoroughly wet?
- Group A
- Group B
- Group C
- Group D (Correct answer)
Correct answer: Group D
Hydrologic Soil Group D consists of soils with very slow infiltration rates (clay soils or soils with a high water table) and produces the highest runoff.
Question 2: The concept of 'bankfull discharge' is significant in watershed management because it:
- Represents the 100-year flood recurrence interval
- Is the flow that does the most geomorphic work and shapes the channel (Correct answer)
- Marks the point at which levee overtopping begins
- Equals the mean annual flood discharge
Correct answer: Is the flow that does the most geomorphic work and shapes the channel
Bankfull discharge, typically corresponding to the 1.5- to 2-year return interval, is responsible for transporting the most sediment and maintaining channel form over time.
Question 3: What is the primary purpose of a unit hydrograph in stormwater hydrology?
- To estimate average annual runoff from long-term precipitation records
- To relate a unit depth of excess rainfall to a direct runoff hydrograph for a watershed (Correct answer)
- To calculate base flow contributions to total streamflow
- To determine the time of concentration using travel time segments
Correct answer: To relate a unit depth of excess rainfall to a direct runoff hydrograph for a watershed
A unit hydrograph represents the direct runoff response of a watershed to one unit (1 inch or 1 cm) of excess rainfall uniformly distributed over the watershed for a specified duration.
Question 4: Which of the following best describes the 'baseflow recession' portion of a streamflow hydrograph?
- The rising limb that occurs immediately after rainfall begins
- The slow drainage of groundwater into the stream after storm runoff has ceased (Correct answer)
- The peak flow portion caused by direct surface runoff
- The falling limb due to decreasing rainfall intensity
Correct answer: The slow drainage of groundwater into the stream after storm runoff has ceased
Baseflow recession represents the gradual depletion of groundwater storage that sustains streamflow between storm events, typically following an exponential decay pattern.
Question 5: A developer proposes to fill a wetland that provides 50 acre-feet of floodwater storage. Under the watershed management perspective, this action would most likely:
- Reduce downstream peak flows by improving channel conveyance
- Increase downstream flood peaks by eliminating natural detention storage (Correct answer)
- Have no effect on downstream flows if compensatory storage is provided upstream
- Decrease sediment transport capacity in downstream reaches
Correct answer: Increase downstream flood peaks by eliminating natural detention storage
Wetlands attenuate flood peaks by temporarily storing floodwater; eliminating this storage increases and accelerates downstream runoff, raising peak flows.
Question 6: The dimensionless unit hydrograph developed by the NRCS is characterized by:
- A time to peak equal to the storm duration
- 37% of the total volume occurring before the peak
- A recession limb that is longer than the rising limb (Correct answer)
- A symmetric shape with equal rising and falling limbs
Correct answer: A recession limb that is longer than the rising limb
The NRCS dimensionless unit hydrograph has a skewed shape where the recession limb extends approximately 1.67 times longer than the rising limb, reflecting slower groundwater drainage.
Question 7: In watershed delineation, a 'pour point' refers to:
- The highest elevation point on the watershed boundary
- The outlet or lowest point through which all watershed runoff exits (Correct answer)
- The location of maximum soil infiltration rate
- A point where two stream tributaries merge
Correct answer: The outlet or lowest point through which all watershed runoff exits
The pour point (or watershed outlet) is the single point at the lowest elevation through which all surface water within the delineated watershed drains.
CSM Hydrology & Watershed Management 3
Which soil hydrologic group has the highest runoff potential and lowest infiltration rate when thoroughly wet?