CPESC Hydrology & Hydraulics 2 — Questions and Answers
Question 1: What is the hydraulic radius of a channel cross-section, and how is it calculated?
- The average depth of flow; calculated as total depth divided by 2
- The ratio of flow area to wetted perimeter; A/P (Correct answer)
- The width of the water surface; top width of flow
- The maximum depth of flow in the channel cross-section
Correct answer: The ratio of flow area to wetted perimeter; A/P
Hydraulic radius (R) = cross-sectional flow area (A) divided by the wetted perimeter (P), and is a key parameter in Manning's equation for channel design.
Question 2: A 2-year, 24-hour storm event is commonly used for which type of erosion and sediment control design?
- Sizing permanent retention ponds for post-construction
- Designing temporary diversion channels and sediment basins during construction (Correct answer)
- Computing 100-year floodplain boundaries
- Determining TMDL allocations for receiving streams
Correct answer: Designing temporary diversion channels and sediment basins during construction
Many state stormwater regulations require temporary BMPs such as diversions and sediment basins to be designed for the 2-year, 24-hour storm to provide adequate protection during construction.
Question 3: In stormwater hydrology, the term 'imperviousness' refers to:
- The ability of soil to resist compaction under load
- The percentage of a watershed surface that does not allow water infiltration (Correct answer)
- The depth of soil available for root zone water storage
- The rate at which sediment accumulates in a basin
Correct answer: The percentage of a watershed surface that does not allow water infiltration
Imperviousness is the fraction of land surface covered by impervious materials (pavement, rooftops) that prevent infiltration and generate direct runoff.
Question 4: Which flow regime is considered most erosive and is characterized by a Froude number greater than 1.0?
- Subcritical (tranquil) flow
- Supercritical (rapid) flow (Correct answer)
- Laminar flow
- Transitional flow
Correct answer: Supercritical (rapid) flow
Supercritical flow (Fr > 1.0) has high velocity and shallow depth, producing significant shear stress on channel beds and banks, making it highly erosive.
Question 5: The unit peak discharge (qu) used in the TR-55 graphical peak discharge method is expressed in units of:
- Cubic feet per second per acre (cfs/acre)
- Cubic feet per second per inch of runoff per square mile (csm/in) (Correct answer)
- Gallons per minute per impervious acre
- Acre-feet per hour
Correct answer: Cubic feet per second per inch of runoff per square mile (csm/in)
In the TR-55 method, unit peak discharge (qu) is expressed as csm/in (cubic feet per second per square mile per inch of runoff), used to compute peak discharge from small watersheds.
Question 6: Detention storage in stormwater management is designed primarily to:
- Permanently retain all runoff on-site to prevent any discharge
- Temporarily store runoff to reduce peak discharge rates to pre-development levels (Correct answer)
- Filter sediment from runoff through a gravel media
- Promote groundwater recharge through infiltration
Correct answer: Temporarily store runoff to reduce peak discharge rates to pre-development levels
Detention basins temporarily store stormwater and release it at a controlled rate to reduce downstream peak flows, protecting channel stability and reducing erosion.
Question 7: Sheet flow, shallow concentrated flow, and channel flow are the three flow regimes used in calculating time of concentration. Which typically has the longest travel time per unit length?
- Channel flow through a well-defined stream
- Shallow concentrated flow over paved surfaces
- Sheet flow across vegetated overland surfaces (Correct answer)
- Pipe flow in a storm drain system
Correct answer: Sheet flow across vegetated overland surfaces
Sheet flow over vegetated surfaces moves very slowly due to high friction, resulting in the longest travel time per unit length and dominating the Tc calculation for small watersheds.
CPESC Hydrology & Hydraulics 2
What is the hydraulic radius of a channel cross-section, and how is it calculated?