SRT Hydraulics & River Classification 2 — Questions and Answers
Question 1: What feature is formed when fast-moving water drops over an obstruction and recirculates back upstream at the surface?
- Eddy
- Hydraulic (hole) (Correct answer)
- Pillow water
- Boil
Correct answer: Hydraulic (hole)
A hydraulic, or hole, forms when water drops over a ledge and the surface current recirculates back upstream, creating a potentially entrapping feature.
Question 2: An eddy line is best described as the boundary between:
- Fast main current and a calm pool below a rapid
- Downstream current and the upstream-flowing water behind an obstruction (Correct answer)
- Two rivers of different water temperatures merging
- Surface water and subsurface flow in a pool
Correct answer: Downstream current and the upstream-flowing water behind an obstruction
An eddy line separates the downstream main current from the upstream-moving water that has swung behind an obstacle, creating a distinct shear zone.
Question 3: Pillow water on the upstream face of a rock is primarily caused by:
- Backwash from the hydraulic downstream
- Water piling up as current is deflected around or over an obstacle (Correct answer)
- Subsurface springs feeding upward pressure
- Eddy currents rotating toward the rock face
Correct answer: Water piling up as current is deflected around or over an obstacle
Pillow water forms when moving current slams into a rock and the water pressure builds and deflects upward and to the sides around the obstacle.
Question 4: Standing waves (haystacks) in a river are formed when:
- Wind opposes the current direction
- Fast water enters a pool and decelerates, causing the water surface to oscillate (Correct answer)
- Two eddies collide at the center of the channel
- A submerged rock deflects flow upward at a consistent angle
Correct answer: Fast water enters a pool and decelerates, causing the water surface to oscillate
Standing waves form at the transition zone where fast water decelerates into slower water, creating stationary wave peaks that oscillate in height.
Question 5: What distinguishes a sieve from a strainer in swiftwater terminology?
- A sieve is above water; a strainer is submerged
- A sieve involves fixed rock gaps; a strainer typically involves debris like logs or branches (Correct answer)
- Sieves flush swimmers through; strainers always trap them
- A sieve is in a rapid; a strainer is only found in flatwater
Correct answer: A sieve involves fixed rock gaps; a strainer typically involves debris like logs or branches
Sieves are typically gaps in fixed rock formations, while strainers are debris (logs, branches, root balls) that allow water to pass but trap solid objects like swimmers.
Question 6: A boil on the river surface indicates:
- Aerated, oxygenated water from a recent drop
- Upwelling of subsurface current striking the riverbed and returning to the surface (Correct answer)
- A recirculating hydraulic that extends below the surface
- Thermal activity increasing water temperature locally
Correct answer: Upwelling of subsurface current striking the riverbed and returning to the surface
Boils occur when a strong subsurface current hits the riverbed or an obstruction and deflects upward, creating an upwelling that appears to boil at the surface.
Question 7: Undercut rocks are particularly dangerous because:
- They create unpredictable surface turbulence that capsizes boats
- Water flows beneath the rock, trapping swimmers in a cavity with no air space (Correct answer)
- They generate the strongest hydraulics on a river section
- Their smooth surfaces prevent any purchase for self-rescue
Correct answer: Water flows beneath the rock, trapping swimmers in a cavity with no air space
Undercut rocks have cavities beneath them where current flows; a swimmer dragged into the undercut faces entrapment with no air, making rescue extremely difficult.
SRT Hydraulics & River Classification 2
What feature is formed when fast-moving water drops over an obstruction and recirculates back upstream at the surface?