SRT Hydrology and River Hazards 3 — Questions and Answers
Question 1: When reading a river, what does an upstream-pointing V between rocks indicate?
- The safest channel for navigation
- A submerged rock or obstruction just below the surface (Correct answer)
- A strong eddy current
- A pillow of water indicating deep water ahead
Correct answer: A submerged rock or obstruction just below the surface
An upstream-pointing V (pointing toward you as you face downstream) indicates converging currents caused by a submerged rock or obstruction beneath the surface.
Question 2: What is 'laminar flow' in the context of river hydraulics?
- Water flowing in parallel layers with minimal mixing between layers (Correct answer)
- Chaotic, turbulent water mixing at a rapid
- The circular recirculation in an eddy
- High-velocity water at the surface of a deep channel
Correct answer: Water flowing in parallel layers with minimal mixing between layers
Laminar flow occurs when water moves in parallel layers with little disruption between them, typically found in slow, deep, smooth sections of a river.
Question 3: Which principle explains why water flows faster in the center of a river channel than near the banks?
- The Bernoulli effect from channel narrowing
- Hydraulic radius and boundary friction reducing velocity near channel edges (Correct answer)
- Increased gradient in the center thalweg
- Greater water depth reducing pressure in midchannel
Correct answer: Hydraulic radius and boundary friction reducing velocity near channel edges
Friction from the riverbed and banks slows water near the edges and bottom, so velocity is greatest away from these boundaries, typically at center and surface.
Question 4: A rescue swimmer enters an eddy to stage for a live-bait rescue. What current direction will they encounter when crossing the eddy line?
- No directional change; current is uniform across the eddy line
- A sudden shift from upstream-moving eddy current to faster downstream main current (Correct answer)
- A lateral current pushing them downstream parallel to the eddy line
- A downward current pulling them under the eddy line
Correct answer: A sudden shift from upstream-moving eddy current to faster downstream main current
The eddy line is a shear zone where the upstream-moving eddy current meets the faster downstream main current, requiring the rescuer to lean and anticipate the force change.
Question 5: What does the term 'hydraulic gradient' refer to in river hydrology?
- The temperature change in water as depth increases
- The drop in elevation of the riverbed per unit of horizontal distance (Correct answer)
- The ratio of sediment load to discharge
- The difference in velocity between surface and bottom water
Correct answer: The drop in elevation of the riverbed per unit of horizontal distance
Hydraulic gradient (or river slope) is the vertical drop divided by horizontal distance, directly influencing current velocity — steeper gradients produce faster water.
Question 6: Which hazard is specifically associated with foot entrapment in moving water?
- Hypothermia from prolonged cold water immersion
- Hydrodynamic force pinning the victim face-down against the current (Correct answer)
- Sudden cardiac arrest from cold water shock
- Disorientation caused by turbulent water
Correct answer: Hydrodynamic force pinning the victim face-down against the current
When a foot becomes trapped on the riverbed, the hydrodynamic force of moving water pushes the victim face-down, making self-rescue nearly impossible and causing rapid drowning.
Question 7: What is the 'thalweg' of a river?
- The line of maximum depth along the river channel (Correct answer)
- The flood plain boundary on either side of the river
- A German term for a shallow bar crossing the channel
- The upstream limit of tidal influence in a river
Correct answer: The line of maximum depth along the river channel
The thalweg is the line connecting the deepest points of the river channel along its length, typically where the fastest current flows.
SRT Hydrology and River Hazards 3
When reading a river, what does an upstream-pointing V between rocks indicate?