Bank cushion and bank suction are strongest when the bank of a channel is

• A. Flat
• B. Steep
• C. Gentle slope
• D. Curved

Answer: (B)

When water moves through a channel, the geometry of the bank controls how strongly the flow has to turn and accelerate near that bank. A steep bank creates a sharper bend, so the flow must deflect more abruptly. This stronger deflection speeds up the water along the outer side of the bend and lowers the pressure there, producing a noticeable suction effect on the outer bank. At the same time, the sharper turn increases the overall forces the water exerts on the near-bank region, intensifying what is felt as a bank cushion as the flow presses against the bank. In contrast, a flat or gently sloped bank doesn't force as sharp a turn, so the velocity changes and pressure differences—and thus those bank forces—are much weaker. A curved bank can cause some of these effects, but not as strongly as a steep bank because the turn isn’t as abrupt.