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

• A. Gentle
• B. Moderate
• C. Steep
• D. Flat

Answer: (C)

The effect comes from how flow behaves when it has to turn around a bend. As the bend becomes sharper, the flow near the outer bank has to accelerate more and change direction more abruptly. That increased turning raises centrifugal forces, lowers the pressure along the outer bank, and makes a fast layer of water hug the bank. That combination is what we call bank suction (the pull of low pressure away from the bank) and the bank cushion (the fast-moving water acting along the bank surface). When the bank is steep, the bend is sharpest, so these effects are strongest; with gentler, moderate, or flat banks, the turn is less abrupt and the suction and cushion are less pronounced.