When you have a situation where a high exit gradient is predicted at the downstream base of a dam what could be done to the dam design to reduce the exit gradient

• A. Lengthen the flow path, install a drain, and drawdown the reservoir
• B. Raise the dam crest
• C. Increase tailwater elevation
• D. Reduce spillway capacity

Answer: (A)

When seepage through the foundation creates a high exit gradient at the downstream base, the goal is to reduce the hydraulic gradient that water experiences as it exits at the toe. You can achieve this by altering the flow path and the pressure driving the seepage. Lengthening the flow path through the dam or foundation dissipates head over a longer distance, lowering the gradient that reaches the downstream base. Installing a drain provides an outlet for seepage, reducing the pressure buildup at the exit and helping to keep the gradient lower. Drawing down the reservoir reduces the upstream water level, which directly lowers the head driving seepage and further reduces the exit gradient.

Raising the crest would increase the head on the downstream side and is not helpful for reducing the gradient. Increasing tailwater elevation changes boundary conditions but is not as direct a dam-design measure for lowering exit gradient. Reducing spillway capacity tends to raise reservoir levels and head, which would worsen seepage pressures rather than alleviate them.