Hydrostatic pressure is based on the height of a column of liquid and acts equally in all directions. You could say that a dam keyway would help the dam to resist or hold a higher hydrostatic pressure. The only way to reduce the pressure at any given point under water is to reduce the amount of water above it. The height of the water column is the only variable here. Hydrostatic pressure always acts normal to a surface. Decreasing the slope on the face of a dam changes the direction of the force the water exerts on it. A shallower slope directs the force more downward. However, the area of the face of the dam also increases as the slope decreases, so that the total sum of the force is greater. Since the change in surface area is based on the sine of the dam face angle, the net result is that there is always the same amount of force trying to push the dam away from the pond, no matter what the angle. It's the product of the pressure across the depth x the cross sectional area of the valley being dammed. Looking forward to some rain in TN this weekend.
