Dams are designed to store runoff resulting
from heavy rain. When storage has reached a specific limit,
spillways are designed to pass the excess water initially through a primary spillway. Many dams are equipped with an emergency spillway
to help pass large flood inflows which exceed the capacity of the primary spillway. In an extreme flood flows can exceed the combined
spillway capacity, spill over the top of the dam and create a very dangerous situation. Over topping flood flows rapidly erode down
into an earthen embankment dam leading to an uncontrolled release of the stored waters
and causing catastrophic flooding down stream.