expected during the design flood is obtained by hydrologic calculations of the upstream watershed. The return period is set by dam safety guidelines, based on the size of the structure and the potential loss of human life or property downstream. The United States Army Corps of Engineers bases their requirements on the probable maximum flood (PMF) and the probable maximum precipitation (PMP). The PMP is the largest precipitation thought to be physically possible in the upstream watershed. Dams of lower hazard may be allowed to have an IDF less than the PMF. Energy dissipation As water passes over a spillway and down the chute, potential energy converts into increasing kinetic energy. Failure to dissipate the water's energy can lead to scouring and erosion at the dam's toe (base). This can cause spillway damage and undermine the dam's stability. To put this energy in perspective, the spillways at Tarbela Dam could, at full capacity, produce 40,000 MW; about 10 times the capacity of its power plant. The energy can be dissipated by addressing one or more parts of a spillway's design. Steps First, on the spillway surface itself by a series of steps along the spillway (see stepped spillway). Flip bucket Second, at the base of a spillway, a flip bucket can create a hydraulic jump and deflect water upwards. Ski jump A ski jump can also direct water horizontally and eventually down into a plunge pool or two ski jumps can direct their water discharges to collide with one another. Stilling basin Third, a stilling basin at the terminus of a spillway serves to further dissipate energy and prevent erosion. They are usually filled with a relatively shallow depth of water and sometimes lined with concrete. A number of velocity-reducing components can be incorporated into their design to include chute blocks, baffle blocks, wing walls, surface boils, or end sills. Safety Spillway gates may operate suddenly without warning, under remote control. Trespassers within the spillway are at risk of drowning. Spillways are usually fenced and equipped with locked gates to prevent casual trespassing within the structure. Warning signs, sirens, and other measures may be in place to warn users of the downstream area of sudden release of water. Operating protocols may require "cracking" a gate to release a small amount of water to warn persons downstream. The sudden closure of a spillway gate can result in the stranding of fish,