/ s . ▁ ▁When ▁cav itation ▁occurs ▁on ▁a ▁sp ill way , ▁it ▁can ▁cause ▁severe ▁damage . ▁This ▁is ▁especially ▁true ▁when ▁the ▁veloc ities ▁exceed ▁ 2 5   m / s . ▁Therefore , ▁protection ▁is ▁needed ▁at ▁these ▁veloc ities . ▁Cav itation ▁can ▁be ▁prevent ed ▁by ▁decre asing ▁the ▁flow ▁velocity ▁or ▁by ▁increasing ▁the ▁boundary ▁pressure . ▁ ▁Energy ▁diss ip ation ▁ ▁Every ▁dam ▁needs ▁some ▁form ▁of ▁energy ▁diss ip ation ▁in ▁its ▁dis charge ▁structure ▁to ▁prevent ▁er os ion ▁and ▁sc our ▁on ▁the ▁down stream ▁side ▁of ▁the ▁dam , ▁since ▁these ▁phen omena ▁can ▁result ▁in ▁dam ▁failure . ▁Pl unge ▁po ols ▁( also ▁called ▁still ing ▁bas ins ) ▁and ▁impact ▁boxes ▁are ▁two ▁examples ▁of ▁energy ▁diss ip ators ▁used ▁on ▁d ams . ▁ ▁Many ▁USB R ▁d ams ▁use ▁energy ▁diss ip ating ▁blocks ▁for ▁ch ute ▁sp ill ways ▁( also ▁called ▁b aff led ▁apr ons ). ▁These ▁blocks ▁help ▁indu ce ▁a ▁hyd ra ul ic ▁jump ▁to ▁establish ▁sub crit ical ▁flow ▁conditions ▁on ▁the ▁down stream ▁side ▁of ▁the ▁dam . ▁ ▁The ▁steps ▁on ▁stepped ▁sp ill ways ▁can ▁be ▁used ▁for ▁energy ▁diss ip ation . ▁However , ▁they ▁tend ▁to ▁be ▁effective ▁only ▁at ▁diss ip ating ▁energy ▁at ▁low ▁flows ▁( i . e . ▁sk imming ▁flow ). ▁ ▁See ▁also ▁ ▁Dam ▁Lake ▁D isch ar ge ▁Problem ▁Sp ill way ▁Ste