Should dams be modified for the probable maximum flood?

The probable maximum flood (PMF) currently serves as the design standard fo r many U.S. dams. Floods used for design have increased and currently thous ands of dams in the U.S. would be overtopped and possibly fail using the la test calculated PMF at each dam site. Some researchers have suggested that modifying dams to accommodate the PMF could be wasteful. Objections to usin g the PMF for dam modification include: (1) larger spillway capacity may in crease annual downstream flood losses, (2) benefit-cost ratios may be low, (3) construction accidents associated with dam modification may cause fatal ities, and (4) the dollar amount spent to save lives by making dame safer i s often very high. Based on these objections, a procedure is presented for evaluating the effectiveness of a proposed dam modification. A change in sp illway design policy is recommended. Accepting the status quo at a dam that cannot accommodate the PMF may be the best course of action.