Propagation of anthropogenic variations in hydroclimate statistics for dynamic modeling of probable maximum precipitation

By the year 2020, 85% of dams in the United States will have surpassed 50 years of service. Ageing dams subjected to a gradual decrease in storage capacity from sedimentation coupled with unforeseen extreme precipitation events may increase the risk of failure during extreme floods. Dams today are overwhelmingly designed under the assumption of stationarity using a static design value, known as Probable Maximum Precipitation (PMP). It is therefore worthwhile to explore the impact of relaxing the assumption of stationarity and re-calculating design PMP values using currently practiced procedures, numerical modeling techniques, and observational climate trends. This study reports the findings of non-stationary PMP re-calculations at three large dam sites (South Holston Dam in Tennessee, Folsom Dam in California, and Owyhee Dam in Oregon). Results indicate that currently accepted PMP values are significantly increased by up to 20% when future changes in dew point temperature from observational trends or numerical models are taken into account. It is plausible that such future changes in these meteorological thresholds, had they been known among the engineering community when PMPs were being designed, would have received the necessary attention regarding the future uncertainty of the stationary PMP values as a dam ages. The following was concluded from the study: 1) Dams designed with static HMR PMP values are at risk of failure during a PMP event. Areas with significant post-dam irrigation development are particularly at risk. 2) Prospective dams should be constructed with the assumption of a dynamic PMP variable, not a static PMP value. 3) The mitigation of anthropogenic propagation of climate variability directly increases the safety of large dams. 4) A re-evaluation of existing and ageing dams designed with static HMR PMP values should be performed, taking into account projected climate trends due to global warming and predicted LULC changes in the post-dam era.