Updating the Probable Maximum Flood: How Updating Meteorologic Conditions Impacts Dam Safety

Pipestem Dam in North Dakota was recently deemed a high hazard dam through analysis during a Periodic Assessment, which warranted an update to the Probable Maximum Flood (PMF). Pipestem Dam’s PMF has not been updated since it was originally developed in 1967, and when the PMF was originally estimated, only rainfall was considered as a runoff contributor. Meteorologic conditions are an integral part of calculating the PMF. Historically, flooding in the Pipestem watershed has been the result of a combination of runoff from snowmelt and rainfall. For this reason, when updating the PMF, an antecedent snow water equivalent (SWE) component was included as well as an update to the Probable Maximum Precipitation (PMP) estimate. Several other assumptions were also updated including contributing drainage area, infiltration rates, and unit hydrograph peaking.

Two runoff conditions were considered in updating the PMF for Pipestem Dam: rainfall only and rain on snow. For a rainfall only PMF, an all-season PMP was developed according to Hydrometeorological Report (HMR) 51 and 52 standards with consideration of more recent events. Tools such as HEC-MetVue and the MMC Precip Tool were used to develop the PMP for the Pipestem watershed. For a rain on snow runoff event, a seasonal May PMP, developed using HMR 53, falls on a 1% annual chance exceedance antecedent SWE. The antecedent SWE component was developed based on historic SWE records using HEC-MetVue and HEC-DSSVue. A temperature sequence was developed to melt the snow that coincided with temperatures needed to produce the seasonal May PMP. The two runoff conditions were compared to determine the driving meteorologic conditions for a PMF event, and the resulting updated PMF was a significant increase from the original estimate.

This presentation will outline the process of updating the PMF for Pipestem Dam, with a focus on development of the antecedent SWE and PMP components. Other updated assumptions will also be discussed, as well as how the update will be incorporated into risk management decisions.