Novel Updates to Tropical Storm Remnant Precipitation Frequency Estimates for Enhanced Dam Safety Decision-Making in the Tennessee Valley

In 2015, the Tennessee Valley Authority (TVA) adopted a risk-informed decision-making approach for dam safety, prompting a Probabilistic Flood Hazard Analysis across the region. This approach evaluates the frequency of hazardous events, dam responses, and consequences of dam failures. A crucial element is the likelihood of external hazards like precipitation. Consequently, a regional precipitation frequency (PF) study was performed, estimating precipitation magnitudes for various annual exceedance probabilities, ranging from common (1-in-10 year) to rare (1-in-10⁶ year) events.

PF estimates were independently calculated for different storm types: Local Storms, Mesoscale Storms with Embedded Convection, Mid-Latitude Cyclones, and Tropical Storm Remnants (TSR). Grouping precipitation events by storm type yielded more homogenous datasets, reducing statistical uncertainty. However, the data for TSRs was limited due to their infrequent occurrence.

Nearly a decade later, additional data justifies updating the TSR PF estimates. Data from the TVA Rain Gauge Network, previously excluded due to its relatively recent establishment and short record length, can now be utilized. This network's dense station coverage allows for spatial precipitation patterns to be resolved in more detail. Furthermore, updates to the International Best Track Archive for Climate Stewardship (IBTrACS) dataset improve storm track accuracy, enhancing TSR event classification.

A significant update involved reassessing the mixed distribution of TSRs. The theta parameter represents the fraction of years without significant TSR events. Previously, a single theta value (45%) was used across the region. The updated analysis, leveraging the TVA Rain Gauge Network data, recalculated theta as a spatially varying parameter. Results showed lower theta values east of the Appalachians and decreasing values from north to south, aligning with meteorological expectations.

The recalculated TSR PF estimates revealed new regional differences, highlighting the importance of the new data and methods, offering improved flood modeling insights and informing future risk-informed decision-making for dam safety.