The Influence of Tropical Cyclones on the Evolution of River Conveyance Capacity in Puerto Rico

Tropical cyclones (TCs) in Puerto Rico and other tropical environments cause some of the world's most intense rainfall rates and pose major challenges to current and future flood resilience. One potential consequence of TC-induced floods is the reconfiguration of river channels due to sediment scour or deposition. This reconfiguration has the potential to alter subsequent flood hazard by changing channel conveyance capacity, violating statistical independence assumptions that underpin conventional recurrence interval concepts. In this study, we examine changes in channel conveyance capacity in Puerto Rico and compare these changes to streamflow trends. Results show that relatively modest long-term changes in river channel capacity are composed of numerous short-term transients which are of much larger magnitude. These transients are most often caused by abrupt scour or deposition during TCs and are comparable in magnitude to long-term trends in peak streamflows. An abrupt change is typically followed by a multiyear recovery period as the channel reestablishes quasi-equilibrium. Flood events with recurrence intervals of approximately 10 years and above appear to be sufficient to cause these changes; channel reconfiguration thus may be more common in Puerto Rico than in less extreme hydroclimates. Short-term conveyance capacity changes are not considered in typical flood hazard assessments, which could substantially overstate or understate flood threat at any particular time, depending on the recent history of TC rainfall and flooding. Improving flood resiliency in Puerto Rico and elsewhere will require better understanding of rapid conveyance capacity changes, their causes, and how they influence flood hazard and risk.