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.
At-a-station hydraulic geometry (AHG), which describes how channel width, depth, and velocity vary with discharge at a river cross section, has long been used to study fluvial processes. For example, identification of landscape and river reach drivers of hydraulic geometry can help to predict channel properties at ungaged sites and to understand channel responses to major floods. Most prior AHG studies have focused on mid-latitude, temperate regions. Tropical zones-including those affected by tropical cyclones (TCs)-have received less attention. This study analyzed spatial and temporal variability in hydraulic geometry at 24 stream gaging sites in Puerto Rico, and identified the watershed and river reach characteristics that correlate with each hydraulic geometry parameter. These characteristics were then used to build regression models of AHG parameters, with relatively high predictive power. The largest flood events from each site were found to cause systematic changes to AHG parameters; most of these floods were caused by major TCs. Upstream drainage area, average watershed elevation, watershed land cover and other characteristics were found to be significant predictors of AHG parameters. Reaches with steeper slopes were found to have limited lateral adjustability, which may reflect consolidated bank materials and valley confinement. Watersheds with high percentages of forested area showed greater changes in roughness but less vertical adjustability than more developed watersheds. These correlation results help inform whether river channel properties in Puerto Rico and similar environments are resistant to the forces of TC-induced flooding, and how these properties are affected by major floods.
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