TROPICAL CYCLONES AND SEDIMENT DYNAMICS IN MOUNTAINOUS ISLANDS: EROSION, TRANSPORT, AND COASTAL DELIVERY IN PUERTO RICO

Abstract

Small, tropical and sub-tropical mountainous islands exposed to episodic tropical cyclones export a disproportionate sediment mass to the world’s coasts. Yet, the multiple mechanisms of sediment erosion via landslides in mountainous landscapes, fluvial transport, and export to the coast that occur during tropical cyclones are difficult to measure, thus limiting our understanding of the role of tropical cyclones in watershed-scale sediment dynamics and their importance in coastal sediment delivery. Sediment supply to coastal landforms in tropical coasts serve as building blocks to sustain beaches and mangrove forests wetlands. These landscapes provide high ecologic and economic value, and often serve as natural infrastructure and barriers to protect human activities along the coast. Despite their value, these ecosystems are threatened by a combination of coastal erosion and sea level rise, hence making sediment supply potentially critical for these systems to adapt. However, we do not know how important episodic tropical cyclone sediment delivery to mangrove forests is. Here I aim to study the role of tropical cyclones in three components of the basin-scale sediment budget dynamics in tropical islands: (1) sediment erosion via landslides, (2) fluvial transport, and (3) sediment export captured in reservoirs stratigraphy. I use Hurricane María (2017) and Fiona (2022) in Puerto Rico (PR) as an example of extreme events. And finally, I aim to quantify the relative role of tropical cyclones in delivering sediments to riverine and tidal fringe mangrove forests and how important minerogenic sediments are in soil carbon stocks and sequestration potential. In Chapter 2, I aim to characterize the sedimentary and stratigraphic signature of María (2017) and Fiona (2022) relative to background sediments and compare how well internal stratigraphic structures reflect temporal patters in extreme flooding and sediment delivery associated with tropical cyclones. Here I collected repeated sediment cores at Caonillas reservoir to measure down-core geochemical and physical sediment properties: X-ray fluorescence (XRF), grain size, and loss-on-ignition (LOI). I found that María in 2017 generated three deposits characterized by relatively inorganic, medium-sized sand that is enriched in Sr and Ca elements relative to background mud. Additional cores collected after Fiona in 2022 revealed an additional event deposit attributed to the event, enriched in Be-7, which suggests recent deposition. Event deposits are linked to multiple flash floods generated during Maria and linked to individual rain bands evident in radar imagery collected during the event. Several rainbands of intense precipitation created the ideal conditions to erode sediments via landslide triggering and generate high transport capacity in the fluvial network to export sediments from upland watershed. In Chapter 3, I aim to quantify the sediment budget from landslide erosion and fluvial transport during extreme events like María, and contextualize the role of tropical cyclones in sediment yields in the Caonillas reservoir, using María as an example of an extreme geomorphic event. Here I used repeated bathymetric surveys from 1990, 1995, 2000, 2012, and 2020 to assess sediment accumulation after María and contrast it with background rates of sediment accumulation. I found that 69% (± 2.9%) of the sediment deposited between 1995-2020 were related to four tropical cyclones, where María represents 23% (3.0 ± 0.1 x 106 Mg) of the total sediment accumulation. To assess sediment delivery, I built two rating curves for the period of 1995-2022 from an upstream hydrograph to assess suspended sediment delivery during María, Fiona, and three other earlier tropical cyclones. I found that 72.5% (± 3.3%) of the suspended sediment delivery were related to these tropical cyclones between 1995-2022, where María delivered 25% (5.3 ± 0.2 x 106 Mg) of the total suspended sediment transported over the entire study period. Sediment deposition and suspended sediment delivery estimates correspond to the equivalent of 20-22 and 23-24 years of long-term, background sediment yields, respectively. In addition, sediment erosion from landslides was calculated using lidar-derived DEMs collected before María in 2015, and after the event in 2018. I found that María mobilized 1.2 ± 0.2 x 106 Mg of sediments via landslides, which represent 8-11 years worth of annual sediment yields and 40% of the total María deposits, assuming all eroded sediment gets exported. Although this 1995-2022 period was particularly active in tropical cyclone impacts relative to the 20th century, our observations show that tropical cyclone-related sediment erosion and transport are the main drivers of sediment mobilization and transport in the 1995-2022 period, more than doubling long-term sediment delivery before 1990. In Chapter 4, I aimed to determine the relative importance of minerogenic sediment supply on soil organic carbon stocks and sequestration rates in semi-arid mangrove systems. I sampled three mangrove forests whose soils represent different levels of sediment supply. Tidal fringe mangrove forests of Los Machos and Jobos Bay represent sites with minimal minerogenic sediment supply evident by their high organogenic, peaty substrate soils, while Rio Yauco Delta represents high sediment supply with minerogenic substrate and low organic-rich peat development. Despite these differences, carbon stocks for both tidal fringe and riverine mangrove soils contained similar soil organic carbon stocks with median values of 362 Mg C ha-1 and 352 Mg C ha-1, respectively. Organogenic mangrove sites in tidal fringe geomorphic setting showed lower accretion rates (1.04 mm yr-1 for MAC and 0.78 mm yr-1 for JOB) than the riverine site (4.5 mm yr-1 for RYD). Tidal fringe mangroves have also been historically prone to extensive die-off events with up to 45% in mangrove cover reduction These fringe systems are also more constrained in space than riverine mangroves, which showed mangrove expansion prior to a reductions in sediment supply likely caused by upstream damming in 1952. Our findings further inform soil carbon stocks accounting in underrepresented semi-arid mangroves environments and documents diverse anthropogenic impacts and their effects on carbon sequestration services by these systems. The findings presented in this dissertation improves our understanding of upland sediment mobilization, transport, and export during large tropical cyclones in sub-tropical mountainous islands as well as their relative role in providing valuable sediments to coastal mangrove forests ecosystems. I found that intense rainfall derived from tropical cyclones, when occurring frequently enough, can be main drivers of sediment export in steep, landslide-prone basins. Landslides are a significant source (40%) of sediments in Caonillas sediment budget during hurricane María (2017), yet other geomorphic sources (e.g. gullies and channels) are important as well. Although outside of the scope of this study, it is acknowledge that gully and channel erosion are two additional sources that have yet to be constrained. Event derived sediment yields from steep upland basins can be 23-25 years of annual background sediment export, yet no significant event sediment deposition from María is observed in the two organogenic, tidal fringe mangrove forests sites. Contrastingly, small mangrove expansion was observed at the river mouth in the minerogenic substrate, riverine mangrove forests. Despite smaller area cover and reduced sediment supply due to upstream dam construction in 1952, high minerogenic sediment accretion rates at this riverine site makes it more effective in sequestering soil organic carbon as sea level increases. This illustrates the trade-offs between securing water and clean hydroelectric energy sources via dams and reservoirs, while the reduced sediment supply to the coast diminishes the carbon sequestration potential of downstream mangrove forests.The work presented in this dissertation was supported by NSF NRT ELEVATE (Elevating Equity Values in the Transition of the Energy System) (Award Number: 2021693), UMass Spaulding Smith Fellowship, Leo M. Hall Memorial Fund, Gloria Radke Memorial Fund, and NSF/GSA Graduate Research Fellowship, and John A. Black award.Doctor of Philosophy (Ph.D.)2026-02-0