Forecasting of Future Flooding and Risk Assessment under CMIP6 Climate Projection in Neuse River, North Carolina

Hydrological extremes associated with climate change are becoming an increasing concern all over the world. Frequent flooding, one of the extremes, needs to be analyzed while considering climate change to mitigate flood risk. This study forecast streamflow and evaluate risk of flooding in the Neuse River, North Carolina considering future climatic scenarios, and comparing them with an existing Federal Emergency Management Agency study. The cumulative distribution function transformation method was adopted for bias correction to reduce the uncertainty present in the Coupled Model Intercomparison Project Phase 6 (CMIP6) streamflow data. To calculate 100-year and 500-year flood discharges, the Generalized Extreme Value (L-Moment) was utilized on bias-corrected multimodel ensemble data with different climate projections. Out of all projections, shared socio-economic pathways (SSP5-8.5) exhibited the maximum design streamflow, which was routed through a hydraulic model, the Hydrological Engineering Center’s River Analysis System (HEC-RAS), to generate flood inundation and risk maps. The result indicates an increase in flood inundation extent compared to the existing study, depicting a higher flood hazard and risk in the future. This study highlights the importance of forecasting future flood risk and utilizing the projected climate data to obtain essential information to determine effective strategic plans for future floodplain management.

Effects of Ferrous Iron and Hydrogen Sulfide on Nitrate Reduction in the Sediments of an Estuary Experiencing Hypoxia

Hypoxia is common feature of eutrophic estuaries and semi-enclosed seas globally. One of the key factors driving hypoxia is nitrogen pollution. To gain more insight into the effects of hypoxia on estuarine nitrogen cycling, we measured potential nitrate reduction rates at different salinities and levels of hypoxia in a eutrophic temperate microtidal estuary, the Neuse River Estuary, North Carolina, USA. We also tested the effect of hydrogen sulfide and ferrous iron additions on the nitrate reduction pathways. Overall, DNRA dominated over denitrification in this periodically hypoxic estuary and there was no correlation between the potential nitrate reduction rates, salinity, or dissolved oxygen. However, when hypoxia lasted several months, denitrification capacity was almost completely lost, and nearly all nitrate added to the sediment was reduced via DNRA. Additions of hydrogen sulfide stimulated DNRA over denitrification. Additions of ferrous iron stimulated nitrate consumption; however, the end product of nitrate consumption was not clear. Interestingly, substantial nitrous oxide formation occurred in sediments that had experienced prolonged hypoxia and were amended with nitrate. Given expanding hypoxia predicted with climate change scenarios and the increasing nitrate loads to coastal systems, coastal sediments may lose their capability to mitigate nitrogen pollution due to DNRA dominating over denitrification during extended hypoxic periods.

Pulsed terrestrial organic carbon persists in an estuarine environment after major storm events

<p>The transport of dissolved organic carbon from land to ocean is a large and dynamic component of the global carbon cycle. Export of dissolved organic carbon from watersheds is largely controlled by hydrology, and is exacerbated by increasing major rainfall and storm events, causing pulses of terrestrial dissolved organic carbon (DOC) to be shunted through rivers downstream to estuaries. Despite this increasing trend, the fate of the pulsed terrestrial DOC in estuaries remains uncertain. Here we present DOC data from 1999 to 2017 in Neuse River Estuary (NC, USA) and analyze the effect of six tropical cyclones (TC) during that period on the quantity and fate of DOC in the estuary. We find that that TCs promote a considerable increase in DOC concentration near the river mouth at the entrance to the estuary, on average an increase of 200 µmol l<sup>-1</sup> due to storms was observed. TC-induced increases in DOC are apparent throughout the estuary, and the duration of these elevated DOC concentrations ranges from one month at the river mouth to over six months in lower estuary. Our results suggest that despite the fast mineralization rates, the terrestrial DOC is processed only to a minor extent relative to the pulsed amount entering the estuary. We conclude that the vast quantity of organic carbon delivered to estuaries by TCs transform estuaries from active biogeochemical processing “reactors” of organic carbon to appear more like passive shunts due to the sheer amount of pulsed material rapidly flushed through the estuary.</p>