1024 / 2024-09-20 08:25:54

Heavy rainfall induced coastal water-quality deterioration -comparison between 2019 typhoon Lekima and 2021 autumn flood

Rainfall,river flood,nutrients,water quality,coastal sea

In the context of global climate change, the frequency and intensity of heavy rainfall events are on the rise. Such heavy rainfall can lead to extreme flooding, which transports substantial amounts of water and nutrients into coastal seas. Super Typhoon Lekima made landfall in Shandong on the night of August 11, 2019, marking it as the strongest typhoon to hit China that year and the fifth strongest in the past 70 years. During this extreme weather event, the water discharge from ten rivers surrounding Laizhou Bay (LZB) increased by 78.7%, and the total nitrogen flux rose by 1.1 times. The average concentrations of dissolved inorganic nitrogen (DIN) in LZB surged by 3.5 times following Typhoon Lekima. From September to November 2021, an exceptionally heavy autumn rainfall occurred in the middle reaches of the Yellow River (YR). In September, the average precipitation in the YR Basin was 1.7 times greater than that of the same period in previous years, marking the highest levels recorded since 1961. The water discharge and total nitrogen flux from major rivers around LZB from September to October were 3.5 and 4.0 times higher, respectively, compared to the same period in 2020. Both events triggered algal blooms in LZB. Modeling studies indicate that physical processes primarily govern water quality recovery following each heavy rainfall event, while biogeochemical processes play a secondary role. Wind-driven bay-shelf exchange is the predominant mechanism for nutrient transport away from LZB, with northwesterly winds being the most favorable in winter and southeasterly winds in summer. In comparison to Typhoon Lekima, the autumn flood resulted in a more prolonged deterioration of water quality. Even in May 2022, DIN concentrations in LZB remained twice as high as those observed during the same period the previous year. Analysis of flow hydrographs from the Lijin station on the lower reaches of the YR revealed a high-flow tail period following the significant stormflow period, indicating a profound and detrimental impact on coastal water quality that has been previously overlooked. The increased baseflow following the autumn flood was identified as the primary factor contributing to the high-flow tail period, while continuous discharge from the Xiaolangdi Reservoir and reduced water intake from the YR also contributed to the prolonged deterioration of water quality.