Abstract
The low-lying coastal basins of Ravenna (Italy) are at or below mean sea level and currently undergoing land subsidence, which exposes the basins to frequent inundation and groundwater and soil salinization. The surface water drainage becomes necessary to lower the water table head and further prevent flooding and waterlogging. The study examines the evolution of drainage apropos to climate change and land subsidence in the three main Ravenna coastal basins. Our findings show that the evolution of drainage is influenced by land subsidence, climate change variability, droughts, vertical seepage, and local water management. Land subsidence causes an increase in upward-directed vertical seepage of saline water through the shallow unconfined aquifer into the drainage channels of the coastal basins, thus leading to an increase in drainage through time. At a seasonal timescale, the rate of pumping depends on antecedent rainfall and soil–water storage. The warming extremes indices, specifically drought indices, show to be more significant than rainfall indices trends to monitor drainage evolution. Drought indices permit easy comparison of dryness or wetness severity with drainage evolution along their time scale. The co-occurring anthropogenic and natural factors involving in the increasing drainage rate will affect decadal and seasonal water management policies in the area. The implications of increasing drainage rates, long periods of drought with limited rainfall, and increasing temperature will further worsen freshwater availability in this coastal area already experiencing soil and water salinization. However, drainage of this low-lying territory has effectively mitigated rising water tables and avoided flooding. Our study has shown that each coastal basin behaves differently in terms of sensitivity to land subsidence and climate extremes. Therefore, when using drainage data time series for water management purposes, one should account for past management practices and for the specific sensitivity of each basin to external factors.
Highlights
The coastal cities worldwide, and more so in the Mediterranean, are threatened by sea-level rise (SLR), inundation, storm surges, and precipitation variability (Essink et al 2010; Nicholls et al 2007)
Past and recent studies have greatly improved our understanding of these drivers, but correlation with seawater seepage and drainage is widely unrecognized in many coastal studies
We show that there is a wide range of factors that affect drainage time series and knowledge gained from a single basin cannot be extrapolated to the whole coastal area
Summary
The coastal cities worldwide, and more so in the Mediterranean, are threatened by sea-level rise (SLR), inundation, storm surges, and precipitation variability (Essink et al 2010; Nicholls et al 2007). Climate change and land subsidence have a profound role in the severity of coastal flooding, with far-reaching water management, flooding infrastructure, ecological and socio-economic implications for low-lying coastal cities and polder systems. These two main drivers of coastal flooding have attracted the interest of many scientists and researchers in recent years (Essink et al 2010; Mimura 2021). Recognizing the role these two drivers have on coastal surface water drainage through time is imperative to strategic water planning and development of coastal urban areas and their drainage waterways. Past and recent studies have greatly improved our understanding of these drivers, but correlation with seawater seepage and drainage is widely unrecognized in many coastal studies
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