Several North African coasts, including low-lying sandy and deltaic areas, are at high risk of coastal hazards due to increased climatic fluctuations. Alexandria, a historic and densely populated Mediterranean port city that is representative of several coastal cities in North Africa, has experienced over 280 building collapses near its shorelines over the past two decades, with the root causes still being investigated. To address the potential causes, we explore several anthropogenic and hydroclimatic drivers along the coastline of Alexandria using a GIS-based multi-criteria analysis in the areas where buildings collapsed from 1974 to 2021. We considered both the coastal physical properties and the buildings' structural elements. Our results suggest that collapses are correlated to severe coastal erosion due to sediment imbalance caused by the decades-long inefficient landscape and urban expansion along the historic city's waterfront. This severe erosion increases seawater intrusion, which in turn raises the groundwater levels in coastal aquifers, disrupting soil stability and accelerating corrosion in building foundations until they collapse. We identified a littoral area of high vulnerability with over 7,000 buildings at risk, surpassing to an extent any other vulnerable zone in the eastern Mediterranean Basin. We conclude that coastal and densely urbanized areas in North Africa are at greater risk of being affected by hydroclimatic extremes, which can lead to higher risks of building collapses. Therefore, we propose a landscape-based coastal mitigation approach to perform adaptive transformation to curb these risks.

Distributed hydrological water quality models are increasingly being used to manage natural resources at the catchment scale but there are no calibration guidelines for selecting the most useful gauging stations. In this study, we investigated the influence of calibration schemes on the spatiotemporal performance of a fully distributed process-based hydrological water quality model (mHM-Nitrate) for discharge and nitrate simulations at Bode catchment in central Germany. We used a single- and two multi-site calibration schemes where the two multi-site schemes varied in number of gauging stations but each subcatchment represented different dominant land uses of the catchment. To extract a set of behavioral parameters for each calibration scheme, we chose a sequential multi-criteria method with 300.000 iterations. For discharge (Q), model performance was similar among the three schemes (NSE varied from 0.88 to 0.92). However, for nitrate concentration, the multi-site schemes performed better than the single site scheme. This improvement may be attributed to that multi-site schemes incorporated a broader range of data, including low Q and NO3- values, thus provided a better representation of within-catchment diversity. Conversely, adding more gauging stations in the multi-site approaches did not lead to further improvements in catchment representation but showed wider 95% uncertainty boundaries. Thus, adding observations that contained similar information on catchment characteristics did not seem to improve model performance and increased uncertainty. These results highlight the importance of strategically selecting gauging stations that reflect the full range of catchment heterogeneity rather than seeking to maximize station number, to optimize parameter calibration.

The low-lying, arid coastal regions of the Southern Mediterranean basin, extending over 4600 km, face daunting sea level rise and hydroclimatic changes due to shifting weather patterns. The impacts of the above on coastal urban buildings and infrastructure still need to be more qualified and understood. Alexandria, a historic and densely populated port city representative of several coastal cities in the Southern Mediterranean, has experienced over 280 building collapses near its shorelines over the past two decades, with the root causes still being investigated. We explore the decadal changes in coastal and hydroclimatic drivers along the city's coastline using a GIS-based multi-criteria analysis in the areas where buildings collapsed from 1974 to 2021. Our results suggest that collapses are correlated to severe coastal erosion due to sediment imbalance caused by the decades-long inefficient landscape and urban expansion along the historic city's waterfront. This severe erosion, combined with sea level rise, upsurges seawater intrusion, which raises the groundwater levels in coastal aquifers, disrupting soil stability and accelerating corrosion in building foundations until they collapse. We identified a coastal area of high vulnerability with over 7,000 buildings at risk, surpassing any other vulnerable zone in the Mediterranean Basin. We conclude that several coastal and densely urbanized areas in the Southern Mediterranean are at greater risk of building collapses due to similar hydroclimatic changes. Therefore, we propose a landscape-based coastal mitigation approach to implement adaptive transformations to curb these risks that apply to Alexandria and other southern Mediterranean cities facing the same challenges.

Excessive dissolved inorganic nitrogen (DIN) added to the urban river systems by point-source inputs, such as untreated wastewater and wastewater treatment plant (WWTP) effluent, constitutes a water-quality problem of growing concern in China. However, very little is known about their impacts on DIN retention capacity and pathways in receiving waters. In this study, a spatially-intensive water quality monitoring campaign was conducted to support the application of the river water quality model WASP7.5 to the PS-impacted Nanfei River, China. The DIN retention capacities and pathway of a reference upstream Reach A, a wastewater-impacted Reach B and an effluent-dominated Reach C were quantified using the model results after a Bayesian approach for parameter estimation and uncertainty analysis. The results showed that the untreated wastewater discharge elevated the assimilatory uptake rate but lowered its efficiency in Reach B; while the WWTP effluent discharge elevated both denitrification rate and efficiency and made Reach C a denitrification hotspot with increased nitrate concentration and hypoxic environment. The effects of the point-source inputs on the DIN retention pathways (assimilatory uptake vs. denitrification) were regulated by their impacts on river metabolism. Despite different pathways, the total DIN retention ratios of Reaches A, B and C under low-flow conditions were 30.3% km-1, 14.3% km-1 and 6.5% km-1, respectively, which indicated the instream DIN retention capacities were significantly impaired by the point-source inputs. This result suggests that the DIN discharged from point-source inputs to urban rivers will be transported downstream with the potential to create long-term ecological implications not only locally but also regionally.