Coastal Megacities Research Articles

Bourgeoning Impacts of the Coastal Megacities and Potential Ecological Risks of Leachable Heavy Metals in Fine-sediments Fractions along the Egyptian Red Sea Shores

Bourgeoning Impacts of the Coastal Megacities and Potential Ecological Risks of Leachable Heavy Metals in Fine-sediments Fractions along the Egyptian Red Sea Shores

Turning peril into opportunity: Construction and validation of a resilience assessment system for China's coastal megacities

Enhancing urban resilience and strengthening risk resistance capacities have emerged as pivotal concerns in the construction and governance of contemporary urban landscapes globally. Given the uniqueness of coastal megacities in terms of geography and population, a more targeted and effective resilience assessment is urgently needed. This investigation develops a comprehensive, multidimensional index system to assess the resilience life cycle of China's coastal megacities. First, we creatively introduce the process perspective of the 4R model of crisis management into urban resilience assessment, revealing the preventive, defensive, recovery, and transformative resilience of China's coastal megacities. Second, we propose a spatio-temporal assessment matrix that integrates four processes of urban resilience and six representative subsystems in coastal megacities: social, economic, ecological, infrastructural, technological, and institutional. Third, we select 59 indicators to construct a three-level assessment index system for China's coastal megacities. This index system is strictly screened and optimized through qualitative and quantitative methods. Fourth, we validate the assessment system using Qingdao's data by calculating an urban resilience index and analyzing trends. We find the most important improvement goals and resilience enhancement suggestions for Qingdao by analyzing the indicator sensitivity. This study contributes to the development of urban resilience theories and identifies weaknesses in different dimensions to strengthen and improve the resilience of coastal megacities.

Water scarcity in the fast‐growing megacity of Lagos, Nigeria and opportunities for managed aquifer recharge

AbstractIn the last three decades, the world has experienced a rapid emergence of megacities. The increasing demographics, urbanization, and water demand brought severe and undesired effects on the quantity and quality of their water resources. A prime example is Lagos (Nigeria, West Africa), which is projected to become the world's largest city by 2100, and faces water scarcity challenges common to other megacities of developing countries. A literature review reveals the vast gap between water demand and regulated supply, inadequate knowledge and information on the current state of water (re)sources, and major, yet unregulated, use of groundwater. It further highlights the impacts of uncontrolled wastewater discharge into surface waters and aquifers, the role of increasing paved surfaces and blocked drainages on flooding, the inadequate supply of potable water, and the unsustainable abstraction of groundwater. Here, we examine the potential of managed aquifer recharge (MAR) to address these recurrent challenges across the megacity. The analysis reveals the opportunities and potential risks associated with the capture of wastewater, storm water, and brackish surface water for MAR. These waters, after appropriate treatment and subsurface storage may bridge the growing water supply–demand gap and mitigate the effects of groundwater (over)exploitation, including aquifer depletion, saltwater intrusion and land subsidence. Immediate efforts should focus on improving the conceptual and quantitative knowledge of Lagos' hydrogeology and groundwater resources through comprehensive spatial–temporal groundwater monitoring and socio‐economic studies of groundwater access and use. The insights provided may inform other fast‐growing coastal megacities in Africa and the wider developing world.This article is categorized under: Science of Water > Water and Environmental Change Engineering Water > Water, Health, and Sanitation Human Water > Value of Water Science of Water > Hydrological Processes

Achieving Carbon Mitigation with Economic Benefits through High-Resolution Analysis of Renewable Resource Integration in Global Coastal Cities.

Coastal regions, home to more than half of the global population and contributing over 50% to the global economy, possess vast renewable resources, such as seawater and solar energy. The effective utilization of these resources, through the seawater-cooled district cooling system (SWDCS), seawater toilet flushing (SWTF), and rooftop solar photovoltaic system (RTPV), has the potential to significantly reduce carbon emissions. However, implementing these technologies in different geographic contexts to achieve the desired carbon and economic outcomes at the city level lacks a clear roadmap. To address this challenge, we comprehensively analyzed 12 coastal megacities worldwide by integrating geospatial building data. Our study evaluated the potential energy savings, carbon mitigation, and levelized carbon abatement costs (LCACs) from a life cycle perspective. The results revealed that using seawater and solar energy within urban boundaries can reduce electricity consumption from 1 to 24% across these cities. The spatial distribution of the LCAC for seawater-based systems exhibited more variation compared to the RTPV. By applying specific LCAC thresholds ranging from 0 to 225 USD/tCO2e, all cities could achieve both carbon reductions and economic benefits. These thresholds resulted in up to 80 million tonnes of carbon emission reductions and 5 billion USD of economic benefits, respectively. Our study provides valuable insights into integrating renewable resource systems, enabling coastal cities to achieve carbon and economic advantages at the city scale simultaneously.

Historical trends and future projections of annual rainfall from CMIP6 models in Ho Chi Minh City, Vietnam

Climate risks have posed a major threat to many local communities living in low-lying coastal megacities across the globe, including Ho Chi Minh City, Vietnam. Hence, this study first aimed to contribute towards a comprehensive understanding of temporal trend patterns of annual rainfall and absolute extremes in Ho Chi Minh City over the last 4 decades (1980-2022) through multiple non-parametric statistical trend tests. We employed the quantile delta mapping (QDM) method to develop daily bias-corrected rainfall data based on the outputs in the latest Coupled Model Intercomparison Project phase 6 (CMIP6) under 8 shared socio-economic pathway (SSP) greenhouse gas emission scenarios. Evaluation of model performance was implemented by repeatedly omitting 5 successive years in turn for estimating testing errors. The outcomes indicate the high applicability of well-calibrated transfer functions, even for high quantiles, to the production of future rainfall scenarios. The projected changes in annual rainfall and absolute extremes were obtained by estimating multi-model medians from CMIP6 models for future periods (i.e. 2021-2040, 2041-2060, 2061-2080, and 2081-2100), with reference to the base period (1995-2014). In general, annual rainfall in Ho Chi Minh City is projected to increase substantially, and Thu Duc station consistently shows the highest increases in annual rainfall. Projected changes are approximately 30.9% (8.3 to 77.8%) under the high-end scenario (i.e. SSP5-8.5) by the end of the 21st century. It is expected that these findings will yield several solid arguments for mitigating climate-related risks in Ho Chi Minh City.

Human activities and sustainability of the sea

The marine system is undergoing profound changes, following global change, rising sea levels as a consequence of rising temperature, significant biogeochemical transformations having multiple impacts in terms of environmental biodiversity and resources. The phenomenon of coastalization represents a fundamental factor of evolution. The coastal population represents more than 60% of the world’s population and continues to grow rapidly through immigration; the population rate expected in 2020 will be 75%. Coastal urbanization is increasing with the creation of coastal mega-cities. This has the consequences of increasing pressure on the coastal terrain and pressure on marine resources. This paper is a short review that reported the danger which face the coastal heritage with a case study on Morocco.

Comparative study on physicochemical characteristics of atmospheric microplastics in winter in inland and coastal megacities: A case of Beijing and Shanghai, China

Atmospheric microplastics (MPs) have received global attention across various sectors of society due to their potential negative impacts. This study aims to understand the physicochemical characteristics of MPs in inland and coastal megacities for raising awareness about the urgent need to reduce plastic pollution. Laser Direct Infrared Imaging (LDIR) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) techniques were employed to characterize atmospheric MPs in megacities (inland megacity Beijing and coastal megacity Shanghai) in China, focusing on their physicochemical characteristics, including compositional types, number concentration, morphology, size, possible sources, and potential health risks. The LDIR analysis identified sixteen different types of MPs present in the atmospheres of Beijing and Shanghai. The number concentration of atmospheric MPs in Beijing (3.0 items/m3) is 1.8 times that of Shanghai (1.7 items/m3). The study found that the variations in MP pollution between Beijing and Shanghai are influenced by the urban industrial structure and geographical location. Morphological analysis indicates that fragment MPs have the highest relative abundance in Beijing, while fibrous MPs dominate the atmosphere of Shanghai. Additionally, the study assessed the potential health risks of atmospheric MPs to urban residents. The results suggest that residents of Beijing face more severe health risks from atmospheric MPs compared to those in Shanghai. These findings underscore the urgency to address the issue of atmospheric MPs and provide crucial evidence for the formulation of relevant environmental and health policies.

People in PowerPoint Pixels: Competing justice claims and scalar politics in water development planning

Coastal megacities all over the world face challenges related to climate adaptation, ecosystem protection and inclusive development. In response, governments develop high-level and long-term climate adaptation plans to guide coastal development. In Metro Manila, a consortium of Dutch and Philippine consultants developed the Manila Bay Sustainable Development Master Plan (MBSDMP). The planning team stressed the importance of inclusive and participatory planning, yet, the pre-set premises of the masterplan, such as the high-level and long-term planning scale and corresponding problem formulation, determined which justice claims were foregrounded in the project, disadvantaging small-scale fishing and informal settlement communities. ‘Justice’ is a contested concept. Hence, we deploy a critical theory and politics of expert knowledge lens to investigate how struggles over competing justice claims unfold in water development planning. The scalar politics as manifested in the MBSDMP planning process hides particular conceptions of justice while privileging others in congruence with the larger scale uneven political-economic development dynamics. We provide three examples of scale framing in the planning process that functioned to legitimize the contested displacement of informal settlements by pointing to economic development, disaster risk reduction, or environmental protection. Planning design choices involving scalar out-zooming enabled the uptake of these justice claims, while backgrounding the justice claims of negatively affected groups: namely, the urban poor and small-scale fishing communities. The case analysis provides conceptual-empirical insights relevant for coastal cities' grassroots and policy action platforms anticipating climate change impacts and strategizing their stance in the politics of climate adaptation planning.

Quantitative attribution framework for urban air pollutant: Investigating policy impact on NO2 emissions of megacities in China and Japan

This comparison study deciphered the intricate dynamics of policy impacts on NO2 air pollution emissions across five populous megacities – Tokyo and Osaka in Japan, Shanghai, Beijing, and Tianjin in China. Utilizing spatiotemporal mapping and time series analysis, we traced pollution trends from 2015 to 2022, underlining the profound influence of rapidly enacted policy measures on these densely populated urban areas. Leveraging daily policy data coupled with NO2 emission records, we developed a quantitative attribution framework to measure policy impacts. The results showed that 'School Closure' emerged as a dominant factor in four out of five cities. Besides, 'International Travel Controls' played a pivotal role in coastal megacities like Tokyo, Osaka, and Tianjin, revealing their unique emission characteristics. The insights demonstrated the essential need for targeted, city-specific policy interventions, cautioning against the over-simplified approach of one-size-fits-all strategies.

Urbanization and its impacts on land surface temperature and sea surface temperature in a tourist region in Mexico from 1990 to 2020

The United Nations World Tourism Organization has stated that Mexico is the sixth most visited country, with nearly 40 million international tourists. However, tourism can be one of the key contributors to rapid urbanization, contributing to climate change and environmental impacts. One of the tourist areas with the highest urban growth in the Mexican Pacific is Mazatlán, receiving more than 2 million tourists each year. The goal of this study was to analyze urban expansion and its impacts on land surface temperature and sea surface temperature in a tourist region in Mexico from 1990 to 2020. A supervised classification was carried out using the support vector machine algorithm on Landsat images taken over the last 30 years, analyzing the relationship of this growth to land surface temperature (LST). We also analyze whether increases in LST on urban land are related to increases in ocean temperature (SST) adjacent to the city. It was found that in the 1990s the urban area, currently 80 km2, was 25 km2. Urban growth has been exponential, and greater than in some other coastal megacities that have 5 times the population of Mazatlán. The median LST in the 1990s was 30.8 and is currently ∼32.9 °C, but in an analysis of the city's temperatures by pixel we found areas with temperatures as high as 40 °C; e.g., malls and residential developments. These LST values are higher than those reported in other coastal cities located at similar latitudes. Median SST values were between 19.2 and 20.8 °C, so SST was not related to LST. From the 1990s to the 2020s, the city's urban area has grown by 303%. Therefore, LST values in summer could reach 40 °C, which would give this coastal city one of the highest LSTs in the world, with a non-permeable surface that would cause flooding throughout the hotel zone.

Low-regret climate change adaptation in coastal megacities – evaluating large-scale flood protection and small-scale rainwater detention measures for Ho Chi Minh City, Vietnam

Abstract. Urban flooding is a major challenge for many megacities in low-elevation coastal zones (LECZs), especially in Southeast Asia. In these regions, the effects of environmental stressors overlap with rapid urbanization, which significantly aggravates the hazard potential. Ho Chi Minh City (HCMC) in southern Vietnam is a prime example of this set of problems and therefore a suitable case study to apply the concept of low-regret disaster risk adaptation as defined by the Intergovernmental Panel on Climate Change (IPCC). In order to explore and evaluate potential options of hazard mitigation, a hydro-numerical model was employed to scrutinize the effectiveness of two adaptation strategies: (1) a classic flood protection scheme including a large-scale ring dike as currently constructed in HCMC and (2) the widespread installation of small-scale rainwater detention as envisioned in the framework of the Chinese Sponge City Program (SCP). A third adaptation scenario (3) assesses the combination of both approaches (1) and (2). From a hydrological point of view, the reduction in various flood intensity proxies that were computed within this study suggests that large-scale flood protection is comparable but slightly more effective than small-scale rainwater storage: for instance, the two adaptation options could reduce the normalized flood severity index (INFS), which is a measure combining flood depth and duration, by 17.9 % and 17.7 %, respectively. The number of flood-prone manufacturing firms that would be protected after adaptation, in turn, is nearly 2 times higher for the ring dike than for the Sponge City approach. However, the numerical results also reveal that both response options can be implemented in parallel, not only without reducing their individual effectiveness but also complementarily with considerable added value. Additionally, from a governance perspective, decentralized rainwater storage conforms ideally to the low-regret paradigm: while the existing large-scale ring dike depends on a binary commitment (to build or not to build), decentralized small- and micro-scale solutions can be implemented gradually (for example through targeted subsidies) and add technical redundancy to the overall system. In the end, both strategies are highly complementary in their spatial and temporal reduction in flood intensity. Local decision-makers may hence specifically seek combined strategies, adding to singular approaches, and design multi-faceted adaptation pathways in order to successfully prepare for a deeply uncertain future.

The potential of open-access data for flood estimations: uncovering inundation hotspots in Ho Chi Minh City, Vietnam, through a normalized flood severity index

Abstract. Hydro-numerical models are increasingly important to determine the adequacy and evaluate the effectiveness of potential flood protection measures. However, a significant obstacle in setting up hydro-numerical and associated flood damage models is the tedious and oftentimes prohibitively costly process of acquiring reliable input data, which particularly applies to coastal megacities in developing countries and emerging economies. To help alleviate this problem, this paper explores the usability and reliability of flood models built on open-access data in regions where highly resolved (geo)data are either unavailable or difficult to access yet where knowledge about elements at risk is crucial for mitigation planning. The example of Ho Chi Minh City, Vietnam, is taken to describe a comprehensive but generic methodology for obtaining, processing and applying the required open-access data. The overarching goal of this study is to produce preliminary flood hazard maps that provide first insights into potential flooding hotspots demanding closer attention in subsequent, more detailed risk analyses. As a key novelty, a normalized flood severity index (INFS), which combines flood depth and duration, is proposed to deliver key information in a preliminary flood hazard assessment. This index serves as an indicator that further narrows down the focus to areas where flood hazard is significant. Our approach is validated by a comparison with more than 300 flood samples locally observed during three heavy-rain events in 2010 and 2012 which correspond to INFS-based inundation hotspots in over 73 % of all cases. These findings corroborate the high potential of open-access data in hydro-numerical modeling and the robustness of the newly introduced flood severity index, which may significantly enhance the interpretation and trustworthiness of risk assessments in the future. The proposed approach and developed indicators are generic and may be replicated and adopted in other coastal megacities around the globe.

Developing and understanding cascading effects scenario of typhoons in coastal mega-cities from system perspectives for disaster risk reduction: A case study of shenzhen, China

Coastal mega-cities provide spaces of economic and social activities in contemporary China. Due to the cumulated effects of rapid urbanization and climate change, typhoon disasters always trigger serious cascading effects by affecting interrelated urban elements in high-density urban areas, thereby amplifying disastrous consequences. Developing cascading effects scenarios of typhoons, as well as understanding cause–effect relationships among the involved cascading events is essential to designing disaster risk reduction strategies in coastal mega-cities. However, high uncertainty of the natural hazards and the increasing complexity of coastal mega-cities present challenges to develop and understand cascading effects of typhoons. This research proposes an innovative approach to develop, model, and analyze cascading effects scenario of typhoon disasters in Shenzhen, China for providing insights of disaster risk reduction from system perspectives. First, considering the uncertainty of typhoons and complexity of the study area, the cascading effects scenario of typhoon disasters is developed by encoding hazard transitions triggered by historical cases, in combination with expert judgment. A network approach based on Social Network Analysis is employed to model, visualize, and analyze cascading effects scenarios of typhoons aiming to explore the underlying complexity of typhoon disasters. Finally, disaster risk reduction strategies are designed and evaluated from network analysis perspectives. This research increases the understanding of the evolution process of cascading effects triggered by typhoons in rapid-urbanizing mega-cities from system perspectives. Furthermore, it provides insights of designing effective disaster risk reduction strategies for urban governments to improve sustainability and resilience of coastal mega-cities to typhoon disasters.

Vulnerable spaces, unequal responses: lessons for transformative climate resilience in Lagos

Urban coastal megacities like Lagos face flooding challenges that may be exacerbated by climate change in the future. Through an urban political ecology lens, this study engages with the dynamics of politics and power that produce differentiated flood impacts and adaptation strategies. Data from telephone interviews of 21 Lagos residents across the mainland and island areas reveal people's understanding of their flood vulnerabilities within the wider socio-political context of Lagos. In particular, state failure in the provisioning of services, amenities, and overall flood protection, shapes flood risk in Lagos. In addition, income and access to material resources inform people's experiences and ability to cope with flooding. Furthermore, citizens apply localized strategies to prepare for and cope with flooding events, particularly through Community Development Associations (CDAs). These localized strategies have implications for transformative resilience. However, these forms of endogenous resilience cannot replace attention to wider urban governance challenges in cities like Lagos.

Climate change and coastal megacities: Adapting through mobility

Climate change poses threats to individuals, communities, and cities globally. Global conversations and scholarly debates have explored ways people adapt to the impacts of climate change including through migration and relocation. This study uses Lagos, Nigeria as a case study to examine the relationship between flooding events, migration intentions as a preferred adaptation, and the destination choices for affected residents. The study draws on a mixed-methods approach which involved a survey of 352 residents and semi-structured interviews with 21 residents. We use a capability approach to analyze mobility decisions following major or repetitive flood events. We found that the majority of affected residents are willing to migrate but the ability to do so is constrained by economic, social, and political factors leading to involuntary immobility. Furthermore, intra-city relocation is preferred to migration to other states in Nigeria or internationally. These findings challenge popular Global South-North migration narratives. Indeed, some residents welcome government-supported relocation plans but others remain skeptical due to lack of trust. Community-based relocation may therefore be preferred by some Lagosians. Overall, this study contributes a nuanced understanding of mobility intentions in response to climate-induced flooding in one of the world’s largest coastal cities.

Latitudinal Trend Analysis of Land Surface Temperature to Identify Urban Heat Pockets in Global Coastal Megacities

Recent global warming has led to increased coastal disturbances through a significant transfer of heat between the land and the ocean surface. The polar regions show excessive temperature changes resulting in massive ice sheet melting. Mid-latitudinal storms pull heat away from the equator towards the poles; therefore, the global sea level is rising, making coastal cities the most vulnerable. In last few decades, rapid urbanization in big cities has drastically changed the land cover and land use due to deforestation, which has led to increased land surface temperatures (LSTs). This eventually leads to urban flooding due to oceanic storm surges frequently created by low pressure over the ocean during summer. This paper considered factors such as drastic unplanned urbanization to analyze coastal cities as the focal point of the generation of heat yielding the annihilation of the natural topography. Urban heat pockets (UHP) were studied for nine megacities, which were selected at an interval of 5° of latitudinal difference in the northern hemisphere (NH) since 70% of densely populated megacities are located in coastal regions. A comparative surface temperature analysis was effectively carried out with the same latitudinal reference for nine mid-sized cities using the derived LST data from Landsat 8. The results provide a comparative classification of surface temperature variations across the coastal cities over the NH. This study infers that the issues pertaining to growing urbanization are very important for analyzing the proportional impact caused by the settlement hierarchy and lays a robust foundation for advanced studies of global warming in coastal urban environments.

Data-driven spatio-temporal analysis of consolidation for rapid reclamation

Spatial extension of lands through rapid reclamation is attractive for congested coastal megacities, although reclamations might suffer from project delay and budget overrun, often due to encountering unforeseen ground conditions. To accelerate reclamations, accurate prediction of soil consolidation over a construction duration of multiple years is needed for reclaimed lands, which often contain spatially varying subsurface stratigraphy and soil parameters. This calls for a spatio-temporal analysis of consolidation with a sound understanding of subsurface stratigraphic alternations of fine/coarse-grained soils and spatial variability of consolidation parameters (e.g. permeability). In this study, a unified framework, capable of simultaneously modelling stratigraphic variation and spatial variability of soil properties through machine learning of limited site investigation data, is combined with the finite-element method and Monte Carlo simulation for spatio-temporal consolidation analysis of reclaimed lands. The proposed method is applied to a real reclamation project in Hong Kong. Results indicate that the proposed method can accurately characterise subsurface geological cross-sections and spatially varying soil permeability with quantified uncertainty. Ignorance of spatial variability of soil permeability may result in an underestimation of consolidation time and an overestimation of undrained shear strength gain, and thus pose significant risks to reclamation projects.

Towards a Combined Physical and Social Evaluation of Climate Vulnerability in Coastal Urban Megacities

Coastal urban megacities across Asia face significant risks from climate change, including coastal flooding, high temperatures, urban heat island impacts and air pollution. These hazards are associated with negative impacts on infrastructure, communities and the environment. To identify the current intensity of climate change impacts in coastal urban megacities, an integrated evaluation method is needed. Firstly, the present study assesses the climate change impacts of Guangzhou, a Chinese coastal urban megacity, for both physical and social aspects. This study includes 60 years of time-series data for 1960–2020 to examine temperatures, precipitation, humidity and air pollution in Guangzhou city. At the same time, a survey was conducted between April and July 2022 in this megacity and collected the views of 336 people on climate change and its associated environmental impacts. Secondly, the Ganzhou city results are compared with existing data from similar nearby cities to evaluate the diverse climate change trends. Results show that during 1961-1990, the city received the most rainfall in May, reaching 283.6 mm. From 1990 to 2020, June recorded the highest rainfall of 356.6 mm and shows an increase of 73 mm during that period. The very severe monsoon season brought an increased risk of flooding. Results also revealed that the warmest month is July, and the coldest month is January, and both months showed increased temperatures of 0.60 ℃. Comparison results revealed that Guangzhou is not the only city which scored increased highest temperatures; other nearby cities including Heyuan, Shantou and Shaoguan also scored increased highest temperatures. The survey reveals that the majority of respondents (75%) perceived the increased frequency of extreme weather, including typhoons, heavy rainfall and multiple days of hot weather, such as higher temperatures and an increased number of hot days. In the responses to the questions related to the heat island effect, more than 80% of residents are aware of the existence of the heat island and its impacts. People believe that the primary causes of the urban heat island problem are industrial production and anthropogenic heat generated by the city. These results will be helpful to local and national policy and decision makers to revise and/or develop new strategies to improve the environment and quality of life in coastal megacities, particularly Ganzhou.

Control and prevent land subsidence caused by foundation pit dewatering in a coastal lowland megacity: indicator definition, numerical simulation, and regression analysis

Land subsidence has to be controlled under global climate change and sea level rising for coastal megacities. Land subsidence caused by lowering groundwater level during underground excavation has become a dominant influence factor for land subsidence during urbanization and city renewal. How to manage the land subsidence induced by foundation pit dewatering (FPD) on an urban scale was urgent. Shanghai was selected as the research background. A subsidence and drawdown double control (SDDC) partition was established. The position of three times excavation depth (3H) horizontally away from the foundation boundary in the plane was defined as the boundary between dewatering subsidence and excavation settlement. Land subsidence of 3H (LS-3H) and groundwater drawdown on 3H (GD-3H) were defined as the evaluating and controlling indicators. An FPD conceptual model was summarized by the estimating and investigating of foundation pit information and numerical simulations were performed. A total of 6540 FPD scenarios were simulated for the LS-3H and GD-3H. Multi-factor regression analysis was conducted to obtain relations between the GD-3H and the shape, area, depth, and curtain depth of foundation pit on the basis of the numerical simulations. The regression models can be used to estimate the GD-3H and compared with the threshold specified by land subsidence prevention and control (LSPC) law, which can provide a reference for similar cases.

Modelling of groundwater recharge in the megacity of Lagos, Nigeria: preliminary results using WetSpass-M

Abstract. Lagos, Nigeria is one of the World's fastest-growing cities. The increasing water demand has led to depletion and deterioration of the groundwater resources contained in its highly productive coastal aquifers. Groundwater recharge processes and drivers, which dictate groundwater replenishment, and their changes associated with rapid urbanization, are yet to be quantified and understood. We used the spatially distributed WetSpass-M model to estimate the spatiotemporal patterns and rates of recharge between 2000 and 2020 in the 5440 km2 coastal aquifer system underlying the megacity of Lagos. The influence of expanding urban areas, from 807 km2 in 2000 to 1183 km2 in 2020, coincide with reduction in the simulated recharge by 50 % in 20 years. The study also revealed diffuse and focused recharge processes associated with, respectively, the urban centres and areas closer to water bodies signalling strong groundwater-surface water interactions. The preliminary findings from this modelling work provide directions for exploring sustainable water management options including Managed Aquifer Recharge (MAR) in Lagos and other fast-growing coastal megacities in the world.