Degrading flood regulation function of river systems in the urbanization process

Identifying changes in flood characteristics and their causes from an event-based perspective in the Central Taihu Basin

Increasing rainstorms induced by climate change and modification in the land surface due to urbanization have greatly altered floods at different spatio-temporal scales. However, investigating flood events in urbanized plains is challenging as anthropogenic behaviors can change river flow without rainfall. In addition, while the frequency and magnitude of floods have been well examined, knowledge about variations in the rate of flood change is still limited. To fill these gaps, we proposed a scheme that focused on flood responses to rainfall to detect changes in flood characteristics in the Central Taihu Basin, a highly urbanized region in the Yangtze River Delta of China. Four characteristic metrics were adopted to summarize the flood hydrograph, including the peak, increment, rising rate, and falling rate. We then examined trends of these metrics based on the selected rainfall-flood events from ten hydrological stations during 1970-2020. Subsequently, the reduction method was used to separate the impacts of regional climate change and human activities on flood characteristics alterations. Furthermore, the importance of fifteen factors was quantified by the random forest model. We found that there is a significant upward trend in the evolution of flood characteristics, except for the increment of floods. Flood characteristics exhibit higher values when rainfall accumulates, indicating stronger responses of floods to a large amount of rainfall. The results also show that human activities dominate and impact the peak, rising rate, and falling rate of floods more than climate change. Meanwhile, although cumulative precipitation is the most important factor, flood characteristics are also susceptible to anthropogenic factors, such as land use change and hydraulic engineering construction. Our findings, which provide insights into flood event identification and enhance the understanding of regional flood changes, will serve as a reference for water resource management and flood mitigation in urbanized areas.

Spatial responses of water quality to river density and connectivity alterations on the Taihu Plain

With the advancement of urbanization, the structure and connectivity of river networks have been changed by the interference of human activities, resulting in a series of water environment problems. Numerous studies have indicated that river networks are associated with water quality; unfortunately, few studies have revealed the contributions of the structure and connectivity of river networks to variations in water quality. Taking one water conservancy region with dense and braided rivers on the Taihu Plain as an example, we depicted the spatial aggregations of water quality using the Getis-Ord Gi* index, quantified the variations in polluted regions using the standard deviational ellipse method, and quantified the influence of river density and connectivity on water quality during the different seasons. The results showed that (1) the water quality during the flood season was better than that during the non-flood season, especially in the western region; (2) the spatial aggregations of most water quality indicators were higher and the polluted regions increased in size during the flood period compared to the non-flood period; and (3) the relative contribution rates of the river density and connectivity exhibited mean values of 62.5% (61.2%) and 37.5% (38.8%) in the flood (non-flood) period. Our results provide theoretical support for enhancing water environment management.

Assessment of Supply and Demand of Regional Flood Regulation Ecosystem Services and Zoning Management in Response to Flood Disasters: A Case Study of Fujian Delta

Global climate change has led to flood disasters increasing in terms of frequency and damage caused, which seriously threatens urban and rural security. The flood regulation (FR) service function of the ecosystem plays an important role in mitigating flood disaster risk. Previous studies on flood regulation ecosystem services (FRES) are still lacking in a cross-scale assessment of supply and demand, refined simulation of regional complex hydrology, and application of spatial zoning management. Taking the Fujian Delta as an example, this study established a cross-scale research framework based on the social-ecosystem principle. The SWAT model was used to simulate the regional hydrological runoff and calculate the macro-scale supply of FRES. Taking patches of land as units, a flood risk assessment model was constructed to calculate the micro-scale demand for FRES for urban and rural society. Through a comparison of supply and demand across spatial scales, a zoning management scheme to deal with flood disaster risk was proposed. The results showed that: (1) The supply of FRES differed greatly among the sub-basins, and the sub-basins with low supply were mostly distributed in the lower reaches of Jiulong River and the coastal areas. (2) The demand for FRES was concentrated in high-density urban built-up areas. (3) By comparing the supply and demand of FRES in sub-basin units, 2153 km² ecological space was identified as the primary ecological protection area, and 914 km² cultivated land and bare land were identified as the primary ecological restoration area. (4) By comparing the supply and demand of FRES of land patch units, 65.42 km² of construction land was identified as the primary intervention area. This study provides a decision-making basis for regional flood disaster management from the perspective of FRES.

Mapping lake ecosystem services: A systematic review

Lake ecosystems are essential for human wellbeing and development for their capacity to provide multiple ecosystem services (ES). However, because of an anthropogenic-driven ecologic degradation, the sustainable supply of lake ES is at risk. It is, therefore, necessary to restore lake ecosystems and their ES for the sustainable support of future generations. There is a lack of knowledge about mapping lakes ES at the global level. This study conducts a systematic literature review focused on mapping lake ES. Initially, 617 studies were identified. However, only 30 studies were considered. The 30 studies identified were published between 2015 and 2021. Most of them were conducted in Asia, followed by Europe and America. The selected works covered all three Common International Classification of ES sections (regulating and maintenance, provisioning and cultural). We identified a similar number of works carried out in all the sections. Most of the studies were performed at a regional scale and focused on ES supply. Methodologically, an equal number of works followed qualitative and quantitative approaches. In total, 13 different methods were identified. The most used were the ES value (ESV) and biophysical modelling. Less than half of the works that applied the ESV method mapped all the ES analysed. Among the studies assessed, most were focused on environmental and socio-economic aspects. Only 1 study attempted to validate the results. Overall, 16 studies assessed the drivers of change impacts on ES supply. Nevertheless, only 1 considered climate change. The other 15 only considered land-use changes. This systematic review showed that a small number of works focused on lake ES mapping. Although challenging, there is an urgent global need for studies in this domain.

Spatially non-stationary relationships between urbanization and the characteristics and storage-regulation capacities of river systems in the Tai Lake Plain, China

Given environmental or hydrological functions influenced by changing river networks in the development of rapid urbanization, a clear understanding of the relationships between comprehensive urbanization (CUB) and river network characteristics (RNC), storage capacity (RSC), and regulation capacity (RRC) is urgently needed. In the rapidly urbanized Tai Lake Plain (TLP), China, various methods and multisource data were integrated to estimate the dynamics of RNC, RSC, and RRC as well as their interactions with urbanization. The bivariate Moran's I methods were applied to detect and visualize the spatial dependency of RNC, RSC, or RRC on urbanization. Geographically weighted regression (GWR) model was set up to characterize spatial heterogeneity of urbanization influences on RNC, RSC and RRC. Our results indicated that RNC, RSC and RRC variables each showed an overall decreasing trend across space from 1960s to 2010s, particularly in those of tributary rivers. RNC, RSC, or RRC had globally negative correlations with CUB, respectively, but looking at local scale the spatial correlations between each pair were categorized as four types: high-high, high-low, low-low, and low-high. GWR was identified to accurately predict the response of most RNC, RSC, or RRC variables to CUB (R²: 0.6-0.8). The predictive ability of GWR was spatially non-stationary. The obtained relationships presented different directions and strength in space. All variables except for the water surface ratio (Wp) were more positively affected by CUB in the middle eastern parts of TLP. Drainage density, RSC and RRC variables were more negatively influenced by CUB in the northeast compared to other parts. The quantitative results of spatial relationships between urbanization and RNC, RSC or RRC can provide location-specific guidance for river environment protection and regional flood risk management.

Temporal and Spatial Characteristics of River Water Quality and Its Influence Factors in the TAIHU Basin Plains, Lower Yangtze River, China

Water quality pollution has been a serious problem in the Taihu Basin plains, which is a highly urbanized area in China. This study aims to detect the interannual and seasonal changes and spatial patterns of water quality in this region. Based on cluster analysis, Moran’s I, and standard deviational ellipses, the site clusters, spatial heterogeneity of water quality characteristics and identified polluted regions were clarified. Results showed that (1) water quality improved since 2002, and nutrient concentrations were lower in summer and autumn than in winter and spring. (2) The monitoring sites were divided into six clusters according to the water quality during the period from 2010 to 2014. Water quality worsened from Cluster 1 to Cluster 4. Cluster 1 sites were mostly distributed beside the Yangtze River and Taihu Lake. Cluster 4 sites were mainly located along the southeast border near Shanghai, while the remaining sites were separately distributed in the main cities. (3) A polluted region of both total nitrogen (TN) and total phosphorus (TP) was present in the southeastern part of the study area near the border from 2010 to 2014. In addition, polluted regions were most likely to form near the junctions of main cities. (4) Anthropogenic factors had greater impacts on water quality than natural factors. More attention should be given to water quality protection around impervious surface areas due to the greatest considerable effect.

Connection between Anthropogenic Water Diversion and Hydrodynamic Condition in Plain River Network

The increase in the rate of water renewal driven by hydrodynamics contributes to improving the water quality of the plain river network. Taking the lakeside river network in Wuxi as an example, through numerical simulation, polynomial fitting, correlation analysis, and principal component analysis, the hydrodynamic responses of urban lake-connected river networks to water diversion and hydrodynamic grouping were researched. Based on numerical model and influence weight analysis, we explored the improvement of hydrodynamic conditions of plain river network with strong human intervention and high algal water diversion. The results showed that: (1) The relationship between water diversion impact on river network flow velocity and water diversion flux was not as simple a linear relationship. It could be reflected by polynomial. The water transfer interval in dry season with high hydrodynamic efficiency (HE) was lower than 10 m3/s and higher than 30 m3/s, and the HE increased significantly when the water transfer flow was higher than 20 m3/s in the wet season. (2) According to the main hydrodynamic driving factors, the channels in the river network could be divided into three types: water conservancy projects, river and lake water level difference, and river channel characteristic. The correlations of rivers’ flow velocity in each group were very high. (3) The influence weights of water conservancy projects, river and lake water level difference, and river channel characteristic on the whole river network dynamics were 65, 21, and 12.4%, respectively, and the other factors contributed 1.6% of the weight.

Urbanization and Increasing Flood Risk in the Northern Coast of Central Java—Indonesia: An Assessment towards Better Land Use Policy and Flood Management

This study explores urbanization and flood events in the northern coast of Central Java with river basin as its unit of analysis. Two types of analysis were applied (i.e., spatial data and non-spatial data analysis) at four river basin areas in Central Java—Indonesia. The spatial analysis is focused on the assessment of LULC change in 2009–2018 based on Landsat Imagery. The non-spatial data (i.e., rural-urban classification and flood events) were overlaid with results of spatial data analyses. Our findings show that urbanization, as indicated by the growth rate of built-up areas, is very significant. Notable exposure to flood has taken place in the urban and potentially urban areas. The emerging discussion indicates that river basins possess dual spatial identity in the urban system (policy- and land-use-related). Proper land use planning and control is an essential instrument to safeguard urban areas (such as the case study area) and the entire island of Java in Indonesia. More attention should be put upon the river basin areas in designing eco-based approach to tackle the urban flood crises. In this case, the role of governance in flood management is crucial.

Conceptual Planning of Urban–Rural Green Space from a Multidimensional Perspective: A Case Study of Zhengzhou, China

The structure and function of green-space system is an eternal subject of landscape architecture, especially due to limited land and a need for the coordinated development of PLEs (production, living, and ecological spaces). To make planning more scientific, this paper explored green-space structure planning via multidimensional perspectives and methods using a case study of Zhengzhou. The paper applies theories (from landscape architecture and landscape ecology) and technologies (like remote sensing, GIS—geographic information system, graph theory, and aerography) from different disciplines to analyze current green-space structure and relevant physical factors to identify and exemplify different green-space planning strategies. Overall, our analysis reveals that multiple green-space structures should be considered together and that planners and designers should have multidisciplinary knowledge. For specific strategies, the analysis finds (i) that green complexes enhance various public spaces and guide comprehensive development of urban spaces; (ii) that green ecological corridors play a critical role in regional ecological stability through maintaining good connectivity and high node degree (Dg) and betweenness centrality index (BC) green spaces; (iii) that greenway networks can integrate all landscape resources to provide more secured spaces for animals and beautiful public spaces for humans; (iv) that blue-green ecological networks can help rainwater and urban flooding disaster management; and (v) that green ventilation corridors provide air cleaning and urban cooling benefits, which can help ensure healthy and comfortable urban–rural environments. In our view, this integrated framework for planning and design green-space structure helps make the process scientific and relevant for guiding future regional green-space structure.

Identifying Urban Flood Regulation Priority Areas in Beijing Based on an Ecosystem Services Approach

Climate change and rapid urbanization have severe impacts on urban flood regulation ecosystem services (UFRES). Quantifying the UFRES has attracted increasing attention for urban sustainable development. However, few studies have focused on how to identify urban flood regulation priority areas. In this study, we simulated urban surface runoff by using the soil conservation services-curve number model, and quantified UFRES supply and demand by using relative indicators (i.e., runoff reduction ratio and urban vulnerability) at the subdistrict scale in Beijing, China. Then, an urban flood regulation priority index was developed by integrating UFRES demand and supply, and further used to identify priority areas. The results show that the mean runoff reduction ratio in Beijing decreased from 38.70% (for a 1-year rainfall return period) to 24.74% (for a 100-year rainfall return period). Subdistricts with low UFRES supply were mainly located in the urban central area and the southeastern zone, while subdistricts with high UFRES demand were mainly located in the urban central region. Meanwhile, places with high priority for flood regulation were mainly located in the inner city, and low priority areas were mainly located in northwestern, southwestern, and northeastern Beijing. Our results also imply that the urban flood regulation priority index is an effective indicator to identify urban flood regulation priority areas. These findings could provide urban planners with a comprehensive understanding of UFRES and scientific guidance to improve them.

Analysis of the Effects of the River Network Structure and Urbanization on Waterlogging in High-Density Urban Areas-A Case Study of the Pudong New Area in Shanghai

In the process of urbanization, high-intensity human activities have seriously disturbed the river networks, especially in the core urban areas of large cities. At present, a series of urban water environmental problems, such as urban waterlogging and non-point pollution, caused by damage to the river network structure and the decrease in surface permeability conditions in high-density urban areas have received widespread attention. In this study, the effects of the river network structure and urbanization on urban waterlogging were analyzed in the Pudong New Area by using the data of waterlogging sites on the Amap. The results showed that the average water surface ratio is 10.9%, the average river network density is 4.59 km/km², and the comprehensive impervious ratio is 42.8%. From northwest to southeast, the impervious ratio of the Pudong New Area decreases gradually, and the water surface ratio and the river density increase gradually, while the areas with high waterlogging density are mainly concentrated in the northwest districts. The correlation coefficients indicate that the river network structure (-0.710 and -0.716) has a greater impact on waterlogging than urbanization (0.608) does. The current water surface ratio (10.9%) of the study area generally meets the requirements of the suitable water surface ratio (10.0%) in a rainfall return period of 50 years. However, the water surface ratio and the river density in about half of the districts did not meet the requirements of the suitable river network structure.

Correlations between water quality and the structure and connectivity of the river network in the Southern Jiangsu Plain, Eastern China

Incorporating the structure and connectivity of the river network to seasonal variations and different land use patterns can help improve the understanding the complex relationship between water quality and environmental factors. The present study first employed the grey relational analysis (GRA) to examine any existing correlations between the water quality and the structure and connectivity of river networks in the Southern Jiangsu Plain in Eastern China. All grey relational degree results were greater than the distinguishing coefficient (ρ = 0.5), and their average value was 0.7551. The average grey relational degrees of the water quality parameters varied between 0.7389 and 0.7744, and those of the characteristic indicators of the river network ranged from 0.6874 to 0.8850. Seasonal variations and different land use patterns were then employed to further analyze these relationships. The average grey relational degrees in the urban, rural, and fringe regions were calculated to be 0.7231, 0.7530, and 0.7124 during the flood season, respectively, and 0.7331, 0.7432, and 0.7052 during the non-flood season. The results suggest strong correlations between the water quality and the structure and connectivity of the river network. The preponderance of the urban land weakened the original correlations more than that of the cultivated land, while the seasonal interactions of the cultivated and urban lands presented opposite. The GRA can be employed as an effective supplement for numerical modeling and statistical analysis of the incomplete data. In addition, the structure and connectivity of the river network should be taken in account to improve water quality.

The Influence of River Channel Occupation on Urban Inundation and Sedimentation Induced by Floodwater in Mountainous Areas: A Case Study in the Loess Plateau, China

River channel occupation has made cities in the mountainous areas more vulnerable to floodwater out of river channels during rapid global urbanization. A better understanding of the influence of river channel occupation on urban flood disasters can serve as a reference in planning effective urban flood control strategies. In this study, taking a flood event that occurred on July 26th, 2017 in a city on the Loess Plateau as an example, field surveys, dynamics detection of the river channel using remote sensing technology, and scenario simulations with a two-dimensional flow and sediment model were utilized to quantitatively analyze the impacts of river channel occupation on urban inundation and sedimentation. The results show that river channel dynamics reduced by construction can be successfully detected using the combination of high-resolution images and Landsat time-series images. The variation of the water level–discharge relationship caused by the narrowing of the river channel and the increase of the flood-water level caused by water-blocking bridges/houses result in a significant reduction of the flood discharge capacity. The contribution of the narrowing of the river channel was 72.3% for the total area inundated by floodwater, whereas 57.2% of urban sedimentation was caused by the construction of bridges/houses within the river channel. Sustainable flood mitigation measures were also recommended according to the investigations and research findings in this study in order to reduce the social, environmental and economic damages caused by floods.

The relative impact of urbanization and precipitation on long-term water level variations in the Yangtze River Delta

The combined and individual hydrological impacts of climate variation and urbanization have been extensively discussed over the past few decades, yet little is known about the relative impact of each. In this paper we took one of the most developed regions worldwide, Yangtze River Delta, as an example to analyse the long-term relative impacts of precipitation and urbanization change on water level alterations, based on precipitation, water level series, and annual impervious area data from 1960 to 2015. Abrupt changes detection in the water level series divided the data into the pre-impact period (1960-1988) and impacted period (1989-2015), and relative impacts of precipitation and urbanization on the water level increase from pre-impacted to impacted period, as well as their spatial and seasonal variations were estimated with the elasticity method. The results indicated that the urbanization change showed no distinct influence on the water level rise in the pre-impact period, while the precipitation played distinct roles only during summer months in the impacted period; the precipitation dominated two thirds of the water level rise in flood season, and in non-flood season the urbanization controlled the two thirds of the water level rise; spatially, the water level variations in old and new urban area were dominated by precipitation and urbanization process respectively; compared with precipitation amount, the water level correlated more strongly to the contribution ratio of precipitation. The results would provide a good reference for flood control and water resource management in the river basin, especially in the economically developed areas.