14.2 Urbanization and its effects on surface processes
4 min read•july 30, 2024
Urbanization drastically alters Earth's surface processes, transforming natural landscapes into concrete jungles. Cities reshape hydrology, sediment dynamics, and local climates, leading to increased runoff, altered stream morphology, and urban heat islands.
These changes have far-reaching consequences for ecosystems and human communities. Understanding urban impacts on surface processes is crucial for developing sustainable cities and mitigating environmental degradation in our increasingly urbanized world.
Impervious Surfaces and Runoff
Characteristics and Effects of Impervious Surfaces
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- prevent water infiltration into soil (concrete, asphalt, buildings)
- Percentage of impervious surface cover in a watershed directly correlates with increased surface runoff and decreased groundwater recharge
- Alter natural hydrologic cycle by reducing infiltration, evapotranspiration, and increasing volume and velocity of surface runoff
- Significantly reduce time of concentration in urban areas
- Time water takes to travel from most distant point in watershed to outlet
- Urban areas with high impervious surface coverage experience more frequent and severe flash floods
- Non-linear relationship between impervious surface area and runoff
- Significant increases in runoff occur at relatively low levels of imperviousness (10-20%)
Hydrological Impacts in Urban Watersheds
- of urban watersheds shows:
- Higher
- Shorter
- Steeper rising and falling limbs
- Increased flood risk in urban areas due to rapid runoff generation
- Reduced groundwater recharge leads to lowered water tables
- Altered
- More frequent high flow events
- Reduced baseflow during dry periods
- Impaired water quality due to increased pollutant transport in runoff
Urban Infrastructure Impact on Sediment Dynamics
Alterations to Sediment Transport Processes
- Urban infrastructure (storm sewer systems, channelized streams) modifies natural sediment transport
- Increased runoff from impervious surfaces leads to higher stream power
- Causes accelerated erosion and channel incision in urban streams
- Urban development results in:
- Initial pulse of sediment delivery during construction
- Long-term decrease in sediment supply due to surface stabilization
- Alteration of natural flow regimes affects:
- Frequency and magnitude of geomorphically effective flows
- Sediment transport and deposition patterns
- Urban infrastructure creates local sediment sinks and sources
- Sinks (stormwater detention basins)
- Sources (construction sites)
- Spatial variability in sediment dynamics within urban watersheds
Channel Stability and Urban Stream Syndrome
- in urban streams characterized by:
- Widening
- Deepening
- Straightening
- Process known as
- Disconnection of streams from floodplains due to channelization and flood control structures
- Impacts sediment storage and channel evolution processes
- Increased erosion of stream banks and bed materials
- Altered sediment size distribution in urban streams
- Often coarser bed material due to washing out of fines
- Changes in channel morphology (pool-riffle sequences, meander patterns)
Green Infrastructure for Mitigation
Types and Functions of Green Infrastructure
- mimics natural processes to manage stormwater and improve water quality
- (rain gardens, bioswales) reduce peak flows and total runoff volume
- Help restore natural hydrologic conditions in urban watersheds
- allow stormwater infiltration
- Reduce surface runoff
- Promote groundwater recharge
- significantly reduce runoff from buildings
- Mitigate
- Influence local climate and weathering processes
- and act as sediment traps
- Reduce sediment loads to urban streams
- Improve water quality
Effectiveness and Implementation Considerations
- Green infrastructure helps restore natural flow regimes and sediment dynamics
- Promotes channel stability and ecological health in urban streams
- Effectiveness in mitigating urban geomorphic impacts depends on:
- Design
- Scale of implementation
- Integration with existing gray infrastructure systems
- Challenges in implementation:
- Cost
- Maintenance requirements
- Space limitations in dense urban areas
- Benefits beyond stormwater management:
- Improved air quality
- Enhanced biodiversity
- Increased property values
- Monitoring and adaptive management crucial for long-term success of green infrastructure projects
Urban Heat Islands and Soil Formation
Urban Heat Island Effects on Weathering
- Urban heat islands (UHIs) significantly warmer than surrounding rural areas
- Increased temperatures in UHIs accelerate chemical weathering processes
- Particularly affects building materials and urban infrastructure
- UHIs alter local precipitation patterns
- Can lead to more frequent and intense rainfall events
- Affects erosion and soil formation processes
- Modification of local climate influences
- Impacts physical and chemical weathering processes
- Affects soil biological activity
Impacts on Soil Properties and Formation
- Urban soils in heat island areas experience accelerated
- Higher temperatures impact soil structure and nutrient cycling
- Increased surface temperatures lead to more rapid evaporation of water from soils
- Can result in and changes in soil structure
- Effects of UHIs on weathering and soil formation vary spatially within cities
- Depends on factors like building density, vegetation cover, proximity to water bodies
- Altered soil pH in urban areas due to increased CO2 concentrations and acid deposition
- Changes in and fauna in UHIs
- Affects nutrient cycling and soil aggregation
- Formation of anthropogenic soils with unique properties ()
- Often contain high levels of contaminants and artificial materials
Key Terms to Review (35)
Air Pollution: Air pollution refers to the presence of harmful substances in the atmosphere that can negatively affect human health, the environment, and the climate. These pollutants can originate from various sources, including industrial emissions, vehicle exhaust, and agricultural activities. As urbanization increases, the concentration of these pollutants often rises, leading to severe impacts on air quality and surface processes such as weather patterns, soil erosion, and water cycles.
Biodiversity in cities: Biodiversity in cities refers to the variety of living organisms, including plants, animals, and microorganisms, that exist within urban environments. This concept emphasizes how urban areas can support diverse ecosystems despite the challenges posed by human activities, such as pollution and habitat fragmentation. Understanding biodiversity in cities is crucial because it highlights the potential for urban areas to provide vital ecological services, enhance quality of life, and promote resilience against climate change.
Bioretention systems: Bioretention systems are landscape features designed to manage stormwater runoff through the use of vegetation, soil, and other natural materials. These systems promote water infiltration, reduce pollution, and enhance groundwater recharge, making them essential in urban environments where impervious surfaces increase runoff. By mimicking natural hydrology, bioretention systems help mitigate the negative impacts of urbanization on surface processes and water quality.
Channel Instability: Channel instability refers to the unpredictable changes in a river or stream's channel morphology, often leading to erosion, sediment deposition, and shifts in flow direction. This phenomenon can be influenced by factors such as urbanization, land use changes, and alterations in water flow patterns, which can exacerbate flooding and habitat destruction.
Constructed wetlands: Constructed wetlands are engineered systems designed to mimic the natural processes of wetlands, using vegetation, soil, and microbial activity to treat wastewater or stormwater. These systems play a crucial role in urban areas by managing water quality, reducing runoff, and enhancing biodiversity, ultimately helping to mitigate the negative impacts of urbanization on surface processes.
Earthquake risk: Earthquake risk refers to the potential for damage and loss caused by seismic events, influenced by factors such as location, infrastructure, and population density. The degree of risk is determined by how vulnerable a community or area is to earthquakes, which can be heightened in urbanized environments where buildings, roads, and other infrastructure may not be designed to withstand significant shaking. Understanding earthquake risk is crucial for developing effective preparedness strategies and mitigating potential impacts.
Erosion control: Erosion control refers to practices and methods aimed at preventing soil erosion, which can be caused by wind, water, or human activities. These methods are particularly important in urban areas where land development can increase the risk of erosion due to the removal of vegetation and alteration of natural landscapes. Implementing effective erosion control strategies helps maintain soil health, protect water quality, and reduce sedimentation in waterways.
Floodplain development: Floodplain development refers to the process by which land adjacent to rivers and streams is altered for various human uses, particularly in areas that are subject to periodic flooding. This development can significantly impact both natural ecosystems and hydrological processes, influencing sediment transport and river channel dynamics while also introducing challenges related to urbanization and land management.
Green infrastructure: Green infrastructure refers to a strategically planned network of natural and semi-natural areas designed to manage stormwater, improve air quality, and enhance urban biodiversity. This approach integrates ecological functions into urban planning, allowing for the creation of green spaces that offer both environmental benefits and recreational opportunities. By using vegetation and permeable surfaces, green infrastructure supports sustainability and resilience in rapidly urbanizing areas.
Green roofs: Green roofs are roof systems that incorporate vegetation and soil, creating a living layer on top of a building. These systems not only improve the aesthetic value of urban environments but also play a crucial role in managing stormwater, reducing urban heat, and enhancing biodiversity in densely populated areas. By integrating natural elements into the built environment, green roofs help mitigate some of the adverse effects associated with urbanization.
Hydrograph Analysis: Hydrograph analysis involves studying hydrographs, which are graphical representations of river discharge over time. This analysis is crucial in understanding how water flow changes in response to various factors, including precipitation, land use, and urbanization, helping to predict flooding events and manage water resources effectively.
Impervious Surfaces: Impervious surfaces are hard, man-made structures that do not allow water to penetrate into the ground, such as asphalt, concrete, and rooftops. These surfaces disrupt the natural hydrological cycle by preventing water infiltration and increasing surface runoff, leading to various environmental issues, particularly in urban areas.
Lag times: Lag times refer to the delay between the onset of a rainfall event and the peak response of a watershed in terms of runoff and streamflow. This concept is crucial for understanding how urbanization impacts surface processes, as developed areas often exhibit shorter lag times due to increased impervious surfaces, altered drainage patterns, and reduced vegetation cover. As a result, urban areas can experience more intense and rapid flooding compared to natural landscapes.
Land subsidence: Land subsidence is the gradual sinking or settling of the Earth's surface, often resulting from human activities such as groundwater extraction, mining, and changes in soil structure. This phenomenon can lead to significant environmental and infrastructure challenges, particularly in urban areas where the demand for resources and space increases. The process affects the landscape and can trigger further issues like flooding, degradation of infrastructure, and alteration of natural drainage patterns.
Land use planning: Land use planning is the process of managing land resources to balance various needs such as development, conservation, and environmental protection. It aims to guide the development and arrangement of land, ensuring sustainable growth while considering factors like urbanization, infrastructure, and ecological impacts. This practice is crucial in mitigating the negative effects of urban expansion on surface processes like erosion, water runoff, and habitat loss.
Organic Matter Decomposition: Organic matter decomposition is the process through which dead plant and animal material is broken down by microorganisms, fungi, and other decomposers into simpler organic and inorganic substances. This process is vital for nutrient cycling, as it releases essential nutrients back into the soil, promoting plant growth and supporting ecosystem health. In urbanized environments, the dynamics of this decomposition can change significantly due to human activities, altering soil composition and affecting overall surface processes.
Peak Flows: Peak flows refer to the maximum rate of discharge or flow of water in a river or stream at a specific location over a defined period, typically occurring during and after precipitation events. These flows are critical for understanding the hydrological response of watersheds to rainfall and snowmelt, particularly in urban areas where changes in land use can significantly alter natural drainage patterns and increase the risk of flooding.
Permeable pavements: Permeable pavements are surfaces that allow water to pass through them, helping to manage stormwater runoff and reduce flooding. These pavements are designed with materials that permit water infiltration, which can recharge groundwater and minimize surface water accumulation, making them crucial for urban areas where impervious surfaces dominate.
Retention ponds: Retention ponds are designed basins that capture and hold stormwater runoff to prevent flooding and improve water quality. These engineered systems allow for the gradual release of water, helping to mitigate the impact of urbanization on local waterways and reducing the risk of erosion and sedimentation.
Roadway runoff: Roadway runoff refers to the water that flows over roads and pavements during rainstorms or snowmelt, collecting various pollutants and debris along the way. This type of runoff is a significant environmental concern, particularly in urban areas, where increased impervious surfaces lead to higher volumes of water rushing into drainage systems. The contaminants in roadway runoff can adversely affect water quality in nearby streams, rivers, and lakes, making it a critical issue in understanding the impact of urbanization on surface processes.
Sediment transport processes: Sediment transport processes refer to the mechanisms by which sediment is moved from one location to another through natural forces such as water, wind, and ice. These processes play a crucial role in shaping landscapes, influencing landforms, and altering ecosystems. Understanding how these processes are affected by human activities like land use changes and urbanization is vital, as they can lead to significant geomorphic consequences and impact surface processes.
Soil desiccation: Soil desiccation is the process through which soil loses moisture, leading to a reduction in its water content and overall fertility. This phenomenon can be exacerbated by human activities, such as urbanization, which can alter the natural landscape, reduce vegetation cover, and disrupt local hydrology. As cities expand, impervious surfaces increase, preventing water infiltration and increasing runoff, further contributing to moisture loss in soils.
Soil microbial communities: Soil microbial communities refer to the diverse assemblages of microorganisms, including bacteria, fungi, archaea, and viruses, that inhabit the soil ecosystem. These communities play a crucial role in nutrient cycling, organic matter decomposition, and overall soil health, influencing surface processes such as erosion, sedimentation, and water retention.
Soil Moisture Regimes: Soil moisture regimes refer to the patterns of soil moisture availability that influence plant growth and ecosystem dynamics over time. These regimes are categorized based on factors such as the amount of water present, the duration of saturation, and the frequency of wet and dry periods, which can significantly affect surface processes, particularly in urban environments where land cover and water management practices change natural hydrology.
Stormwater runoff: Stormwater runoff refers to the water that flows over surfaces during and after rainfall events, which does not infiltrate into the ground. This runoff is significantly influenced by urbanization, as impervious surfaces like roads, parking lots, and rooftops prevent water from soaking into the soil, leading to increased volume and speed of runoff. The consequences of stormwater runoff can be profound, contributing to flooding, erosion, and the transport of pollutants into waterways.
Stream flow regimes: Stream flow regimes refer to the variations in the flow of water in rivers and streams over time, influenced by factors like climate, geology, land use, and human activities. Understanding these regimes is crucial for predicting how streams will respond to changes such as urbanization, which can alter the natural flow patterns and impact water quality, habitat availability, and flood risks.
Sustainable drainage systems: Sustainable drainage systems (SuDS) are designed to manage surface water runoff in a way that mimics natural drainage processes. These systems aim to reduce flooding, improve water quality, and enhance biodiversity by using techniques such as green roofs, permeable pavements, and swales to control and treat stormwater. By integrating SuDS into urban landscapes, cities can mitigate the impacts of urbanization on the hydrological cycle and maintain healthier ecosystems.
Technosols: Technosols are a type of soil that forms in urban or industrial areas, characterized by the presence of anthropogenic materials like concrete, bricks, or other construction debris. These soils often reflect the impact of human activities on the landscape, serving as a testament to urbanization and its effects on natural surface processes. Technosols can influence water drainage, plant growth, and nutrient cycling, highlighting how human modification of the environment alters traditional soil development.
Transportation Corridors: Transportation corridors are designated pathways or routes that facilitate the movement of people, goods, and services across different regions. These corridors are essential for urban development, linking residential, commercial, and industrial areas, and significantly influence surface processes by altering land use, hydrology, and ecosystem dynamics.
Urban Ecology: Urban ecology is the study of the relationships between living organisms and their environment in urban settings. This field examines how urbanization affects natural processes, biodiversity, and ecosystem dynamics. It highlights the interactions between human activities and the natural environment, emphasizing the need for sustainable practices to mitigate negative impacts on ecosystems.
Urban Forestry: Urban forestry refers to the management and study of trees, forests, and green spaces within urban environments. This practice is crucial for enhancing the ecological and social functions of cities, providing benefits like improved air quality, increased biodiversity, and enhanced aesthetic appeal. By integrating trees and greenery into urban planning, urban forestry aims to create healthier and more sustainable living spaces for residents.
Urban heat island effect: The urban heat island effect refers to the phenomenon where urban areas experience significantly warmer temperatures than their rural surroundings due to human activities and alterations in land use. This effect arises from factors like the concentration of buildings, roads, and other infrastructure that absorb and retain heat, as well as reduced vegetation and soil that would otherwise provide cooling through evapotranspiration. Consequently, this leads to increased energy consumption, elevated emissions of air pollutants, and enhanced heat-related health risks.
Urban soil: Urban soil refers to the type of soil found in urban areas, often modified and affected by human activities. This soil typically contains a mix of natural soil components and materials introduced through construction, landscaping, and waste management practices. The unique characteristics of urban soil can influence local ecosystems, water drainage, and plant growth.
Urban Stream Syndrome: Urban stream syndrome refers to the degradation of stream ecosystems caused by urbanization, which alters natural hydrology, water quality, and habitat conditions. As cities expand, the increased impervious surfaces lead to higher runoff, which in turn causes flashier flow regimes, erosion, and the introduction of pollutants into waterways. This syndrome highlights the significant ecological impacts of urban development on stream health and function.
Water quality degradation: Water quality degradation refers to the decline in the quality of water due to pollution, contaminants, and changes in physical, chemical, or biological characteristics. This degradation can affect freshwater bodies, coastal areas, and groundwater resources, often resulting from human activities, especially in urban areas where runoff can carry harmful substances into water systems.