14.2 Urbanization and its effects on surface processes

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

Top images from around the web for Characteristics and Effects of Impervious Surfaces

• 

  Basics--Streams View original

  Is this image relevant?

• 

  Stormwater - Wikipedia View original

  Is this image relevant?

• 

  Urbanization - LID SWM Planning and Design Guide View original

  Is this image relevant?

• 

  Basics--Streams View original

  Is this image relevant?

• 

  Stormwater - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Characteristics and Effects of Impervious Surfaces

• 

  Basics--Streams View original

  Is this image relevant?

• 

  Stormwater - Wikipedia View original

  Is this image relevant?

• 

  Urbanization - LID SWM Planning and Design Guide View original

  Is this image relevant?

• 

  Basics--Streams View original

  Is this image relevant?

• 

  Stormwater - Wikipedia View original

  Is this image relevant?

1 of 3

• Impervious surfaces 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

• Hydrograph analysis of urban watersheds shows:
  • Higher peak flows
  • Shorter lag times
  • 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 stream flow regimes
  • 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

• Channel instability in urban streams characterized by:
  • Widening
  • Deepening
  • Straightening
• Process known as urban stream syndrome
• 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

• Green infrastructure mimics natural processes to manage stormwater and improve water quality
• Bioretention systems (rain gardens, bioswales) reduce peak flows and total runoff volume
  • Help restore natural hydrologic conditions in urban watersheds
• Permeable pavements allow stormwater infiltration
  • Reduce surface runoff
  • Promote groundwater recharge
• Green roofs significantly reduce runoff from buildings
  • Mitigate urban heat island effect
  • Influence local climate and weathering processes
• Constructed wetlands and retention ponds 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 soil moisture regimes
  • Impacts physical and chemical weathering processes
  • Affects soil biological activity

Impacts on Soil Properties and Formation

• Urban soils in heat island areas experience accelerated organic matter decomposition
  • Higher temperatures impact soil structure and nutrient cycling
• Increased surface temperatures lead to more rapid evaporation of water from soils
  • Can result in soil desiccation 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 soil microbial communities and fauna in UHIs
  • Affects nutrient cycling and soil aggregation
• Formation of anthropogenic soils with unique properties (Technosols)
  • Often contain high levels of contaminants and artificial materials