Getting Started
Urban environments fundamentally alter the natural movement of water. In a process central to local hydrology, cities replace porous, natural landscapes with vast areas of non-porous surfaces, disrupting the water cycle. This chapter focuses on the problem of urban runoff—water that flows over these sealed surfaces—and explores key methods designed to mitigate its negative environmental consequences by restoring a city's ability to absorb rainwater.
What You Should Be Able to Do
After completing this section, you should be able to:
Explain how urban development alters the natural pathways of the hydrologic cycle.
Describe the primary environmental problems associated with high volumes of urban runoff.
Identify four distinct methods for reducing urban runoff.
Connect each mitigation method to the core principle of increasing water infiltration.
Compare the ecological functions of a traditional urban landscape with one incorporating green infrastructure.
Key Concepts & Mechanisms
The generation and management of urban runoff is a classic environmental process of cause and effect. By understanding the inputs, mechanisms, and impacts, we can better appreciate the strategies designed to intervene and mitigate the problem.
Inputs & Preconditions
The process begins with a necessary input: precipitation (rain or snowmelt). The key precondition is the landscape upon which it falls. Urban areas are characterized by a high percentage of impermeable surfaces—materials like asphalt, concrete, and roofing that do not allow water to pass through them. In a natural ecosystem like a forest or grassland, these surfaces are rare.
Key Steps / Mechanism: The Urban Water Pathway
Reduced Infiltration: When precipitation hits impermeable surfaces, it cannot soak into the ground. Infiltration, the process of water seeping into the soil, is drastically reduced. This also prevents the groundwater recharge that naturally replenishes underground aquifers.
Accelerated Flow: Unable to go down, the water flows sideways across the landscape as urban runoff. The smooth, engineered surfaces of roads and parking lots allow this water to move much faster and in greater volumes than it would in a natural setting.
Pollutant Collection: As it travels, the runoff acts as a solvent and a transport medium, picking up pollutants from the urban environment. This includes oil, gasoline, and heavy metals from vehicles; fertilizers and pesticides from lawns; sediment from construction sites; and litter. This is a form of nonpoint source pollution, where contaminants come from a broad area rather than a single identifiable source.
Overwhelming Infrastructure: This large, fast-moving, polluted volume of water is channeled into storm drain systems, which are often directly connected to local streams, rivers, and lakes.
Outputs & Impacts
Water Pollution: The direct discharge of untreated, polluted runoff degrades the water quality of aquatic ecosystems. Excess nutrients from fertilizers can cause eutrophication, while toxins like heavy metals can harm or kill aquatic life.
Flooding: During heavy precipitation events, the sheer volume of runoff can exceed the capacity of storm drains, leading to localized and downstream flooding.
Stream Degradation: The high velocity of runoff entering streams causes bank erosion, widens channels, and destroys aquatic habitats. The lack of groundwater recharge can also cause streams to run low or dry up during periods of no rain.
Mitigation: Increasing Infiltration
The primary goal of mitigating urban runoff is to interrupt the mechanism described above by increasing the amount of water that soaks into the ground. This approach, often called green infrastructure, mimics natural hydrology. The table below outlines key methods.
| Mitigation Method | Mechanism for Reducing Runoff | Primary Environmental Benefit | Additional Co-benefits |
|---|---|---|---|
| Permeable Pavement | Constructed with porous materials (e.g., porous asphalt, pervious concrete, interlocking pavers) that allow water to pass through the surface into a gravel base, infiltrating the soil below. | Reduces runoff volume at the source and promotes groundwater recharge. | Filters pollutants from water as it passes through; can reduce the urban heat island effect compared to dark, non-porous asphalt. |
| Planting Trees | Tree canopies intercept a portion of rainfall, reducing the amount that hits the ground. Root systems create channels in the soil that enhance its capacity for infiltration. | Slows and reduces runoff, stabilizes soil to prevent erosion, and absorbs pollutants. | Improves air quality, provides shade and cooling, creates wildlife habitat, and increases property values. |
| Building Up, Not Out | This "smart growth" strategy focuses on developing compact, multi-story buildings rather than sprawling single-story ones. It minimizes the total land area dedicated to a development. | Preserves surrounding natural, permeable landscapes (forests, wetlands) that can absorb rainwater, preventing that land from being converted to impermeable surfaces. | Reduces habitat fragmentation, encourages walkable communities, and can reduce reliance on cars. |
| Increased Public Transit | An efficient public transportation system reduces the number of personal vehicles on the road. | Fewer cars require less infrastructure, such as massive parking lots and wide roads, thereby reducing the total percentage of impermeable surfaces in a city. | Reduces air pollution and greenhouse gas emissions from vehicle exhaust; less traffic congestion. |
Key Models & Diagrams
The diagram below illustrates the fundamental shift in the water cycle caused by urbanization and the restorative effect of mitigation strategies.
Flowchart: Water Cycle Pathways in Different Landscapes
1. Natural Ground Cover (e.g., Forest)
Precipitation → High Infiltration (to soil & groundwater) → High Evapotranspiration (from plants) → Low Surface Runoff
2. Impermeable Urban Cover (e.g., Parking Lot)
Precipitation → Low Infiltration → Low Evapotranspiration → High Surface Runoff (collecting pollutants) → Discharge to Water Body
3. Mitigated Urban Cover (e.g., Permeable Pavement & Trees)
Precipitation → Increased Infiltration (through pavement & around roots) → Increased Evapotranspiration → Reduced Surface Runoff → Groundwater Recharge
Key Components & Evidence
Urban Runoff: Precipitation that flows over impervious urban surfaces rather than soaking into the ground. It is a major carrier of nonpoint source pollution.
Impermeable Surfaces: Materials such as concrete, asphalt, and rooftops that prevent the infiltration of water into the soil.
Infiltration: The process by which water on the ground surface enters the soil. This is a critical step in the natural water cycle that is disrupted by urbanization.
Groundwater Recharge: The process through which water moves downward from surface water to groundwater. It is essential for replenishing aquifers.
Permeable Pavement: A type of pavement with a high porosity that allows water to pass directly through it into the ground below, reducing runoff.
Urban Sprawl: The expansion of low-density development outward from city centers. This pattern of growth consumes large areas of natural, permeable land.
Watershed: An area of land that drains all the streams and rainfall to a common outlet such as a river, lake, or ocean. Urban runoff affects the health of the entire watershed.
Nonpoint Source Pollution: Pollution that comes from many diffuse sources, rather than a single, identifiable "point." Urban runoff is the primary example.
Green Infrastructure: An approach to water management that protects, restores, or mimics the natural water cycle. The methods described in this chapter are all forms of green infrastructure.
Skill Snapshots
Causation
Cause: Replacing forests and fields with concrete and asphalt results in a dramatic decrease in water infiltration.
Cause: The rapid flow of urban runoff into streams leads to increased streambank erosion and habitat destruction.
Cause: Implementing smart growth principles like "building up, not out" causes a reduction in the rate of land conversion, preserving permeable surfaces.
Comparison
Traditional pavement is designed to shed water quickly into storm drains, whereas permeable pavement is designed to absorb water on-site.
Urban sprawl increases the total area of impermeable surfaces, while urban densification (building up) minimizes the footprint of development.
A natural landscape manages rainwater through high infiltration and evapotranspiration, in contrast to a traditional city which manages it through rapid collection and discharge.
Change & Continuity Over Time (CCOT)
Baseline: A pre-development watershed features high levels of infiltration, stable stream flows, and naturally filtered water.
Change 1: Following urbanization in the 20th century, the landscape is covered with impermeable surfaces, leading to flashy floods, polluted runoff, and depleted groundwater.
Change 2: In recent decades, cities have begun implementing green infrastructure, such as permeable pavement and urban forests, to restore some of the area's natural hydrologic function.
Continuity: Throughout this entire process, the fundamental driver of the local hydrologic system remains precipitation; what has changed is the pathway that water takes after it falls.
Common Misconceptions & Clarifications
Misconception: Storm drains lead to a water treatment plant.
- Clarification: In most systems, storm drains are separate from the sanitary sewer system. They are simply pipes that convey untreated runoff directly into the nearest stream, river, or lake, along with any pollutants it has collected.
Misconception: Urban runoff is just clean rainwater.
- Clarification: Runoff from urban areas is a significant source of water pollution, often containing a toxic mix of oil, heavy metals, pesticides, fertilizers, bacteria from pet waste, and sediment.
Misconception: Public transportation is only about reducing air pollution.
- Clarification: While reducing air pollution is a major benefit, public transit also has significant land-use benefits. By reducing the need for cars, it also reduces the demand for vast, impermeable parking lots and roads, which are primary generators of urban runoff.
Misconception: The only way to prevent floods is to build bigger pipes and channels.
- Clarification: This "gray infrastructure" approach only moves the problem downstream. Modern "green infrastructure" methods aim to manage water where it falls by increasing infiltration, which reduces the total volume of water that needs to be managed by pipes.
One-Paragraph Summary
Urbanization drastically alters the natural water cycle by replacing permeable soil with impermeable surfaces like concrete and asphalt. This change prevents rainwater infiltration, leading to increased urban runoff, which causes localized flooding, stream erosion, and the transport of nonpoint source pollutants into aquatic ecosystems. To combat these issues, cities can implement green infrastructure solutions designed to increase infiltration and manage water on-site. Key methods include using permeable pavement, planting trees, and adopting smart growth principles like building vertically ("up, not out") and promoting public transportation to reduce the land area devoted to roads and parking. These strategies work to mimic natural hydrology, thereby reducing pollution, recharging groundwater, and creating more resilient and sustainable urban environments.