What Are Impervious Surfaces?
Impervious surfaces are constructed surfaces that do not allow water to infiltrate into the underlying soil. These surfaces, common in urban and suburban landscapes, include roads, parking lots, rooftops, and sidewalks. Within the broader field of environmental finance, understanding impervious surfaces is crucial due to their significant hydrological and economic impacts. As urban development expands, the proliferation of impervious surfaces fundamentally alters natural water cycles, leading to increased stormwater runoff and reduced groundwater recharge.
History and Origin
The concept of impervious surfaces gained prominence with the rise of widespread urban development and modern infrastructure. Historically, land surfaces largely allowed for natural water infiltration. However, as human settlements grew and infrastructure advanced, the construction of paved roads, buildings, and other hardened surfaces became integral to modern living. This transformation, driven by industrialization and population growth, accelerated the conversion of permeable land to impervious areas.
The environmental and economic consequences of this shift have become increasingly recognized over recent decades. Researchers and policymakers have intensified their focus on how impervious surfaces contribute to issues like urban flooding, water pollution, and heat island effects. For instance, the U.S. Environmental Protection Agency (EPA) developed tools like the Impervious Surface Growth Model (ISGM) to help communities estimate and compare the potential impacts of proposed development scenarios on impervious cover, reflecting a growing awareness and need for better planning6.
Key Takeaways
- Impervious surfaces are human-made structures that prevent water absorption into the ground.
- They significantly increase stormwater runoff, reduce groundwater recharge, and contribute to water pollution.
- The economic impacts include increased costs for stormwater management and potential damages from flooding.
- Financial markets, particularly the municipal bonds sector, are increasingly considering the climate risks associated with high imperviousness.
- Strategies like green infrastructure aim to mitigate the negative effects of impervious surfaces.
Interpreting Impervious Surfaces
The extent of impervious surfaces within a given area is a critical indicator of its hydrological health and susceptibility to water-related issues. A higher percentage of impervious cover typically correlates with increased rates and volumes of stormwater runoff, leading to more frequent and severe urban flooding and degraded water quality. For urban planners and environmental economists, interpreting the distribution and density of these surfaces helps in assessing environmental risk and planning for resilient infrastructure.
For instance, studies show that stream quality can begin to degrade when more than 10% of a watershed is covered by impervious surfaces5. This understanding informs decisions regarding new construction, requiring mitigation strategies like permeable pavements or green roofs to manage runoff. Investors in real estate or infrastructure investment may consider the existing imperviousness of an area as a factor in long-term asset valuation due to potential future costs associated with flood mitigation or environmental compliance.
Hypothetical Example
Consider two hypothetical towns, Riverbend and Greenfield, both of similar size and population. Riverbend has aggressively pursued traditional urban planning with extensive paved roads, large parking lots, and commercial rooftops, resulting in 40% impervious surfaces across its developed areas. Greenfield, in contrast, has adopted policies promoting smart growth and green infrastructure, maintaining only 15% impervious surfaces.
During a heavy rainstorm, Riverbend experiences significant surface runoff, overwhelming its traditional drainage systems. This leads to localized flooding, causing property damage and disruptions to businesses. The town faces substantial capital expenditure for emergency services and repairs, and potentially higher insurance premiums for residents. Greenfield, with its lower imperviousness, sees more rainwater infiltrate the ground, reducing surface runoff and mitigating flood risks. While Greenfield might have invested more upfront in green infrastructure solutions, its long-term economic impact from avoided flood damage and improved environmental quality is more favorable.
Practical Applications
Impervious surfaces have significant practical implications across several sectors, particularly in finance, urban development, and risk management.
- Municipal Finance: Local governments issue municipal bonds to fund infrastructure projects. The increasing frequency and intensity of extreme weather events, exacerbated by impervious surfaces, pose growing financial risks to these issuers. Investors are increasingly scrutinizing how municipalities manage these risks, with some bonds explicitly linked to environmental outcomes, such as targets for managing impervious surface areas through green infrastructure initiatives4. This reflects a shift towards incorporating climate change resilience into public financing.
- Property Development and Valuation: Developers and investors assess the percentage of impervious surfaces in proposed projects or existing properties. High imperviousness can lead to increased costs for stormwater management systems, potential regulatory hurdles, and elevated property value risks due to flood exposure. Conversely, projects that reduce impervious cover through sustainable designs can enhance long-term value and marketability.
- Insurance and Risk Assessment: Insurance companies evaluate the flood risk of properties, which is directly influenced by the prevalence of impervious surfaces in a watershed. Regions with higher imperviousness may face higher insurance premiums or reduced coverage availability. Governments and agencies, such as the National Oceanic and Atmospheric Administration (NOAA), highlight how natural infrastructure solutions can be more cost-effective for flood risk reduction compared to traditional built infrastructure, indirectly emphasizing the costs associated with extensive imperviousness3.
Limitations and Criticisms
While the concept of impervious surfaces is a useful metric, its application has limitations. Simply measuring the total area of impervious surfaces does not always capture the full hydrological impact. The "connectedness" of these surfaces, or how directly they channel water into drainage systems, can be a more accurate predictor of environmental degradation than total imperviousness alone. For example, a large parking lot with permeable pavers and effective stormwater management systems may have a different impact than a similarly sized lot with conventional asphalt and direct runoff into a stream.
Furthermore, efforts to mitigate the effects of impervious surfaces can involve substantial capital expenditure. Implementing green infrastructure solutions, while beneficial long-term, requires significant upfront investment and ongoing maintenance. Critics sometimes point to the challenges in retrofitting existing urban areas, where space for such solutions may be limited and the costs prohibitive. The economic impact of these interventions on existing property values and local tax bases is a complex area, with studies showing varied effects depending on design and location2. Therefore, comprehensive risk mitigation strategies must balance economic feasibility with environmental goals.
Impervious Surfaces vs. Pervious Surfaces
The distinction between impervious and pervious surfaces lies in their ability to allow water to penetrate the ground.
| Feature | Impervious Surfaces | Pervious Surfaces |
|---|---|---|
| Water Infiltration | Prevents water from soaking into the soil. | Allows water to infiltrate the soil. |
| Examples | Asphalt roads, concrete sidewalks, rooftops, most buildings, compacted soils. | Vegetated areas, forests, grasslands, permeable pavers, gravel, uncompacted soil. |
| Stormwater Runoff | Significantly increases runoff volume and velocity. | Reduces runoff, allowing natural absorption and filtration. |
| Environmental Impact | Contributes to urban flooding, water pollution, reduced groundwater recharge, urban heat island effect. | Supports groundwater recharge, filters pollutants naturally, reduces flood risk, cools ambient temperatures. |
Confusion often arises because some urban features, like certain types of sidewalks or driveways, can be designed to be either impervious or pervious depending on the materials used (e.g., traditional concrete vs. permeable concrete pavers). The key differentiator is the surface's capacity to absorb or transmit water to the subsurface.
FAQs
How do impervious surfaces affect water quality?
Impervious surfaces prevent rainwater from filtering through the soil, causing it to flow over hardened areas and pick up pollutants such as oil, chemicals, sediment, and litter. This contaminated runoff then flows directly into rivers, lakes, and oceans, degrading water quality.1
Are impervious surfaces only found in cities?
While most prevalent in densely populated urban and suburban areas due to extensive urban development, impervious surfaces can be found anywhere human construction hardens the landscape. This includes rural roads, farm buildings, and even driveways in less developed regions.
What is the financial impact of impervious surfaces on communities?
The financial impact includes increased costs for stormwater management infrastructure, potential property value depreciation due to flood risk, higher insurance premiums, and expenses related to cleaning up polluted waterways. Communities may also incur costs for risk mitigation efforts like building green infrastructure.
Can existing impervious surfaces be modified to be more environmentally friendly?
Yes, existing impervious surfaces can be modified. Techniques include replacing traditional pavements with permeable materials, installing green roofs on buildings, creating rain gardens, and establishing bioswales to capture and infiltrate runoff. These retrofitting projects fall under green infrastructure initiatives.
How do investors consider impervious surfaces?
Investors, especially those focused on sustainable investing or assessing climate risk, consider the extent of impervious surfaces when evaluating real estate, municipal bonds, or infrastructure investment opportunities. High imperviousness may signal increased environmental liabilities or susceptibility to climate-related financial impacts.