What is “impervious cover?”
Impervious cover is any built surface that prevents water from seeping or being absorbed into the ground. Impervious cover includes driveways, sidewalks, roofs, patios, pools, parking lots, and other areas of pavement. (You might also see or hear these surfaces called “impermeable.”)
On the other hand, pervious or permeable cover are surfaces that absorb water or let water slowly seep into the ground. Examples of permeable surfaces include natural ground cover such as parks, lawns, gardens, and green roofs.
You can think of pervious or permeable surfaces as a sponge, absorbing and filtering water that slowly flows into the ground. Impervious cover, on the contrary, can be compared to a tin roof – water is not absorbed or filtered through the surface; instead, the water quickly flows off the surface, picking up and carrying anything on the surface with it.
Why is it important to reduce impervious cover?
Sidewalks, roads, buildings, and roofs are critical elements of our cities. And, it is crucial that our cities have plenty of green spaces, trees, and the right infrastructure to absorb water and heat, filter pollutants, and increase the health of urban ecosystems. Too much concrete, pavement, and other non-absorbent (impervious) surfaces can negatively impact our communities and environment in various ways:
- Urban Heat Islands: Hard, non-absorbent surfaces reflect and retain more heat than green spaces which help reduce the ambient temperature. Increased temperatures in urban areas result in an "urban heat island effect." Hotter temperatures lead to negative health impacts such as higher prevalence of heat stroke, and economic impacts such as more expensive electricity bills to cool buildings.
- Water Quality: When water (e.g. rain, melted snow, etc.) flows over impervious surfaces, it collects oil, chemicals, and other pollutants. These pollutants are then carried into Denver's waterways, reducing the quality of water in the South Platte River and other lakes and streams. When water has the opportunity to first pass through soil or vegetation, these absorbent surfaces filter out pollutants, improving overall water quality.
- Flooding: Stormwater flows over hard surfaces at faster speeds than when it flows over absorbent surfaces, increasing the likelihood and consequences of flooding. Floods damage buildings, pose safety risks, and interrupt travel.
- Strain on Drainage Systems: When it rains in the city, runoff water is directed to stormwater infrastructure (e.g. drains and sewer pipes). Excessive rain can overwhelm drainage systems, especially where upgrades are needed. Preserving and expanding green spaces and absorbent areas, as well as developing rain runoff areas and retention ponds, we can extend the capacity and lifetime of our drainage systems.
- Ecosystem Health: Developing land (e.g. constructing buildings, paving new roads) displaces plants and animals in the natural landscape. This affects overall ecosystem health and decreases biodiversity. The additional heat reflecting off of paved surfaces also makes it more challenging to grow trees, grasses, and other landscape elements, especially in Denver's dry climate.
What does it mean to have greener, less paved neighborhoods?
A neighborhood becomes greener when it has more area dedicated to plants and soil that can absorb water—instead of areas covered in pavement, concrete, and other built surfaces. The “PRT framework” describes three ways to make this happen:
- Preserve areas with soils and plants (“pervious” areas) and remaining green spaces;
- Reduce or remove existing pavement and concrete (“impervious” cover) where possible; and
- Treat stormwater runoff from pavement and concrete that cannot be removed.
Which neighborhoods have the most impervious cover?
Dense neighborhoods downtown and areas with industrial properties tend to have the most impervious cover. In Denver, the ten neighborhoods with the highest proportional amount of impervious cover are:
- Sun Valley
- Central Business District
- Valverde
- Lincoln Park
- Westwood
- Elyrisa Swansea
- Baker
- Jefferson Park
- Five Points
- Union Station
Who will benefit from the dePave project?
dePaving Denver will bring the benefits of green infrastructure to many types of Denverites, property owners, and institutions across many neighborhoods. Strategies and programs will be implemented on different property types, including city-owned spaces, public right-of-way, private residences, community institutions, and commercial corridors.
It’s important to note that green infrastructure benefits nearby areas and neighborhoods too, not just the property it’s on. Problems with heat, flooding, and water quality don’t stop at property lines or city boundaries, so reducing pavement in one spot may help the community as a whole.
What kinds of programs and strategies will be implemented?
Using community input, DGI organized possible strategies into four main programs:
- Residential Rain Gardens,
- Greening Public Right-of-Way,
- Institutional Partnerships & Projects, and
- Business & Development Programming.
Each of these programs will support strategies for community participation, educational/technical support and awareness building, stronger data collection, intentional goal setting, and alignment of city policies.
How can I get involved?
Community support and input is essential to deliver programming which meets the needs of all Denverites. The project team is committed to offering many different ways for Denverites to be involved in this effort. To get and stay engaged in the dePaving a Greener Denver initiative:
- Sign up for our quarterly newsletter announcing opportunities for feedback.
- Check the dePaving Denver web page for ongoing updates.