Main methods to reduce urban runoff focus on increasing infiltration and capturing stormwater (CED STB-1.B). Key techniques: permeable pavement (porous asphalt, permeable pavers), green roofs, rain gardens/bioswales, bioretention cells and infiltration trenches, plus stormwater retention ponds and detention basins. Rainwater harvesting and vegetated buffer strips capture water for reuse and slow flow. Urban tree canopy, planting more vegetation, and low-impact development/smart growth (build up, not out; more public transit) reduce impervious area and runoff volume. These approaches lower peak flows, filter pollutants, recharge groundwater, and reduce erosion—the sorts of solutions AP asks you to describe on free-response questions for Unit 5 (Topic 5.13). For a quick review, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and the unit overview (https://library.fiveable.me/ap-environmental-science/unit-5). Practice more with Fiveable practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Permeable pavement reduces runoff by letting water pass through the surface instead of flowing off impervious streets. The top layer is porous (porous asphalt, permeable pavers, or concrete with voids) so rainwater moves down into a gravel storage layer beneath, where it’s held temporarily and either slowly infiltrates into native soil (recharging groundwater) or is released gradually to storm drains. That slows peak flows, reduces erosion and flooding, and filters out sediments and some pollutants as water moves through the pavement layers and underlying soil (a form of bioretention). For the APES CED, this is an example of increasing infiltration in EK STB-1.B.1 and a low-impact development technique—know the types (porous asphalt, permeable pavers) and the benefits (reduced runoff volume, pollutant filtration, less stress on stormwater systems). For more on Topic 5.13, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Regular (traditional) pavement—like concrete or asphalt—is impermeable: water runs off its surface quickly, increasing stormwater volume and speed, which leads to erosion, flooding, and carries pollutants into drains and waterways. Permeable pavement (porous asphalt, permeable pavers, or permeable concrete) is designed with void spaces or gaps so rainfall can infiltrate into the ground below. That reduces surface runoff, recharges groundwater, and filters some pollutants through the sub-base and soil (part of low-impact development/green infrastructure strategies listed in EK STB-1.B.1). For the AP exam, remember permeable pavement is an example of increasing infiltration to mitigate urban runoff (Topic 5.13). For a quick review, see the Topic 5.13 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu). More unit review and practice questions are at the Unit 5 page (https://library.fiveable.me/ap-environmental-science/unit-5) and the APES practice bank (https://library.fiveable.me/practice/ap-environmental-science).

Planting trees reduces urban runoff because trees increase infiltration and slow water before it becomes surface runoff—exactly what EK STB-1.B.1 lists as a mitigation method. Tree canopies intercept rainfall (some water evaporates back to the air), trunks and leaf litter slow runoff, and roots create pores in soil that let water soak in. That lowers peak flows into storm drains, reduces erosion, and filters out pollutants (sediment, nutrients) before they reach streams. Trees also use water via evapotranspiration, lowering the volume that reaches drains and reducing urban heat island effects that can worsen storm intensity. On the AP exam, you can cite “urban tree canopy” or “increased infiltration” as specific low-impact development solutions when asked to propose or justify runoff-reduction methods (see the Topic 5.13 study guide for quick facts) (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu). For broader review, check Unit 5 (https://library.fiveable.me/ap-environmental-science/unit-5) and practice FRQ-style scenarios (https://library.fiveable.me/practice/ap-environmental-science).

Public transportation reduces urban runoff mainly by cutting the amount of impervious surface and vehicle use that create and move stormwater and pollutants. Fewer cars means less need for wide roads and huge parking lots (both are impervious), so cities can “build up, not out,” keep more permeable surfaces, and maintain urban tree canopy and green infrastructure (EK STB-1.B). Fewer vehicles also mean less oil, rubber, heavy metals, and de-icing salts deposited on pavement that wash into storm drains during rain. So transit = less polluted runoff and lower peak stormwater volumes, which makes low-impact development measures (rain gardens, bioswales, permeable pavement) more effective. For AP exam hooks: this connects directly to Topic 5.13 methods to increase infiltration and STB-1.B on the CED. If you want a concise review, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice more problems at (https://library.fiveable.me/practice/ap-environmental-science).

“Building up, not out” means growing cities vertically—more apartments, taller buildings, and denser development—instead of expanding outward into new land. That reduces total impervious surface (roads, parking lots, rooftops spread over a wide area), preserves surrounding green space, and keeps more land available for infiltration features like rain gardens, bioswales, and tree canopy. Dense, mixed-use neighborhoods also make public transportation and walking more practical, lowering parking lots and paved roads that cause runoff. In AP terms this is a smart-growth/low-impact development strategy tied to EK STB-1.B.1: it increases infiltration and reduces stormwater volume and pollutant loading. For quick review, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and Unit 5 overview (https://library.fiveable.me/ap-environmental-science/unit-5). Want practice? Try problems at (https://library.fiveable.me/practice/ap-environmental-science).

Trees increase water infiltration in cities mainly by changing how rain hits and moves through the surface. Their leaves and branches intercept rainfall, slowing drop speed and spreading out runoff. Roots create channels and fracture compacted soil, increasing soil porosity and permeability so more water soaks into the ground (helpful for bioretention cells, rain gardens, and infiltration trenches). Root systems and leaf litter also boost organic matter, which improves water-holding capacity and reduces surface runoff. Finally, evapotranspiration returns some water to the atmosphere, lowering the total volume that must run off. These processes are listed in the CED as part of methods to increase infiltration (EK STB-1.B) and tie into urban tree canopy and low-impact development strategies. For AP review, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and more Unit 5 resources (https://library.fiveable.me/ap-environmental-science/unit-5).

Urban runoff’s a big problem because cities replace permeable soil with impermeable surfaces (roads, roofs, parking lots), so rain can’t infiltrate. That increases the volume and speed of runoff, which causes erosion, floods, and transports pollutants—oil, heavy metals, nutrients, sediments—straight into streams and lakes. Nutrient loads fuel algal blooms and hypoxia; sediments smother aquatic habitat; fast flows damage stream banks and infrastructure. Runoff also reduces groundwater recharge, lowering base flow during dry periods. On the APES exam you should connect this to solutions in Topic 5.13 (STB-1.B)—like permeable pavement, green roofs, rain gardens, bioswales, retention ponds, urban tree canopy, and low-impact development—which reduce peak flows and increase infiltration. For a focused review, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu); for overall unit review and lots of practice, check Unit 5 (https://library.fiveable.me/ap-environmental-science/unit-5) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Water infiltration is just water soaking into the ground instead of running off the surface. Think of pouring water on a sidewalk (it runs off) vs. on a patch of soil (it soaks in). Infiltration reduces urban runoff, recharges groundwater, and filters pollutants. Ways cities increase infiltration (these are in Topic 5.13): use permeable pavement (porous asphalt or permeable pavers), plant trees/expand urban tree canopy, build rain gardens, bioswales, infiltration trenches, green roofs, and design low-impact development or “build up, not out” to limit impervious surfaces. These methods are key for free-response questions asking for realistic solutions to runoff problems (STB-1.B in the CED). For a quick topic review, check the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice more questions at (https://library.fiveable.me/practice/ap-environmental-science).

Permeable pavement lets water soak through instead of running off—good for reducing urban runoff (EK STB-1.B.1). Common materials/examples you should know for the APES exam: - Porous (pervious) asphalt—similar to regular asphalt but with connected voids that let water through. - Pervious concrete—concrete mix designed with gaps between aggregate so water infiltrates. - Permeable pavers / interlocking concrete pavers—individual units with gaps or sand joints that allow infiltration (open-joint pavers). - Grass/soil-filled pavers and turf grids—plastic or concrete grids filled with soil and vegetation (good for low-traffic areas). - Porous rubber or recycled-material pavers—used for sidewalks or light-use surfaces. - Reinforced gravel or open-graded aggregate systems—combine a structural layer with void space for infiltration. These are part of low-impact development and smart growth strategies (CED keywords). For more on methods to reduce urban runoff and examples, check the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) or the Unit 5 overview (https://library.fiveable.me/ap-environmental-science/unit-5).

Less pavement means more water soaks in instead of running off. When you replace impervious surfaces with permeable pavement, rain gardens, bioswales, or add urban tree canopy (all CED keywords), infiltration increases and runoff volume and velocity drop. That reduces the amount of pollutants—oil, heavy metals, nutrients, and sediment—carried to storm drains and waterways, lowers erosion and thermal pollution, and cuts the chance of combined-sewer overflows. More infiltration also recharges groundwater and supports baseflow in streams. These are exactly the low-impact development/green-infrastructure strategies the CED lists (permeable pavement, rain gardens, green roofs, bioretention cells). For AP prep, know cause → effect links (less impervious area → more infiltration → less pollutant transport) for free-response and multiple-choice questions. Review Topic 5.13 study guide here (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice related problems at (https://library.fiveable.me/practice/ap-environmental-science).

Think of runoff as water that can’t soak in. Urban development replaces permeable soil with roofs, roads, and parking lots (impervious surfaces), so rain that used to infiltrate now flows quickly across surfaces. That increases total runoff volume, speeds flow (causing erosion), and carries pollutants (oil, nutrients, sediment) into streams and lakes—raising flood risk and degrading water quality. AP methods to fix this focus on increasing infiltration and slowing water: permeable pavement, green roofs, rain gardens, bioswales, infiltration trenches, detention/retention ponds, rainwater harvesting, and expanding urban tree canopy (CED keywords). On the exam, you may be asked to describe or propose these low-impact development solutions (STB-1.B / EK STB-1.B.1), so practice identifying how each reduces runoff or pollution. For a focused study guide, see Topic 5.13 (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and more practice questions at (https://library.fiveable.me/practice/ap-environmental-science).

When rain hits regular concrete (impervious surface) most of the water becomes surface runoff: it flows quickly into gutters and storm drains, carries pollutants (oil, metals, nutrients), raises peak flow and erosion downstream, and can overload sewer systems causing floods or combined-sewer overflows. By contrast, permeable surfaces (permeable pavement, porous asphalt, permeable pavers, vegetated areas) let much more water infiltrate into the soil. That infiltration recharges groundwater, slows and reduces peak runoff, and allows soil and plants to filter out pollutants (rain gardens, bioswales, bioretention cells). These are exactly the low-impact development/green infrastructure approaches the CED lists for mitigating urban runoff (STB-1.B.1). For AP review, focus on how permeable options reduce runoff volume and pollutant transport and which practices (green roofs, infiltration trenches, retention ponds) are used (study guide: https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu). More practice problems are at (https://library.fiveable.me/practice/ap-environmental-science).

Building up instead of out is better because it reduces the amount of land covered by impervious surfaces (roads, parking lots, low-permeability pavement). Less sprawl means more open ground for infiltration (rain gardens, bioswales, vegetated buffer strips, urban tree canopy) so stormwater soaks in instead of running off quickly, carrying pollutants to streams and overwhelming storm systems. Taller, denser development also makes public transit and walkability more viable, cutting paved area per person and reducing stormwater per capita. On the APES exam, this idea shows up in Topic 5.13 as a low-impact development/smart-growth strategy to mitigate urban runoff (CED EK STB-1.B.1: “building up, not out”). For a quick review of these methods, see the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

They’re both effective tools for reducing urban runoff, but how well they work depends on design, climate, and maintenance. Green roofs (vegetated rooftop systems) reduce and delay rooftop runoff by retaining rainfall in their growing medium and releasing it more slowly; they’re great for lowering peak discharge from buildings. Rain gardens (bioretention cells) capture and infiltrate stormwater at ground level, treating the “first flush” of pollutants and reducing volume that reaches storm drains. Both are core low-impact development (LID) methods aligned with EK STB-1.B.1 (increase infiltration) and help cut peak flows, pollutant loads, and combined-sewer overflow risks. Effectiveness varies with soil/substrate depth, plant type, storm size, and maintenance—so a well-designed green roof or rain garden will outperform a shallow/poorly maintained one. For AP review, focus on their roles (infiltration, detention, pollutant uptake) and where they fit in urban runoff management (permeable surfaces, bioswales, rainwater harvesting). See the Topic 5.13 study guide (https://library.fiveable.me/ap-environmental-science/unit-5/methods-reduce-urban-runoff/study-guide/rqaZZb3TDbQl3j66bpbu) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).