On this page
In this lesson
What you'll learn
- Explain how impervious surfaces change runoff volume, peak flow and quality
- Compare detention, retention and infiltration practices
- Describe common low impact development and green infrastructure practices
- Understand long-term maintenance responsibilities
Why developed land needs permanent controls
Roofs, roads and parking lots are impervious: water can't soak in. Compared with the meadow or forest they replace, developed sites produce:
- More runoff volume, because less water infiltrates.
- Higher and faster peak flows, because runoff travels quickly over smooth surfaces and through pipes.
- More pollutants: oil, metals, nutrients, bacteria, trash and heat.
Without controls, downstream channels erode, flooding increases and water quality declines. That's why MS4 permits require communities to run a post-construction stormwater program for new development and redevelopment, enforced through local codes and plan review.
Detention, retention and infiltration
| Practice | How it works | Main benefit |
|---|---|---|
| Detention (dry) pond or vault | Temporarily stores runoff and releases it slowly through an outlet; empties between storms | Reduces peak flow and flooding |
| Retention (wet) pond | Keeps a permanent pool; new runoff displaces treated water | Peak control plus settling and biological treatment |
| Infiltration basin or trench | Stores runoff until it soaks into the ground | Reduces runoff volume and recharges groundwater |
| Underground storage | Pipes, chambers or vaults under parking areas | Saves surface space |
See detention vs. retention ponds for a deeper comparison.
Low impact development and green infrastructure
Low impact development (LID) manages rain close to where it falls, mimicking natural hydrology:
- Bioretention and rain gardens: shallow planted depressions with engineered soil that filter and infiltrate runoff.
- Permeable pavement: pervious concrete, porous asphalt or permeable pavers over a stone reservoir.
- Vegetated swales and filter strips: grassed channels and slopes that slow and filter flow.
- Green roofs: vegetated roof systems that retain rainfall.
- Rainwater harvesting: cisterns and rain barrels that capture roof runoff for reuse.
- Disconnecting impervious areas: directing downspouts and pavement runoff to vegetated areas instead of pipes.
Many states and cities set performance standards, such as retaining or treating a specified design storm on site. Requirements vary widely, so check the local stormwater manual.
Manufactured treatment devices
Proprietary devices such as hydrodynamic separators, cartridge filters and catch basin inserts remove sediment, trash and oil in tight urban sites. They need regular cleanout to keep working.
Maintenance: the part that fails
Permanent practices only work if they're maintained. Common failures: clogged outlets, sediment-filled ponds, dead plants in bioretention, and sealed permeable pavement. Most programs require:
- A maintenance plan and agreement recorded with the property.
- Regular inspections and records, often annually.
- Clear responsibility for the owner or association, with municipal oversight.
Key takeaways
- Impervious surfaces increase runoff volume, peak flow and pollutants.
- Detention reduces peaks; retention adds a permanent pool for treatment; infiltration reduces volume.
- LID practices manage rain where it falls: bioretention, permeable pavement, swales, green roofs, harvesting.
- Standards vary by state and city, set through MS4 programs and local codes.
- Maintenance plans, agreements and inspections keep permanent controls working.
Knowledge check
Lesson 11 quiz: 8 questions
Choose one answer for each question, then select Check my answers. Score 80% or higher to pass. Your answers stay in your browser and are never saved.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.