Low Impact Development is a stormwater management approach that mimics the natural hydrology of a site by managing rainfall at the source, using small-scale, distributed practices that infiltrate, filter, store, evaporate, and detain runoff close to where it falls. It is not a single BMP or product. It is a design philosophy that shapes the entire site plan.

LID emerged in the 1990s as a response to the limitations of conventional stormwater management, which relied on end-of-pipe detention ponds to manage peak flows but did nothing to address runoff volume or water quality. The insight was simple: instead of collecting all the water at one point and trying to treat it, keep the water distributed across the site and let natural processes handle it.

The Core Principles

1. Preserve Natural Features

The single most effective LID strategy is to leave the site's natural drainage features in place. Existing trees, native vegetation, topsoil, and natural depressions provide stormwater management for free. Every tree you remove is infiltration capacity you have to replace with engineered BMPs. Every natural swale you fill is a conveyance channel you have to rebuild.

In practice, this means:

• Map the existing drainage patterns before you start the site plan. Design the building footprint and parking layout to work with the natural drainage, not against it.
• Preserve trees and vegetation in buffers along the site perimeter and between buildings where possible.
• Avoid grading areas that do not need to be graded. Limit the disturbance footprint to the minimum necessary for construction.

2. Minimize Impervious Surfaces

Impervious surfaces (roofs, concrete, asphalt) are the direct cause of increased runoff. Every square foot of impervious surface you add generates approximately 0.95 cubic feet of runoff per inch of rainfall, compared to approximately 0.1 to 0.3 cubic feet per inch for vegetated surfaces. Reducing impervious area reduces the stormwater management burden.

Strategies include:

• Narrower streets and drive aisles (within fire access requirements)
• Reduced parking ratios where zoning allows
• Shared driveways and parking courts
• Compact building footprints with vertical rather than horizontal building expansion
• Pervious pavement for overflow parking, fire lanes, and low-traffic areas

3. Disconnect Impervious Surfaces

When impervious surfaces are necessary, disconnect them from the storm drain system by routing their runoff to pervious areas where it can infiltrate or be filtered. Instead of piping roof downspouts directly to the storm drain, discharge them to landscape areas. Instead of collecting parking lot runoff in curb and gutter and piping it to a single outfall, sheet-flow it across a vegetated buffer or into distributed bioretention cells.

4. Manage Stormwater at the Source

Instead of one large detention basin at the low point of the site, use multiple small facilities distributed across the site, each treating a small drainage area. A parking lot island that is actually a bioretention cell. A sidewalk planter that is actually a flow-through treatment facility. A landscaped setback that is actually an infiltration area.

Distributed facilities are more resilient than centralized ones because a single failure does not take the entire system offline. They are also easier to maintain because each facility is small enough to inspect and clean quickly.

5. Slow, Spread, and Sink

This is the LID mantra. Slow the runoff velocity so that sediment settles and water has time to infiltrate. Spread the runoff across the widest possible area to maximize contact with soil and vegetation. Sink the runoff into the ground through infiltration wherever soil conditions allow.

LID in the Permit Framework

Most modern stormwater permits require or strongly encourage LID. In California, C.3 requires that LID measures be used to treat stormwater to the maximum extent practicable before resorting to conventional treatment. The permit hierarchy is:

1. Infiltration — the preferred approach where soils and groundwater allow
2. Biotreatment (bioretention, flow-through planters) — required where infiltration is infeasible
3. Proprietary treatment devices — allowed only where biotreatment is infeasible, with documented justification

Oregon's BES Stormwater Management Manual has a similar hierarchy, prioritizing on-site infiltration and treatment before allowing off-site facilities or fee-in-lieu payments.

LID and Site Design

The most important thing to understand about LID is that it is not something you add to the site plan after the architect and civil engineer have designed everything else. It must be integrated into the site plan from the beginning. If the parking lot is fully designed with conventional curb and gutter draining to a centralized pipe system, you cannot easily retrofit LID bioretention into the parking lot islands — the grading, drainage, and utility layout are already set.

LID works best when:

• The civil engineer is involved from the schematic design phase, not just the construction document phase
• The landscape architect designs the bioretention and planters as landscape features, not as engineering afterthoughts
• The architect understands that roof drainage may go to landscape areas rather than direct pipe connections
• The project team accepts that LID may increase landscape area slightly but reduce underground pipe and vault costs

Common Misconceptions

• "LID costs more." Some individual LID BMPs cost more than conventional alternatives, but the system-level cost is often comparable or lower because LID reduces pipe sizes, vault volumes, and outfall infrastructure.
• "LID does not work on clay soils." LID facilities on clay soils use underdrains and still provide water quality treatment. They do not infiltrate, but infiltration is only one of several LID mechanisms.
• "LID requires more maintenance." LID facilities do require maintenance, but so do conventional systems. The maintenance is different (landscape maintenance vs. vault pump-out), not necessarily more.
• "LID only works on large sites." LID is scalable. A flow-through planter treating a single building downspout is LID. A rain barrel is LID. The principles apply at any scale.