The Infiltration Basin
This pattern is shaped by
Problem
When a single property generates more runoff than a rain garden can absorb — a parking lot, a large roof, a cluster of townhouses — the water must go somewhere. If it flows to the street, it joins the cumulative flood. If it pools on the surface, it drowns the landscape. The lot needs a way to handle large volumes of water without sending them downstream, but it also needs usable space, not a permanent pond.
Evidence and Discussion
Rain gardens work beautifully for residential lots, but they are sized for modest roof areas — typically 5-10% of the impervious surface they serve. When the catchment grows larger, the math changes. A 500-square-meter parking lot receiving a 25mm storm event generates 12,500 liters of runoff. A residential rain garden cannot absorb this; you need a basin.
The infiltration basin is the middle ground between the rain garden and the engineered detention pond. Prince George's County, Maryland — where bioretention was invented in the early 1990s — developed the infiltration basin as a scaled-up rain garden for commercial and multi-family sites. Their Low-Impact Development Design Manual (1999) specifies basins sized to the 1-year, 24-hour storm, with a maximum ponding depth of 300mm and a drawdown time of 48-72 hours. The basin looks like a landscaped depression — a shallow bowl planted with native grasses and shrubs — but beneath the surface, an engineered soil mix allows water to percolate into the subsoil at rates far exceeding natural ground.
The critical difference between a basin and a pond is residence time. A detention pond holds water for days, breeding mosquitoes and algae, requiring fencing for safety. An infiltration basin empties within two to three days, appearing dry most of the time. Minneapolis parks have installed infiltration basins in public spaces since the 2010s as part of their Stormwater Management Program — at Webber Park and Bryn Mawr Meadows, these basins double as playing fields during dry weather, visible only as subtle depressions in the landscape. The dual use is the key: the basin earns its space by serving two purposes.
Soil conditions determine feasibility. Infiltration rates below 15mm per hour require either soil amendment or an underdrain system that allows the basin to function even when the native soil drains slowly. The Philadelphia Water Department's Green Stormwater Infrastructure program, which has installed over 500 green infrastructure projects since 2011, requires infiltration testing at every site — and where rates are too slow, they add a perforated pipe beneath the basin that releases water slowly to the storm system, still filtering pollutants and reducing peak flows even when full infiltration is impossible.
Alexander's patterns did not address stormwater management directly — his Pattern 64, Pools and Streams, treats water as a visual and social amenity, not as infrastructure. The infiltration basin bridges this gap: it is infrastructure that becomes landscape, a working system that looks like a garden.
Therefore
where runoff from a parking lot, large roof, or multi-building site exceeds what a rain garden can absorb, create an infiltration basin — a planted depression sized to hold 75mm of water over 10% of the contributing impervious area. Excavate 450-600mm deep, fill with an engineered soil mix (60% sand, 20% compost, 20% topsoil), and plant with deep-rooted native grasses and shrubs that tolerate both wet and dry conditions. Slope the sides gently — no steeper than 3:1 — so the basin reads as landscape, not excavation. Place the basin where it can serve a second purpose: a playing field, a meadow, a picnic area. The test: after a 25mm rainstorm, the basin should drain completely within 48 hours and show no standing water within 72 hours. If water remains longer, the soil mix or underdrain needs adjustment.