Site and Stormwater

Every square foot of roof, parking and drive you draw makes water, and that water needs land or depth before the building gets either. Rain that used to soak into the ground now runs off as surface flow, more of it and faster, and downstream watersheds get the flooding, erosion and water quality problems. In a regulated jurisdiction, stormwater rules may call for the site to detain, infiltrate or treat that new runoff before it leaves, most often by holding it and releasing it slowly. So the basin isn't a civil detail added to a finished site plan. It's a slice of the site plan, sized by the hardscape you drew, competing with the building for the same ground.

When the pond is full, the rain keeps coming

Illustrative event-volume model on a fixed 200,000 SF site. Held fixed: the site area, the runoff coefficients (0.9 for roofs and paving, 0.2 for lawn and soil), a pond on its own land, and an outlet that passes 2,160 ft³ during the storm (an assumed 0.1 cfs for 6 hours). Edited: storm rain depth, roofs-and-paving area, pond surface area and pond working depth. Runoff volume = rain depth / 12 × (impervious area × 0.9 + pervious area × 0.2), where pervious area is the site less roofs and paving and the pond. Pond capacity = pond area × working depth. Overflow = max(0, runoff − capacity − outlet release). No infiltration, routing, hydrograph, freeboard, treatment volume or jurisdiction-specific design storm is modeled, and the 100-year depth is an assumed figure, not a rainfall record.

Volume first, then a footprint

The arithmetic fits on a napkin. Runoff volume is hardscape area × rain depth × a runoff coefficient (the share of rain that leaves the surface instead of soaking in). Divide that volume by how deep you're willing to store water and you get the basin's footprint:

basin SF = hardscape SF × (rain inches ÷ 12) × runoff coefficient ÷ storage depth in feet

Everything after the hardscape term is a ratio: square feet of basin per square foot of hardscape. That ratio is the stormwater tax on every square foot you draw, and you pay it in land.

A worked example: one 200,000 SF site

These are illustrative assumptions, not a design event for any jurisdiction. Take a 200,000 SF site (about 4.6 acres). Roofs and paving shed 0.9 of the rain that lands on them, and lawn and soil shed 0.2. A detention pond on its own land has a four-foot working depth, and its outlet passes 2,160 ft³ while the rain falls (an assumed 0.1 cfs for 6 hours). Whatever the pond cannot hold and the outlet cannot pass goes over the edge:

overflow = runoff − pond capacity − outlet release, and never less than zero

Start with a modest storm. With 80,000 SF of roofs and paving, 114,000 SF of lawn and a 6,000 SF pond, 1.5 inches of rain gives 1.5 ÷ 12 × (80,000 × 0.9 + 114,000 × 0.2) = 11,850 ft³ of runoff. The pond holds 24,000 ft³ (6,000 SF × 4 ft). After the outlet passes 2,160 ft³, 9,690 ft³ is standing in the pond and it is 40% full.

Make the storm larger. At 3 inches the same site makes 23,700 ft³ of runoff, double the first storm. The pond stores 21,540 ft³ of it, 90% of its capacity, with 2,460 ft³ of room left. Nothing spills, but the water is close to the rim.

At 4 inches the pond runs out of room. Runoff is 31,600 ft³. The pond takes 24,000 ft³, the outlet passes 2,160 ft³, and 5,440 ft³ has nowhere to go. That is 17% of the runoff, and it goes over the rim and off the site.

Now hold the storm at 3 inches and change the site instead: 120,000 SF of roofs and paving in place of 80,000 SF. Runoff rises from 23,700 to 30,700 ft³, and the pond that was 90% full overflows by 4,540 ft³, 15% of the runoff. The storm did not change. The site plan did.

A bigger pond fixes it, and it takes land to do it. At 12,000 SF and the same 4 feet, the pond holds 48,000 ft³. It takes another 6,000 SF of lawn, so runoff eases slightly to 30,400 ft³, 28,240 ft³ of it is stored, and the pond is 59% full with nothing over the rim. The price is 12,000 SF of the site, 6% of it, given to water.

Then a larger storm arrives. Assume a 100-year storm of 8 inches (the real depth depends on the place). Runoff is 81,067 ft³ against 48,000 ft³ of pond, so even the bigger pond floods: 30,907 ft³, 38% of the runoff, goes over the rim. A pond is sized for one storm. A larger storm ignores the size, and where the overflow goes is a site-plan decision too.

What the interactive changes, and what it holds fixed

Four assumptions move. Drag the rain gauge to change the storm depth, drag the edge of the orange bar to change the roofs and paving, drag the pond's right wall to change its surface area, and drag its floor to change the working depth. Three things stay fixed: the 200,000 SF site, the runoff coefficients of 0.9 and 0.2 with the 2,160 ft³ outlet release, and a pond on its own land. In the site bar, orange is roofs and paving, green is lawn and soil, and the blue sliver is the pond.

Watch the water. It rises in the pond as the storm grows. When it reaches the rim it spills down the slope and off the bottom of the drawing, and the text below goes with it. The model leaves out infiltration, outlet design, routing, treatment volume, freeboard and every jurisdiction's actual design storm, so read it for proportions, not for a permit set.

Where the basin can go

The most common answer is detention: capture runoff in a surface or underground basin and release it slowly to approximate pre-development flow rates. In the worked example above that is 24,000 ft³ held in a 6,000 SF pond, before any cost, which depends on the system and the market. A detention system also creates maintenance obligations, needs access for inspection and cleanout, and has to sit somewhere that doesn't conflict with utilities, parking or building foundations.

The interactive puts the basin on its own land on purpose. An underground vault under the parking escapes that assumption, storing water beneath a surface you were paving anyway, but you buy the overlap with excavation, coordination, access and maintenance. A surface basin may take land that could serve another use. Either the basin takes land or it takes budget. It doesn't take nothing. Soil, slope, groundwater and local standards decide which of these actually works on a given site.

Green infrastructure attacks the coefficient instead of the depth. Bioswales, permeable pavement, green roofs and rain gardens all aim to hold or soak up rain where it falls, and local standards decide how much credit each earns. The interactive has no coefficient control, but its roofs-and-paving edge does the same job from the other side: in a 3-inch storm, 80,000 SF of roofs and paving makes 23,700 ft³ of runoff and 120,000 SF makes 30,700 ft³, the difference between a pond with room left and one that overflows. These choices have to be made early. They can't be added after the site plan and building footprint are fixed.

Water quality is a second, separate constraint. Local standards may require treating runoff, not just slowing it, before it leaves the site. That can mean more cost and more space, with detention (quantity) and treatment (quality) each needing its own infrastructure. The interactive counts only storage, so treatment space comes on top of the pond.

And the permit sequence makes all of this early. Local standards set when stormwater approval is needed, and a project that discovers its stormwater approach is wrong partway through review faces delays and redesign. Bringing civil engineering into site planning at concept, before impervious surfaces are placed and footprints are fixed, is how you avoid paying for the same site plan twice.

The judgment

Draw the basin before you draw the building. Pick the storage depth and the location (a deeper pond, a vault you're willing to pay for, or less runoff to begin with) at concept, then let the footprint take what's left. Do it the other way around and the basin takes what's left instead, which on a tight site is the open space, the amenity or the last building. Stormwater isn't something the civil engineer fits in later. It's the first tenant on the site, and it doesn't negotiate. The slogan this page started as, “your site plan is actually a stormwater plan,” is a provocation, though: a site plan is not only a stormwater plan. It still has to coordinate water with access, building program, ecology and public requirements, and the engineering approach should be checked against real site conditions rather than picked from a generic storage volume.