Residential subdivision in the woods, with bioretention

The job. A 12.25 acre wooded tract in Chatham County, North Carolina, becomes a subdivision of half-acre lots with curbed streets. Part of the woods stays. The design checks the 2-year, 24-hour storm and a bioretention area at the low corner.

About this example

This is a real run on app.curvenumber.com, made under the CurveNumber Examples account. The place is real. The developer, the plans and the geotechnical work are fictional, and the run says so wherever they are named.

What this example shows

Woods on the existing side, and land cover classes too small to keep apart.
Residential lots whose curve number already includes their roofs and driveways, and the guard that stops you counting them twice.
CurveNumber's own time of concentration estimate, for both conditions.
A bioretention area with a field infiltration test and a water table depth.
A sizing check that does not pass a practice that is too small for what drains to it (loading ratio above 20 to 1).
Every storm in one table, before and after, and the larger size CurveNumber suggests.
A published link whose report carries a site map.

01The boundary and the public data

We drew the 12.252 acre tract on the aerial photograph and read the public data. The soil survey puts the whole site in hydrologic group B. The land cover map sees mostly forest, with a few classes smaller than one 30 m map square; CurveNumber folds each of those into the largest class on the same soil and says so in plain words, so nothing is lost without a note.

The existing condition is woods in fair condition on group B, CN 60.

We entered the existing woods as Fair condition; CurveNumber now starts woods at Good.

A rectangular boundary drawn over dense woods beside a highway, with a readout of 12.252 acres enclosed and the line saved boundary, 12.25 acres. — **The wooded tract.** 12.252 acres, drawn on the aerial photograph.

What the public data says: the note 2 land cover classes folded, with its ? open, explaining that small areas of developed land on group B were folded into deciduous forest, and the hydrologic soil group table showing Georgeville silt loam and Georgeville-Badin complex in group B. — **Small classes folded, in plain words.** Each class smaller than one map square joins the largest class on the same soil.

Today's ground: a small measured impervious area at CN 98, and woods in fair condition on group B at CN 60 from deciduous, evergreen and mixed forest, each marked as changed by the account holder, totaling 12.252 acres. — **The existing condition.** Woods in fair condition on group B, CN 60.

02The proposed lots, streets and woods

We entered three proposed areas:

Area	Acres	Curve number
Half-acre lots (TR-55: 25 percent impervious)	8.500	70
Streets, paved, curbs and storm sewers	1.400	98
Woods kept, good condition	2.352	55

When we tried to add the lots' roofs and driveways as a separate impervious area, CurveNumber stopped us: the TR-55 residential curve number already includes the 25 percent impervious cover, so adding it again would count it twice.

Adding half-acre lots of 8.500 acres at CN 70 with 25 percent impervious. An ochre note says this TR-55 row already includes its impervious share in CN 70, so the 25 percent typed is not added to it. — **The guard against counting twice.** The residential curve number already includes the impervious cover.

The proposed condition: half-acre lots, 17 homes, 8.500 acres at CN 70; streets with curb and gutter, 1.400 acres at CN 98; preserved stream buffer woods, 2.352 acres at CN 55; total 12.252 acres. — **The proposed areas.** Lots, streets and the woods kept, 12.252 acres in all.

03Time of concentration

The elevation model gives a slope and a flow path, and CurveNumber estimates the time of concentration by the TR-55 segmental method. For the proposed condition it offers its own estimate on the same flow path with the new cover, labeled an estimate, and you can accept it or type your own.

Estimated inputs, under Check these 4 first: the existing woods condition, Fair, CN 60, and the time of concentration, 0.2217 hours, computed, its details open: TR-55 segmental sheet flow over woods and shallow concentrated flow, with the slope from the USGS elevation model and the flow length it used. — **The estimate, with what it rests on.** Time of concentration by the TR-55 segmental method.

The Compute form with design storm 3.57 inches over 24 hours, and a proposed time of concentration of 0.1278 hours. Its ? is open: this is CurveNumber's estimate for the proposed cover on the same flow path, and to accept it or enter your own. — **The proposed estimate.** Offered for the new cover, labeled an estimate; accept it or type your own.

04Results at the 2-year storm

At 3.57 inches of rain (NOAA Atlas 14, 2-year, 24-hour, NRCS Type II):

Condition	Runoff depth	Peak
Existing woods	0.575 in	7.17 cfs
Proposed subdivision	1.182 in	18.79 cfs

Runoff roughly doubles, and the peak rises by about 2.6 times. We checked both depths by hand with the TR-55 runoff equation.

05Bioretention, with a field test

Under Practices and green roof we added a 9,000 square foot bioretention area and entered what the geotechnical work found (fictional, for this example): a double-ring infiltrometer test at TP-1 on September 10, 2026, measuring 0.9 inches per hour, and a seasonal high water table 5 feet below the rim. A measured rate carries its method, location and date onto the report; a rate with no test behind it is marked as an assumption.

The bioretention form: 9,000 square feet of surface, its contributing impervious area and time of concentration, and Infiltration rate set to Field test: native rate 0.9 inches per hour, test date 09/10/2026, test method double ring infiltrometer ASTM D3385, and test location TP-1 at the proposed bottom elevation (example, fictional). — **A field test on this site.** Method, location and date go onto the report with the rate.

06When the practice is too small

The sizing check compares the area draining to the bioretention with its footprint. Here the ratio is 53.7 to 1, and the design guidance CurveNumber holds sets a ceiling of 20 to 1. CurveNumber does not pass it quietly: it says the loading ratio is exceeded and voids the sizing conclusion, while still computing what the practice would do.

With the bioretention in place, proposed runoff falls to 0.760 inches (about 18,760 cubic feet soaked into the ground), and the peak leaving the site falls to 16.34 cfs. The report also says that 64.3 percent of the water reaching the practice went over the overflow untreated. That is the point of the example: the numbers show a practice working hard and still too small, which is what a reviewer needs to see.

The sizing check for the bioretention: it says this sizing check cannot be relied on, above the storage to the crest against the treatment volume required. The loading ratio reads 53.7 to 1 on a bar that runs past its limit; the stage against the crest, 3.47 feet, is above it. — **The sizing conclusion, voided.** A loading ratio of 53.7 to 1 against a ceiling of 20 to 1.

An ochre panel headed Not computed: a practice check is in force, the drainage area is too large for the practice's footprint, 431,244 square feet to 8,034 square feet of bed, a loading ratio of 53.7 to 1, above the 20 to 1 ceiling. Below, the results summary with a peak leaving the practice of 16.34 cfs. — **Results with the bioretention.** The reason sizing was not computed leads the page, above the figures.

07Every storm, before and after

We pressed Run all storms. CurveNumber ran the water quality storm (1.0 inch) and the 1, 2, 10, 25 and 100-year, 24-hour storms, each depth read from NOAA Atlas 14 at the site. Each storm is run three ways: the existing woods, the proposed lots, and the lots with the bioretention. Peak flow at the site outlet, in cfs:

Storm	Rain, in	Existing	Proposed	With practices	Check
Water quality	1.00	0.06	1.54	0.00	Pass
1-year	2.95	3.20	12.48	3.76	Fail
2-year	3.57	7.17	18.79	16.34	Fail
10-year	5.17	21.14	38.29	37.75	Fail
25-year	6.12	31.01	51.11	50.45	Fail
100-year	7.64	48.45	72.85	72.06	Fail

The General check asks one question per storm: is the peak with practices at or below the existing peak? Here it passes only for the water quality storm. The bioretention overflows in 5 of the 6 storms, and from the 1-year storm up the peak leaving the site stays above the woods. The 2-year row matches the run above: 7.17, 18.79 and 16.34 cfs.

Suggest a fix then ran the bioretention at 1.25, 1.5, 2 and 3 times its footprint, every storm each time, and kept the smallest size that passes the most storms: 27,000 square feet in place of 9,000. That passes 4 of the 6; the 25 and 100-year storms still fail. Nothing on the site changes until you change it. CurveNumber picks the size; the words only report it.

The General check is a plain, generic test. It is not your county's rule.

We ran the storms in the current version of CurveNumber, on the same site and the same inputs as the published run. The published link shows the single 2-year run.

The storms panel: a table of six storms with existing, proposed and with practices peaks, and a red 1 of 6 pass mark. Only the water quality row passes. A line says the bioretention basin at the low corner overflows in 5 of 6 storms. Below, a chart of flow over time for the 2-year storm: the proposed line peaks at 18.79 cfs, the line with practices at 16.34 cfs, and the dashed existing line at 7.17 cfs. — **Every storm in one table.** One pass, five fails, and the flow over time for the 2-year storm.

08The published report, with a map

The published page and the printed report carry a site map on page 1: the boundary over aerial imagery, with a north arrow and a scale bar. The map is embedded in the report, so a published link keeps the map it was issued with.

The top of the published page for Example 2: Wooded tract to half-acre lots, Chatham County, North Carolina (bioretention, 2-year storm). Under Provisional: the practice check, marked not computed, and the 3 changes to the existing areas (the woods entered as Fair). The runoff summary says this is an increase, not a reduction. — **The published page.** Example 2 rebuilt and published in the current version, as the reviewer sees it.

Page 1 of the printed report: the report identifier and status, the site name Wooded tract to 1/2-acre lots, a site map with the boundary in yellow, a north arrow and a 250 foot scale bar, then the site result summary opening with the storms: 1 of 6 storms pass the General check, the 2-year being the site result's storm at 3.57 inches. — **Page 1 of the report, with the site map.** The boundary, a north arrow and a scale bar; this rebuild had no aerial photograph, and the report says so.

The published example

A fixed, read-only page with the full report, exactly as it was published. It needs no account.

https://app.curvenumber.com/p/urVBMNyHRohmiH1SpmW2C6_2qe8xtd6a

Open the published example