CurveNumber

Stormwater runoff for small sites

Before and after runoff, a first answer in minutes.

Draw the site. See existing, proposed and with practices for six storms, pass or fail.

Water quality, 1, 2, 10, 25 and 100-year, from NOAA Atlas 14 at your site. Then a report that says where every figure came from.

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The storms table in CurveNumber for a 4.8 acre wooded site planned as quarter acre lots with two rain gardens, after Suggest a fix. Six rows, water quality through 100-year, with rain depth and peak flow in cubic feet per second for existing, proposed and with practices, and a check for each. Five pass and the 1-year is not checked. The 100-year peak is 15.12 existing, 26.69 proposed and 6.16 with practices. Below: water quality 1,332 of 1,332 cubic feet treated, 17 cubic feet bypass. The same storms table in the dark theme.
After Suggest a fix: 5 of 6 pass, 1 not checked. After the first compute, every storm in one press. A real run: a wooded site near Pittsboro, North Carolina, with drawn lots, a road and two rain gardens. How the fix was found.

How it works

Three steps

  1. Draw the site. Soils, land cover, slope and rainfall are read for you, each with its source.
  2. Draw what you will build. Land use, practices, and a drainage area for each, suggested from the ground.
  3. Run every storm. One table and one chart, before and after. Then the report.

Watch each step in a short video.

Words used here
Curve number (CN)
A number, from about 30 to 98, for how much rain runs off the ground: higher for pavement and clay, lower for woods and sand.
TR-55
The USDA NRCS method for runoff from small watersheds, which CurveNumber follows.
Time of concentration (Tc)
How long water takes to travel from the far end of an area to its outlet. A shorter Tc gives a higher peak.
Design storm
A rain depth for a return period, such as the 10-year storm, from NOAA Atlas 14.
Practice
Something built to hold or slow runoff, such as a rain garden or permeable pavement.

When a storm fails

Suggest a fix

CurveNumber reruns every storm with the practice at larger sizes and shows the smallest that passes the most.

Nothing on the site changes until you change it.

The suggested fix: Bioretention 1 from 9,400 square feet to 14,100 square feet, 5 of 6 storms pass. A table gives each storm's peak now, with the change, and existing: the 2-year falls from 1.39 to 1.32 cubic feet per second against 1.33 existing, so it now passes. The 1-year is not checked. A size menu and an Open practice button sit below. The same suggested fix in the dark theme.
Every storm checked passes. The sizes tried are behind its "?".

Drainage areas

Suggested from the ground

One press suggests a drainage area for each practice, from USGS elevation. Use these keeps them, or draw your own.

On flat or paved ground it says so and asks you to draw.

The map of the same site with two drainage areas from Suggest drainage areas: Bioretention 1 in blue, 1.49 acres, and Bioretention 2 in green, 2.05 acres. The legend reads treated 73.1 percent, bypass 1.30 acres, on a 4.845 acre site. The same drainage areas on the aerial photograph, in the dark theme.
73 percent treated, 1.30 acres bypass. Read from the ground, not guessed.

The report

A report a reviewer can check

Every storm, its depth and its source, on the page. Every figure says where it came from.

Open a published example
The storms block on page 1 of the same site's report: 5 of 6 storms pass the General check and 1 is not checked, a table of depth and existing, proposed and with practices peaks for each storm with its check, and lines on the water quality capture and on overflow.
From the report itself. Printed reports stay light in either theme.

The assistant

It guides you to the number, and helps you prove it

  • Explain this result. Why a storm fails, in plain words.
  • Suggest a fix. CurveNumber runs the sizes; the assistant only explains.
  • Draft text for the report. You edit it and sign it.

Every number comes from CurveNumber, not from the AI. A check reads every draft and blocks a number not in the run or your record. The number check is one of 16 rules the assistant cannot break.

Status and the reference library

The assistant is switched on for invited accounts. It has not yet had an expert review. It quotes manuals from a reference library of 124 documents and about 20,800 pages, and growing: federal and state manuals, published agency studies and CurveNumber's own published examples. It gives you the page. Reports by private firms are left out. What is sent, and to whom, is on the privacy page.

Why a reviewer can trust it

Every number says its source, or that it is an assumption; where the data cannot support a number, it says the number is not computed, gives the hydrology reason, and says what it needs, instead of guessing. How it handles what it does not know.

Worked examples

Four real sites

All examples

Pricing

Two free sites, then USD 100 per site

No subscription. Quick checks are free with an account.

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