Runoff and green infrastructure reduction, for small sites
The curve number is not the hard part.
CurveNumber computes NRCS curve number (TR-55) runoff and peak flow for a site from its boundary.
Defending it is the hard part. Draw a boundary and CurveNumber reads the soils, land cover, slope and design rainfall from public data, computes existing and proposed conditions, and attaches where every figure came from.
Every number shows how and why it was chosen. Where the public data cannot support one, the tool tells you what it needs instead of guessing.
Worked examples
See it worked through
Four real sites, run step by step in the product, each ending in a published link you can open without an account.
The places are real. The clients, plans and geotechnical reports are fictional, and each run says so.
- 01 Commercial redevelopment on urban fill Ballston, Arlington County, Virginia
- 02 Residential subdivision in the woods, with bioretention Chatham County, North Carolina
- 03 Special rainfall and unit hydrograph cases Georgetown, Delaware, and Hillsboro, Oregon
- 04 A small lot, the aerial photograph check and a quick check Olney, Montgomery County, Maryland
What it replaces
Four agency portals, a shelf of manuals, and a spreadsheet
The alternative is not another product. The soil survey, the land cover raster, the elevation service and Atlas 14 are each their own portal, their own projection and their own chance to mistype a number, and a spreadsheet holds the answer together afterwards. The spreadsheet usually works. What it does not carry is its own evidence.
Here the four are read against one boundary in one step, each figure carrying its edition and the date it was retrieved. Alongside them you get:
- A document library of federal, state and local stormwater manuals, searchable by the state your site is in.
- An AI assistant that explains where each number came from and quotes the manual page it relies on.
- Aerial imagery of your site, sorted into impervious cover, vegetation, bare soil and water, for you to accept or reject.
The quick check in the video is free. It needs an account, and the account is for the terms rather than for the money.
What is actually being sold
Every quantity is one of three things, and it says which
Nothing here is a bare number. A quantity carries how it came to be, and construction fails if that is missing.
Retrieved from a published source
The citation carries an edition, because TR-55 1986 and TR-55 1975 disagree.
Tables 2-2a to 2-2d
An assumption, priced in the answer
CurveNumber states what it is worth, in curve number units or inches of runoff.
worth 5 to 12 CN units
Changed by a person, with a reason
Who, when, the prior value and the reason, on a record that cannot be edited.
soil boring log, 2 borings, tested
When none of the three can be said honestly, the tool refuses and tells you what would resolve it. See how refusals work, and every kind of refusal. The full list is also published as data for software.
The part that is unusual
It refuses, in writing, rather than filling the gap
The figure that costs an engineer an afternoon in review is the one that looks reasonable and is quietly wrong, and it is nearly always something filled in silently: an unrated soil map unit read as B, or a pond given CN 98. CurveNumber says so instead, in ninety-three named refusals, each carrying what would resolve it.
Structural, not waivable.
Policy, waivable with a reason that names the threshold you are arguing with.
Evidence, waivable as an attestation that states what it rests on.
One hundred twenty-six values in CurveNumber are defaults rather than anything read off your site. Fifty-four of them say no external source exists on their face, because nobody publishes the figure. Seven more say the source exists and was not checked, a different admission with a different remedy, and the two are never merged. Twenty-four practices, seventeen modeled; the other seven are refused in writing.
Ninety-three refusals in one published list, and the list of default values, are described in the white paper: the refusals and the default values.
The assistant
It guides you to the number, and helps you prove it
Ask what a figure rests on and it walks the chain: the table row, the soil group, the land cover, the override somebody typed and why. Ask what would change and it runs that scenario through CurveNumber rather than guessing at it.
It cannot invent a figure. A check reads every draft before it reaches your screen and blocks a numeral not grounded in the run or your record: one of sixteen numbered violations, each refused by id and in writing.
It quotes from seventy-four documents and 23,180 passages across twenty-nine states, and growing, and gives you the page. Ten are federal method documents, held in full text as works of the United States government. The other sixty-four are state, regional and municipal manuals published openly by their agencies.
Status. The assistant is switched on for invited accounts. It has not yet had an expert review.