A small lot, the aerial photograph check and a quick check

The job. A homeowner in Olney, Montgomery County, Maryland, wants a house addition and a wider driveway on a 0.71 acre lot. Before that, they asked whether a back yard patio and shed would matter at all.

About this example

This is a real run on app.curvenumber.com, made under the CurveNumber Examples account. The place is real. The homeowner and the house location survey are fictional, and the run says so wherever they are named.

What this example shows

What CurveNumber says when a site is too small for the national land cover map.
The aerial photograph check: reading hard surface and planting off a 1 m photograph, and accepting it under your name.
Building the existing condition from the photograph.
A quick check, and the small area rule that stops it from reporting a curve number answer on a very small area.
The Reference page with the TR-55 tables.

01A site smaller than the map

The land cover map reads the ground in 30 m squares, and this lot is about 3 of them. CurveNumber marks the land cover as thin and says why: one square read as the wrong class would move a cover share by about 31 percent of the site. It suggests the aerial photograph check, or entering the surfaces by hand.

An open question: this site is small for 30 m land cover (about 3.2 squares), marked thin. It explains that each 30 m square is read as one class, and suggests running the aerial photograph check or entering the cover surfaces by hand. — **Thin land cover.** About 3 map squares, and what that means.

02The aerial photograph check

Classify the photograph fetches the public domain aerial photograph of the lot (here flown July 12, 2023) and sorts its pixels into hard surface, planting, bare soil, water and shadow. On this lot it read 38.9 percent hard surface. It then compares that with the land cover map (29.5 percent), within the 10 point tolerance.

CurveNumber is plain that this is a guess about an image, not a measurement of the ground. Nothing uses it until a person accepts it with a reason.

We accepted it, noting that a (fictional) house location survey gives about 0.27 acres of roof, driveway, walks and patio, against 0.277 acres read off the photograph. The acceptance is recorded with the name, the time and the photograph it applies to.

An ochre panel headed What was read off it, a guess, not a measurement: 38.9 percent of this boundary is hard surface, then the classes impervious, vegetation, bare soil, water and shadow, and the cross check against the published land cover map at 29.5 percent, a difference of 9.4 points against a tolerance of 10. — **A guess, not a measurement.** 38.9 percent hard surface, read off the photograph.

The panel headed Nobody has decided about this yet, with a reason typed: checked against the owner's (fictional) 2024 house location survey, roof, driveway, walks and patio total about 0.27 acres, which matches the 0.277 acres read off the photograph. Buttons read Accept the photograph and Reject the photograph. — **Accepting it, with a reason.** Recorded with the name, the time and the photograph.

03Areas from the photograph

On a small site the photograph's classes can replace the existing areas. The photograph cannot tell grass from trees, so you choose the cover for the planted part (we chose open space in good condition, a mowed lawn) and confirm with a reason. The soil group comes from the soil survey (group B here).

A section headed Areas from the photograph, for you to confirm: impervious 38.9 percent, 0.277 acres, and grass or trees 61.1 percent, 0.435 acres, with the cover for the grass or trees set to Open space, Good, soil group B from the soil map, a reason, and a button Replace the existing areas with these. — **Areas from the photograph.** Choose the cover for the planted part, and confirm.

04The addition and driveway

The proposed condition adds 0.060 acres of roof and paving and takes the same area out of the lawn. At the 10-year, 24-hour storm (4.87 inches, NOAA Atlas 14):

	Curve number	Runoff depth	Runoff volume	Peak
Existing	75.4	2.591 in	6,690 cu ft	2.48 cfs
Proposed	78.5	2.872 in	7,419 cu ft	2.73 cfs

The addition adds about 730 cubic feet of runoff in that storm. We checked both depths by hand.

The results summary for the lot: existing CN 75.4, 2.591 inches, 6,690 cubic feet, peak 2.48 cfs; proposed CN 78.5, 2.872 inches, 7,419 cubic feet, 2.73 cfs. Large figures read plus 0.282 inches, plus 10.9 percent and plus 728 cubic feet. — **Results for the lot.** Plus 0.282 inches of runoff depth.

The top of the published page for Example 4, the house addition and driveway in Olney, Maryland, with the report identifier and the runoff summary cards. — **The published page.** The house addition and driveway, as published.

The published example

The house addition and driveway, published: a fixed, read-only page with the full report, exactly as it was published. It needs no account.

https://app.curvenumber.com/p/6dJRYt_bZwus57VwoGpC7iXLks4ppxQs

Open the published example

05A quick check, and the small area rule

Quick check answers a question without setting up a site. You type the storm and the areas, before and after; nothing is saved or charged. Here: a 0.10 acre back lawn, of which 0.02 acres becomes a patio and shed, at the 2-year storm (3.17 inches).

CurveNumber would not give a curve number answer. Below 0.5 acres it treats the curve number as the wrong primary method and asks for Green-Ampt instead, which needs the soil's measured infiltration or its Green-Ampt figures. It still shows the curve number figure for reference (0.432 inches for the lawn), labeled not reportable, and offers Open as a site to carry the areas and storm into a full site.

The printed quick check says on every page that it is a quick check, not a site record and not for submission, carries your Prepared by details and the label you typed, and cannot be published.

The Quick check: design storm 3.17 inches over 24 hours, before a back lawn of 0.10 acres at CN 61, after a new patio and shed of 0.02 acres at CN 98 and the back lawn after the patio of 0.08 acres. An ochre panel says the area of 0.1 acres is below the 0.5 acre floor at which the curve number is the primary method, and that the curve number method would give 0.432 inches, for reference, not reportable. — **The small area rule.** Below 0.5 acres the curve number is not the primary method.

The printed quick check, watermarked Quick check, not a site record, not for submission. It carries the Prepared by line, the label Patio and shed, back yard, and a box saying no runoff depth is reported for the existing or the proposed condition. — **The printed quick check.** Not a site record, not for submission, on every page.

06The Reference page

Every curve number CurveNumber reads out of TR-55 comes from the tables on the Reference page, by cover, condition and soil group, with the edition named.

The Reference page, Curve number tables, from USDA NRCS TR-55 (1986, 2nd edition) Tables 2-2a to 2-2d, with columns for cover, condition and soil groups A to D: open space poor, fair and good at 68 79 86 89, 49 69 79 84 and 39 61 74 80, impervious paved and roofed at 98, then gravel and dirt roads, woods, pasture, meadow, desert shrub, row crops and brush. — **The curve number tables.** By cover, condition and soil group, with the edition named.