CurveNumber

Examples Example 3 of 4

Example 3 · Georgetown, Delaware, and Hillsboro, Oregon

Special rainfall and unit hydrograph cases

Two sites, one on each coast, where the rainfall source or the shape of the hydrograph is not the usual one.

Part A: Farm field to warehouse, Georgetown area, Delaware Part B: Pasture to townhomes, Hillsboro, Oregon

About this example

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

What this example shows

  • Part A: wetland land cover, which has no row in TR-55, and the presets CurveNumber offers for it.
  • Part A: a dual soil group (B/D) and how the drainage question is answered.
  • Part A: naming the rule that sets the design storm.
  • Part A: the NRCS Delmarva unit hydrograph, and a comparison run with the standard one, published side by side.
  • Part B: NOAA Atlas 2 rainfall, which still governs in Washington and Oregon.
  • Part B: the NRCS Type IA storm, and its early peak, with a dual soil group (C/D) answered both ways.

Part A · Sussex County, Delaware

Farm field to warehouse, Georgetown area

The job. A 19.10 acre field and scrub tract near Georgetown in Sussex County, Delaware, becomes a warehouse with a truck court and parking. A forested wetland along the stream stays as a buffer. The design checks the 100-year, 24-hour storm.

A1Two questions before any number

After reading the public data, CurveNumber shows no curve number for this site yet, and two questions.

  • Wetlands. 2.705 acres (14.2 percent) are woody wetlands. TR-55 has no wetland row, so CurveNumber will not pick one. You can supply a curve number from your local manual, choose a TR-55 row, or leave the wetland out of the drainage area as storage. It also offers presets, each with a note on which way it errs: a high number is conservative in the proposed condition, a low one in the existing.
  • Drainage. The Longmarsh and Indiantown soils are published as B/D: group B if drained, D if not. On this site the difference is CN 58.5 against CN 78.3.
A red panel headed No curve number for this site yet: 14.2 percent of the site is land cover with no TR-55 row, woody wetlands, 2.705 acres. Below it the open question: is there artificial drainage on Longmarsh and Indiantown soils, marked B/D.
No curve number yet, and two questions. 14.2 percent woody wetland, and a B/D soil.
The woody wetlands question, 2.705 acres, 14.2 percent of the site, with three choices (use a curve number I supply, use a TR-55 row I choose, leave it out of the drainage area) and presets such as saturated or ponded CN 98, meadow on a hydric soil CN 78 and woods in good condition on a hydric soil CN 77, each with a note on which condition it is conservative in.
The wetland presets. Each preset says which way it errs.

A2Answering them

For the drainage question we chose D, marked the source Observed on site, and wrote why: the (fictional) site walk found no drain tile, no outlet ditches and standing water along the stream.

For the wetland we started from the preset Woods, good condition, on a hydric soil (group D), CN 77 and wrote that the forested wetland is a buffer left untouched in both conditions. CurveNumber drafts the wording from the preset, but nothing is entered until you confirm it.

With both answered, the areas add up to the full 19.098 acres and the site has a curve number.

The drainage question answered with the source Observed on site selected and a reason: the site walk (fictional survey for this example) found no drain tile, no outlet ditches and standing water in the low ground along the stream.
The drainage answer. Group D, observed on site, with the reason.
The woody wetlands question with Use a curve number I supply selected, curve number 77, and the reason: wetland taken as woods in good condition on a hydric soil, TR-55 Table 2-2c, CN 77, a buffer left untouched in both conditions. A button reads Apply and read the public data again.
The wetland at CN 77. Drafted from the preset; entered only when confirmed.
The existing condition list: pasture, brush and woods on groups A, B and D, and two woody wetland areas at CN 77 marked entered, one on group B and one on group D, totaling 19.098 acres.
The existing areas. They add up to the full 19.098 acres.

A3What else CurveNumber estimated

The Estimated inputs panel lists every figure CurveNumber filled in for you, ranked by how much it can move the answer, with what it rests on and what would displace it. Here the three to check first are the storm's time distribution, the slope, and the time of concentration.

The Estimated inputs panel: check these 3 first, temporal distribution, slope and time of concentration, each with the factor it can move the answer by. The first entry shows the temporal distribution, what it rests on, what it is worth, and what would displace it.
Estimated inputs, ranked. Check these three first.

A4The storm, with the rule that requires it

The rainfall table shows the NOAA Atlas 14 depths at the site, each with its 90 percent confidence range. Choosing the 100-year storm (9.20 inches) opens a box that asks which rule requires that event. We cited the Delaware Sediment and Stormwater Regulations' flooding event. The citation prints on the report next to the depth.

The design rainfall table from NOAA Atlas 14: 1-year 2.81 inches, 2-year 3.41, 5-year 4.44, 10-year 5.33, 25-year 6.68, 50-year 7.86 and 100-year 9.20, each marked derived with a Choose this storm button.
The rainfall table. NOAA Atlas 14 depths at the site, 1-year to 100-year.
The storm chooser with the 100-year depth of 9.20 inches selected and its confidence range. Below, name the return period the ordinance requires, 100, and the rule that requires this storm: Delaware Sediment and Stormwater Regulations, flooding event, the 100-year, 24-hour storm, checked for this example site in Sussex County.
Naming the rule. The citation prints on the report next to the depth.

A5The proposed warehouse

We copied the existing areas to the proposed condition, kept the wetland and the group D woods and brush, and replaced the upland field and scrub with 8.0 acres of roof and pavement (CN 98) and 7.366 acres of lawn in good condition (CN 39 on group A, 61 on group B).

A6The Delmarva unit hydrograph

The Compute form names the unit hydrograph before you run: This site: NRCS Delmarva (peak rate factor 284), the site is on the Delmarva coastal plain. The Delmarva curve was developed for the flat, storage-rich coastal plain of Delaware and the Eastern Shore, and gives lower peaks than the standard curve. CurveNumber uses it by default inside the Delmarva coastal plain (less the Piedmont north of the Fall Line), and a local rule you enter overrides it.

Curve numberRunoff depthRunoff volumePeak
Existing53.13.427 in237,571 cu ft39.07 cfs
Proposed74.86.104 in423,128 cu ft90.60 cfs

We checked both depths by hand. The peak section says which unit hydrograph was used, why, and where its ordinates come from. It also warns that the existing time of concentration is CurveNumber's estimate while the proposed one (0.25 hours) was typed by us, so part of the change in peak may come from that difference and not from the development.

The Compute form: design storm 9.2 inches over 24 hours, proposed time of concentration 0.25 hours, storm distribution NRCS Type II, unit hydrograph As the site decides, and the line: This site: NRCS Delmarva (peak rate factor 284), the site is on the Delmarva coastal plain.
The unit hydrograph, named before you run. NRCS Delmarva, peak rate factor 284.
The results summary: existing CN 53.1, runoff depth 3.427 inches, 237,571 cubic feet, peak 39.07 cfs; proposed CN 74.8, 6.104 inches, 423,128 cubic feet, peak 90.60 cfs. Large figures read plus 2.677 inches, plus 78.1 percent and plus 185,557 cubic feet.
The results summary. Existing CN 53.1 against proposed CN 74.8.
The Peak discharge section: it names the NRCS Delmarva dimensionless unit hydrograph, peak rate factor 284, why it was used, and the source of its ordinates, then a table of existing 39.07 cfs and proposed 90.60 cfs, and a note that the two times of concentration came from different sources.
The peak section. Which unit hydrograph, why, and where its ordinates come from.

A7The same site on the standard curve

To show what the choice is worth, we ran the site again with the NRCS standard unit hydrograph. Choosing the other curve takes a written reason, which prints with every peak.

PeakDelmarva (284)Standard (484)
Existing39.07 cfs60.43 cfs
Proposed90.60 cfs127.53 cfs

The volumes do not change; only the peaks do, by about a third.

The Compute form with unit hydrograph set to NRCS standard (peak rate factor 484) and a box headed why this unit hydrograph, if it is not the one the site uses, recorded with your name and printed with every peak. The reason reads: comparison run for this example only.
A reason for the other curve. It prints with every peak.
The Peak discharge section for the comparison run on the NRCS standard unit hydrograph: existing 60.43 cfs, proposed 127.53 cfs, with the reason for the choice printed beneath.
Peaks on the standard curve. Existing 60.43 cfs, proposed 127.53 cfs.

A8Publishing with a comparison

When you publish, you can pick an earlier run to compare against. The published page then carries a table of both runs' figures side by side.

The publish form with the saved run and, under Compare with an earlier run, a second run chosen. The publication title reads Example 3: Farm field to warehouse, Georgetown area, Delaware (Delmarva unit hydrograph).
Publish with a comparison. Pick an earlier run to compare against.
The top of the published page for Example 3, the farm field to warehouse near Georgetown, Delaware, with the report identifier, the marks the result carries, and the runoff summary cards.
The published page. Part A, as the reviewer sees it.
Page 1 of the printed report: the site name Farm field to warehouse, Georgetown area, a site map with the boundary over aerial imagery, the section This result rests on listing the override, the wetland treatment chosen for 14.2 percent of the site and the soil group chosen for 16 percent, and the headline increase of 185,557 cubic feet.
Page 1 of the report. The headline figures and what they rest on.

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/3qCqHE8T_0fWinS0P38FeX9LQeFz6b8x

Open the published example, part A

Part B · Washington County, Oregon

Pasture to townhomes, Hillsboro

The job. A 12.87 acre pasture on the west side of Hillsboro in Washington County, Oregon, becomes townhomes. The design checks the 25-year, 24-hour storm.

B1NOAA Atlas 2 rainfall

In Washington and Oregon, CurveNumber reads the NOAA Atlas 2 (1973) grids instead of Atlas 14, and says so on the rainfall table. Atlas 2 publishes 6 and 24 hour depths for 2 to 100 years only and no confidence interval, and the storm chooser says that too.

Return period2-yr10-yr25-yr100-yr
24-hour depth2.25 in3.23 in3.72 in4.62 in
The design rainfall table for the Oregon site: 2-year 2.25 inches, 5-year 2.84, 10-year 3.23, 25-year 3.72, 50-year 4.18 and 100-year 4.62, with a note that these are NOAA Atlas 2 partial duration depths read from the grid cell under the site.
NOAA Atlas 2, named on the table. Depths for 2 to 100 years.
The storm chooser for the Oregon site, headed 24-hour depths at this point from NOAA Atlas 2 (1973), which publishes no confidence interval, with the 25-year depth of 3.72 inches selected and the rule that requires the storm typed below.
No confidence interval. The storm chooser says so.

B2One soil drained, one not

Two map units are published as C/D. For the Aloha silt loam we chose C, citing a (fictional) drain tile plan: 4 inch tile at 40 foot spacing under the pasture, found working on the site walk. For the Wapato silty clay loam in the low swale, which the tile does not reach, we chose D. Each answer shows how much it moves the site's curve number before you save it.

The question is there artificial drainage on Aloha silt loam, published C/D, with C drained selected. The reason cites the owner's drain tile plan (fictional, dated 1998, for this example): 4 inch clay tile at 40 foot spacing under the whole pasture, found working on the site walk.
Drained, so group C. The answer shows what it moves before you save it.

B3Townhomes and the Type IA storm

We replaced the pasture and open lawn with townhomes (TR-55: 1/8 acre or less, 65 percent impervious; CN 90 on group C, 92 on group D) and computed with the NRCS Type IA distribution used west of the Cascades.

Curve numberRunoff depthRunoff volumePeakTime to peak
Existing82.72.065 in96,489 cu ft6.15 cfs8.03 hr
Proposed88.32.539 in118,658 cu ft8.05 cfs8.00 hr

The peak comes at about 8 hours into the storm, not 12 as with Type II, because the Type IA storm puts its heaviest rain early. We checked both depths by hand.

The Peak discharge section for the Oregon site under the NRCS Type IA storm: existing 6.15 cfs at 8.03 hours, proposed 8.05 cfs at 8.00 hours. Below, bars compare the runoff depths, 2.065 inches existing and 2.539 inches proposed.
A peak at 8 hours. The Type IA storm puts its heaviest rain early.
The top of the published page for Example 3 part B, the pasture to townhomes in Hillsboro, Oregon, with the report identifier and the runoff summary cards.
The published page. Part B, as the reviewer sees it.

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/JiRxJNXc-VgJ069M-b2XmQf0-zseuf5P

Open the published example, part B

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