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This copy of report CNR-48CDE392BFE5 is published at https://app.curvenumber.com/p/dtvh06WjvwtHftfeJ_c0dIbOkvfA3H7m. Open that address to compare this printout with the published document. Reports are shown as they print, on a light page. CurveNumber
Runoff report
Report CNR-48CDE392BFE5 Inputs CNI-2CE963B01C3A Computed Sep 26, 2026, 7:24 PM EDT Version CurveNumber 0.9.0 Status PROVISIONAL, read at Sep 26, 2026, 7:24 PM EDT
Office pad redevelopment, Ballston areaExample 1: Commercial redevelopment, client Example client (fictional), jurisdiction Arlington County, Virginia. Prepared by CurveNumber Examples, CurveNumber, as entered by the account holder. Located in Arlington County, Virginia (boundary centroid at latitude, longitude 38.8815, -77.1114). Site boundary Site boundary 3.094 ac, centroid at latitude 38.881500, longitude -77.111350. North is up. Imagery: USGS The National Map, public domain, flown November 13, 2023. This result rests on
The marks in square brackets are repeated beside the figures they affect. Each item is set out in full directly after the headline figures. Read this before the figures
This report was produced using CurveNumber.com, a computational aid for estimating NRCS curve numbers and direct runoff. It is not an engineering deliverable and it is not a certification of compliance with any regulation, ordinance, or design standard. The curve number method, as published in USDA NRCS Technical Release 55 and National Engineering Handbook Part 630 Chapter 10, is an empirical method with substantial uncertainty, and its applicability to any specific site is a professional judgment that this software has not made. Input data were retrieved from third party sources listed at https://curvenumber.com/data-sources and are provided by those sources without warranty. All results require review, verification, and adoption by a qualified professional before use in design, permitting, or construction. CurveNumber.com is not the engineer of record for any project. No Data Quality Grade is stated on this report. The grade is a defined measure of how good the inputs behind a result are, this version does not compute one, and an absent grade is not a good grade. What this report carries instead is the provenance of each input individually, in the dataset and assumption sections below. 2,989 cu ft [OVERRIDDEN] [SOIL CHOSEN] Reduction in runoff volume, existing against proposed, at the 4.85 in design storm: 0.069 ac-ft, 6.1 percent of the existing runoff volume. Runoff depth falls by 0.266 in (6.1 percent of the existing depth). Existing depth 4.345 in [SOIL CHOSEN] Proposed depth 4.079 in [OVERRIDDEN] Existing volume 48,803 cu ft (1.120 ac-ft) [SOIL CHOSEN] Proposed volume 45,814 cu ft (1.052 ac-ft) [OVERRIDDEN] Design storm 4.85 in over 24.0 hr, NRCS Type II. Peak discharge by the NRCS standard dimensionless unit hydrograph (peak rate factor 484): existing 18.12 cfs; proposed 17.24 cfs with no practice, 13.85 cfs leaving Underground detention vault under the parking deck. Proposed peak leaving Underground detention vault under the parking deck against existing: a decrease of 4.27 cfs. Overflow. Underground detention vault under the parking deck overflowed in this storm: 2,342 cu ft (5.1 percent of what reached it) passed over the crest untreated; the stage reached 5.22 ft against a crest at 5.00 ft. The practice section gives the detail. Summary, page 2: what was entered
Time of concentration used for existing and proposed: 0.100 hr (6.0 min) (entered by the engineer). Practices: Underground detention vault under the parking deck (underground detention vault), storage to the crest 10,000 cu ft, outlet: 10 inch low-flow orifice 10 in at 0.00 ft, routed at a 0.0125 hr (0.75 min) time step. Summary, page 3: what a person changed, and why
The full record follows this summary, unchanged: every section, with each figure's source. The full recordWhat this result rests on, in full
Existing and proposed both total 3.094 ac; they differ by 1 sq ft (0.00 percent), which is within the 0.015 ac tolerance and is shown as information, not as a finding. Volumes and peaks are computed over the areas entered: 3.094 ac existing and 3.094 ac proposed. The drawn boundary is 3.094 ac. Land cover and soil were read from 30 m map squares. Each square counts only for the part of it inside your boundary, so the derived areas are shares of the 3.094 ac you drew. Your boundary is about 13.9 of those squares. The areas total 3.094 ac. Land cover folded into a larger class on its soil group (each below CurveNumber's minimum share for a class of its own; waiving the fold keeps each class at its own share): On soil group C, 0.003 ac of Developed, medium intensity (0.1% of that group) was folded into Developed, high intensity; both are CN 79, so the site figure does not change. The state and county named by the datasets were compared with every geographic statement typed onto this file, and they agree. Provisional: assumptions not accepted 14 assumptions carried this answer and a person has accepted 0 of them. 14 carry no acceptance, so this report is provisional. Provisional means that the figures on this report were computed and are not withheld, and that the account holder has not yet submitted an acceptance of every assumption underneath them. The assumptions are listed further down with what each one is worth in the answer and what evidence would displace it. 9 of the 14 outstanding are an entry for which CurveNumber's list of default values states that no external source exists, or which it rates Low confidence. Those are accepted one at a time, cannot be accepted in bulk, and each one asks for a written reason. An acceptance on this report is an account holder submitting one, at the time printed with it. This product does not verify professional licensure: it holds no license number and no state of licensure, and it records no evidence that an entry was read as distinct from accepted. An acceptance is not a seal, a signature or a certification, and the absence of one is not a finding that anything is wrong. This count is as of , when this copy of the page was produced. It can change after that; the report identifier does not, because it names the computation. Two printouts with the same identifier are the same computation; if their counts differ, the later one is current. An entry is accepted on its own when it has no published source (a judgment, or a threshold CurveNumber chose) or when its confidence is low. Every other entry (a method constant, a published table, a local manual's figure or a research value rated moderate or better) may be accepted together with others in one act, and the record says so. Site mapThe boundary is the one saved on this site when it was computed, and the practice footprint is the one drawn for the practice, if any. The aerial photograph was taken from the public service when this report was made and is kept with the report, so it shows the ground as it was photographed, not as it is today. The map is for locating the site; the areas on this report are the figures, not measurements off this picture. Where this site is
This boundary was drawn or loaded on the map. It was NOT geocoded: this product does not turn an address into an area of interest, because the Census geocoder returns a point interpolated along a street centerline and runoff computed around such a point is runoff for a piece of road. The coordinates below are the polygon a person drew, and every dataset on this report was read over that polygon and over nothing else. The boundary every dataset on this report was read over
What is claimed about this ground, and on whose evidence
Annual NLCD Land Cover (CONUS) 2024: A national map is sampled at this boundary's own coordinates, so it agrees with the boundary whatever the boundary is and is not evidence about where the boundary is. Where the water went in the 4.85 in NRCS Type II storm. The reduction above is a single figure and it has a composition, and the composition is what a reviewer is actually approving. Water that infiltrated left the site and does not come back: it recharges groundwater and it is retention. Water that left through the underdrain was filtered and then discharged, at full volume, to the same place the runoff was going: it is filtration, and it reduces nothing downstream. Water that was metered out through the outlet structure was detained and then discharged: it arrives later and more slowly, at full volume, and it passed through no media at all, so it is neither of the other two. Water that was drawn out of the practice for a use left the site as neither runoff nor recharge: somebody used it, and folding it into the infiltrated volume would credit the ground with water that never reached it. Many jurisdictions credit the first and not the others. The figures below are therefore reported separately and are never added together anywhere in this report, because a single treated volume would tell a reviewer something that is not true. Retained on site. Infiltrated into the native soil beneath the practice.retention 0 cu ft 0.0 percent of what reached the practice This water left the site and does not come back: it recharges groundwater, and it is the only figure a retention credit is claimed against. RETENTION IS NOT FILTRATION AND THE TWO ARE NOT ADDED. The figure above went into the ground and stayed. The figure below went down a pipe to the same place the runoff was going, cleaner and at exactly the same volume. Left the site, filtered. Through the underdrain, under the media bed.filtration 0 cu ft 0.0 percent of what reached the practice Treated and then discharged at full volume to the same place the runoff was going. It recharges nothing, and a jurisdiction that credits retention and not filtration credits none of it. FILTRATION IS NOT DETENTION AND THE TWO ARE NOT ADDED. The figure above passed through a media bed and was treated. The figure below passed through an opening in a wall and was treated by nothing: it was slowed down, which reduces a peak and reduces a volume by nothing. Left the site, detained. Metered out through the outlet structure.detention 43,472 cu ft 94.9 percent of what reached the practice Every cubic foot of this arrives downstream, later and more slowly than it would have without the structure but at full volume. It is quantity control, it is not a volume reduction, and nothing here passed through media, so it is neither retention nor filtration. Left the site, untreated. Over the emergency crest, without entering the structure.no credit 2,342 cu ft 5.1 percent of what reached the practice It passed the crest without entering the media and without passing through any outlet. Nobody credits it, and it is printed because a practice that overflows in a storm is a design finding a reviewer has to see rather than an omission. Still in the practice when the routing stopped. Not a destination. 0 cu ft Held, not yet disposed of. It is not capacity: it is what is in the practice when the next storm arrives. It is not retained either: nothing credited removes it except the outlets, so it is counted in the volume leaving the site. THESE FIGURES ARE NOT ADDED, HERE OR ANYWHERE IN THIS REPORT. Retention, filtration, detention and a use are four different claims about where the water went. Check which of them the governing jurisdiction credits before any of these figures is carried into a compliance run. Reaching the practice: 45,814 cu ft. Leaving the site after it, which is the underdrain plus the outlet structure plus the overflow plus what was still in the practice when the routing stopped, and is the figure the change above is taken against: 45,814 cu ft. Volume balance residual 0 cu ft, to the six decimal places this document carries: the inflow plus whatever the practice held before the storm, less the FIVE outflows, less what is still held. The five are infiltration, the underdrain, the outlet structure, the overflow, and water drawn out for a use. On a practice with no demand schedule the drawn term and the antecedent term are both exactly zero and the balance is the four it always was. Zero is the expected value and it is the arithmetic closing rather than a modeling allowance. Maximum stage 5.22 ft against a crest at 5.00 ft. Infiltration: no infiltration (lined) No infiltration (lined). This practice is declared lined or not intended to infiltrate, so no native rate is claimed, no field test is required, and the infiltrated volume is reported as zero. Lined: no infiltration is claimed. The curve number method, chosen by the storm depth rule: Judged area by area on each area's own curve number: P of 4.85 in is above 5 Ia on every area that yields a material share of the runoff. The curve number governs. Sensitivity: A five CN unit uncertainty either way on every area's curve number moves the site's area weighted runoff at this depth from 3.80 in to 4.61 in, against 4.34 in as computed, a factor of 1.2 from end to end. Proposed condition: routing through the practice, chosen by the practice rule: A practice is present, so the site's runoff is routed through it. The inflow is computed by the curve number method, chosen by the storm depth rule: Judged area by area on each area's own curve number: P of 4.85 in is above 5 Ia on every area that yields a material share of the runoff. The curve number governs. Method by area. Each area is judged on its own curve number and initial abstraction (TR-55 Chapter 2); impervious cover is computed at its TR-55 curve number. The site figure is the area weighted mean of the runoff depths below.
Storm
The distribution table has not had a second check
The four NRCS 24 hour distributions in this product are the published NRCS tables at a 0.1 hour step (as issued with WinTR-55 version 2.00), read from the published files by a script and compared by machine with a second copy of the same tables. They have NOT yet had a second person's check against the NRCS program files. This matters more here than a curve number would: a distribution ordinate changes the peak intensity, and the peak intensity drives the inflow hydrograph, the stage the practice reaches and therefore the volume split reported below. Where the jurisdiction publishes its own distribution, supply it. NRCS Type II (built in) from USDA NRCS WinTR-55 tabular rainfall distributions (NEH Part 630 Ch. 4) (WinTR-55 version 2.00) Type I, IA, II and III 24 hour mass curves at 0.1 hr; applicability per TR-55 (1986) Appendix B, Figure B-2; not transcribed; parsed by script from the WinTR-55 v2.00 tabular files and machine cross-checked against the HEC-HMS 4.13 copy of the same tables (agreement within 0.0006); checked by NOBODY; a mass curve read as one rainfall Peak dischargeNRCS unit hydrograph peak, lag = 0.6 Tc (NEH Part 630, Chapter 16). Unit hydrograph: the NRCS standard dimensionless unit hydrograph (peak rate factor 484), because the site is not on the Delmarva coastal plain and no local rule asks for another. The site with no practice on it, under the NRCS Type II storm distribution, which was chosen for this run; its fit to this region was not assessed. Time of concentration used for existing and proposed: 0.100 hr (6.0 min) (entered by the engineer). Time step 0.0125 hr. The peak is built from runoff computed for each area separately and added (distributed). A hand check at the composite curve number gives a lower peak where impervious and pervious areas are mixed, which TR-55 itself warns a composite understates. The peak leaving the practice is the same storm routed through it; its detail is shown with the practice, and the note on the distribution table below applies to its routed peaks too.
Distribution: NRCS Type II, chosen for this run. TR-55 Figure B-2 assigns the NRCS type by county; this product does not hold that map, so the choice has not been checked against it. Same Tc. The same time of concentration (0.1 hr) is used for existing and proposed. A shorter developed Tc would raise the proposed peak. Not checked against the source The unit hydrograph shape table was transcribed and has not been checked against the source. The NRCS Type II rainfall distribution was read from the published NRCS table by script and has not yet had a second person's check. The four NRCS 24 hour distributions in this product are the published NRCS tables at a 0.1 hour step (as issued with WinTR-55 version 2.00), read from the published files by a script and compared by machine with a second copy of the same tables. They have NOT yet had a second person's check against the NRCS program files. This matters more here than a curve number would: a distribution ordinate changes the peak intensity, and the peak intensity drives the inflow hydrograph, the stage the practice reaches and therefore the volume split reported below. Where the jurisdiction publishes its own distribution, supply it. The datasets these areas were read fromThis section is the record for this report: the editions it read and the dates they were read. Every dataset CurveNumber reads, with the edition in use, is listed at https://curvenumber.com/data-sources. A dataset's retrieved date can be earlier than the day the areas were read. CurveNumber keeps a copy of each answer a publisher's service gives it, and a later request for the same map tile, soil survey area or storm point is answered from that copy rather than from the publisher again. The read date is when this site's areas were read; the retrieved date is when CurveNumber fetched that answer from the publisher, and the edition printed with it is the one that answer carries.
n is the boundary's area measured in land cover map squares: the area divided by the area of one square. Every square that touches the boundary is counted only for the part of it inside, so the derived areas add up to the boundary. n says how many independent readings of the ground the land cover rests on; a site of a few squares is coarse for this dataset whatever its areas add up to. What the datasets said about themselves. 1 land cover class was folded into a larger class on the same soil group, because each was smaller than one 30 m land cover pixel (0.222 ac) on that soil group, below what the map resolves: On soil group C, 0.003 ac of Developed, medium intensity (0.1% of that group) was folded into Developed, high intensity; both are CN 79, so the site figure does not change. Waiving the fold keeps every class at its own share. Where each source can be read.
Refusals CurveNumber raised on this runCurveNumber raised no refusal on either condition or on a practice during this run. That is a statement about this run and not a warranty: a refusal is raised where a named condition is met, and the conditions CurveNumber tests are the ones in its list of refusals. The practice, and the two checks a reviewer performsThese dimensions came from the kind template, not from a section on a drawing: top length, top width, ponding depth, media depth, gravel depth. The others were entered. They are a starting point for a cell described by its area alone, and every one of them is replaced by a designed profile before anything is stamped.
single orifice control structure, stage by stage. The stages of an outlet structure discharge IN PARALLEL to one barrel. Every stage with head on it is flowing at once and the discharges add, which is why a riser passes far more at the top of its range than any one opening in it. The emergency spillway is not one of these stages: it is the overflow crest and weir length on the practice itself, and the two are kept apart because water through the structure drained the practice and water over the spillway left without entering it.
Design finding: exceeded in the 4.85 in NRCS Type II storm 2,342 cu ft, 5.1 percent of what reached the practice, passed over the crest and left the site untreated. The stage reached 5.22 ft against the emergency overflow crest at 5.00 ft, which is 0.22 ft over it. EXCEEDED in the 4.85 in NRCS Type II storm: 2,342 cu ft, 5.1 percent of the inflow, passed over the crest at 5.00 ft and left the practice untreated. The maximum stage was 5.22 ft. That is not by itself a failure: every practice overflows in a large enough storm, and the design question is whether it overflows in the storm it is being sized against. CurveNumber refused no figure on this practice. Every result above is reported as computed. What CurveNumber said about this routing. The practice had not emptied when the routing stopped 48 hr after the storm ended, and was still holding 0.2 cu ft still draining. It did not empty within the drawdown limit, so that volume is not capacity: it is what is in the practice when the next storm arrives. It is not retained either: nothing credited removes it except the practice's outlets, so the site's runoff volume counts it as leaving. The loading ratio of 67.4 to 1 above is REPORTED AND NOT CHECKED on an underground detention vault. The ceiling in CurveNumber is a filter bed criterion: it reasons about clogging, about scour across a media surface and about pretreatment capacity, and this practice has no media surface to load. Two acres onto the reference extended detention basin is 21.7 to 1 and would be refused by that ceiling, which is a small drainage area for a basin. What actually sizes a detention practice is the release rate against the allowable discharge, the residence time, the forebay volume and the embankment, and CurveNumber computes none of the four. The volume check below and the drawdown are sound and they are not the whole of the design. Both aggregation paths
With a practice, the proposed distributed figure is what leaves the site after the practice, and the proposed composite is the land surface before it, so the two proposed figures do not compare like with like. The two paths differ because runoff is a non-linear function of the curve number, so averaging the curve numbers first and averaging the runoff first do not give the same answer; the distributed path is reported because it applies the method to each surface as the method was fitted, and the composite path is printed beside it because it is what a reviewer recomputing by hand from a single area weighted curve number will obtain. What was estimated, from what, and what each estimate is worthCheck these 3 first: temporal distribution, a factor of 8.37 on peak one hour rainfall fraction; irrigation demand, July, a factor of 4.00 on irrigation demand in the governing month; antecedent dry period, a factor of 3.00 on antecedent dry period. The other 1 move their own figures by a factor of 2.50 or less and are reported rather than put in front of you. RANK 1 HEADLINE Temporal distribution derived 0.277 fraction of total depth Can change the peak one hour rainfall fraction by up to 8.4 times. Rests on: NOAA Atlas 14 Volume 2 Version 2.1 temporal distribution file for Volume 2 (Ohio River Basin and Surrounding States), area general, 24h duration, fetched and hashed rather than transcribed. WHICH of its 45 curves is a judgment and is CurveNumber's default value “Which of the NOAA Atlas 14 quartile cases and probability of occurrence lines a design storm shape is taken from”. The selection spans a factor of 5.10 on the peak one hour fraction inside this file, and a factor of 8.37 against the NRCS Type II curve the product uses without it. The volume is untouched. Displaced by: The quartile case and probability line the governing jurisdiction names, which supersedes outright. Failing that, NOAA's own guidance on which case a design should use, which is published alongside the files and which this product has not read. A local gauge record of the storms that actually produced the design flows displaces the whole question. NOAA Atlas 14 (Volume 2 Version 2.1) first quartile case, 10 percent cumulative probability of occurrence line, 24h duration, Volume 2 (Ohio River Basin and Surrounding States), area general retrieved RANK 2 HEADLINE Irrigation water demand defaulted 228 cu ft/day Can change the irrigation demand in the governing month by up to 4.0 times. Rests on: gridMET monthly reference evapotranspiration (archive built 03 March 2026) as a landscape water budget: reference rate times a plant factor of 0.5, divided by an irrigation efficiency of 0.75, over 18,659 sq ft. Both coefficients are CurveNumber defaults, not figures from a published page. In July the demand is 228.4 cu ft per day at the shipped coefficients, 114.2 at the generous end of both and 456.9 at the lean end, so a 1,500 gallon tank of 200.5 cu ft empties in 21.1 hours, 1.8 days or 10.5 hours. Displaced by: A plant factor from the WUCOLS listing or from the state model ordinance that governs the site, for the species actually specified on the landscape plan, which is a document the project already has. A metered irrigation record displaces both. A distribution uniformity test on the system as installed, which is the measurement this number stands for and which irrigation auditors run as a matter of course. A drip system designed and certified to a stated efficiency displaces it directly. CurveNumber default value (“Plant factor in a landscape water budget”), which is an assumption and not a measurement. RANK 3 HEADLINE Antecedent dry period derived 12 days Can change the antecedent dry period by up to 3.0 times. Rests on: NOAA NCEI GHCN-Daily (station record through ) station USW00013743, 1499 interevent dry periods over to , plus the choice of the 90th percentile, which is a judgment and not a measurement. P50 is 4 days and P90 is 12 days at this station, a factor of 3.00, and on a green roof whose media empties in between the two it is the difference between the whole credit and half of it. Displaced by: A continuous simulation over the local record, which answers the question this percentile is a proxy for: how often the roof is full when a storm arrives, rather than how dry a typical gap is. A jurisdiction that names an antecedent dry period supersedes outright. Failing both, a local gauge record in place of the station this one reads, which is 4.8 km away and is a different point. NOAA NCEI GHCN-Daily (station record through ) station USW00013743, 90th percentile of interevent dry period length retrieved RANK 4 Green roof evapotranspiration rate defaulted 0.132 in/day Can change the green roof recovery time by up to 2.5 times. CurveNumber default value (“Ratio of an extensive green roof's evapotranspiration to the grass reference”), which is an assumption and not a measurement. Time of concentration SERVICE NOT ESTIMATED BECAUSE A DATASET DID NOT ANSWER. An estimate is not made from a service that was not read. Not estimated: the elevation model (USGS 3DEP) did not answer, so there is nothing to estimate from. The USGS elevation service (3DEP) did not answer. This is the service failing, not an answer about this site. Try again in a few minutes. Design storm depth REGULATORY A REGULATORY CHOICE, refused rather than defaulted. The answer exists and is written in an ordinance; nothing in a climatological dataset says which one applies here. No return period was named, and CurveNumber does not pick one. The return period is a REGULATORY choice and not a climatological one: it is written into an ordinance, it differs between water quality, channel protection and flood control on the same site, and nothing in NOAA Atlas 14 says which applies here. NOAA Atlas 14 Volume 2 Version 3 gives these depths for the 24-hr duration at this point, each with its ninety percent interval in brackets. 1 yr: 2.61 in (2.38 to 2.91). 2 yr: 3.16 in (2.87 to 3.52). 5 yr: 4.06 in (3.68 to 4.51). 10 yr: 4.85 in (4.39 to 5.37). 25 yr: 6.06 in (5.44 to 6.68). 50 yr: 7.15 in (6.36 to 7.84). 100 yr: 8.37 in (7.39 to 9.15). 200 yr: 9.76 in (8.52 to 10.7). 500 yr: 11.9 in (10.2 to 12.9). 1000 yr: 13.8 in (11.7 to 15). That is a factor of 5.29 on the depth between the ends. Name the return period the jurisdiction requires, or enter the manual's own design depth under Local rules, which then takes precedence. Slope of the receiving area SERVICE NOT ESTIMATED BECAUSE A DATASET DID NOT ANSWER. An estimate is not made from a service that was not read. Not estimated: the elevation model (USGS 3DEP) did not answer, so there is nothing to estimate from. The USGS elevation service (3DEP) did not answer. This is the service failing, not an answer about this site. Try again in a few minutes. Contributing flow path length SERVICE NOT ESTIMATED BECAUSE A DATASET DID NOT ANSWER. An estimate is not made from a service that was not read. Not estimated: the credit area figures are read from the same services as the slope, and they did not answer. Indoor water demand NO DATASET NOT IN ANY DATASET THIS PRODUCT READS. There is nothing to estimate from and a plausible figure would be an invention. An indoor water demand cannot be estimated from anything this product reads: it depends on occupancy, which no dataset holds. The irrigation part of a cistern's demand is estimated; the indoor part is not, because a guessed continuous demand would give a cistern its whole credit on a number nobody supplied. To credit it, enter a metered monthly demand schedule with a written commitment that the use continues. Overrides of the derived and defaulted valueshydrologic soil group on map unit Urban land-Udorthents complex, 2 to 15 percent slopes (12) overridden hydrologic soil group decision recorded; the letter used for each map unit is in the soil map unit table in the datasets section Not recorded against any one area: this override is against the site. Changed by CurveNumber Examples on Sep 26, 2026, 5:28 PM EDT Reason given: Geotechnical report: Soil group C for the Urban land-Udorthents complex, which covers the whole site. Basis: the site's geotechnical report (Boring logs B-1 to B-4, example report, fictional) found 4 to 6 feet of silty clay fill over residual silt loam, with infiltration tests of 0.08 to 0.12 inches per hour. Those rates fall in the TR-55 group C range (0.05 to 0.15 in/hr). The borings are spread across the pad, so group C applies to the whole site. This reason was drafted from a CurveNumber template, then edited and signed by CurveNumber Examples. This reason was drafted from a CurveNumber template, then edited and signed by CurveNumber Examples. CN, Construction staging lawn, compacted overridden FLAGGED: override exceeds the material band 79 → 86 CN Size: CN 79 to 86 (7 points higher) on Construction staging lawn, compacted, proposed condition, 0.428 ac: about 2.3 percent more proposed runoff at this storm. On the proposed condition, area Construction staging lawn, compacted. Changed by CurveNumber Examples on Sep 26, 2026, 5:30 PM EDT Reason given: This strip stays as a construction staging area and will be heavily compacted by equipment. The geotechnical report (example, fictional) measured 0.02 inches per hour on similar compacted fill, which behaves like group D. TR-55 Table 2-2a open space in fair condition on group D is CN 84; we use 86 to allow for gravel patches, and we will restore it to lawn after construction. Areas, covers and soil groups a person changed. Changes to the proposed condition are the design and are listed here for the record. A curve number change is listed once, above, with the record it wrote. Proposed building, drives and parking, proposed condition edited area 2.666 to 2.200 ac, name Impervious (measured), soil C to Proposed building, drives and parking Changed by CurveNumber Examples on Sep 26, 2026, 5:29 PM EDT Reason given: Site plan sheet C-3 (example, fictional) reduces paved and roofed area from 2.67 to 2.20 acres by adding a landscaped courtyard and tree islands. Construction staging lawn, compacted, proposed condition edited name Open space, fair condition, soil C, from land cover Developed, high intensity to Construction staging lawn, compacted Changed by CurveNumber Examples on Sep 26, 2026, 5:30 PM EDT Reason given: This strip stays as a construction staging area and will be heavily compacted by equipment. The geotechnical report (example, fictional) measured 0.02 inches per hour on similar compacted fill, which behaves like group D. TR-55 Table 2-2a open space in fair condition on group D is CN 84; we use 86 to allow for gravel patches, and we will restore it to lawn after construction. Records that are not overrides. The product wrote these when a condition was copied, or they are the design of a practice. Proposed condition seeded from existing by CurveNumber Examples on Sep 26, 2026, 5:29 PM EDT: 3.094 ac copied. Since then 2 of its areas were edited, listed above, so the two conditions are no longer the same surfaces. Practice storage to the crest, Underground detention vault under the parking deck: 10,000 cu ft, set by CurveNumber Examples on Sep 26, 2026, 5:33 PM EDT. A design input of the practice, not a change to a derived value. Reason: Adding the 2,000 square foot underground vault from sheet C-5 (example, fictional) so the run routes the proposed runoff through it. The aerial photograph, and what was read off itNo aerial photograph of this site was classified. Nothing on this report rests on an aerial photograph. This is the ordinary case and it is stated rather than left to be inferred from the absence of a section, because a page that does not mention imagery and a page that says no photograph was classified read identically and mean different things. The curve number fitted to an observed record, and what it is aboutNo curve number was fitted to an observed record for this site. Nothing on this report rests on a gauged rainfall and discharge record. This is the ordinary case and it is stated rather than left to be inferred from the absence of a section. It is also the expected case: a curve number fitted at a stream gauge describes the whole watershed above that gauge, and this product will not print such a figure against a site that is not inside that watershed and a large enough fraction of it, which most sites are not. Refusals a supplied input resolvedNo refusal was resolved by a supplied input. Nothing on this report rests on a figure a user handed CurveNumber to answer a refusal with. Where a refusal fired, it fired and it is printed as a refusal. Refusals a person waivedNo refusal was waived. Every figure on this report is one CurveNumber produced. Nobody waived a refusal, and no figure here rests on a signature in place of a computation. The assumptions that carry this answerHydrologic condition defaulted Fair (a judgment with no published source; low confidence) PENDINGno acceptance recorded Worth in the answer: 5 to 12 CN units. One of the two largest silent levers in the method. Displaced by: A site visit, current aerial imagery, or a measured ground cover fraction against the NEH Chapter 9 mapping. Curve number suppression band, lower defaulted 2 multiples of Ia (a threshold CurveNumber chose; low confidence) PENDINGno acceptance recorded Worth in the answer: A five CN unit uncertainty is worth a factor of 18.1 at 1.5 Ia, 4.4 at 2 Ia, 2.3 at 3 Ia and 1.4 at 9 Ia. Displaced by: Expert panel calibration. Curve number caution band, upper defaulted 5 multiples of Ia (a threshold CurveNumber chose; low confidence) PENDINGno acceptance recorded Worth in the answer: Above this the method is in the regime it was fitted for. Displaced by: Expert panel calibration. Impervious connectivity defaulted 0 (a judgment with no published source; low confidence) PENDINGno acceptance recorded Worth in the answer: Bounded at 10.2 CN units. Figure 2-4 adjusts the composite as CN = CNp + (Pimp/100)(98 - CNp)(1 - R/2), so the whole span from fully connected to fully unconnected is 0.5 (Pimp/100)(98 - CNp), largest at the 30 percent total impervious ceiling the figure is valid to and at the lowest pervious curve number TR-55 publishes, which is 30: 0.15 x 68 = 10.2 units. At a pervious CN of 61 the same bound is 5.55 units, and at 80 it is 2.7. Displaced by: A delineated drainage path showing impervious area discharging to pervious ground. Automated detection from LiDAR routing is an open gap. Layer depths that the kinds added after the first four start from defaulted (a judgment with no published source; low confidence) PENDINGno acceptance recorded Worth in the answer: The kind chosen moves the answer further than any coefficient in CurveNumber does. On the same 1,000 sq ft footprint the seven dry templates give effective storage depths over their own bottom of 15.1 in for micro-bioretention, 15.9 in for a tree box, 24.0 in for a drywell, 33.3 in for a sand filter, 45.7 in for an infiltration basin and 60.0 in for a detention vault, which is a factor of 4.0 from end to end, against the 20.3 in the bioretention template gives on the same ground. The extended detention basin template is the one to read twice: at 1,000 sq ft the 3H:1V side slopes leave 116.5 sq ft of bottom and the effective depth reads 155.8 in, which is not storage worth having but a footprint that has almost entirely become side slope, and below 648 sq ft the same template has no bottom at all. The three wet templates supply geometry and no permanent pool, so they have no effective depth until a pool is declared, which is the point of not defaulting one. Displaced by: The section on the drawing. These are starting dimensions for a practice described by its area alone, and every one of them is replaced by a designed profile before anything is stamped. Kinds with no filter bed are exempt from the loading ratio ceiling defaulted (a judgment with no published source; low confidence) PENDINGno acceptance recorded Worth in the answer: Applying the ceiling to these kinds would refuse every one of them. Two acres of impervious cover onto the reference extended detention basin is 87,120 sq ft over a 4,022.3 sq ft bottom, which is 21.7 to 1, above the 20 to 1 ceiling, and that is a small drainage area for a basin: the ratio for a real one runs past 100 to 1 and nothing is wrong with it. The same two acres onto the reference wet pond is 9.1 to 1 and passes, which is the clearest evidence the ceiling is measuring the wrong thing here: it separates the two practices by their plan area and not by anything either of them does. What is given up by exempting them is not small, because nothing replaces it: the checks that govern a detention practice are the release rate against the allowable, the residence time, the forebay volume and the embankment, and CurveNumber computes none of the four. Displaced by: A jurisdictional loading limit written for detention rather than for filtration, which supersedes, or a release rate criterion, which is what actually sizes these practices and which CurveNumber would need an allowable discharge to check against. 8 further default values applied to this computation and are left out here, because a reader shown thirty defaults checks none of them. Those above are ordered by the strength of the evidence behind them, weakest first. The complete list, with what each is worth and what would replace it, is on the Reference page of CurveNumber. Across all 14 of them, 0 have been acknowledged and 14 have not; the machine readable companion to this report lists every one with its state. Assumptions with no external sourceThese values carry the answer and nothing external supports them: some are judgments for which no source exists at all (a judgment with no published source), and others are product thresholds that were reasoned rather than measured (a threshold CurveNumber chose). They are assumptions, recorded as assumptions. The text below is CurveNumber's own statement of each value, verbatim. Hydrologic condition defaulted Fair (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. Fair is the mid-range assignment and avoids systematic bias. Recorded as a judgment, not a citation, because a user told this is 'the published default' will not think to change it, and a user told it is a mid-range assumption worth 5 to 12 CN units will. Worth in the answer: 5 to 12 CN units. One of the two largest silent levers in the method. Displaced by: A site visit, current aerial imagery, or a measured ground cover fraction against the NEH Chapter 9 mapping. Curve number suppression band, lower defaulted 2 multiples of Ia (a threshold CurveNumber chose) REASONED, NOT MEASURED. Proposed default awaiting calibration. Worth in the answer: A five CN unit uncertainty is worth a factor of 18.1 at 1.5 Ia, 4.4 at 2 Ia, 2.3 at 3 Ia and 1.4 at 9 Ia. Displaced by: Expert panel calibration. Curve number caution band, upper defaulted 5 multiples of Ia (a threshold CurveNumber chose) REASONED, NOT MEASURED. Worth in the answer: Above this the method is in the regime it was fitted for. Displaced by: Expert panel calibration. Impervious connectivity defaulted 0 (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. The conservative assumption. Worth in the answer: Bounded at 10.2 CN units. Figure 2-4 adjusts the composite as CN = CNp + (Pimp/100)(98 - CNp)(1 - R/2), so the whole span from fully connected to fully unconnected is 0.5 (Pimp/100)(98 - CNp), largest at the 30 percent total impervious ceiling the figure is valid to and at the lowest pervious curve number TR-55 publishes, which is 30: 0.15 x 68 = 10.2 units. At a pervious CN of 61 the same bound is 5.55 units, and at 80 it is 2.7. Displaced by: A delineated drainage path showing impervious area discharging to pervious ground. Automated detection from LiDAR routing is an open gap. Layer depths that the kinds added after the first four start from defaulted (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. A second entry rather than an edit to “Layer depths that each practice kind starts from”, and the reason is that the first entry quotes the effective depth of each of its four templates and a test pins those four figures: folding seven more into it would either leave that worth stale or reword a number a report has already printed. The templates follow. Micro-bioretention: 6 in ponding over 24 in media over 6 in gravel on CurveNumber's default side slopes, with an underdrain. Infiltration basin: 24 in ponding over prepared subgrade, with no media, no stone and no underdrain. Sand filter: 24 in ponding over 18 in sand over 12 in stone, vertical walled, with an underdrain. Tree box: 6 in ponding over 30 in media over 6 in stone, vertical walled, with an underdrain. Drywell: 60 in of stone in a square vertical walled pit. Detention vault: 60 in of open chamber, carried as ponding at a side slope of zero because it is chamber volume and not stone voids. Dry extended detention basin: 36 in of ponding on CurveNumber's default side slopes, with no underdrain and no outlet. Wet pond: 72 in on CurveNumber's default side slopes at a 3 to 1 aspect. Wet swale: 18 in at a 10 to 1 aspect. Constructed wetland: 36 in at a 2 to 1 aspect. They are the middle of what is commonly drawn and they are not a recommendation. Neither the detention vault nor the extended detention basin template carries an outlet structure, and both are refused as unsizeable without one, because the invert elevations in a riser are the release rates the jurisdiction asked for and there is no default release rate. Worth in the answer: The kind chosen moves the answer further than any coefficient in CurveNumber does. On the same 1,000 sq ft footprint the seven dry templates give effective storage depths over their own bottom of 15.1 in for micro-bioretention, 15.9 in for a tree box, 24.0 in for a drywell, 33.3 in for a sand filter, 45.7 in for an infiltration basin and 60.0 in for a detention vault, which is a factor of 4.0 from end to end, against the 20.3 in the bioretention template gives on the same ground. The extended detention basin template is the one to read twice: at 1,000 sq ft the 3H:1V side slopes leave 116.5 sq ft of bottom and the effective depth reads 155.8 in, which is not storage worth having but a footprint that has almost entirely become side slope, and below 648 sq ft the same template has no bottom at all. The three wet templates supply geometry and no permanent pool, so they have no effective depth until a pool is declared, which is the point of not defaulting one. Displaced by: The section on the drawing. These are starting dimensions for a practice described by its area alone, and every one of them is replaced by a designed profile before anything is stamped. Kinds with no filter bed are exempt from the loading ratio ceiling defaulted (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. The loading ratio in CurveNumber's list of default values is a filter bed criterion and says so: “Contributing area to practice area ratio above which the practice is flagged” reasons about clogging, scour across the media surface and pretreatment capacity, and all three are properties of a bed that water passes through. A detention vault, a dry extended detention basin, a wet pond, a wet swale and a constructed wetland have no bed to load. The exemption is granted to those five kinds by name and to nothing else: an infiltration basin and a drywell have no media either and keep the ceiling, because their bottom is the outlet and clogging it is exactly how they fail. The ratio is still computed and still reported for the exempt kinds, with a note saying it was not checked and what would have to be checked instead, because a ratio that disappears from the output is a ratio nobody notices was never applied. Worth in the answer: Applying the ceiling to these kinds would refuse every one of them. Two acres of impervious cover onto the reference extended detention basin is 87,120 sq ft over a 4,022.3 sq ft bottom, which is 21.7 to 1, above the 20 to 1 ceiling, and that is a small drainage area for a basin: the ratio for a real one runs past 100 to 1 and nothing is wrong with it. The same two acres onto the reference wet pond is 9.1 to 1 and passes, which is the clearest evidence the ceiling is measuring the wrong thing here: it separates the two practices by their plan area and not by anything either of them does. What is given up by exempting them is not small, because nothing replaces it: the checks that govern a detention practice are the release rate against the allowable, the residence time, the forebay volume and the embankment, and CurveNumber computes none of the four. Displaced by: A jurisdictional loading limit written for detention rather than for filtration, which supersedes, or a release rate criterion, which is what actually sizes these practices and which CurveNumber would need an allowable discharge to check against. Routing step to practice response time ratio at which the peak is flagged defaulted 0.25 (a threshold CurveNumber chose) REASONED, NOT MEASURED, and weaker than it looks. The response time is the practice's emptying time at the crest, and it is the right scale for the recession and the wrong one for the peak: the peak is governed by the local time constant dS/dz over dO/dz at the stage the routing actually reaches, which at an overflow weir or just above an orifice invert is orders of magnitude shorter than the emptying time and cannot be known before the routing is run. This ratio is therefore a cheap screen that catches the gross case, and the practice routing additionally routes the same storm at half the step and reports the difference in the peak, which is the check that catches what this one misses. Worth in the answer: Measured on two 400 cu ft trenches with response times of 0.823 and 3.080 hr, routing a triangular inflow sampled identically at every step so that only the routing changes: the peak discharge runs 0.65 and 3.00 percent below the converged value at this ratio, 2.17 and 6.10 percent below at 0.5, 5.54 and 18.22 percent below at 1.0 and 20.96 and 42.72 percent below at 2.0. The error is a clipped peak and it is one sided, so it is not conservative for a downstream pipe. The volumes are far less sensitive but they are not untouched, and the difference between those two statements matters: the volume BALANCE closes exactly at any step, by construction, while the volume SPLIT shifts a little because how much infiltrates depends on how long the water stands. Across a factor of forty in the step on the first of those trenches, the volume leaving the site moved by 1.5 percent and the infiltrated share by 10 percent of itself, against 2.3 percent on the peak over the same range. Displaced by: The step halving check the practice routing runs on the case in hand, which measures the sensitivity of this practice and this storm instead of predicting it from a ratio. Routing step to practice response time ratio at which routing is refused defaulted 1 (a threshold CurveNumber chose) REASONED, NOT MEASURED. Set at the ratio where one step drains the whole practice from its crest, because a recession described by its two endpoints is not a routing, and because that is also where the measured peak error passes a twentieth on the better behaved of the two practices and a fifth on the other. Where the line falls inside that range is a judgment and nothing published says where it is. Refused rather than warned because the peak is what a downstream pipe is sized on. Worth in the answer: On the same two measurements, the peak discharge is 5.54 and 18.22 percent below the converged value at this ratio and 20.96 and 42.72 percent below at twice it. The spread between the two practices is as large as the error itself, which is the reason this is a screen and not a correction. Displaced by: Not evidence-displaceable; it is a product policy line. Cover type, NLCD 24 developed high intensity defaulted Open space, Fair, on the pervious part (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. Maps to open space for the sixth of the area that is pervious, which is street tree pits, verge and lot edges rather than lawn. Confidence is LOW on the cover row and that matters little, which is the finding: on this class the measured impervious fraction carries the answer. Worth in the answer: The cover row is nearly irrelevant on this class and the impervious measurement is nearly everything. Against the commercial row used as the pervious cover with the measured 83.1 percent added on top, 97.0 against 91.7 and 2.657 in against 2.138 in, 24 percent high. Against the class definition midpoint of 90 percent impervious, 94.3 against 91.7, which is 2.55 CN units, 11.3 percent of the runoff depth and 27.6 percent at P = 1.2 in. Displaced by: A high resolution land cover product, or a site plan. This is the class where a 1 meter product changes the answer most. Inputs, for recomputationExisting condition
Proposed condition
How to read the provenance marksentereddouble borderA person typed this area: its area, and its cover and soil group where named. Who and when are printed on the row. Where the curve number is the TR-55 table value for that cover and soil group, the row also says the curve number was looked up; the area and the choice of cover are still the person's.editeddouble border, underlinedA person changed the area, cover, soil group or curve number of this area after it was derived, entered or copied. The change, who, when and the reason are printed on the row and in the overrides section.copiedheavy solid borderCopied from the other condition. The row says whether the area it was copied from was read from the datasets or typed by a person.derivedsolid borderRetrieved from a published source. The citation and its edition are printed with the value, because TR-55 1986 and TR-55 1975 disagree and a citation without an edition cannot be checked.defaulteddashed borderAn assumption, not a measurement. CurveNumber's list of default values entry is named, and what the assumption is worth in the answer is stated with it. Where no external source exists for the assumption, this report says so in those words.overriddendouble border, hatched, boldA person changed the value. Who, when, the prior value and the reason given are recorded in the overrides section and cannot be edited afterwards, because the parameter record is append only.supplieddouble border, boldA person gave CurveNumber an argument and it recomputed from it: a measured flow path length in place of the geometric bound, the flow surface the ground actually is, the month the design storm is taken in. Nothing the product had stored was overwritten, which is why this mark is not the override mark; what the argument displaced is CurveNumber's own figure, and the estimates section prints it. Who, when and the reason given are in the overrides section and the record is append only.accepteddouble border, dotted inner rule, boldA person examined a figure a machine produced and adopted it unchanged. It carries a person's mark because a person put it into this record under their own name, and it is not the supplied mark because nothing was handed to anything and nothing recomputed: the magnitude is the machine's own. Two acts carry it. The acceptance of a classification of an aerial photograph, which is a guess about an image and not a measurement of the ground, and which the imagery section explains; and the acceptance of a curve number fitted to a gauged rainfall and discharge record, which is a property of the whole watershed above that gauge and not a measurement of this site, and which the calibration section explains. The mark is also on an area of ground the soil survey does not rate, computed on the most likely soil group a person accepted (an estimate, not a survey rating), and on an impervious area sized by an accepted photograph. Who, when and the reason given are in the overrides section and the record is append only.fitteddashed border, dotted inner ruleCurveNumber fitted this magnitude to an observed record. It carries no person's mark, because no person put it here, and it is not the computed mark, because a computed value is a function of the inputs printed beside it and a fitted one is a function of the inputs AND of conventions: change the baseflow separation and not one input moves while the magnitude moves by curve number units. It is never printed without its interval, and a fitted figure becomes an input to a design only when a named person accepts it, which is a separate act carrying the accepted mark above. The marks are separated by the word, by the border style and by the fill, in that order of reliability, so this report can be read correctly from a black and white photocopy and from a browser printing with background graphics switched off. Reproducibility
Decision support. Every figure in this report requires independent verification, and a licensed professional is responsible for the design. Values marked DERIVED carry a citation with an edition. Values marked DEFAULTED carry an assumption rather than a measurement, and for some of them no external source exists, which CurveNumber's list of default values states on its face and which this report repeats rather than hides. Values marked OVERRIDDEN were changed by a named person, and the prior value and the reason given are printed in the overrides section. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||