Stormwater Atlas

hydrology.methods

Runoff and Peak-Flow Calculation Methods

Rational Method, NRCS/TR-55, continuous simulation, and when each is required or allowed.

ReviewedPrevalence 46%~274 manuals tagged14 citationsModel: heuristic+editorial
Provisional practice synthesis for discussion — not a design manual or adopted regulation. Controlling state, regional, and MS4 criteria apply when more stringent. Verify citations against the linked jurisdiction manuals.

Practice

Peak-flow and runoff methods in atlas extracts cluster into Rational Method for small catchments, NRCS/SCS curve-number and hydrograph methods for most site design, and continuous or multi-event models where volume control or complex routing is required. Unit hydrograph and Green-Ampt variants also appear. Local manuals often set maximum drainage areas for Rational Method.

When to use common peak-flow methods

Draft national guidance grounded in atlas practice — not a design manual or adopted regulation.

Method familySuggested national guidanceAtlas note
Rational MethodAllow for small, largely impervious catchments only; document Tc and intensity assumptions; respect local area limits41% of manuals with methods (111)[1,2,4,6,7]
NRCS / SCSDefault for most site design; document CN, rainfall distribution, and Tc64% of manuals with methods (174)[1,2,3,4,5]
Continuous / event modelsRequire where permits demand volume control, complex routing, or LID credit verification14% of manuals with methods (38)[1,7,9,10,11]
Unit hydrograph / otherAllow when documented in the controlling jurisdiction manual5% of manuals with methods (14)[8,12,13,14,15]

Guidance

  1. 5.1Field-verified

    Allow Rational Method only for small, largely impervious catchments with documented time-of-concentration and intensity assumptions.[1,4,6,7]

  2. 5.2Field-verified

    Use NRCS/SCS methods for most site design; document CN, rainfall distribution, and Tc.[1,2,3,4]

  3. 5.3Field-verified

    Require continuous or multi-event simulation where permits demand runoff reduction or complex routing.[1,7,11,2]

  4. 5.4Editorial

    Defer to stricter local method rules when they conflict with this guidance.[1,2,3,4]

Regional notes

Some West Coast and cold-climate manuals restrict Rational Method more tightly or require continuous simulation for LID volume credit.

Open questions

National area limit for Rational Method. Whether continuous simulation is guidance or a requirement for volume-based standards.

References

Numbered footnotes for guidance clauses and atlas notes. Field-verified entries come from structured atlas evidence; corpus-pattern entries are keyword-matched excerpts.

  1. [1]AlaskaAlaskaField-verifiedSection 3.2, page 3-5
    “A partial list of hydrologic modeling software accepted by some Alaskan municipalities includes the following: ILLUDAS TR55-1986 WinTR553 WinTR20 SCS HEC-HMS Rational Method”
  2. [2]AlabamaAlabamaField-verifiedPage 181, Diversion
    “The peak rate of runoff should be determined using the Natural Resources Conservation Service runoff curve number (RCN) method or other equivalent methods.”
  3. [3]MontanaMontanaField-verifiedSection 3.3, Page 3-2
    “This procedure relies on the volume of runoff computed using Equation 3-1 and uses an adaptation of the NRCS TR-55 Graphical Peak Discharge Method”
  4. [4]ConnecticutConnecticutField-verifiedPage 59, Appendix D
    “using a hydrologic/hydraulic routing model based on the NRCS Curve Number method (e.g., HydroCAD or similar software)... Graphical Peak Discharge Method... routing employed by TR-20... Rational Formula”
  5. [5]GeorgiaGeorgiaField-verified154
    “What is the SCS curve number for this sub-watershed”
  6. [6]State of Hawaii Department of Transportation, Highways DivisionHawaiiField-verifiedSection 7.3.2, Page 7-7
    “Flow rate calculation shall be based upon the Rational Method as follows: WQFR = C x i x AT”
  7. [7]IdahoIdahoField-verifiedSections 3.6.3–3.6.3.2, Pages 53–59
    “The rational method and NRCS Technical Release-55 (TR-55) method are simple and widely used for performing single event design storm analysis... continuous simulation models... HSPF... HEC-HMS, and... SWMM... For rain-on-snow events, an energy balance approach using a computer model such as HEC-HMS is recommended... de”
  8. [8]PennsylvaniaPennsylvaniaField-verifiedPage 2-53
    “Methods include the NRCS hydrologic soil-cover complex method, Green and Ampt equation, Snyder Unit Hydrograph, and WINSLAMM small storm hydrology method.”
  9. [9]WashingtonWashingtonField-verifiedGlossary, page 1303
    “Rational method”
  10. [10]West VirginiaWest VirginiaField-verified3.4.4
    “utilizes the NRCS TR55 runoff equations 2-1 through 2-4 to derive a reduced curve number”
  11. [11]City of ChicagoIllinoisField-verifiedPages 7, 15, 16, B-6, B-7
    “Modified Rational Method to calculate the maximum volume of stormwater... Rational Method... hydrograph methods with regional rainfall data... HEC-1 and HEC-HMS... TR-20... TR-55... SWMM”
  12. [12]City of HoustonTexasField-verifiedSection 2A, 9.2.01.B.2.a, Page 9-12
    “Rational Method: The Rational Method will be used to estimate peak flows for individual drainage areas up to 200 acres in size, and for project areas up to 640 acres in size.”
  13. [13]City of San AntonioTexasField-verifiedSection 5.2.2
    “For drainage areas less than 200 hundred (200) acres, the basis for computing runoff shall be the rational formula (as defined in Section 5.3) or some other method provided it is acceptable to the Director of TCI.”
  14. [14]Municipality of AnchorageAlaskaField-verifiedTable of Contents / Chapter 4
    “4.5.2 Rational Method... 4.5.3 NRCS (SCS) Peak Flow Method (Tabular Method)... 4.5.4 USGS Regression Equations... 4.5.5 NRCS (SCS) Tabular Hydrograph Method”
  15. [15]City of MesquiteTexasField-verified§3.2
    “current edition of the NCTCOG Integrated Stormwater Management (iSWM™) Hydrology Technical Manual”
  16. [16]City of DetroitMichiganCorpus patternpages 98-98
    “basin to the point of interest. The tc must be determined to be able to use the Rational Method to estimate peak flow for sizing storm sewer systems, or for applying unit hydrographs and NRCS curve number methods to generate and route runoff hydrog”
  17. [17]MontanaMontanaCorpus patternpages 23-23
    “hydrograph developed using one of the methods listed in this chapter (modified rational method or SCS Curve Number (TR-55)). Outflow values in this method must be based on a stage-discharge relationship for detention facility outlet structures and”
  18. [18]MinnesotaMinnesotaCorpus patternpages 189-189
    “/or event hydrograph calculations only TR-55 (original or DOS) Low No Yes -- -- Rational Method (equation) Low No Yes -- -- Hydrologic Models rainfall-runoff simulation, reservoir & channel routing HEC-1 Medium No Yes -- -- HEC-HMS Medium Yes Yes -”
  19. [19]IdahoIdahoCorpus patternpages 72-72
    “(IDF) curves can be used to acquire storm duration and intensity for use in the rational method. IDF curves for Idaho have been developed by ITD and are found in the Roadway Design Manual, Appendix B. NRCS TR-55 presents a single event hydrograph m”
  20. [20]Clark County Regional Flood Control DistrictNevadaCorpus patternpages 124-126
    “603.2 - SCS Curve Number Method 608 603.2.1 - CN Determination 609 604 MODIFIED RATIONAL FORMULA METHOD 610 604.1 - Methodology 610 604.2 - Assumptions 610 604.3 - Limitations on Methodology 611 604.4 - Rainfall Intensity 611 604.5 - Runoff Coeffic”
  21. [21]City of BellevueWashingtonCorpus patternpages 64-65
    “1 Acceptable Uses of Runoff Computation Methods TYPE OF COMPUTATION Allowed For Rational Method Single Hydrograph Method Ecology-Approved Continuous Simulation Model PEAK FLOW CONVEYANCE SIZING (DESIGN FLOWS) Tributary Areas < 10 acres (measured to”
  22. [22]West VirginiaWest VirginiaCorpus patternpages 467-467
    “stem design are NRCS TR‐55 Curve Number (CN) methods (NRCS TR‐55, 1986) and the Rational Formula. The Rational Formula is highly sensitive to the time of concentration and rainfall intensity, and therefore should only be used with reliable Intensit”
  23. [23]WashingtonWashingtonCorpus patternpages 484-484
    “dvantages over the single event-based meth- ods such as the SCSUH, SBUH, or the Rational Method. HSPF is a continuous simulation model that is capable of simulating a wider range of hydrologic responses than the single event models. Single event mo”
  24. [24]Rhode IslandRhode IslandCorpus patternpages 57-57
    “m events. This is more appropriate than the traditional NRCS CN Methods and the Rational Formula, which have been widely used for peak runoff calculations and drainage design. The traditional NRCS TR-55 CN methods are valuable for estimating peak d”
  25. [25]MaineMaineCorpus patternpages 15-15
    “orate the SCS-TR-20 or HEC-1 procedures, or other routing methods. The Modified Rational Method, while not a true routing procedure, can be used for preliminary design of detention storage for watersheds up to 20 or 30 acres. 2.4 Hydrologic Data fo”
  26. [26]District of ColumbiaDistrict of ColumbiaCorpus patternpages 93-93
    “Storage-Indication Routing  HEC-1, WinTR-55, TR-20, and SWMM Computer Models  Rational Method (limited to sites under five acres) These methods are given as valid in principle, and are applicable to most stormwater management design situations in”

Supporting jurisdictions

11 jurisdictions cited in this section (outline tags ~274 manuals; full atlas has ~299).

detroit-mimt-deq8mn-state-2008idaho-bmp-cataloglas-vegas-nvbellevue-wawest-virginia-manualwa-state-westernri-statemaine-manualdc-guidebook