Stormwater Atlas

hydrology

Hydrology and Design Storms

Return periods and storm definitions used for water quality, channel protection, and flood control.

ReviewedPrevalence 49%~295 manuals tagged14 citationsModel: heuristic+editorialRegional: arid_west, coastal, cold_climate
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

Hydrology chapters in U.S. manuals almost always separate storms by purpose: water-quality or treatment capture, channel or overbank protection, and flood or emergency conveyance. Among manuals that list return periods (~81% of the atlas; ~91% of Tier A), the 100-, 10-, and 2-year events appear most often. Peak-flow methods are dominated by NRCS/SCS and Rational Method families, with continuous or event models required for more complex sites.

Design storms by purpose

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

Design purposeSuggested national guidanceAtlas note
Water quality / treatmentPrefer a rainfall or runoff depth, or a percentile WQ storm — not a long return period aloneWQ method field present in 95% of all manuals (88% of Tier A)[1,2,3,5,6]
Channel / overbank protectionCommonly select a short return period in the 1–10 year range; document local channel criteria2-yr: 64% of listing manuals (155). 10-yr: 74% of listing manuals (178)[4]
Peak matching / detentionCommonly use a 2–25 year suite (sometimes through 100-year); match pre-development peaks unless volume-based alternative applies2-yr: 64% of listing manuals (155). 25-yr: 43% of listing manuals (104). 100-yr: 88% of listing manuals (212)[4,7]
Flood / emergency conveyanceCommonly design for the 100-year event with freeboard and a safe overflow path100-yr: 88% of listing manuals (212)[7]

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)[8,9]
NRCS / SCSDefault for most site design; document CN, rainfall distribution, and Tc64% of manuals with methods (174)
Continuous / event modelsRequire where permits demand volume control, complex routing, or LID credit verification14% of manuals with methods (38)[9,15,16,19]
Unit hydrograph / otherAllow when documented in the controlling jurisdiction manual5% of manuals with methods (14)[13,22,25,26,27]

Software allow-list posture (capability, not a mandated product)

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

Capability needSuggested national guidanceAtlas note
Event / watershed hydrologyAccept tools that implement approved methods and export reviewable inputs/outputs (e.g. HEC-HMS class)29% of manuals with software lists (22)[10,12,18,21]
Urban runoff / LID continuousAccept SWMM-class tools where continuous or multi-event simulation is required17% of manuals with software lists (13)[18,20,21,24]
Site detention / pond routingAccept HydroCAD-class or equivalent tools used by the review agency21% of manuals with software lists (16)[11,12,17,21]
Open channel / floodplainAccept HEC-RAS-class tools for conveyance and floodplain checks when required13% of manuals with software lists (10)[10,14,18,23]
Submittal hygieneRequire version disclosure and native input files with the drainage report — do not mandate a single brandAtlas lists named tools; product mandates are uncommon

Guidance

  1. 3.1Editorial

    Require designers to name the design purpose (water quality, channel protection, peak matching, or flood conveyance) before selecting a storm or method.[1,2,3,4]

  2. 3.2Field-verified

    Present return periods and water-quality depths as guidance with jurisdiction footnotes; more-stringent local criteria control.[4,7,1,2]

  3. 3.3Field-verified

    Match methods to site scale—Rational with documented area limits, NRCS for most site design, continuous simulation where permits require volume control.[8,9,13,15]

  4. 3.4Editorial

    Address arid West, coastal, and cold-climate forks explicitly; karst and high groundwater may disallow infiltration assumptions.[1,2,3,4]

Regional notes

Arid West: larger capture depths and infiltration limits. Coastal: tide and surge interactions. Cold climate: frozen ground and snowmelt hydrographs. Karst and high groundwater may disallow infiltration assumptions.

Open questions

Which return periods belong in a national MOP versus guidance-only. Single national WQ depth versus climate bands. Software: prescribe versus allow-list.

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-verifiedTable 3-4; Section 3.3.1
    “Treat the runoff from 90% of the storms that occur in an average year. For Alaska, this equates to providing water quality treatment for the runoff resulting from a rainfall depth of 1.25 inches or less. The goal is to reduce average annual post-development TSS loadings by 80%. WQv = (Rv) × (A) × (P) / 12”
  2. [2]AlabamaAlabamaField-verifiedPage 9, Chapter 2
    “the first flush equates to the first 1" of runoff which carries 90% of the pollution load from a storm (USGS, 1984)”
  3. [3]MontanaMontanaField-verifiedSection 1.3.1, Pages 1-2 to 1-3
    “infiltrate, evapotranspire, and/or capture for reuse the post-construction runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation... remainder... remove 80 percent total suspended solids (TSS)”
  4. [4]California Department of Transportation (Caltrans)CaliforniaField-verifiedSection 7.1.5 NS-5 Clear Water Diversion, Page 7-5
    “A 2-year, 24-hour storm event has been used by many as a default event, but recent studies have shown that this may oversize the system”
  5. [5]ConnecticutConnecticutField-verifiedPage 46, Water Quality Volume Calculation
    “WQV = (P)(R)(A)/12 where: WQV = water quality volume (cubic feet) P = 1.3 inches (90th percentile rainfall event) R = volumetric runoff coefficient = 0.05+0.009(I) I = post- development impervious area (percent) A = post-development total drainage area of site or design point (square feet)”
  6. [6]FloridaFloridaField-verifiedChapter 3, Page 103
    “temporary (or permanent) sediment or wet detention basin providing 3,600 cubic feet of storage per acre drained must be provided”
  7. [7]GeorgiaGeorgiaField-verified105, 112
    “the 1-year, 25-year, and 100-year return intervals... 2-year through the 25-year return frequency storm events”
  8. [8]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”
  9. [9]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”
  10. [10]MinnesotaMinnesotaField-verifiedChapter 8, Table 8.1 Modeling Tool Selection
    “TR-55 (original or DOS) ... HEC-1 ... HEC-HMS ... WinTR-20 (or TR-20) ... WinTR-55 ... HEC-RAS”
  11. [11]New York StateNew YorkField-verifiedPage 1.5
    “These data can be imported into HydroCAD, USDA NRCS TR20, and other computer models”
  12. [12]New York StateNew YorkField-verifiedSection 4.9
    “Hydraulic and hydrologic modeling software, such as USDA NRCS TR20, HydroCAD, Pond Pack, StormCAD, Hydraflow, and Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) and others, shall be used”
  13. [13]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.”
  14. [14]VermontVermontField-verified2-14
    “The models TR-55 and TR-20 (or approved equivalent) will be used for determining peak discharge rates”
  15. [15]WashingtonWashingtonField-verifiedGlossary, page 1303
    “Rational method”
  16. [16]West VirginiaWest VirginiaField-verified3.4.4
    “utilizes the NRCS TR55 runoff equations 2-1 through 2-4 to derive a reduced curve number”
  17. [17]Charlotte-MecklenburgNorth CarolinaField-verifiedChapter 1, Section 1.3, Page 7
    “The City will typically use HEC software to verify designs submitted by the development community. In addition, the City may check designs using HydroCAD and StormCAD. Designers may use any software tools that are based on the methods described in this manual.”
  18. [18]City of ChesapeakeVirginiaField-verifiedp. 5
    “25-yr. 7.0” 50-yr. 8.17” 100 9.44” C. Acceptable methods for runoff analysis are tabulated below: Model Major Sys. Minor Sys. Hydrologic Hydraulic Water Quality Hydrology Software HEC-GeoHMS X X HEC-HMS X X TR-55 X TR-20 X Pond Pack X X X WMS X X Watershed Modeling X X Hydrauli”
  19. [19]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”
  20. [20]District of ColumbiaDistrict of ColumbiaField-verifiedAppendix H, Section H.4
    “HEC-1, WinTR-55, TR-20, and SWMM Computer Models”
  21. [21]City of DetroitMichiganField-verifiedChapter 4, Table 4-5, Page 4-9
    “Rational Method, EPA SWMM, HydroCAD, StormCAD, or other DWSD-approved model... NRCS Curve Number Approach, EPA SWMM, HEC-HMS, TR-55, TR-20, HydroCAD, StormCAD, or other DWSD-approved model”
  22. [22]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.”
  23. [23]County of Los AngelesCaliforniaField-verifiedSection 6.3, Page 6-3
    “LACPW developed a hydrologic calculator (HydroCalc)... Use of this program is encouraged”
  24. [24]City of MinneapolisMinnesotaField-verifiedSection 4.3.1.6, Page 26
    “The hydrologic and hydraulic models accepted by the City of Minneapolis include: HydroCAD; XP-SWMM”
  25. [25]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.”
  26. [26]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”
  27. [27]City of MesquiteTexasField-verified§3.2
    “current edition of the NCTCOG Integrated Stormwater Management (iSWM™) Hydrology Technical Manual”
  28. [28]PennsylvaniaPennsylvaniaCorpus patternpages 60-60
    “tained in 25 Pa. Code § 102.8(g)(3), the PCSM Plan preparer has two options: 1. Design storm analysis approach; and 2. Storm of record approach. Peak rate management analysis requires the use of a unit hydrograph-based hydrologic model in conjunction w”
  29. [29]MinnesotaMinnesotaCorpus patternpages 191-191
    “state are included in TP-40 and reproduced in Appendix B of this Manual: 1-Year design storm• 2-Year design storm• 5-Year design storm• 10-Year design storm• 25-Year design storm• 100-Year design storm• Design engineers typically make use of precipitat”
  30. [30]ConnecticutConnecticutCorpus patternpages 556-556
    “Climate Change Considerations 549 Options Evaluated Stormwater Quantity Control Design Storm As described above, the Workgroup evaluated several alternative approaches for updating the stormwater quantity control design storm precipitation (24-hour des”
  31. [31]MassachusettsMassachusettsCorpus patternpages 162-164
    “n multiply that value by a scaling factor of 0.9. Steps to Obtain Site-Specific Design Storm Depths 1) Navigate to NOAA Atlas 14: https://hdsc.nws.noaa.gov/hdsc/pfds/ (Nationwide) or https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=ma (Mass”
  32. [32]City of FlagstaffArizonaCorpus patternpages 24-25
    “2025 STORMWATER MANAGEMENT DESIGN MANUAL February 2025 Page 12 • Identify design storm recurrence intervals • Discussion and justification of other criteria or calculation methods used that are not presented in or referenced by this Manual • Summ”
  33. [33]AlaskaAlaskaCorpus patternpages 23-23
    “ruction phases and afterwards. 1.1.2.1 New Development Management Measure 1. By design or performance a. After construction has been completed and the site is permanently stabilized, reduce the average annual total suspended solid (TSS) loadings”
  34. [34]AlabamaAlabamaCorpus patternpages 24-24
    “street gutters and ditches, storm sewers, culverts, and open channels which are designed to prevent inconvenience and minor property damages from relatively frequent storm events. Of course, the most effective strategy for flood control at this l”
  35. [35]City of AkronOhioCorpus patternpages 40-40
    “(Pond) addressing issues such as: • Ponds must incorporate emergency spillways designed to safely convey flows exceeding design storm flows. • Outlet structures should be built to withstand floatation and incorporate anti-vortex and debris or tr”
  36. [36]City of AlbanyNew YorkCorpus patternpages 30-30
    “standard SMPs with RRv capacity. The objective is to replicate pre-development hydrology and provide groundwater recharge by maintaining pre-construction infiltration, peak runoff flow, discharge volume, as well as minimizing concentrated flow by u”

Supporting jurisdictions

9 jurisdictions cited in this section (outline tags ~295 manuals; full atlas has ~299).

pa-statemn-state-2008ct-statema-state-draftflagstaff-azak-statealabama-handbookakron-ohalbany-ny