Stormwater Atlas

hydrology.design-storms

Design Storm Selection by Purpose

Separate WQ, channel, detention, and flood storms; avoid collapsing all purposes into one number.

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

Design storms are selected by purpose rather than collapsed into one number. Water-quality sizing is often a rainfall or runoff depth (or a percentile storm), while channel protection and detention use short-to-medium return periods, and flood conveyance commonly uses the 100-year event. Atlas extracts show 100-, 10-, and 2-year storms as the most frequently listed return periods among manuals that publish a list.

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]

Guidance

  1. 4.1Field-verified

    For water quality, prefer a rainfall or runoff depth or a percentile storm—not a long return period alone.[1,2,3,5]

  2. 4.2Field-verified

    For channel or overbank protection, commonly select a short return period in the 1–10 year range and document local channel criteria.[4,7,1,2]

  3. 4.3Field-verified

    For detention or peak matching, commonly use a 2–25 year suite (sometimes through 100-year).[4,7,1,2]

  4. 4.4Field-verified

    For flood or emergency conveyance, commonly design for the 100-year event with freeboard and a safe overflow path.[4,7,1,2]

  5. 4.5Corpus pattern

    Prefer NOAA Atlas 14 or successor precipitation where available; document any climate adjustments required locally.[1,2,3,4]

Regional notes

Arid West manuals often emphasize retention of a larger capture depth. Humid East manuals emphasize multi-storm peak matching. Coastal projects may need tide or surge checks beyond inland Atlas 14 depths.

Open questions

Whether to publish a single national WQ storm depth or climate-banded ranges. How to reference climate-change precipitation adjustments without prescribing one method.

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]MaineMaineCorpus patternpages 11-11
    “ing stormwater management facilities, the designer usually selects one or more "design storms". The most common approach is to use a design storm that relates the rainfall intensity, duration, and frequency (return period). Intensity-duration-frequency”
  9. [9]MontanaMontanaCorpus patternpages 29-29
    “for volume made available through infiltration or designed outlet. The minimum design storm event for retention facilities is the 2-year 24- hour storm for standard plans and the 100-year 24-hour storm for simplified plans. The capacity of a retention”
  10. [10]New York StateNew YorkCorpus patternpages 221-221
    “except for the inlet opening at the top, or a Qp = the peak inflow rate for the design storm. (The mini- dewatering opening. There shall not be other holes, mum design storm will be a 10 year, 24 hour storm under leaks, rips, or perforations in the str”
  11. [11]Rhode IslandRhode IslandCorpus patternpages 58-58
    “ention of the post- development runoff volume from the 1-year, 24-hour Type III design storm event. If a stormwater discharge is proposed within 200 feet of streams and any contiguous natural or vegetated wetlands in watersheds draining to cold-water f”
  12. [12]MarylandMarylandCorpus patternpages 165-165
    “y subtracting the runoff treated by ESD practices from the total 1-year 24-hour design storm runoff. Table 5.3 was developed using the “Change in Runoff Curve Number Method” (McCuen, R., MDE, 1983) to determine goals for sizing ESD practices and reduci”
  13. [13]City of DetroitMichiganCorpus patternpages 59-60
    “same outlet structure as used for flood control. d. FLOOD CONTROL i. Determine Design Storm: 1. If the Regulated Area is less than 5 acres, the proposed design must limit the 10-Year, 24-Hour peak flow to 0.15 cfs/acre. 2. If the Regulated Area is equ”
  14. [14]MinnesotaMinnesotaCorpus patternpages 202-202
    “d States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. U.S. Weather Bureau, Technical Publication 40. Huff and Angels, 1992. Bulletin 71. Midwest Climate Center. Metropolitan Council, 1995. Precipitation Frequenc”
  15. [15]PennsylvaniaPennsylvaniaCorpus patternpages 26-26
    “ons are incorporated in this Manual by using increased rainfall volumes for the design storms and more recent rainfall records with some accommodation for climate change when using continuous simulation. It is expected that future research will provide”
  16. [16]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”
  17. [17]VermontVermontCorpus patternpages 187-187
    “𝑝𝑝 × 𝑃𝑃1𝑦𝑦𝑦𝑦 12 � Where: TV = maximum design volume (ft3) P1yr = 1 year design storm depth (inches) Where rooftop runoff can be routed to a permeable pavement system with available treatment capacity (Tv), the surplus treatment capacity can be”
  18. [18]Clark County Regional Flood Control DistrictNevadaCorpus patternpages 86-86
    “R 513 McCARRAN AIRPORT RAINFALL AREA 514 DEPTH-AREA REDUCTION CURVES 515 6-HOUR DESIGN STORM DISTRIBUTIONS 516 DEPTH-DURATION-FREQUENCY CURVES FOR McCARRAN AIRPORT RAINFALL AREA 517 TIME-INTENSITY-FREQUENCY CURVES FOR McCARRAN AIRPORT RAINFALL AREA 518”

Supporting jurisdictions

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

maine-manualmt-deq8ny-bluebookri-statemd-statedetroit-mimn-state-2008pa-statect-statevt-statelas-vegas-nv