Stormwater Atlas

channel-flood

Channel Protection, Detention, and Flood Control

Post-development peak matching, extended detention, and floodplain/channel criteria.

ReviewedPrevalence 46%~273 manuals tagged14 citationsModel: heuristic+editorialRegional: arid_west, urban_infill
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

Channel and flood criteria separate erosive-flow control from flood conveyance. Manuals that list return periods commonly include short storms for channel protection and the 100-year event for flood or emergency design. Coordination with floodplain managers and FEMA mapping is routine for larger conveyances.

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. 9.1Field-verified

    Separate channel-protection storms from flood conveyance; do not collapse both into one number.[4,7,1,2]

  2. 9.2Editorial

    Limit post-development increases in erosive flows where stable channels are a goal.[1,2,3,4]

  3. 9.3Field-verified

    Size conveyance and freeboard for the flood design storm (commonly 100-year) with emergency overflow paths.[4,7,1,2]

  4. 9.4Editorial

    Coordinate with floodplain managers and FEMA mapping where applicable.[1,2,3,4]

Regional notes

Arid ephemeral channels and coastal tidally influenced systems need region-specific velocity and freeboard checks.

Open questions

Default channel-protection return period (1- versus 2- versus 10-year). National freeboard guidance versus deferral to local floodplain ordinances.

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]VermontVermontCorpus patternpages 11-11
    “pter 4.0 to meet the specified treatment standards for recharge, water quality, channel protection, overbank flood protection, and extreme flood control. Each of these standards and their exemptions are discussed in more detail in the following sections. Tab”
  9. [9]Rhode IslandRhode IslandCorpus patternpages 269-269
    “Standards Manual Amended March 2015 as well as reduce the runoff volume for the channel protection volume, and to a lesser extent, overbank controls. • Rev = (1”)(F)(I)/12: Recharge is required at the site based on land use and soils. Use exfiltrating bioret”
  10. [10]West VirginiaWest VirginiaCorpus patternpages 55-55
    “rmwater practices to provide peak rate control for larger storms for downstream channel protection and/or flood control. In West Virginia, this is likely to be a local stormwater standard or requirement. The Runoff Reduction Method allows for the annual Runo”
  11. [11]GeorgiaGeorgiaCorpus patternpages 121-121
    “lled through a combination of extended detention for the 1-year, 24-hour event (channel protection) and control of the 25-year peak rate for overbank flood protection. These design standards, therefore, are intended to be used in unison. This standard may be”
  12. [12]MassachusettsMassachusettsCorpus patternpages 107-107
    “the water back into the ground. Infiltration practices typically cannot provide channel protection and overbank or extreme flood detention storage. Infiltration SCMs include: Exfiltrating Bioretention Areas and Rain Gardens, Dry Wells, Infiltration Basins, I”
  13. [13]New York StateNew YorkCorpus patternpages 239-239
    “requires that projects meet three separate stormwater quantity criteria: 1. The Channel Protection (CPv) requirement is designed to protect stream channels from erosion. This is accomplished by providing 24 hours of extended detention for the 1-year, 24-hour”
  14. [14]MarylandMarylandCorpus patternpages 46-47
    “Chapter 2. Unified Stormwater Sizing Criteria ...............Channel Protection Storage Volume 2.8 Section 2.3 Channel Protection Storage Volume Requirements (Cpv ) The method for determining the Cpv requirement is detailed in Appendix D.1”
  15. [15]MinnesotaMinnesotaCorpus patternpages 173-173
    “ermit, so this is seldom a major factor in BMP selec- tion. Can the BMP provide channel protection?• BMPs must provide extended detention for long periods at sites where channel protection (Vcp) is required to protect streams, which means that only a short l”
  16. [16]IndianaIndianaCorpus patternpages 729-729
    “e of potential flood conditions resulting from development. They include stream channel protection, conveyance protection, peak runoff attenuation, and emergency outlet sizing. Percolation. The movement of water through soil. Percolation Rate. The rate, usua”
  17. [17]MaineMaineCorpus patternpages 21-21
    “is evenly distributed between three ponds in series, the reduction may be 40%. Channel Protection Volume: Wet ponds must detain, above the permanent pool, a runoff volume equal to 1.0 inch times the subcatchment's impervious area plus 0.4 inch times the lan”

Supporting jurisdictions

10 jurisdictions cited in this section (outline tags ~273 manuals; full atlas has ~299).

vt-stateri-statewest-virginia-manualga-statema-state-draftny-statemd-statemn-state-2008indiana-manualmaine-manual