Stormwater Atlas

channel-flood.release

Allowable Discharge and Release Rates

Pre- vs post-development peaks and site-specific release limits.

ReviewedPrevalence 46%~273 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

Release-rate or peak-matching criteria typically require post-development peaks not to exceed pre-development for a specified storm suite, unless a regional facility provides equivalent control. Some manuals replace multi-storm peak matching with volume reduction or duration control, especially where LID retention is the primary standard.

Release-rate / peak-matching criteria

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

Design purposeSuggested national guidanceAtlas note
Pre- vs post-development peaksMatch or reduce post-development peaks to pre-development for the specified storm suite unless a regional facility provides equivalent controlCommon storm suite anchors: 2-yr 64% of listing manuals (155); 10-yr 74% of listing manuals (178); 100-yr 88% of listing manuals (212)[1,2,3,4,5,6]
Volume-based alternativeAllow duration or volume-reduction compliance where the local manual endorses it in lieu of multi-storm peak matchingSome GI manuals replace peak matching with retention standards
Hydrograph documentationRequire pre/post hydrographs and routing assumptions in the drainage reportSubmittal expectation across plan-review manuals

Guidance

  1. 10.1Field-verified

    Match or reduce post-development peaks to pre-development for the specified storm suite unless a regional facility provides equivalent control.[1,2,3,4]

  2. 10.2Editorial

    Document pre- versus post-hydrographs and routing assumptions in the drainage report.[1,2,3,4]

  3. 10.3Editorial

    Allow alternative volume-based or duration-control compliance where local manuals endorse it.[1,2,3,4]

Regional notes

Some manuals emphasize volume reduction or duration control over multi-storm peak matching. Arid regions may emphasize retention over release-rate matching.

Open questions

Which storm suite belongs in national guidance. How to treat redevelopment sites with no reliable pre-development condition.

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, Unified sizing criteria, Section 3.3
    “Provide 24 hours of extended detention of the runoff from the 1-year, 24-hour duration storm event... Provide peak discharge control of the 5-year storm event... Some jurisdictions may require peak discharge control for the 2-year storm event... Evaluate the effects of the 100-year storm”
  2. [2]AlabamaAlabamaField-verifiedPage 181, Table DV-1
    “Temporary Construction Areas 2-year... Building Sites 5-year... Agricultural Land 10-year... Isolated Buildings 25-year... Urban areas, Residential, School, Industrial Areas, etc. 50-year”
  3. [3]MontanaMontanaField-verifiedSection 3.4 Flood Control, Page 3-4
    “Generally, a 10- or 25-year return design storm is used to size storm drainage infrastructure and a 100-year return design storm is used to address upstream and downstream flooding”
  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 35, Table 4-1
    “Control the 2-year, 24-hour post-development peak flow rate... Control the 10-year, 24-hour post-development peak flow rate... Potentially control the 100-year, 24-hour post-development peak flow rate”
  6. [6]FloridaFloridaField-verifiedChapter 3, Page 106
    “peak runoff expected from the drainage area for a 10-year storm”
  7. [7]District of ColumbiaDistrict of ColumbiaCorpus patternpages 94-95
    “d will determine the site’s predevelopment peak rate of discharge, or allowable release rate, qo2, for the 2-year storm, whereas the preproject hydrology for the 15-year storm is based on existing conditions, and will determine the site’s preproject pe”
  8. [8]PennsylvaniaPennsylvaniaCorpus patternpages 334-335
    “ts are summarized in Table 3-75. Determination of ponding time, 1.2-inch/2-hour release rate, and 2-year/24-hour peak rate parameters are demonstrated in Figure 3-80 and Figure 3-81. Ponding time in the MRC SCM for the 10-, 50-, and 100-year/24-hour st”
  9. [9]MaineMaineCorpus patternpages 108-108
    “needed for peak rate control.  The artificially high estimates could result in release rates higher than existing conditions, resulting in adverse effects downstream.  The method assumes that the rainfall intensity is uniform over the entire watershe”
  10. [10]City of SeattleWashingtonCorpus patternpages 90-90
    “Stormwater Code Language References SMC 22.805.080.B.5 a. The post-development release rates shall be limited as follows: 1) The peak flow with a 50 percent annual probability (2-year recurrence flow) shall not exceed 0.07 cubic feet per second per ac”
  11. [11]AlabamaAlabamaCorpus patternpages 208-209
    “expected from a 10-year 24-hour duration storm. Adjustments should be made for release rates from structures and other drainage facilities. Swales shall also be designed to comply with local stormwater ordinances and should be designed for greater cap”
  12. [12]VermontVermontCorpus patternpages 17-17
    “where the water is discharged. 2.2.4.1. Water Quality Peak Flow Calculation The peak rate of discharge for the water quality design storm is needed for the sizing of rate based treatment practices and off-line diversion structures. Conventional Natu”
  13. [13]GeorgiaGeorgiaCorpus patternpages 121-121
    “n 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 adjusted by a local jurisdic- tion fo”
  14. [14]Rhode IslandRhode IslandCorpus patternpages 59-59
    “ation (see example sizing calculations in Appendix D). To determine the average release rate, use the following equation: Average release rate = Vr / T where Vr = defined above; and T = extended detention time (24 hours). The CPv criterion can be waive”
  15. [15]City of FlagstaffArizonaCorpus patternpages 28-28
    “tical points for the storm runoff events and frequencies required. 11. Volumes, release rates, and locations for detention storage facilities and information on outlet works. This shall include design drawings, consisting of plan views, cross-sections,”
  16. [16]WashingtonWashingtonCorpus patternpages 1297-1298
    “and to such water pollution control struc- tures. Overflow rate Detention basin release rate divided by the surface area of the basin. It can be thought of as an average flow rate through the basin. Overtopping To flow over the limits of a containment”
  17. [17]MinnesotaMinnesotaCorpus patternpages 841-841
    “0 or equivalent to route the 2-year storm through the Vcp orifice. Check 2-year peak rate of discharge. If discharge is equal to or less than pre-development discharge rates, then outlet sizing is complete. If not, then raise the elevation of the Vp”

Supporting jurisdictions

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

dc-guidebookpa-statemaine-manualseattle-waalabama-handbookvt-statega-stateri-stateflagstaff-azwa-state-westernmn-state-2008