History and Evolution of the PMP/PMF

1. History and Evolution of the PMP/PMF David F. Kibler and Michael Bliss Civil and Environmental Engineering Virginia Tech

2. PMP and PMF Definitions • PMP – Probable Maximum Precipitation: “theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographic location during a certain time of year” • PMF – Probable Maximum Flood: “the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the drainage basin under study” Source: FEMA Interagency Committee on Dam Safety, Oct., 1998

3. 1900 2000 1950 PMP Development Timeline 1970s-Present: PMP is formalized and becomes standard of practice 1920 -1950: Flood prediction linked to storm precipitation 1910 – 1940: Hydrology methods begin to emerge Time Periods 1953: Yarnell publishes rainfall intensity charts for U.S. Pre-1900: No consistent data from U.S. Weather Bureau – used “worst known flood” Key Events 1978: HMR 51,52 is published; PMP east of 105th Meridian 1950: Creager and Justin develop “Creager” envelop curve equation 1961: Tech. Paper 40 links storm recurrence to depth of precipitation By 1930: Formal statistical methods are applied to hydrologic problems 1942: C.S. Jarvis develops “Myers” envelope curve equation

4. Popular Maximum DischargeFormulas of the 1940-60s • Myers Equation (C.S. Jarvis, 1942) • Q = 10,000A0.5 • where A = drainage area, sq. miles for watersheds • larger than 4 sq. miles • Q = ultimate maximum flood flow (cfs) • Creager Equation (Creager and Justin, 1950) • Q = 46CA(0.894A-0.048) • where C = coefficient with maximum value of 100 for many areas

5. Creager Envelop Curve

6. PMP Timeline (cont.) 1998: FEMA publishes procedure for selecting inflow design floods – introduces “acceptable incremental consequences” for selecting design floods By 1970: All federal agencies use PMP estimation for spillway design 1970 1990 2000 1980 1988: ASCE Task Committee proposed 3 hazard categories: Cat 1 (high) = PMP Cat 2 (medium) = PMF or smaller base on risk analysisCat 3 (low) = Q10 – Q100 1985:NRC’s Dam Safety criteria stated it would be acceptable for non-PMP design on certain projects 1973: ASCE Task Committee proposes an alternative approach to PMP – economic analysis of risk 1986: FEMA workgroup favored PMP, but gave guidance that Federal Agencies could develop agency specific criteria 1979: Published Fed. Dam Safety guidelines acknowledged risk analysis, but favored PMP methods

7. PMP vs. Extreme Rainfall Events Source: “PMPs Never Happen – or Do They?” (Harrison, 2002)

8. Do actual observed rainfalls ever approach the PMP? • The Mid-Atlantic Region has experienced three of the top five most intense 12-hr storms in US. • At Smethport, PA (1942), the PMP was exceeded by 19%. • The other two occurred in VA and approached 81 and 86% (Madison County, 1995) of PMP for areas less than 10 mi2. • See Fig. 5, Harrison, 2002.

9. Heaviest 12-Hr Rainfalls Source: “PMPs Never Happen – or Do They?” (Harrison, 2002)

10. Do actual floods ever approach the PMF? • Harrison and Paxson have developed Fig. 6 for floods in the Northern Appalachian area (PA). • The PMFs lie above the envelop curve for maximum observed floods in PA. • See Fig. 6

11. Northern Appalachian Region Source: “Ballpark PMFs” (Harrison, 2004)

12. Central Appalachian Region • However, for the Central Appalachian region, the PMF curve is very close to the envelop line representing the most severe observed (historical) floods on record for the area • See Fig. 7 from Harrison and Paxson, 2004

13. Central Appalachian Region Source: “Ballpark PMFs” (Harrison, 2004)

14. Recent PMF Relationships • Using data from PA, Harrison and Paxson (2002) developed statistical relationships for the PMF, the best of which is: Q = 8148A0.69 where Q = est. PMF in cfs; A = drainage area, sq mi • Looks a lot like the old Myers equation -- we have come full-circle! • But -- statistical PMF relations intended only for checking purposes in areas where there is little hydrologic data available. See Fig 7. Harrison and Paxson.

15. PMF vs. Drainage Areas Source: “Ballpark PMFs” (Harrison, 2003)

16. Summary and Current Status of PMP/PMF • The PMP and PMF concepts have been with us for at least 50 years and perhaps longer. • The PMF has been recognized as a standard of practice since about 1970. • We have experienced near-PMPs in mid-Atlantic states. • However, there remain uncertainties regarding the estimation of the PMP, its time distribution and its probability of occurrence.

17. Summary and Current Status of PMP/PMF (cont.) • PMFs in the southern mid-Atlantic are seen to approach the envelop of observed maximum (historical) floods. • Nevertheless, questions remain about the PMF, because it is a hydrologic estimate based on the PMP and the saturation condition of the watershed.

18. Summary and Current Status of PMP/PMF (cont) • Because it has almost zero exceedance probability, the PMF is difficult to fit into current-day risk analysis • The use of fractional parts of the PMF has led to an inconsistent standard across US • Other approaches have been adopted, such as the use of “incremental analysis”