§ 2.6.2. Probable Maximum Flood (PMF)

The PMF is calculated by obtaining the PMP for a specific storm duration and a specific drainage area. To determine the PMF, each of the possible storm durations (1, 2, 3, 6, 12, 24, 48, and 72 hour storms) needs to be analyzed in order to determine the critical duration. The critical duration is the storm duration that produces the highest water surface elevation behind the dam. The PMF for each storm duration is derived using the PMP depths from the TCEQ PMP tool (see 2.6.1) and using a rainfall-runoff model (i.e. HEC-HMS, TR-20).

The Soil Conservation Service (SCS) Type III distribution must not be used for PMP analyses. Instead, the rainfall-runoff model should use the temporal distribution as provided in the Hydrologic and Hydraulic Guidelines for Dams in Texas. The temporal distribution for each storm duration has been reproduced in Figure 2-4, Appendix D. Figure 2-4 provides the temporal distribution ordinates to be multiplied by the associated storm depths for use in the various rainfall-runoff models. The runoff parameters used in the PMF model are the same as those used for runoff analyses of the more frequent storm events, with the exception of curve numbers and the temporal distribution of rainfall.

Runoff curve numbers for the PMF need to reflect the assumption that the soils will be saturated. Therefore the runoff curve number should be based on ARC III. The appropriate curve number should be chosen using the tables provided in the DCM Section 2.5.2. These are ARC II values which can be converted to ARC III values using Table 10.1 in Part 630, Chapter 10 of the National Engineering Handbook. Note that the ARC was previously referred to as the Antecedent Moisture Condition (AMC) in older NRCS publications.

Source: Rule No. 161-19.01 , 3-14-19.