An Introduction to Rainfall Management for Professional Engineers

The "design storm" is the estimate of the rainfall amount and its distribution over a particular drainage basin, with respect to both time and area which is used in the development of a PMF hydrograph or; in some instances, a flood hydrograph representing a specific frequency event. The purpose of this chapter is to provide the Bureau hydrologic engineer a basic familiarity with the atmospheric processes that produce extreme precipitation events, and the methods by which these processes are used in computing individual drainage PMS (Probable Maximum Storm) values and in developing regionalized criteria used to compute such values for particular drainage basins. In 1981, the Bureau, NWS, and COE adopted a mutually acceptable, uniform definition of the widely used term PMP (Probable Maximum Precipitation) as "theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year." Complete technical application of this definition results primarily in a storm isohyetal pattern; isohyets are lines of equal precipitation that are analogous to contour lines on a topographic map. This pattern is overlaid and critically centered on a drawing of the drainage basin outline, and then the average precipitation is computed. The average basin precipitation is then distributed over time, which results in incremental basin average precipitation values that are used to generate the PMF hydrograph. The average basin precipitation is highly dependent on the total understanding of atmospheric processes as to the cause of severe event precipitation and the analysis of accumulated basic storm data. As hydrometeorologists expand their knowledge of severe storm meteorology, future revisions to present PMP estimates can be expected. However, at least for the conterminous United States, only minimum modification to current values of PMP is expected in the foreseeable future because knowledge of severe storm phenomena has reached a plateau.