Laserfiche WebLink
<br />e <br /> <br />. <br /> <br />,e <br /> <br />. <br />i <br /> <br />e <br /> <br />CHAPTER 4 <br /> <br />HYPOTHETICAL PRECIPITATION DATA <br /> <br />4,1 <br /> <br />Introduction <br /> <br />HYpothetical rainfall data are used when rain gage data are not <br />available or when records are too short to develop rainfall-frequency <br />relationships. Hypothetical storms are also used as a basis of design for <br />projects which pose a threat to property or would result in loss Df life. <br />General use of these data is discussed in this chapter, Detailed guidelines <br />for hypothetical storm development are given in the Appendix, <br /> <br />Sources of storm Data, The primary sources of hypothetical storm <br />information for the United states are various technical publications (TP) of <br />the NatiDnal Weather service (~S) and the hydrometeorological reports (HR) <br />of the National Oceanic and Atmospheric Administration (NOAA) (NOAA, 1973; <br />I\WS, 1961; for example), The development of hypothetical-frequency storms <br />from data given in these publications is based on generalized isopluvial <br />rainfall maps and regression equations. Other methods, such as statistical <br />analysis of nearby long-record rain gages to derive the hypothetical storms <br />of particular frequencies, are used extensively in some parts of the United <br />States but are not discussed here, <br /> <br />The United States is divided into two major geographical areas based on <br />type of precipitation. The 35 states east of the Rocky Mountain area are <br />essentially free of significant Drographic effects and thus are included in <br />one region. The 13 mountain states (those containing the Rocky Mountains and <br />states to the west) are covered on a state-by-state basis as well as in <br />site-specific publications. Similar data are available for Alaska and <br />Hawaii. The NWS procedures are based on statistical evaluations of long-term <br />rainfall-gage records in a region. These evaluations include estimates of <br />the frequency of accumulated rainfall-depth versus storm duration at each <br />rain gage. Rainfall maps made from these depth-duration values were used to <br />define isopluvial lines on the maps to produce consistant rainfall-depth <br />relationships through B region for specific storm durations. A typical <br />isopluvial map is shown in Figure 4.1. Derivation of the rainfall-frequency <br />depth-duration relationships is detailed in each of the publications. <br /> <br />17 <br />