FLOOD00540 - Laserfiche WebLink

Home Browse Search

The geographical areas to whicl1 tt1ese publications apply include NWS Hydro 35, TP 40 and TP 49 for the 35 states east of the l'locky Mountain area which are essentially free of significant orographic effects. The particulal' pulJlications to be used depends on the storm duration under examination. The 13 rnountair states containing the Rocky Mountains and those areaS, to the west are covered by NOAA Atlas 2 and other sile- specific publications. The procedures described in all tile NWS publications are based on statistical evaluations of long-term rainlall-gage records in a rWlion. These evaluations include estimates of the frequency of accumulated rainfall-depttl versus storm duration at each rain gage. Rainfall maps were macle frorn these depth-duration values, and isopluvial lines were drawn on these maps to define constant rainfall-depth relationships througrl a region for a specific storm duration. Each of the NWS publications gives a detailed discussion of the derivation of the rainfa.lI-frequency dept~-duration relationships. It is strongly recommended that users of NWS data thoroughly read the pertinent sections and be familiar with the applications and limitations of Ute NWS material. A hypothetical storm developed from NWS data is also referred to as a "ba,anced storm," because a consistent depth-frequency relation is used for each peak duration interval of the storm. That's, for a hypothetical 1 OO-year return period 48-hour-cluration storm, the rainfall depths for the peak :lO-minute, 1.hour, i)-hour, 24-hour or other peak period would each be equal to the 100-year depth for tllat duration. This consistent frequency-depth-duration relationship throughout a storm would not occur in nature, because of the randomness of the rainfall events. The balanced storm concept, however, does allow for logical construction and arrangement of a storm event for a particular return period. Balanced storms are discussed in more detail later in this section. The Probable Maximum Precipitation Storm WMF') is cleveloped primarily from NOAA and NWS criteria. The area east of the 105t11 meridian is covered by HMR 51 and HMR 52. For areas between the 103rd meridian anlj the continental divide, HMR 55A is used. HM R 49 covers the Colorado Ftiver basin and the Great Basin areas. Special studies for site-specific areaS are prepared by the NWS upon request. For example HMR :l:l was prepared for the desdgn of Chatiield Fleservoir and covers the upper South P-a~e River Basin. 3. HYPOTHETICAL-FRECIUENCY-STORM DERIVATION. Development of a storm from NWS clata is straighl:forwarcl and systematic. Hie individual performing the study must: 1) establish Ute appropriate storrn duration and the time interval for subdividin9 the storm ra.infall, 2) extract the rainfall values from I\IWS publications for his area of interest, 3) make adjust:l1ents to the rainfall depth for size 01 drainage area if needed, 4) adjust for partial to annual sE!ries (if required). 5) compute incremental rainfall amounts, and 6) arrange tile storm rainfall increments in tirne. E:acJI of these steps is described In the following paragraplls. 3.1, STORM DURt.TION. Before corstructlng any hypothetical event (inclu,jing the SPS and PMS), one must estimate two storm parameters: total duration and ':irne interval for each rainfall increment. BOul parameters must reflect ttle type and size of Hie drainage areaS being examined, the type of bas.in features one intends to analyze. and Hie Colorado Flood Hydrology Manual lJRill'T 7.2