FLOOD03304 - Laserfiche WebLink

Home Browse Search

The geographical areas to which these publications apply include NWS Hydro 35, TP 40 and TP 49 for the 35 states east of the Rocky Mountain area which are essentially free of signnicant orographic effects, The particular publications to be used depends on the storm duration under examination, The 13 mountain states containing the Rocky Mountains and those areas to the west are covered by NOAA Atlas 2 and other site- specnic publications, The procedures described in all the NWS publications are based on statistical evaluations of long-term raintall-gage records in a region, These evaluations include estimates of the frequency of accumulated rainfall-depth versus storm duration at each rain gage, Rainfall maps were made from these depth-duration values, and isopluvial lines were drawn on these maps to define constant rainfall-depth relationships through a region for a specnic storm duration, Each of the NWS publications gives a detailed discussion of the derivation of the rainfall-frequency depth-duration relationships, It is strongly recommended that users of NWS data thoroughly read the pertinent sections and be familiar wnh the applications and Iimnations of the NWS material. A hypothetical storm developed from NWS data is also referred to as a "balanced storm," because a consistent depth-frequency relation is used for each peak duration interval of the storm, That is, for a hypothetical 100-year retum period 48-hour-duration storm, the rainfall depths for the peak 30-minute, 1-hour, 6-hour, 24-hour or other peak period would each be equal to the 100-year depth for that 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 (PMP) is developed primarily from NOAA and NWS criteria, The area east of the 105th meridian is covered by HMR 51 and HMR 52, For areas between the 103rd meridian and the continental divide, HMR 55A is used, HMR 49 covers the Colorado River basin and the Great Basin areas, Special studies for sne-specnic areas are prepared by the NWS upon request. For example HMR 33 was prepared for the design of Chatfield Reservoir and covers the upper South Plalle River Basin, 3. HYPOTHETICAL-FREQUENCY-STORM DERIVATION. Development of a storm from NWS data is straightforward and systematic, The individual performing the study must: 1) establish the appropriate storm duration and the time interval for subdividing the storm rainfall, 2) extract the rainfall values from NWS publications for his area of interest, 3) make adjustments to the rainfall depth for size of drainage area if needed, 4) adjust for partial to annual series (if required), 5) compute incremental rainfall amounts, and 6) arrange the storm rainfall increments in time, Each of these steps is described in the following paragraphs, . 3.1. STORM DURATION. Before constructing any hypothetical event (including the SPS and PMS), one must estimate two storm parameters: total duration and time interval for each rainfall increment. Both parameters must reflect the type and size of the drainage areas being examined, the type of basin features one intends to analyze, and the Colorado Flood Hydrology Manual DRAFI' 7,2