Need for new rainfall intensity atlas analyses in the West.

Estimates of short-duration rainfall intensities provide critical information for the design of highway drainage systems. Rapid growth of urban population in the arid west has exposed more drivers to the effects of undersized drainage structures. Contemporary stormwater computations on small, semiarid or urban watersheds require the estimation of rainfall intensities at ten-minute intervals in runoff-producing storms. Earlier methods, such as the rational formula, required less complex rain data. Precipitation-frequency atlases (noaa-2) for eleven western states were prepared from data prior to 1970.These atlases used daily rainfall observations, regression against topographic parameters, and hydrometeorological considerations to draw isohyetal maps for 24- and 6-hour intervals. The 24-hour maxima may follow topographic features because they usually occur in winter.Visual observation in arizona does not confirm this to be true with summer convective storms. These produce design floods on semiarid watersheds smaller than 100 or 200 square miles. Regional variation of short-duration rainfall may be less sensitive to desert mountains than the 24-your maps developed in noaa-2. The shortest duration analysed for noaa-2 was one hour, used at about 10% of the sites. For example, in arizona only 38 autographic recorders were used, over periods averaging less than 20 years. Re-analysis in that state could consider recording gages from over 100 additional sites, for durations as short as 10 minutes. Two decades of new data at noaa-2 sites could now reduce the error of estimate at these points. Improved statistical methods are also available. Highway and other engineers in ten western states would gain from new analyses of short-duration intensity data available from recording rain gages operated by many agencies. This paper attempts to serve administrators, drainage engineers, and others who may consider the implementation of re-studies in a western state. The 16-year-old western-rainfall intensity atlas should be replaced after the short-duration recorder data now available have been analysed.