Laserfiche WebLink
<br />CH^PTER 3. STNlD^RI) PROJECT Fl')()I)S <br /> <br />Section 3.01. Oefinition <br /> <br />The standard project flood represents the flood that can be ex~ <br />pected from the most severe combination of meteorologic and hydrolo- <br />, <br />gic conditions that are considered reasonably characteristic of the <br />geographic region involved, excluding extremely rare combinations. <br />It is usually computed by examinin~ all of the major storms that have <br />occurred in the region and selecting a storm Magnitude and pattern that <br />is as severe as any of the transposed storms, vlith the possible excep- <br />tion of any storm or storms that are exceptionally larger than others <br />and are considered to be extremely rare events. In the case of snow- <br />melt floods, it is ordinarily the flood that would result from the most <br />reasonably severe combination of snowpack, rainfall and sno\~elt factors. <br />~round conditions in either type of flood should be reasonably condu- <br />cive to high runoff, hut not necessarily the most extreme observed. <br />Standard project flood estimated completed to date in the United States <br />indicate that SPF discharge on detailed studies usually are about 40 <br />to 60 percent of the probable maximum flood for the same hasin. (See <br />Chapter 4.) The standard project flood is intended as a practicable <br />expression of the degree of protection that should be sought as a gen- <br />eral rule in the design of flood control works for communities where <br />protection of human life and unusuallY high-valued property is in- <br />volved. The SPF procedure is used in lieu of the discharge-frequency <br />approach because of the unre1iabi1ity inherent in estimating large mag- <br />nitude infrequent events from short record. or even regional, discharge <br />frequency analyses. <br /> <br />Section 3.02. Storm severity <br /> <br />Storm rainfall severity is not simply a function of the average <br />total rainfall for a drainage basin, but is a function of the area and <br /> <br />3-01 <br />