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<br /> <br />Hydrology <br /> <br />I <br />I <br />I <br />, <br />I <br />I <br />I <br />'t <br />I <br />,I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br /> <br />. The duration of rainfall, particularly the length of time that sto <br />form or remain over a given geographic area <br /> <br /> <br />· The areal extent of rainfall (This factor is considered to a lesser <br />than the previous two factors.) <br /> <br />· Radar processing and interpretation (The Z-R relationship used y the <br />NWS [National Weather Service] to convert NWS radar reflecti 'ty to <br />rainfall intensity is not constant in the spatial domain. Differe t Z-R <br />relationships may be correct for the same evening, for different p s of <br />the state, or for different storm types. An example of this occurred n the <br />evening of July 28, 1997. Several storm cells developed across Col rado <br />on that evening, one over Fort Collins [killing 5 people] and oth rs in <br />eastern Colorado. The single Z-R relationship used to estimate r infall <br />intensity underestimated the rainfall in Fort Collins, but overes' ated <br />rainfall to the east.) <br /> <br />The following are the most important factors in assessing flooding po ential <br />(from a hydrological perspective): <br /> <br />· Soil moisture levels in the watershed (How much rainfall <br />watershed experienced prior to the current storm event?) <br /> <br /> <br />· Hydraulic aspects of the watershed (How many canals and gates re in <br />the watershed, and what is their operational status? How much w ter is <br />currently flowing in the canals and streams, and how much flow ca acity <br />remains within the banks? How much storage capacity remains av ilable <br />in retention basins?) <br /> <br />· The occurrence of man-made structures transecting watershed and <br />causing obstructions to natural drainage paths (The majority of d ge <br />across the state occurred at roads and road embankments where verts <br />were too small to handle flow and debris, at railroads and road <br />embankments [again, culverts were too small], at irrigation canal that <br />lacked adequate drainage by-passes or enough conveyance for coll . g <br />overland flows, and at bridges that underwent damage to piers, p . gs, <br />or approaches. Debris was a major flow-limiting factor or a so ce of <br />structural impacts at many locations.) <br /> <br />· Urbanization (Sheet flooding in highly urbanized areas can pr duce <br />heavy damage, and development of low-lying areas can exacerbate flood <br />impacts. ) <br /> <br />d479/chap1.doc 08/12/98 <br /> <br />1-6 <br />