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<br />r <br /> <br />Introduction <br /> <br />. <br /> <br />The duration of rainfall, particularly the length of time that storm cells <br />form or remain over a given geographic area <br /> <br />. <br /> <br />The areal extent of rainfall (This factor is considered to a lesser degree <br />than the previous two factors.) <br /> <br />. <br /> <br />Radar processing and interpretation (The Z-R relationship used by the <br />NWS [National Weather Service] to convert NWS radar reflectivity to <br />rainfall intensity is not constant in the spatial domain. Different Z-R <br />relationships may be correct for the same evening, for different parts of <br />the state, or for different storm types. An example of this occurred on the <br />evening of July 28, 1997. Several storm cells developed across Colorado <br />on that evening, one over Fort Collins [killing 5 people] and others in <br />eastern Colorado. The single Z-R relationship used to estimate rainfall <br />intensity underestimated the rainfall in Fort Collins, but overestimated <br />rainfall to the east.) <br /> <br />1.5.2 Hydrology <br /> <br />The following are the most important factors in assessing flooding potential <br />(from a hydrological perspective): <br /> <br />. Soil moisture levels in the watershed (How much rainfall has the <br />watershed experienced prior to the current storm event?) <br /> <br />. Hydraulic aspects of the watershed (How many canals and gates are in <br />the watershed, and what is their operational status? How much water is <br />currently flowing in the canals and streams, and how much flow capacity <br />remains within the banks? How much storage capacity remains available <br />in retention basins?) <br /> <br />. The occurrence of man-made structures transecting watersheds and <br />causing obstructions to natural drainage paths (The majority of damage <br />across the state occurred at roads and road embankments where culverts <br />were too small to handle flow and debris, at railroads and railroad <br />embankments [again, culverts were too small], at irrigation canals that <br />lacked adequate drainage by-passes or enough conveyance for collecting <br />overland flows, and at bridges that underwent damage to piers, pilings, <br />or approaches. Debris was a major flow-limiting factor or a source of <br />structural impacts at many locations.) <br /> <br />. Urbanization (Sheet flooding in highly urbanized areas can produce <br />heavy damage, and development of low-lying areas can exacerbate flood <br />impacts.) <br /> <br />d479/chapl.doc 08/12/9B <br /> <br />1-6 <br />