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199B. Colorado Flood Hazard Mitigation Plan (409; Chapter 2 - Hazard Identification and Evaluation ""_../10.".' 2.1 People and Hazards Relationships between flood hazards and population identify pattems of risk, as shown in figure 2-1. Relationships between pattems of risk and steps taken toward preparedness explain degrees of vulnerability to which various Coloradans are exposed. Such relationships are not new to Colorado. The natural phenomena involved have occurred here long before people settled near them and were impacted by them. Risk grows from the increasingly close associa- tion between natural phenomena and a growing popu- lation. People become vulnerable to hazards when they choose (knowingly or unknowingly) to live near the areas where these extreme events occur. Vulnerability is also related to preparedness. People who prepare for the occurrence of an extreme event are less vulner- able to it than those who do not. The vulnerability of Colorado's population is rooted in a relationship be- tween the occurrences of extreme events, the proxim- ity of people to these occurrences, and the degree to which these people are prepared to cope with these extremes of nature. Today, flood prone areas have been identified in 268 cities and towns and in all of the 63 counties in Colorado. Using information supplied from local units of government, there are estimated to be approxi- mately 250,000 people now living in Colorado's flood- plains. The Colorado Water Conservation Board (CWCB) estimates that approximately 65,000 homes and 15,000 commercial and industrial business struc- tures are located in Colorado's floodplains. The total """"'," ...-.--...-..--, Webster's Definitions Flood: a temporary condition of inundation of normally dry land areas a source of danger to cause to become less harsh or hostile, to make less severe Hazard: Mmgate: value of property, including structures and contents, exposed to the 1 OO-year flood in Colorado is estimated to be over $11 billion dollars. Cumulative flood losses from the tum of the century to 1993 from the state's most damaging floods are $3.3 billion (1995 dollars). 2.2 Types of Hazards 2.2.1 Floods Floods in Colorado occur on "riverine" systems con- sisting of a basin (or watershed) and a hierarchical order of stream channels which convey the normal flow of water through the watershed. The area adja- cent to the channel is the floodplain. Flooding results when the flow of water is greaier than the normal carrying capacity of the stream channel. Rate of rise, magnitude (or peak discharge), duration and fre- quency of floods are a function of specific physio- graphic characteristics. Generally the rise in water surface elevation is quite rapid on small (and steep gradient) streams and slow in large (and flat sloped) streams. CountieslCitieslT owns with Flood Prone Areas Population of 100-Year Floodplain Homes in 100-Year Floodplain Commercialllndustrial Businesses in 100-year Floodplain Total Value of Property in 100-Year Floodplain Cumulative Flood Losses from Turn of Century to 1997 268 250,000 65,000 15,000 $11 Billion $ 3.3 Billion Soun:e: ewes FIgure 2.1 H'.'-"V' ,'"v~, FlIe:FHMP-ChapfN-2.pub ChaptBr 2 . 1 112911810.12 AM