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CHAPTER 11. FLOOD <br />11-11 <br />Source/Location <br />Discharge (cubic feet/second) <br />10-Year 50-Year 100-Year 500-Year <br />Middle Fork Teanaway River at mouth 1,250 1,570 1,700 2,020 <br />West Fork Teanaway River at Mouth 1,300 1,640 1,780 2,080 <br />a. Discharges reflect regulated conditions <br />b. Includes overflow from Yakima River, Reecer, and Currier Creeks <br />c. Includes overflow from Reecer Creek <br /> <br />11.2.8 Warning Time <br />Floods are the number one natural disaster in the United States in terms of loss of life and property. Floods <br />are generally characterized as either slow-rise or flash floods. Slow-rise may be preceded by a warning time <br />from several hours, to days, to possibly weeks. Evacuation and sandbagging for a slow-rise flood may <br />lessen flood damage. Flash floods are the most difficult to prepare for, due to the extremely short warning <br />time, if any is given at all. Flash flood warnings usually require evacuation within an hour. <br />Each watershed has unique qualities that affect its response to rainfall. A hydrograph, which is a graph or <br />chart illustrating stream flow in relation to time (see Figure 11-4), is a useful tool for examining a stream’s <br />response to rainfall. Once rainfall starts falling over a watershed, runoff begins and the stream begins to <br />rise. Water depth in the stream channel (stage of flow) will continue to rise in response to runoff even after <br />rainfall ends. Eventually, the runoff will reach a peak and the stage of flow will crest. It is at this point that <br />the stream stage will remain the most stable, exhibiting little change over time until it begins to fall and <br />eventually subside to a level below flooding stage. <br /> <br />Figure 11-4. Yakima River Hydrograph at Umtanum (USGS Station 12484500) <br />The potential warning time a community has to respond to a flooding threat is a function of the time between <br />the first measurable rainfall and the first occurrence of flooding. The time it takes to recognize a flooding <br />threat reduces the potential warning time to the time that a community has to take actions to protect lives