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<br />flood warning system is to identify the various sources of flood threat. <br /> <br />Sources can vary from large, sluggish rivers that take days or weeks to crest <br /> <br />to small creeks that crest in minutes. Each watershed has a unique set of <br /> <br />hydrologic characteristics that describe the watershed's response to <br /> <br />rainfall. This response is typically represented hy a hydrograph, which is a <br /> <br />graph of streamflow and/or stage in relation to time at a given location. <br /> <br />Figure 4 illustrates a hydrograph. <br /> <br />As rainfall or snowmelt occurs over the watershed, runoff begins and the <br /> <br />stream rises. The streamflow continues to increase as the rainfall continues <br /> <br />(see rising limb of the hydrograph). <br /> <br />~ <br /> <br />Even after the rainfall or snowmelt ends, the stream will continue to rise in <br /> <br />response to runoff. Eventually the runoff will reach a peak and the stream <br /> <br />will crest. The stream will then begin falling and eventually recede to a <br /> <br />level below flood stage. The potential warning time (Twp) is the maximum lead <br />time available for warning communities of flooding. An effective local flood <br /> <br />warning system should identify the specific areas that will flood, the time at <br /> <br />which flooding is first expected, when the flood is expected to crest, and the <br /> <br />flood crest elevation. <br /> <br />- <br /> <br />Areas subject to damage and evacuation must be identified in the warning <br /> <br />statement. Specific identification will lessen the possibility that the <br />National Weather Service (NWS) flood warning messages -- which give projected <br />flood levels -- may be misunderstood. Both local officials and the general <br /> <br />public need to understand how the projected flood levels relate to their <br /> <br />particular areas of concern. It is vitally important that the NWS forecast <br /> <br />11 <br />