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<br />Awarning was issued by the National Weather Service (NWS) with excellent lead-time prior to <br />the flooding that occurred along the mainstem of the Arkansas River. Depending on the location, <br />the warning was issued 12 to 24 hours prior to the onset of flooding and up to 48 hours prior to <br />the onset of major flooding problems. River stage and flood information provided in NWS <br />warnings and statements provided valuable information to the public. The reliability of the <br />information provided by the NWS to local agencies and the public throughout the flood event <br />determined the usefulness of the information. In this regard, crest forecasts were very accurate <br />making them extremely valuable for emergency planning and flood operations. An advance <br />warning with excellent lead-time was also issued by the NWS prior to the flooding that occurred <br />along the Purgatoire River. The warning was issued about 12 hours in advance of flooding and <br />up to 24 hours in advance of major flooding. <br /> <br />Flood Measurement Systems <br /> <br />The flood runoff produced stages in the Arkansas River that exceeded the reliable portion of the <br />stage-flow rating tables. Peak gage heights on the Purgatoire River between Trinidad and <br />Thatcher were also exceeded. The staff gage on Fountain Creek at the Pinon Bridge in Pueblo <br />County between Manitou Springs and Colorado Springs was lost. This gage was a key part of <br />the Pueblo early flood warning and response system. Other key gages within the Arkansas River <br />Basin became inoperative during the event. The United States Geological Survey (USGS) and <br />the State of Colorado performed fIeld measurements to determine the actual flow. The Colorado <br />Division of Water Resources was especially helpful providing timely and regular stage data at <br />several points along the Arkansas where critical river gages had been damaged or destroyed. <br />This data was essential to the integrity of the river stage forecasts issued by the River Forecast <br />Center. Nevertheless, as a result of gages being overwhelmed, damaged, or destroyed, flow data <br />accumulated for this flood event may not be as accurate as desired. For this reason, an accurate <br />estimate of the frequency of the event is diffIcult to determine. <br /> <br />The peak from Fountain Creek along with tributary contributions combined to create a river stage <br />of record on the Arkansas River at the Avondale gauge on May 1, 1999. This peak went on to <br />cause extensive flooding in La Junta by May 3, 1999. Initial estimates ranked the flood on the <br />Arkansas River as a 15 to 30 year event, although some estimates ranged as high as a 40 to 50 <br />year event. The USGS issued provisional estimates of peak flow on the Arkansas River just <br />downstream of La Junta that ranged from 40,000 cubic feet per second (c.f.s.) to 27,900 c.f.s. <br />The Colorado Water Conservation Board (CWCB) has equated this peak discharge to be roughly <br />a 20-year to 30-year event. <br /> <br />The heaviest rain fell in the Colorado Springs area. The runoff entered Monument Creek, which <br />is a left bank tributary to Fountain Creek just downstream of Manitou Springs. The USGS <br />issued provisional estimates of peak flow on Fountain Creek at Fountain between 26,000 c.f.s. <br />and 18,900 c.f.s. The CWCB has equated this peak discharge to equate roughly to a 10-year <br />event. Despite the fact that larger floods have occurred on Fountain Creek, few have caused such <br />extensive erosion damage. The high volume of water produced by the flood was a key factor in <br />causing the unusually high erosion damage along Fountain Creek. <br /> <br />Post Flood Assessment Report <br /> <br />16 <br /> <br />Chapter 3 - Flood of April 29, 1999 <br />Draft Revised 09/09/99 <br />