FLOOD11630 ‎(2)‎

• County: Statewide
• Basin: Arkansas
• Title: Post Flood Assessment Report - Arkansas River
• Date: 9/15/1999
• Floodplain - Doc Type: Flood Documentation Report

A warning was issued by the National Weather Service (NWS) with excellent lead-time prior to
the flooding that occurred along the mainstem of the Arkansas River. Depending on the location,
the warning was issued 12 to 24 hours prior to the onset of flooding and up to 4E hours prior to
the onset of major floDding problems. River stage and flood information provided in NWS
warnings and statements provided valuable information to the public. The reliat~ility of the
information provided by the NWS to local agencies and the public ttuoughout the flood event
determined the usefulness of the information. In this regard, crest forecasts were very accurate
making them extremely valuable for emergency planning and flood operations. .~n advance
waming with excellent lead-time was also issued by the NWS prior to the flooding that occurted
along the Purgatoire River. The warning was issued about 12 hours in advance af flooding and
up to 24 hours in advance of major flooding.
Flood Measurement Systems
The flood runoff pr.oduced staaes in the Arkansas River that exceeded the reliable; portion of the
stage-flow rating tables. Peak gage heights on the Purgatoire River ~etween Trinidad and
Thatcher were also exceeded. The staff gage on Fauntain Creek at the Pinon Bricige in Pueblo
County between Manitou Springs and Colorado Springs was lost. This gage was a key part of
the Pueblo early flood warning and response system. Other key gages within ti~e Arkansas River
Basin became inoperative during the event. The United States Geological Survey (USGS) and
the State of Colorado performed field measurements to determine the actual flow. The Colorado
Division of Water Resources was especially helpful providing timely and regular stage data at
several points along the Arkansas where critical river gages had been damaged or destroyed.
This data was essential to the integrity of the river stage forecasts issued by the River Forecast
Center. Nevertheless, as a result of gages being overwhelmed, damaged, or destrayed, flow data
accumulated for this flood eyent may not be as accurate as desired. For this reasom, an accurate
estimate of the frequency of the event is difficult to determine.
T'he peak from Fountain Creek along with tributary contributions combined to create a river staQe
of record on the Arkansas River at the Avondale gauge on May l, 19g9. This pealc went on to ~
cause extensive flooding in La Junta by May 3, 1999. Initial estimates ranked the fl~od on the
Arkansas River as a I S to 30 year event, although some estimates ranged as high as a 40 to 50
year event. The USGS issued provisional estunates of peak flow on the Arkansas River just
downstream of La Junta that ranged from 40,000 cubic feet per second (c.f.s.) to 2'7,900 c.f.s.
The Colorado Water Conservation Boazd (CWCB) has equated this peak dischazge: to be roughly
a 20-year to 30-yeaz event.
The heaviest rain fell in the Colorado Springs area. The runoff entered Monument Creek, which
is a left bank tributary to Fountain Creek just downstream of Manitou Springs. Th~ USGS
issued provisional estimates of peak flow on Fountain Creek at Fountain between 16,040 c.f.s.
and 18,900 c.f.s. The CWCB has equated this peak discharge to equate roughly to a 10-year
event. Despite the fact that larger floods have occurred on Fountain Creek, few have caused such
extensive erosion damage. The high volume of water produced by the flood was a key factar in
causing the unusually high erosion damage along Fountain Creek.
Chapter 3- Flood ofApril2!~, 1999
Post Flood Assessment Report ~ 16 Dra}'t Revised 1)9/09/99