FLOOD11630 ‎(2)‎

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

CHAPTER 3
FLOOD OF APRIL 29, 1999
INTRODUCTION
The flooding that occurred from Apri129, 1999 through May 5, 1999 was the res~ilt of an
unusually long period of moderate to heavy rainfall over a broad azea of southerri Colorado. Up
to 13 inches of rain was reported west of Interstate 25 and 5 inches to 9 inches of rain in the
eastern plains. A lazge cutoff low-pressure azea that stalled aver the Great Basin caused the
extended period of heavy rain. The large upper low stalled over the Great Basin fbr three days,
from Apri129, 1999 through May 1, 1999. The low-pressure area pulled in south~ast winds up
against the eastern face of the Rocky Mountains, transporting large amounts of m~isture up from
the Gulf of Mexico at tremendous speeds. Wind profilers indicated sustained sou~thheast winds at
40 to 50 knots just off the surface for much of the event. Since the teirain of southeast Colorado
generally rises to the northwest, a strong southeast wind is a strong ugslope wind. The strong
orographic lifting of this strong southeast wind was the primary cause of the intense rainfall that,
occurred during this event. In fact, the azeas that received the heavies~t rainfall coincided with
azeas that create the greatest orographic lifting during southeast wind events. This area runs
along, and adjacent to, the Rocky Mountains from the Pikes Peak regian south to the Wet
1Vlountains. It includes Manitou Springs and Colorado Springs in westem El Paso County and
many small mountain towns in western Pueblo County. These are the areas that r~;ceived the
highest rainfall that led to the extreme flooding downstream. Although local preci.pitation totals,
on the order of 12 to 13 inches, aze not uncommon within 24 hour peri.ods, they generally occur
over smaller geographic azeas. The 1999 event extended over a much lazger area and was not a
localized flash flooding event like the one that occurred in 1997.
Analyses of data provided by the network of cooperators (COOP), the Automated Local
Evaluation in Real Time (ALER`1~, and spotter data after the event in~'icated that t~otal rainfall
from the storm around Manitou Springs and Colorado Springs, in wesi:em El Paso County,
ranged from 10 to 14 inches over an elliptical area parallel to the Rocky Mountain.~. A center of
6 to 10 inch rainfall extended southwazd from this area, along the Wet Mountains in western
Pueblo County. Elsewhere, total rainfall for this event was generally L~a the 3 to 6~nch range
across southeast Colorado. These rainfall figures reflect an extreme event for sout~east
Colorado. Many of the rainfall reports received were indicated to be d~ily or storni total records.
What were extremely unusual.about this event were the duration of the heavy rainf'all and the
time of year that it occurred. Rains of this magnitude occurring for over a day or sm are very
rare. In addition, rainfall of this magnitude normally occurs later in the season; usually in the
late May to late August rime frame. This storm was a rarity in both duration and time of
occurrence.
Chapter 3- Fload ofApri12:9, 1999
Post Flood Assessment Report~ 13 ~raft Revised i~9109199