FLOOD03369

• County: Prowers
• County: Bent
• County: Otero
• Community: Southeastern Colorado
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Post Flood Assessment Report Arkansas River
• Date: 9/15/1999
• Prepared For: State of Colorado
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Flood Documentation Report

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TABLE 3-3

CHANGE IN CHANNEL CONVEYANCE CAPACITY
ARKANSAS RIVER AT LA JUNTA

Period Covered

10/1/63 - 9/30/64
10/1/64 - 6/14/65
6/14/65 - 9/30/65
11/1/67 - 9/30/71
10/1171 - 9/30/80
10/1/80 - 2/28/91
3/1/91 - 11/1997

Gauge Height = 5.5 feet

Q (C.F.S.)
1,200 (est)
1,200
1,230
895
660
320
98

Gauge He:ight = 7.0 feet

Change in Q
N/A
o
30
-305
-540
-880
-1,102

Change in Q
N/A
o
o
o
-510
-1,840
-2,970

The elevation of incorporated La Junta on the south overbank area of the Arkansas River is about
twenty feet higher in elevation than North La Junta. Nevertheless, the City of La Junta is still
subject to frequent flooding along U.S. Highway 50 (First Street) near Brandish A venue as a
result of stormdrain backup. Drainage from the Anderson Arroyo on the west and King Arroyo
on the east have begun to face near adverse northward gradients into the Arkansas River as a
result of aggradation in the streambed. The First Street Stormdrain running eastward also backs
up to pond along the highway when the flap gates shut as a result of Arkansas River flow and
aggradation related ponding. The Fifth Street Storm drain, further south of First Street, also runs
eastward and enters King Arroyo; however surface flows northward along the streets by pass the
stormdrain and contribute to flooding along the highway. The Santa Fl: Street Stormdrain runs
northward into the First Street Stormdrain and further contributes to interior flooding along the
highway. Commercial and residential properties are often affected by this interior drainage
problem. Fortunately the arroyos and local drainage supplied little flow during this event. Flood
water depths still exceeded three to four feet along First Street and Second Street near their
intersections with Brandish and Harriet Streets just west of King Arroyo.

The Arkansas River crested on May 2, 1999. According to a USGS estimate, the peak flow was
approximately 40,000 c.f.s. just downstream of La Junta. The spoil bank levee performed well
until the levee was overtopped. The overtopping flows created breaches at four points along tne
levee. The county worked around the clock to repair the breaches and cutoff the floodwaters.
The entire levee upstream ofthe State Route 109 (Main Street) Bridge was overtopped. Over
200 homes and commercial properties were flooded. As many as 300 residents were evacuated
from their homes. Floodwaters from the Arkansas River had also entered the Fort Lyons
irrigation channel in North La Junta. The channel subsequently breached and flooded North La
Junta. It was unclear whether the irrigation channel breached prior to the Corps levee being
overtopped; however, a local newspaper account did indicate that this was the case. Water from

Post Flood Assessment Report

21

Chapter 3 - Flood of April 29, 1999
Dmft Revised 09/09/99