FLOOD08745

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Colorado Flood Hazard Mitigation Plan
• Date: 12/1/1999
• Prepared For: State of Colorado
• Prepared By: Kistner and Associates
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

I

I

I

I

I
I
I

I
I

I

I

I

I

I

I

I
I
I
I

Colorado Flood Hazard Mitigation Plan -1999

rain and snow storms are generally a resu~ of moist air
masses which originate in the cooler northern Pacific
Ocean and move inland across the Pacific Northwest.

Floods caused by failure of man-made structures are a
resu~ of:

. Hydrologic deficiencies

. Structural deficiencies

. Improper Operation or Sabotage (1 case in CO)

Each of these causes resu~s in floods that have dis-
tinct characteristics relatve to rate of rise. volume, du-
raton, and flood season.

2.2.1.1 General Rain Floods
General rain floods can result from moderate to heavy
rainfall occurring eNer a wide geographic area lasting
several days, They are characterized by a slow steady
rise in stream stage and a peak flood of long duration.
As various minor streams empty into larger and larger
channels, the peak discharge on the mainstream chan-
nel may progress upstream or downstream (or remain
stationary) over a considerable length of river,

General rain floods can result in considerably large
volumes of water, The general rain flood season is

historically trom the beginning of May through October.
Because the rate of rise is slow and the tme available
for warning is great. few lives are usually lost. but mil-
lions of dollars in valuable public and private property
are at risk.

The October 5. 1911, floods in Pagosa Springs and
Durango were a result of a general rain system eNer
tributaries of the San Juan River Basin in southwestern
Colorado. The June 3. 1921, flood in Pueblo was a
result of a general rain system in the Upper Arkansas
River Basin, The damaging floods of June 1965 in the
Denver-metro area were a resu~ of heavy to torrential
rainfall over large portions of the South Platte River
Basin that lasted several days.

2.2.1.2 Thunderstorm Floods

Damaging thunderstorm floods are caused by intense
rain over basins of relatively small area They are
characterized by a sudden rise in stream level. short
duration, and a relatively small volume of runoff, Be-
cause there is little or no warning tme. the term 'f1ash
flood" is often used to describe thunderstorm floods,
The average number of thunderstorm days per year in
Colorado varies from less than 40 near the western

..-

~
~
I

Chapter 2 - 3

ll1ro5:3DPM