BOARD01805

• Board Meeting Date: 5/23/2005
• Description: Flood Section - 2005 Flood Update
• Board Meetings - Doc Type: Memo

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. Link to FEMA's Spring Flood Center
. Link to Urban Drainage and Flood Control Center's ALERT website (flood warning system for the .
Denver metropolitan area)
. Information on the use of sandbags to protect property
. Link to FEMA's Map Service Center to obtain Flood Insurance Rate Maps online
. Information regarding the Flood Mitigation Assistance (FMA) Program

Please view the Flood Section's website link at htto://cwcb.state.co.us/Flood Intro.htm for additional information
regarding online service.

E. Basinwide Summaries

Starting in mid-March 2005, staff has been monitoring snowpack in Colorado watersheds, as measured by inches of
Snow-Water Equivalent (SWE) and % of 30-year average of SWE. SWE is measured daily at the NRCS automated
SNOTEL sites, and reports are posted on the Intemet. The inches of SWE measured at the SNOTELs represent the
total amount of water equivalent stored as snow in the portion of a particular watershed represented geographically
by each SNOTEL. The % of average SWE represents the amount of water stored as snow in the portion of a
watershed represented by each SNOTEL as compared to a 30-vear average amount for that watershed. Both the
absolute number (inches of SWE) and the relative number (% of average) are needed to paint a comprehensive
picture of conditions in the state's watersheds.

Staff has monitored these numbers on a frequency ranging from every four or five days to every day, depending on
weather conditions in the mountains. Approximately once a week the SWE numbers have been assembled into
tables and incorporated into the Weekly Snowmelt Flood Updates, described in Subsection B of this memorandum.
Staff has been using a threshold value of 120% of average SWE to define a moderate risk oLsnowmelt flooding, a .
next threshold value of 140% of average SWE to define a high risk of snowmelt flooding, and an upper threshold
value of 160% of average SWE to define an extremelv high risk of snowmelt flooding. The table below lists the
total number of SNOTELs in each of Colorado's major river basins and the number of those SNOTELs with
preliminary readings on May 12,2005 that fel! into each of those risk categories.

Rive~Basffi : ' .
..:". '.'.: " '.
Gunnison River
UDDer Colorado River
South PlaUe River
Laramie and North Platte
Rivers
YamDa and White Rivers
Arkansas River
Upper Rio Grande
San Miguel, Dolores,
Animas and San Juan Rivers

.....

Total. #. of .,
SNOTELS., .
12
27
15
13

. #ofSNOTELs with.:'
1206;.< SWE < 140% .
2
3
3
2

#ofSNOTELs with
140%<SWE < 160%'
6
1

#. of SNOTELs with
i60% <'SWE
3
3
1

17
5
10
14

1
1
1
1

2
3

2
5
6

Clearly three basins (Gunnison River, Upper Rio Grande, and San MigueVDolores/Animas/San Juan Rivers)
have a very high proportion of SNOTELs reporting high relative numbers. Another measure of regional snowmelt
risk is basinwide SWE as a percentage of 30-year average. The list below summarizes that percentage of average,
for May 12,2005, for all 8 major river basins in Colorado:

. Gunnison River
. Upper Colorado River

150%
102%

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Flood Protection. Water Project Planning and Finance. Stream and Lake Protection
Water Supply Protection. Conservation Planning