FLOOD08135

• Designation Number: 256
• County: La Plata
• Community: Unincorporated La Plata County
• Stream Name: Pine River
• Basin: San Juan/Dolores
• Title: Floodplain/Floodway Study, Pine River
• Date: 6/1/1986
• Designation Date: 9/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

1

miles long. The headwaters of the drainage is at elevation 13,9~0
feet so the average slope of the river is 200 feet of drop per mile.
The slope is actually much steeper in the upper half of the drainage
and flatter in the lower half.
The drainage area is separated into two major subbasins which
are the Pine River and Vallecito Creek; the size of the two
subbasins are about 150 and 100 square miles repectively.
The vegetation in the drainage area is alpine trees, brush,
grasses. The area is well vegetated and a significant amount
runoff will be retained in the drainage because of that.
The only major structure in the r'iver basin upst>'eam of the
subdivision is Vallecito Reservoir, which is a 120,000 acre-foot
reservoir, constructed by the Bureau of Reclamation in 1940. The
dam is operated by the Pine River Irrigation District. When the dam
was constructed there was very 1 i tt Ie development below the darn so
thet'e were no appreciable flood ber,efits to justify flood stot'age in
the reservoir. As a resylt there is no flood capacity in the
reservoir to contain a 100 year fle,od unless the fle,od occured when
the reservoir was not full. Generally, major flood events OCcur in
the late summer and fall from rainstorms and since this is the
pet'iod when the reset'voir is riot full due to SUmmer irrigatiorl
releases, there is an excellent chance that the reservoir would be
able to significantly reduce the 100 year flood peak. In the past
the Colorado Water Conservation Board (CWCB) has said that
incidental reservoir flood storage carlrlot be considered LlYIless there
is a flood storage allocation.
The reservoir and spillway were designed to pass the "inflow
design flood" which is a flood eve'nt far greater than the 100 year
flood. To pass this flood the reservoir has a few feet of surcharge
above the normal higH water surface and the spillway is capable of
passing 30,000 cfs. The 100 year flood is sufficiently small,
compared to the "inflow design flood", that the surcharge would not
be used but the flood would pass through the reservoir and the
spillway if the reservoir were full. As will be described, the 100
year flood is less than the spillway capacity.

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FLOOD ESTIMATION

Based on maps prepared by the F'ederal I::mergency Management
Agency for the National Flood Insurance Program dated December 15,
1981, the entire subdivision is in the 100 year flood plain. This
map is very general and does not show the depths of water nor the
elevations of the floodplain boundary. In order to determine the
flood depths and boundaries, the first step is to estimate the
instantaneous 100 year peak-f'lood flow. The flow will be estimated
in cfs; the volume of the flood is not important and will not be
estirnated.
The exist ing flood data for the ar'ea ir,cludes: 1) "Floe,d Hazard
Information" study prepared by the Corp of Engineers for Vallecito
Creek in April 1975 arid 2) "Flood Insur'ance Study" prepared for the
Town of Bayfield for the Federal Insurance Administration. The
first study was prepared to delineate the flood plain for Vallecito
Creek above the the reservoir where there is a significant amount of
development. This study estimated a 100 year flood peak of 11,000
cfs where Vallecito Creek enters the reservoir. Vallecito Creek
drains about 40%, 100 square miles of the 250 square mile drainage