FLOOD10343

• County: La Plata
• Stream Name: Pine River
• Basin: San Juan/Dolores
• Title: Pine River Flood Plain Study
• Floodplain - Doc Type: Correspondence

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Greiner Engineering is under contract to the Colorado Department of
Transportation to design a new Highway 160 bridge at Bayfield. Mr. Knowlton estimates
that the existing Highway 160 bridge capacity is approximately 6,800 cfs. Steve Smith
previously reported that Greiner determined the 100-year Pine River discharge at
Bayfield of 7,600 cfs using unpublished USGS regional regression equations for western
Colorado. Greiner compared the results with other regional equations for the area. A
draft USGS report, co-sponsored by the CWBC, contains the unpublished equations. Mr.
Browning will contact Ed Vail with the USGS to obtain a copy of the draft report for the
COE. It should be noted that Greiner is using the entire Pine River above Bayfield
drainage area of approximately 340 square miles and ignoring the effect of Vallecito
Dam based on the assumption that the reservoir does have not flood control storage. Mr.
Browning will contact Mr. Smith to inform him of the results of this meeting.
The COE produced a 1997 flood plain delineation study for a six-mile reach of
the Pine River at Ignacio, Colorado. For that study, the COE developed a HEC-1 model
for the Pine River watershed below Vallecito Dam to the New Mexico state line. The
COE calibrated the HEC-1 model for the unregulated Spring Creek, with the rainfall
reduced for Spring Creek and using infiltration rate adjustments, to discharges computed
by regional regression equations for the northwest plateau reported in USGS report 96-
4112 Analysis of the Magnitude and Frequency of Peak Discharge and Maximum
Observed Peak Discharge in New Mexico. The calibrated infiltration rate of 1.18" was
then applied to the HEC-l model for the entire watershed, with the rainfall reduced for
300 square miles, resulting in a 100-year discharge of 18,000 cfs at Ignacio and 14,000
cfs at Bayfield. The 18,000 cfs was used in the 1997 Ignacio flood plain analysis.
To determine the discharges for the La Plata reach, the COE calibrated the HEC-l
model for the unregulated Bear Creek, with the rainfall reduced for Bear Creek, to
discharges computed from the regional equations in USGS report 96-4112. The COE
then applied the calibrated infiltration rate of 1.50" to the HEC-1 model above Bayfield,
with the rainfall reduced for the 70 square miles above Bayfield, and produced a 100-year
discharge of 9,670 cfs at Bayfield. In the model for the entire 300-square-mile watershed
with the rainfall reduced for the entire area, the COE applied the 1.50" infiltration rate for
the area above Bayfield and the 1.18" infiltration rate for the area below and produced a
100-year discharge of 7,060 cfs at Bayfield and 13,300 cfs at Ignacio.
Using the regression equations alone for the 70-square-mile drainage area below
Vallecito dam at Bayfield, the 96-4112 report provides a discharge of 5,800 cfs which
produces a discharge of 8,900 cfs with a dam release of 3,100 cfs added. Mr. Browning
provided a copy of the CWCB report Guidelines for Determining lOO-year Flood Flows
(January 1999) that contains a regression equation specific to western Colorado. This
equation provides a 100-year discharge of 5,200 cfs for the 70-square-mile watershed,
producing a discharge of 8,300 cfs with an added Vallecito Dam release of 3, I 00 cfs.
We agreed that the COE should proceed by calibrating the HEC-1 model to the
discharges computed using the USGS 96-4112 report. This report provides
approximately the same results as the CWCB report. In addition, the USGS report
provides equations for other frequencies that the CWCB report does not. The COE study
will probably produce 100-year discharges of approximately 10,000 cfs at Bayfield and
13,000 cfs at Ignacio; these discharges include the 3,100 cfs Vallecito Dam release.
Everyone agreed that these discharges are reasonable.

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