FLOOD10343

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

jSENJ BY: 60R-WCAOS SO

2-17-99 10:56j

970 385 6539 =>

303 866 4474j

#6/11

C. PROBABLE MAXIMUM: FLOOD STUDY AND ROUTING

A study approved February 8, 1985. identified three probable maximum flood (PKF)
scenarios. These scenarios are:

1. June rain-on-snowmele with & peak inflow of 76,000 cf. and a lS-day
volume of 305,000 acre-feet.

2. Local thunderstorm with a peak inflow of 71.600 cfs anel a 2-day volume of
38.400 acre-feet.

3. Fall general storm with a peak inflow of 94.700 cfs and a 5-day volume of
186,000 acre-feee.

The most critical flood is the fall general storm PKF. RO\lting of this flooel.
assuming the dam did not fail, resulted in the dam being overtopped for 7.5 hours
with a Maximum depeh of 1.4 feet. A maximum discharge of 79.200 cfa occurred
chrough the outlet structures and over t:he dam. An overtopping depth of 0.2 foot
and a total maximum discharge of 59,900 cfs occurred for the flood equal to 90
percent of this PMF. Overtopping for this flood occurred for 2.6 hours. Results of
the routing of a flood equal to 70 percent of ehls PKF were a freeboard of 2.8 feet
and a maximum spillway discharge of 46.700 cfs. The maximum ouelet wora discharge,
which 1s between 2.950 and 3,050 cfs for .11 of ehe routed floods, was 3.000 cfs for
chis flood. A flood equal to 50 percene of ehe fall general seorm PKF vas rou~ed ~o
determine the mlntmum perceneage flood that would produce flows through ~he spillway
which eould cause det:rimental erosion to ehe dovn.treara alope of ehe clam. The
maxiDIWD. spillway discharge for thb flood was 37,400 efs. Spillway discharges above
about 40.000 cfs would overtop the lefe spillway chute wall upstream of the junccion
with the outlet works. This overeopping flow viII cause eros10n of ehe backfill
adjacent: eo ehe spillway chute vall and the earth maeerial adjacent to and
unde-rneaeh the ouclet works chute and stilling basin. It would also travel alone
the ouelee works to the dam slope. Undenainlng of the outlet works chute could
eause a dIfferential settlemene of the chute. which could cause backwater that vould
come in coneact with the dam slope. Erosion of the downstream dam slope. which
occur froll either or boeh of the.e actions, would be detrimental to the safet:y of
che dam.

The oeher ewo PKF's also produce spillway discharees 'chat: would overtop the left
sp1llway chute wall upstream of t:be outlee vorks junction. A freeboard of only 2.S
feet occurs du.ring rO\.lting of the June rain-on-snow PKF. R.outing of the local
thunderstorm PKF produces a freeboard of 5.4 feet.

The inundation maps prepared for areas downsereAm of Valleclto Dam were developed
using the fall general storm PMr. The re.ervoir .levation was assumed eo be 7665
when the storm began. 'nle high-pre.aure gate outlet works and spillway were \laed to
help pass the flood. Using these assumptions, the probable maximum flood would
overtop the clam. Therefore the inundation maps prepared using che probable maxtlllWll
flood assWled the dam would faU. The lnundation map. are in the Emergency Action
Plan section of this SOP. The folloving table give. information on the resulting
flood at key loca~lons along the Pine River.

111.3

Rev1sed 03/94