FLOOD10343

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

:SENT BY: BOR-WCAOS SO

2-17-99 12:58;

970 385 6539 ->

303 866 4474;

#7/12

C. PROBABLE MAXIMUM FLOOD STUDY AND ROUTING

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

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

2. Local thunderstona with a peak inflow of 71,600 cfa and 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.f.et.

lbe most critical flood is the fall ieneral acorm PMF. Routing of this flood,
assuming the dam did not fail, resulted in the dam being overtopped for 7.5 hours
with a maximum depth of 1.4 feet. A maximum discharge of 79,200 cf. occurred
through the outlet structures and over the 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 PKF. Overtopping for this flood occurred for 2.6 hours. Results of
the routing of a flood equal to 70 percent of this PKF were a freeboard of 2.8 feet
and a maximum spillway discharge of 46,700 cfs. The 1IIaXimUII outlet works discharge,
which is between 2,950 and 3,050 cf. for .11 of the routed floods, was 3,000 cfs for
this flood. A flood equal to 50 percent of the fall general seom PKF vas routed to
de~ermine ~he minimum percentage flood that would produce flows through the spillway
which could cause detrimental erosion to ~he downstream slope of the dam. The
maximum spillway discharge for thb flood vas 37.400 cfs. Spillway cU.scharges above
aboue 40,000 cf. would overtop ehe lefe sp1.11way chute vall upstream of the juncdon
with the outlet works. This overtopping flow w111 cause erosion of the backfill
adjacent to the spillway chute wall and dbe earch materlal adjacenc to and
underneath the outlet works chute and stilling basin. Ie would also travel along
the outlet works to the dam slope. Uncle mining of the outlet works chute could
cause a differential settlement of the chute. which could cause backwater that would
come in contact: with the dam slope. Erosion of the downstream dam 8lop~, which
occur from either or both of these actions, would be detrimental to the safety of
the dam.

The other two PKF's also produce spillway discharges that would overtop the left
sp111way chute wall upstre811 of the outlet works junction. A freeboard of only 2.5
feet occurs during routing of the June rain-on-snow PKF. Routing of the local
thunderstorm PKF produces a freeboard of 5.4 feet.

The inundation maps prepared for areas downstream of Valleclto Dam were developed
using the fall general storm PKF. The reservoir elevation was assumed to be 7665
when the stona began. The high-pressure gate outlet works and spillway were used to
help pass the flood. Using these assumption., th. probable maximum flood would
overtop the dam. Therefore the inundat:ion maps prepared. using the probable lIlax1mum
flood assumed the dam would fail. The inundation map. are in the Emergency Action
Plan section of thi.. SOP. The following table glves informat:lon on the resulting
flood at key locacions aloni the rtne River.

111-3

Revised 03/94