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Milltown Dam History

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					                             Milltown Dam History
             Chronology Compiled by Missoula City-County Health Dept.
                                December 3, 2001

1906-7 Clark’s dam built by William A. Clark. Original construction included a rock-
       filled timber crib wing wall at the right abutment, concrete and masonry
       powerhouse, rock-filled timber crib wall separating the powerhouse from
       spillway, and timber crib spillway. Owned by Clark Montana Realty Company. 1
       Three power generating units installed. 2

1908 Flood estimated at 48,100 cfs damages spillway adjacent to left abutment.
     Downstream end of spillway washes out over entire length of spillway. 3,4

1909 Repairs completed. Concrete placed in breach at left side of spillway, down to
     bedrock. Rock-filled timber cribs constructed up to finished elevation of spillway.
     Longer timber crib toe along length of spillway. Steel rails driven into bedrock
     along toe. Rock cliff extending into river on left bank blasted and removed.
     Waste rock used for fill of timber cribs. Timber crib structures at right
     embankment raised five feet. 5,6


1908-1925       Undercutting and erosion occur 7

1920 Fourth generator installed 8

1925 Concrete wall installed to reduce seepage under the dam. Sluice gates installed at
right end of spillway. 9
        River bed material under the spillway had been removed by leakage through the
dam and erosion by currents at the toe of the spillway. Repairs included driving steel



1
  Raytheon Infrastructure Services Inc. (1996) Inspection of Project Works, Milltown Hydroelectric
Project, Prepared for Montana Power Company, Butte Montana.
2
  Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
3
  Raytheon Infrastructure Services Inc. (1996) Inspection of Project Works, Milltown Hydroelectric
Project, Prepared for Montana Power Company, Butte Montana
4
  Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
5
  Raytheon Infrastructure Services Inc. (1996) Inspection of Project Works, Milltown Hydroelectric
Project, Prepared for Montana Power Company, Butte Montana
6
  Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
7
  DeHeer, Lee - CH2M Hill (2000) Milltown Dam – Review of 1996 and 1999 FERC Documents. Memo
to Russ Forba, EPA Montana Office.
8
  Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
9
  DeHeer, Lee - CH2M Hill (2000) Milltown Dam – Review of 1996 and 1999 FERC Documents. Memo
to Russ Forba, EPA Montana Office.
rails into the river bed material along the tow, and pouring a concrete wall filling the
washed out space up to 6 feet back from the toe to 3 feet downstream.10

1926 Fifth generator installed 11

1929 Ownership transferred to Montana Power Company 12

1930 Timber crib non-overflow structure at wing of right abutment replaced with
concrete structure 13
       further undercutting and erosion occur under concrete wall installed in 1925.
Undercutting reaches 10 feet back under tow of spillway. Repairs included additional
concrete around that poured in 1925.14
                                                                        15
1957 Magnitude 5.7 earthquake occurs in the Swan Range.

1967 Seepage found in upstream left corner of powerhouse, and in adjacent concrete
gravity block. Grouting of timber cribs done to reduce seepage. 16

1971 “Large cavity” found under the spillway accompanied by significant leakage17

1972 Safety Inspection Report by Engineering Management Inc. reports several signs
     of “deterioration” in the dam, including seepage in right abutment, large
     transverse crack through concrete gravity wall and surface decay of concrete in
     several places. Large cavity discovered in base of concrete wall at toe of
     spillway, extending back beneath the spillway, with considerable flow coming out
     of it. Reinforced concrete slab installed in 1930 was “badly broken up” and
     “undercut by holes in the underlying material.” “The flow may not be actively
     washing out material at this time, but it could conceivably increase in the future.
     This could wash out the alluvium underlying the spillway and the sluiceway, and
     if not checked could lead to and eventual failure of these structures.” In
     conclusion, Engineering Management said, “The dam shows signs of
     deterioration that can be expected in a structure of this age and type.” Citing

10
   Engineering Management Inc. (1972) Dam Safety Investigations, Milltown Dam, January, 1972. San
Francisco CA.
11
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
12
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
13
   Washington Infrastructure Inc. (2000) Ice Jam Evaluation, Milltown Hydroelectric Project, for Montana
Power Company, Butte MT
14
   Engineering Management Inc. (1972) Dam Safety Investigations, Milltown Dam, January, 1972. San
Francisco CA.
15
   Montana Bureau of Mines and Geology (2002) Quaternary Faults and Seismicity in Western Montana.
Butte MT
16
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
17
   DeHeer, Lee - CH2M Hill (2000) Milltown Dam – Review of 1996 and 1999 FERC Documents. Memo
to Russ Forba, EPA Montana Office.
        “considerable settlement” in the spillway and movement of other parts of the dam,
        and appreciable seepage flows under the spillway and sluiceway, and substandard
        stability factors for the dam, Engineering Management stated that “These several
        deficiencies could ultimately lead to partial failure of the dam. However. Cost
        considerations by the Montana Power Company have indicated that repairing the
        structures cannot be justified economically when compared to the value of
        producing power by the installation.” The report also stated, “In the past the
        action of seepage and eddy currents at the toe appears to have removed
        considerable amounts of the alluvium foundation materials, as indicated by
        the1930 and 1971 divers’ reports. At present this erosive action may be in a
        quiescent state but it is entirely possible that the rate of erosion could accelerate in
        the future. If this occurred it could eventually wash out enough of the riverbed
        materials underlying the right end of the spillway and the adjacent sluiceway to
        seriously jeopardize the stability and safety of these structures.18
        In December, a failure of a 54-foot section of flashboard occurred during
        reservoir drawdown. Inspection revealed settlement in the crest and timbers in
        the cribs and facing in “poor condition” 19,20

1973 Repairs included replacement of flashboards, bringing crest to level grade,
     concrete slab to protect against erosion and to prevent upstream face from
     raveling into the reservoir. Sheet piling also installed. 21,22

1974 January Ice floe occurs on Blackfoot River, and Ice Jam/debris flows occur on
     Clark Fork23
     Highway 200 flooded by ice jam on Blackfoot river. Ice floe comes to base of
     footbridge at Blackfoot Tavern, and family forced to evacuate home in Marco
     Flats Area.24

1975 FERC reports ice jam/debris flows. 25

1981 Ice and debris flows reported by FERC 26


18
   Engineering Management Inc. (1972) Dam Safety Investigations, Milltown Dam, January, 1972. San
Francisco CA.
19
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
20
   Washington Infrastructure Inc (2001) Spillway Leakage Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT.
21
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT.
22
   Washington Infrastructure Inc (2001) Spillway Leakage Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT.
23
   Wheeler, Robert J. (1974) Water Resource and Hazard Planning Report for the Clark Fork River Valley
Above Missoula. Montana Water Resource Report 51. University of Montana, Dept. Geology, Missoula
MT
24
25
 Hall, Harry T. – Regional Director Federal Energy Regulatory Commission (2000) letter to Jim Stilwell,
Montana Power Company October 31, 2000.
        May flood damages spillway. About 30% of spillway and timber superstructure
        was damaged by flood flows. 65-foot width of spillway sloping deck was
        removed from crest to upstream end of apron. Repairs in summer included
        concrete panels on apron. 27,28

1986 February Ice jam event causes minor flooding and damage to spillway. MPC
     spokesman Dean Conklin reports 24 of 69 flashboard panels broken by ice floes.29
     Reservoir drawn down 6.5 feet. MPC spokesman Cort Freeman reports 47 of 67
     flashboards destroyed.30 MPC spokesman Cort Freeman reports “living room
     sized hole” in the spillway caused by ice and debris. 50 of 67 flashboards
     destroyed by ice and debris. Freeman says loss of power productions would not
     significantly affect MPC’s service, because the dam represents a small fraction of
     the utility’s generating capacity. All flow diverted to sluiceway to prevent further
     damage to spillway.31 Repairs include 37 tensioned soil anchors in spillway and
     installation of new stanchions for flashboards. A concrete wall was installed
     upstream of timber crib spillway and divider block to reduce seepage through the
     dam. 32
                                                              33
1988-89 Radial gate installed to replace sluiceway.

1989 Concrete cutoff wall installed in the entrance of powerhouse to reduce under-
     seepage. 34Plastic concrete cutoff wall installed to reduce leakage through crib
     sections of spillway. 35 Six post tensioned tendons installed in the intake and right
     abutment sections of dam. 36

1994 EPA releases a risk assessment, entitled “Continuing Releases Risk Assessment”,
     which concludes that the failure of the dam at very high flows and “release of
     substantial quantities of buried sediments”, would result in “concentrations of


26
   Hall, Harry T. – Regional Director Federal Energy Regulatory Commission (2000) letter to Jim Stilwell,
Montana Power Company October 31, 2000.
27
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT
28
   Washington Infrastructure Inc (2001) Spillway Leakage Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT
29
   Moore, Micheal (1986) Missoula Mops Up. Missoulian, February 26, 1986.
30
   Woodruff, Steve (1986) Flooding eases, dam faces repairs. Missoulian, Feb. 27, 1986.
31
   Woodruff, Steve (1986) Flood damage to Milltown Dam forces MPC to plan repairs. Missoulian, Feb
28, 1986.
32
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT
33
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT
34
   DeHeer, Lee - CH2M Hill (2000) Milltown Dam – Review of 1996 and 1999 FERC Documents. Memo
to Russ Forba, EPA Montana Office.
35
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT
36
   Washington Infrastructure Inc (2000) Overtopping Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT
        arsenic and copper in downstream surface waters and deposited sediments that
        pose a risk to the health of select human receptors and the aquatic environment.”37

1995 Leakage occuring in a vertical construction joint in the right abutment concrete
     gravity wall, about 144 feet north of powerhouse. This location coincides with
     change in underlying foundation from bedrock to alluvium. 38

1996 Raytheon safety inspection report finds two of three spillway sections to be
     slightly below FERC minimum safety factors for low hazard dams (1.0). Report
     uses outdated seismic safety criteria from 1991 report. Raytheon also reports less
     than adequate safety factors for a large flood. 39
     February ice floe on Blackfoot River and Ice Jam/debris flows on Clark Fork.
     MPC conducts drawdown to protect the dam, causing downstream water
     pollution. 40

1999 FERC re-classifies dam as “Significant Hazard Potential” due to the polluted
     sediments behind the dam. 41

1999 Raytheon report finds that safety factors for timber crib spillway were below
     those required by FERC for significant hazard dams. Report also found that right
     abutment of dam is built on sand and gravel alluvium, subject to erosion and
     possible failure during a large flood. Raytheon and MPC argue that the spillway
     is adequately safe, because it was rehabilitated prior to 1997, when FERC
     modified requirements for such dams, and the repairs were made in compliance
     with the standards in effect at that time. 42

2000 Increased rate of leakage reported to FERC by MPC43
     FERC re-classifies dam as “High Hazard Potential” 44
     Report by Raytheon addresses overtopping potential, and proposes new 30 foot
     earthen dam from right abutment to I-90 45
37
   Environmental Toxicology International (1994) Continued Releases Risk Assessment, Milltown
Reservoir Operable Unit, Milltown Reservoir Sediments Site. Prepared for U.S. Environmental Protection
Agency, Region 8.
38
   Thomas, Dean and Hoskins, Inc. (1995) Milltown Dam Draft Safety Analysis Report. Prepared for
ARCO, Anaconda MT
39
   Raytheon Infrastructure Services Inc. (1996) Inspection of Project Works, Milltown Hydroelectric
Project, Prepared for Montana Power Company, Butte Montana
40
   Washington Infrastructure Inc. (2000) Ice Jam Evaluation, Milltown Hydroelectric Project, for Montana
Power Company, Butte MT
41
   Hall, Harry T. – Regional Director Federal Energy Regulatory Commission (2000) letter to Jim Stilwell,
Montana Power Company May 7, 1999.
42
   DeHeer, Lee - CH2M Hill (2000) Milltown Dam – Review of 1996 and 1999 FERC Documents. Memo
to Russ Forba, EPA Montana Office.
43
   Washington Infrastructure Inc (2001) Spillway Leakage Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT
44
   Hall, Harry T. – Regional Director Federal Energy Regulatory Commission (2000) letter to Jim Stilwell,
Montana Power Company September 12, 2000.
45
   Washington Infrastructure Inc (2000) Overtopping Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT
        June – magnitude 4.5 earthquake centered about 20 miles east of the dam. 46
        Washington Infrastructure (formerly Raytheon) submits report on Ice Jam study.
        47
           The report finds that the radial gate and flashboard structure could be damaged
        by ice. FERC agrees that drawdowns should be conducted to protect the dam
        against ice damage. 48

2001 Spillway leakage evaluation reports settlement of 3.32 inches in center of spillway
     from 1987 to 2000, caused by compression of structure and underlying alluvial
     material caused by the tensioned tendons. 49
     FERC writes letter to Missoula County, reporting that it would not be unusual for
     the dam to require complete replacement at some time in the future. 50
     MPC conducts temporary repairs to plug leaks in spillway apron panel. Water
     spouts at base of spillway drains reach height of approx. 18”
     MPC requests extension of time until January, 2002 to complete its required five-
     year safety inspection report for FERC.




46
   Stickney, Micheal. Director Montana bureau of Mines and Geology (2001) letter to Len Schombel,
April 18, 2001
47
   Washington Infrastructure Inc. (2000) Ice Jam Evaluation, Milltown Hydroelectric Project, for Montana
Power Company, Butte MT
48
   Hall, Harry T. – Regional Director FERC. (2001) letter to Peter Nielsen, Missoula City-County Health
Dept. June 20, 2001
49
   Washington Infrastructure Inc (2001) Spillway Leakage Evaluation, Milltown Hydroelectric Project.
Prepared for Montana Power Company, Butte MT
50
   Hall, Harry T. – Regional Director FERC. (2001) letter to Peter Nielsen, Missoula City-County Health
Dept. June 20, 2001

				
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