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The Engineering Background to theOldman River Dam and ReservoirDavid B. ChalcroftPublished online: 23 Jan 2013.

To cite this article: David B. Chalcroft (1988) The Engineering Background to the Oldman River Damand Reservoir , Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 13:3,6-14, DOI: 10.4296/cwrj1303006

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The Engineering Background to the Oldman RiverDam and Reservoirl

David B. Chalcroft2

Abstract.The Government of Alberta announced in August, 1gB4 that a dam and reservoir wourobe constructed on the oldman River, about 10 kilometres northeast of the Town olPinche'Creek. T"rs anrouncement culrn.nsls6 years o'study dating back to the early1960s and triggered a seven-year program of exploration, design, and constructionthat will see the prolect complete and storing water by 1991.

Rdsum6.Le gouvernement de I'Alberta a annonc6 au mois d'ao0t 1984 qu'un barrage et unreservolr sera ent construits sur la rividre o dman, a environ 1o km au nord-est de la vrllede P ncher creek. cette annonce couronnait de longues 6tudes remontant au debutdes annees 60, et mettait en oeuvre un programme de sept ans d'exploration, d'etudetechnique et de construction, qui d6bouchera sur l'achdvement du proiet et la retenuede l'eau d' ci 1 991 .

IntroductionThe need for the Oldman River Dam results fromthe geography, hydrology and population dis-tribution of Western Canada. The malor riverbasrns that drain the three prairie provinces andthe Northwest Territor es cornpr se an area ofabout 4,0OO,OOO km2 with a total average runoffof 2O,3OO m3/s While thls flow may seemgenerous for Western Canada's 4.5 million peo-ple, the natural geographic and seasonal dis-tribution of these water resources often renderthem nadequate to .neet the needs of thepoou ation.

The Slave-Mackenzie River basin, the thirdla'geSt . vg. In \orth A:nerica, o.ar.tS an area o'1,800,000 square kilometres, has a total lengtfio' 4.200 k 'loraetres, a.rd discharges ar averageflow of 10,OOO m3/s into tbe Arctic Ocean, about50 percent of the totaldrainage from the region,but has a very sparse population. In contrast,the Saskatchewan-Nelson river basin drains anarea ot slightly under 1,000,000 km2, with anaverage discharge of approximately 2,400 m3/s.

Over 90 percent of the Prairie provinces' 4.5million people live in the Saskatchewan-Nelsondrainage basin which accounts for only 12 per-cent of the West's freshwater resources.

In Alberta, almost 90 percent of the province'ssurface water is found in the Peace and Atha-basca River drainage basin, Only 1O percent,about 480 m3/s on average flows out of theprovince in the North Saskatchewan and SouthSaskatchewan Rivers, while over BO percent ofAlberta's population lives within the Sas-katchewan River basin. Here, as elsewhere n

Weste'n Canada, there is an imbalance be-tween population growth areas and the supp yo' water needed to support the agriculturat,industrial, and municrpal needs of the population.

Oldman River BasinThe Oldman River basin is located in southwes-tern Albqrta and drains an area of 23,000 km2.The maximum dimensrons of the basin areapproximately 2OO km east-west, and 240 kmnorth-south, The principal tributaries of the O dman

1 An earlier version of this paper was presented at the Canadian Water Resources Association 40thAnnual Conference, June 15-18, 1987, in Winnipeg, Manrtoba.

2 P.Eng,, Director of Civil & Industrial Engineering for UMA Engineering Ltd., 17007-107 Avenue,Edmonton, Alberta T5S 1G3, and UMA's Proiect Co-ordinator for the Oldman River DamProiect.

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River are the Little Bow River, the St. Mary Rlver,rhe Waterlon Rive., Wi ow Cree<, the CastleRiver and the Crowsnest R ver

The oasr topogr6p^y va.es 'rom ruggedmountain terrain along the B,C. border withelevatror-s over 3.000 merres lo ge11le olairseast ot Lethbridge with a general topographicelevation of 600 to 700 met.es above sea evel.The mean annua precipitation varies from over600 mm rn the Rockies to about 350 mm towardsthe eastern plains. The main popu at on centrein the basin is the City of Lethbridge with about60,000 resrder-ts - 40 pe.cert of the lotal pop-ulation in the Oldman River bas n

Seasonal flows in the Oldman River varywidely with over 65 percent of the annual totaoccurring in May, June and July. lrrgation-supported agriculture requires rrrigation flows

FIGURE 1: Oldmen River Basin

'o. the penod May thrgsg6 October each year.Natural rrver flows in the months of Augustlnrgggh October are rnSuflrc ent to SUppo(irrigation requirements, and annual flow volumescan vary over the years by a factor of 10 fromhighest to lowest annual volumes.

Sir-ce the late 1800s. pro.ects such as theOldman River Dam have been constructed in

soJtnerr Alberta to ove'cone the yearly andSeaSonal i-ba arCeS in water SUpply and tO

rnake lhe de ivery o{ water poss'ble whenneeded for munic pal, industria and irrigationpLJrposes. T^,s ^as enabled a slrong rrrigat'on-supported agriculture-based economy to de-velop and flourish, Crop yields are not onlysubstantial y higher on irrigated land, but southernAlberta also enjoys greater economic stabilitylhan rt otherwise would. since rrrgated 'and ismore immune to the efie '*s of drouqht.

,Alfuto

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As of 1981 there were about 1.2 million acresof irrigated and in southern Alberta, of which600,000 were located in tbe Oldman Riverbasin The total water consumption in the bas n

for irrigation was about 650,000 DAMS or 530,000

FIGURE 2: Flow Variations in the Oldman River

NEAR LETHBRIOGE

acre-feet. Prolects currently approved andunderway will increase the amount of irrigatedland in the Oldman River basin to about 863,000acres.

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FIGURE 3: lrrigation in the Oldman River Basin

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Oldman River Flow RegulationStudies carr ed out by Alberta Environment dur-ing the mid-1970s and early 19BOs concludedthat an on-stream storage reservoir should bebuilt on the Oldman River, The best site overallwas determined to be the "Three Rivers Site," 1 0km northeast of Pincher Creek, and now knownas the Oldman River Dam site.

At this location the reservorr formed will con-tain a live storage volume of 490,000 DAM3(400,000 acre-feet), and will control a drainageareaol 4,400 km2,When completed in l991,theproject w ll enab e irrigat on in the Oldman Riverbasin to be expanded by about 1 70,000 acres,will improve water supply and quallty tor municipal

FIGURE 4: Oldman River Reservoir

and industrial uses, will provide a degree offlood protection, w ll offer new recreation oppor-tunitres n the area, and has the potential for a

modest hydro-electric development of up to 30MW at a tuture date.

The reservotr which will be created behind thedam w I be about 22kmlong and 2 km wrde atits widest point. The reservoir wrll have a surfaceareaoI2,42a ba (5,970 acres)at the FullStorageLeve of elevation 1,1.1 8.6 metres. In order todeliver the total live storage vo ume, tne reser-voir evelwrll operate through its full range of 54metres. This will occur infrequent y; only rn tnoseyears when very dry cond tions prevat .

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The Project ArrangementThe Oldman River Dam s te is located in anarrow rock-controlled valley on the main stemof the Oldman River, approximately B km down-stream of the confluence of the Castle River,and 1 2 km downstream of the confluence of theCrowsnest River, At this point, the mean annualrunoff is 1,240,000 DAM3 which corresponds toan average tlow of 39.3 m3/s,

The valley at the damsite is about 60 to 70metres deep and about 350 metres wide at thebase of the valley walls. The river itself is about50 metres wide and generally 1 to 3 m in depth.The principal structures at the dam site consist

FIGURE 5: Principal Project Structures

of an embankment dam, twin diversion tunnelsto pass the river flow during construct on of thedam, and later to serve as low level outiets, alda gated spillway structure to pass flood flows in

the river after completion of the prolect. Variousintrastructure facilities were constructed to sup-port the construction of the principal structures,including a 4.5 km access road trom Highway+? t^ tha nr^ra.i aila en AE, -J metre IWln Span pre-stressed concrete bridge across the OldmanRiver, a construction camp to house up to 500workers, a temporary b.rdge. and an accessroad to the west bank of the river.

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Diversion TunnelsThe twin diversion tunnels are each 900 metreslong, have a finished internal diameter of 6.5metres, and can pass a total 1ow of 1,000 m3/s.

After completion of the dam, the tunnels willeach be equipped with 2 metre diameter hollowcone valves to requlate 'utu'e downstreamflows. The regulatinj valves can handle the fullrange of flows up to 200 m3/s. Flood flowsabove thrs will be handled by the spillway.

The tunnels were excavated using the drrlland blast method. Initial roof support is pro-vided by a pattern rockbolt system and steelfibre reinforced shotcrete installed close to therock face at all times. Excavation was carriedout full-face, on four headings simultaneously.The final concrete liner was placed using twohydraulically automated steel forms, The con-crete liner is generally 300 mm thick, with spe-cial sections at the inlet, outlet. and valvechamber thickened to about 1,000 mm. Theliner upstream of the valve chamber containssteel reinforcement, while downstream of thevalve chamber the liner is not reinforced.

Provrsion has been made in the design of thetunnels and valve system so that at some futuredate. a hydroelectric generating sration car'be added.

Tunnel Hydraulic ModelHydraulic model studies of the diversion tunnelsystem were carried out at a scale of 1:30. Thetunnels in the model were approximately 20 cmin diameter and the total model length was over35 metres. lnlet, outlet, and in-tunnel conditronswere care'ully tested to yslrty lhdt hydraulicconditions were acceptable over the totalrange of design flows, and to illustrate wheredesign improvements could be made thatwould lower the cost of the prototype struc-tures, Several million dollars were trrmmedout of the construction costs by des gn im-provernents that were identified in the modellingprocess,

The hollow cone control valve was modelledby the valve contractor, at a scale of 1:15 inorder to develop an effective energy dissipationsystern downstream o' lhe valve. Drschargevelocities trom the valve under full reservoirconditions are in the order of 30 m/s and requ re38 metres of the tunnel to be steel-lined, andequipped with an energy dissipation deflectorlng and bafr'e blocks, which reduce the outgo-ing velocity to less than B m/s.

Test TunnelBesides conventional core drilling and geophysicalsurveys from the ground surface, the exploration

program for the tunnel contract included thedriving of a "test tunnel" near the site ot thediversion tunnels. The test tunnel was a totai of100 metres in length.

The frrst 65 rnetres or access section wasdriven as a 3.5 metre high horseshoe shape,through the sandstone, siltstone and mudstonerocks of the Porcupine Hills geologic formation.Beyond the access section, the tunnel wasturned through a 45" curve and enlarged to a7.5 metre dianerer circular shape - ll^e size o'the excavation for the diversion tunnels. A 30metre length was driven by heading and benchat the full prototype size,

The main ourposes of the test tunnel were toobtain additional information for design pur-poses related to rock swelling or squeezingproperties, and the strengths of the rock units;toprove the horizontal continuity of the rock strata;and to illustrate to contractors bidding on themain tunnels, what the rock conditions andexcavation conditions would be, to reduce thelevel of uncertainty in the tender process.

SpillwayThe Oidman River Dam Spillway is sized to passthe routed Probable Maximum Flood (PMF) of7600 m3/s. The 85 m crest width is dlvided intoseven gate openings each housing a 10 m wideby 8.5 m high vertical lift roller gate,

The chute converges to a 40 metre widthhaltway down its 350 metre length, and ter-minates in a flip bucket designed to throw thedischarge about BO metres from the end ot thespillway at PfVF.

Headblock stabillty is achieved by a com-prehensive grout curtain and drainage tunnelsystem along the upstream line of the head-block to reduce hydrostattc uplift pressures onthe underside of the headblock concrete.

The chute slab is underlaiaby a2 metre thickblanket of free-draining granular material and anetwork of drain pipes to ensure that uplift pres-sures do not occur on the chute slab due eitherto seepage through the underlying rock or toleakage through cracks or joints in the chuteslab. The 2 m gravelblanket will also insulate theunderlying rock against freezing and therebyprevent frost-heave conditions under the chuteslaDS.

To reduce the maintenance probiems whichhave been caused by transverse loints in thechute slabs of other structures, the number ofjoints is minimized. Continuous concrete slabsup to 275 metres in length run from the flip bucketup to the headblock structure, with adequatereinforcing steel to distribute thermal shrinkageuniformly throughout the slabs in hairline cracks.

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FIGURE 6: Spillway

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PROFILE

Spillway Hydraulic ModelsA nurnber of hydraul c rnodels of the spillwaywere constructed and tested to ass st in evaiu-ating the cost-effectiveness of various des gnoolrons. Inrtial'y, a 1:200 sca e.node was testedto evaluate in a general way, the scour patternsand potential scour depths downstream of theil p bucket, and to ensure that no hazard existsto the downstream face of the dam under thePM F condition. Scour depths of up to 40 metresh^,,^ A^^^ ^/^^ ^+^! ^ +LI ave uee,r p'eo,c e(l n tne otunge poo area.The 1:2OO scale mode was also used to rl us-trate the hydraulic advantages of chang ng thespillway to a converging chute confrgurationwrth a constant 1 vertica to 6 horizontal s ope,rather than the initia configurat on proposedwhich featured an 85 metre constant widthchute and a broken s ope. The convergingchute has signif cantly superor flow charac-teristics and lower side walls when compared tothe constant w dth chute.

Later models of the sp I way at a scale of l:75were tested to refine and final ze the spillwaygeornetry. The fl p bucket radius and width wereboth reduced in order to lower the f ow at whichthe "tlip action" wou d be inltiated to about 4OO

m3ls with a sustainable llip action flow down to200 m3/s

SOOm CHUTE SLAB

Main Embankment DamThe Main Embankment Dam spans the river atts narrowest point, and has a maximum heightof 76 metres above the existing rverbed. Thecrest length is 600 metres in the main valley sec-tion and about 3,000 metres overall when theeft bank closure dyke is tncluded. The max-imum base wldth of the structure is 615 metresand the averaoe slopes a'e close to 4 6ott/on-tal to 1 vertical. The structure contains about B

mrllion cubic metres of f ll.

The dam cross-section consists of a corezone of impervious glacial til , surrounded bythin zones of highly processed granular filterand drain material, and supported by shellzones consisting of pit run alluvial gravels androck eycavated from the spillway approachcr-a1nel. Tre upslrear face 's protected by rip-rao which w ll be ouarried and hauled in from theCrowsnest Pass area. The core zone of the damextends downward through the alluvia gravelsand into the underlying bedrock. A grout curtainfollows the centerline of the dam through closelyspaced dril ho es.

Dam Exploration ProgramsA number of explorat on programs were under-taken to def ne the foundat on conditions at the

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FIGURE 7: Dam Cross-Section

dam site and to locate, quantify and classitysurtable borrow materials with wh ch to build theembankment. The exploration programs con-slsted of conventional augering in overburden,and coring in bedrock uslng the recently devel-oped Strato-pac drill bit, which has revolutionrzedcoring in soft sedimentary rocks as are prev-alent at this site. The Strato-pac bit achievedexcellent core recoveries, including the clayinf i I zones between hard rock strata wflich wereusually not recovered using earlter dri ling equip-ment. Downhole geophysics were employed In

all deep core .oles to augment +he geo'ogicallog of each hole, and geophysical surface sur-veys defined overburden depths and types,

Four vertical inspection shafts were drilled atthe site using a cable tool drlll rig and bit. Eachshaft was 1.l m diameter and up to 6O metres indepth. This technique allowed geological per-

sonnel to inspect the n-situ rock formationsfrom the safety of a cage lowered down theshaft. Cored B ,nch diamerer sarnp es were alsoobtained from the walls of the shaft tor aboratoryanalysis and shear box testing.

An extensive granular borrow investigationwas carr ed olt whicr- resLrlleo in an irve'itory o'15 m llion cubic metres of naturally occurr ngg'a1ula' r"ale'ials wit^ n lhe reservoir area to be{looded and for a distance of about 4 kmdownstream of the dam site.

Project Schedule andDiversion SequenceThe proiect duratlon, from the government'sannouncement in 1984, tofirstfilling of the reser-voir in 1991, is seven years. Stte exp oration,prellm nary engineering studies, and the prepa-ration of a prolect cost esttmate occurred during 1985. Design of the infrastructure works andthe principa structures was carried out durlng1986 and 1987. Construction began in February

1986 on access roads, bridge and campfacrlrt,es. The Divers on Tunnels were corstr-ctedfrom October 1986 to June

.1

98B. Main Embank-ment fill placing occurs during

.1

9BB, 1989 and1990. Tne Sp'l way const.uction o,oceeds 'romearly 1989 to late 1990, Gates and controlvalves were tendered and awarded in late 1986with delivery to the site in early 1990 for installa-tion before March 1991.

Once the two dtvers on IUnnels are complete,the Stage I upstream cofferdam is placed ln July19BB to force the river to flow througb the tun-nels. This enables the dam foundations to bedewatered so that the core trencn zone can De

excavated and the foundation grouting com-pleted. By April 1 989, the upstream cofferdam ts

raised to elevation 1,OB0 m. Rock excavationfrom the spillway and the spillway approacnchannel is placed in the enbalkrrspl 95s11

zones, By November 1989, the upstream shellzone is ra jsed to elevation 1 091 m. All remainingfi I is placed in the upper third of the damduring 1990.

Commission ng of the facilities and nitial fjl -

ing of the reservo r starts in the second quarterof 1991 and could take two years to comp etedepending on natural river f ows in those years

Construction ManPowerThe total prolect, including works related toretocatron of roads ano .lt,tities in the reservoir

area, is est mated to cost $349 million in 1986dollars. Construction of the princlpal structuresshould generate about 2,1 00 man-years ot con-struction employment with a peak work force ofabout 800 du'ng the summe' constructionseason in 1989.

AcknowledgementsThe author wishes to acknowledge the supportof Alberta Environment 1n publishing this paper,

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FIGURE 8: Construction Sequence

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and the cont'ibutro'r o'othe. memoerS of theeng neering team composed of personnelfrom:UMA Eng neerlng Ltd, Acres International Ltd.,Goder Associates, Thurber Consultants Ltd.,Geo-physicon Ltd., Brown Okarura Ltd., North-west Hydraulic Consu tants.

ReferencesAlberta Environment Planning Dvislon. 1976.Oldman Rrver F ow Regulation, Prel minary Plan-ning Studies. June.

Canada West Foundation. 1982. "Nature's Life-Ine: Pra rie and Northern Waters,"

Marv Anderson & Assoc ates Ltd. 1986, "Od-man River Dam, Economic Analys s." January.

UMA Engineer ng Ltd. 1986. Oldman R ver Dam,Prel minary Engineering Report. March.

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