PRELIMINARY ENGINEERING REPORT

RAISING OF BIG COULEE DAM

PROJECT NO. I4I8

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NORTH DAKOTA Fl o o'S \o STATE WATER COMMISSION E' ul E N O V. l98l F r{ PRELIMINARY ENGINEERING REPORT

RAISING OF BIG COULEE DAM Sl,lC PROJECT #1418

NOVEMBER, IgSi

Nont,h DakoLa State l,Iater .Commission State 0ffice Buiì_ding 900 East Boulevard Bisnarck, North Dakota 58505

PREPARED BY:

GJ AssÍst.ant nvestigation neer

SUBMITTED BY:

ID A. SP E Director of Engineering

APPROVED BY:

VERNON. FAHY E State Enþineer"

Prepared for the Towner County I'IaLer Resource Board TABI.E OF CONTENTS

page 1. TNTRODIJCTION..,.... .,.,. rr. DtscRIPTrqN OF SITE. ;... ¡ 3

III. STATEMENT OF PROBLEM Ò.¡.. ¡.. i.

Baekground.:.. . ¡... g

. ¡...... r ¡.. ¡ - 11 ÏV. SYDROLOGT

1y...... :.. 1q. St.udy Results. '...1,o..e ....f .?.....i.. 16 V. AITEmûåTI\¡ES..,.....i..¡.d.. .r... !.qri .. zz

Al.t.e¡native 1 Àllernat.ive Z. r,... ALt.ennat.ive 3,...

VT. SUU!..{A8T AND AECOMMENDATTÔNS. Sumary.

Reeor¡r¡endatione. .,. . i

TABLES . TABLE 1. Routing Results, for Existing Conditions. 17 TARLE 2. Rout ing Resu1t.s proposed for ModÍfications. . . . . i, . . 19 TÁBIE 3" Preliminary Cost, Est.inat.e-Aj_ternative A, . . 26 TABLE 4. Prelini¡ra¡y Cost EetimaLe-Alternative 3. 2B TABLE 5. sunmary of Routing Results proposed for Art.ernat.ives... 31

1 North Dakota State Water Commission 900 EAST BOULEVARD . BISMARCK, ND SB50s-0187 . 17011224-2750 o FAX (701)224-3696

WÀTER DEVELOP}IENT DTVISION (701) 224-27s2 February L4, 1991 Honorable Margo Helgerson Mayor of I{esthope PO Box 412 Westhope, ND 58793

RE: SWC ProjecE #L267 Dear Mayor Helgerson:

In considering your request for cost-sharing for the I{esthope Dam project, I have reviewed the Preliminary Engineering Report prepared by Toman Engineering. Before t,he project can be considered for cost-sharing, there are several concerns which must be addressed Às stated in the report, the dam, as proposed, would be classified as a Class fIf dam under the North Dakota Dam Design Guidelines. The freeboard requirement for a Class III dam in North Dakota is a 30 percent PMP flood. This means that the dam must be abl-e to pass a flood equal to 30 percent of the Probable Maximum Precipitation without overtopping. This would be approximately a 6 1/2-inch rainfall. The engineering report mentioned nothing about an emergency spillvray or the performance of the dam during floods. f should also point out that there are veLocity requirements for emergency spillways l-isted in the North Dakota Dam Design Guidelines as wel1. An adequate emergency spillway could vastly increase the cost of the proposed dam. The main concern is that the preliminary engineering report be sufficiently detailed to provide a good cost estimate. This cost estimate is needed to determine the level of cost-share required from each entity. Once r{e are satisfied that aII requirements have been addrelsed, your specific cost-share request wilt be submitted io the State I{ater Commission for consideration.

Sincerel Yt

s Lenn , P.E. ater Resources Engineer JL:ùn cc: Toman Engineering

GOVEBNOR GEORGE A. SINNER DAVIO A. SPRYNCZYNATYK. P.E. CHAIRMAN SECRETARY E STAIE ENGINEER MESSÀGE DTSPLAY

To Dale To Dave To Cary BC JIM LENNINGTON

FTOn: JIM LENNTNGTON Postmark: Apr 30r9I 2:11 PM Delivered: Apr 30r91 2:l-1 PM

Subject: I{esthope Dam Message: Ilesthope is considering constructing a multi-purpose reservoir to the south of town. They have been discussing cost sharing for this project with us. There is a big problem with this proposal in that there is a strong probability that a water use pennit application would be denied. The location they are proposing for the dam is on a tributary to the Souris. I discussed the possibility of transferring al1 or some of the appropriation the town currently has out of the refuge pool to the dam site with Gordon and Bob lùhite. They informed me that this would not be possible. I have a meeting scheduled tomorrow in Westhope and I'm wondering what f should teII them about their proposal. Àny suggestions would be welcome. RÀLEIGH DAM - S.WC--P.8.A]ECT ÀBSTRÀCT OF BIDS OPENED I:30 Pl I 7oo 53 ¿S

BTDDER Dakota Dirt Àfoving Box I3l ITEM Mclaughlin, SD 57642 ri- / ?<.q

I. Mobilization s7 . 200 .00 0

2- ControL and Removal of vùater LS 2,000.00 2 ,000.00

3. Stri lopsoil 60,000 sY 24 14,400.00

Excavation 1.40 15, 260.00

90 84,600.00

6. ÍIater For Compaction 2,5OO M GAL 5. 50 13 ,750.00

7. Foundat ation LS 6,840.00 6,840.00

266.OO

Reinforc Steel 59 650 LB 49 29 228.5 lO. Rock Ri Pilter Beddi 700 cY 6.00

11. Rock Riprap 1,400 cY 12.00 16 800.00

L2 cel LS 3.900-oo .00

Drawdown fnstallation LS q qqfì ôô

500.00 500.00

7.5 AC. 395.00 2,962.50

TOTAL $ 290,633 .00 t,t:/\t, r,r.,t,t t,til:t;¡i. l,/\t\l , .\l!(, l,l(O,Jl:(,'l ll l(t7 I All.S'l'llA(;'l' Ol; llll)Íi Ol,t;Nt:t) 2:00 t).¡,l., l, 22, l9S3 ^tltìl *(lot'l'r'ct crl lll.l)r)t;R llo<¡ ¡'lrcitx l(,7,1 I'loorllca

12. l'VC Pipc 4 50 l,¡; 11.1.3 5,008.5t) 11.13 5,00tÌ.50 l0.,.lt) 4, ()80.l)() 13. - 6'r l)crforatcd 390 Llj 9.s7 3,732.30 ^Cl) 9.s7 3,732.30 12.!)0 .5, ()31 . ()0 14. - 6t' No¡r-Pcrforatcd S2S Llr 7 .46 3,91(i.50 7 .46 3,91(r . ^Clt 50 f).50 I , !)87.50 15. [)r¿r i ¡r l;i I I l,lateri al 7(r0 CY 7!¡ .17 11,.529.20 9.00 (r,840.00 t 7 .2t, I3,II().()0 16. llock fl i 5 (r75 ¡rrap , CY 14.8(r 84,330.50 30.00 I 70, 250. 00 20.00 113,500.0(ì l7 lì i . llock pra¡r Fi I tcr Bcd I ,900 Cy 4 (')0 .02 7 ,63t\ . 10.00 1 !) , 00(). 00 l5 ,9,075.00 lB. Catc Instnllation I,S 1,873.00 1 , 873. 0t) I , B73.00 1, tì73.00 .100. 00 300. 00

19. llisccl lnrrcous l.lctals I,S 1 t ,20¡t.00 11,208.00 1 1 ,209.00 I I ,208.00 I () , l()r) . ()0 1(),200.0() 20 . Scccli rrg -50 Âct'cs 214.40 I0, 72(). ()() 1 ()0 50.00 7,5r)(ì. 2 3tr ()() ll 800 . 00 l()l'¡\1, ii1) lli L),1 . l,l , lil).I , 11J7, .;1) .)1.{tll.lll.r)tl TABLE OF CONTENTS (CONT. }

Pagje

FIGURES

FIOURE 1. GeneraL Locat.ion of Bíg eoulee Dam., FÏGU8E 2. Area CapaeÍLy Curve...... rr.5¡¡...... 5 FIGURE 3. 25-Tear. Eydrograph.. .,. . ...

F'ÍGURE ¡I. Proposed Alterations to Downst.ream Enbankment.. 23 FIGURE 5. J0-Fooü l{ide Chut-e...... 25

PLATES

PLATS 1. As-Built Damsit.e Topography. 35 Ptr,ATE 2. 'Drainage Area...... 36 PIATE 3. Area rnundat.ed by proposed Reservoir for ^arternatives.. 3T PIATE 4. proposed area of Downstream Ennbanknlent for !{odification 3g

APPENDIX

APPENDIX A. Invest.Ígation Agreement

¡1 I. INTRODUCTION

Big Coulee Dam is a multi-purpose dam located in Towner County. It provides the munÍcipal water supply for the City of Bisbee, North Dakota and also is a recreational- area for the cornmunity. Since it.s init.iaL fiJ.J-ing in 1969, t,he damsit.e has required a large amount ofl maintenance. The service spillway has a rêlatively small- capaciLy, result.ing in frequent use of the emergency spii-l.,¿au.

This has caused some erosion t.o t.he emergeney spiLlway. In addit.ion t.o Lhese problems, t.he qualit.y of the water in the reservoir has been det.eniorating.

On April 30, 1980, t.he St.ate Water Commission enlered into an agreement. with t.he Towner Count.y l'lat.er Resource Dist.rict-. The purpose of this agreement. was to study t.he feasibil-ity of raising t.he control elevat.ion of Big Coulee Dam in order t.o improve condit.ions for boat.ing and fishing. This involved studying the cost.s to buil-d a new principal- spirrway, plug t.he ord drawdown pipe, and possibÌe modifications t.o the emergeney spillway. To improve the wat.er quality, the Board want.ed t.he modifications to inelude a low-level (hypolimnet.ie) wat.er qualit.y control struct.ure. A copy of the agreement. is included in

Appendix A.

The hydrology of t.he waLershed was re-evaluated in order to det.ermine the infl-ow to the reservoir. The infl-ow was flood rout.ed through several sizes of service spillways set. at'different. control elevat.ions. This was done t.o find the nost feasible st-ruct.ure capable of adequately handling the infLow.

The as-built. topographic map is included in t.his repcrt. as Plat.e 1. This reflect,s the various changes whieh happened over t.he Iife of the st.ructure.

The hist.ory of t.he problems associated with t.he dam was reviewed. A description of the geology.and hydrology of t.he watershed was also presented. This report summarizes the resuLt.s obt.ained from t.he fl-ood routing of the reservoir.

-1- Frelininary eost. estir-{at.es r¡ere presented for the atrLernat.ives proposed. These aLt:ernat,ives handied the design flows with a mininun of discharge over the emergency spitiway. Final pLanq and. a &ore conlplete cost. est.irnat.e would be pnepared after the project çouLd be aut.horÍ1ed and construetion ls imminen:t.

-2- II. DESCRIPTION OF SITE

Big Cou1ee Dam is located in Section 36, Township 160 North, Range 68 lvest in Towner County. The City of Bisbee, NorLh Dakota, is approximately 3/4 mile sout.hwest from t.he dam. Big Coulee Dam ls on the Big Coulee, which is a trjbu- t.ary of the Mauvais Coulee and in t.he Devils Lake Basin. Figure 1 is a map showing Lhe general location of t.he dam and t.he surfical geology of t.he area. Big Coulee Dam is a 37-foot. high earth-fil-I dam. At. the present, cont.rol elevat.ion of 1572 MSL, it. has a reservoir capaeity of 1,630 acre-feet while inundat.ing an area of 195 acres. The reservoir has a maximum depth of 24 feet. and an average depLh of 8.4 feel. A'drainage area of lOB square miles supplies infl-ow to the reservoir. An area capacit.y curve of the resenvoir is shown in Figure 2.

The slope of t.he main channel above t.he dam is approximat.el-y Z feet. per slile. Downst'ream the slope fl-at.tens out. to 4 feeL per mile. Ìüithin t.wo or t'hree miles downst.neam t.he topography of the land also fl-at.tens ouL. This causes Big Coulee t.o become wide and sJ-uggish as it. flows in a sout.h-sout.heast.erly di rect i on .

The very upper reaches of t.he drainage area extend t.o the eastern slopes of t'he Turtle Mount.ains near t.he town of St. John. The Turt.Ie Mountains, in t.his area, have so many Floughs and ket.ttes t.hat. very little of the area act.ually cont'ributes t.o t.he runoff collect.ed at Big Coulee Dam. Plat.e 2 depict.s Big Coulee Dam and it.s t.otal wat.enshed. The surfical geology of t.he Basin Area is primarity of glacial origin. This area is. locat.ed in t.he centrar Lowland province (Fenneman, 1938, p.559) and the Drift. Prairie as designated by Simpson (1929, p.4). Surfieal deposit.s in the dam site area consist entirely of glacial drift. The drift deposits are composed predominant.Ly of fiII and are similar in visual appearanee, but. differ

-3- S A. S K.A T C H E W A N MAN IT OBA I TU MI I

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Fisure t- LocATloN .oF BIG COULEE DAM AND PHYSIÓGRAPHIC PROVINCES OF THE AREA ii. : ,¡\ !¡. ii\'. I ¿ UUb gPIFFl- a 1 T -l-F

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'vs .of, qr" - n Nt toYn uf ssl e 13ljn3y ã \\ il Ç¡ qy TBlo lHcNl . Hf,l. Ol_ C 5\r¡ somewhat' in texture and composit.ion. Pierre Shal-e of the Cretaceous Age is the predominant t.ype of bedroek in the area. The present creek oecupying Big

Coulee is a recent. st.ream wit.h a drainage area of 108 square miles above t.he irnpoundment.

The embankment. is a zoned rolled eart.h-fill darn with a clay core. It. originally was 931 feet' Iong with a top widt.h of 12 feet. and a volune of

E2,191 c.y' The upst.ream slope was 3H:'1V while t.he downst.ream siope was 2H:1V. A 1O-foot wide bern was added about. 12 feeL below t.he fop along the right. half of the embanknent. on the downst.ream side. This berm has a 4H:1V slope. The t.op of the embankment is at. an el_evat.ion of 1585.0 MSL. A 1O-floot. wide cì_ay core trench runs t.he ent.ire lengt.h of the embankment. and ext.ends bel-ow the permeable sands int'o a elay loam soil. Riprap covers the upst.ream slope from elevation 1566.0 MSL to 1579.0 MSL. Plat.e 1 shows the t.opography of the entÍre damsi t.e area.

The emergency spillway is at. an elevation of 15TT.O MSL. It. is an uneon- trolled grassed spil-Iway with 3H:1V side slopes. Although the majorit.y of t.he emergency spillway is 200 feet. wide, t.he cont.rolling point. is'181 feet. wide. The steepest. portion of the emergency spiJ_l_way has a slope of 3.5 percent. A concrete drop inlet, a 54-ineh eorrugated st.eel pipe, and a concrete out.let st.ruct.ure are the components of the se¡"vice spitlway. The Ínlet. struct.ure controls the eievat.ion of t.he reservoir at. 1572.0 MSL. ÍL consists of a reinforced concrete drop struct.ure wit.b inside dimensions of I feet by I feet.

The floor of the st.ruct.une is 8 feet bel-ow t.he spillway crest.. The conduit. consists of 109 leet of 54-inch,1O-gauge HeIicalty CorrugaLed GaLvanized Steel Pipe. An open coneret.e box, 10 feet. long and 6 feet. wide, serves as the outlet. st.ruet.ure. No energy dissipat.ors are present in this structure. There is a

A-foot. cut.off wall on the outlet. end to prevent the st.ruct.ure from being

-6- ¿; *

undermined. A low level out.let, consisting of 156 feet. of tO-inch, lg-gauge

welded st,ee,I Fipe, disehrarges into t.he out.let. sL.ruct.ure. a gate va1ve, locat,ed I feet. flio.r¡ r[he oublet., cont.rols t.he. fl<¡w through t,his pipe. Double rail pi;plng and fit.tings serve as the trash raek.

¿

-T- III. STATEMENT OF PROBLEM

BACKGROUND

Initial int.erest. in the construction of Big Cou1ee Dam began with a request. for survey on Oct.ober 8, 1965. Borings were taken during mid-July of 1966 and the bid opening was on JuIy 11, 1968. AI1 work, excepl for seeding, was eompJ-et.ed on December 4, 1968. The project. was dedicat.ed on June 21, .|969.

Const.ruction of Big Coulee Dam was sponsored by the City of Bisbee, the

Towner County Park Board, the St.ate Game and Fish Department, the SbaLe Outdoor

Recreat.ion Agency, the Bureau of Outdoor Recreat.ion, and t.he StaLe l.iat.er

Commission. Actual const.ruct.ion of the dam cost. $ 107 ,693, recrealion facilit.ies cost Û17,214, invest.igat.ion and design costs were $6,4t+2, and Lhe cost of land was $27,500. Included in the t.ot.aL cost was þ32,251 allocated for cont.ract and related cost.s incurred during the 1969 fLood damage repair.

The init.ial fitling of the dam was on April 11, 1969. Water was flowing over the emengency spillway five hours aft.er the service spillway had been overtopped. The roar of t.he water v'/as not.iceable t.o t.he residents of Bisbee.

The flow peaked with a seven-fool head on t.he service spilì-way weir and Lwo feet of flow over t.he emergency spillway. The maximum fl-ow over the ernergeney spillway, which was 130 feet wide wit.h 3H:lV side slopes, was est.imated t.o be bet.ween two and t.hree t.housand cubic feet. per second. This large discharge caused serious erosion to t.he unseeded earthen emergeney spÍl-lway. Towner

County equipment. made emergency repairs to t.he spilrway. The spiì-lway $Jas closed by dozing large piles of frozen malerial across t-he spillway. The equipmenL was in operation from AprÍl 15 through April 17. After closure, t.he service spiì-lway was able to handl-e t.he receding flow.

-8- Foll-owing this emergency act.ion, the prime contract was extended to include repairing the badly seoured emergency spill-way. The bot.tom width of Lhe emergency spillway was widened.from 130 feet to 200 fleet.. (Alt.hough the main portion is 200 feet, wide, t.he controlling widt.h L¡as measured t.o be 181 feet'.) tfie slope of t.he.emergency spillway was redueed in order to lower the velocit.y of any water flowing over it. Tot.al costs for repair and revision of Lhe ernergency spilJ-way amounted t.o 632,2j1.

Grout.ing t.he ent.ire lengt.h of t.he service spiì-J-way pipe in order t.o fitl all voids which had been created by leakage of t.he joints v,¡as recommended aft.er an inspect.ion on June 19, 1969. It was also recommended that. t.he joint.s be sealed wít.h eit.her internal- expanding pressure bands or welding t.he joint.s.

Int.ernal expanding bands were installed on the joint.s of the helical corrugat.ed st.ee1 pipe in JuIy of 1969.

An inspect'ion on April 17,19TO, revealed t.hat the black dint. blanket had sLid down t.he slope of t.he clay sub-base. The ernbankment. loeat.ion is ideal for colÌect.ing huge snow drifts on t.he downstream sIope. The grass eover had not obtained sufficient root depLh to st.op t.he black dirt. from sliding when sat,urated by t.he melting drifts.

It. was discovered on May 22, 19TO, t.hat. there were two cracks in t.he headwall- of the service spillway. These L¡ere repaired Ín 19Zl by driving steel piling along each edge o1 t.he headwall and bracing across the t.op t.o support. and strengt.hen t.he headwall. llater had again overt.opped the emergency spi1lway during t.he spring of 1970.

During t.he spring of '1971, a flow of 6 inehes, or 200 cfs, was observed over the emergency spillway. An inspection on August 18, 1971 revealed that. the 54-ineh diameter service spillway pipe had deformed to an average size of 50 inehes by 57 inches.

-9- An inspection on May 9,1974, revealed that the Lopsoil had slid a1_ong a

150-foot. sect.ion of the downst.ream slope. l,'lat.er at a dept.h of i.5 feet had flowed over the emergency spillway during that. spring causing scme Camage. The eroded areas were repaired during t.hat. year-

IL was revealed that the t.opsoil on t.he downstream slope had again slid when an inspect.ion ulas rnade on May 6, 1976. The ND Arny National Guard commenced repairs on t.he embanknrenl cn August. 28, 1976- They st.arted repairing t.he emergency spillway and st.abiJ-izing the downst.ream embankment. by adding malerial to change the slope to 4H:lV. This fill extended from the right side of t.he service spillway outl-et. t.o the right. abutment. I,lork cont-inued through lale fal-l wit.h weekend dritls and was cont.inued t.he foLlowing year during June and July. The compacted fill was brought. up to wit.hin 12 feet of the top of t.he embankment and lefl as a berm. The topsoiJ- was then replaced over t.he new fiLl materÍaI.

An inspect.ion on June 14, 1977, revealed t.hat almost. all of the asphaÌt coatÍng had peel-ed off the service spillway pipe. Corrosion was t.aking plaee along the bot.tom 1/3 of the pipe. A Bitumastic eoat. was applied during jg7T. !'lat.er at a dept.h of 2.5 feeL had flowed over the emergency spillway during the spring of 1979. This large discharge eaused a great. deal of erosion t.o the emergency spiJ-lway. Over half of the topsoil was washed away. Sevenal pits were formed wit.hin'the emergeney spillway. Repairs Írere made at a cost of $10,180.

There was a discrepancy in what was thought to be t.he elevaLions of the different component.s of the dam. The correct. elevaLions and dimensions of t.he damsite were obt.ained from a survey eonducted in August of 1980 for this re- port, An inspect.ion of t.he spillway pipe on December 2p, '1980 revealed that t.he first. joint required addit.ional seaLer. A eoncent.rat.ion of rust was presenL on Joint. 3.

- 10- EXISTING AND ANTICIPATED PROBLEMS The maintenance cost's for Big Coulee Dam have been high. In the dam,s 13 year exist'ence, the emergency spiJ.lway has been used at Least. five times. Water has fl-owed over t.he emergency spiJ-lway Ín the spring of 1969, 1970, 1gT1, 1g7U, and 1979' The emengency spÍllway suf.eered a large amount ofl scour damage during the 1969, 197t1, and 1979 ftoods. Even wit.h a good grass cover, the high velocit'y of flow eaused by a large snowmeLL is eapable of eausing considerable erosi on.

Aeconding t'o it's history, the service spill-way for Big Coutee Dam cannot handle anything great'er than a 3-year snor¡melt. without the emergency spÍrlway being used. At' the present. elevation of the service spirrway, t.he emergeney spiJ'l-way is used too frequentJ-y. Simply raising the lever of t.he exist.ing service spilJ-way woul-d eause water t.o flow oven the emergency spÍl1way more frequent'ly, endangering Lhe dam. rf t.he cont.rol elevat.ion of Lhe reservoir were t'o be raised, a complete new service spillway woul-d have t.o be const.ruct.ed. It should be designed so that. t.he emergency spill.way is not used durjng t.he Z5-year snowmelt'- For largen runoffs, t.he velocity of fl-ow over the emergeney spillway should be kept. smarr in order i.o p"euent. scouring.

The downstream embankment was originatty const.ructed with a ZH:1V slope. The right' hal-f of this enbankment suffered an annuar sroughing probrem until correcLive measures were t.aken in l9T6 and 1977. The embankment has apparently been stabilized by t.hese efforts. RaisÍng t.he control elevation of the reser- voir could cause t.he port.ion of t.he embankment. t.hat. js still at. a 2H:lv slope t'o become unstabLe. Att.hough minor seepage exists currentry, t.he increased water pressure may eause greater seepage through t.he dam. The service spillway consists of an 8-foot. by 8-foot weir wit.h a 54-inch CMP leading from iL. The pipe, set. at a st.eep slope of 13.6T percent., cannot maint.ain full pipe flow for any length of t.ime. llhen flowing under partiarly

-11- full- condit.ions, hígh vel-ocities are obtained due to t.he steepness of the pipe.

This hígh veloeity causes the wat.er t.o entrain air, thus swelì.ing in vol-ume and filì-ing the pipe. The increased wetted perimet.er creales greater frictional resist.ance and reduces t-he ilow rat.e. This causes the air t.o become separated from the wat.er. The flow then returns to a partially fulI condition. This process' called slugging, is repeated cont.inuously. It. is t.he cause of fhe l-oud roar occasionally heard by Bisbee resident.s. The slugging effect. causes a large pnessure differenLial wit.hin the pipe. Strong vibrations ane produced, which ean jan t-he pipe fnom its posit.ion.

There has been some deteriorat.ion of t.he met.al spilJ-way pipe. Rust.ing has been evident on the bott.om 1/3 of t.he pipe and at the joints. The Bitumastic coating appears to be retarding t.his process at. the present time. There has been some leakage at t.he joinls which could eause the soil support.ing the pipe to be washed away. Due to t.he weight of t.he soil above it, t.he pipe has deformed t.o an average size of 50 inches by 57 inches. This pipe will probably have t.o be replaeed in the fulure.

The low level drawdown pipe also present.s a problem. The gate valve is located near t.he out-Iet. end of the dam. This allows t.he pipe, whieh extends below Lhe dam, t.o eontain water under pressune. There is the pot.ential for t'his pipe t.o rust. out and cause a large amount. of seepage. This could easily eome to t.he surface and cause exlensive erosion or failure of the st.ructure. The Cit.y of Bisbee has receÍved poor qualÍty wat.er fnom the reservoir.

This is due Lo eutrophicaLion and the strat.ifieat.ion of t.he wat.er in t.he reservoir- St.agnant wat.er concentrates on t.he bott.om while the fresher wat.er stays on t.op and is discharged downstream t.hrough t.he spilJ.way. A low Level

(hypolimnetic) wat.er quality cont.rol struct.ure could be const.ructed to help alleviate this problem. This struet.ure would discharge the stagnant water from

-12- t.lre bs1.tsn, allowing t.he reservoln t.o be fi'eshened by inflows. .The ¡¡ater

. quality of the neservoir woutrd improve, due to an lncrease in the dissolve.d oxygen and a decrease in nutrients which cause.algae At. the present el-evat.ion of t.he reset'voir, pleasure boat.ing ls only possible in t.he area of t.he original coulee. Thè average depth at present is 8.4 feel. would be inereased to 8.9 f,eet .if lhe cont.roÏ elevat.ion ¡¡ere .Thís raised 2 fe,et,. An average depth o.f 9.7 feet.'¡ouLd be obtaÍned ifl Lhe reservoir "$rere raised 4 feet. Tbere çrould still be a J.arge amounl of shallow âFêâs. However, the area accessible to boaLing t¡ouÌd ínerease.

Und'en t.he reservoirrs present..condiLions, many of t.he fish do Rot survive through the wint.er. The poor qualit"y of the water nay be as big ä eon:tribl¡tór to this problem as t.he shallow dept.h is.

î

- 13- IV. HYDROLOGY

DISCUSSION OF THE STUDY

The hydrology of the ent.ire Big coulee wat.ershed was analyzed using the TR-20 comput'er prograrn which was developed by the u.s. conservat.icn service- rt r¡as used to deternine the peak erevat-ions, peak infllow, and peak discharges of various frequency storms. The program forrnulates a mat.hematieal, hydroì_o3ie model ol the walershed. This is based on t,he forlowing daLa: t.he amount. of rainfarl-, rainfatl distribution, soiì_ type, land use, and the hydrauric eharacterist.ics of t-he st.ream channers and drainage area. The Big couree watershed ext-ends from the dan nort.hwestward to incl_ude the eastern sropes of the Turt.le Mountains near t.he t.own of sainL John. rt ineludes 108 square niles of drainage area. rn order to obt.ain mone accurate data t.o input int'o the TR-20 program, t.he watershed was divided into 22 sub-basins- Each of these sub-basins has it.s own distinct drainage boundaries exeept for four divisions wit-hin t'he TurLle Mountains. These regions r^rere separated from t'he remaining sub-basins beeause of the differences in topography. They are comprised of sevenar sloughs and ket.t.les which are capable of storing most of t'he runoff' For t'he input on t.he TR-20 program, a reduct.ion in the effeet.ive runoff r^¡as made in order to aceount. for this storage capacit.y. Cross sect'ions F¡ere,taken at. several point.s along t.he main channel of Big coulee' The surface roughness and type of vegeLative growt.h was not.ed for each cross-sect.ion in order to obtain a roughness eoeffieient (n). The slope, n_value, and the dimensions of the channel for each cross-sect.ion were Ínput- int.o an open channel program utilizing Manning's formula. The output from Lhis program provided the area and discharge at 1/2 foot int.ervars fon each cross-seet.ion. This was put int.o the TR-20 pnogram.

- 14- No special allowances were made for point.s where t.he ereek crossed roads utilizing culvert.s for drainage. These erossings do not impede t.he flow t.o any measurabl-e degree because t,he water can easily overtop t.he road. The only crossing t.haL could back up t.he flow an apprecÍabl-e amount is t.he t.wo 7-foot. diamet-er culverLs under the railroad tracks in Section 1, Township 161 North, Range 69 Ítest. The rat.e of ftow through t-hese culvert.s was t.aken into accounl on t-he TR-20 program.

A general land use deseript.ion of t.he v¿at.ershed, wit.h t.he except.ion of t.he Turtle Mountain Sub-basins, is as follows: Past.ure 41í

Small Grain 39?l FaIIow 18í

Farmst.eads and Roads 2f

The divisions wit.hin the Turt.le Mountains are predominant.ly woods and past.ure land. About. 4O% of the area is sloughs and kelt.les.

The rat.e of flow over t.he emergeney spillway was det.ermined by inputting an n-valve and'the dimensions int.o an emergency spiJ-Iway rat.ing curve program. The discharge through t.he service spillway was deLermined by using forrnulas for weir flow, orifice flow, and full pipe flow.

. The TR-20 program produced out.put. of t.he infiow into the resenvoir. This inflow was flood rout,ed t-hrough several shapes of service spillways set at different cont.rol elevat-ions. This was done t.o find the most- feasible structure t-hat. wilI meet Lhe recommended guidelines. These result.s showed t.he peak elevat.ions and discharges for eaeh sit.uation_

-15- r:L, i:.{, i r3. ùr:,v+sìr.i#.tuìiÉ-É jú-jilÀ*rirájae

STUDY RESULTS

The TR-20 eomput.en program showed that. all floods due t.o spring snowmelts were greater than t'hose due to rainfall. These result.s are supported by the fact' that' aLl the larger fl-oods at Big Coulee Dam have oecurred during t.he snowmelt period- The eomput.er results also confirm t.hat t.he t.wo 7-foot diamet.er eulverts in Sect.ion 1, Township 161 North, Range 69 West cause wat.er t.o back up behind them' The dischange raLe from the drainage area upstream from Lhis point. is eont'rolled by t.he fl-ow¡"ate t.hrough these culvert.s. This control poinl causes the peak inflow at Big Coulee Dam t.o be reduced.

TabLe 1 shows t'he peak inflow to t-he reservoir for various frequency sno!¡me1t,s as obtained from Lhe Tn-20 model, The peak reservoir elevat.ion and peak dis- charges through the service spillway and emengeney spillway are arso shown for t'he present condit.ions af Big Coulee Dam. The veloeit.y of flow over the emer- gency spillway is also shown in order t.o see if scouring would occur. The result's obtained from the computer program appear to cl-osely coineide wit.h t.he act.ual condit.ions experienced at Big Coul_ee Dam.

Any revisions made t.o t.he service spiltway at. Bíg Coulee Dam should neet. cert.ain design requirements in order t.o ensure the safety of the dam. No flow should be allowed to pass over the emergeney spillway during a 25-year snowmel.t.

For events larger t.han t.he 25-year snou¡meit., t.he velocit.y of flow over t.he emergency spillway should be limit.ed so that scouring does not oceur. The velocity at which seouring occurs is dependent on t.he t.ype of soil- and grass cover. For the conditions at Big CouLee Dam, scouring will- occur when the vefocit'y is in the range of 4 feet.per second t.o 6 feet per second. The velocÍt.y of flow should not. be allowed to exceed t.his range. The present. weir and conduit are not eapable of meeting t.hese requirements. An ent.irely new service spillway would have to be const.ructed if the reservoir elevation were t.o be raised.

- 16- TABLE 1

NOUTING RESULTS FOR EXISTING CONDITIONS

Diseharge of Discharge of VeLoci t.y 1n Cont.noL Etrevation Frequency of Snowmelt. Inflow Elevat lon of Servjce Spillway Energency Splllway Emergency (cfs (cfs (ofs ( ) Reservolr ) ) 8pi 1i¡¡ay f ps ì.

10 year sno¡¡melt, 11622 1578.8 561 960 6. lt

1572 MSL 25 year snowmelt 1,822 1 579.0 565 1 ,17A 7.0

50 yeâr snowmelt. 2,436 1579.5 654 1'' 700 8. r

100 yeãn snowmeLt. 3, 143 1580,1 740 2,480 9.4

I { I

All values shown are t.he peak values for fhat. frequeney snowmelt,.

Emengency spillway at an elevat.ion of 1577.0 MSL. A service spiì-lway, aL eaeh proposed control efevat,ion, vJas found that would meet the suggested requirements. The resuLt.s obtained by rout.ing t.he inflow of t.he reservoir t.hrough each service spiltway showed whet.her iL coul_d adequately handle t.he flow. These result.s are shown in Table 2. A cost esLÍmat.e was completed for the construct.ion of a service spillway at. each cont.rol eleva- tion. ft. was found that. t.here would be a relat.ively smal1 difference in Lhe cost of const'ruct.ing a service spillway which woutd raise t.he contrcl eievaLion of the reservoir by 1 foot or 2 feet.- Therefore, t.he 1 foot rÍse was disnegarded.

Addit-ional land would have to be aequired if t.he cont.rol el-evation $rere raised more than 2 feet. Pl-afe 3 shows t.he boundaries of the land that. have been acquired around the reservoir. It also shows the land inundat.ed under t,he present. condit.ions and with a Lwo foot, and four foot rise in the reservoir elevat.ion. The emergeney spiJ-Iway would al-so have Lo be raised in order to cont'rol the amount. of water flowing over it.. This would increase t.he ehance of ,¡60 water overLopping its banks in the Nl'¡ 1/4 of Sect.ion 3'l , Township Nort.h, Range 67 vlest. This water could spread over a large area. A smal-t dike could be built at the low area to prevent. t.his from happening. The sane sit.uation exists whet'her t.he reservoir were raised 3 feet. or 4 feet, Therefore, raising t.he reservoir 3 feet will not. be Iist.ed as an alt.ernat.ive.

Figure 3 is a hydrograph of the inflow int.o Big Coulee Dam for the 25-year sno¡¡melt'. The dischargeà under current. eondit.ions and wi th a 2 foot. and 4 foot. rise in the reservoir elevat.ion are also shown in t.he hydrograph. The out.flow rates l¡Iere nearly ident.ical- whet.her the reservoir hras raised 2 feet or 4 feet. One line represents bot.h discharges on the hydrograph.

A dam is considered unsafe if it cannot pass one-hal-f of t.he probable

Maximum Flood (PMF) wit.hout overt.opping occurrÍng. About. ?5 percent. of t,he pMF passes before overtopping occurs under the eurrent condit.ions of t.he dam. A

- 18- TABLE 2

NOUTINb RESI]ITS FON PFOPOSED MODIFTCATTONS

Discharge of Dlsoharge of Velocity in Cont.nol Elevat.lon Frequeney of Snowmelt, Inflow Elevat,lon of, Servi ce SpiJ.Iway Bmergency Splllway Emergency (cfs) Reservofr o (cfs) Spi tl-way ( f ee ).

10 year snowmeLt. 1 ,622. 1576.9 1 ,600 0 0

t5?4 MSL1 25 year snowrnel t: 1r82? 1577 .1 1,7TO 30 x.?

50 year sno¡melt, 2,u36 1577 .6 ?,230 1'60 3.2 '!l{3 100 year snowmelt 3 , 1578.2 2,540 u75 4.E

a 10 year sno¡¡melt 1,622 1578.9 1,580 0 0 t 15?6 MSr2 25 yaar snoüflmelt '1,822 1579.1 1,735 3o 1.7

50 year snohtmelt 2,436 1579,6 2 1200 160 3.2

100 year snohmelt 3,143 1580.0 2,667 353 4,4

AlI values shown are t.he peak values f,or that frequency'snowmeLt.

1Émergency spÍIlway aL an elevat,lon af 157V.0 MSL.

2E spillway at an elevatl.on ot 1579.0 MSt. ""genoy I

I

I

ii I I il 1 ï

I

v T -l -t- -l ...1 # t i ,il I 1 -- IF tl +$ '.l-l- -l- :] {t + l

asTr looJ tf pue asTr looJ ¿ e L{1Tû1 a8;eqoslq

c suoT? Japun a8;eqoslq H Ipuoc luesa.rd U -t

¡4oïJuï _ll I I

I t- -t. l I

I NVC g!1n0C ÐIg

HdvuÐouoÃH uvgà 9z

-l E sunÐrJ I l- l-it i.H

Y t rì l(v* c:ttt -¡l ll o1 H:lfilìll v I I1 lllly 1r\l TJ è,81Õ tl¡/t . t{tl . Í)I I üaxÍmun ol 0.85 feet, oventops the dar¡ upon oeeurrence of ttre pMF. I¡aler ç¡ould fLow over the top of. the dam fon 11.2 bours. The'FMF ¡¡as rout.ed t.hrough the serviee spillways proposed for construction Íf t,he re,servojr was rajsed 2 feeL or 4 feet above the exist.ing cont.rol elevat.ions. I,laLer did noL flo+¡ over t,he lop of t.he daln for eit:her of these situaLjons.

-?1- :a< r - :@!-r4< ¿: @\rtffi j,tì-1ô::.¡siÉijÀSJr¿.rtåb-Þ

V. ALTERNATIVES

AlI the possible combinations of spillways have been narrowed down to t.he t.hree most. feasible solutions. These alternatives are dependent on t.he el-evation of t.he reservoir. The Towner Count.y tlat.er Resource Distriet. has the oplion of doing nothing at.the present. t.ime, inst,all a service spillway which would raise the elevat.ion of the reservoir by 2 feet., or to conslruct a spillway with a conLrol- elevat.ion 4 feet. higher i.han Lhe current. el_evaLion.

Several changes should be made if a serviee spillway is constructed. The port.ion of t.he downsLream embankment. which is currenlly at a 2H:1V slope should be changed to a 3H:'lV slope in order t.o ensure t.he st.abiJ-it.y of t.he embankment..

Plat.e 4 shows t-he area of t-he embankment. where the slopes are Lo be changed. Figure 4 shows the changes t,haL shoutd be made for t.he different condit.ions of the downstream embanl:nent.. The 54-inch CMP and the low level- drain should be removed. A low level (hypolimnetic) wat.er qualit.y control slruet.ure could'be const.ruct.ed. A Saint. Ant.hony Fall-s (SAF) st.iì-ling basÍn, complet.e wit.h energy dissipat.ors, should be const.ructed for t.he out.Iet. st.ruct.ure.

ALTERNATIVE 1

The first. choice available to t-he Board is t.o do nothÍng at. t.he presenL t.ime. Alt.hough t.here are no init.ial cost,s, t.he costs to maint.ain the dam in good eondition could becpme veny high. The service spillway pipe has been de- t.eriorating. It will most. likely have to be replaced in t.he fut.ure. The energency spillway is vulnerable to seouring damage due t.o t.he high velocit.y of s¡at.er frequent.ly flowing over it.. Raising t.he conLrol elevat.ion of t.he reservoir by inst.all-ing flashboards on t.he serviee spillway, would jeopardize the safet.y of t.he dam. this procedure would increase t.he frequeney that the emergency spillway would be used. The velocit.y of flow, and chance of scouring occurring on it., would also be increased.

-22- UPSTREAM DOWNSTREAM

\l 5 2 3 New Embonkment

Exlstlng Embonkment

TYPICAL CROSS-SECT|ON BETWEEN STA. O+OO AND STA. 6+gO

New Embonkment UPSTREAM DOWNSTR EAM 2 N- Embankment 4 5 Erlstlng 7

TYPICAL CROSS - SECTION BETWEEN STA. 5+OO AND STA. 9+3I

Flgure 4 Therefore, the likelihood ol the da;o being washed out would inerease.

åLTERNATIVE 2

A 30 foot by 3o foot. weir with a 3O-foot wÍde chute coul-d be constructed at. a cont.rol elevation of l5Z4 MSL. This chut.e Ís shown in Figure 5. This woul_d const.itut'e a 2 foot rise in t.he eLevalion of the reservoir. The area Lhat the

wat.er inundat.es would increase fron 195 acres Lo 226 aeres while the storage

capaeity wourd change from 11630 acre-feeL t.o 1r99g aere-feet.. This would

increase the average dept.h of the reservoir from 8.4 feeL t.o g.9 feet.. The peak

elevat'ion of the reservoir and peak discharge through t,he serviee spillway and

emergeney spillway for different- frequeney st.orms are shown in Table 2.

This alLernat.ive is estimaled io eosL $3ZO,0OO. The breakdown of these costs are shown in Table 3. The cost. of acquirÍng land is not. Íneluded in this

est.imat.e- TiLle woul-d have t.o be acquired for an addit.ional 17.33 acres. A flood easement. is currently in effect. for t.his land. lJat.er would permanently cover Lhis area if the spillway were const.ruct.ed at an elevaLion of t5Z4 MSL. A

flood easement should be obtained for land which would be temporariJ_y inundat.ed by the '100 year snowmelt. Approxinat.eì-y 50 acres is bel-ow this elevation of

1578.2 MSL. As shown in Plate l, the najority of t,his Land is located along the upper reaches of the reservoir, alt-hough there are severat smal1 areas whieh extend beyond Lhe boundaries of the land originall,y acquired for t.he resenvoir.

ALTERNATIVE 3

A 30 foot. by 30 foot' weir with a 3O-foot. wide chut,e could al-so be installed at. an elevat.ion of 1576 MSL. This would allow a 4 foot. rise in t.he elevation of t.he reservoÍr. A 2 foot rise Ín the emergency spil1way, t.o l5Z9 MSL, would be required in order t.o meet. the reeornmended guidelines for t.his project. The

-24- elr Chute S.A.F. Stllllng Bosin-+

Top of Chute Er. t574.O 5 Llne ìfEÐpì E= E Elr r= ÈB =G@ = =, : = =¡ =:E =,=: =,= :.: :,:,: I Erhilng tO,' Steel plpe -_:: El. 1548.0

ELEVATION

30 24' o 8.5' 32' 23

ttr trl trt Itr E r E¡ ]_ ¡tl

Hypollmneilc Struclurc

PLAN

Flgure 5 - 30 FOOT WIDE CHUTE - Scqle: l";30' TABLE 3

PRELIMINARY COST ESTIMATE - ALTERNATTVE 2

2 FOOT RISE ]N THE CONTROL ELEVAT]ON OF BIG COULEE DAM

ITE},I QUANTITY UNIT UNIT PRICE TOTAL

1. Drainage L. S. 5,000 .00 $ 5,ooo 2 Si t.e Preparat i on and Res i.oraii on L. S. 4,000 .00 4 ,000

3 Coffer Dam 8,OOO c.Y. r .50 1 2,000

4 Salvaging and Spreading Topsoil 1 Z,OOO S.Y. o.20 2 ,400

5 Earthwork (pipe recovery & chut.e preparation) 3,500 c.Y. 2.00 7 ,000 6. Earthwork (achieve 3:1 slopes) 4,OOO c.Y. 1.50 6,ooo

7. Concnet.e (sidewall-s) 3OO c.Y. 300.00 90,0oo 8. (base Concret.e slab) 3OO c.ï. 250.00 75,ooo

9. Rest.eel 84 , OOO Lbs 0.50 42 ,000 't0. Sheet. PilÍng 300 L. F. 20.00 6 ,0oo

11. Structural Piling L. S. 7,000.00 7,000

12. Rock Riprap 400 c.Y. 20.00 8,000 13. Rock Fiprap Filt.er Material 150 c.Y. 10.00 I ,500 14. Low Level Drawdown Syst.em L.S. 8,000.00 I,0oo 't5. Underdrai nage L. S. 6,000. oo ó ,000 16- Seeding z Acres 150.00 300 17. Sodding 3OO s.Y. 8.00 2,400

18. I'Iat er 2OO 1 ,000 Ga1. 3.00 600

19. Demolition and Removal L. S. 2,500.00 2,5OO

Subt.ot al 9285 ,200 Cont.ingeneies 2T,3OO Const.ruet.ion Inspect.ion 28,¡OO Cont.ract. Admini st.rat ion 28,500 TOTAL $37O|OO0 Does not include cost. of acquiring land

-26- st.eepest. slope ofl the emergency spil-Iway should not. be Íncreased when the fill is added. A steepen slope on the emergency spillway would allow the velocity of fLow to increase. The area inundat.ed by t.he reservoir woul-d increase from

'195 acres l.o 25T acres whire the capaeity would increase from 1630 acre_feet. to

2Ll8l acre-feet. This aÌLe¡ native woul-d increase the average depth of t.he reservoir from 8.4 feet. to 9.7 feeL. Table 2 shows the results obtained from nouting the inflow of various frequeney snowmelts through t.his sLrucLure.

A low area exists in t.he lir¡i 1/\ of Sect.ion 3i, Township 160 Nort.h, Range 6Z t'lest. (See Plat'e 3)' l,Iat.er could overtop t.his area when t.he reservoir reaches it's peak elevat.ion during fhe annual snoumelt.. This water would t,hen spread across the count.ryside. Apparenlly there was no problem wit.h wat.er overt.opping t.his point. during t.he 1979 snowmelt. Wat,er was flowing over the emergeney spillway at. a depth of 2.5 feet during that spring. Therefore, no overt.opping should oecur aL this l-ow area below an elevalion of 1579.5 MSL. Írrit.h this alLernative in operation, the 1O0-year snowmelt would produce a peak reservoir elevat.ion of 1580 MSL. A snall dike could be built. to prevent any high wat.er from flowing over t.his area.

The increased cont-rol elevation of the reservoir would cause t.he beach and boat launching ramp at Lhe recreaLional site t.o be inunda[ed by the normal poot.

The wat.er level would be nean the edge of the parking area. Modificat.ion could be made at an est.inat.ed .cost. of $41000.

ThÍs alt.ernative is estimated t.o cost $390,000. The costs are broken down int'o individual items in Table 4. The eost. of acquiring land and modifying the beach area are not, included in this est.imale. At. t.his stage of planning, the only difference in cost betr¿een the const.ruction of alternative 2 and 3 is due t.o the earthwork involved in raising the elevation of the emergency spillway and construct.ing the dike. The higher control elevation also requires more land to

_27 _ TABLE 4

PRELIMINARY COST ESTIMATE - ALTERNATIVE 3

4 FOOT RISE IN THE CONTROL ELEVATION OF BIG COULEE DAM

ITEM QUANTITY UNIT UNIT PRTCE TOTAL

Drai nage L.S 5,000.00 $ 5,ooo 2 Sit.e Preparat.ion and Restor aLion L.S 4 ,0oo - oo 4 ,000

3 Coffen Dam 8,ooo c. Y. 1 .50 1 2,000

4 Salvaging and Spreading Topsoi I 1 7 ,000 S.Y. 0.20 3, 400

5 Eart.hwork (pÍpe recovery and chute preparat.ion) 3,500 c. Y. 2.00 7 ,000 6 Eanthwork (achieve 3:1 slopes, dike, emergency spillway revision) i0,000 c. Y. 1 .50 I 5,000 (sidewalts) 7. Concret.e 300 c. Y. 300.00 90 , ooo

8. Concret.e (base slab) 300 c. Y. 250.00 75 ,0oo

9, Rest.eeI 84, 000 Lbs. 0.50 42 ,000

10. Sheet Piling 300 L.F 20.00 6 ,000

11. St.ructural PiIing L.S 7,000 . oo 7,ooo

'12. Rock Riprap 700 c. Y. 20.00 't4 ,000 13. Rock Riprap FiIt.er Material 250 c. Y. 10.00 2,500

14. Low Level Drawdown System L. S. 8,ooo.0o 8,o0o

15. Underdrâi nage L. S. 6,000.00 6,ooo

16. Seedi ng 4 Acres 150.00 600

17. Soddi ng 300 S. Y. 8.00 2 ,4oo

18. I'lat.er 300 1,000 GaI. 3. oo ç'00

19. Demolition and Removal L. S. 2 ,500 . 00 2 ,50o Subt.ot.al $303,300 Cont.ingenci es Z6 ,iOO Construction Inspection 30,000 Cont ract. Admi ni st.rat i on__39r999 TOTAL $390,000 Does not include cost. of acquiring land

-tA- be purehased'. As shor¿n in PLat.e 3, the reservoÍr elevat.ion of 1576 l,,lSL ¡¡ould exfend beyond t'he bsundaries of the Land originally purchased fo:¡" Lhe reservoÍr.

ApproxÍmatel-¡¡ l$ acres of la¡ld r¿ill have io be aeguii.ed a-t varjous poinis along the neservQír. eurrenLJ.y, 17.33 aeres of thÍs a¡nouRt is included Ín flsod easemen¿. Flood easenent.s should be obtained for any other land surrounding t.he reservoir which is o-eLor.¡ an etrevation of 1580 MSL. This is t,he- elevation that. a 100-yean snomnelt would at.t.ain. Eighty acres of lanC are included in Lhis designation. The najority of this aereage is in t.he upper port.ions of the reservoi r.

-29- VI. SUMMARY AND RECOMMENDATIONS

SUMMARY

Big Coulee Dam provides t.he municipal water supply for the City of Bisbee.

The qualily of the water has been poor. The reservoir al-so serves as a recreat.ional area for the community. Reeently some problerns have arisen- Lccal resident.s have compl-ained t.hat, pleasure boat.ing is only possible in t.he area of the original coulee. Ì"lany of the flish do not survive t.hrough the wint-er- The

54-inch metaL spillway pipe has been deteriorat.ing. It has deformed due t.o the weight of the soil above it.. Rust. is evident on the bot.t.om 1/3 of t.he pipe.

A dangerous situat.ion exists because t.he val-ve fon t.he dnawdown pipe is on t.he downstream side of the structure. It. could cause severe damage if it. breaks. The emergency spillway is suscept,ible to scouring since wat.er frequent.l_y flows over it at high velocities during the spring snowmelL.

The inflow for various frequency snowmel-ts was obLained with t.he use of t.he

TR-20 comput.er program. Fl-ows from a Z5-year snounnel-t. were routed t.hrough several combinat.ions of serviee spillways set. at. different. cont.rol elevations.

This was done in order t.o find t.he nost feasible st.ruet.ure which could adequately handl-e the inflow while allowing an increase in the normal reservoir elevation. Tabl-e 5 summarizes the results oblained for the three alt.ernatives. Three alt.ernat.ives !'tere looked at. in lhis sludy. The first alt.ernat.ive involves doing nothing aT t.he present. time. This may resul-t in high maint.enance costs in t.he fuLure due to erosion on t.he emergency spiJ-Iway. The service spillway pipe has been det.eriorat.ing and may have t.o be replaced in the future.

Similar1y constructed spiltways in t.he St.ate have expenienced problems and have had to be replaced.

The second alt.ernative would raise the control elevat.ion two feet to an elevation of 1574 MSL' In onder to raise the cont.nol el-evation of t.he reservoir,

-30- . ji: . 1..qil4rtuÈtt1+1

TAELE 5

SI'I,IMARY OF ROUT'ING RESULTS FON PROPOSED ATTERNATIVES

Diilcharge ln ' Velocit.y in Frequency of Snowmelt Besenvoir Elevat.lon Emergeney Spillway Ernengency Splllway (cfs )

25 year 1579.0 1'1?0 ?.0 Alternat.ive 11 tl8CI 1O0 year 1580. 1 2 , 9.4

.l 25 year 1517.1 3o 1.7 Alt.ennat i ve 2' 100 year 1578.2 [tt5' 4.8 wI I 25 year 1,579 .1 30 1,7 Allernatlve 32 .l00 year 1 580.0 353 4.4

All t¡alues shown are the peak values f,on t.hat frequeney sno!ümelt..

lEhergency sp11.Lway at. an elevation of 1577.0 MSL.

2Emergency spillway at an elevaLion of 1579.0 MSL. a complet'ely new service spilrway wil-l have t,o be const.ruct.ed. A 30 foot by 3o foot weir wit'h a 3o-foot. wide chut.e was found t.o be t.he best spitrway for this sit'uation' Average depth of the reservoir would increase from 8.4 feet to g.9 feet' The increased elevation of the reservoir wouLd require thaf 17.33 acres, únder 'currently flood easement, be bought.. A fl-ood easement shoutd be obtained for 50 acres of l_and belors t.he 1OO_year flood elevat.jon of .15Tg.2. This alt.ernat.ive is est.imat.ed to cost $3Z0,OOO, not including land cost.s. Art'ernat'ive I consist.s of eonstruct.ing the same lo foot. by 30 foot. weir and 3O-foot wide chut.e at. a cont.rol elevat.ion of 1526 MSL. This would provide an average dept'h of 9'7 feet'- The emergency spilrway would have to be raised 2 feet' t'o an el-evat.ion of 1579 MSL. A smart dike wourd have t.o be eonstruct.ed in the Nl'l 1/4 of section 31, Township 160 North, Range 6z !,lest. prat.e 3 shows the locat'ion of t'his low area. l,IiLhout a dike, water could overt.op t.his regÍon and spread out' over a large area of land during the 100-year snowmel-t. Thirt.y-six acres of l-and wirl have t'o be purchased to accommodate t.he rarger reservoir area. A flood easemenL is currently in effect for 12.33 acres of this. Fl_ood easements shourd be obt'ained for 80 acres of tand below t.he loo-year frood erevalion of 1580. The est.imated cosl for this alt.ernative, not including the cost of obtaining 1and, is $39O,OOO. rncluded in the costs for alt-ernat'ive 2 and I are sevenal ehanges which should be made if a new service spillway is constructed. The 54-inch CMp and low level drain should be removed. Inst.al'at.ion of low (hypolimnetic) 'eve' water quarit'y cont'ror strueture would help improve Lhe water quarity by removing st.agnant water fnom the bot.tom of the reservoin. A Saint Ant.hony Fatls (SAF) st.illing basin, complet.e with energy dissipat.ons, should be const.ructed for the out.Iet. st.ruct,ure. port,ion The of the downstream embankment. which is current.ry at a 2H:1v slope should be nodified so the sropes are 3H:lv t.o ensure the stability of t.he embankment.

-32- RECOMMENDATIONS

The three alternatives pnesent.ed in t.his report. are al-l viable, alt.hough some may be more desirable Lhan ot,hers. Several problems current,ly exist wiLh the water quaLit.y and t.he operat.ion of t.he Big Coulee reservoir. Therefore, alternat.ive 1, do noLhing, wouLd be of no val_ue_

Alternafive 2, the two foot increase, would improve t.he waLer quaì-ity of the reservoir- peak The water Ìevel-s, due t.o runoff, would be l-ess than t.hose for the exist'ing condiLions since t.he capaciLy of fhe spillway wourd be iner^eased. In additÍon, use of t.he emergency spiÌlway would be redueed and t.he velocit.y fl-ow of over it' woutd be reduced during t,he larger snovmerts. Tne greater reservoir dept,h would increase the area avairabre t.o boat.ing. ALternative 3, the four foot increase, wouJ-d arso improve the water quaì.it.y of t'he reservoir' rt' would provide a langen area for boating t.han alt.ernative 2' The average dept'h of the reservoir wourd arso increase. use of the emergency spillway and t.he veiocity of flow over it. during the larger snow- nelts would be reduced t.o the same revel as obLained in alternat.ive Z. Ex- cluding t'he cost of acquiring land, this alt-ernative woul-d cost abouL six percent. more than al_t.ernat,ive 2. Al-though alt'ernative 3 appears t,o be the best choice, a pnoblem exisLs due Lo the higher water level. The normäl poor wourd complet.ely inundat.e the beach and boat' launch area. The water revel woutd extend crose to the bath houses and parking (see lot Prate 3). The peak wat.er revels wouLd be about the same as for the exist.ing eondit.ions. A port.ion of t.he parking lot. nay be inundat'ed during t'he peak level- The remainder of the recreation area shourd not. be affeet'ed. Possibly, t.he beach could be ext.ended t.o a highen erevation or moved to a different' loeation. This modificat.ion ü¡as est.imat.ed to cost $4,ooo.

-33- the ?ou¡ner County ïlate¡'Resounce Board must. deaide ç¡Aåt alternat.ive i.ü

deens the nsst. feasible. Det.eriorat.ion may cause sorne spiì"lway improvemenL to be required in t.he near fuLure. Therefore, the first. al[ernalive ls.not recommended; Al.though alternati"ve J is the ¡nosL costly, it ie neeommended as fhe þreferred a.lternative. The Boa¡'d nust. weigh t,he value of inproving the

.. depth of the reservoir while losing part, of the exist.fng recreation area.

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la o'f PLÂTE I

EIIERGENCY SPILL WAY tt.õ NORTH DAKOTA STATE WATER COMMISSIOH t562 ioRt¡ oAxoÌa aèl- - ittHAtct. ú.5 Éo PROJECT tlo. l4lS -tzo---- ta DAM rlt BIG COULEE t578 ,2t SITE TOPOGRAPHY- AS EUILT ?!t ù ,ó ÊñrßtY lowNFF DÄ18 A-7-¡O Stc. 36, Twp. 160 N., Rgc.6E W. -'

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I I c , sa i/ I rl ==t==:= R il r" lt Allcn è M,orrts f ¡7ts-toThzrton ' õ€ LEGENO ( (ForcsÞ) z ;: Ac.rÈ ftd '-ÉvrM 5l: )+ æ 8o*¡r r.,"æ s¡a Êæfr.¡ .SiEþo^ DB E x o À N Elrf E llolh Racapiäc ra XA g NEJ4 Í ql Bæ+ oo¿k PARKIN6 IIE B".l L¡u¡'ch . ÂRÉA ñ Í) X Ôtgo ì.1qt6¿¡'q I ¿¡tl â 6tsbcc) N [_t.k. 'x^ 016 Ae. x./ g I

l I ¡¡¡"1 f Morris CårisloPhørson PROPO$ED þiebca; N. Þck I óJ5 I K+

¡ I G' I I I I l. I ec. I Ac. ) I I StN )z+ _oB I W./t I 7 /59 i{. * r^l RECREATION AREA I R. 67 L scers t". loo' N \^,'J4 I sw,4 -r- I svl)t+

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I I 'ï t60 N. R. ø8W CONTROL ELEVATION 1572.0 MSL - / t= A¿- CONTROL ELEVATION 7574.0 MSL 7 t59 ü- R.. €8 w' CONTROL ELEVATION 7576.0 MSL ;'úP taU L at + A.A3 Ac ].OO YEAR FLOOD EASEMENT EL. 1580.0 MSL (T'OR CONTROL EL. 7576) I I 3EX NE/+ 3c-Y4 I sbce, N Da I L or nwy . * _= 1OO YEAR FLOOD EASEMENT EL. 1578.2 IT,ISI (FOR CONTROL EL. 7574)- 2--í I

R66Uf R.6r W PRESENT CONDITIONS: ACÔUIRTD 326,Itì ACRES PAêæSEO ÐA É 5'7É TOÎAL ACREAGE fz- È ôt Ro& TOTAL ÀCR¿À6E PERMÀ}'EI{T EASEMEMT 1.35ACRE9 Þ f ¡æE TOTAL ACFIEÄGE FLOOO EÀsEJ'4ËNT 17.33 ACRES DECK oEcx EL 1585 5 :R CEeK ÉL t5=6.É- EL. ur{oER DF-CÍ eL r!84 o --(2 ''5A,I 'C. t6t f, - -=- àetzo l1 eoe -1,I Locnt¡oN M¡e PLATE 5 q4x ær7aM Ê1. ¡547.1 eFEEK Bo.ÍroM EL ta7t.o t' . ¿ M¡L¿s ): WJDE 2ô' ,vúIOE GAPACITY TABLE -- ^ffÁ |-L ÆBE: NORTH OAKOTA ôR I DGE iuo- I B R lD6¡ No. 2- STAÏE WATER COMMISSION Nê gc'ALÊ NO SCAL€. 25 t-\ 275.8 I ,998. 2 ErsrÁFûx- trôÂli oa

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EXÉ,ßGENCY SPILLWAY PLATE 4 fta ãf NORÍH DAKOTA A.a----'¿' STATE WATER COMMISSION 156e iotfx olfo aa! ato PRoJ€CT llo. l4t8 -tzo---- r.c no6 BIG COULEE DAM tja ,57e AREA OF DOIIINSTREAM EMBANKMENT nl 7+a ,à PROPOSED FOR MODIFICAT]ON 16' src. 36, ÎrP. 160 N., Rgc.68 W HO, 9 t4 -23