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<br />During feasibility in vestigBtions, other features were. studied as
<br />potentinl additions to tbe pion of development, These lllelude tbe
<br />Bijou Creek diversion channel to provicle control of floods Qriginating
<br />on Bijou Creekj tbe Riverside, BijOll, and Sterling {Jumping plnnts;
<br />and the O,-id weil field, Descriptions of these potcntuu additions are
<br />presented Bubsequently in this part of the repol't. .
<br />No provisi.ons for hydroelectric power development ha.ve been lll-
<br />eluded since the conditions which rendered (,bis potentiality infellSihle
<br />lIS dctermined in the 1951 definite plan in,estigntions, have not
<br />changed, ,
<br />M unicipaJ and indust.rial wnter supply bns not been meluded as a
<br />unit function for lack of my forsecable need as descnbed subsequently,
<br />
<br />NARROWS DAM ANn RESERVOrR
<br />
<br />elevation 4,453,0, A cutoff trench backfilled with impervious material
<br />would extend under the leCt abutment sou across the river cha.nnel.
<br />Tbe upstream face of tile dam would he protected by ~ 3-foot layer of '
<br />rock rlpra.p plt\ced on an IS-inch gravel blanket. The downstream face
<br />would be covered wit1112 inches of topsoil and seeded t.o grass. Total
<br />volume of embaukment in the dam would be ..bout 8,840,000 cubic
<br />yards,
<br />An llncontrol1ed concrete spillway di::3charging into a. chute and
<br />stilling basin would be located io tbe Iclt abutment. A 30-foot-wide
<br />concrete bridge would be provided ..cross tbe spillway for relocated
<br />State Higbway 144, The spillway, with a crest elev..tion of 4,428,5,
<br />the top of tbe flood control pool, was designed with a capacity of
<br />27,600 cubic feet per second with the reservoir water surface at
<br />elevation 4,447,0, tile top of surcharge capacity, In tbe design, a
<br />surcharge of .176,000 acre-feet plus 384,000 acre-feet of exclusive
<br />flood control capacity in combination with ll. spillway discharge of
<br />27,600 cubic feet per second and a river outlflt discharge of 8,600
<br />cubic feet per second was provided to protect against all inflow design
<br />flood of 344,000 cubic feet per second and a 20-day volume of 1,363,000
<br />acre-feet. Subsequent to this design the reservoir alloca.tions werfl
<br />adjusted to allon" 475,000 8.Cre-feet ror exclusive Rood conlrol and
<br />7.1,000 ..cre-feet. for joint use lor /lood control and irrigation, Adjust-
<br />ment of tbe spillway capacity would be minor, and the design w..s not
<br />changed. Area-capacity data and storage allocations for the re5ervoir
<br />are shown on exhibit 4. Corresponding data shown on exhibit 3 are
<br />rounded to the nearest 1,000 acre-feet.
<br />The river outlet works would be located /lear the left abutment.
<br />Two 10-Ioot~diameter circulo.l" concret.e conduits would lead from tbe
<br />intake to it shaft and gate structure under the darn. Two lO-Ioot-
<br />diameter steeJ pipes inside two 14-foot-diameter concrete horseshoe
<br />cOrlduit!'l would exlend from the gate struc.ture t.o a. control house at
<br />the downst.ream toe of the dam, High-prcssure slide gates would be
<br />pro\;ded for each conduit jn the gate chamber and the control house.
<br />Tbe outlct " arks would be capable of discharging 8,600 cubic feet
<br />per second with t.he reservoir water surface at elevation 4,447, t.he top
<br />of 5w-charge capacity, and would be lIsed tllong with the spillway to
<br />prof.eet the dam during tloo&~. During normal operation, t.he outlet
<br />works \\.'ould be used to make releases to the river for downstream
<br />users. In a letter dat.ed September 14, 1965, which is appended to this
<br />report, the Corps of Engineer5 indicated that an out.let capacity up
<br />to ]0,000 cubic feet per second \\'ould ha,'e to be availahle, The corps
<br />later explamed ihat tbe 10,000 cubic fcet per secoTld was intended to
<br />represent the maximum allowable downstream. discharge to obtain
<br />tbe ev~uatcd benefits nnd that the designed rker outlet capacity
<br />was 58 tlsf Rctory.
<br />An outlet for tbe Fort Morgan Callal would be located near the
<br />right abutment and a short distance to the lefL of tbe existing Fort
<br />Morgan Ctlllal which it \\'ould serve. A 9-foot 6-inch circular concrete
<br />conduit would lea.d from the intRke to a gat.e chumber under the dam.
<br />A 6-foot-d.i~meter steel pipe il\side a IO-root-diameter concrete Lorse-
<br />ghoe condUIt. would extend from the gate chamber and control houge.
<br />The capacity would be 350 cubic feet per second with the reservoir
<br />water surface at e.levaUon 4,3.51, the bottom of the conser.....ation pool.
<br />
<br />The Narrows Dam site is located in sections 7 tlnd 18, T. 4 N., R.
<br />58 W, and sections 13, 23, 24, 25, and 26, T',4 N" R. 59 W" about
<br />7 miles northwest. of Fort .Morgan. Th~ locatIOn (If the dan.ls!te With
<br />proposed r~ilroad and highway relocatIOns is shown on exhIbIt 2,
<br />
<br />Geology of d<L7nsite
<br />Types of subsurface materials of imp~rtance at. the Na.rrows Dam
<br />site are alluvial va.lle.y-fill sand, i.errace silt, nnd PIArre sh~le bedrock,
<br />The present South Platte River channel has eroded into the north wall
<br />of the oldcr and much deeper channel, the thalweg 01 ,which ,lies ~~-
<br />proximately 3 miles to t.he south. The aUllvlal ~and With whlC~ thIS
<br />ancestral cbanoel is filled is exposed belo>y the blgh water level m the
<br />reservoir area. Since the sand bos n slgm.fic.a.nt coeffiCIent or perme-
<br />ability, it ig expected t.hat an appreciable seepage J~ss wouId occ.ur
<br />through the sand into the ancestral ch..nne!. Overl.l'lIlg the allU\'~al
<br />sand, on t.he right side of the present channel, is a layel' of terrll.ce silt
<br />of vnrying t.hickness.
<br />At the left. abutment.nnd beneath the river cbaru1el, tbe shale is at
<br />shallow deptbs, but bas undergone weathering ~ffcct.s to considerable
<br />depth, Where the impervious cmbankment., spillway, and the outlet
<br />condu,"s ,,,e on Picrre shale exeavatlOn should be made to un-
<br />wcnthered rock, Because of ~welling tendencies wben subjected to
<br />saturution, a spraved protective coating o~ mortnr w.ould be reqUIred
<br />on excavated shale surfaces {or tbe condmt.s and spillway structure.
<br />In ..ddition there will be some rebound as the load is removed by
<br />excavation, 'and both design A.nd construction prac.tices should take
<br />t,hh; into considerp.,tion.
<br />The Pierre sbale formation slopes down from the river cho.nnel
<br />toward tbe right end of the dam to such depth that. it is not. fel1.Sible
<br />to seal the impervious embankment, to bedrock. Probable water losses
<br />through the sand and gravel overlaying the shale along the right abut-
<br />ment cannot be fully controUeo; but \elier weBs and toe drnins.. ~s
<br />included in t.be dam design, would provl,de structural safety and hmlt
<br />downstream seepage to tolerable q uan tItles.
<br />
<br />Dam and reservoir
<br />Thc dam .hown on the feasibility de.ign drawing, exhibit 3, would
<br />be a zoned, rolled earthfill structure v..rith a height o.boye strea.mbed of
<br />146 feet, a crest width of 30 feet, and a Icngth of 22,120 feet at crest
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