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<br />Works of I:nprovar.cnt <br />Structure B-3 <br /> <br />I <br />, <br />I <br /> <br />Works of Improvement. <br />St.ruct.ure B-3 and B.4 <br /> <br />is at or near the surface in both abutments of the dam and in the emer- <br />gency spillway area. The valley bott(Jll at the site contains alluvial <br />deposits consisting df a few feet of silty sand and silty clay overlying <br />penneable sands and gravels. The maxilwm thickness of the alluviUIII is <br />probably about 40 feet. The emergency spillway will be excavated into <br />silty and sandy shale which will require sane protectlOll irern weathering. <br />Adequate amounts of satisfactory borrow materials are available fnn the <br />emergency spillway excavation and from the reservoir area. <br />Classification, Drainage Area and Storage <br />This structure has (b) classification. The total drainage area above <br />the structure is 63.3 square.miles. The runoff fran 11.4 square-miles <br />of the area is stored by the existing Round Butte Resenroir leaving the <br />nmoff fran 50.9 square-miles to be controlled by B-3. The structure <br />has a lOO-year sediment storage of 524 acre-feet and the SO-year stonn <br />runoff storage plus 544 acre-feet added to reduce emergency spillway <br />excavation for a floodwater capacity of 3,258 acre-feet. The total <br />capacity is 3,782 acre-feet. <br /> <br />covered riser about 23 feet high. The riser crest will be at the SO- <br />year sed~nt pool level. Release tiJr.e for the retarded volume is about <br />nine days. The 50-year sed:iJnent pool will be drained by a 24-incll dia- <br />meter ungated drawdown pipe. The principal spillway will have an impact <br />basin outlet structure. <br /> <br />Emergency Spillway <br />The emergency spillway will be vegetated earth. It will be constructed <br />arOWld the east end of the daIlI. The spillway depth will be 7,0 feet, <br />the bottOOl width will be 410 feet and the level section length will be <br />about 1,000 feet. The frequenc::y of emergency spillway flow is once in <br />50 years. <br />The spillway excavation will be in shale. The spillway bottern will be <br />cut one foot below grade and backfilled with topsoil 50 that vegetation <br />c:an be established and so the shale will not be exposed. <br />Land Rights and Land Use <br /> <br />Embankment <br /> <br />, <br />I <br />I <br />, <br />, <br /> <br />The site requires about 290.0 acres for the dam, emergency spillway and <br />flood pool. About 267 acres of the site are presently rangeland. <br />Except for about 0.2 acres of roadway needed for the dam, the dam,emer- <br />geney spillway and sediment pool are on rangeland. There are about 20 <br />acres of dry cropland and about 3.3 acres of roadway in the floodwater <br />pool with the remainder rangeland. <br /> <br />The cll'Vankment will consist of about 364,000 cubic-yards of rolled <br />earthfill. The 1IIllJ(:iJm.mJ height of the dam is about 51 feet and the top <br />length is about 2,675 feet. The embankment will be a zoned fill section <br />consisting of a center core with coarse materials for the outer zones <br />of the section. The downstream zone will extend dlMl1 through the natu- <br />ral blanket of 3 to 6 feet of silt-clay material and connect with gravels <br />underlying the site to provide embankment drainage. l2nbanJ:ment slopes <br />will be 3:1 upstream and 2:1 downstream. <br /> <br />roondation Treatment <br /> <br />A county road and an adjacent power line cross the west side of the pool <br />area, The power line will be modified by installing and anchoring taller <br />poles. The road grade at the west end of the dam will he raised to the <br />ettbanlonent top elevation. The road will be posted with depth g:!.gcs and <br />closed for short periods during flood runoff storage. The road has <br />light use and during flood periods an alternative route is available <br />and is customarily used during flood flows. <br /> <br />Foundation treatll\ent consists of removing the silt-clay natural blanket <br />under the downstream zone of the ernbanlanent and replacing it with highly <br />permeable sand and gravel =terial. This will take about 19.000 cubic- <br />yards of excavation and earthIill. <br />A cutoff to bedrock will be made in each abutment where shale is near <br />the surface. An upstream blanket will be used across the majority of <br />the dam. This will require supplemental blanketing in the present chan- <br />nel and flood channels to obtain a continuous blanket a mintmum of 3 <br />feet thick extending at least 500 feet upstream fran the upstream toe <br />of the dam. This is estimated to take about 60,000 cubic-yards of <br />eat'thfill. <br /> <br />Floodwater RetardillR Structure 3-4 <br /> <br />Floodwatet' retarding structure 8-4 is located on Indian Creek about one <br />mile north of the Mountain Supply Reservoir northeast of Wellington, <br />Colorado. <br /> <br />Principal Spillway <br />The principal spillway will be a 36-inch diameter reinforced concrete <br />pipe. The inlet structure will be a single staRe standard two-way <br /> <br />Site Geology <br />Bedrock Wlderlying the B-4 dam site consists of silty clay shale of the <br />upper part of tne PierTe Formation of Upper Cretaceous age. The shale <br />is firm with little or no fracturing and is relatively impermeable. It <br />is within a few feet of the surface in both abutments and in the eoor- <br />genc::y spill....ay area.. The valley bott(lll at the site contains alluvial <br />deposits consistillR mainly of silty and sandy o:::lay with a few thin sandy <br />layet's. The maxinun thickness of the alluvilJlll is probably about 30 feet. <br /> <br />-25- <br /> <br />-26- <br />