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Works of Improvement Structure 8-2 , I I , , Works of Iaq>rovement Structure B-2 and B-3 Floodwater Retardi~ Structure B-2 Floodwater retarding structure B-2 is located on Boxelder Creek just below juncture with Rawhide Creek. Most of the site is on rangeland. Site Geology Bedrock underlying the B-2 dam site consists of silty clay shale of the upper part of the Pierre Formation of Upper Cretaceous age. The shale is firm with little or no fracturing and is relatively impermeable. It is at or near the surface in both abutments of the daJn and in the emer- gency spillway area. The valley bottan at the site contains alluvial deposits consisting mainly of a few feet of sandy silt and clay over- lying permeable sands and gravels. Wells downstream fran the darn indi- cate that the maxiJrurn thickness en the alluvium is about SO feet. The emergency spillway will be excavated into silty and sandy shale. Ade- quate amounts of satisfactory borrow materials are available fran the emergency spillway excavation and fran the reservoir area. Classification, Drainage Area and Storage This structure has (b) classification. The total drainage area above the structure is 111.1 square-miles of which 34.6 square-miles is con- trolled by B-5 and 8-6. The remaining area controlled by B-2 is 76.5 square-miles. The structure has a lOa-year sedilllent storage of 1,162 acre-feet and the SO-year frequency storm runoff storage of 5,304 acre-feet for a total storage of 6,466 acre-feet. cnbankment The embankment will consist of about 865,000 cubic-yarOs of rolled earthfill with a maxinrull height of about 46 feet and a top length of about 5,92D feet. The embankment will be a zoned fill section having a center core and upstream and downstream zones CQIlSisting of the coarse permeable material available at the site. The downstream zone will ex- tend down through the natural blanket of 4 to 6 feet of fine material and connect to gravels underlying the site to provide embanl:ment drain- age. Embankment slopes will be 3:1 upstre3lD and 2:1 dolmstreron. Foundation Treatment A alt-off core will be made for a short distance in each abutment where shale is near the surface. About 35,OOD albic-yards of the natural blanket of fine material under the downstream zone of the embankment will be I"elllOved and replaced with permeable IlIaterial. Suppl~ntal blanketing will be done in the present channel and old channel areas to obtain a continuous blanket a mininwm of 3 feet thick extending at least 500 feet upstream fran the upstream toe of the daro. This is estimated to take &boot 9O,ODO cubic-yards of earth fill. Principal Spillway The principal spillway conduit will be a 48.inch diameter reinforced concrete pipe. The inlet structure will be a single stage standard two-way covered riser 20 feet high. The riser crest will be at the SO-year sediment pool level. The release time for the retarded vollX1le is 10 days. The 50-year sediment pool will be drained by a 30-inch diameter Utlgated drawdown pipe. The principal spillway will have an impact basin outlet structure. Canal Crossing ln addition to the principal spillway, a 48-inch diameter gate" conduit 110 feet long will be constructed through the dam to pass the North Peudre Irrigation Company diversion canal flows. Canal bottom elevation at the dam centerline station 67 + 10 is 5514.7 feet. It will have a slide gate ....ith control WQrks on top of the dam. It will have an im- pact basin outlet strncture. The irrigation canal and diversion are above the HlD-year sediment pool and will not be relocated. Emergency Spillway The emergency spillway will be vegetated earth. It ....ill be constructed at the left abutment of the dam. The bottom width will be 875 feet, the spillway depth is 7.0 feet and the length of level section is about 1,300 feet. The frequency of emergency spillway fleM is once in 5D years. The spillway excavation ....ill be in earth and shale. The botton will be cut one foot below grade and backfilled with topsoil so that vegetation can he established and the sh:lle will not be exposed. Land Rights and Land Use The site requires about 520 acres for the dam, emergency spillway a:ld flood pool. The majority of the area is presently rangeland. There are 22 acres of irrigated cropland and 120 acres of dry cropland in the floodwater pool. There are no improvements in the site area except the canal and diversion structure. Floodwater RetardiNl Structure B-3 Floodwater retarding structure B-3 is located on Coal Creek about seven miles above the town of Wellington, ColoraJo. Site Geology BeJrock underlying the B-3 dam site consists of silty clay shale of the upper part of the Pierre Formation of Upper Cretaceous age. The shale is firm with little or no fracturing anJ is relatively impermeable. It -24- -23-