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Works of Improvement StructureB-4 Works of Improvement Structure B-4 and B-S The emergency spillway will be excavated into silty clay lying over silty clay shale which will require sane protection frorn erosion. Adequate . amounts of satisfactory borrow materials are available from the reservoH area close to the dam. About 12.0 acres of dry cropland is in the floodwater pool with the remainder being rangeland. Floodwater Retaroinr, Structure B-S Floodwater retarding structure B-5 is located on upper Boxelder Creek about 2-1/2 miles above juncture with Sand Creek. Site Geology Bedrock underlying the B-5 dam site consists of red shales. siltstones, rmd sandstones of the Lykins Fonnation of Permian Age. The rock layers dip about 15 degrees to the east and crop out only in the upper part of the right abutment. An alluvial fan consisting of sandy silt and clay with a few thin layers of cobbles and boulders forms the left abutment of the dam site. The valley bottan at the site contains alluvial deposits cOll5isting mainly of sandy and clayey silt. The alluviun material appears to bave a fairly 101<1 shear strength and a high consoli- dation potential. The maximLan thickness of the alluvllvn in the left abutment and in the valley bottom is probably about 40 feet. The emer- gency spillway will be excavated into red silty shales and shaly silt- stones which should be moderately resistant to erosion. Adequate alnOWlts of borrow materials are available from the emergency spillway excavation and fran the reservoir area close to the dam. Classification, Drainage Area and Storage The StructUI~ has (a) classification. The drainage area above the struc- ture is 19.6 square-miles. The structure has 100-year sediment storage of 198 acre- feet. Floodwater capaci ty is the average of the 2S-year and the SO-year frequency storm runoff which is 941 acre-feet. Total capadtyisl,139a.cre-feet. Embankment Classification, Drainage Area and Storage Structure B-4 has (a) classification. The drainage area above the structure is 13.5 square-miles. The structure has 100-year sediment capacity of 188 acre-fect. Floodwater capacity is the average of the ZS-year and 50-year frequency stom nmoff tohich is 720 acre-feet. Total capacity is 908 acre-feet. Embankment The embankment will consist of about 114,000 cubic-yards of compacted earthfill. The maximun height of the embankl1ient is about 27 feet and the top length is about 2,170 feet. The embankment will be a hCllloge- neous fill having an upstream slope of 3:1 and a downstream slope of 2.5:1. A gravel filter blanket will be installed unde~ the d~TIstream toe of the dam to provide embankment drainage. Foundation Treatment A core to bedrock will be constructed to provide a positive cutoff. This is estimated to ~equire about 32,000 cubic-yards of excavation and compacted earthfill. Principal Spillway 'fhe principal spillway will be a 30-inch diameter reinforced concrete pipe about 135 feet long. The inlet structure will be a single stage standa~d two-way covered riser about 12 feet high. The riser crest will be at the 50-year sediment pool level. Release time for the retarded voluoe is about 5 days. The SO-year sediment pool will be drained by a 24-inch diameter ungated drawdown pipe. The principal spillway will have an impact basin outlet structure, The embanlonent will consist of about 318,000 cubic-yards of compacted earthfill including about 22,000 cubic-yards of gravel filter material. The 1IIaXi.rrun height of the emankment is about 67 feet and the top length is about 1,240 feet. The embankment will be a homogeneous fill except for a chimney drain system. Elltankment will have a 3.5:1 upstream slope and a 3:1 downstream slope. F~rgency Spillway The emergency spillway will be vegetated earth. It will be constructed around the east end of the dam. The ~pillway depth will be 6.0 feet, the hettan width will be Z20 feet and the level section is about 400 feet. The frequency of emerllency spillway flow is once in 37 years. 1"0unJation Treatment 1he site requires flood pool area. the dam, the dam, about 137 acres for the dam, emergency spillway and Except for ahoot 0.1 acres of dry cropland needed for emergency spillway and sediment pool are rangeland. Foundation treatment for the B-S structure will consist of rem:wing and recompacting the foundation material beginning at the ground surface and the embankment toes and extending downward on a 2:1 slope to the water table. Below the water table a core with 2:1 slopes and 12 foot bottan width will be constructed to bedrock. This is estimated to require moVing about 138,000 cubic-yards above the water table and 37,000 cubic yards below the water table. Land Rights and Land Use -27- -28-