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- 22 - 0:22C9 " Eng~neering Investigations Field investigations to determine structural measure needs were conducted on the two drainages, Fishers Peak and Carbon Arroyos, above the City of Trinidad. These investigations resulted in the selection of two sites, FPC-l and FPC-2, for floodwater-retarding storage. Structure FPC-I is on Fishers Peak Arroyo and FPC-2 is on Carbon Arroyo. A diversion, FPC-I Div., was also determined to be needed in the upper southwest corner of the Fishers Peak drainage to divert floodwater from the upper portion of a small uncontrolled tributary into the floodwater detention area of structure FPC-I, Site locations included consideration of: (1) topography, (2) site conditions, and (3) existing facilities including roads and culverts. An existing closed conduit conveys the runoff from Fishers Peak Arroyo through the City of Trinidad. The conduit was built of rubble masonry, grout, and reinforced concrete-and the floor has poor hydraulic characteristics. The floor will be replaced with new reinforced concrete slab, FPC_l Chan., to insure its margin of operating safety. A reinforced concrete and rock stilling basin will be constructed at the lower end of the closed conduit to stabilize the flow within the conduit and to control erosion downstream. Detailed topographic maps of the dam sites and pool areas were obtained by planet able surveys on both of the proposed floodwater- retarding structures. Geologic investigations were made and soil samples were taken in the respective borrow areas. Based on results of soil tests by the Soil Mechanics Laboratory, it was interpreted that a zoned embankment with a core of CL material 11 placed at 90 per cent of modified AASHO would be most feasible. It is believed that 3: I upstream and 2t: 1 downstream slopes would be satisfactory with berms as needed for erosion protection and with drains under the downstream toe. Shear tests will be made on the CL material at 90 per cent of modified and on the GC-GM material before final design. ! The floodwater-retarding structures are classified as class "c" based on the interpretation of Engineering Memorandum SCS No. 27, revised. Capacity for sediment storage is provided in each structure to contain the estimated sediment accumulation at the sites for the next 50 years. The floodwater-retarding capacity of both structures is based on the runoff of four inches occurring on the wat~rshed less frequently than once in 100 years. A typical structure is shol~ on Figure 3. " 11 Soil type based on Unified Soil Classification System.